ALBERT

Description: 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.

Description: This study reports neodymium isotopic (e-Nd) variability at a time resolution of 0.5 to 3 Ma since the Late Cretaceous as recorded in a marine sedimentary core from the Western Pacific (ODP 807; 3°36'N, 156°3?E; Ontong Java Plateau). Our core is mainly fine-grained and composed of continuous sequences of nannofossil oozes. The e-Nd measured in the carbonate fraction was used as a proxy of e-Nd of seawater of the Western Pacific. On a long term, our results indicate a general increase in e-Nd of seawater by 4.5 e-Nd units from the Late Cretaceous (e-Nd = - 6) to modern times (e-Nd = - 1.7). This pattern was related to the emergence of the West Pacific margin and the progressive isolation of the Pacific Ocean from the other oceanic basins, resulting in its progressive shift to more radiogenic values through the Cenozoic. This long-term pattern is in accordance with previously published Fe-Mn crusts data from the same study area. Nonetheless, by being at higher time resolution, our data records additional sharp and pseudo-cyclic variations (~ 7?11 Ma periods) superimposed on this long-term pattern from ~ 40 Ma to modern times. These oscillations might reflect the alternating dominance of the two main deep water masses (NPDW and UCDW) bathing our study area. In the same core, we also measured the e-Nd in the detrital fraction in order to trace back the local terrigenous inputs. The terrigenous record shows a significant variability up to + 12 e-Nd units. This was linked to the emergence of the west Pacific subduction zone ~ 50 Ma ago causing a higher input of radiogenic isotopes. In conclusion, the large variability observed in both seawater and detrital e-Nd records most probably result from a major tectonic and oceanic circulation reorganization of the Pacific Ocean.

Description: 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.