Climatic variation in the Linxia basin, NE Tibetan Plateau, from 13.1 to 4.3 Ma: The stable isotope record

The δ¹³C and δ¹⁸O values of carbonates and δ¹³C_org values, % C, and C/N ratios of organic matter from lacustrine and fluvial sediments were measured from two stratigraphic sections in the Linxia basin on the northeastern margin of the Tibetan Plateau. Diagenesis, when present, is early, limited to small spatial scales and restricted stratigraphic intervals. A strong correlation exists between dolomite content and the δ¹⁸O values of micritic carbonate suggesting that carbonates are primary precipitates from lake water and that δ¹⁸O values reflect changes in the lake environment, with more positive values derived from evaporated and more saline lake water. The first part of this record is characterized by strong oscillations between dry and wet conditions accompanied by changes in the lake system between hydrographically closed and open states (13.1-8.0 Ma). The most severe aridity occurred from 9.6 to 8.5 Ma. After ∼ 8.0 Ma, a more stable and less arid climate dominated the region and the drainage system was open (8.0-5.3 Ma). After 5.3 Ma, climate became gradually drier and/or cooler (5.3-4.3 Ma). The organic matter preserved in the Linxia basin is most likely a mixture of terrestrial C₃ plant matter and lake algae. A correlation between C/N and δ¹³C_org indicates that a significant percentage of the organic matter is derived from terrestrial sources. The organic matter has undergone selective degradation during which the C/N ratios and organic carbon percentage decreased. The δ¹³C_org values are relatively stable throughout the 13.1 to 4.3 Ma interval ranging from - 24 to - 29‰ VPDB, suggesting that C₄ grasses were either absent or insignificant in the Linxia region prior to 4.3 Ma.