Dynamic behavior of dissolved and soluble titanium along the salinity gradients in the Pará and Amazon estuarine system and associated plume

Abstract

Ultratrace concentrations of Ti were determined by catalytic differential pulse adsorptive stripping voltammetry (cDPAdSV) in samples collected in the Pará and Amazon estuaries and plume into the Atlantic Ocean. Different end members including rivers Tocantins, Amazon and Pará and Atlantic seawater were sampled as well as the salinity gradients in the mixing zone between the river outflows and waters from the North Brazil Current during several transects. The Mangrove Belt southeast of the Pará river mouth with its extensive groundwater discharge was also sampled. Most samples were taking during cruise M147 (GEOTRACES process study GApr11) during the high discharge period in April and May 2018. In addition to high resolution determination of dissolved Ti distributions in this region, size fractionation was investigated by using several filtration steps with pore sizes (0.2 μm, and 0.015 μm) and ultrafiltration (10 kDa and 1 kDa) at four selected stations. Dissolved Ti varied significantly between different river end members and showed a non-conservative behavior along the mixing gradients with strong removal at low salinities and some enrichments at higher salinity ranges. The results suggest that there was both adsorption and desorption of Ti from suspended particles from both riverine and marine sources or flocculation and aggregation of colloids and particulate matter from end member rivers as well as resuspension at particular salinity ranges. The 0.015 μm filtered and 10 kDa and 1 kDa ultrafiltered aliquots showed variable distributions of Ti in the different size fractions, depending on the sampling zone. This very complex behavior of Ti along the mixing gradient and the dynamic system of the Amazon estuary, which comprises a fifth of the global freshwater flux into the ocean, is the key to controlling the fluxes of Ti into the Atlantic.