Diagenetic overprint on authigenic Nd isotope records: A case study of the Bering Slope

Abstract

Neodymium isotope ratios (143Nd/144Nd, εNd) have been used as a quasi-conservative water mass tracer in the Atlantic and Southern oceans. However, boundary exchange and diagenesis can limit their use in areas where extensive interactions between seawater, pore water and bottom sediments can overwrite the original water mass signature. To avoid the misapplication of this proxy and to provide a new perspective on the application of Nd isotopes to authigenic phases, we present εNd records covering the past 2.4 Myr for the authigenic (εNd, auth) and detrital (εNd, det) phases of sediments from Integrated Ocean Drilling Program site U1343 (∼1950 m water depth) on the Bering Slope. The εNd, det values range from −9.0 to −2.3 (n=70), showing binary mixing between Alaskan and Aleutian sediments. Glacial–interglacial variations are muted in the εNd, det record because sediment delivery pathways from both source areas exist regardless of climatic conditions. The εNd, auth values range from −5.9 to −0.2 (n=153) and show a strong correlation (r=0.58, n=60) with εNd, det in the deeper part of the core (>160 ka), where authigenic carbonate layers are frequently observed. We attribute this covariation to diagenetic overprinting by pore water Nd during late diagenesis. The pore water Nd that is incorporated into authigenic carbonates was initially released by marine silicate weathering, therefore indicating that sediment lithology exerts a first-order control on the εNd values of the pore water in deeply buried sediments. In the shallower part of the core (<160 ka), there is no correlation between εNd, auth and εNd, det (r=−0.23, n=10). Here, εNd, auth is typically radiogenic during interglacial periods, which may be due to the diagenetic flux from the relatively stagnant N. Pacific or the incongruent chemical weathering of freshly ground sediments on the Bering Shelf. The effect of dense water formation on εNd, auth seems to be minimal when we compare the εNd, auth records at three different depths of the Bering Sea. Our εNd dataset provides a unique perspective on the behavior of εNd, auth during late diagenesis. To apply εNd, auth as a water mass tracer to long cores containing volcanogenic sediments or in productive regions, we recommend testing for deep authigenesis using pore water profiles.