Features of the Quaternary 1031 General timeline

How many major ice advances and retreats occurred over the Quaternary is unclear and there are discrepancies to be resolved between continental and deep-sea chronologies. Ice-rafted detritus (IRD) in marine sediments dated to around 2.6 Ma marks the initiation of glaciation at sea level in the circum-North Atlantic region (Ehlers and Gibbard, 2003). This implies that the base of the Pleistocene epoch (and the Quaternary period) might be better placed earlier than the international stratigraphic definition of 1.8 Ma. Harris (2001) suggests that four major cold events during the Pliocene and eleven during the Quaternary affected the Cordillera of North America (including those of Alaska and the Yukon). Over the past 400000 years, which are better known, there have been at least four major global-scale ice advances. During the last 0.9 million years, there were eight ice age cycles with durations of about 100 000 years and these fluctuations appear to have increased in amplitude over the last 430 000 years. Prior to about 0.9 Ma, ice age cycles had a 41 000 year duration. The reason for this shift is not understood. The latest Antarctic ice core from Dome C (EPICA Community Members, 2004) spans eight glacial cycles and clearly shows the increased amplitude of ice volume over the last four cycles. Between 750 and 430 ka, warm intervals were longer (up to 28 000 years duration) but these interglacials had less pronounced warmth than later ones.

The Last Glacial Cycle, which we know most about, started about 124 ka at the peak of the Last Interglacial (the Eemian), and extended to the start of the Holocene. The paleoclimate community often refers to history in terms of marine "stages", based on 18O in benthic foraminifera. Odd-numbered stages correspond to warmer conditions and/or less terrestrial ice, while even-numbered stages correspond to colder conditions and/or more terrestrial ice. There are also substages. The Eemian interglacial (Kolfschoten et al., 2003; Shackleton et al., 2003) is associated with Marine Isotope Substage 5e (MIS 5e also known as MIS5.5). Figure 10.2 is a composite MIS chronology extending back to 300 ka (in normalized units). In this figure, substages 5a-e correspond to 5.1-5.5. While 5a, 5c and 5e (5.1,5.3,5.5) were periods of reduced ice volume and/or higher temperature within stage 5, 5b and 5d (5.2 and 5.4) were periods of cooler conditions and/or terrestrial ice growth, but on a smaller scale than in stage 4 (Bradley, 1999).

Kukla et al. (2002a) identify the Eemian with widespread temperate mixed forest cover in Europe and temperatures comparable with the present day or even higher. In January, mean temperatures were 2-3 °C higher than now in northwestern Russia and 12 °C higher in the lower Lena River Basin. In the central Siberian Arctic coastal area, July temperatures were 4-8 °C warmer than now and the frost-free season was up to 50% longer (Kukla et al., 2002a). Based on ice core evidence, the Greenland Ice Sheet was markedly reduced in size (Koerner, 1989). Modeling of the response of the ice sheet to inferred Eemian climatic conditions indicates that its partial melt may have

Mis Stages
Figure 10.2 The SPECMAP (Spectral Mapping Project) composite chronology for a set of seven stacked (superimposed) S 18O records from different ocean basins of the world. Dating involves tuning of the marine isotope records by orbital forcing (from Martinson et al., 1987, by permission of Elsevier).
Figure 10.3 Modeled extent and elevation of the Greenland Ice Sheet during the Eemian interglacial under three different temperature reconstructions (a)-(c) based on the GRIP ice core records (d) (adapted from Cuffey and Marshall, 2000, by permission of Nature).

contributed up to 4.5 m of the interglacial rise in sea level above that of the present day (Letreguilly et al., 1991; Cuffey and Marshall, 2000). Figure 10.3 illustrates the modeled extent and elevation of the Greenland Ice Sheet during the Eemian, based on three different interglacial temperature reconstructions.

As is evident from Figure 10.2, the Eemian interglacial was followed by a cold glacial, which was itself characterized by fluctuations between periods of extensive ice (stades) and less extensive ice (interstades). Temperatures in northern middle and high latitudes dropped in a series of steps between MIS 5 and 2. Ice cover began to build up in northern North America, Scandinavia and the Arctic Archipelagos around 110 ka. Major ice sheets were present over northern North America, and Fennoscandinavia

lb' 2b" " 3b" " ' " 40 50 ' 60 70 80 90

Age in thousands of years

Figure 10.4 Reconstructions from several different data sources illustrating climate changes since 93 ka. The top two panels give the abundance of N. Pachyderma from two North Atlantic ocean cores (DSDP 609, approx. 50o N, 45o W, Vema 23-81, approx. 54o N, 18o W) while the bottom panel is the S 18O record from the GRIP Summit ice core. The dotted lines on the bottom panel illustrate long-term cooling trends. H1-H6 indicate Heinrich Events, while YD indicates the Younger Dryas event. The age scale is approximate (courtesy of G. Bond, Lamont-Doherty Earth Observatory, Palisades, New York).

lb' 2b" " 3b" " ' " 40 50 ' 60 70 80 90

Age in thousands of years

Figure 10.4 Reconstructions from several different data sources illustrating climate changes since 93 ka. The top two panels give the abundance of N. Pachyderma from two North Atlantic ocean cores (DSDP 609, approx. 50o N, 45o W, Vema 23-81, approx. 54o N, 18o W) while the bottom panel is the S 18O record from the GRIP Summit ice core. The dotted lines on the bottom panel illustrate long-term cooling trends. H1-H6 indicate Heinrich Events, while YD indicates the Younger Dryas event. The age scale is approximate (courtesy of G. Bond, Lamont-Doherty Earth Observatory, Palisades, New York).

by about 90 ka but their extent is poorly known. In northern Europe and northwest Russia, four glacial stades are apparent within the last glaciation. Recent work in northern Russia indicates that the most extensive glaciation there may have been during the so-called Middle Valdai, prior to 40 ka, contrasting with the maximum in global ice volume 25-18 ka, as discussed below.

There were large fluctuations in sea level (marine transgressions and regressions) associated with ice sheet growth and decay. These represent the direct influence of changes in terrestrial ice volume on eustatic sea level, as well as the response of the Earth through glacio-hydro-isostatic effects. The latter involve vertical movements of the land induced by varying loads of ice and water and the redistribution of mass. These processes are still occurring today. During the marine regressions and transgressions, zones of frozen ground in the coastal and continental shelf areas of the Arctic, especially Eurasia, underwent successive north-south displacements (Rozenbaum and Shpolyanskaya, 2000).

Figure 10.4, based on the original work of Bond et al. (1993), gives a closer focus on climate changes from about 93 ka through the early Holocene on the basis of the relative abundance of left-coiling (sininstral) N. Pachyderma in two ocean cores in the North Atlantic and in the S 18O record at the GRIP Summit ice core. These records go back to stage 5b (5.2). Recall that a more negative (i.e., lighter) S 18O in ice cores means colder conditions over Greenland. A higher percentage of left-coiling N. Pachyderma is interpreted as lower sea surface temperatures. The scale for the percentage of N. Pachyderma is reversed, providing easier comparison with the ice core record. Changes often began and ended rapidly - much faster than indicated in this low-resolution figure. The GISP2 ice core records indicate that temperature changes roughly equivalent to those associated with glacial-interglacial contrasts often occurred over periods of decades. Some transitions may have occurred even more rapidly. We will return to this issue shortly.

The LGM occurred anywhere from 18 to 25 ka, depending on the region and interpretation. In Figure 10.2, we see a minimum in S 18O (i.e., coldest conditions) at stage 2.2, around 18 ka. Figure 10.5 summarizes the approximate extent of Northern Hemisphere ice sheets during the LGM. While some aspects of this figure are controversial and some have been revised (see later discussion), it makes the fundamental point that glaciation was widespread. The Laurentide Ice Sheet was the largest after that of Antarctica. It covered most of Canada, merging in the west with the Cordilleran Ice Sheet and reaching into the northern United States. Moraine features such as Cape Cod and Long Island are geomorphological relics of the Laurentide Ice Sheet. In Europe, the Fennoscandian (or Scandinavian) Ice Sheet covered Scandinavia and part of western Russia. Glacier ice covered parts of the ocean shelf areas off northwest Russia but most of Siberia was ice free. During the LGM, sea level was about 135 m lower than

Lgm Last Glacial Maximum
Figure 10.5 Extent of Northern Hemisphere glacial ice during the Last Glacial Maximum (from Denton and Hughes (eds.), 1981, by permission of John Wiley and Sons).

today. The Bering land bridge, connecting eastern Siberia with western Alaska, last emerged at about 70 ka. The land bridge is of great anthropological importance in that it allowed the migration of humans into North America.

Deglaciation - the transition from the LGM to the Holocene, was not a period of uniform warming, but rather was interrupted by cold events. The most notable was the YD, from about 13 to 11.7 ka, which represented a return to near glacial conditions. The YD is labeled in Figure 10.4. It corresponds clearly to a S 18O minimum in the GRIP record as well as a high percentage of sinistral N. Pachyderma (implying low sea surface temperatures) at the VEMA 23-81 ocean core site. The YD had variable regional expressions.

The Holocene (starting at MIS 1,11.5 ka), considered to start at the end of the YD, has been a generally warm period, and while more stable than the last glacial, still exhibited strong temperature variability. Reconstructions from many different sources indicate that during the so-called Holocene Thermal Maximum or Climatic Optimum (roughly 9.5-6.3 ka) global average temperatures were considerably higher than those of the twentieth century. The period at which maximum warmth occurred varied by region. The Holocene Thermal Maximum was followed by general cooling starting around 5 ka. The most notable cooling since the YD is represented by the LIA. Details of the LGM, deglaciation and the Holocene are examined in Sections 10.5 through 10.7.

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