Chemical weathering index and magnetic susceptibility of deposits and their responses to the Quaternary climate in Dongting Basin

Abstract：The drilling of ZKC1 borehole was carried out at Lianghucun in southeastern Anxiang sag. The Quaternary alluvial and lacustrine sediments in the borehole are composed of Pliocene-Early Pleistocene Huatian Formation, Early Pleistocene Miluo Formation, early-middle Middle Pleistocene Dongtinghu Formation, Late Pleistocene Potou Formation and Holocene strata. The major elements and magnetic susceptibility of the core from the borehole were analyzed systematically. According to the idea that the chemical weathering index (CWI) of terrestrial sediments has a positive correlation with temperature and humidity, and that the magnetic susceptibility has a negative correlation with temperature and humidity, the authors probed into the Quaternary climate evolution of the Dongting basin. Combined with other materials, the curve of CIA indicates that the Quaternary climatic evolution of the Dongting basin was from early to late in order of cool and dry → tropical and humid → cool and dry → tropical and humid during Early Pleistocene; cool and dry → tropical and humid → cool and dry-warm and humid→ tropical and humid during Middle Pleistocene; cool → warm and humid → cool during Late Pleistocene, and warm and humid-tropical and humid during Holocene. This climatic evolution process coincides basically with climatic change indicated by sporopollen assemblages of the core from the ZKC1 borehole, and also accords with Quaternary climatic variations of eastern China, which suggests that the CWI of the sediments shows a good responses to the Quaternary climate evolution. Curves and correlation coefficient of －0.32 between CWI and magnetic susceptibility of the sediments show that the magnetic susceptibility has a negative correlation with CWI, which suggests that temperature and humidity controlled evidently the magnetic susceptibility. Impacted probably by the uplift and subsidence of the basin, the curve of the magnetic susceptibility fails to show the same phasic variations as CWI.