内蒙古达里湖沉积记录的中晚全新世干旱事件

The semi-arid areas in Northern China at the northern margin of the East Asian summer monsoon(EASM) have experienced an increased frequency and severity of drought in recent two decades. However, little is known about the characteristics and mechanisms of past drought events in this region on different timescales. In this study, the upper 6.5 m of a sediment core(DL04) (43°15.68'N, 116°36.26'E) recovered from Dali Lake(43°13'~43°23'N, 116°29'~116°45'E) in central-eastern Inner Mongolia is analyzed for Ca and Mg concentrations, δ18O and δ13C values of < 38-μm carbonates; and for total organic carbon(TOC) and total nitrogen(TN) concentrations and TOC/TN(C/N) ratio of organic matter in bulk sediments. Based on the precise AMS14C dates, these proxy data document a detailed process of changes in the hydrology and climate in the region during the last 6000 cal.a. The results indicate that the < 38-μm carbonates in the Dali Lake sediments are predominated by endogenic calcites precipitated within the water body of the lake. Increases in the Ca and Mg concentrations, δ18O and δ13C values are interpreted as the intensive evaporative losses overwhelming the water input to the lake and thus the drought events in the region. Meanwhile, decreases in the TOC and TN concentrations and C/N ratios generally reflect decreases in the terrestrial organic matter input to the lake and declines in the primary productivity of the lake induced by weakened surface runoffs. The geochemical data of endogenic carbonates and organic matter from the Dali Lake sediments suggest that significant weakening in the surface runoffs, decreases in the lake level and phytoplankton productivity and drying in the regional climate, and thus significantly weakening in the EASM intensity occurred at the intervals of 6000~5300 cal.a B. P., 3200~2800 cal.a B. P., 1650~1150 cal.a B. P. and 650~300 cal.a B. P. Large declines in the EASM intensity during the last 6000 cal.a correspond, within age uncertainties, to the occurrences of ice rafted debris in the North Atlantic, but not coincide well with the changes in the sea surface temperature of the western tropical Pacific and in the solar activity, indicating that millennial and centennial abrupt changes in the EASM intensity are mainly triggered by climatic cooling occurring in the northern high latitudes.