Paleoenvironmental significance of the magnetic fabrics in Chinese loess-paleosols since the last interglacial (<130 ka)

摘要

We compare new anisotropy of magnetic susceptibility (AMS) data with previously-determined paleoclimatically sensitive parameters for a loess profile at Lingtai, central Loess Plateau, China, to unravel factors that control loess AMS. Magnetic lineations are clustered along NE–SW and NW–SE orientations during cold and warm periods, respectively, which probably corresponded to paleowind directions of both winter and summer monsoons. Anomalously steep magnetic foliations coincidentally occur at the paleoclimatic stage boundaries, indicating sediment disturbance. Lineation strength appears to be sensitive to millennial-scale rapid paleoclimatic fluctuations. Generally, the winter monsoon produces a lower degree of lineation than the summer monsoon. Furthermore, by studying lineation subsets for the cold and warm periods, we find that the cold-interval lineation strengths co-vary with grain size over the last ～70 ka. During the last interglacial (about 74–128 ka), the lineation and declination of the susceptibility maximum (Dec-K_(max)) are sensitive to rapid fluctuations in winter and summer monsoons. Long-term trends of magnetic foliation and anisotropy degree replicate grain size variations, with superimposed short-term peaks related to post-depositional compaction, as suggested by density. The AMS shape parameter (T) indicates oblate fabrics. These new findings will not only improve our understandings of loess AMS, but also expand the possibilities of paleoclimatic studies in the Chinese loess.