Geographic control in Pb isotope distribution and sources in Indian Ocean Fe-Mn deposits

摘要

High-precision Pb isotope data have been obtained on Indian Ocean Fe-Mn deposits with a large geographic coverage. These data reveal a strong geographic control on the distribution of Pb isotopes as well as sources of Pb within this basin. The provinciality of Pb isotopes at the scale of the whole Indian Ocean, as well as that of individual basins, broadly matches the pattern of deepwater flow. The existence of several sources of Pb is best illustrated by the presence of three well-defined Pb isotopic arrays, each of which is confined to clear-cut geographic domains. These arrays imply a dominance of binary mixing of Pb sources within each of these domains. The domains consist of the North-Indian (N-Indian; north of 20°S), Southwest-Indian (SW-Indian; 20°S to 50°S, west of 45°E), South-Indian (S-Indian; 20°S to 50°S, east of 45°E) and Antarctic-Indian (A-Indian; south of 50°S), and clearly exhibit a strong control by latitude. The S-Indian domain and Mozambique Channel samples form a cluster, suggesting that there are more than just two sources contributing in these regions. We show that the N-Indian is dominated by sources of Pb derived from the High Himalayas and the Trans-Himalayan Complex, and most likely inherited from interaction at the water interface with Bengal and Indus Fan sediments. The SW-Indian and A-Indian domains share a common unradiogenic component, represented by circumpolar waters derived from the Pacific Ocean and flowing through the Drake Passage into the Atlantic and Indian Oceans. However, the radiogenic sources of Pb in these two domains are clearly distinct. In the SW-Indian, the radiogenic Pb signal reflects the tongue of North Atlantic Deep Water flowing around the tip of South Africa, while in the A-Indian, the source of radiogenic Pb is less certain, but quite probably originates in the Weddell Sea region. These data illustrate the complexity of the spatial Pb isotopic distribution, and Pb sources, within a single ocean basin at the present day, which should be borne in mind when interpreting long-term radiogenic isotope paleorecords from Fe-Mn crusts.