Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary

Abstract

Rare earth element (REE) concentrations and neodymium isotopic composition (eNd) are tracers for ocean circulation and biogeochemistry. Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated. Here we present the first simultaneous observations of dissolved (o0.45 mm), colloidal and particulate REE and eNd in the Amazon estuary. A sharp drop in dissolved REE in the low-salinity zone is driven by coagulation of colloidal matter. At mid-salinities, total dissolved REE levels slightly increase, while eNd values are shifted from the dissolved Nd river endmember ( 8.9) to values typical of river suspended matter ( 10.6). Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.