Extracting Authigenic εNd Signals from Gulf of Alaska Sediments

Abstract:

The isotopic signature of neodymium (ε_Nd) extracted from authigenic phases has been widely used for reconstructing past deep water circulation. In particular, sediment Fe-Mn oxide coatings have the advantage of providing high resolution records, but application of this proxy is weakened by uncertainty in both signal extraction methodology and, more importantly yet underappreciated, how authigenic signals relate to bottom water. Here, we present data from the extraction of authigenic ε_Nd signals from sediments in the Gulf of Alaska and explore systematically the influence of volcanic material on leaching, which has been argued to bias leachates to more radiogenic values. We tested how reagent type, concentration and leaching duration affect the resultant ε_Nd and then applied one method to a set of coretop samples, monitoring major and trace element concentrations and strontium isotope ratios to identify the source of leachate Nd. Leachates are enriched in elements characterizing Fe-Mn crust (Co, Cu, Mn, Ni and Zn) and depleted in elements typically associated with volcanic materials (Cr, Sc, Nb and Ta). Such evidence supports the authigenic source of leachate Nd: however, extracted ε_Nd values are still consistently ~1.5 units higher than bottom water below 500 m. We conclude that the authigenic phases record pore water, not bottom water, ε_Nd. Authigenic phases likely reflect bottom water in locations where pore water and bottom water are similar: however, in places where they are considerably different, e.g., because of volcanic particle diagenesis, interpreting εNd records demands care. Further study of the diagenetic cycle of Nd will improve our ability to explain authigenic ε_Nd records, but presently we suggest the best paleoceanographic practice is to consider ε_Nd gradients instead of absolute values.