(began approximately 2.588 million years BP to 11.700 years BP):
➽ General Characteristics of Pleistocene
➵ The climate was characterised by repeated glacial cycles during during 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 end of Pleistocene corresponds with end of the last glacial period.
➵ Claim by climatologist to be similar to 'a continuous El Nino with trade winds in South Pacific weakening or heading east, warm air rising near Peru, warm water spreading from West Pacific and Indian Ocean to East Pacific, and other El Nino markers.'
➵ Can be divided into four stages or ages: Gelasian, Calabrian, Ionian, and Tarantian period.➵ Concluded with the Young Dryas: The Younger Dryas is the youngest and longest of three stadials that resulted from typically abrupt climatic changes that took place over the last 16,000 calendar years. 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.
Link: Younger Dryas
➵ Modern continents were essentially at their present positions during Pleistocene, plates upon which they sit probably having moved no more than 100 km relative to each other since the beginning of Pleistocene/Quaternary.
➵ Each glacial advance tied up huge volumes of water in continental ice sheets, 1500 to 3000 metres thick, resulting in temporial sea level drops of 100 metres or more over the entire surface of the earth. During interglacials, such as the present, drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions.
Effects of Quaternary Glaciation
➵ Formation of Glacial (Baltic Sea, Great Lakes of America which was probably Lake Agassiz originally) and Pluvial Lakes during glaciation. Playa lakes during interglacials. Most pluvial lakes developed in relatively arid regions where there typically was insufficient rain to establish a drainage system to the sea. Instead, stream runoff in those areas flowed into closed basins and formed playa lakes. With increased rainfall, the playa lakes enlarged and overflowed. Pluvial lakes were most extensive during glacial periods. During interglacial stages, when less precipitation fell, the pluvial lakes shrank to form small salt flats.
Pluvial lakes represent changes in the hydrological cycle — wet cycles generate large lakes, whereas dry cycles cause the lakes to dry up leaving large flat plains.[citation needed] Accumulated sediments show the variation in water level. During glacial periods, times when the lake level is fairly high, mud sediments will settle out and be deposited. At times in between glaciers (interglacial), salt deposits may be present due to the arid climate and evaporation of lake water.[4]
Several pluvial lakes formed in what is now the southwestern United States during the glaciation of the late Pleistocene epoch. One of these was Lake Bonneville in western Utah which covered roughly 19,000 square miles (49,000 km2). When Lake Bonneville was at its maximum water level, it was 1,000 feet (300 m) higher than the Great Salt Lake.
When warm air from arid regions meets chilled air from glaciers, it creates cloudy, cool, rainy weather beyond the terminus of the glacier. This type of humid climate was present during the last glacial period in North America, and causes more precipitation than evaporation. The increase in rainfall fills the drainage basin and forms a lake.[5]
During interglacial periods, the climate becomes arid once more and causes these lakes to evaporate and dry up.[1]
Recall: Cold air reduces evaporation and ice prevented outflow of water from pluvial lakes, thus resulted in an endorheic (drainage) basin. This resulted in the existence of the lake. While glacials ended (i.e. interglacials begin), then no longer have cold air from glacial margins to reduce evaporation, and no longer have that much ice to prevent outflow of water.
Isostatic adjustment[edit]
Main article: Post-glacial rebound
Major isostatic adjustments of the lithosphere during the Quaternary glaciation were caused by the weight of the ice, which depressed the continents. In Canada, a large area around Hudson Bay was depressed below sea level, as was the area in Europe around the Baltic Sea. The land has been rebounding from these depressions since the ice melted. Some of these isostatic movements triggered large earthquakes in Scandinavia about 9,000 years ago. These earthquakes are unique in that they are not associated with plate tectonics.
Studies have shown that the uplift has taken place in two distinct stages. The initial uplift following deglaciation was rapid (called "elastic"), and took place as the ice was being unloaded. After this "elastic" phase, uplift proceed by "slow viscous flow" so the rate decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm per year or less. In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST GPS network.[16] Studies suggest that rebound will continue for about at least another 10,000 years. The total uplift from the end of deglaciation depends on the local ice load and could be several hundred meters near the center of rebound.
Winds[edit]
The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation. Winds near the glacial margins were strong and persistent because of the abundance of dense, cold air coming off the glacier fields. These winds picked up and transported large quantities of loose, fine-grained sediment brought down by the glaciers. This dust accumulated as loess (wind-blown silt), forming irregular blankets over much of the Missouri River valley, central Europe, and northern China.
Sand dunes were much more widespread and active in many areas during the early Quaternary period. A good example is the Sand Hills region in Nebraska, USA, which covers an area of about 60,000 km2 (23,166 sq mi).[17] This region was a large, active dune field during the Pleistocene epoch, but today is largely stabilized by grass cover.[18][19]
Ocean currents[edit]
Thick glaciers were heavy enough to reach the sea bottom in several important areas, thus blocking the passage of ocean water and thereby affecting ocean currents. In addition to direct effects, this caused feedback effects as ocean currents contribute to global heat transfer.
Specifically, it disrupted ocean currents such as the thermohaline current, which plays an important role in distributing heat from the energy surplus at equator to energy deficit at polar regions. The inception of these glaciers restricted this flow by blocking deep ocean undercurrents and served as a feedback for the 'glaciation'. From personal speculation, it could lead to decreased sea levels due to increased volume stored in glaciers, also known as eustatic sea level decreases/eustatic cause of sea level fall.
Recall:
Thermohaline circulation is the movement of water due to global density differences. It is affected by temperature and salt content, which determines the density of seawater. The two mechanisms which regulate it are upwelling and downwelling.
Thermohaline circulation is the movement of water due to global density differences. It is affected by temperature and salt content, which determines the density of seawater. The two mechanisms which regulate it are upwelling and downwelling.
Effects of thermohaline circulation on global climate[edit]
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean.[18] Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in the concentration of carbon dioxide in the atmosphere. While it is often stated that the thermohaline circulation is the primary reason that Western Europe is so temperate, it has been suggested that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because of the effect of atmospheric waves bringing warm air north from the subtropics.[19] However, the underlying assumptions of this particular analysis have likewise been challenged.[20]
Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas.[21]
Shutdown of thermohaline circulation[edit]
Main article: Shutdown of thermohaline circulation
See also: Deglaciation and Cold blob (North Atlantic)
In 2005, British researchers noticed that the net flow of the northern Gulf Stream had decreased by about 30% since 1957. Coincidentally, scientists at Woods Hole had been measuring the freshening of the North Atlantic as Earth becomes warmer. Their findings suggested that precipitation increases in the high northern latitudes, and polar ice melts as a consequence. By flooding the northern seas with lots of extra fresh water, global warming could, in theory, divert the Gulf Stream waters that usually flow northward, past the British Isles and Norway, and cause them to instead circulate toward the equator. If this were to happen, Europe's climate would be seriously impacted.[22][23][24]
Downturn of AMOC (Atlantic meridional overturning circulation), has been tied to extreme regional sea level rise.[25]
Nonmenclature of Quaternary Glacial cycles➽ Effects of Glaciation
➵ The effects of glaciation were global, Antarctica was ice-bound throughout the Pleistocene, Andes were covered in south by 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 the east and central Africa were larger. Glaciers existed in the mountains of Ethiopia and to the west in the Atlas mountains.
➵ In Northern Hemisphere, many glaciers fused into one. The Cordilleran ice sheet covered the North America northwest, and the east was covered by Laurentide. The Fenno-Scandian ice sheet rested on northern Europe, including the 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 Ice sheets large lakes accumulated because outlets were blocked and cooler air slowed evaporation. When 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 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 decrease in oceanic and other evaporation.
➵ It has been estimated that during the Pleistocene, East Antarctic Ice sheet thinned by at least 500 metres, and thinning of the Last Glacial maximum is less than 50 meters and probably started after ca 14 ka.
➽ Reasons for Climate Change
➽ Milankovitch cycles
Main article: Milankovitch cycles
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. The timing of our present interglacial interval (known as the Holocene, Postglacial, or the Present Interglacial) to that of the previous interglacial, beginning about 130,000 years ago (The Eemian Interglacial), suggests that the next glacial might begin in about 3,000 years.
➽ Oxygen isotope ratio cycles[edit]
Main article: Oxygen isotope ratio cycle
In oxygen isotope ratio analysis, variations in the ratio of 18O to 16O (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 18O, 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 18O than sea water, the snow that fell on the glacier year by year nevertheless contained 18O and 16O 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, Aurochs, Short-faced bear, Gigantopithecus and others.
The extinctions were especially severe in North America where native horses and camels were eliminated.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 southward 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, ground sloths, Irish elk, cave bears, 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.
- Asian land mammal ages (ALMA) include Zhoukoudianian, Nihewanian, and Yushean.
- European land mammal ages (ELMA) include Gelasian (2.5—1.8 Ma).
- North American land mammal ages (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.
- South American land mammal ages (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.
Wikipedia claims:
Over 11 major glacial events have been identified, as well as many minor glacial events.[13]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.
Another website claims:-Hard to verify veracity of either sources. They may have confused ice ages, glaciations, and glacial events.
https://www.wunderground.com/climate/Glaciers.asp
https://en.wikipedia.org/wiki/Quaternary_glaciation
Personal confusion resolved:
-Initially, read that during interglacials, greater moisture in polar regions resulted in the disappearance of steppe tundra climatic type due to more water circulating in global hydrological cycle.
=> This implies that during glacials, lack of moisture in polar regions due to increased ice volume which decreased water circulating in global hydrological cycle resulted in flourishing of steppe tundra climatic types.
-Then, read about how increased preciptation during glaciers led to the formation of kettle and pluvial lakes. This contradicted with earlier point of decreased precipitation.
SO is there greater precipitation during glaciation or not?
Upon greater analysis, discovered that increased precipitation triggers glaciation (i.e. in the Quaternary, or Pleistocene specifically). The formation of glaciers was merely a response to increased rainfall. This coincides with both the earlier two points and is supported by Le Chatelier's principle, just that it's for the Earth's climatic system. Initially, increased precipitation led to formation of glaciers and pluvial lakes. Due to this, therefore less water in circulation and thus disappearance of the Steppe-Tundra climatic type.
Summary of statement: More water initially (which triggers glaciation), but less water after glaciers form!
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