夏季风降水对黄土高原现生蜗牛壳体碳酸盐氧同位素组成的影响

MONSOON PRECIPITATION EFFECT ON OXYGEN ISOTOPE COMPOSITION OF LAND SNAIL SHELL CARBONATE FROM THE LOESS PLATEAU

为了深入了解黄土地层中丰富的陆生蜗牛壳体碳酸盐化石氧同位素组成的气候环境指示意义,文章对黄土高原典型气候带的现生蜗牛壳体文石氧同位素进行了测量。结果显示:高原半干旱地区蜗牛壳体氧同位素组成相对于半湿润地区显著富集18O;壳体1δ8O值与夏季降水量具有密切负相关关系。对照黄土高原气候特征以及夏季风降水氧同位素组成和降水量的关系,认为:1)夏季风降水18O丰度是影响蜗牛壳体氧同位素组成变化的主要因素;2)进一步研究黄土沉积物中蜗牛壳体化石氧同位素组成对重建黄土高原夏季风强度变化的历史具有重要的意义。

Oxygen isotope composition of biogenic carbonates has the potential to indicate changes of paleoclimate. Fossil land-snail carbonate shells are plentiful in the wind blown loess sequences in the Loess Plateau. However, their ^18O/^16O ratio had not been broadly used to reconstruct paleoclimatic history until present. In this paper, we discuss the paleoclimatic reconstruction significance of oxygen of living land snails from the Loess Plateau. isotope measurements based on analyses on carbonate shells Oxygen isotope analyses have been carried out on carbonate （aragonite） shells of the living snail samples collected from different sites in the Loess Plateau. Based on the oxygen isotope measurements, the following results have emerged ： 1 ） There is no obvious isotopic difference between different species from a given site; 2） δ^18O value in a range of - 7.69‰～ - 1.78‰ for the multi-shell samples varies from one site to another site ; and 3 ） δ^18O values of the snail shells from semiarid regions are more positive than those from the semi-humid areas in the Loess Plateau. The climate in the Loess Plateau is essentially controlled by Asian monsoon system. It is warm and wet during summer when Asian summer monsoon prevails over the plateau and brings heavy rainfall. Since land snails are active only during warm and wet season, it is suggested that the shell δ^18O values of land snails would be correlated to oxygen isotope composition of the monsoon precipitation. The fact that δ^18O values of both snail shells and monsoon rain are strongly correlated supports the following conclusions： 1 ） Abundance of ^18O in land snail carbonate shells in the Loess Plateau is dominantly controlled by oxygen isotope composition of the summer monsoon precipitation. 2） It is significant to analyze the oxygen isotope composition of fossil snail shells deposited in the Loess Plateau for reconstructing the history of the Asian monsoon in this area.