Climate changes in northeastern margin of Tibetan Plateau since 8ka BP --- The speleothem records

We analyzed two stalagmites (XN-2 and XN-5) collected from Xiannv (Fairy) Cave, southeast of Qinghai Province. Twenty 230Th dating results shown that stalagmite XN-2 grew through 7600-130 yr. BP except a hiatus between 4600-920 yr. BP and XN-5 deposited through 4590-2470 yr. BP. Tests for equilibrium conditions show that their δ18O variations are primarily caused by climate change. Thus, we propose that calcite δ18O were dominated by the isotope composition of drip water, i.e., amount weighted mean of precipitation δ18O, because the 3.5‰ shift of δ18O value within the Holocene could not be ascribed to temperature change in cave alone, as it needs 11 °C change which is practically too large. Comparison of our record with adjacent δ18O record of Hongyuan peat cellulos and speleothem records of Asian summer monsoon (ASM), indicates that all these records correlated remarkably from 4000 yr BP to 7600 yr BP, implying that ASM, was the primary climate system influencing the precipitation in the study region, and monsoon front could reach this region and even northwestward during this period. However, since 4000 yr BP, our speleothem and peat records sharing similar pattern disagree with that of speleothem records of ASM. As our study cave and Hongyuan peat site located in the north border of summer monsoon front, it is plausible to suggest that this difference may be caused by the southward retreat of monsoon front as ASM weakening with the NH insolation decreasing, and the summer precipitation might not be dominated by ASM during this period. Further study need to clarify which climate systems, including the westerlies and air masses activity over the Tibetan Plateau, was the primary system influencing summer precipitation in this region during the late Holocene. Moreover, climate oscillations on centennial time-scales in all these records well correlate, implying that driving mechanisms for these variations might be the same during the Holocene.