Quaternary glaciation in Northern Central Asia. | Вестн. Том. гос. ун-та. Биология. 2012. № 2 (18).

Quaternary glaciation in Northern Central Asia.

In Central Asia, Pleistocene glaciations occurred in two climatic regimes: arid regions where annual precipitation was <150 mm, and more humid regions where it was greater. In the former, the precipitation controlled the ELA and size of the glaciers; in the latter it was temperature. Temperatures are less variable spatially than precipitation, and therefore the glaciers of the arid regimes have a wide range of ELAs. This leads to highly local, idiosyncratic glacial chronologies because of local rain-shadow effects as well as nuances in the pattern of moisture distribution by storms affected by topography and the jet stream. However, southern Siberia appears to have followed the global pattern of glacial advances, while the complexities are largely farther south.Mountain glaciers in arid Central Asia are important in and of themselves because their meltwater is necessary to sustain some communities through dry seasons when rainfall is slight. Furthermore, glaciers are commonly associated with ice-dammed lakes that can rupture to release dangerous outburst floods downstream. However, because of their widespread distribution in Central Asia, the most significant role for glaciers may be as a warning system for climate change and a signal for the degradation of permafrost and consequent release of greenhouse CH4 into the atmosphere.

Четвертичное оледенение в северной части Центральной Азии.pdf IntroductionGlaciers develop in response to many climatic forcing functions, especially winter precipitation and summer temperatures. However, summer cloudiness, wind speed and humidity all play important if commonly secondary roles in modifying the loss of ice (ablation) from the glacier's surface. Topographic factors such as shadowing by ridges also may be important. Glaciers (fig. 1) grow when the accumulation of ice exceeds the ablation. Because for many glaciers summer temperatures are the dominant factor in ablation, and because temperatures decrease with elevation about 6.5°C km-1 (environmental lapse rate), except in polar regions glaciers are preferentially found in mountains today. At high elevations, accumulation exceeds ablation and the glacier grows. As the ice thickens, it also flows downslope to lower elevations and higher temperatures. There, ablation exceeds accumulation and the glacier melts. Thus the glacier exists in a dynamic state, balancing accumulation and loss. If accumulation increases, the glacier will advance downslope; if it is reduced, the glacier will melt back and appear to retreat upslope. The elevation or altitude atwhich accumulation balances ablation is known as the equilibrium line altitude, or ELA. This is an important parameter characterizing glaciers (fig. 2).Fig. 1. Photograph of the South Cascade Glacier in retreat, Cascade Mountains. The landscape has been modified by repeated glaciation. The steep cliffs above the lateral margins of the glacier were produced by glacial erosion and mark the height of the glacier during the Ice Age. Ablation zone is the darker part of the glacier near its terminus, where light-toned snow has melted. View to SE. Photograph: Kurt Parker, 2007Fig. 2. Schematic cross section of glaciated mountain showing the ELA for glaciers of two sizesThe ELA represents a loosely characterized long-term average of the actual elevation of equilibrium, which fluctuates daily and seasonally and on longer time scales as well. In late summer this line is easy to see on a glacier, because above it the snow is preserved and appears white, but below it darker bare ice is exposed.In Central Asia the fundamental observation that must be explained is the high variability in the ELA from place to place, on length scales of 500 km or less. This is quite unlike, for example, the Sierra Nevada in California (USA) in which the ELA rises systematically with latitude for hundreds of km [1]. This lack of systematic behavior in some places, and with it the lack of asynchronism of the local LGM, was noted by [2], but it has taken years of careful dating of glacial deposits by many different researchers to confirm.In this paper, the response of glacial ELAs to climate is discussed first. Then, examples of dated glacier systems from northern Mongolia, southern Siberia, and the Kyrgyz Tien Shan are summarized. Finally, some implications for modern climate-change studies are discussed.Climate and glacier dynamicsClimate is long-term weather and, on long time scales, the common short-term annual or decadal oscillations in weather tend to average out. It is this average that is indicated by the ELA. Climate has fluctuated throughout the Quaternary Period, and on a wide range of time scales. Some climatic changes have led to expanded high-latitude ice sheets, and in these times the eustatic sea level is lowered accordingly. Along with lowered sea levels, the ratio of 18O and 16O in the remaining seawater is changed because the lighter 16O is preferentially evaporated. The isotopic ratio from fossils in marine sediment cores reflects the changing sea-water values, and the oscillations in the fossil oxygen are interpreted as climatic changes (fig. 3).Along with growth and decay of the high-latitude ice sheets, mountain glaciers advance and retreat - but not necessarily all in synchrony with the polar ice, or with each other [2]. This is probably because of local or regional changes in weather systems and the jet stream: climate cannot be well-characterized by simple global averages. The southern border regions of Siberia, and Central Asia to the south, are locations in which the variability of paleo-precipitation especially was pronounced, with strong effects on the glacial landscape.Given this complexity, how can we use the glacial record to infer anything of value concerning paleoclimate? We first must understand how glaciers form and behave under the influence of climate, and how to 'read' geologic deposits and landforms to learn the distributions of vanished glaciers that occupied the landscape in more favorable times. Glaciers commonly, but not always, erode the landscape on which they are found. The erosion is strongest near their heads, where the ever-increasing mass drives the ice downward onto the bed and sides of the glacier, grinding awaythe rock. Below the ELA, the load - lightened by ablation from the top of the glacier - is lessened, and the flow lines are up and out from the base of the glacier. This upward and outward flow of ice carries with it the rocky rubble that was eroded from its sides and base by the glacier, and this material is deposited as till along the sides of the glacier, forming lateral moraines, and at the snout, forming end or terminal moraines (fig. 4). Lateral moraines are found only below the ELA. Therefore, the ELA of long-vanished glaciers can be estimated from the highest elevation of the moraines it left behind when it melted away. The highest lateral moraines can be seen just inside the mountain front in Figure 4, and from this observation it is possible to infer a local paleo-ELA of ~2400 m asl. In the Hoyt Agaya Uul (Fig. 4), the Pleistocene moraines have not been eroded and their highest occurrence can be identified in the field to within a few meters or tens of meters, but commonly lateral moraines often occur within steep mountain valleys where preservation is poor. Here the highest lateral approach cannot be used to find the paleo-ELA, and other techniques must be employed. The ELA can be also estimated by from the toe and headwall altitudes, or from the areas of the accumulation and ablation zones. In both these approaches, the elevations (or areas) are ratioed. Fig. 3. Climatic record of the past 200,000 years, from oxygen isotopic data for planktonic foraminifera. Marine oxygen isotope stages (MIS) 1-6, defined by these data, are shown in the horizontal bar above the curve and by color bands belowthe curve itself. Even-numbered stages are considered to be glaciations; odd-numbered stages are interglaciations. δ18O is a measure of the ratio of oxygen isotopes 18O and 16O(relative to modern seawater) and increases in sea water when evaporation lowers sealevel, and thus when continental ice caps are large. Warm intervals of low ice-sheet volume are indicated by δ 18O

Ключевые слова

экологический прогноз, палеогляциологические показатели, палеоклиматические условия, четвертичное оледенение, climate-change, paleoclimate, paleo-precipitation, quaternary glaciation

Авторы

ФИООрганизацияДополнительноE-mail
Гиллеспи Алан Р.Вашингтонский университет (США)профессор наук о Земле и Космосе Центра четвертичных исследований
Всего: 2

Ссылки

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 Quaternary glaciation in Northern Central Asia. | Вестн. Том. гос. ун-та. Биология. 2012. № 2 (18).

Quaternary glaciation in Northern Central Asia. | Вестн. Том. гос. ун-та. Биология. 2012. № 2 (18).