Spatial and temporal variations of Rb/Sr ratios of the bulk surface sediments in Lake Qinghai

Rb generally coexists with K in the K-rich minerals, such as K-feldspar, and biotite, etc.; while Sr tends to enrich in Ca-bearing minerals, e.g. the limestone and the Ca-bearing silicates such as hornblende, plagioclase and picrite [1]. Since the Ca-bearing minerals are easy to breakdown compared with the K-bearing minerals, the K-Rb pair and Ca-Sr pair are easy to fractionate during natural processes [1,2]. For example the chemical weathering can leach the Ca-Sr much easier than K-Rb, which leads the residue enriched in K-Rb but depleted in Ca-Sr [1,2].The different behaviors between Rb and Sr are useful in identifying material provenance and indicating the intensity of chemical weathering [1,3]. One example is the use of Rb/Sr ratio in the Chinese loess/paleosol profile as an indicator of intensity of Asian summer monsoon [3]. Since Sr-Ca can be leached much easier than Rb-K, the relict would have higher Rb/Sr ratio compared with the leached fraction. Therefore, as shown by Chen et al. [3,4], higher Rb/Sr ratios in a loess/paleosol profile correlated to higher degree of weathering, and thus to stronger monsoon precipitation on long-term timescales. On the other hand, the dissolved material is relatively rich in Sr but depleted in Rb compared with the parent rocks. These dissolved materials will be eventually transported into lakes/oceans and will therefore lead to lower Rb/Sr ratios in lake/ocean sediments. As a result, sediments with higher fraction of chemical/biogenic deposits have lower Rb/Sr ratios, while those with higher fraction of terrigenous detritus have higher Rb/Sr ratios. Some studies thus use the Rb/Sr ratios of lake sediments as a potential indicator of chemical weathering intensity in the catchment (e.g. [5]).However, factors that influence the material load to lakes are quite complex, such as the local climates, the chemical/physical property of the bedrock, vegetation cover, and human activities, etc. These variable factors may complicate the e