干旱区生态环境知识资源中心

In the study of loess-soil sequences from monsoonal northern China, magnetic susceptibility (MS) and chemical weathering indexes (such as the ratio of free Fe2O3 versus total Fe2O3, FeD/FeT) are usually used as proxies of the summer monsoon, and grain-size of bulk samples as a proxy of the winter monsoon. In this report, orbital signals recorded in these climate proxies are compared, showing that they all yield frequency patterns essentially similar to that of the marine delta O-18 record. This is attributable to the interactive effects of the winter (dust accumulation intensity and winter monsoon) and summer (summer monsoon) foreings, and particularly to the predominance of the winter signals in the frequency patterns of these proxies. Loess deposition in northern China and the resulting stratigraphic structure, as is reflected by the alternations between loess and soil layers, were primarily controlled by ice-boundary conditions through influencing dust accumulation intensity. We also developed a geochemical proxy (SiO2/Al2O3) to reflect the changes of original eolian grain-size prior to post-depositional weathering, and compare the orbital signals with those recorded in the grain-size of bulk samples. The results reveal a near-lack of the similar to 100-ka cycle and a predominance of similar to 40-ka and similar to 23-ka cycles in the variations of original eolian grain-size. These indicate a relative independent dynamic link between dust grain-size and ice-boundary conditions. Rather, the combination of the moderate similar to 40-ka obliquity signals and the strong similar to 23-ka precessional signals suggests a factor relative to low-latitude insolation changes. We thus believe that original eolian grain-size bears strong summer signals, probably through the influence of summer moisture on the conditions of the southern margins of the deserts. The clear orbital signals in all of these climate proxies from Chinese loess greatly support the Milankovitch theory of paleoclimate.