Sources of Pb for Indian Ocean ferromanganese crusts: a record of Himalayan erosion?

A high resolution Pb isotope time-series for the last 26 Ma, dated by 10Be/9Be chronology, is reported for a north Indian Ocean ferromanganese crust. This record is compared with available Pb isotope time-series of six other crusts from the Atlantic, Indian and Pacific Oceans, each of which is based on 10Be/9Be chronology. The seven Pb isotope records reveal some remarkable features. In contrast to the Nd isotope time-series of these crusts which show a long-term (∼60 Ma) provinciality between the three main ocean basins, the Pb isotopes only show comparable provinciality over the last ∼5 Ma. Prior to about 15 Ma ago no distinct Indian Ocean Pb isotope signal existed. Within this established framework of Pb isotope distribution in the oceans the 208Pb/206Pb data for the north Indian Ocean crust reported here are anomalous. The 208Pb/206Pb ratio is particularly high and exceeds a value of 2.08 during the time interval from 20 to 8 Ma ago. Consideration of potential sources of Pb in the Indian Ocean which might provide such high 208Pb/206Pb ratios suggests that this crust most probably has recorded a time-varying erosional input of Pb from the Himalayas. The timing of the isotopic shift is in good agreement with maximum Himalayan exhumation rates deduced from crystallisation and cooling ages of synorogenic granites (20–14 Ma) and the sedimentation history of the Bengal Fan.