The Amazon estuary as an interface for trace metal fluxes from the river into the Atlantic – a study along a salinity gradient in the mixing zone

Due to its magnitude and high discharge, the Amazon River is a major exporter of trace elements into the Atlantic. Geochemical reactions occurring in the Amazon estuarine water can significantly affect the riverine flux of these components, including biological modification, adsorption/desorption on/from particles, precipitation, dissolution of particles, and changes in solution speciation. In September 2012 (low discharge) we sampled the tributaries to the larger estuarine region of the Amazon and a transect in the mixing zone of the Rio Pará into the Atlantic reaching up to 25% of seawater salinity in the samples. We found a large variability in trace metal concentrations in the different river types, with extremely low concentrations of all metals including rare earth elements (REE) in the clearwater river Rio Tocantins compared to significantly higher concentrations in the white water Amazon river close to Macapá, coupled with lower dissolved organic carbon (DOC). In the salinity transect, complex processes seem to control the behaviour of elements such as Fe, Zr, Ti, Th, REE+Y and of DOC, which do not follow straight trends. In this highly dynamic system, REE and Y concentrations were largely unaffected up to 10% seawater salinity, while in the mid-salinity samples these elements display known seawater patterns with negative Ce anomalies, positive La, Gd, and Y anomalies and HREE>LREE. Size fractionation by gel permeation-HPLC coupled to ICP-MS showed a strong association of metals such as Cu with the organic phase in river endmembers and indicated a change in association of some trace metals in the mixing zone. Notably, in the more saline samples the colloids become smaller. Possible reasons are photo-oxidation of the metal-binding organic matter, or disaggregation of larger particles as a result of the increasing ionic strength in the estuary. Our data highlight the significant modification of trace metal fluxes in the Amazon estuarine mixing zone.