Chapter 104: Pleistocene Part 2


    Another loess-paleosol session urs in the Czech Republic, Slovakia, and Austria, where loess nkets terraces of the major rivers that drained eastward and southward from the principal ciated areas in the Alps and northern Europe.


    As in China, buried soils aremon in the loess session and, along with gastropod shells, provide paleoclimatic data and evidence for climatic change.


    The climatic cycles varied from cold and dry conditions when loess umted to warm and wet conditions with hardwood forests and well-developed soils. In thest 730,000 years, eight climatic cycles have been delineated; these corrte with the eight oxygen-18 cycles that urred in the marine record during the same time interval.


    During the entire Pleistocene, about 17 cial episodes alternated with 17 intercials.


    cial till, which was directly deposited by ciers, covers extensive areas of northern Eurasia and northern North America and urs as well in many mountain regions and other areas that currently are not covered by cial ice. Soils of warm climate origin buried between tills were recognized long ago and provided the basis for the development of the idea of multiple ciation during the Pleistocene. However, because direct dating of the deposits generally is not possible and the cial sequence is notplete as a result of erosion or nondeposition or abination of the two, the development of long chronological records and corrtion to the oxygen-18 record are difficult. Corrtions generally are possible for thest two climatic cycles. They also are feasible in areas where the cial session contains interbedded volcanic rocks from which radiometric ages can be obtained.


    In the mid-continental region of the United States, early work recognized tills that were interpreted to represent four major ciations and three major buried soils that were viewed as representing interciations.


    Subsequent work showed that the ciated record was moreplex and that parts of the older record were miscorrted. Consequently, the older portion of the record is informally referred to as the pre-Illinoian, and the older cial and intercial terms are no longer used except locally. Volcanic ash urs within the session in Iowa, Kansas, and Nebraska and is useful for corrtion and dating.


    In one core, till urs below ash that has been dated at about 2.2 million years old, suggestingte Pliocene ciation. Other tills of the pre-Illinoian sequence probably are corrtive with oxygen-18 stages 22, 16, and 12, and possibly others.


    The Illinoian corrtes with oxygen-18 stage 6 and possibly stage 8, and the Sangamonian corrtes with stage 5. Thest cial interval, the Wisconsinan, is subdivided into three parts, an early stade (substage) of ciation, a middle interstadial, or time of restricted ciation, and ate stade of ciation. These intervals generally corrte with oxygen-18 stages 4, 3, and 2, respectively. Deposits of the early and middle Wisconsinan are poorly known in the mid-continental region of the United States; the area probably was not ciated.


    Tills of the early Wisconsinan and even some that are corrtive with oxygen-18 substages 5d or 5b, however, aremon in the Canadian Arctic and on Baffin Ind, where the ice sheet developed much earlier.


    It was not until thete Wisconsinan, about 18,000 years ago, that the southern ice sheet margin reached its maximum extent in the United States and eastern and western Canada.


    The ice sheet margin began to retreat and downwaste (i.e., thin out) soon after reaching its maximum position, and the United States was deciated by about 10,000 years ago. Hudson Bay, near the centre of the ice sheet, was open to the ocean by 8,000 years ago, and, except for the Barnes and Penny ice caps on Baffin Ind, the ice sheet had dissipated from the und areas of central Canada by 6,000 years ago, well into the Holocene and oxygen-18 stage 1.


    A somewhat simr chronology has been developed for the ciated areas of Eurasia and the British Isles based on a variety of criteria. In addition to tills and buried soils, marine deposits, permafrost features, and fossil pollen and beetles have been used to subdivide the session on a climatic basis. As elsewhere, the earlier portion of the record is not well established, and corrtions among different geographic areas, as well as to the marine oxygen-18 record, are uncertain.


    The first cold period, known as the Pretiglian and based on pollen data from the Nethends, began about 2.3 million years ago, soon after extensive ice-rafted material first appears in North Antic deep-sea cores. The Pretiglian was followed by a session of warm and cold intervals, which also are based on pollen and on other flora and fauna evidence and which have been given different names in different areas. Although several old gravels with cial erratics are known, the oldest major cial episodes with extensive till deposits are the Elsterian in northern Germany and the Anglian in Ennd. These ciations probably are corrtive with oxygen-18 stage 12, and local evidence suggests the possibility of earlier cial events. Along coastal areas, these tills are ovein by the marine Holstein deposits, which also may represent more than one high sea-level stand.


    The next major cial sequence is the Saalian of Germany, which is subdivided into the Drenthe and the Warthe; these probably corrte with oxygen-18 stages 8 and 6, respectively. Deposits and soils of thest interciation, the Eemian and Ipswichian, are corrtive with oxygen-18 stage 5e, and those of thest ciation, the Weichselian and Devensian, corrte with oxygen-18 stages 5da, 4, 3, and 2. As in central North America, tills and other deposits are well known only from thest part of this interval.


    The decial history generally is simr, except for a widespread but short interval of renewed cial activity and cold climatic conditions that is known as the Younger Dryas in Scandinavia and Loch Lomond in the British Isles. This event urred about 11,000 years ago, some 2,000 years before the dissipation of the ice sheet.


    A rtively short but importantte Pleistocene and Holocene climatic record is derived from ice cores that have been taken from the ice sheets of Antarctica, Greend, and Arctic Canada. The ice record in several cores extends back to thest interciation (oxygen-18 stage 5) and, in one case, to the next-to-thest ciation (stage 6).


    Although dating of the lower portions of the ice cores is difficult, annualyers of snow and ice can be counted in the upper parts and an urate time scale reconstructed. Because the air temperature at the time when moisture condenses to fall as snow controls the oxygen and hydrogen isotopicposition of the snow, investigators are able to reconstruct temperature variations through isotopic studies of the ice cores.


    Data from the Vostok core taken from the East Antarctic Ice Sheet indicate that the climatic record of the Southern Hemisphere is simr to that interpreted from Northern Hemisphere records with respect to times of ciation and interciation.


    It also is possible to measure the amount of microparticles (very fine dust) in the ice, and studies of this kind show that there are many more particles in the portions of the core that umted during periods of extensive ciation, apparently reflecting greater atmospheric cirction and dust in the atmosphere at those times.


    Trapped air preserved in small bubbles in the ice gives an indication of theposition of the atmosphere at the time the ice (snow) umted.


    An important result from this work indicates that the amount of carbon dioxide in the atmosphere during thest cial (stages 2, 3, and 4) was substantially less than during the Holocene (stage 1) and thest interciation (stage 5e).


    This observation has significant implications with respect to climate and climatic change during cial and intercial transitions.


    Environments during the Pleistocene were dynamic and underwent dramatic change in response to cycles of climatic change and the development ofrge ice sheets. Essentially all regions of the Earth were influenced by these climatic events, but the magnitude and direction of environmental change varied from ce to ce. The best-known are those that urred from the time of thest interciation, about 125,000 years ago, to the present.


    The growth ofrge ice sheets, ice caps, and long valley ciers was among the most significant events of the Pleistocene.


    During times of extensive ciation, more than 45 million square kilometres (or about 30 percent) of the Earth''snd area were covered by ciers, and portions of the northern oceans were either frozen over or had extensive ice shelves.


    In addition to the Antarctic and Greend ice sheets, most of the cial ice was located in the Northern Hemisphere, whererge ice sheets extended to midtitude regions.


    Thergest was the Laurentide Ice Sheet in North America, which at times stretched from the Canadian Rocky Mountains on the west to Nova Scotia and Newfounnd on the east and from southern Illinois on the south to the Canadian Arctic on the north.


    The other major ice sheet in North America was the Cordilleran Ice Sheet, which formed in the mountainous region from western ska to northern Washington.


    ciers and ice caps were more widespread in other mountainous areas of the western United States, Mexico, Central America, and ska, as well as on the inds of Arctic Canada where an ice sheet has been postted.