The last glacial/interglacial cycle at two sites in the Chinese Loess Plateau: Mineral magnetic, grain-size and 10Be measurements and estimates of palaeoprecipitation

Abstract

To elucidate the way in which past climatic changes are encoded magnetically by wind-blown silt, we have investigated two stratigraphic sections in the Chinese Loess Plateau (Xiagaoyuan, in the cool, dry western plateau and Houzhuang, in the warmer, wetter central plateau). In view of the anticipated importance of magnetic particle size, we have determined grain-size distributions from > 50 μm down to ∼10 nm by sequential sieving, sedimentation and centrifugation of typical loess and palaeosol material from the two sites. For the essentially unaltered loess, the main susceptibility contribution lies in the 20–50 μm fraction, with only ∼20% of the signal residing in the < 2 μm fraction. In the well-developed palaeosol, ∼60% of the susceptibility comes from the < 2 μm fraction, with a strong peak in the 0.1–0.4 μm fraction. Low-temperature experiments confirm this magnetic enhancement, and also exhibit Verwey transitions characteristic of magnetite. Magnetic hysteresis parameters show simple relationships to susceptibility, which can be interpreted in terms of a uniform mineral ingredient which increases in amount as pedogenesis intensifies. Beryllium-10 content was determined for a profile spanning palaeosol S 1 (corresponding to oxygen isotope stage 5) at Houzhuang, and a peak value of 4.8 × 10 8 atoms/g was observed, almost identical to that found by other workers for the same horizon at Luochuan. A similar profile at Xiagaoyuan reveals three peaks corresponding to palaeosols S 1S 1 (2.6 × 10 8 atoms/g), S 1S 2 (3.3 × 10 8 atoms/g) and S 1S 3 (3.3 × 10 8 atoms/g). Conversion of these concentrations into 10Be fluxes indicates low dust accumulation during warmer climate episodes. At these times pedogenic susceptibility enhancement is favoured, the amounts of which imply palaeoprecipitation slightly higher than today for S 1S 3 at Xiagaoyuan, but somewhat lower than today for S 1 at Houzhuang.