<HW>Quaternary glaciations
<AU>P.L.Gibbard1, Jürgen Ehlers2, and P.D. Hughes3
<AF>1University of Cambridge, UK
2Independent scholar, Germany
3University of Manchester, UK
<ABS>The Quaternary is synonymous with extensive glaciation of earth’s mid- and high latitudes. Although there were local precursors, significant glaciation began in the Oligocene in eastern Antarctica. It was followed by glaciation in mountain areas through the Miocene (in Alaska, Greenland, Iceland, and Patagonia), later in the Pliocene (e.g., in the Alps, the Bolivian Andes, and possibly in Tasmania) and in the earliest Pleistocene (New Zealand, Iceland, and Greenland). Today evidence from both the land and the ocean floors demonstrates that the major continental glaciations, outside the polar regions, rather than occurring throughout the 2.6thinsp>Ma of the Quaternary, were markedly restricted to the last 1thinsp>Ma–800thinspKa or less. Marine isotope stage (MIS) 22 (ca. 870–880thinspMa) included the first of the “major” worldwide events with substantial ice volumes that typify the later Pleistocene glaciations (i.e., MIS 16, 12, 10, 6, 4–2).
<KW>Africa; Antarctica; Australia; Central Asia; Europe; Marine Isotope Stage (MIS); Miocene; North America; Oligocene; Pleistocene; Pliocene; Plio-Pleistocene boundary; South America
<A>Introduction
<P>Since the recognition in the mid-nineteenth century that glaciers had been considerably more extensive than at present, the Quaternary has been synonymous with glaciation of the mid-latitudes. Today evidence from both the land and ocean floor sediment sequences demonstrates that the major continental glaciations occurred repeatedly over what are now temperate regions of the earth’s surface. Knowledge about the number of glaciations has increased, and also knowledge about the extent of the Pleistocene ice sheets. Progress has been rapid since the 1970s and can be seen in the INQUA Work Group 5 project “Extent and Chronology of Quaternary Glaciations” (Ehlers and Gibbard 2004a, 2004b, 2004c) and the “Extent and Chronology of Quaternary Glaciations, a Closer Look” (Ehlers, Gibbard, and Hughes 2011) (Figure 1). While the maximum extent has undergone comparatively few changes, the Late Weichselian ice sheet has changed fundamentally. Today, a single ice sheet is envisioned instead of the previous three major glaciation centers. Major differences include the (i) glaciation of the Bering Sea, (ii) non-glaciation of the North Sea, (iii) non-glaciation of the northern Urals, and (iv) limitation of glaciation in Siberia to the Putorana Massif and the Taimyr coast. Extensive ice-rafting, an indication that glaciers had reached sea -level, is found from the earliest cold stage (2.6–2.4thinsp>Ma) in both the North Atlantic and North Pacific oceans. The initiation of conditions that resulted in glaciation resulted from the long-term declining cooling trend in world climates that began early in the previous Tertiary period. Apart from some limited activity in the Eocene, significant glaciation began in the Late Oligocene (ca. 35thinsp>Ma) in eastern Antarctica. It was followed by mountain glaciation through the Miocene (23–5.3thinsp>Ma) in Alaska, Greenland, Iceland, and Patagonia and later in the Pliocene (5.3–2.6thinsp>Ma) in the Alps, the Bolivian Andes, and possibly in Tasmania. From the Neogene, glacially derived ice-rafted debris is found in ocean-sediment cores from the North Atlantic region, including the Barents Sea, and areas adjacent to Norway, north and southeast Greenland, Iceland and northern North America, and in the Southern Ocean off Antarctica (De Schepper et al. 2014).
FIG1
<A>Glaciation during the quaternary
<P>The climatic variation of that characterizes the earth’s climate during the Late Cenozoic and indeed before, is controlled by variations of the planetary orbit around the sun that therefore controlled the receipt of solar energy at the earth’s surface. These Milankovitch variations, named after their discoverer, are responsible for the cyclic climate changes that characterize the Quaternary and indeed much of earth’s history. One of the most critical ways they are expressed is through the development of “ice ages” or periods when glaciation extended across large areas of the earth. The Early Pleistocene (2.6–0.8thinsp>Ma) was characterized by climatic fluctuations dominated by the 41thinspKa precession cycle, during which relatively few cold periods were sufficiently cold and long to allow the development of substantial ice sheets. Only 14 of the 41 cold stages of that period currently show evidence of major glaciation. They include the Plio-Pleistocene boundary events Marine isotope stage (MIS) 104, 100, and 98, together with Early Pleistocene MIS 82,?78, 68, 60, 58, 54, 52, 36, 34,?30, and 26 which reach δ18Oocean‰ of circa 4.6–5. It is not until the transition in dominant orbital cyclicity to the 100thinspKa cycles, that began circa 1.2thinsp>Ma and was fully established by about 800thinspKa (“Middle Pleistocene transition”), that the cold periods (glacials) were regularly cold and long enough to allow ice-sheet development on a continental scale, outside the polar regions. However, it is during MIS 22 (ca. 870–880thinspKa) that the first of the “major” cold events that reached critical values of circa 5.5 or above δ18Oocean‰ equivalent to substantial ice volumes that typify glaciations of the later Pleistocene (i.e., MIS 16, 12, 10, 6, 4–2). Potentially therefore, it is likely that there were a minimum of 20 periods during which extensive glaciation could have developed during the last 2.6thinsp>Ma, with the most extensive (ca. 5–6 periods) being limited to the last 900thinspKa (Table 1, Table 2, and Table 3).
Precisely where these glaciations occurred and how far they extended is very difficult to determine, given that the remnants of less extensive early glaciation tends to be obliterated and mostly removed by later, more extensive advances. Although this is so in all terrestrial areas, it is especially difficult in mountain regions where the preservation potential of older sequences rapidly diminishes with time and subsequent glaciation. However, examination of the frequency of glaciation through the Cenozoic indicates that glaciation in the Southern Hemisphere having been established first, principally in Antarctica and southern South America, occurred continually from the Early Neogene to the present day. By contrast, Northern Hemisphere glaciation, although initially somewhat restricted, increased markedly at the beginning of the Quaternary, increasing again in frequency in the latest Early Pleistocene and reaching very high levels in the Middle–Late Pleistocene. While this pattern is not unexpected, the striking increase in ice sheets through the Quaternary clearly emphasizes that worldwide glaciation is in effect a northern-hemispheric phenomenon.
Examination of the evidence accumulated in the INQUA project (Ehlers and Gibbard 2004a, 2004b, 2004c; Ehlers, Gibbard and Hughes 2011), supported by other published sources, demonstrates the current state of knowledge. When examining the resulting tables (Table 1, Table 2, Table 3), it is important to bear in mind that the stratigraphical control between regions, beyond the range of radiocarbon dating, is weaker than might be desired. This is particularly a problem outside Europe where biostratigraphy is less developed and the sheer extent of the unexplored regions makes future discoveries likely. Thus the presentation below can only be a first step toward a comprehensive overview. The correlations applied here are based on the Global Correlation Chart (Gibbard and Cohen 2013).
TAB1
TAB2
TAB3
<A>Plio-Pleistocene glaciation
<P>Evidence of glaciation is widespread from throughout the Quaternary and indeed the Neogene in the Northern Hemisphere. The longest sequences are restricted to Alaska and the adjacent northwest territories of Canada which, together with Greenland and the Rockies, preserve evidence of glaciation from the Neogene to the present. In northern Canada and Alaska, the oldest till and accompanying ice-rafted detritus in marine settings, dates from the Early Miocene, with regionally widespread glaciation occurring in the Pliocene and regularly throughout the Pleistocene. In adjacent British Columbia a comparable sequence is found, particularly in the north. Similarly, in Greenland and Iceland glaciation began in the Miocene, occurring regularly through the Pliocene and onwards to the present day in the mountains. Likewise, in Norway, its adjacent offshore and the neighboring Barents Sea, glaciation is recorded from the Early Miocene, Early Pliocene and Plio-Pleistocene. By the Late Miocene, inland ice shields were periodically present in Greenland, especially in the mountainous east, with ice reaching the sea in southeast Greenland, although contiguous ice sheets have occurred since the earliest Pleistocene (ca. 2.3thinsp>Ma). In the eastern Rockies of the United States a much shorter glacial sequence occurs. Here Plio-Pleistocene-aged till is known from Montana, North Dakota, and California. On Mount Kilimanjaro in East Africa the first glaciation is recognized at circa 2.0thinsp>Ma (ca. MIS 68). In Europe glaciation before the Middle Pleistocene glaciation is represented only by ice-rafted material, outside the mountain regions (e.g., in the Netherlands, lowland Germany, European Russia, and Britain). Substantial glaciation of the Baltic region late in the Early Pleistocene is indicated by erratic materials in the Netherlands (1–1.2thinsp>Ma; MIS 34–36). Glaciation is also established in the Alps from the Plio-Pleistocene.
In the Southern Hemisphere, glaciation is much longer established, as noted above. Here the ice already formed in the Late Eocene–Early Oligocene in East Antarctica and built up in a step-like pattern through the Neogene (De Schepper et al. 2014). The present polar conditions were already established by the Early Pleistocene after 2.5thinsp>Ma. A similar history is known from the Piedmont areas of Argentina and Chile where substantial ice caps were established by 14thinsp>Ma. Widespread lowland glaciation between 2.05–1.86thinsp>Ma (ca. MIS 68–78) followed by the 'Great Patagonian Glaciation' took place at 1.15–1.00thinsp>Ma (ca. MIS 30–34). Further north, the earliest glaciation recorded in the Bolivian Andes dates from 3.27thinsp>Ma, with extensive events at 2.2thinsp>Ma (ca. MIS 82). In Columbia the record also begins at 2.5thinsp>Ma. The earliest records in Australia are found in Tasmania and New Zealand from the Plio-Pleistocene (2.6thinsp>Ma: MIS 98–104). Only slightly younger is New Zealand’s oldest known glacial event (the Porika Glaciation).
<A>The “glacial” pleistocene
<P>The “glacial” Pleistocene effectively begins with ice sheets spreading over vast lowland areas, particularly around the North Atlantic region, and the intensification of global cold period (glacial) climates in general. It coincides with the “Middle Pleistocene transition” (1.2–0.8thinsp>Ma) when the transition from the dominant 41 to 100thinspKa Milankovitch orbital cyclicity resulted in periods sufficiently cold and long to allow the development of continental-scale ice sheets.
The till sheets of the major glaciations of the “glacial Pleistocene” are found throughout large parts of Europe and North America and especially in the lowlands and under the sea. Widespread lowland glaciation began in the early Middle Pleistocene shortly after the Brunhes/Matuyama palaeomagnetic reversal (780thinspKa). In Europe, the phases represented include the Weichselian (Valdaian, MIS 4–2), Saalian (Dniepr and Moscovian, MIS 6, 8 and 10), Elsterian (Okan, MIS 12), and the Donian (Narevian, MIS 16). More limited glaciation may also have occurred in the circum-Baltic region during the latest Early Pleistocene (MIS 20 and 22). Curiously, evidence for early Middle Pleistocene glaciation is absent from the North Atlantic and Norway, while it is certainly present in Denmark, the Baltic region, and European Russia. In the Italian Dolomites, glaciation becomes established in MIS 22. Comparable evidence is also found from north of the Alps in Switzerland and southern Germany. Further to the west, in the Pyrenees, the oldest glaciation identified is of Late Cromerian age (MIS 16 or 14). Widespread lowland glaciation again is first seen throughout North America in MIS 22. From this point onwards, major ice sheets covered large regions of the continent during the Middle Pleistocene pre-Illinoian events MIS 16, 12, 8 and 6 (Illinoian sensu stricto) and the Late Pleistocene MIS 2–4 (Wisconsinan). In Mexico, the oldest moraines on volcanoes have been dated to 195thinspKa and probably relate to a pre-MIS 6 glaciation. Evidence from east Greenland suggests that its quasi-permanent ice sheet may have almost disappeared during the Eemian Stage interglacial (ca. MIS 5e). Glaciation of Tibet and Tianshu is not recorded before the Middle Pleistocene, of which that during MIS 12 was the most extensive. In Tainshu older glaciation (?MIS 16) may have also occurred. This apparently delayed glaciation of the Himalayan chain might reflect a late uplift of high Asia. Subsequent events took place during MIS 8, 6 and 4–2, and continue today in the highest peaks.
As in Europe and North America, glaciation increased in intensity throughout the Andean chain from 800thinspKa to the present day, but in the south it was less extensive than during the Early Pleistocene events. In Australasia, following a 1thinsp>Ma break, the glacial record continues in MIS 12, followed by MIS 6, 4 and 3. In Tasmania, an early Middle Pleistocene event, possibly during MIS 16, is followed by glaciations during MIS 6 and 3. The glacial record during this time in Africa is restricted to the East African mountains, Mount Kilimanjaro, Mount Kenya, and the High Atlas, where glaciations appear to be broadly equivalent to those elsewhere, that is, during MIS 12, 6 and 2.
<A>Last glaciation
<P>The term last glacial maximum or LGM is widely accepted as referring to the maximum global ice volume during the last glacial cycle corresponding with the trough in the marine isotope record centered on circa 18–14CthinspKa bp (Martinson et al. 1987) and the associated global eustatic sea level low also dated to 18–14 CthinspKa bp (Yokoyama et al. 2000). It has also been assigned chronozone status (23–19 or 24–18thinspKa cal bp dependent on the dating applied), the event being centred on the calibrated date at 21thinspKa cal bp (i.e., LGM sensu stricto). However, since the last maximum glaciation after MIS 5 occurred much earlier in some areas than in others, the term LGM should be used with caution. For the purposes of this entry it is defined as the interval 27.5–23thinspKa (= Greenland Stadial 3) (see Hughes, Gibbard, and Ehlers 2013; Hughes and Gibbard 2015).
During the LGM, the extent of the glaciation of the Southern Hemisphere differed very little from that of the Pleistocene glacial maximum. Glaciers in Antarctica still reached to the shelf edge and on New Zealand, Tasmania, and in South America the glacier tongues were only slightly smaller than during earlier events. On mainland Australia, local mountain glaciation occurred. It seems that the LGM in the Southern Hemisphere began earlier than in the Northern Hemisphere, probably around 27thinspKa. The high mountains of East Africa were glaciated. There is no unequivocal evidence of glaciation in South Africa, although minor glaciers have been postulated by various authors. However, it must be borne in mind that South Africa is located relatively close to the equator to the north. Were it in the Northern Hemisphere, Cape Town would be situated at the same latitude as Atlanta in Georgia, USA or, if placed relative to the European ice sheet, it would be south of Tunis. Consequently, sea ice cover did not reach the southern end of Africa (Figure 2b).