At a number of localities in the Chinese Loess Plateau, the boundary between the polygenetic soil complex SI and the overlying loess unit LI is exceptionally sharp. Both units formed on the near-horizontal surfaces by subaerial deposition of eolian dust and are unaffected by erosion. Since the sedimentation across the boundary is essentially continuous, the feature points to a relatively rapid decline of pedogenic activity, which would otherwise mix the sediment and smooth the transition. In this respect, the boundary is not dissimilar to the "markers" in Central and Western Europe, which separate humous steppe soils of the Early Glacial interstadials from the sterile slope sediments and are interpreted as deposits of a major continental-scale dust storm. The reconstructed rate of environmental change across the boundary, whose age within the precision Limits of available dating corresponds to the MIS 5/4 shift, is by an order of magnitude greater than the rate of change of orbital parameters. The deflection of atmospheric circulation in response to the build-up of continental ice sheets would also be likely to take a considerably longer time. We suggest that the most probable cause of the rapid monsoon retreat in the Loess Plateau, documented by the onset of the LI loess deposition, is the sudden rearrangement of oceanic conveyor belt possibly triggered by the Toba volcanic explosion. (C) 2000 Elsevier Science B.V. All rights reserved.
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