博斯腾湖全新世岩芯沉积物碳酸盐氧同位素气候意义

西风影响区全新世气候变化逐渐受到重视。但目前该区高精度定年和气候意义明确的代用指标仍然缺乏。利用西风影响区新疆博斯腾湖BST04H钻孔岩芯沉积物全新世自生碳酸盐(细颗粒沉淀碳酸盐和介形虫壳体)氧同位素,结合该区降水同位素分布特征和流域水文状况,分析该区湖泊沉积物碳酸盐氧同位素的气候意义,并与湖泊不同位置钻孔碳酸盐氧同位素指标进行对比。结果表明,博斯腾湖深水区沉积物自生碳酸盐氧同位素主要反映了区域的降水(入流)/蒸发比,并保留了入流水体同位素组成的变化信息。全新世自8100cal.aB.P.以来,氧同位素呈现出在波动中逐渐偏正的长期变化趋势,中全新世以后波动幅度加大,其间氧同位素值偏负的波动并不完全指示有效湿度增加,而是入湖水体同位素变化,并与湖泊的水文状况有关。这种波动在湖泊西部距离入湖河口位置较近的钻孔中表现得更为明显。不同位置钻孔同位素变化有很好的对应性,但不同位置相同时段内的波动幅度不一样。实际研究中应充分对比分析,深入理解指标的意义,防止仅依靠某一个位置的指标将这种局地波动信息解释成代表大范围区域的气候或环境信息。

PALEOCLIMATIC SIGNIFICANCE OF OXYGEN ISOTOPE COMPOSITION OF CARBONATES FROM A SEDIMENT CORE AT BOSTEN LAKE, XINJIANG, CHINA

More and more attention has been paid to Holocene climate changes in the westerly-influenced regions.However,high-resolution climatic proxies with unambiguous implications are still lacking.We analyzed the oxygen isotope composition of authigenic carbonates,fine-grained calcite and ostracode shells from a sediment core BST04H recovered from the deep part of Bosten Lake,Xinjiang,China in this study.Based on detailed analyses of isotopic compositions of the regional rainfall and hydrological settings of the lake region,climatic implications of the oxygen isotope composition of authigenic carbonates deposited in Bosten Lake was investigated.The data suggest that the oxygen isotope composition of carbonates in the lake sediments reflects changes in both the regional precipitation/evaporation ratio and the long-term isotopic composition of the input water controlled by the proportion of winter precipitations to the annual precipitation.The oxygen isotope composition has become heavier from the early Holocene before ca.8100cal.aB.P.to the Late Holocene after ca.1300cal.aB.P.Such a pattern was interrupted by shifts towards very negative δ18 O occurring 4500~3500cal.aB.P. and 1300~500cal.aB.P.although the climate was relatively dry during both intervals as indicated by fine grain sizes and high carbonate contents of the same sediment core.The negative δ18 O values might be related to the input of snowmelt and/or groundwater into the lake,implying decreases in the summer precipitation.However some short periods characterized by the input of increased summer precipitation can also be seen as demonstrated by the oxygen isotope composition.We therefore suggest that attention should be paid to the paleoclimatic implication of oxygen isotope composition of lacustrine sediments in the arid regions where the local hydrology may be variable.It is essential to understand climatic proxies precisely when regional paleoclimatic reconstructions are undertaken.