Abstract
Rare earth element (REE) concentrations and neodymium isotopic composition (ɛNd) 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 (<0.45 μm), colloidal and particulate REE and ɛNd 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 ɛNd 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.
Highlights
Rare earth element (REE) concentrations and neodymium isotopic composition are tracers for ocean circulation and biogeochemistry
Understanding rare earth element (REE) speciation, dynamics and mass balance in natural waters is important because REE and neodymium (Nd) isotopes are used as tracers of biogeochemical processes, water mass transport and mixing in the modern ocean[1,2,3,4] and as paleoproxies for past oceanic circulation patterns[5,6]
We find that Nd release from Amazon river suspended sediments is rapid, and larger than the dissolved Nd river input to the Atlantic Ocean
Summary
Rare earth element (REE) concentrations and neodymium isotopic composition (eNd) are tracers for ocean circulation and biogeochemistry. Dissolved and particulate river inputs of REEs to the oceans were recognized early on as being potentially important[7,8,9]. This study showed that all REE concentrations slightly increase again in the salinity range between 6.6 and 34.4 This was interpreted as being due to the release of REE from sediments and resuspended particles. Goldstein and Jacobson[31] recognized early on that estuarine REE removal is important in balancing marine REE budgets They documented that the eNd of dissolved and suspended river loads can differ by up to four units, and that replacing one with the other in river eNd budgets is not trivial. We estimate that on a global scale the release process may dominate all other Nd inputs
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