A Rb/Sr record of catchment weathering response to Holocene climate change in Inner Mongolia

Variation in the rubidium to strontium (Rb/Sr) ratio of the loess-palaeosol sequences has been proposed to reflect the degree of pedogenesis and weathering in the northwestern region of China. To characterize the Rb/Sr ratio of the dissolved loads of a single catchment, we analysed a 12(.)08 m sediment core from Daihai Lake in Inner Mongolia, north China. Dating control was provided by Pb-210, Cs-137 and AMS-C-14. Sequential extraction experiments were conducted to investigate the concentrations of Rb and Sr on various chemical fractions in the lake sediments. Down-core variation in the Rb/Sr ratios provides a record of Holocene weathering history. From 9 to 3(.)5 ka BP, accelerated chemical weathering was experienced throughout the Daihai catchment under mainly warm and humid conditions, and this reached a maximum at c. 5 ka in,. However, weathering was reduced between c. 8(.)25 and 7(.)90 ka BP, which may reflect the global 8(.)2 ka cooling event. After c. 2(.)5 ka BP, increased Rb/Sr ratios with higher frequency of fluctuations indicate reduced weathering within the Daihai catchment. The highest Rb/Sr ratios in the Little Ice Age lake sediments indicate the weakest phase of Holocene chemical weathering, resulting from a marked reduction in Sr flux into the basin. The Rb/Sr record also shows an enhancement of chemical weathering under today's climate, but its intensity is less than that of (he Medieval Warm Period. Increased Rb/Sr in lake sediment corresponding to reduced catchment weathering is in striking contrast to Rb/Sr decrease in the glacial loess layers in the loess-palaeosol sequence. Copyright (c) 2005 John Wiley & Sons, Ltd.