植物叶蜡单体氢同位素重建非洲和东亚大陆古降水：亮点与难点

RECONSTRUCTION OF PALEO-RAINFALL IN CONTINENTAL AFRICA AND EAST ASIA USING LEAF WAX HYDROGEN ISOTOPE RATIOS： HIGHLIGHTS AND DIFFICULTIES

植物利用周围环境中的水分进行光合作用合成有机质，其叶蜡氢同位素记录了源水中的氢同位素组成，被认为是重建古水循环的替代性指标。然而从源水（降水）到合成叶蜡脂类化合物，降水中的氢同位素会发生多步分馏作用。本文综述了对氢同位素分馏造成影响的因素及其校正方法，通过有效的数据处理，叶蜡氢同位素可以比较直接地反映降水的氢同位素组成。由于在热带地区降水氢同位素受“雨量效应”影响较大，所以可以较好地反映降水量变化，在非洲大陆常被用于重建水循环和气候变化；在东亚大陆，“雨量效应”不再那么显著，水汽的来源可能在影响降水氢同位素组成的过程中也起到了重要作用。

Plants use environmental water during photosynthesis to produce organic compounds. Therefore, hydrogen- isotopic composition （SD） of source water is recorded in leaf wax lipids, which has become a new proxy to reconstruct paleohydroclimate. However, hydrogen isotopic fractionation is observed between source water （precipitation water） and leaf wax lipids. Here we review the factors and processes that are known to influence the hydrogen-isotopic composition of lipids. Meanwhile, we present the method for data corrections. Leaf wax δD values generally follow those of the precipitation with proper corrections. In tropical Africa, as ＂amount effect＂ has a major influence on the precipitation δD, leaf wax 6D mainly reflects precipitation amount. Therefore, it is widely used to reconstruct paleohydrological and paleoclimatic changes of continental Africa. By comparing leaf wax δD records from marine and lake sediments in tropical Africa, we found that leaf wax δD records from different lakes share the same pattern in general but have difference in details, which can be explained by the changes of water raper source. However, in East Asia, there is no＇ strong correlation between precipitation δD and rainfall amount indicated by modern observation data. We further compared the leaf wax δD record from ODP 1146 in northern South China Sea with records of environmental humidity proxies. The discrepancies indicate that there are other factors besides the ＂amount effect＂ controlling the precipitation δD in East China. In order to investigate if the changes of water vapor source may have an influence on the leaf wax δD in East China. We compare the leaf wax $D record from ODP 1146 with the intensity of East Asian summer monsoon, South Asian summer monsoon and East Asian winter monsoon. We found that leaf wax δD has heavy values when the Asian summer monsoon is weak or/and the East Asian winter monsoon is strong. During these periods, the continental East Asia receives less water vapor from ocean in the South transported by Asian summer monsoon but more from continent in the North transported by East Asian winter monsoon. Water vapor from the continent is more D-isotope enriched than that from the Ocean which leads to heavy values of leaf wax δD. Consequently, we suggest taking the change of water vapor source into account when interpreting the leaf wax δD record in East China.