ALBERT

Description: Barium (Ba) isotopes are a promising new tracer for riverine freshwater input to the ocean and marine biogeochemical cycling. However, many processes that affect Ba cycling at continental margins have not yet been investigated with respect to Ba isotope fractionation. Here, we present a comprehensive data set of Ba concentration and isotope data for water column, pore water and sediment samples from Kiel Bight, a seasonally stratified and hypoxic fjord in the southwestern Baltic Sea. The surface water Ba concentration and Ba isotope inventory of the water column can generally be explained by mixing of riverine freshwater and Atlantic seawater. However, the deep-water below the seasonal pycnocline (10 - 15 m water depth) is characterized by a pronounced positive Ba concentration anomaly (up to 915 nM) that is accompanied by a δ138Ba of ~+0.25 ‰, which is lighter than expected from the seawater-freshwater mixing line (Ba: 77 nM, δ138Ba: +0.32 ‰ at a salinity of 18). Pore water profiles indicate a Ba flux across the sediment-water interface, which contributes to the enrichment in isotopically light Ba in the deep-water. Pore waters of surface sediments and deep-waters are oversaturated with respect to barite. Therefore, barite dissolution is unlikely to account for the benthic Ba flux. Water column Ba concentrations closely correlate with those of the nutrients phosphate and silica, which are removed from surface waters by biological processes and recycled from the sediment by diffusion across the sediment-water interface. As nutrient-to-Ba ratios differ among sites and from those observed in open-marine systems, we propose that Ba is removed from surface waters by adsorption onto biogenic particles (rather than assimilation) and regenerated within surface sediments upon organic matter degradation. Pore water data for subsurface sediments in Kiel Bight indicate preferential transfer of isotopically heavy Ba into an authigenic phase during early diagenesis. Quantifying the burial flux associated with this authigenic Ba phase along continental margins could potentially help to settle the isotopic imbalance between known Ba source and sink fluxes in the ocean.

Description: Iron (Fe) speciation in marine sediments and suspended particulate matter (SPM) are widely used to investigate the role of Fe minerals in marine biogeochemical cycling and to reconstruct paleo-redox conditions. A traditional method for the determination of Fe speciation are sequential extractions, in which operationally defined Fe pools are dissolved by specific extraction solutions. More recently, many studies make use of synchrotron radiation-based X-ray absorption near edge spectroscopy (XANES), especially if little sample material is available or a high spatial resolution is required. However, few studies have systematically compared Fe speciation obtained by the two methods. Here, we report Fe speciation for marine sediments (both wet and freeze-dried) and SPM from contrasting marine environments (western Baltic Sea and Amazon shelf) obtained by sequential extraction and XANES. The two methods yield comparable results regarding the approximate proportion of reactive Fe ((oxyhydr)oxides, carbonate and sulfide minerals) in the total Fe pool and the extent to which reactive Fe minerals have been converted to pyrite. However, methodological issues associated with both approaches complicate a direct comparison of specific mineral groups. Pyrite concentrations obtained by XANES are lower than those obtained by extraction, which is attributed to an underestimation by XANES due to self-absorption effects and an overestimation by the chromium reduction method due to dissolution of reduced sulfur phases that are not associated with Fe. As substantial amounts of pyrite are oxidized during freeze-drying and sample storage, we recommend to analyze wet sediments in modern marine environments. The XANES spectra of Fe (oxyhydr)oxide and Fe(III)-containing (ferric) clay minerals are similar, which is why these components cannot be differentiated by XANES. Similarly, sequential extraction schemes are known to dissolve a variable proportion of ferric clay minerals along with Fe carbonates and Fe (oxyhydr)oxides. Further investigating the role of ferric clay minerals in the low-temperature marine Fe cycle may therefore require the use of additional methods. Our findings may help to assess the transferability of XANES-based Fe speciation for small sediment and SPM samples to those reported in more traditional studies on Fe biogeochemical cycling and paleo-redox conditions.

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: Reactive iron (Fe) oxides in marine sediments may represent a source of bioavailable Fe to the ocean via reductive dissolution and sedimentary Fe release or can promote organic carbon preservation and long-term burial. Furthermore, enrichments of reactive Fe (sum of Fe oxides, carbonates and sulfides normalized to total Fe) in ancient sediments are utilized as a paleo-proxy for anoxic conditions. Considering the general importance of reactive Fe oxides in marine biogeochemistry, it is important to quantify their terrestrial sources and fate at the land-ocean interface. We applied sequential Fe extractions to sediments from the Amazon shelf to investigate the transformation of river-derived Fe oxides during early diagenesis. We found that ∼22 % of the Amazon River-derived Fe oxides are converted to Fe-containing clay minerals in Amazon shelf sediments. The incorporation of reactive Fe into authigenic clay minerals (commonly referred to as reverse weathering) is substantiated by the relationship between Fe oxide loss and potassium (K) uptake from sedimentary pore waters, which is in agreement with the previously reported Fe/K stoichiometry of authigenic clay minerals. Mass balance calculations suggest that widely applied sequential extractions do not separate Fe-rich authigenic clay minerals from reactive Fe oxides and carbonates. We conclude that the balance between terrestrial supply of reactive Fe and reverse weathering in continental margin sediments has to be taken into account in the interpretation of sedimentary Fe speciation data. Key Points - Reactive Fe is transferred from river-derived Fe oxides into Fe-containing silicate minerals during early diagenesis - Standard sequential extraction schemes do not separate Fe oxides and carbonates from authigenic silicate minerals in Amazon shelf sediments - Terrigenous supply of reactive Fe and reverse weathering need to be considered in the interpretation of sedimentary Fe speciation

Description: Sedimentary molybdenum (Mo) and uranium (U) enrichments have been widely used as a proxy for redox conditions in oxygen-depleted marine paleo-environments. However, in a dynamic upwelling system the seasonal fluctuations from oxic to completely anoxic-sulfidic bottom waters and lateral sediment transport can modify the primary Mo and U signal of the sediment, which in turn may impact paleo-redox interpretations. In this study we present pore water and solid phase data collected at two cross shelf transects during the ‘more oxygenated’ austral winter and ‘anoxic’ austral summer to study the influence of spatially and seasonally contrasting redox conditions on the formation of authigenic Mo and U enrichments in organic carbon (TOC) rich mud belt sediments on the Namibian shelf. A mass balance was established for each element based on diffusive fluxes and element mass accumulation rates to evaluate the respective mechanisms of trace metal delivery, accumulation and recycling. Mo is delivered to the sediment in its dissolved form via diffusion across the sediment–water interface, especially during austral summer when bottom waters are anoxic and surface sediments are highly sulfidic. In the center of the inner shelf mud belt, the benthic Mo fluxes of up to 37 nmol cm−2 yr−1 into sulfidic surface sediments are the highest ever reported for reducing sulfidic systems and agree with the rate of Mo accumulation in the solid phase. Concurrently, high sedimentation rates and low terrigenous input limit solid phase Mo accumulation on the Namibian shelf. In ancient marine sediments, this mode of Mo cycling can be identified by low Mo/TOC ratios of ∼2 similar to those found in sediments deposited below the perennial oxygen minimum zone on the Peruvian shelf and to those found in deposits of the Cretaceous Oceanic Anoxic Event 2. Diffusive U fluxes into the sediment are generally too low to account for the sedimentary enrichment leading to the conclusion that U is delivered mainly in particulate form. In areas with anoxic bottom water, shallow dissolved U maxima directly below the sediment water interface and rather low sedimentary U content indicate that particulate U is recycled and largely released back into the bottom water. At sites where bottom water oxygen concentrations vary from anoxic to completely oxic on seasonal timescales, the depth at which Mo and U are removed from pore waters moves vertically within the sediment column thus defining a layer between the sediment surface and ∼20 cm depth, in which Mo and U accumulate in the solid phase. Our results emphasize the importance of short-term redox fluctuations in the bottom waters and underlying sediments, as well as lateral sediment transport for the authigenic enrichment of redox-sensitive trace metals in reducing shelf sediments. The relative enrichment patterns identified might be useful for the reconstruction of open marine anoxia in the geological past.

Description: Particulate organic carbon (POC), total particulate nitrogen (TN) and total particulate sulfur (TS) were determined by using an element analyzer (Euro EA, HEKAtech). POC was determined after acidifying the sample with 0.25N HCl to remove carbonate. POC content refers to the dry weight percent (wt %) of C (detection limit 〈0.1 wt% and relative error of 3%). Total iron (TFe) concentrations in the solid phase was determined by ICP-OES following digestion in nitric acid, hydrofluoric acid and perchloric acid. Porosity was determined from the water loss after freeze-drying assuming a seawater density of 1.023 g cm-3 and a particle density of 2.5 g cm-3.

Description: Particulate organic carbon (POC), total particulate nitrogen (TN) and total particulate sulfur (TS) were determined by using an element analyzer (Euro EA, HEKAtech). POC was determined after acidifying the sample with 0.25N HCl to remove carbonate. POC content refers to the dry weight percent (wt %) of C (detection limit 〈0.1 wt% and relative error of 3%). Total iron (TFe) concentrations in the solid phase was determined by ICP-OES following digestion in nitric acid, hydrofluoric acid and perchloric acid. Porosity was determined from the water loss after freeze-drying assuming a seawater density of 1.023 g cm-3 and a particle density of 2.5 g cm-3.