Geochemistry of ODP Sites 121-758 and 121-757

Nd isotopes are useful tracers for paleoceanography due to the short Nd residence time in seawater and the large differences between the isotopic signatures of various geological reservoirs. Therefore, eNd variations reflect the geological history of individual oceanic basins.
Using a differential dissolution technique, which extracts Nd isotopes of seawater trapped in MnO2 coatings and carbonates in marine sediment, we measured almost two hundred samples from ODP Sites 758 and 757 in the Northern Bay of Bengal covering the last 4 Ma. For the first time, we have shown a covariation between epsilon-Nd and d18O over at least the last 800 ka. We also show that from 4 Ma to 2.6 Ma, epsilon-Nd is almost constant and starts to fluctuate at 2.6 Ma when northern glaciations increased. From 2.6 Ma to 1 Ma the fluctuation period is close to 40 ka while from 1 Ma to present it is dominantly 100 ka.
We attribute these findings to mixing between Himalayan river water (that ultimately originates as Indian summer monsoon rain) and normal Bay of Bengal seawater. Previous studies on seawater, using epsilon-Nd, d18O analyzed on planktonic foraminifera and sedimentary data, can be integrated into this model.
A simple quantitative binary mixing model suggests that the summer monsoon rain was more intense during interglacial than glacial periods. During last glacial episode, the monsoon trajectory was deviated to the east. At a large scale, the Indian monsoon is fully controlled by the variations in Northern Hemisphere climate but with a complex response function to this forcing.
Our study clearly establishes the large potential of Nd isotope data to evaluate the hydrological river regime during the Quaternary and its relationship with climate fluctuations, particularly when the sediment archive is sampled close to sediment sources.

钕（Nd）同位素因在海水中的停留时间较短，且不同地质储库的同位素组成差异显著，因此是古海洋学研究中极具价值的示踪剂。ε钕（epsilon-Nd）的变化可反映单个大洋盆地的地质演化历史。
本研究采用差速溶解法，提取海洋沉积物中二氧化锰（MnO₂）包膜及碳酸盐矿物内赋存的海水钕同位素，对孟加拉湾北部大洋钻探计划（Ocean Drilling Program, ODP）758与757站位的近200个沉积物样品开展分析，样品时间跨度覆盖过去4百万年（Ma）。本研究首次证实，至少在过去80万年（ka）内，ε钕与δ¹⁸O（d18O）存在协同变化关系。研究同时发现，在4 Ma至2.6 Ma期间，ε钕数值基本保持恒定；自2.6 Ma北半球冰川作用增强起，ε钕开始出现波动。其中，2.6 Ma至1 Ma期间的波动周期约为4万年（ka），而1 Ma至今的波动周期则以10万年（ka）为主。
本研究将上述观测结果归因于喜马拉雅河河水（其最终溯源为印度夏季风降水）与正常孟加拉湾海水的混合作用。既往基于ε钕、浮游有孔虫δ¹⁸O分析及沉积数据开展的海水相关研究，均可纳入本研究构建的模型框架。
基于简易定量二元混合模型的结果表明，印度夏季风降水在间冰期的强度高于冰期。末次冰期时段，季风输送轨迹发生东偏。从大尺度而言，印度季风完全受控于北半球气候波动，但对该气候强迫因子的响应机制较为复杂。
本研究明确证实，钕同位素数据在评估第四纪（Quaternary）河流径流状况及其与气候波动的关联方面具备巨大应用潜力，尤其当沉积物记录的采样点位靠近沉积源区时，该潜力将更为凸显。