Intensity Variation in the Asian Monsoon and the Westerly during the Last 140kyr Deduced from Grain Size Analysis of Japan Sea Sediments

Abstract

The hemipelagic sediments of the Japan Sea contain a significant amount of eolian dust (Kosa) derived from inland Asia. The grain size and flux of Kosa are considered to reflect wind speed and aridity in the source area, and these parameters have been used to estimate the intensities of the Asian monsoon and the westerly. In this study, we analyzed the samples from a piston core, KT94-15-PC-5, retrieved 150km off Akita, to reconstruct 140kyr variations in the Asian monsoon and westerly intensities on the basis of grain size analysis.
After removing the biogenic fraction, we measured the grain size distribution of detrital sediments using a laser diffraction-scattering grain size analyzer (Horiba LA-920). The results showed negatively-skewed unimodal grain size distribution on a logarithmic scale. We resolved the original grain size distribution curve into two log-normal populations. The finer population, whose median diameter is approximately 3μm, corresponds to the detrital material derived from the Japanese Islands, and the coarser one, whose median diameter is roughly 5 to 8μm, corresponds to Kosa derived from inland Asia. Temporal variations in grain size and content of Kosa during the last 140kyr indicate that glacial-interglacial changes and millennial-scale abrupt oscillations are strikingly similar to those of the δ¹⁸O records of a GRIP ice core from Greenland. Smaller grain size and lower content of Kosa during glacial interstadials may suggest a stronger summer monsoon and/or a weaker westerly, while larger grain size and abundance of Kosa during glacial stadials may suggest a weaker summer monsoon and/or a stronger westerly. It is worth noting that the similarity between the variations in Kosa grain size and δ¹⁸O records of the GRIP ice core can be traced back to the marine isotope stage (MIS) 5e. Our results suggest millennial-scale large climatic changes in East Asia during MIS 5e.