Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

High-resolution Hf isotopic records are presented for hydrogenetic Fe-Mn cr usts from the North Atlantic and Indian Oceans. BM1969 from the western Nor th Atlantic has previously been shown to record systematically decreasing N d isotopic compositions from about 60 to similar to 4 Ma, at which time bot h show a rapid decrease to unradiogenic Nd composition, thought to be relat ed to the increasing influence of NADW or glaciation in the northern hemisp here. During the Oligocene, North Atlantic I-If became progressively less r adiogenic until in the mid-Miocene (similar to 15 Ma) it reached +1. It the n shifted gradually back to an epsilon(Hf) value of +3 at 4 Ma, since when it has decreased rapidly to about -1 at the present day. The observed shift s in the Hf isotopic composition were probably caused by variation in inten sity of erosion as glaciation progressed in the northern hemisphere. Ferrom anganese crusts SS663 and 109D are from about 5500 m depth in the Indian Oc ean and are now separated by similar to 2300 km across the Mid-Indian Ridge . They display similar trends in Hf isotopic composition from 20 to 5 Ma, w ith the more northern crust having a composition that is consistently more radiogenic (by similar to 2 epsilon(Hf) units). Paradoxically, during the l ast 20 Ma the Hf isotopic compositions of the two crusts have converged des pite increased separation and subsidence relative to the ridge. A correlata ble negative excursion at similar to 5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan ma in central thrust. Changes to unradiogenic Hf in the central Indian Ocean a fter 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reducti on of Pacific flow through the Indonesian gateway. In either case, these re sults illustrate the utility of the Hf isotope system as a tracer of paleoc eanographic changes, capable of responding to subtle changes in erosional r egime not readily resolved using other isotope systems. (C) 2000 Elsevier S cience B.V. All rights reserved.