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3.3.1 Ice Sheets - Biology

3.3.1 Ice Sheets - Biology



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Warming of the Earth’s surface poses a particular threat for the Earth’s ice systems including ice sheets, sea ice, permafrost (frozen ground), and glaciers. Antarctica and Greenland hold the Earth’s two ice sheets, and both lose hundreds of gigatonnes (109 tonnes) of ice mass each year to melting (Fig 3.3.1.1).

Figure (PageIndex{1}): Mass loss in Antarctic (left) and Greenland (right) ice sheets due to warming. Images from NASA1.

In addition to persistent melting, the patterns of ice loss in Figure 3.3.1 lead to instability in portions of the ice sheets that can also result in major catastrophic events. A series of such events have occurred in the Larsen Ice Shelf on the Antarctic Peninsula. In ice shelf is a mass of ice that is anchored to land, but which juts out over the ocean, like a shelf. The Larsen A portion of the ice shelf collapsed and melted into the ocean in 1995. The Larsen B portion collapsed in 2002, dropping a sheet of ice the size of the US state of Rhode Island into the Southern Ocean, where it melted (Fig 3.3.1.2). In 2015, the Larsen C portion developed a 120-mile long crack from which, in 2017, a sheet of ice the size of the US state of Delaware collapsed into the Southern Ocean and melted.

Figure (PageIndex{2}): The Larsen B ice shelf (left) in the Antarctic Peninsula with a scale comparison to the US state of Rhode Island which collapsed and melted into the Southern Ocean in February of 2002. The location of the Larsen A, B, and C shelves in the Antarctic Peninsula (right) prior to the collapse of portions B and C. Images from Wikimedia Commons2,3.

Recently, both Greenland and Antarctica have experienced short intense heat waves which have significantly increased localized melting. In July 2012, NASA’s Earth Observatory documented surface melting across 97% of the Greenland ice sheet in response to a week-long heat wave. On June 18th 2019, Greenland experienced a 40℉ temperature increase which led to massive surface warming and an estimated loss of 11 billion tons of surface ice in a single day. Similarly, Antarctica set its highest temperature record of 65℉ on February 7th, 2020, leading to significant ice loss along the Antarctic peninsula, and associated islands, such as Eagle Island.


Ice age

An ice age is a period of long-term reduction in the temperature of Earth's climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers.

Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist).

More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents: in this sense, the most recent ice age ended about 10,000 years ago.

There are three main types of evidence for ice ages: geological, chemical, and paleontological.

Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics.

Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret.

It took some time for the current theory to be worked out.

There have been at least four major ice ages in the Earth's past.

Outside these periods, the Earth seems to have been ice-free even in high latitudes.


Contents

Charles Lyell introduced the term "Pleistocene" in 1839 to describe strata in Sicily that had at least 70% of their molluscan fauna still living today. This distinguished it from the older Pliocene epoch, which Lyell had originally thought to be the youngest fossil rock layer. He constructed the name "Pleistocene" ("Most New" or "Newest") from the Greek πλεῖστος (pleīstos, "most") and καινός (kainós (latinized as cænus), "new") [7] [8] [9] this contrasts with the immediately preceding Pliocene ("newer", from πλείων (pleíōn, "more") and kainós) and the immediately subsequent Holocene ("wholly new" or "entirely new", from ὅλος (hólos, "whole") and kainós) epoch, which extends to the present time.


The Pleistocene has been dated from 2.580 million (±0.005) to 11,650 years BP [10] with the end date expressed in radiocarbon years as 10,000 carbon-14 years BP. [11] It covers most of the latest period of repeated glaciation, up to and including the Younger Dryas cold spell. The end of the Younger Dryas has been dated to about 9640 BC (11,654 calendar years BP). The end of the Younger Dryas is the official start of the current Holocene Epoch. Although it is considered an epoch, the Holocene is not significantly different from previous interglacial intervals within the Pleistocene. [12] In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Calabrian, Chibanian (previously the unofficial "Middle Pleistocene"), and Upper Pleistocene (unofficially the "Tarantian"). [13] [14] [note 1] In addition to these international subdivisions, various regional subdivisions are often used.

In 2009 the International Union of Geological Sciences (IUGS) confirmed a change in time period for the Pleistocene, changing the start date from 1.806 to 2.588 million years BP, and accepted the base of the Gelasian as the base of the Pleistocene, namely the base of the Monte San Nicola GSSP. [16] The start date has now been rounded down to 2.580 million years BP. [10] The IUGS has yet to approve a type section, Global Boundary Stratotype Section and Point (GSSP), for the upper Pleistocene/Holocene boundary (i.e. the upper boundary). The proposed section is the North Greenland Ice Core Project ice core 75° 06' N 42° 18' W. [17] The lower boundary of the Pleistocene Series is formally defined magnetostratigraphically as the base of the Matuyama (C2r) chronozone, isotopic stage 103. Above this point there are notable extinctions of the calcareous nanofossils: Discoaster pentaradiatus and Discoaster surculus. [18] [19] The Pleistocene covers the recent period of repeated glaciations.

The name Plio-Pleistocene has, in the past, been used to mean the last ice age. Formerly, the boundary between the two epochs was drawn at the time when the foraminiferal species Hyalinea baltica first appeared in the marine section at La Castella, Calabria, Italy [20] however, the revised definition of the Quaternary, by pushing back the start date of the Pleistocene to 2.58 Ma, results in the inclusion of all the recent repeated glaciations within the Pleistocene.

Radiocarbon dating is considered to be inaccurate beyond around 50,000 years ago. Marine isotope stages (MIS) derived from Oxygen isotopes are often used for giving approximate dates.

Pleistocene non-marine sediments are found primarily in fluvial deposits, lakebeds, slope and loess deposits as well as in the large amounts of material moved about by glaciers. Less common are cave deposits, travertines and volcanic deposits (lavas, ashes). Pleistocene marine deposits are found primarily in shallow marine basins mostly (but with important exceptions) in areas within a few tens of kilometers of the modern shoreline. In a few geologically active areas such as the Southern California coast, Pleistocene marine deposits may be found at elevations of several hundred meters.

The modern continents were essentially at their present positions during the Pleistocene, the plates upon which they sit probably having moved no more than 100 km (62 mi) relative to each other since the beginning of the period. In glacial periods, the sea level would drop by over 100 m (330 ft) during peak glaciation, exposing large areas of present continental shelf as dry land.

According to Mark Lynas (through collected data), the Pleistocene's overall climate could be characterized as a continuous El Niño with trade winds in the south Pacific weakening or heading east, warm air rising near Peru, warm water spreading from the west Pacific and the Indian Ocean to the east Pacific, and other El Niño markers. [21]

Glacial features Edit

Pleistocene climate was marked by repeated glacial cycles in which continental glaciers pushed to the 40th parallel in some places. It is estimated that, at maximum glacial extent, 30% of the Earth's surface was covered by ice. In addition, a zone of permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometres in North America, and several hundred in Eurasia. The mean annual temperature at the edge of the ice was −6 °C (21 °F) at the edge of the permafrost, 0 °C (32 °F).

Each glacial advance tied up huge volumes of water in continental ice sheets 1,500 to 3,000 metres (4,900–9,800 ft) thick, resulting in temporary sea-level drops of 100 metres (300 ft) or more over the entire surface of the Earth. During interglacial times, such as at present, drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions.

The effects of glaciation were global. Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene. The Andes were covered in the south by the Patagonian ice cap. There were glaciers in New Zealand and Tasmania. The current decaying glaciers of Mount Kenya, Mount Kilimanjaro, and the Ruwenzori Range in east and central Africa were larger. Glaciers existed in the mountains of Ethiopia and to the west in the Atlas mountains.

In the northern hemisphere, many glaciers fused into one. The Cordilleran Ice Sheet covered the North American northwest the east was covered by the Laurentide. The Fenno-Scandian ice sheet rested on northern Europe, including much of Great Britain the Alpine ice sheet on the Alps. Scattered domes stretched across Siberia and the Arctic shelf. The northern seas were ice-covered.

South of the ice sheets large lakes accumulated because outlets were blocked and the cooler air slowed evaporation. When the Laurentide Ice Sheet retreated, north-central North America was totally covered by Lake Agassiz. Over a hundred basins, now dry or nearly so, were overflowing in the North American west. Lake Bonneville, for example, stood where Great Salt Lake now does. In Eurasia, large lakes developed as a result of the runoff from the glaciers. Rivers were larger, had a more copious flow, and were braided. African lakes were fuller, apparently from decreased evaporation. Deserts, on the other hand, were drier and more extensive. Rainfall was lower because of the decreases in oceanic and other evaporation.

It has been estimated that during the Pleistocene, the East Antarctic Ice Sheet thinned by at least 500 meters, and that thinning since the Last Glacial Maximum is less than 50 meters and probably started after ca 14 ka. [22]

Major events Edit

Over 11 major glacial events have been identified, as well as many minor glacial events. [23] A major glacial event is a general glacial excursion, termed a "glacial." Glacials are separated by "interglacials". During a glacial, the glacier experiences minor advances and retreats. The minor excursion is a "stadial" times between stadials are "interstadials".

These events are defined differently in different regions of the glacial range, which have their own glacial history depending on latitude, terrain and climate. There is a general correspondence between glacials in different regions. Investigators often interchange the names if the glacial geology of a region is in the process of being defined. However, it is generally incorrect to apply the name of a glacial in one region to another.

For most of the 20th century only a few regions had been studied and the names were relatively few. Today the geologists of different nations are taking more of an interest in Pleistocene glaciology. As a consequence, the number of names is expanding rapidly and will continue to expand. Many of the advances and stadials remain unnamed. Also, the terrestrial evidence for some of them has been erased or obscured by larger ones, but evidence remains from the study of cyclical climate changes.

The glacials in the following tables show historical usages, are a simplification of a much more complex cycle of variation in climate and terrain, and are generally no longer used. These names have been abandoned in favor of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognized since the historical terminology was established. [23] [24] [25]

Historical names of the "four major" glacials in four regions.
Region Glacial 1 Glacial 2 Glacial 3 Glacial 4
Alps Günz Mindel Riss Würm
North Europe Eburonian Elsterian Saalian Weichselian
British Isles Beestonian Anglian Wolstonian Devensian
Midwest U.S. Nebraskan Kansan Illinoian Wisconsinan
Historical names of interglacials.
Region Interglacial 1 Interglacial 2 Interglacial 3
Alps Günz-Mindel Mindel-Riss Riss-Würm
North Europe Waalian Holsteinian Eemian
British Isles Cromerian Hoxnian Ipswichian
Midwest U.S. Aftonian Yarmouthian Sangamonian

Corresponding to the terms glacial and interglacial, the terms pluvial and interpluvial are in use (Latin: pluvia, rain). A pluvial is a warmer period of increased rainfall an interpluvial, of decreased rainfall. Formerly a pluvial was thought to correspond to a glacial in regions not iced, and in some cases it does. Rainfall is cyclical also. Pluvials and interpluvials are widespread.

There is no systematic correspondence of pluvials to glacials, however. Moreover, regional pluvials do not correspond to each other globally. For example, some have used the term "Riss pluvial" in Egyptian contexts. Any coincidence is an accident of regional factors. Only a few of the names for pluvials in restricted regions have been stratigraphically defined.

Palaeocycles Edit

The sum of transient factors acting at the Earth's surface is cyclical: climate, ocean currents and other movements, wind currents, temperature, etc. The waveform response comes from the underlying cyclical motions of the planet, which eventually drag all the transients into harmony with them. The repeated glaciations of the Pleistocene were caused by the same factors.

The Mid-Pleistocene Transition, approximately one million years ago, saw a change from low-amplitude glacial cycles with a dominant periodicity of 41,000 years to asymmetric high-amplitude cycles dominated by a periodicity of 100,000 years. [26]

However, a 2020 study concluded that ice age terminations might have been influenced by obliquity since the Mid-Pleistocene Transition, which caused stronger summers in the Northern Hemisphere. [27]

Milankovitch cycles Edit

Glaciation in the Pleistocene was a series of glacials and interglacials, stadials and interstadials, mirroring periodic changes in climate. The main factor at work in climate cycling is now believed to be Milankovitch cycles. These are periodic variations in regional and planetary solar radiation reaching the Earth caused by several repeating changes in the Earth's motion.

Milankovitch cycles cannot be the sole factor responsible for the variations in climate since they explain neither the long term cooling trend over the Plio-Pleistocene, nor the millennial variations in the Greenland Ice Cores. Milankovitch pacing seems to best explain glaciation events with periodicity of 100,000, 40,000, and 20,000 years. Such a pattern seems to fit the information on climate change found in oxygen isotope cores.

Oxygen isotope ratio cycles Edit

In oxygen isotope ratio analysis, variations in the ratio of 18
O to 16
O (two isotopes of oxygen) by mass (measured by a mass spectrometer) present in the calcite of oceanic core samples is used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in 18
O , which is included in the tests of the microorganisms (foraminifera) contributing the calcite.

A more recent version of the sampling process makes use of modern glacial ice cores. Although less rich in 18
O than sea water, the snow that fell on the glacier year by year nevertheless contained 18
O and 16
O in a ratio that depended on the mean annual temperature.

Temperature and climate change are cyclical when plotted on a graph of temperature versus time. Temperature coordinates are given in the form of a deviation from today's annual mean temperature, taken as zero. This sort of graph is based on another of isotope ratio versus time. Ratios are converted to a percentage difference from the ratio found in standard mean ocean water (SMOW).

The graph in either form appears as a waveform with overtones. One half of a period is a Marine isotopic stage (MIS). It indicates a glacial (below zero) or an interglacial (above zero). Overtones are stadials or interstadials.

According to this evidence, Earth experienced 102 MIS stages beginning at about 2.588 Ma BP in the Early Pleistocene Gelasian. Early Pleistocene stages were shallow and frequent. The latest were the most intense and most widely spaced.

By convention, stages are numbered from the Holocene, which is MIS1. Glacials receive an even number interglacials, odd. The first major glacial was MIS2-4 at about 85–11 ka BP. The largest glacials were 2, 6, 12, and 16 the warmest interglacials, 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under the articles for those names.

Both marine and continental faunas were essentially modern but with many more large land mammals such as Mammoths, Mastodons, Diprotodon, Smilodon, tiger, lion, Aurochs, short-faced bears, giant sloths, Gigantopithecus and others. Isolated landmasses such as Australia, Madagascar, New Zealand and islands in the Pacific saw the evolution of large birds and even reptiles such as the Elephant bird, moa, Haast's eagle, Quinkana, Megalania and Meiolania.

The severe climatic changes during the Ice Age had major impacts on the fauna and flora. With each advance of the ice, large areas of the continents became totally depopulated, and plants and animals retreating southwards in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food supply. A major extinction event of large mammals (megafauna), which included mammoths, mastodons, saber-toothed cats, glyptodons, the woolly rhinoceros, various giraffids, such as the Sivatherium ground sloths, Irish elk, cave bears, Gomphothere, dire wolves, and short-faced bears, began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period. At the end of the last ice age, cold-blooded animals, smaller mammals like wood mice, migratory birds, and swifter animals like whitetail deer had replaced the megafauna and migrated north. Late Pleistocene bighorn sheep were more slender and had longer legs than their descendants today. Scientists believe that the change in predator fauna after the late Pleistocene extinctions resulted in a change of body shape as the species adapted for increased power rather than speed. [28]

The extinctions hardly affected Africa but were especially severe in North America where native horses and camels were wiped out.

    (ALMA) include Zhoukoudianian, Nihewanian, and Yushean. (ELMA) include Gelasian (2.5–1.8 Ma). (NALMA) include Blancan (4.75–1.8), Irvingtonian (1.8–0.24) and Rancholabrean (0.24–0.01) in millions of years. The Blancan extends significantly back into the Pliocene. (SALMA) include Uquian (2.5–1.5), Ensenadan (1.5–0.3) and Lujanian (0.3–0.01) in millions of years. The Uquian previously extended significantly back into the Pliocene, although the new definition places it entirely within the Pleistocene.

In July 2018, a team of Russian scientists in collaboration with Princeton University announced that they had brought two female nematodes frozen in permafrost, from around 42,000 years ago, back to life. The two nematodes, at the time, were the oldest confirmed living animals on the planet. [29] [30]


Contents

The water in subglacial lakes remains liquid since geothermal heating balances the heat loss at the ice surface. The pressure from the overlying glacier causes the melting point of water to be below 0 °C. The ceiling of the subglacial lake will be at the level where the pressure melting point of water intersects with the temperature gradient. In Lake Vostok, the largest Antarctic subglacial lake, the ice over the lake is thus much thicker than the ice sheet around it. Hypersaline subglacial lakes remain liquid due to their salt content. [5]

Not all lakes with permanent ice cover can be called subglacial, as some are covered by regular lake ice. Some examples of perennially ice-covered lakes include Lake Bonney and Lake Hoare in Antarctica's McMurdo Dry Valleys as well as Lake Hodgson, a former subglacial lake.

Hydrostatic seals Edit

The water in a subglacial lake can have a floating level much above the level of the ground threshold. In fact, theoretically a subglacial lake can even exist on the top of a hill, provided that the ice over it is thin enough to form the required hydrostatic seal. The floating level can be thought of as the water level in a hole drilled through the ice into the lake. It is equivalent to the level at which a piece of ice over it would float if it were a normal ice shelf. The ceiling can therefore be conceived as an ice shelf that is grounded along its entire perimeter, which explains why it has been called a captured ice shelf. As it moves over the lake, it enters the lake at the floating line, and it leaves the lake at the grounding line.

A hydrostatic seal is created when the ice is so much higher around the lake that the equipotential surface dips down into impermeable ground. Water from underneath this ice rim is then pressed back into the lake by the hydrostatic seal. The ice rim in Lake Vostok has been estimated to a mere 7 meters, while the floating level is about 3 kilometers above the lake ceiling. [5] If the hydrostatic seal is penetrated when the floating level is high, the water will start flowing out in a jökulhlaup. Due to melting of the channel the discharge increases exponentially, unless other processes allow the discharge to increase even faster. Due to the high hydraulic head that can be achieved in some subglacial lakes, jökulhlaups may reach very high rates of discharge. [7] Catastrophic drainage from subglacial lakes is a known hazard in Iceland, as volcanic activity can create enough meltwater to overwhelm ice dams and lake seals and cause glacial outburst flooding. [12]

Influence on glacier movement Edit

The role of subglacial lakes on ice dynamics is unclear. Certainly on the Greenland Ice Sheet subglacial water acts to enhance basal ice motion in a complex manner. [13] The "Recovery Lakes" beneath Antarctica's Recovery Glacier lie at the head of a major ice stream and may influence the dynamics of the region. [14] A modest (10%) speed up of Byrd Glacier in East Antarctica may have been influenced by a subglacial drainage event. The flow of subglacial water is known in downstream areas where ice streams are known to migrate, accelerate or stagnate on centennial time scales and highlights that subglacial water may be discharged over the ice sheet grounding line. [15]

Russian revolutionary and scientist Peter A. Kropotkin first proposed the idea of liquid freshwater under the Antarctic Ice Sheet at the end of the 19th century. [2] [16] He suggested that due to the geothermal heating at the bottom of the ice sheets, the temperature beneath the ice could reach the ice melt temperature, which would be below zero. The notion of freshwater beneath ice sheets was further advanced by Russian glaciologist Igor A. Zotikov, who demonstrated via theoretical analysis the possibility of a decrease in Antarctic ice because of melting of ice at a lower surface. [5] As of 2019, there are over 400 subglacial lakes in Antarctica, [7] and it is suspected that there is a possibility of more. [5] Subglacial lakes have also been discovered in Greenland, [6] Iceland, and northern Canada. [17]

Early exploration Edit

Scientific advances in Antarctica can be attributed to several major periods of collaboration and cooperation, such as the four International Polar Years (IPY) in 1882-1883, 1932-1933, 1957-1958, and 2007-2008. The success of the 1957-1958 IPY led to the establishment of the Scientific Committee on Antarctic Research (SCAR) and the Antarctic Treaty System, paving the way to formulate a better methodology and process to observe subglacial lakes.

In 1959 and 1964, during two of his four Soviet Antarctic Expeditions, Russian geographer and explorer Andrey P. Kapitsa used seismic sounding to prepare a profile of the layers of the geology below Vostok Station in Antarctica. The original intent of this work was to conduct a broad survey of the Antarctic Ice Sheet. The data collected on these surveys however was used 30 years later and led to the discovery of Lake Vostok as a subglacial lake. [18]

Beginning in the late 1950s, English physicists Stan Evans and Gordon Robin began using the radioglaciology technique of radio-echo sounding (RES) to chart ice thickness. [19] Subglacial lakes are identified by (RES) data as continuous and specular reflectors which dip against the ice surface at around x10 of the surface slope angle, as this is required for hydrostatic stability. In the late 1960s, they were able to mount RES instruments on aircraft and acquire data for the Antarctic Ice Sheet. [20] Between 1971 and 1979, the Antarctic Ice Sheet was profiled extensively using RES equipment. [20] The technique of using RES is as follows: 50-meter deep holes are drilled to increase the signal-to-noise ratio in the ice. A small explosion sets off a sound wave, which travels through the ice. [7] This sound wave is reflected and then recorded by the instrument. The time it takes for the wave to travel down and back is noted and converted to a distance using the known speed of sound in ice. [20] RES records can identify subglacial lakes via three specific characteristics: 1) an especially strong reflection from the ice-sheet base, stronger than adjacent ice-bedrock reflections 2) echoes of constant strength occurring along the track, which indicate that the surface is very smooth and 3) a very flat and horizontal character with slopes less than 1%. [21] [22] Using this approach, 17 subglacial lakes were documented [23] by Kapista and his team. RES also led to the discovery of the first subglacial lake in Greenland [1] and revealed that these lakes are interconnected. [3]

Systematic profiling, using RES, of the Antarctic Ice Sheet took place again between 1971–1979. During this time, a US-UK-Danish collaboration was able to survey about 40% of East Antarctica and 80% of West Antarctica – further defining the subglacial landscape and the behavior of ice flow over the lakes. [4]

Satellite exploration Edit

In the early 1990s, radar altimeter data from the European Remote-Sensing Satellite (ERS-1) provided detailed mapping of Antarctica through 82 degrees south. [24] This imaging revealed a flat surface around the northern border of Lake Vostok, and the data collected from ERS-1 further built the geographical distribution of Antarctic subglacial lakes.

In 2005, Laurence Gray and a team of glaciologists began to interpret surface ice slumping and raising from RADARSAT data, which indicated there could be hydrologically “active” subglacial lakes subject to water movement. [25]

Between 2003 and 2009, a survey of long-track measurements of ice-surface elevation using the ICESat satellite as a part of NASA's Earth Observing System produced the first continental-scale map of the active subglacial lakes in Antarctica. [25] In 2009, it was revealed that Lake Cook is the most hydrologically active subglacial lake on the Antarctic continent. Other satellite imagery has been used to monitor and investigate this lake, including ICESat, CryoSat-2, the Advanced Spaceborne Thermal Emission and Reflection Radiometer, and SPOT5. [26] [27]

Gray et al. (2005) interpreted ice surface slumping and raising from RADARSAT data as evidence for subglacial lakes filling and emptying - termed "active" lakes. [28] Wingham et al. (2006) used radar altimeter (ERS-1) data to show coincident uplift and subsidence, implying drainage between lakes. [29] NASA's ICESat satellite was key in developing this concept further and subsequent work demonstrated the pervasiveness of this phenomenon. [30] [31] ICESat ceased measurements in 2007 and the detected "active" lakes were compiled by Smith et al. (2009) who identified 124 such lakes. The realisation that lakes were interconnected created new contamination concerns for plans to drill into lakes (see the Sampling expeditions section below).

Several lakes were delineated by the famous SPRI-NSF-TUD surveys undertaken until the mid-seventies. Since this original compilation several smaller surveys has discovered many more subglacial lakes throughout Antarctica, notably by Carter et al. (2007), who identified a spectrum of subglacial lake types based on their properties in (RES) datasets.

Sampling expeditions Edit

In March 2010, the sixth international conference on subglacial lakes was held at the American Geophysical Union Chapman Conference in Baltimore. The conference allowed engineers and scientists to discuss the equipment and strategies used in ice drilling projects, such as the design of hot-water drills, equipment for water measurement and sampling and sediment recovery, and protocols for experimental cleanliness and environmental stewardship. [20] Following this meeting, SCAR drafted a code of conduct for ice drilling expeditions and in situ (on-site) measurements and sampling of subglacial lakes. This code of conduct was ratified at the Antarctic Treaty Consultative Meeting (ATCM) of 2011. By the end of 2011, three separate subglacial lake drilling exploration missions were scheduled to take place.

In February 2012, Russian ice-core drilling at Lake Vostok accessed the subglacial lake for the first time. [32] Lake water flooded the borehole and froze during the winter season, and the sample of re-frozen lake water (accretion ice) was recovered in the following summer season of 2013. In December 2012, scientists from the UK attempted to access Lake Ellsworth with a clean access hot-water drill [33] however, the mission was called off because of equipment failure. [34] In January 2013, the US-led Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) expedition measured and sampled Lake Whillans in West Antarctica [35] for microbial life. [36] On 28 December 2018, the Subglacial Antarctic Lakes Scientific Access (SALSA) team announced they had reached Lake Mercer after melting their way through 1,067 m (3,501 ft) of ice with a high-pressure hot-water drill. [9] The team collected water samples and bottom sediment samples down to 6 meters deep.

Antarctica Edit

The majority of the nearly 400 Antarctic subglacial lakes are located in the vicinity of ice divides, where large subglacial drainage basins are overlain by ice sheets. The largest is Lake Vostok with other lakes notable for their size being Lake Concordia and Aurora Lake. An increasing number of lakes are also being identified near ice streams. [1] An altimeter survey by the ERS-2 satellite orbiting the East Antarctic Ice Sheet from 1995 to 2003 indicated clustered anomalies in ice sheet elevation [37] indicating that the East Antarctic lakes are fed by a subglacial system that transports basal meltwater through subglacial streams.

The largest Antarctic subglacial lakes are clustered in the Dome C-Vostok area of East Antarctica, possibly due to the thick insulating ice and rugged, tectonically influenced subglacial topography. In West Antarctica, subglacial Lake Ellsworth is situated within the Ellsworth Mountains and is relatively small and shallow. [38] The Siple Coast Ice Streams, also in West Antarctica, overlie numerous small subglacial lakes, including Lakes Whillans, Engelhardt, Mercer, and Conway. [38] Glacial retreat at the margins of the Antarctic Ice Sheet has revealed several former subglacial lakes, including Progress Lake in East Antarctica and Hodgson Lake on southern Alexander Island near the Antarctic Peninsula. [39]

Greenland Edit

The existence of subglacial lakes beneath the Greenland Ice Sheet has only become evident within the last decade. Radio-echo sounding measurements have revealed two subglacial lakes in the northwest section of the ice sheet. [1] These lakes are likely recharged with water from the drainage of nearby supraglacial lakes rather than from melting of basal ice. [40] Another potential subglacial lake has been identified near the southwestern margin of the ice sheet, where a circular depression beneath the ice sheet evidences recent drainage of the lake caused by climate warming. [41] Such drainage, coupled with heat transfer to the base of the ice sheet through the storage of supraglacial meltwater, is thought to influence the rate of ice flow and overall behavior of the Greenland Ice Sheet. [40]

Iceland Edit

Much of Iceland is volcanically active, resulting in significant meltwater production beneath its two ice caps. This meltwater also accumulates in basins and ice cauldrons, forming subglacial lakes. [7] These lakes act as a transport mechanism for heat from geothermal vents to the bottom of the ice caps, which often results in melting of basal ice that replenishes any water lost from drainage. [42] The majority of Icelandic subglacial lakes are located beneath the Vatnajökull and Mýrdalsjökull ice caps, where melting from hydrothermal activity creates permanent depressions that fill with meltwater. [7] Catastrophic drainage from subglacial lakes is a known hazard in Iceland, as volcanic activity can create enough meltwater to overwhelm ice dams and lake seals and cause glacial outburst flooding. [43]

Grímsvötn is perhaps the best known subglacial lake beneath the Vatnajökull ice cap. Other lakes beneath the ice cap lie within the Skatfá, Pálsfjall and Kverkfjöll cauldrons. [7] Notably, subglacial lake Grímsvötn's hydraulic seal remained intact until 1996, when significant meltwater production from the Gjálp eruption resulted in uplift of Grímsvötn's ice dam. [44]

The Mýrdalsjökull ice cap, another key subglacial lake location, sits on top of an active volcano-caldera system in the southernmost part of the Katla volcanic system. [43] Hydrothermal activity beneath the Mýrdalsjökull ice cap is thought to have created at least 12 small depressions within an area constrained by three major subglacial drainage basins. [7] Many of these depressions are known to contain subglacial lakes that are subject to massive, catastrophic drainage events from volcanic eruptions, creating a significant hazard for nearby human populations. [43]

Canada Edit

Until very recently, only former subglacial lakes from the last glacial period had been identified in Canada. [45] These paleo-subglacial lakes likely occupied valleys created before the advance of the Laurentide Ice Sheet during the Last Glacial Maximum. [46] However, two subglacial lakes were identified via RES in bedrock troughs under the Devon Ice Cap of Nunavut, Canada. [47] These lakes are thought to be hypersaline as a result of interaction with the underlying salt-bearing bedrock, and are much more isolated than the few identified saline subglacial lakes in Antarctica. [47]

Unlike surface lakes, subglacial lakes are isolated from Earth's atmosphere and receive no sunlight. Their waters are thought to be ultra-oligotrophic, meaning they contain very low concentrations of the nutrients necessary for life. Despite the cold temperatures, low nutrients, high pressure, and total darkness in subglacial lakes, these ecosystems have been found to harbor thousands of different microbial species and some signs of higher life. [9] [36] [48] Professor John Priscu, a prominent scientist studying polar lakes, has called Antarctica's subglacial ecosystems "our planet's largest wetland.” [49]

Microorganisms and weathering processes drive a diverse set of chemical reactions that can drive a unique food-web and thus cycle nutrients and energy through subglacial lake ecosystems. No photosynthesis can occur in the darkness of subglacial lakes, so their food webs are instead driven by chemosynthesis and the consumption of ancient organic carbon deposited before glaciation. [36] Nutrients can enter subglacial lakes through the glacier ice-lake water interface, from hydrologic connections, and from the physical, chemical, and biological weathering of subglacial sediments. [9] [50]

Biogeochemical cycles Edit

Since few subglacial lakes have been directly sampled, much of the existing knowledge about subglacial lake biogeochemistry is based on a small number of samples, mostly from Antarctica. Inferences about solute concentrations, chemical processes, and biological diversity of unsampled subglacial lakes have also been drawn from analyses of accretion ice (re-frozen lake water) at the base of the overlying glaciers. [51] [52] These inferences are based on the assumption that accretion ice will have similar chemical signatures as the lake water that formed it. Scientists have thus far discovered diverse chemical conditions in subglacial lakes, ranging from upper lake layers supersaturated in oxygen to bottom layers that are anoxic and sulfur-rich. [53] Despite their typically oligotrophic conditions, subglacial lakes and sediments are thought to contain regionally and globally significant amounts of nutrients, particularly carbon. [54] [12] [55] [56] [57]

At the lake-ice interface Edit

Air clathrates trapped in glacial ice are the main source of oxygen entering otherwise enclosed subglacial lake systems. As the bottom layer of ice over the lake melts, clathrates are freed from the ice's crystalline structure and gases such as oxygen are made available to microbes for processes like aerobic respiration. [58] In some subglacial lakes, freeze-melt cycles at the lake-ice interface may enrich the upper lake water with oxygen concentrations that are 50 times higher than in typical surface waters. [59]

Melting of the layer of glacial ice above the subglacial lake also supplies underlying waters with iron, nitrogen, and phosphorus-containing minerals, in addition to some dissolved organic carbon and bacterial cells. [9] [12] [50]

In the water column Edit

Because air clathrates from melting glacial ice are the primary source of oxygen to subglacial lake waters, the concentration of oxygen generally decreases with depth in the water column if turnover is slow. [60] Oxic or slightly suboxic waters often reside near the glacier-lake interface, while anoxia dominates in the lake interior and sediments due to respiration by microbes. [61] In some subglacial lakes, microbial respiration may consume all of the oxygen in the lake, creating an entirely anoxic environment until new oxygen-rich water flows in from connected subglacial environments. [62] The addition of oxygen from ice melt and the consumption of oxygen by microbes may create redox gradients in the subglacial lake water column, with aerobic microbial mediated processes like nitrification occurring in the upper waters and anaerobic processes occurring in the anoxic bottom waters. [50]

Concentrations of solutes in subglacial lakes, including major ions and nutrients like sodium, sulfate, and carbonates, are low compared to typical surface lakes. [50] These solutes enter the water column from glacial ice melting and from sediment weathering. [50] [57] Despite their low solute concentrations, the large volume of subglacial waters make them important contributors of solutes, particularly iron, to their surrounding oceans. [63] [57] [64] Subglacial outflow from the Antarctic Ice Sheet, including outflow from subglacial lakes, is estimated to add a similar amount of solutes to the Southern Ocean as some of the world's largest rivers. [57]

The subglacial water column is influenced by the exchange of water between lakes and streams under ice sheets through the subglacial drainage system this behavior likely plays an important role in biogeochemical processes, leading to changes in microbial habitat, particularly regarding oxygen and nutrient concentrations. [50] [60] Hydrologic connectivity of subglacial lakes also alters water residence times, or amount of time that water stays within the subglacial lake reservoir. Longer residence times, such as those found beneath the interior Antarctic Ice Sheet, would lead to greater contact time between the water and solute sources, allowing for greater accumulation of solutes than in lakes with shorter residence times. [57] [56] Estimated residence times of currently studied subglacial lakes range from about 13,000 years in Lake Vostok to just decades in Lake Whillans. [65] [66]

The morphology of subglacial lakes has the potential to change their hydrology and circulation patterns. Areas with the thickest overlying ice experience greater rates of melting. The opposite occurs in areas where the ice sheet is thinnest, which allows re-freezing of lake water to occur. [22] These spatial variations in melting and freezing rates lead to internal convection of water and circulation of solutes, heat, and microbial communities throughout the subglacial lake, which will vary among subglacial lakes of different regions. [50] [60]

In sediments Edit

Subglacial sediments are primarily composed of glacial till that formed during physical weathering of subglacial bedrock. [50] Anoxic conditions prevail in these sediments due to oxygen consumption by microbes, particularly during sulfide oxidation. [50] [17] [57] Sulfide minerals are generated by weathering of bedrock by the overlying glacier, after which these sulfides are oxidized to sulfate by aerobic or anaerobic bacteria, which can use iron for respiration when oxygen is unavailable. [58]

The products of sulfide oxidation can enhance the chemical weathering of carbonate and silicate minerals in subglacial sediments, particularly in lakes with long residence times. [50] [57] Weathering of carbonate and silicate minerals from lake sediments also releases other ions including potassium (K + ), magnesium (Mg 2+ ), sodium (Na + ), and calcium (Ca 2+ ) to lake waters. [57]

Other biogeochemical processes in anoxic subglacial sediments include denitrification, iron reduction, sulfate reduction, and methanogenesis (see Reservoirs of organic carbon below). [50]

Reservoirs of organic carbon Edit

Subglacial sedimentary basins under the Antarctic Ice Sheet have accumulated an estimated

21,000 petagrams of organic carbon, most of which comes from ancient marine sediments. [55] This is more than 10 times the amount of organic carbon contained in Arctic permafrost [67] and may rival the amount of reactive carbon in modern ocean sediments, [68] potentially making subglacial sediments an important but understudied component of the global carbon cycle. [56] In the event of ice sheet collapse, subglacial organic carbon could be more readily respired and thus released to the atmosphere and create a positive feedback on climate change. [69] [55] [56]

The microbial inhabitants of subglacial lakes likely play an important role in determining the form and fate of sediment organic carbon. In the anoxic sediments of subglacial lake ecosystems, organic carbon can be used by archaea for methanogenesis, potentially creating large pools of methane clathrate in the sediments that could be released during ice sheet collapse or when lake waters drain to ice sheet margins. [70] Methane has been detected in subglacial Lake Whillans, [71] and experiments have shown that methanogenic archaea can be active in sediments beneath both Antarctic and Arctic glaciers. [72]

Most of the methane that escapes storage in subglacial lake sediments appears to be consumed by methanotrophic bacteria in oxygenated upper waters. In subglacial Lake Whillans, scientists found that bacterial oxidation consumed 99% of the available methane. [71] There is also evidence for active methane production and consumption beneath the Greenland Ice Sheet. [73]

Antarctic subglacial waters are also thought to contain substantial amounts of organic carbon in the form of dissolved organic carbon and bacterial biomass. [12] At an estimated 1.03 x 10 −2 petagrams, the amount of organic carbon in subglacial lake waters is far smaller than that contained in Antarctic subglacial sediments, but is only one order of magnitude smaller than the amount of organic carbon in all surface freshwaters (5.10 x 10 −1 petagrams). [12] This relatively smaller, but potentially more reactive, reservoir of subglacial organic carbon may represent another gap in scientists’ understanding of the global carbon cycle. [12]

Biology Edit

Subglacial lakes were originally assumed to be sterile, [74] but over the last thirty years, active microbial life and signs of higher life have been discovered in subglacial lake waters, sediments, and accreted ice. [9] [60] Subglacial waters are now known to contain thousands of microbial species, including bacteria, archaea, and potentially some eukaryotes. These extremophilic organisms are adapted to below-freezing temperatures, high pressure, low nutrients, and unusual chemical conditions. [9] [60] Researching microbial diversity and adaptations in subglacial lakes is of particular interest to scientists studying astrobiology, as well as the history and limits of life on Earth.

Food web structure and sources of energy Edit

In most surface ecosystems, photosynthetic plants and microbes are the main primary producers that form the base of the lake food web. Photosynthesis is impossible in the permanent darkness of subglacial lakes, so these food webs are instead driven by chemosynthesis. [36] In subglacial ecosystems, chemosynthesis is mainly carried out by chemolithoautotrophic microbes. [75] [62] [76]

Like plants, chemolithoautotrophs fix carbon dioxide (CO2) into new organic carbon, making them the primary producers at the base of subglacial lake food webs. Rather than using sunlight as an energy source, chemolithoautotrophs get energy from chemical reactions in which inorganic elements from the lithosphere are oxidized or reduced . Common elements used by chemolithoautotrophs in subglacial ecosystems include sulfide, iron, and carbonates weathered from sediments. [9]

In addition to mobilizing elements from sediments, chemolithoautotrophs create enough new organic matter to support heterotrophic bacteria in subglacial ecosystems. [36] [62] Heterotrophic bacteria consume the organic material produced by chemolithoautotrophs, as well as consuming organic matter from sediments or from melting glacial ice. [12] [52] Despite the resources available to subglacial lake heterotrophs, these bacteria appear to be exceptionally slow-growing, potentially indicating that they dedicate most of their energy to survival rather than growth. [62] Slow heterotrophic growth rates could also be explained by the cold temperatures in subglacial lakes, which slow down microbial metabolism and reaction rates. [77]

The variable redox conditions and diverse elements available from sediments provide opportunities for many other metabolic strategies in subglacial lakes. Other metabolisms used by subglacial lake microbes include methanogenesis, methanotrophy, and chemolithoheterotrophy, in which bacteria consume organic matter while oxidizing inorganic elements. [71] [78] [36]

Some limited evidence for microbial eukaryotes and multicellular animals in subglacial lakes could expand current ideas of subglacial food webs. [48] [79] If present, these organisms could survive by consuming bacteria and other microbes.

Nutrient limitation Edit

Subglacial lake waters are considered to be ultra-oligotrophic and contain low concentrations of nutrients, particularly nitrogen and phosphorus. [50] [80] In surface lake ecosystems, phosphorus has traditionally been thought of as the limiting nutrient that constrains growth in the ecosystem, although co-limitation by both nitrogen and phosphorus supply seems most common. [81] [82] However, evidence from subglacial Lake Whillans suggests that nitrogen is the limiting nutrient in some subglacial waters, based on measurements showing that the ratio of nitrogen to phosphorus is very low compared to the Redfield ratio. [36] An experiment showed that bacteria from Lake Whillans grew slightly faster when supplied with phosphorus as well as nitrogen, potentially contradicting the idea that growth in these ecosystems is limited by nitrogen alone. [62]

Biological diversity in explored subglacial lakes Edit

Biological exploration of subglacial lakes has focused on Antarctica, but the financial and logistical challenges of drilling through the Antarctic Ice Sheet for sample collection have limited successful direct samplings of Antarctic subglacial lake water to Lake Whillans and Lake Mercer. Volcanic subglacial lakes under Iceland's Vatnajökull ice cap have also been sampled.

Antarctica Edit

In subglacial Lake Whillans, the WISSARD expedition collected sediment cores and water samples, which contained 130,000 microbial cells per milliliter and 3,914 different bacterial species. [36] The team identified active bacteria that were metabolizing ammonia, methane, and sulfur from the 120,000-year-old sediments. [78] The most abundant bacteria identified were related to Thiobacillus, Sideroxyans, and pscyhrophilic Polaromonas species. [36] [78]

In January 2019, the SALSA team collected sediment and water samples from subglacial Lake Mercer and found diatom shells and well-preserved carcasses from crustaceans and a tardigrade. [48] Although the animals were dead, the team also found bacterial concentrations of 10,000 cells per milliliter, suggesting the potential for animals to survive in the lake by consuming bacteria. [48] The team will continue analyzing the samples to further investigate the chemistry and biology of the lake.

Lake Vostok is the best-studied Antarctic subglacial lake, but its waters have only been studied through analysis of accretion ice from the bottom of ice cores taken during Russian drilling efforts above the lake. Actively growing bacteria and thousands of unique DNA sequences from bacteria, archaea, and eukaryotes have been found in Lake Vostok's accretion ice. [83] [51] [79] Some DNA appeared to come from multicellular eukaryotes, including species seemingly related to freshwater Daphnia, tardigrades, and mollusks. [79] These species may have survived in the lake and slowly adapted to the changing conditions since Vostok was last exposed to the atmosphere millions of years ago. However, the samples were likely contaminated by drilling fluid while being collected, so some of the identified organisms probably did not live in the lake. [84]

Other subglacial sampling efforts in Antarctica include the subglacial pool of anoxic, hypersaline water under Taylor Glacier, which harbors a microbial community that was sealed off from the atmosphere 1.5 to 2 million years ago. [85] Bacteria under Taylor Glacier appear to have a novel metabolic strategy that uses sulfate and ferric ions to decompose organic matter. [85]

Greenland Edit

No direct sampling of subglacial lakes has been attempted on the Greenland Ice Sheet. However, subglacial outflow waters have been sampled and found to contain methanogenic and methanotrophic microbes. [73] Bacteria have also been discovered within the ice sheet itself, but they are unlikely to be active within the ice. [86]

Iceland Edit

Subglacial lakes under Iceland's Vatnajökull ice cap provide unique habitats for microbial life because they receive heat and chemical inputs from subglacial volcanic activity, influencing the chemistry of lower lake waters and sediments. [87] Active psychrophilic, autotrophic bacteria have been discovered in the lake below the Grímsvötn volcanic caldera. [88] A low-diversity microbial community has also been found in the east Skaftárketill and Kverkfjallalón subglacial lakes, where bacteria include Geobacter and Desulfuosporosinus species that can use sulfur and iron for anaerobic respiration. [89] In the western Skaftá lake, the anoxic bottom waters appear to be dominated by acetate-producing bacteria rather than methanogens. [80]

Refugia for ancient life Edit

In some cases, subglacial lake waters have been isolated for millions of years, and these “fossil waters” may harbor evolutionarily distinct microbial communities. [85] Some subglacial lakes in East Antarctica have existed for about 20 million years, but the interconnected subglacial drainage system between lakes under the Antarctic Ice Sheet implies that lake waters have probably not been isolated over the entire lifespan of the lake. [12]

During the proposed Snowball Earth period of the late Proterozoic, extensive glaciation could have completely covered Earth's surface in ice for 10 million years. [90] Life would have survived primarily in glacial and subglacial environments, making modern subglacial lakes an important study system for understanding this period in Earth's history. More recently, subglacial lakes in Iceland may have provided a refuge for subterranean amphipods during the Quaternary glacial period. [91]

Subglacial lakes are an analog environment for extraterrestrial ice-covered water bodies, making them an important study system in the field of astrobiology, which is the study of the potential for life to exist outside Earth. Discoveries of living extremophilic microbes in Earth's subglacial lakes could suggest that life may persist in similar environments on extraterrestrial bodies. [11] [10] Subglacial lakes also provide study systems for planning research efforts in remote, logistically challenging locations that are sensitive to biological contamination. [92] [93]

Jupiter's moon Europa and Saturn’s moon Enceladus are promising targets in the search for extraterrestrial life. Europa contains an extensive ocean covered by an icy crust, and Enceladus is also thought to harbor a subglacial ocean. [94] [95] Satellite analysis of an icy water vapor plume escaping from fissures in Enceladus' surface reveals significant subsurface production of hydrogen, which may point towards the reduction of iron-bearing minerals and organic matter. [96]

A subglacial lake on Mars was discovered in 2018 using RES on the Mars Express spacecraft. [97] This body of water was found beneath Mars’ South Polar Layered Deposits, and is suggested to have formed as a result of geothermal heating causing melting beneath the ice cap. [98]


3.3.1 Ice Sheets - Biology

Monday 2/2 Tuesday 2/3
Snow Day

Wednesday 2/4 Thursday 2/5
Today's Plan!
https://docs.google.com/document/d/1lqGUrXC5_b4502sdjkkGjMlbSXH3AufXkjERclMIexM/edit

Friday 2/6
Types of Speciation:
https://docs.google.com/document/d/1b4jkaUfWf2X6VC8Ehmuyo
QBvVbpCI5OTBvhtYdfU3jc/edit?usp=sharing

Friday 2/13 Monday 2/23 Tuesday 2/24
Finishing Compounds of Living Things Lab

Monday 3/2 Tuesday 3/3 Wednesday 3/4
Atomic Modeling Practice if finished with other work:
http://phet.colorado.edu/en/simulation/build-an-atom
textbook pgs. 39-41 (pH)

Thursday 3/5 Friday 3/6
Class data for Homeostasis Lab:
https://docs.google.com/spreadsheets/d/1vgHyIcAP7tfAdJuFPyzoXV3TR2Qz-80wObY7p92-Mq8/edit#gid=0

Homeostasis pages 804-807 in textbook :)

Cell Organelles Review (Structure, Function & Cell Type)

Protists and Disease (see resources from Thursday and Friday

Class Demo of DNA Replication

Modeling. (DNA Replication, Transcription, Translation, Evidence of DNA as the genetic material)


Chapter Summary

Ecology is the study of the interactions of living things with their environment. Ecologists ask questions across four levels of biological organization—organismal, population, community, and ecosystem. At the organismal level, ecologists study individual organisms and how they interact with their environments. At the population and community levels, ecologists explore, respectively, how a population of organisms changes over time and the ways in which that population interacts with other species in the community. Ecologists studying an ecosystem examine the living species (the biotic components) of the ecosystem as well as the nonliving portions (the abiotic components), such as air, water, and soil, of the environment.

35.2 Biogeography

Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution. Endemic species are species that are naturally found only in a specific geographic area. The distribution of living things is influenced by several environmental factors that are, in part, controlled by the latitude or elevation at which an organism is found. Ocean upwelling and spring and fall turnovers are important processes regulating the distribution of nutrients and other abiotic factors important in aquatic ecosystems. Energy sources, temperature, water, inorganic nutrients, and soil are factors limiting the distribution of living things in terrestrial systems. Net primary productivity is a measure of the amount of biomass produced by a biome.

35.3 Terrestrial Biomes

The Earth has terrestrial biomes and aquatic biomes. Aquatic biomes include both freshwater and marine environments. There are eight major terrestrial biomes: tropical wet forests, savannas, subtropical deserts, chaparral, temperate grasslands, temperate forests, boreal forests, and Arctic tundra. The same biome can occur in different geographic locations with similar climates. Temperature and precipitation, and variations in both, are key abiotic factors that shape the composition of animal and plant communities in terrestrial biomes. Some biomes, such as temperate grasslands and temperate forests, have distinct seasons, with cold weather and hot weather alternating throughout the year. In warm, moist biomes, such as the tropical wet forest, net primary productivity is high, as warm temperatures, abundant water, and a year-round growing season fuel plant growth. Other biomes, such as deserts and tundra, have low primary productivity due to extreme temperatures and a shortage of available water.

35.4 Aquatic Biomes

Aquatic ecosystems include both saltwater and freshwater biomes. The abiotic factors important for the structuring of aquatic ecosystems can be different than those seen in terrestrial systems. Sunlight is a driving force behind the structure of forests and also is an important factor in bodies of water, especially those that are very deep, because of the role of photosynthesis in sustaining certain organisms. Density and temperature shape the structure of aquatic systems. Oceans may be thought of as consisting of different zones based on water depth and distance from the shoreline and light penetrance. Different kinds of organisms are adapted to the conditions found in each zone. Coral reefs are unique marine ecosystems that are home to a wide variety of species. Estuaries are found where rivers meet the ocean their shallow waters provide nourishment and shelter for young crustaceans, mollusks, fishes, and many other species. Freshwater biomes include lakes, ponds, rivers, streams, and wetlands. Bogs are an interesting type of wetland characterized by standing water, lower pH, and a lack of nitrogen.

35.5 Climate and the Effects of Global Climate Change

The Earth has gone through periodic cycles of increases and decreases in temperature. During the past 2000 years, the Medieval Climate Anomaly was a warmer period, while the Little Ice Age was unusually cool. Both of these irregularities can be explained by natural causes of changes in climate, and, although the temperature changes were small, they had significant effects. Natural drivers of climate change include Milankovitch cycles, changes in solar activity, and volcanic eruptions. None of these factors, however, leads to rapid increases in global temperature or sustained increases in carbon dioxide. The burning of fossil fuels is an important source of greenhouse gases, which plays a major role in the greenhouse effect. Long ago, global warming resulted in the Permian extinction: a large-scale extinction event that is documented in the fossil record. Currently, modern-day climate change is associated with the increased melting of glaciers and polar ice sheets, resulting in a gradual increase in sea level. Plants and animals can also be affected by global climate change when the timing of seasonal events, such as flowering or pollination, is affected by global warming.


Contents

The presence of a distinct cold period at the end of the LGM interval has been known for a long time. Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, as in the Allerød clay pit in Denmark, first recognized and described the Younger Dryas. [6] [7] [8] [9]

The Younger Dryas is the youngest and longest of three stadials, which resulted from typically abrupt climatic changes that took place over the last 16,000 years. [10] Within the Blytt–Sernander classification of north European climatic phases, the prefix "Younger" refers to the recognition that this original "Dryas" period was preceded by a warmer stage, the Allerød oscillation, which, in turn, was preceded by the Older Dryas, around 14,000 calendar years BP. That is not securely dated, and estimates vary by 400 years, but it is generally accepted to have lasted around 200 years. In northern Scotland, the glaciers were thicker and more extensive than during the Younger Dryas. [11] The Older Dryas, in turn, was preceded by another warmer stage, the Bølling oscillation, that separated it from a third and even older stadial, often known as the Oldest Dryas. The Oldest Dryas occurred about 1,770 calendar years before the Younger Dryas and lasted about 400 calendar years. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calendar years BP. [12]

In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain it has been called the Loch Lomond Stadial. [13] [14] In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a). [15]

Since 1916 and the onset and then the refinement of pollen analytical techniques and a steadily-growing number of pollen diagrams, palynologists have concluded that the Younger Dryas was a distinct period of vegetational change in large parts of Europe during which vegetation of a warmer climate was replaced by that of a generally cold climate, a glacial plant succession that often contained Dryas octopetala. The drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the forest vegetation that had been spreading northward rapidly. The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna, but also led to regional glacial advances in Scandinavia and a lowering of the regional snow line. [6]

The change to glacial conditions at the onset of the Younger Dryas in the higher latitudes of the Northern Hemisphere, between 12,900 and 11,500 calendar years BP, has been argued to have been quite abrupt. [16] It is in sharp contrast to the warming of the preceding Older Dryas interstadial. Its end has been inferred to have occurred over a period of a decade or so, [17] but the onset may have even been faster. [18] Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that its summit was around 15 °C (27 °F) colder during the Younger Dryas [16] [19] than today.

In Great Britain, beetle fossil evidence suggests that the mean annual temperature dropped to −5 °C (23 °F), [19] and periglacial conditions prevailed in lowland areas, and icefields and glaciers formed in upland areas. [20] Nothing of the period's size, extent, or rapidity of abrupt climate change has been experienced since its end. [16]

In addition to the Younger, Older, and Oldest Dryases, a century-long period of colder climate, similar to the Younger Dryas in abruptness, has occurred within both the Bølling oscillation and the Allerød oscillation interstadials. The cold period that occurred within the Bølling oscillation is known as the intra-Bølling cold period, and the cold period that occurred within the Allerød oscillation is known as the intra-Allerød cold period. Both cold periods are comparable in duration and intensity with the Older Dryas and began and ended quite abruptly. The cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, European lacustrine sediments, Atlantic Ocean sediments, and the Cariaco Basin, Venezuela. [21]

Examples of older Younger Dryas-like events have been reported from the ends (called terminations) [22] of older glacial periods. Temperature-sensitive lipids, long chain alkenones, found in lake and marine sediments, are well-regarded as a powerful paleothermometer for the quantitative reconstruction of past continental climates. [23] The application of alkenone paleothermometers to high-resolution paleotemperature reconstructions of older glacial terminations have found that very similar, Younger Dryas-like paleoclimatic oscillations occurred during Terminations II and IV. If so, the Younger Dryas is not the unique paleoclimatic event, in terms of size, extent, and rapidity, as it is often regarded to be. [23] [24] Furthermore, paleoclimatologists and Quaternary geologists reported finding what they characterized as well-expressed Younger Dryas events in the Chinese δ 18
O records of Termination III in stalagmites from high-altitude caves in Shennongjia area, Hubei Province, China. [25] Various paleoclimatic records from ice cores, deep-sea sediments, speleothems, continental paleobotanical data, and loesses show similar abrupt climate events, which are consistent with Younger Dryas events, during the terminations of the last four glacial periods (see Dansgaard–Oeschger event). They argue that Younger Dryas events might be an intrinsic feature of deglaciations that occur at the end of glacial periods. [25] [26] [27]

Analyses of stable isotopes from Greenland ice cores provide estimates for the start and end of the Younger Dryas. The analysis of Greenland Summit ice cores, as part of the Greenland Ice Sheet Project-2 and Greenland Icecore Project, estimated that the Younger Dryas started about 12,800 ice (calendar) years BP. Depending on the specific ice core analysis consulted, the Younger Dryas is estimated to have lasted 1,150–1,300 years. [6] [7] Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40 to 50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration and snow accumulation, suggest an even more rapid transition, which would require about 7 °C (13 °F) of warming in just a few years. [16] [17] [28] [29] Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F). [30]

The end of the Younger Dryas has been dated to around 11,550 years ago, occurring at 10,000 BP (uncalibrated radiocarbon year), a "radiocarbon plateau" by a variety of methods, mostly with consistent results:

Years ago Place
11500 ± 50 GRIP ice core, Greenland [31]
11530 + 40
− 60
Krakenes Lake, western Norway [32]
11570 Cariaco Basin core, Venezuela [33]
11570 German oak and pine dendrochronology [34]
11640 ± 280 GISP2 ice core, Greenland [28]

The International Commission on Stratigraphy put the start of the Greenlandian stage, and implicitly the end of the Younger Dryas, at 11,700 years before 2000. [35]

Although the start of the Younger Dryas is regarded to be synchronous across the North Atlantic region, recent research concluded that the start of the Younger Dryas might be time-transgressive even within there. After an examination of laminated varve sequences, Muschitiello and Wohlfarth found that the environmental changes that define the beginning of the Younger Dryas are diachronous in their time of occurrence according to latitude. According to the changes, the Younger Dryas occurred as early as AROUND 12,900–13,100 calendar years ago along latitude 56–54°N. Further north, they found that the changes occurred at roughly 12,600–12,750 calendar years ago. [36]

According to the analyses of varved sediments from Lake Suigetsu, Japan, and other paleoenvironmental records from Asia, a substantial delay occurred in the onset and the end of the Younger Dryas between Asia and the North Atlantic. For example, paleoenvironmental analysis of sediment cores from Lake Suigetsu in Japan found the Younger Dryas temperature decline of 2–4 °C between 12,300 and 11,250 varve (calendar) years BP, instead of about 12,900 calendar years BP in the North Atlantic region.

In contrast, the abrupt shift in the radiocarbon signal from apparent radiocarbon dates of 11,000 radiocarbon years to radiocarbon dates of 10,700–10,600 radiocarbon years BP in terrestrial macrofossils and tree rings in Europe over a 50-year period occurred at the same time in the varved sediments of Lake Suigetsu. However, this same shift in the radiocarbon signal antedates the start of Younger Dryas at Lake Suigetsu by a few hundred years. Interpretations of data from Chinese also confirm that the Younger Dryas East Asia lags the North Atlantic Younger Dryas cooling by at least 200 to 300 years. Although the interpretation of the data is more murky and ambiguous, the end of the Younger Dryas and the start of Holocene warming likely were similarly delayed in Japan and in other parts of East Asia. [37]

Similarly, an analysis of a stalagmite growing from a cave in Puerto Princesa Subterranean River National Park, Palawan, the Philippines, found that the onset of the Younger Dryas was also delayed there. Proxy data recorded in the stalagmite indicate that more than 550 calendar years were needed for Younger Dryas drought conditions to reach their full extent in the region and about 450 calendar years to return to pre-Younger Dryas levels after it ended. [38]

In Western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period. [39] Cooling in the tropical North Atlantic may, however, have preceded it by a few hundred years South America shows a less well-defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas and has no clearly defined start or end Peter Huybers has argued that there is a fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania. [40] Timing of the tropical counterpart to the Younger Dryas, the Deglaciation Climate Reversal (DCR), is difficult to establish as low latitude ice core records generally lack independent dating over the interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and was characterized by a shift to much wetter and likely colder conditions. [41] The magnitude and abruptness of the changes would suggest that low latitude climate did not respond passively during the YD/DCR.

Effects of the Younger Dryas were of varying intensity throughout North America. [42] In western North America, its effects were less intense than in Europe or northeast North America [43] however, evidence of a glacial re-advance [44] indicates that Younger Dryas cooling occurred in the Pacific Northwest. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon's Klamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas. [45]

Other features include the following:

  • Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant Dryas octopetala) or increased snow in mountain ranges around the world
  • Formation of solifluction layers and loess deposits in Northern Europe
  • More dust in the atmosphere, originating from deserts in Asia
  • A decline in evidence for Natufian hunter gatherer permanent settlements in the Levant, suggesting a reversion to a more mobile way of life [46]
  • The Huelmo–Mascardi Cold Reversal in the Southern Hemisphere ended at the same time
  • Decline of the Clovis culture while no definitive cause for the extinction of many species in North America such as the Columbian mammoth, as well as the Dire wolf, Camelops, and other Rancholabrean megafauna during the Younger Dryas has been determined, climate change and human hunting activities have been suggested as contributing factors. [47] Recently, it has been found that these megafauna populations collapsed 1000 years earlier [48]

North America Edit

East Edit

The Younger Dryas is a period significant to the study of the response of biota to abrupt climate change and to the study of how humans coped with such rapid changes. [49] The effects of sudden cooling in the North Atlantic had strongly regional effects in North America, with some areas experiencing more abrupt changes than others. [50]

The effects of the Younger Dryas cooling impacted the area that is now New England and parts of maritime Canada more rapidly than the rest of the present day United States at the beginning and the end of the Younger Dryas chronozone. [51] [52] [53] [54] Proxy indicators show that summer temperature conditions in Maine decreased by up to 7.5°C. Cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north. [55]

Vegetation in the central Appalachian Mountains east towards the Atlantic Ocean was dominated by spruce (Picea spp.) and tamarack (Larix laricina) boreal forests that later changed rapidly to temperate, more broad-leaf tree forest conditions at the end of the Younger Dryas period. [56] [57] Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene. [57] West of the Appalachians, in the Ohio River Valley and south to Florida rapid, no-analog vegetation responses seem to have been the result of rapid climate changes, but the area remained generally cool, with hardwood forest dominating. [56] During the Younger Dryas, the Southeastern United States was warmer and wetter than the region had been during the Pleistocene [57] [50] [58] because of trapped heat from the Caribbean within the North Atlantic Gyre caused by a weakened Atlantic meridional overturning circulation (AMOC). [59]

Central Edit

Also, a gradient of changing effects occurred from the Great Lakes region south to Texas and Louisiana. Climatic forcing moved cold air into the northern portion of the American interior, much as it did the Northeast. [60] [61] Although there was not as abrupt a delineation as seen on the Eastern Seaboard, the Midwest was significantly colder in the northern interior than it was south, towards the warmer climatic influence of the Gulf of Mexico. [50] [62] In the north, the Laurentide Ice Sheet re-advanced during the Younger Dryas, depositing a moraine from west Lake Superior to southeast Quebec. [63] Along the southern margins of the Great Lakes, spruce dropped rapidly, while pine increased, and herbaceous prairie vegetation decreased in abundance, but increased west of the region. [64] [61]

Rocky Mountains Edit

Effects in the Rocky Mountain region were varied. [65] [66] In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places. [67] [68] [69] [70] That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation [67] [71] as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons. [72] There were minor re-advancements of glaciers in place, particularly in the northern ranges, [73] [74] but several sites in the Rocky Mountain ranges show little to no changes in vegetation during the Younger Dryas. [68] Evidence also indicates an increase in precipitation in New Mexico because of the same Gulf conditions that were influencing Texas. [75]

West Edit

The Pacific Northwest region experienced 2 to 3 °C of cooling and an increase in precipitation. [76] [58] [77] [78] [79] [80] Glacial re-advancement has been recorded in British Columbia [81] [82] as well as in the Cascade Range. [83] An increase of pine pollen indicates cooler winters within the central Cascades. [84] On the Olympic Peninsula, a mid-elevation site recorded a decrease in fire, though forest persisted and erosion increased during the Younger Dryas, suggesting cool and wet conditions. [85] Speleothem records indicate an increase in precipitation in southern Oregon, [79] [86] the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin. [87] Pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range, [88] but the pollen record is less chronologically constrained than the aforementioned speleothem record. The Southwest appears to have seen an increase in precipitation, as well, also with an average 2° of cooling. [89]

Effects on agriculture Edit

The Younger Dryas is often linked to the Neolithic Revolution, the adoption of agriculture in the Levant. [90] [91] The cold and dry Younger Dryas arguably lowered the carrying capacity of the area and forced the sedentary early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While relative consensus exists regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated. [92] [93]

Sea level Edit

Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in the rates of sea level rise have been reconstructed for the postglacial period. For the early part of the sea level rise that is associated with deglaciation, three major periods of accelerated sea level rise, called meltwater pulses, occurred. They are commonly called meltwater pulse 1A0 for the pulse between 19,000 and 19,500 calendar years ago meltwater pulse 1A for the pulse between 14,600 and 14,300 calendar years ago and meltwater pulse 1B for the pulse between 11,400 and 11,100 calendar years ago. The Younger Dryas occurred after meltwater pulse 1A, a 13.5 m rise over about 290 years, centered at about 14,200 calendar years ago, and before meltwater pulse 1B, a 7.5 m rise over about 160 years, centered at about 11,000 calendar years ago. [94] [95] [96] Finally, not only did the Younger Dryas postdate both all of meltwater pulse 1A and predate all of meltwater pulse 1B, it was a period of significantly-reduced rate of sea level rise relative to the periods of time immediately before and after it. [94] [97]

Possible evidence of short-term sea level changes has been reported for the beginning of the Younger Dryas. First, the plotting of data by Bard and others suggests a small drop, less than 6 m, in sea level near the onset of the Younger Dryas. There is a possible corresponding change in the rate of change of sea level rise seen in the data from both Barbados and Tahiti. Given that this change is "within the overall uncertainty of the approach," it was concluded that a relatively smooth sea-level rise, with no significant accelerations, occurred then. [97] Finally, research by Lohe and others in western Norway has reported a sea-level low-stand at 13,640 calendar years ago and a subsequent Younger Dryas transgression starting at 13,080 calendar years ago. They concluded that the timing of the Allerød low-stand and the subsequent transgression were the result of increased regional loading of the crust, and geoid changes were caused by an expanding ice sheet, which started growing and advancing in the early Allerød about 13,600 calendar years ago, well before the start of the Younger Dryas. [98]

The current theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. Geological evidence for such an event is not fully secure, [99] but recent work has identified a pathway along the Mackenzie River that would have spilled fresh water into the Arctic and thence into the Atlantic. [100] [101] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic. However, simulations indicated that a one-time-flood could not likely cause the new state to be locked for 1000 years. Once the flood ceased, the AMOC would recover and the Younger Dryas would stop in less than 100 years. Therefore, continuous freshwater input was necessary to maintain a weak AMOC for more than 1000 years. Recent study proposed that the snowfall could be a source of continuous freshwater resulting in a prolonged weakened state of the AMOC. [102] An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation. [103] There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, but that cannot explain the apparent variability in the extinction across all continents. [104]

Impact hypothesis Edit

A hypothesized Younger Dryas impact event, presumed to have occurred in North America about 12,900 years ago, has been proposed as the mechanism that initiated the Younger Dryas cooling. [105]

Among other things, findings of melt-glass material in sediments in Pennsylvania, South Carolina and Syria have been reported. The researchers argue that the material, which dates back nearly 13,000 years, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as the result of a bolide impact. They argue that these findings support the controversial Younger Dryas Boundary (YDB) hypothesis that the bolide impact occurred at the onset of the Younger Dryas. [106] The hypothesis has been questioned in research that concluded that most of the results cannot be confirmed by other scientists and that the authors misinterpreted the data. [107] [108] [109]

After a review of the sediments found at the sites, new research has found that the sediments claimed by hypothesis proponents to be deposits resulting from a bolide impact date from much later or much earlier times than the proposed date of the cosmic impact. The researchers examined 29 sites commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only three of those sites actually date from then. [110]

Charles R. Kinzie, et al. looked at the distribution of nanodiamonds produced during extraterrestrial collisions: 50 million km 2 of the Northern Hemisphere at the YDB were found to have the nanodiamonds. [111] Only two layers exist showing these nanodiamonds: the YDB 12,800 calendar years ago and the Cretaceous-Tertiary boundary, 65 million years ago, which, in addition, is marked by mass extinctions. [112]

New support for the cosmic-impact hypothesis of the origin of the YDB was published in 2018. It postulates Earth's collision with one or more fragments from a larger (over 100-km diameter) disintegrating comet (some remnants of which have persisted within the inner solar system to the present day). Evidence is presented consistent with large-scale biomass burning (wildfires) following the putative collision. The evidence is derived from analyses of ice cores, glaciers, lake- and marine-sediment cores, and terrestrial sequences. [113] [114]

Evidence that adds further to the credibility of this hypothesis includes extraterrestrial platinum, which has been found in meteorites. There are multiple sites around the world with spikes in levels of platinum that can be associated with the Impact Hypothesis, of which at least 25 are major. [115] Although most of these sites are found in the Northern Hemisphere, a study conducted in October 2019 has found and confirmed another site with high platinum levels located in the Wonderkrater area north of Pretoria in South Africa. [116] This coincides with the Pilauco site in southern Chile which also happens to contain high levels of platinum as well as rare metallic spherules, gold and high-temperature iron that is rarely found in nature and suspected of originating from airbursts or impacts. [117] [118] [119] These Southern Hemisphere high platinum zones further add to the credibility of the Younger Dryas impact hypothesis.

Laacher See eruption hypothesis Edit

The Laacher See volcano erupted at approximately the same time as the beginning of the Younger Dryas, and has historically been suggested as a possible cause. Laacher See is a maar lake, a lake within a broad low-relief volcanic crater about 2 km (1.2 mi) diameter. It is in Rhineland-Palatinate, Germany, about 24 km (15 mi) northwest of Koblenz and 37 km (23 mi) south of Bonn. The maar lake is within the Eifel mountain range, and is part of the East Eifel volcanic field within the larger Vulkaneifel. [121] [122] This eruption was of sufficient size, VEI 6, with over 20 km 3 (2.4 cu mi) tephra ejected, [123] to have caused significant temperature change in the Northern Hemisphere.

Currently available evidence suggests that the hypothesis that the Laacher See eruption triggered the Younger Dryas has considerable merit. Earlier, the hypothesis was dismissed based on the timing of the Laacher See Tephra relative to the clearest signs of climate change associated with the Younger Dryas Event within various Central European varved lake deposits. [123] [124] This set the scene for the development of the Younger Dryas Impact Hypothesis and the meltwater pulse hypothesis. However, more recent research places the very large eruption of the Laacher See volcano at 12,880 years BP, coinciding with the initiation of North Atlantic cooling into the Younger Dryas. [125] [126] Although the eruption was about twice size as the 1991 eruption of Mount Pinatubo, it contained considerably more sulfur, potentially rivalling the climatologically very significant 1815 eruption of Mount Tambora in terms of amount of sulfur introduced into the atmosphere. [126] Evidence exists that an eruption of this magnitude and sulfur content occurring during deglaciation could trigger a long-term positive feedback involving sea ice and oceanic circulation, resulting in a cascade of climate shifts across the North Atlantic and the globe. [126] Further support for this hypothesis appears as a large volcanogenic sulfur spike within Greenland ice, coincident with both the date of the Laacher See eruption and the beginning of cooling into the Younger Dryas as recorded in Greenland. [126] The mid-latitude westerly winds may have tracked sea ice growth southward across the North Atlantic as the cooling became more pronounced, resulting in time transgressive climate shifts across northern Europe and explaining the lag between the Laacher See Tephra and the clearest (wind-derived) evidence for the Younger Dryas in central European lake sediments. [127] [128]

Although the timing of the eruption appears to coincide with the beginning of the Younger Dryas, and the amount of sulfur contained would have been enough to result in substantial Northern Hemisphere cooling, the hypothesis has not yet been tested thoroughly, and no climate model simulations are currently available. The exact nature of the positive feedback is also unknown, and questions remain regarding the sensitivity to the deglacial climate to a volcanic forcing of the size and sulfur content of the Laacher See eruption. However, evidence exists that a similar feedback following other volcanic eruptions could also have triggered similar long-term cooling events during the last glacial period, [129] the Little Ice Age, [130] [131] and the Holocene in general, [132] suggesting that the proposed feedback is poorly constrained but potentially common.

It is possible that the Laacher See eruption was triggered by lithospheric unloading related to the removal of ice during the last deglaciation, [133] [134] a concept that is supported by the observation that three of the largest eruptions within the East Eifel Volcanic Field occurred during deglaciation. [135] Because of this potential relationship to lithospheric unloading, the Laacher See eruption hypothesis suggests that eruptions such as the 12,880 year BP Laacher See eruption are not isolated in time and space, but instead are a fundamental part of deglaciation, thereby also explaining the presence of Younger Dryas-type events during other glacial terminations. [126] [136]

Vela supernova hypothesis Edit

Models simulating the effects of a supernova on the Earth, most notably gamma-ray bursts and X-ray flashes, indicate that the Earth would experience depletion of the ozone layer, increased UV exposure, global cooling, and nitrogen changes on the Earth's surface and in the troposphere. [47] In addition to evidence of global cooling during the Younger Dryas, the presence of carbon-rich “black mats” around 30 cm in thickness across faunal and paleoindian hunting sites suggests that an abrupt change to more aquatic conditions occurred in a small time window. Brakenridge also discusses pollen-core research that suggests global cooling conditions did not only occur in northern latitudes, but also latitudes reaching 41°S. Tree-ring evidence shows an increase in cosmogenic 14 C in ice cores. The time frame of this increase also overlaps with the increase of another cosmogenic isotope, 10 Be. [47]

The only supernova known to have occurred at the beginning of the Younger Dryas, and in close enough proximity to the solar system to have affected the Earth, is the Vela supernova, of which only the Vela supernova remnant remains. [47]

However, most geologists regard this hypothesis as an academic exercise by astronomers with little knowledge of earth system science. [137]


3.3.1 Ice Sheets - Biology

Since scientists cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, they must instead indirectly measure temperature. To do this, scientists rely on historical evidence of Earth’s past climate.

Antarctic ice cores are a key example of such evidence for climate change. These ice cores are samples of polar ice obtained by means of drills that reach thousands of meters into ice sheets or high mountain glaciers. Viewing the ice cores is like traveling backwards through time the deeper the sample, the earlier the time period. Trapped within the ice are air bubbles and other biological evidence that can reveal temperature and carbon dioxide data. Antarctic ice cores have been collected and analyzed to indirectly estimate the temperature of the Earth over the past 400,000 years (Figure 1a). The 0 °C on this graph refers to the long-term average. Temperatures that are greater than 0 °C exceed Earth’s long-term average temperature. Conversely, temperatures that are less than 0 °C are less than Earth’s average temperature. This figure shows that there have been periodic cycles of increasing and decreasing temperature.

Figure 1. Scientists drill for ice cores in polar regions. The ice contains air bubbles and biological substances that provide important information for researchers. (credit: a: Helle Astrid Kjær b: National Ice Core Laboratory, USGS)

Before the late 1800s, the Earth has been as much as 9 °C cooler and about 3 °C warmer. Note that the graph in Figure 2b shows that the atmospheric concentration of carbon dioxide has also risen and fallen in periodic cycles. Also note the relationship between carbon dioxide concentration and temperature. Figure 2b shows that carbon dioxide levels in the atmosphere have historically cycled between 180 and 300 parts per million (ppm) by volume.

Figure 2. Ice at the Russian Vostok station in East Antarctica was laid down over the course 420,000 years and reached a depth of over 3,000 m. By measuring the amount of CO2 trapped in the ice, scientists have determined past atmospheric CO2 concentrations. Temperatures relative to modern day were determined from the amount of deuterium (an isotope of hydrogen) present.

Figure 2a does not show the last 2,000 years with enough detail to compare the changes of Earth’s temperature during the last 400,000 years with the temperature change that has occurred in the more recent past. Two significant temperature anomalies, or irregularities, have occurred in the last 2,000 years. These are the Medieval Climate Anomaly (or the Medieval Warm Period) and the Little Ice Age. A third temperature anomaly aligns with the Industrial Era. The Medieval Climate Anomaly occurred between 900 and 1300 AD. During this time period, many climate scientists think that slightly warmer weather conditions prevailed in many parts of the world the higher-than-average temperature changes varied between 0.10 °C and 0.20 °C above the norm. Although 0.10 °C does not seem large enough to produce any noticeable change, it did free seas of ice. Because of this warming, the Vikings were able to colonize Greenland.

Figure 3. The atmospheric concentration of CO2 has risen steadily since the beginning of industrialization.

The Little Ice Age was a cold period that occurred between 1550 AD and 1850 AD. During this time, a slight cooling of a little less than 1 °C was observed in North America, Europe, and possibly other areas of the Earth. This 1 °C change in global temperature is a seemingly small deviation in temperature (as was observed during the Medieval Climate Anomaly) however, it also resulted in noticeable climatic changes. Historical accounts reveal a time of exceptionally harsh winters with much snow and frost.

The Industrial Revolution, which began around 1750, was characterized by changes in much of human society. Advances in agriculture increased the food supply, which improved the standard of living for people in Europe and the United States. New technologies were invented that provided jobs and cheaper goods. These new technologies were powered using fossil fuels, especially coal. The Industrial Revolution starting in the early nineteenth century ushered in the beginning of the Industrial Era. When a fossil fuel is burned, carbon dioxide is released. With the beginning of the Industrial Era, atmospheric carbon dioxide began to rise (Figure 3).


Watch the video: Ice sheet collapse: The greatest unknown in climate science. Jon Gertner. Big Think (August 2022).