Abstract:
Pb- and Nd- isotopic time-series from the authigenic fraction of Central Indian Ocean sediments have been interpreted as responding to changes in the relative amount of Himalayan erosion during the Cenozoic [1,2]. Detrital records of Nd- and Sr- isotopes from the Bengal deep-sea fan suggest a source of sediment dominated by the High Himalaya Series (HHS) for the last 20 Ma [3,4]. Associated variations of Pb-isotopes are not known, and a more precise reconstruction is hampered by the lack of information about temporal changes in the isotopic composition of detrital Pb and Nd carried by rivers draining the Himalayas. We present new Pb- and Nd-isotope time series, together with rare earth elements, from the bulk detrital and silt-sized fractions as well as the authigenic fraction of deep-sea sediment over the last 20 Ma from Ocean Drilling Program Sites 717 and 718 on the Bengal fan, along with Pb- and Nd- isotopic compositions of the bedloads of Himalayan rivers. The oldest bulk detrital and silt-sized fraction samples (7-17 Ma) show similar and relatively uniform Pb- and Nd- isotopic compositions characteristic of a stable input from the HHS. The youngest samples (<1Ma) show the same uniformity with a shift towards more radiogenic values, implying a greater contribution of the Lesser Himalaya Series. However, over the Pliocene (1-7 Ma), strongly marked shifts in both isotopes are observed, along with a decoupling between the bulk detrital and silt-sized fractions. These results imply a strong variability in the erosion and weathering regime of the Himalaya over the Neogene, and we will discuss them in the context of tectonic and climatic changes. We will discuss as well the implications of these changes for the interpretation of the deep water evolution of these isotope systems in the Central Indian Ocean.