第四纪热带西太平洋表层海水氧同位素的岁差周期

Quaternary precession cycles of sea-surface oxygen isotope records from the tropical western Pacific

表层海水氧同位素记录是重建低纬水循环演化历史的一个重要手段,但受到区域降水-蒸发平衡、洋盆间水汽再分配、陆地淡水输入、洋流混合等众多因素的影响,其气候意义的解读十分复杂。文章收集和计算了过去45万年以来热带西太平洋-南海数个站位剩余海水氧同位素的波动历史。发现这些记录存在显著的岁差周期,并且在岁差周期上呈现出同相变化,即在北半球夏季辐射量的高值期,表层海水同位素出现正偏移现象。结合同位素数值模拟结果,推测岁差低值促发了两个机制:一是热带辐合带平均位置北移,导致从热带大西洋往太平洋的水汽输送量下降,二是太平洋向亚洲大陆输出的水汽增多,陆地液态水储库增大。这两个机制共同导致热带西太平洋淡水减少,使得表层同位素出现偏重现象。并且这两个机制的效应超过了大洋内部降水变化的影响,导致表层海水同位素出现大空间尺度上的一致性变化。

Sea-surface δ^(18)O is an important approach to understand the history of the low-latitude hydrological cycle.However,the climate interpretation of sea-surface δ^(18)O can be complicated by a set of factors,including regional precipitation-evaporation balance,water-vapor redistribution among ocean basins,continental runoff and ocean-current homogenization.In this study,through using parallel records of planktonic foraminiferal δ^(18)O and Mg/Ca-sea surface temperature reconstructions,we calculated residual δ^(18)O changes at several sites in the tropical western Pacific-South China Sea over the past 450 thousand years.These sites(MD05-2904,ODP1145,MD05-2901,KX973-22-4,MD01-2386,MD06-3067 and GeoB17426)together cover a latitude range from 2.19°S to 19.46°N,and a longitude range from 110.74°E to 159.23°E.We find these residual δ^(18)O records show strong and in-phase variance at the precession bands,which are relatively enriched when Northern Hemisphere summer insolation is strong.This is contrast to stalagmite δ^(18)O records from southern China and residual δ^(18)O reconstructions from the tropical Atlantic,where δ^(18)O were relatively depleted during precession minima.The reconstructed sea-surface δ^(18)O variations are well matched with previously published simulation results.We infer that the precession minima can have triggered two mechanisms in the tropical Pacific.First,a northward migration of the Intertropical Convergence Zone has resulted in a reduced moisture transport from the tropical Atlantic to the Pacific via the Central America.Second,strengthened summer monsoons have increased a moisture flux from the Pacific to Asian landmasses,giving rise to an expansion of liquid-water reservoir in the land.Both mechanisms can reduce the freshwater amount in the tropical western Pacific and thus increase sea surface δ^(18)O.Moreover,the effect of these two mechanisms can overwhelm that of regional precipitation changes within ocean basins,generating uniform changes in residual δ^(18)O at a large spatial extent.