Neodymium isotopes in authigenic iron-manganese oxyhydroxides from core top samples in the Gulf of Alaska

The isotopic composition of neodymium (epsilon-Nd) extracted from sedimentary Fe-Mn oxyhydroxide offers potential for reconstructing paleo-circulation, but its application depends on extraction methodology and the mechanisms that relate authigenic epsilon-Nd to bottom water. Here we test methods to extract authigenic epsilon-Nd from Gulf of Alaska (GOA) sediments and assess sources of leachate Nd, including potential contamination from trace dispersed volcanic ash. We show that one dominant phase is extracted via leaching of core-top sediments. Major and trace element geochemistry demonstrate that this phase is authigenic Fe-Mn oxyhydroxide. Contamination of leachate (authigenic) Nd from detrital sources is insignificant (<1%); our empirical results are consistent with established kinetic mineral dissolution rates and theory. Contamination of extracted epsilon-Nd from leaching of volcanic ash is below analytical uncertainty. However, the epsilon-Nd of core-top leachates in the GOA is consistently more radiogenic than bottom water. We infer that authigenic phases record pore water epsilon-Nd, and the relationships of epsilon-Nd among bottom waters, pore waters, authigenic phases and detrital sediments are primarily governed by the exposure time of bottom water to sea-floor sediments, rate of exchange across the sediment-water interface and the reactivity and composition of detrital sediments. We show that this conceptual model is applicable on the Pacific basin scale and provide a new framework to understand the role of authigenic phases in both modern and paleo-applications, including the use of authigenic epsilon-Nd as a paleo-circulation tracer.