Holocene hydroclimatic variations on the Tibetan Plateau: An isotopic perspective

There is much debate regarding the temporal and spatial pattern of hydroclimate variations on the Tibetan Plateau during the Holocene, focusing especially on the dipolar pattern of precipitation and moisture between the southern and northern regions, on multiple timescales. Modern observations reveal that the δ18O in precipitation is an integrated tracer of the atmospheric processes of the Indian summer monsoon (ISM), the East Asian summer monsoon (EASM), and the westerlies, which dominate the climate of the Tibetan Plateau. Here, we summarize and compare 20 Holocene lacustrine authigenic carbonate δ18O records from lakes distributed across the Tibetan Plateau. After carefully considering the potential effects of temperature, evaporation, site elevation, meltwater supply, and moisture sources, we eliminate their influences on the isotopic composition of Tibetan lakes, and suggest that the long-term variations of the lacustrine δ18O records were dominated by the moisture sources. The δ18O values of the lake sediments in the central and western Tibetan Plateau increased since the early Holocene, which agrees with the trend of δ18O in speleothem records from the plateau, as well as with the variations of monsoon precipitation reconstructions for the region. Overall, this evidence indicates the substantial influence of the monsoon in the abovementioned regions. However, the δ18O records from lakes in the northeastern Tibetan Plateau show a decreasing trend during the second half of the Holocene (after ∼5 ka), highlighting the operation of a different hydrological process, with enhanced moisture availability, in the regions. It has been suggested that the increased contribution from westerlies-derived moisture in the non-monsoon season, with depleted δ18O (e.g., from melting snow or frozen soil in spring), is responsible for such variations. We propose that the interplay between the Asian summer monsoon and the westerlies was the major control of the hydroclimatic variations on the Tibetan Plateau during the Holocene, which led to the dipolar pattern of moisture variation between the northeastern Tibetan Plateau and other parts of the plateau, on the multi-millennial timescales during the Holocene.