最近130ka来黄土物质化学风化的空间观察

SPATIAL CHANGES OF CHEMICAL WEATHERING RECORDED BY LOESS DEPOSITS IN THE CHINESE LOESS PLATEAU DURING THE PAST 130000 YEARS

选取黄土高原48个末次间冰期以来沉积剖面中的248个黄土、古土壤和弱发育土壤层的代表性样品,对它们的酸不溶物进行了主量元素的分析。结果表明,在已有的几个不受粒度影响的化学风化指标中,(CaO+MgO+Na2O)/TiO2比值对气候条件变化具有相对较高的敏感性,且受沉积速率的影响较小。在源区风化状况相对稳定的前提下,它是一个反映夏季风变化较为有效的指标。根据该指标在黄土高原上的空间等值线图,发现其从北到南间冰期的变幅大于冰期,这可能表明间冰期时夏季风降水在黄土高原的变化梯度要大于冰期。

Chemical weathering intensity of loess deposits is largely determined by three factors： Chemical weathering in source regions, grain size, and post-depositional weathering. The third factor is influenced by climatic conditions such as precipitation and temperature, and the dust sedimentation rate in the area of deposition. Previous studies have shown that the （CaO ＋ MgO ＋ NaEO）/TiO2 ratio of decarbonated residue from loess is independent of grain size changes and thus is a reliable proxy for chemical weathering. However, the validity of （CaO ＋ MgO ＋ Na2O）/ TiO2 to describe changes in monsoon intensity requires further study. In this study, 48 sections over the last glacial-interglacial cycle on the Chinese Loess Plateau during the past 130ka were sampled, and the major elemental concentrations of 248 decarbonated residue samples were measured to investigate the utility of the （CaO ＋ MgO ＋ Na2O）/TiO2 ratio as a proxy for changes in monsoon intensity. Results show that the （CaO ＋ MgO ＋ Na2O）/TiO2 ratio is relatively more sensitive to climate change than other indexes independent of grain size, and is not affected substantially by sedimentation rate. Assuming the weathering regime is relatively stable in the loess source regions, the （CaO ＋ MgO ＋ Na2O）/TiO2 ratio is a reliable proxy for the intensity of summer monsoon. A decreasing （CaO ＋ MgO ＋ Na2O）/TiO2 ratio from northwest to southeast both in loess and paleosols indicates that the Chinese Loess Plateau is in the control of the East Asian summer monsoon during both interglacial and glacial times. In addition, the spatial distributions of （CaO ＋ MgO ＋ Na2O）/TiO2 ratios show a greater north-south gradient during interglacial periods than during glacial periods. This may suggest that the spatial precipitation gradient, controlled by the summer monsoon, is steeper during interglacials than in glacials.