Nd and Sr isotope record of ODP Sites 121-756 and 122-762 sediments

We have analyzed the Nd isotopic composition of both ancient seawater and detrital material from long sequences of carbonated oozes of the South Indian Ocean which are ODP Site 756 (Ninety East Ridge (-30°S), 1518 m water depth) and ODP Site 762 (Northwest Australian margin, 1360 m water depth). The measurements indicate that the epsilon-Nd changes in Indian seawater over the last 35 Ma result from changes in the oceanic circulation, large volcanic and continental weathering Nd inputs. This highlights the diverse nature of those controls and their interconnections in a small area of the ocean.
These new records combined with those previously obtained at the equatorial ODP Sites 757 and 707 in the Indian Ocean (Gourlan et al., 2008, doi:10.1016/j.epsl.2007.11.054) established that the distribution of intermediate seawater epsilon-Nd was uniform over most of the Indian Ocean from 35 Ma to 10 Ma within a geographical area extending from 40°S to the equator and from -60°E to 120°E. However, the epsilon-Nd value of Indian Ocean seawater which kept an almost constant value (at about -7 to -8) from 35 to 15 Ma rose by 3 epsilon-Nd units from 15 to 10 Ma. This sharp increase has been caused by a radiogenic Nd enrichment of the water mass originating from the Pacific flowing through the Indonesian Passage. Using a two end-members model we calculated that the Nd transported to the Indian Ocean through the Indonesian Pathway was 1.7 times larger at 10 Ma than at 15 Ma.
The Nd isotopic composition of ancient seawater and that of the sediment detrital component appear to be strongly correlated for some specific events. A first evidence occurs between 20 and 15 Ma with two positive spikes recorded in both epsilon-Nd signals that are clearly induced by a volcanic crisis of, most likely, the St. Paul hot-spot. A second evidence is the very large epsilon-Nd decrease recorded at ODP Sites 756 and 762 during the past 10 Ma which has never been previously observed. The synchronism between the epsilon-Nd decrease in seawater from 10 to 5 Ma and evidences of desertification in the western part of the nearly Australian continent suggests enhanced weathering inputs in this ocean from this continent as a result of climatic changes.

本研究分析了南印度洋两套长序列碳酸盐软泥沉积物中的古海水与碎屑物质钕（Nd）同位素组成，两套沉积物分别取自大洋钻探计划（Ocean Drilling Program, ODP）756站位（九十度东海岭，南纬30°，水深1518米）与ODP 762站位（澳大利亚西北陆架，水深1360米）。测试结果显示，过去35百万年（Ma）间印度洋海水的εNd（epsilon-Nd）变化受控于大洋环流演化、大规模火山活动以及大陆风化输入的钕物质通量变化，这一发现揭示了该小型海域内上述控制因素的多样性及其相互关联机制。
将本次新获得的记录与此前在印度洋赤道海域ODP 757、707站位获取的数据（Gourlan等，2008，doi:10.1016/j.epsl.2007.11.054）相结合，研究证实，在35 Ma至10 Ma期间，南纬40°至赤道、西经60°至东经120°的绝大部分印度洋海域内，中层海水εNd分布均一。但在35 Ma至15 Ma期间，印度洋海水εNd值基本维持恒定（约为-7至-8），而在15 Ma至10 Ma期间，其值上升了3个εNd单位。这一显著升高源于流经印度尼西亚海道的太平洋源水团的放射性钕富集作用。通过双端元模型计算，本研究发现，10 Ma时通过印度尼西亚海道输入印度洋的钕通量是15 Ma时的1.7倍。
古海水与沉积物碎屑组分的钕同位素组成在部分特定地质事件中呈现显著相关性。第一处证据出现在20 Ma至15 Ma期间，两套沉积物的εNd信号均记录到两处正异常峰，该异常极有可能由圣保罗热点（St. Paul hot-spot）的火山活动事件引发。第二处证据为过去10 Ma间在ODP 756与762站位记录到的大幅εNd降低事件，这一现象此前从未被观测到。10 Ma至5 Ma期间海水εNd降低事件，与澳大利亚大陆西部的荒漠化证据在时间上高度吻合，这表明气候变化导致该大陆向印度洋输入的风化物质通量显著增加。