The potential influence of sedimentary diagenetic processes on the ocean’s εNd signature

Abstract:

The neodymium isotope signature (εNd) of ocean water can be used as a tracer of past ocean circulation. However, the utility of εNd as a tracer is predicated upon a full understanding of the processes and sources that define the ocean’s εNd signature. To date, sources of Nd to the ocean have been assumed to be rivers, with a minor contribution from atmospheric deposition. However, recent studies imply that other sources (e.g. pore fluids, groundwater) are important to the oceanic Nd budget and exchange between particles and marine fluids along continental margins could also influence oceanic Nd.

To examine the influence of sedimentary processes on the oceanic Nd signature we collected sediment, pore fluid, and ocean water column samples at 4 sites along the Oregon margin. Here we present the εNd and Nd concentrations of a weak sediment leach (labile), a strong sediment leach (refractory), and the total bulk sediment. We find that the εNd of the labile phases (weak leach, εNd = 0.8 to -2.3; strong leach, εNd = 0.4 to -2.2) is indistinguishable from the εNd of the bulk sediment digest (-0.2 to -2.8) with no systematic variation in εNd between the components. These data suggest that the εNd labile component is not fractionated from the primary sediment source. Furthermore, we do not observe any changes in εNd that correspond to concentration variability in the pore fluids. The Nd concentrations of the labile components (weak leach: 0.7 - 3.7 μg g^-1; strong leach 2.1 - 8.2 μg g^-1) are only an order of magnitude lower than the Nd concentration of the bulk sediment (13 - 22 μg g^-1) suggesting a large potential reservoir of reactive Nd in the sediment pool with implications to the oceanic budget of Nd as well as the use of εNd.