Pleistocene loess in the humid subtropical forest zone of East Asia

Abstract

Loess deposits in Asia have been used as indicators of palaeoclimate, because they are usually found bordering deserts. This paper reports extensive and thick deposits of loess in tropical southwest China, between latitudes 18 and 23°30′N, which is 1300 km south of known, and extensively researched loess deposits in north China. The present climate of the reported loess areas is hot and humid, with mean annual rainfall of 1000-2000 mm, and vegetation of subtropical evergreen broadleaf forest. This compares with less than 400 mm rainfall and vegetation of semi-desert steppe, in areas of current loess accumulation on desert margins in north China. The source area of the loess, which is dated by optical luminescence to the late Pleistocene, from 90-222 ka, is thought to be the exposed East Asian Shelf, which was up to 140 m below present sea level during Quaternary arid phases. Recent research on the nature of the shelf environment, and the relatively large particle size of the loess, suggests a local origin. The reported loess is not interbedded with palaeosols, and small amounts of soil cover the loess, compared with well developed soils in loess in semiarid regions in north China. This is explained by elimination of the supply source by sea level rise following each arid phase, as continued dust supply appears necessary for soil to form. This preliminary report of loess in southwest China conflicts with palynological evidence, and suggests that recent reconstructions of Pleistocene aridity in east Asia may be conservative.