HOLOCENE MILLENNIAL-SCALE OSCILLATIONS OF EAST ASIAN MONSOON REVEALED BY LAKE-LEVEL AND VEGETATION CHANGES ON THE NORTHEASTERN TIBETAN PLATEAU

We here present preliminary data from a 688-cm sediment core (KL05-2) from Keluke Lake (37^o17'N, 96^o54'E) in NW China, near the edge of modern influence of East Asian summer monsoon. It is a shallow (max depth: 13 m), freshwater (0.8 g/L salinity) lake located in a dry desert environment (annual precipitation <200 mm) at an elevation of 2817 m in Qaidam Basin of the NE Tibetan Plateau. The coring site has a water depth of 270 cm and is ~300 m from the reed-dominated north shore. Two AMS dates of 4780±50 and 9270±50 ¹⁴C BP provide tentative chronology. Loss-on-ignition analysis indicates that organic matter shows little variations of 4-10%, but carbonate content oscillates greatly between <20% and >70%. Most low carbonate intervals correspond with clay-rich, “soil-textured” sediments containing abundant plant remains and roots, likely deposited in very shallow water or wetland environments during reduced monsoon precipitation and dry climates. On the other hand, high carbonate intervals are dominated by silt-size calcite or aragonite precipitates, indicating relatively deep open water under wet climates, associated with intensified monsoon precipitation. Pollen analysis from selected low and high carbonate intervals supports this interpretation, indicating that dry periods are dominated by desert pollen types including Chenopodiaceae (>60%), Ephedra (>5%) and Nitraria (>2%), while wet periods by Chenopodiaceae (~50%), Artemisia (~10%) and Poaceae (~20%; mostly Phragmites-type). The Artemisia-to-Chenopodiaceae ratio, an index of relative steppe and desert plant dominance, is <5 during dry periods but ~20 in wet periods. Wet-dry climate oscillations of great magnitude occurred mostly from 11,300 to 3000 cal BP, while climate appears to be generally moist during the last 3000 cal BP (with high carbonates of 60-70% with little variations). Visual inspection and spectral analysis indicate that dry periods during the Holocene show a regularity of 1200-year time spacing on average, ranging from 600 to 1700 years. Our results may provide one of the strongest evidence in central Asia for millennial-scale Holocene wet-dry climate cycles, suggesting the pervasive nature of Holocene climate variations and the strong moisture response in continental interior as mediated by East Asian summer monsoon dynamics.