Seasonal and Spatial Variations of Stable Strontium Isotope in the Suspended Sediments of the Changjiang ( Yangtze) River

Abstract:

In recent years, remarkable stable strontium fractionation has been found in many geological and biological processes such as chemical weathering and carbonate precipitation. The stable strontium isotope (δ^88/86Sr) compositions in various natural materials have been studied, yet its behavior in river water and sediment remains to be clarified. The Changjiang is the largest river originating from the Tibetan Plateau, and bridges the Eurasian continent and East Asian marginal sea by delivering a large volume of dissolved and particulate materials into the sea. It has complicated tributary system and source rock types and is subject to strong Asian monsoon impacts, which makes it an ideal river for the study of sediment weathering, recycling and source-to-sink transport processes.

In this study, spatial and seasonal suspended sediments were collected from the Changjiang mainstream by filtering with 0.45 μm Millipore membrane in the field. The seasonal samples were taken from Datong hydrological station in the lower reaches during a whole hydrological year. All these samples were dissolved with 4N HNO₃ to separate them into two different phases: leachate and residue. The stable strontium isotope ratios in these two phases were analyzed using SSB method described in Ma et al. (2013) with MC-ICP-MS. The reference material was SRM 987. The external precision of our method is ±0.013‰ (1SD).

The results show that δ^88/86Sr values in the residue phase are much higher (0.071−1.172‰) than those in the leachate phase (0.133−0.281‰). Together with major and trace elemental data, we suggest that heavy Sr prefers to enrich in the silicates, rather than carbonate minerals, during the fractionation process. Regular variations of δ^88/86Sr are observed in the spatial samples, with the leachates showing overall decreasing trends towards the lower reaches. The values of δ^88/86Sr are higher in the flood season than in the dry season. We infer that parent rock types, sediment dynamic sorting and chemical weathering may all contribute to the spatial and seasonal variations of δ^88/86Sr in the Changjiang river sediments. More close examinations on stable Sr isotope in the river water and sediments and parent rocks as well are needed.