The Nd isotope composition of oxic pore waters of marine sediments and implications for its use as a past water mass proxy

Abstract

The neodymium (Nd) isotope signature (ε Nd ) has been widely used as a proxy to reconstruct past ocean circulation. Recently it has been increasingly discussed which archives can be reliably used to extract authigenic ε Nd applied for the reconstruction of past deep ocean circulation and under which environmental conditions bottom seawater ε Nd signals are altered and the original water mass signature is overprinted. Pore waters of marine sediments are the key environment in which early diagenetic exchange processes between seawater-derived Nd and terrigenous solid phases take place. Here we present the first comprehensive open ocean ε Nd as well as rare earth element and yttrium (REY) data set obtained from oxic pore waters, the benthic boundary layer, and different sediment phases from the Northeast Atlantic and the central North Pacific. The sediments from the central North Pacific are characterized by low total organic carbon contents, low sedimentation rates, and low carbonate contents, while in the Northeast Atlantic higher carbonate contents occur. In the Northeast Atlantic, the ε Nd values of the pore waters (−9.8 to −12.8) are within 1 ε Nd unit identical to that of corresponding near-bottom seawater and sedimentary authigenic and detrital solid phase ε Nd signatures of the surface sediment. In the central North Pacific, pore water ε Nd values (−3.5 to −5.4) also match bottom seawater as well as authigenic phase signatures and are 4.5 to 6 ε Nd units offset from detrital signatures towards more radiogenic isotope compositions. The combination of dissolved Nd and Mn concentrations, shale normalized REY patterns, and ε Nd values suggests transport of adsorbed REY from within the water column to the sediment and subsequent release of the seawater-derived Nd to the pore water in the surface sediment. Neodymium is then re-released back to the bottom seawater through benthic fluxes without major additions to this Nd pool from other sedimentary phases under oxic conditions. There are no indications of significant REY inputs from dissolution of Mn- or Fe-(oxyhydr)oxides under oxic conditions. The benthic fluxes of Nd are widespread in the deep sea but only range from −0.2 to −3.3 pmol cm −2 yr −1 , which is small compared to ocean margin sites. The ε Nd values of the pore waters, bottom seawater, and authigenic phases of the uppermost sediments agree within a narrow range and suggest preservation of a bottom seawater signature in the authigenic phase. Based on our data, deviations between pore waters and authigenic sedimentary phases below 1 ε Nd unit may have indeed been caused by early diagenetic processes whereas changes exceeding 1 ε Nd unit can be interpreted as changes in bottom seawater signatures. Our results provide important constraints on the conditions under which authigenic phases reliably record past bottom seawater Nd isotope signatures and water mass mixing.

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