Neodymium isotopes in planktonic foraminifera: a record of the response ofcontinental weathering and ocean circulation rates to climate change

The neodymium (Nd) isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on ti mescales of a few ka. Here we present the first Nd isotope record for seawa ter using sedimentary foraminifera cleaned using standard oxidative-reducti ve techniques. The data, along with Mn/Ca ratios, suggest that cleaned fora minifera provide a reliable record of Nd in seawater and hold out the prosp ect of using Nd in foraminifera to examine changes in seawater that accompa ny glacial-interglacial climatic cycles. The principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic Fe-Mn coatings. These contain large amounts o f Nd and other trace elements but can be cleaned off using highly reducing reagents. Mn/Ca ratios for the majority of the cleaned sedimentary foramini fera analysed here lie within the range (10-100 mu mol/mol) that has yielde d success in studies of transition elements in forams. Mass-balance modelli ng suggests that for residual Mn/Ca ratios <100 mu mol/mol, Nd added to the foram in the coating will never shift the measured Nd isotope composition significantly away from the seawater value acquired by the foram test in th e water column. Additionally, Nd concentrations measured in cleaned sedimen tary foraminifera are comparable with those for a modern sample that has ne ver encountered diagenetic fluids. Finally, core-top planktonic foraminifer a for two sites have Nd isotope compositions that are identical to local su rface seawater. The data we present here for Labrador Sea forams over the p ast 2.5 m.y. are interpreted in terms of changes in the seawater isotopic c omposition. The data show a pronounced shift from epsilon(Nd) values of sim ilar to-12 to similar to-19 in the period 2.5-1.5 Ma. This change is interp reted to result from the initiation of Northern Hemisphere glaciation and t he increased derivation of Labrador Sea Nd via ice-rafting from Archaean te rranes in central Canada. In combination with stable isotope and foraminife ral relative species abundance data, the new Nd data are consistent with th e surface hydrography of the Labrador Sea being dominated by a fluctuating balance between cold, polar waters containing unradiogenic Nd and warm, sub tropical waters containing more radiogenic Nd. The major change in Labrador Sea Nd that is observed in the past 2.5 Ma can, on its own, account for th e change in the Nd isotope composition of North Atlantic Deep Water over th e same time period. (C) 1999 Elsevier Science B.V. All rights reserved.