Nd isotope systematics on ODP Sites 756 and 762 sediments reveal major volcanic, oceanic and climatic changes in South Indian Ocean over the last 35Ma

摘要

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), 1518m water depth) and ODP Site 762 (Northwest Australian margin, 1360m water depth). The measurements indicate that the ε _(Nd) changes in Indian seawater over the last 35Ma 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) established that the distribution of intermediate seawater ε Nd was uniform over most of the Indian Ocean from 35Ma to 10Ma within a geographical area extending from 40°S to the equator and from -60°E to 120°E. However, the ε Nd value of Indian Ocean seawater which kept an almost constant value (at about -7 to -8) from 35 to 15Ma rose by 3 ε _(Nd) units from 15 to 10Ma. 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 10Ma than at 15Ma.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 15Ma with two positive spikes recorded in both ε 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 ε _(Nd) decrease recorded at ODP Sites 756 and 762 during the past 10Ma which has never been previously observed. The synchronism between the ε Nd decrease in seawater from 10 to 5Ma 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.