Water salinity and productivity recorded by ostracod assemblages and their carbon isotopes since the early Holocene at Lake Qinghai on the northeastern Qinghai–Tibet Plateau, China

•The hatched time of ostracods may be controlled by temperature in Lake Qinghai.•Species, δ13Costracod used to infer fluctuations in salinity and productivity•The early Holocene in Lake Qinghai is marked by highest salinity and productivity.•The early Holocene climate may be the warmest and wettest part of the Holocene.•The high salinity and productivity are related to low lake level and high temperature.

Lake Qinghai, the largest saline lake in China, is a closed-basin lake with a salinity of 16 g/L and is situated in the sensitive semi-arid zone between the Asian monsoon-controlled area and the westerly jet stream-influenced area. With the support of the International Continental Drilling Program (ICDP), Lake Qinghai was drilled in 2005 using the ICDP GLAD800 drilling system. Two ostracod species, i.e., Limnocythere inopinata and Eucypris mareotica, were found only in the upper 5.15 m of the drilled core 1F, covering 12 ka, where the depths of 5.15 to 3.15 m (12.0–7.4 ka) are dominated by E. mareotica and the depths of 3.15 to 0 m (7.4 ka–present) are dominated by L. inopinata.The presence of the ostracod Eucypris mareotica was related to the high salinity of the water (exceeding 30 g/L), and the dominant species of ostracod (Limnocythere inopinata) was related to the brackish water (3–16 g/L) in the Lake Qinghai area (Li et al., 2010). The changes in the carbon isotope in the modern ostracod shells were related to the water salinity and productivity on the lake bottom in Lake Qinghai (Li et al., 2012). On this basis, this study evaluated the changes in lake salinity and productivity and their relation to climatic change using changes in the relative abundances of ostracod fossil species assemblages and the stable carbon isotope in ostracod shells from core 1F and other published data (e.g., total organic carbon content, δ13CTOC and ice core δ18O).The single occurrence of ostracod Eucypris mareotica and the highest average δ13C values (− 0.2‰) in their shells indicated that the lake salinity and productivity were very high, a phenomenon that was related to the high temperature and low lake level (caused by intense evaporation even though the precipitation increased sharply) in the early Holocene. The deceased abundance of ostracod E. mareotica and the lowest average δ13C values (− 2.0‰) in ostracod shells showed that the lake salinity and productivity decreased because of the increased lake level and decreased temperatures in the middle Holocene. Finally, the lake level decreased, but the water salinity and the lake productivity gradually increased because of the high E/I ratio related to the increased temperature in the late Holocene.