Magnetic properties and geochemistry of the Xiashu Loess in the present subtropical area of China, and their implications for pedogenic intensity

摘要

Variations in the magnetic susceptibility of Chinese loess have been used to indicate pedogenic intensity and therefore past climate across the Chinese Loess Plateau (CLP). However, the relationship between climate and magnetic properties has not been fully resolved, and therefore a study of loess sections distributed over a wider climatic region is timely. In this study, we have investigated a 22.1 in loess section (named the Xiashu Loess) at Dagang, China, by integrating both rock magnetic and geochemical proxies. This loess profile is situated in a modem subtropical climate region with annual mean precipitation and temperature of I 100 min and 15.4 degrees C, respectively, and therefore has experienced a stronger weathering intensity due to its lower latitude, higher temperature and rainfall. Geochemical and magnetic evidence indicate the same source area for the Xiashu Loess as for loess in northern China. The magnetic mineral assemblage of the Xiashu Loess is dominated by a higher concentration of superparamagnetic (SP)/single domain (SD) magnetite/maghemite grains in strongly weathered paleosols than in the loess layers, consistent with the production of fine-grained ferrimagnetic minerals during pedogenic processes. Nevertheless, compared to loess in the CLP, the Xiasbu Loess shows: 1) reduced contrasts in magnetic susceptibility (chi), Hard Isothermal Remanent Magnetization (HIRM) and remanence of coercivity(B-cr) between loess and paleosols units, which indicates that the loess units have undergone stronger pedogenic alterations; 2) lower peak values of HIRM and chi in paleosols. In addition, the ratios SIRM/chi, chi(ARM)/SIRM (where SIRM and chi(ARM) are Saturation Isothermal Remanent Magnetization and the susceptibility of Anhysteretic Remanent Magnetization, respectively) and B,r are found to be significantly correlated with the quotient (CaO*+Na2O+MgO)/TiO2, a geochemical indicator of pedogenic intensity. Overall, it seems that pedogenic processes in the Xiashu Loess lead to a relative increase in fine-grained ferrimagnetic SP/SD grains but depletion of anti-ferromagnetic minerals. Magnetic and geochemical proxies indicate strengthened East Asia summer monsoon during Marine Isotope Stages 13, 11 and 5, which is in good agreement with the records from the CLP and the South China Sea. (c) 2007 Elsevier B.V. All rights reserved.