Geochemistry of sediment core HE337/001-1, North Sea

Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for d56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine-HCl, reducible oxides (e.g. goethite and hematite) by dithionite-citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the Island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore d56Fe trend. This iron pool was most depleted in 56Fe close to the sediment-water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5 M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide-range of environmental settings.

(Table 2) Layer-by-layer chemical composition of barite crusts from the Atlantis II Deep of the Red Sea rift zone

Results of studies of hydrothermal sulfide-sulfate rocks occurring in the Atlantis II Deep of the Red Sea are reported in the paper.

Ocean acidification increases cadmium accumulation in marine bivalves: a potential threat to seafood safety

To date, the effects of ocean acidification on toxic metals accumulation and the underlying molecular mechanism remains unknown in marine bivalve species. In the present study, the effects of the realistic future ocean pCO2 levels on the cadmium (Cd) accumulation in the gills, mantle and adductor muscles of three bivalve species, Mytilus edulis, Tegillarca granosa, and Meretrix meretrix, were investigated. The results obtained suggested that all species tested accumulated significantly higher Cd (p<0.05) in the CO2 acidified seawater during the 30 days experiment and the health risk of Cd (based on the estimated target hazard quotients, THQ) via consumption of M. meretrix at pH 7.8 and 7.4 significantly increased 1.21 and 1.32 times respectively, suggesting a potential threat to seafood safety. The ocean acidification-induced increase in Cd accumulation may have occurred due to (i) the ocean acidification increased the concentration of Cd and the Cd2+/Ca2+ in the seawater, which in turn increased the Cd influx through Ca channel; (ii) the acidified seawater may have brought about epithelia damage, resulting in easier Cd penetration; and (iii) ocean acidification hampered Cd exclusion.

Concentrations of Fe, Mn, Cu, Zn, Co, Ni, Pb, Cd, B, and SiO2 in interstitial waters from bottom sediments of the Atlantis II and Discovery Deeps, Red Sea rift zone

Chemical analyzes show that interstitial waters from ore-bearing bottom sediments of the Atlantis II and Discovery Deeps are enriched in Fe, Mn, Cu, Ni, Co, Zn, Pb, and Cd compared to sea water. Enrichment factors of these trace elements in the interstitial waters of the Atlantis II Deep relative to the sea water vary within the following ranges: for Fe from 100 to 7000, for Mn from 19047 to 32738, for Zn from 500 to 1600, for Pb from 78333 to 190000, for Cu from 107 to 654. Comparison of average weighted concentrations of Fe, Mn, Zn, Pb, Cu, Ni in the bottom sediments and the interstitial waters of the Atlantis II Deep indicates common regularities and good relationship in distribution of these elements along sediment cores. Differences in concentrations and distribution of the studied trace elements in the interstitial waters of the Atlantis II and Discovery Deeps result from different chemical compositions of hydrothermal fluids entering these deeps.