ALBERT

Description: Highlights • Amazon shelf sediments take up seawater potassium (K) due to reverse weathering. • Amazon shelf sediments release terrigenous phosphorus (P) during resuspension. • Updated estimates of sedimentary K uptake and P release on Amazon shelf are presented. • Sedimentary K uptake on Amazon shelf corresponds to 13% of global riverine K input. • Sedimentary P release is ~5 times higher than dissolved P discharge of Amazon River. Abstract In this study, we identify and quantify processes that lead to sedimentary potassium (K) sequestration and phosphorus (P) release on the Amazon shelf. To this end, seven short sediment cores were recovered from the Amazon shelf during R/V Meteor cruise M147. All of the sediment cores investigated in this study are characterized by elevated K to aluminum (Al) ratios compared to Amazon riverine suspended matter, which indicates that seawater K+ is incorporated into the solid phase on the entire Amazon shelf. Pore water silica (Si) profiles are characterized by irregularly increasing concentrations and plateaus, thus, deviating from the asymptotic shape that is typically found in continental margin sediments. At one site, a dissolved Si plateau coincides with a K+ minimum suggesting that these solutes are incorporated into authigenic minerals, a process referred to as reverse weathering. Previous flux estimates for elements that participate in reverse weathering on the Amazon shelf were derived from pore water diffusive fluxes, reaction rates estimated from sediment incubations and solid phase extractions. In this study, we took an alternative approach, which is based on the concentration difference between shelf sediments and river suspended particles. The resulting K flux due to reverse weathering of 1.7 ∙ 1011 mol yr−1 is in agreement with previous estimates and corresponds to 13% of the global riverine dissolved K+ input. Previous studies demonstrated that Amazon riverine particulate P is partly solubilized on the Amazon shelf. However, these results are exclusively based on sediment data close to the river mouth and no distinction between terrestrial and marine sediment components was made. Here, we quantify P release from Amazon shelf sediments by comparing terrestrial P concentrations in shelf sediments with P concentrations in river suspended particles. The resulting solubilized P flux of 2.2 ∙ 1010 mol yr−1 is about five to six times higher than previous estimates and about seven times the Amazon riverine dissolved P discharge. The magnitudes of the presented fluxes imply that the alteration of riverine shelf sediments significantly affects the mean concentrations of dissolved K+ and P in the global ocean.

Description: The Amazon and the Pará are two major rivers that carry dissolved and suspended particulate trace metals to the Atlantic Ocean. In the dynamic mixing zone of the estuary, competing processes of trace metal sorption and release play a role, which might affect transport to the open ocean. Here we investigate the behavior of dissolved (〈0.2 μm), soluble (〈0.015 μm) and truly dissolved (〈10 kDa and 〈 1 kDa) molybdenum (Mo), uranium (U), and vanadium (V) during estuarine mixing between river water (S 〈 1) and seawater (S 〉 35) end members during the high discharge period, as well as during aging of the plume in its northward flow along the coast. Molybdenum behaved conservatively during estuarine mixing and showed no colloidal fraction, suggesting Mo is solely present in the soluble or even truly dissolved fraction. Uranium behaved mostly conservatively but showed removal in the low salinity range (ca. S 〈 9). This is potentially due to colloidal flocculation at low salinities, as indicated by colloidal (0.015–0.2 μm) fractions of up to 30% for U but decreasing with increasing salinity until no significant difference could be discerned at S 〉 10. Vanadium shows a general conservative mixing, but with more scatter in the data than for Mo and U and potential removal at low to mid-salinities. Removal of V to the sediments is also indicated by surface sediment data from the mid-salinity region of the estuary but no size fractionation in the dissolved phase could be observed. Hence, V seems to be predominantly present in the soluble or even truly dissolved phase and export to the sediments might take place through particles 〉0.2 μm. No considerable removal or release of Mo, U and V was observed in their water column depth profiles, indicating a conservative behavior in the water column of the estuaries studied here. Additionally, we present a comparison of differential pulse adsorptive stripping voltammetry and inductively coupled plasma – mass spectrometry analyses for Mo and V, which showed excellent agreement within analytical uncertainty in this challenging sample material covering the full salinity range from freshwater to seawater.

Description: Highlights • First successful in situ DGT application in the deep ocean. • DGT-lability of dissolved (〈0.2 μm) Cu, Ni, Cd, Mn, As, V, REY differs depending on chemical speciation. • REY in deep ocean water can be almost quantitatively assessed with DGT. • Low Cu availability reflects dominating organic speciation. Abstract Geochemical behaviour and bio-availability of trace metals are closely related to their physical fractionation and chemical speciation. The DGT speciation technique allows the challenging assessment of labile concentrations of Mn, Cd, Cu, Ni, V, As, and REY in ocean waters. In this first deep-water in situ study of DGT-lability, we demonstrate the approach in bottom waters of the Clarion-Clipperton Zone in the central NE Pacific. In the dissolved fraction (〈0.2 μm), 70% to 100% of Cd, Ni, V, and REY, but only 25% of Cu and less than 50% of As were determined, reflecting their prevailing dominance of organic vs. inorganic complexation. This study demonstrates the applicability and sensitivity of DGT-passive samplers for trace metals as a suitable technique in monitoring of anthropogenic activities, such as deep seabed mining, as well as for natural process studies in abyssal environments.