Deciphering drainage reorganization in the NE Tibet: Insights from 50 Ma-long solute Sr isotopic records of Cenozoic lacustrine-fluvial sequences

Drainage evolution and source-to-sink history across the Tibetan plateau and the surroundings as the consequence of changes in landscape and climate, provide a window to understand the growth of the Tibetan Plateau and its environmental impacts. Previous studies focus mostly on the reconstruction of siliciclastic sediment, but less on the dissolved load that displays distinct transport dynamics. Here we provide a solute perspective for deciphering source-to-sink history in NE Tibet through the carbonate ⁸⁷Sr/⁸⁶Sr ratio. The modern observations around NE Tibet exhibit a remarkable contrast of the solute Sr isotopic regime. That is, the Qilian Shan supplies high ⁸⁷Sr/⁸⁶Sr ratios caused by the exhumation of high-grade metamorphic rocks, while the East Kunlun Shan and West Qinling provide low ⁸⁷Sr/⁸⁶Sr ratios.

Here, we reconstruct solute Sr evolution of basin paleowater using bulk carbonate from parallel lacustrine and fluvial sections in the Qaidam, Lanzhou, and Xining Basins, NE Tibet. In the Qaidam Basin, solute ⁸⁷Sr/⁸⁶Sr ratios of the paleo-Qaidam Lake and fluvial sediments remain constant at ~0.713 in most Cenozoic periods, indicating that the solute Sr sourced solely from the Qilian Shan. But two low ⁸⁷Sr/⁸⁶Sr ratio periods at ~44.5 - ~32 Ma and after ~16 Ma might be caused by hydrological connections between the northern source (Qilian Shan) and the southern source (dominantly from East Kunlun Shan) in the Paleogene western basin and Neogene eastern basin, respectively. The two time periods correspond to two stages of the paleo-Qaidam Lake that developed following a south-eastward migration of basin depocenter due to the tectonic evolution of the surrounding mountains.

In the Lanzhou and Xining Basins, ⁸⁷Sr/⁸⁶Sr ratios of the paleo-Longzhong Lake and fluvial sediments maintained similarly low values and turned into an increase after ~32 Ma, reflecting an elevated proportion of solute supply from high ⁸⁷Sr/⁸⁶Sr source area, i.e., Qilian material input. A diverse evolution of solute ⁸⁷Sr/⁸⁶Sr ratio in the Lanzhou and Xining Basins after ca. 25-23 Ma, suggests that there was a drainage reorganization in response to the growth of NE Tibet. Our study implies that radiogenic Sr is a useful proxy for tracing drainage adjustment in response to large-scale tectonics.