Geochemistry Of Basalt Weathering In Eastern China

Basalt weathering acts as an important atmospheric CO2sink, and is a key to understand the links between carbon cycle and global climate change. Climate conditions have important effect on basalt weathering rate and the mineral composition of residual soils. Clay minerals and iron oxides minerals are major products of basalt weathering. Understanding the climate response style from clay minerals and iron oxides in residual soils of weathered basalts is significant to reconstruct palaeoclimate.In this study, typical weathering profiles of Cenozoic basalts from different climatic zones in eastern China were selected to conduct clay mineral analysis, major elements composition and iron oxides measurement. These data were used to explore the transport of elements and the climate response style of clay minerals and iron oxides from temperate to subtropical zones. Then the CO2comsuption rate was calculated based on soil profiles of basalt weathering. The main results of this study are listed as below:1. Because of the combined effect of dust input and weathering leaching, elements display relatively leached or enriched in the basaltic soil profiles in the different climate zones. Ca, K, Na and Si in the soils are relatively enriched due to dust input compared to the concentrations of overlying parent basaltic rocks in Inner Mongolia under dry-cold climate. Ca, K and Na begin to be leached in the soils in Shandong and Si is leached in Jiangsu and Anhui under temperate climate. Si is leached heavily while the alkaline elements are almost lost in soils due to intensive weathering in Hainan under humid-hot climate. 2. The CO2consumption rate calculated with relative change of alkaline earth metals in soil profiles of weathered basalts is comparable to the rate calculated from water chemistry of basalt drainage basin. According to relative percentage change of elements and normalization using average composition of dust in northern China, CO2consumption rate from basalt weathering in eastern China can be estimated about5.37～181.00t km-2yr-1, which is within the similar range provided by Dessert et al.’s (2003) based on water chemistry of global basaltic watersheds.3. The composition of clay minerals exhibits regular change in weathering profiles of basalts. As climate changes from dry-cold in the north to humid-hot in the south in the studied region, the assemblages of clay minerals in the profiles transfer as follows:smectite+illite+kaolinite in Inner Mongolia, Shandong, Anhui and Jiangsu provinces; smectite+kaolinite in Zhejiang province; and kaolinite+gibbsite in Hainan province.4. Redness is a better climate proxy than magnetic susceptibility (MS) because it is less influenced by the composition of parent basalts. Redness is a good indicator of content of coloring iron oxides including hematite, while MS reflects the content of magnetic iron oxides including magnetite and maghemite. There is a good correlation between redness and annual mean precipitation, while MS is relatively less correlated with precipitation.