How Diagenesis Determines the Deep Ocean’s εNd Composition

Abstract:

We measured the Nd isotope composition (εNd) of the water column, sedimentary pore fluids, and sediment leaches of cores from a transect through the oxygen deficient zone of the northeast Pacific. Our results imply that pore fluids represent the dominant supply term for Nd to seawater. Based on a suite of analysis including rare earth element concentrations and εNd, we predict that the difference in grain size between the bulk sediment (pre leach) and the refractory particulates (post leach) is a key variable in determining the magnitude of the source of Nd to seawater, and that both the refractory phase (~20%) and labile phase (~80%) contribute to the pore fluid Nd concentrations during diagenesis. Our results suggest that pore fluid εNd ultimately determines the εNd of the labile phase because the labile phase appears to be formed during early diagenesis. The implication of our work is that the εNd recorded in the labile coatings, which is often utilized as a proxy for past seawater εNd, is not a bottom water signature. Instead, similarities between labile εNd and bottom water εNd occur where the pore fluid εNd controls both the bottom water εNd as well as the labile εNd, thus producing the apparent utility of the coatings as a seawater archive. Therefore, the labile εNd and bottom water εNd will be similar in regions with a large benthic flux. We suggest that this improved understanding of the factors influencing εNd as a tracer will make reconstructions more precise.