Sedimentology of surface samples from the Weddell Sea, Antarctica

Mineralogical and granulometric properties of glacial-marine surface sediments of the Weddell Sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from Antarctic sources. The silt fraction shows marked spatial differences in quartz contents. In the sand fractions heavy-mineral assemblages display low mineralogical maturity and are dominated by garnet, green hornblende, and various types of clinopyroxene. Cluster analysis yields distinct heavy-mineral assemblages, which can be attributed to specific source rocks of the Antarctic hinterland. The configuration of modern mineralogical provinces in the near-shore regions reflects the geological variety of the adjacent hinterland. In the distal parts of the study area, sand-sized heavy minerals are good tracers of ice-rafting. Granulometric characteristics and the distribution of heavy-mineral provinces reflect maxima of relative and absolute accumulation of ice-rafted detritus in accordance with major iceberg drift tracks in the course of the Weddell Gyre. Fine-grained and coarse-grained sediment fractions may have different origins. In the central Weddell Sea, coarse ice-rafted detritus basically derives from East Antarctic sources, while the fine-fraction is discharged from weak permanent bottom currents and/or episodic turbidity currents and shows affinities to southern Weddell Sea sources. Winnowing of quartz-rich sediments through intense bottom water formation in the southern Weddell Sea provides muddy suspensions enriched in quartz. The influence of quartz-rich suspensions moving within the Weddell Gyre contour current can be traced as far as the continental slope in the northwestern Weddell Sea. In general, the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice-rafted detritus beyond the shelves of the study area.

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.