干旱区生态环境知识资源中心

A 12.87-m-long sediment core was retrieved from closed-basin Lake Daihai in the monsoon-arid transition zone of north-central China. Oxides of major elements and their ratios normalized to Al in the AMS-(14)C-dated core were employed to evaluate chemical weathering intensity (CWI) in the lake drainage basin, which reflects hydrothermal conditions in the study area. Lower CWI periods occurred prior to 14.5 ka BP, and during the intervals ca. 11.7-10.3, 3.5-3.2, 2.6-1.7 ka BP, and 1.2-0 ka BP, indicating relatively low temperatures and moisture availability. Greater CWI during the intervening periods ca. 14.5-11.7, 10.3-9.0, 3.2-2.6, and 1.7-1.2 ka BP, with the maximum CWI at ca. 6.7-3.5 ka BP, imply ameliorated hydrothermal conditions in the lake basin, i.e. higher temperatures and precipitation. Exceptionally low CWI, associated with high CaO/MgO ratio during ca. 9.0-6.7 ka BP, suggests higher evaporation rates in the area under warmer temperature. Overall, CWI displays in-phase variations with changes in organic matter (TOC, TN), carbonate (CaCO(3)) and pollen assemblages, all of which are related to variations in monsoon effective precipitation. High CWI indicates strong monsoon-induced precipitation, whereas low CWI reflects a weak precipitation regime. The optimum hydrothermal status, recorded by the strongest CWI and maximum monsoon effective precipitation during ca. 6.7-3.5 ka BP defines the Holocene climate optimum (HCO) in the Lake Daihai region. These results indicate that the HCO prevails after the early Holocene in the monsoon-arid transition zone of north-central China. Temperature and precipitation variations during most of the Holocene, inferred from the lake sediments, are due largely to insolation forcing. Dry but warm conditions ca. 9.0-6.7 ka BP, however, probably reflect the complex interactions between insolation and geography (e.g. altitude and local topography).