Brent L. Lewis

The biogeochemistry of manganese and iron in the Black Sea

The solution speciation and solid-phase suspended particulate fractionation of Mn and Fe were investigated in the Black Sea in an effort to understand the biogeochemical cycling of Mn and Fe across redox boundaries and to study the scavenging/precipitation reactions affecting their distributions. The redox cycling of Mn (in a distinct “suboxic” zone from 15 to 50 m thick) and the redox cycling of Fe (coincident with total sulfide concentrations exceeding 0.4 μM) occur along isopycnal surfaces which deepen sharply towards the Turkish coast. Dissolved Mn behaves primarily as the free hydrated Mn2+ species and approaches saturation with respect to MnS2 (haurite) in the deep anoxic waters. In the oxic zone, colloidal and organically-complexed Fe species account for 10–30% of the total dissolved Fe, while colloidal Fe-sulfides account for 30–60% of the total in the mid-depth dissolved Fe(II) maximum. The deep waters are close to saturation with respect to FeS (mackinawite) or Fe3S4 (greigite). A weak-acid soluble Mn phase dominates in the broad particulate Mn maximum in the suboxic zone and appears to be associated with Mn-oxidizing bacteria. More resistant Mn and Fe phases, presumed to be sulfide precipitates, were found in the deep anoxic waters. Particulate Al showed a broad maximum below the sulfide interface, presumably due to offshore transport of resuspended sediment. A vertical mixing model for dissolved Mn in the central basin of the Black Sea yields removal rates consistent with measured bacterial Mn oxidation rates. The redox cycling of Fe occurs somewhat deeper in the water column. The vertical supply of oxygen cannot account for the Mn oxidation rate. However, horizontal advection and/or seasonal vertical mixing could provide enough oxidizing equivalents.

THE BEHAVIOR OF BARIUM IN ANOXIC MARINE WATERS

Abstract The present day distributions of Ba in the water columns at three anoxic marine sites, namely the Cariaco Trench, Framvaren Fjord, and Black Sea, are presented. Dissolved Ba levels generally increase with depth, ranging from 45–85, 64–280, and 180–460 nM in surface and bottom waters for the three basins, respectively. Small maxima are observed in the vicinity of the in redox interface in both the Framvaren Fjord and Black Sea. Comparison of the dissolved and particulate Ba, Fe, and Mn distributions show that the maxima do not result from adsorption onto freshly precipitated Fe and/or Mn oxyhydroxides. As for the open ocean, Ba cycling in all three basins is dominated by its uptake, primarily in the form of barite, into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface promotes the breakdown of settling particulate matter and the release of barite just above the O 2 H 2 S interface in the Black Sea, and most likely in the Framvaren Fjord, thus providing in part for the observed maxima. Dissolution of such barite in the marginal sediments of these basins probably also contributes to the maxima. Thermodynamic calculations show deep Black Sea Ba concentrations exceed saturation with respect to pure barite by at least a factor of 2. However, the uniformity of the deep water concentrations suggests thermodynamic control by some phase; it is likely that impurities, incorporated into barite during its rapid formation near the surface in microenvironments provided by decaying organisms, are responsible for the levels observed. Additional factors controlling the distributions of Ba in each basin are also discussed.

The investigation of dissolved and suspended-particulate trace metal fractionation in the Black Sea

Abstract A sequential filtration-ion-exchange column scheme has been developed for the investigation of dissolved trace metal fractionation. The method is designed to separate water column metals rapidly into particulate and colloidal-sized species, anionic dissolved metal-organic and/or metal-sulfide complexes, and ‘free’ (hydrated) metal cations and/or labile major ion complexes. The suspended-particulate matter (0.4 μm filterable material) is further subjected to a sequential leaching procedure to separate particulate metals into weak-acid leachable, strong-acid leachable, and refractory fractions. These methods are evaluated with respect to laboratory experiments and field results. These methods have been applied to the study of trace metal chemical fractionation in the Black Sea, the worlds largest anoxic basin. We present here the results for Co, Ni, Cu, Zn, Cd, and Pb. The dissolved Co distribution is best explained in terms of a scavenging-regeneration cycle with Mn-oxyhydroxides across the sulfide interface and coprecipitation of Co with Fe-sulfides in the deep waters. Nickel displays nearly constant dissolved metal concentrations with depth and is apparently unaffected by redox processes. The Class B metals (Cu, Zn, Cd, and Pb) have high dissolved metal concentrations in the surface waters then decrease rapidly across the sulfide interface to low deep water concentrations, consistent with metal-sulfide precipitation below the interface. The dissolved Class B metal fractionation was generally dominated in the oxic zone by ‘free’ metal species, shifting below the interface to anionic species, probably dissolved metal-sulfide complexes. The suspended matter trace metal fractionation is dominated by weak-acid soluble forms throughout most of the water column. For particulate Co, strong-acid leachable forms, probably metal-sulfide phases, are important in the anoxic deep waters. The Class B metals form relatively reactive (weak-acid leachable) metal-sulfide precipitates just below the sulfide interface.

Thermodynamic Modeling of Trace Metal Speciation in the Black Sea

The solution speciation and saturation state for dissolved Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in the Black Sea were modeled using a modified version of the MINEQL equilibrium modeling program. The calculations included the effects of major cations and anions, pH, sulfide, phosphate, carbonate, and ammonia. The effects of potential organic complexation were assessed using two model ligands; nitrilotriacetic acid and cysteine. The results of these calculations were compared to the trace metal speciation measured using a unique ion-exchange chromatography column sequence designed to separate colloidal species, “inert” anionic metal complexes, and “free” metal cations. The possible role of Fe-sulfide precipitation in the scavenging of other trace metals is also addressed.

Sources and sinks of methylgermanium in natural waters

Abstract New data have extended our understanding of the distribution and behavior of methylgermanium in the environment. Laboratory attempts to induce aerobic methylation with known biological and abiotic methylating agents were unsuccessful; this confirmed previous field observations of methylgermaniums unreactive behavior. However, biomethylation of inorganic germanium was observed in the anaerobic digestor of a sewage treatment plant, which suggested a terrestrial methanogenic source. Attempts to locate such a source in methanogenic swamps and their drainages reveal very low methylgermanium concentrations typical of other remote, pristine rivers. Polluted rivers have monomethylgermanium (MMGe) and dimethylgermanium (DMGe) concentrations 3–100 times higher than those of pristine rivers, which suggests an anthropogenic source of methylgermanium as a result of the synergistic effects of sewage treatment and coal-ash derived inorganic germanium contamination. A new high-precision profile of MMGe and DMGe in the Sargasso Sea shows conservative behavior with no vertical gradients. However, marine anoxic basins have both inorganic germanium enrichment and methylgermanium depletion, which suggests that of marine anaerobic processes are responsible for demethylating marine organogermanium. These results all suggest that methylgermanium is produced on the continents, is unreactive in the open ocean, and is destroyed in marine anoxic environments. The residence time of organogermanium in the sea, based on a continental source (pristine rivers), is at least 1 Ma, consistent with its unreactive nature, its observed distribution in the ocean, and rates of destruction in anoxic basins.