Annual Rainfall (mm)
FIGURE 7.6 The record of magnetic susceptibility at Xifeng, Loess Plateau, north central China, interpreted in terms of variations in annual rainfall. As rainfall increased the amount of magnetic material produced by weathering processes increased, leading to higher susceptibility levels (Maher and Thompson, 1995).
last glacial-interglacial cycle. They concluded that rainfall fell to much lower levels in glacial times (<200 mm a"1) than estimated by Maher and Thompson (1995). Further studies with a much more extensive set of modern data may resolve these differences.
Increased wind speed, possibly coupled with an expanding desert margin during glacial periods, is reflected in variations of grain size in loess sections (An et al., 1991). This is clearly seen in the general change from low median grain size during interglacials to higher values during glacial periods, but there are higher frequency variations superimposed on that pattern which may be of significance (Xiao et al., 1995). Porter and An (1995) argue that many of these peaks correspond (within dating error limits) to Heinrich events (episodes of ice rafting) in the North Atlantic. They believe that the ice sheets of North America and Europe in some way are fun-
damentally linked to the winter monsoon regime, leading to stronger winds and large particles being deposited on the Loess Plateau during times when catastrophic draw-down of the ice sheets took place. It is difficult to envision the mechanism for such a linkage and it may be that the dating imprecision in both systems make the connection more apparent than real. The importance of precise dating in linking the loess record to other regions is well-illustrated by detailed studies of the Younger Dryas interval. In the North Atlantic area (at least) this is characterized by a return to near-glacial conditions. It might therefore be expected that this would correspond to an increase in loess deposition at that time in China. Indeed there was a late glacial oscillation on the Loess Plateau, but careful 14C dating reveals that conditions actually became more humid during the Younger Dryas interval, though temperatures remained low (Zhou et al., 1996). Evidently, the insolation maximum during that period led to higher levels of summer monsoon rainfall while cold northeasterly winter winds continued to deposit loess across the region (An etal., 1993).
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