Award: OCE-2037556

Deep-sea sediments from the Central Pacific Ocean were collected to study rare earth element (REE) fluxes from these sediments into the overlying deep Pacific seawater. The chief objective was to investigate whether a deep benthic flux could control REE concentrations and Nd isotope composition of these waters. The primary task of the UMass Boston team was to study REE sorption onto these sediments and use the resulting data to develop surface complexation models (SCM). The SCMs can then be employed in reactive transport models to evaluate REE fluxes as a function of changes in porewater compositions. The project supported the PhD research of a UMass Boston student. The results of these endeavors reveal that the REEs are very strongly adsorbed onto the deep-sea sediments, which may challenge our hypothesis of a deep benthic flux of the REEs from the deep-sea sediments into the bottom waters. We expected the REEs to be strongly sorbed to these sediments, but our new data and the subsequent FITEQL/MINTEQ models appear to indicate that the REEs are nearly immobile in these sediments. That said, if the REEs are chiefly sorbed onto Fe(III)/Mn(IV) oxides/oxyhydroxides in these deep-sea sediments, they could still be mobilized upon reductive dissolution of these sparingly soluble metal oxides/oxyhydroxides. Future work should probably try to evaluate where the "labile" REE fraction occurs in these sediments. This may be possible by EXAFS, although I suspect the low REE contents of the sediments would make EXAFS analysis challenging. Last Modified: 05/21/2024 Submitted by: KarenHJohannesson