Holocene Variations in the Asian Monsoon Inferred from the Geochemistry of Lake Sediments in Central Tibet

Morrill, C., J.T. Overpeck, J.E. Cole, K-b Liu, C. Shen, and L. Tang

2005 Quaternary Research (in press)



ABSTRACT

We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 years and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of: % carbonate, % organic carbon, C/N and d13C of bulk organic matter, d13C and d18O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (~7500 cal yr BP). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr BP and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ~4000 years. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period.