Response of middle and late Pleistocene ostracod δ¹⁸O and δ¹³C values to Indian summer monsoon and lake productivity on the southeastern Tibetan Plateau

The chemical compositions of biogenic carbonates preserved in sediments have been used to reconstruct paleoclimate and paleoenvironmental history. Yet, long and high-resolution biogenic proxy records have not been available for the Indian summer monsoon (ISM), owing to the lack of well-dated and continuous sediment sequences. To investigate the ISM conditions since the middle and late Pleistocene, we here examined the oxygen and carbon isotopic compositions (δ¹⁸O and δ¹³C) of the shells of two ostracod species (benthic Lineocypris jiangsuensis and nektobenthic Ilyocypris microspinata), carbonates smaller than < 63 µm and bulk sediment from the upper part (ca. 274–64 ka) of a sediment core (HQ-2002) drilled from the central Heqing Basin located at the southeastern margin of the Tibetan Plateau. The concordant δ¹⁸O and δ¹³C variations among ostracod shells, < 63 µm carbonates and bulk sediment indicate that they recorded the variations of the water compositions of the Heqing paleolake. Furthermore, the δ¹⁸O values are synchronized with those of stalagmites from Xiaobailong Cave in southwestern China and pace the ISM during the middle and late Pleistocene period, with both responding to precipitation. As a nektobenthic species, the δ¹⁸O_ostr values of I. microspinata shells are more sensitive to ISM precipitation than that of L. jiangsuensis shells. Meanwhile, the δ¹³C_ostr values of the I. microspinata shells are lower than those of the L. jiangsuensis shells, which were determined by the lake-water layers in which they lived, and their co-variations reveal lake productivity associated with ISM intensity. The δ¹⁸O_ostr and δ¹³C_ostr values for the shells of these two ostracod species indicate that the paleolake experienced a shallow-deep-shallow evolution of the lake-water level, pacing the ISM in the middle and late Pleistocene. The study highlights the sensitivity of the chemical compositions of ostracod shells to monsoon and lake productivity at glacial-interglacial cycles.