The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Crystalline iron oxides stimulate methanogenic benzoate degradation in marine sediment-derived enrichment cultures

Elevated dissolved iron concentrations in the methanic zone are typical geochemical signatures of rapidly accumulating marine sediments. These sediments are often characterized by co-burial of iron oxides with recalcitrant aromatic organic matter of terrigenous origin. Thus far, iron oxides are predicted to either impede organic matter degradation, aiding its preservation, or identified to enhance organic carbon oxidation via direct electron transfer. Here, we investigated the effect of various iron oxide phases with differing crystallinity (magnetite, hematite, and lepidocrocite) during microbial degradation of the aromatic model compound benzoate in methanic sediments. In slurry incubations with magnetite or hematite, concurrent iron reduction, and methanogenesis were stimulated during accelerated benzoate degradation with methanogenesis as the dominant electron sink. In contrast, with lepidocrocite, benzoate degradation, and methanogenesis were inhibited. These observations were reproducible in sediment-free enrichments, even after five successive transfers. Genes involved in the complete degradation of benzoate were identified in multiple metagenome assembled genomes. Four previously unknown benzoate degraders of the genera Thermincola (Peptococcaceae, Firmicutes), Dethiobacter (Syntrophomonadaceae, Firmicutes), Deltaproteobacteria bacteria SG8_13 (Desulfosarcinaceae, Deltaproteobacteria), and Melioribacter (Melioribacteraceae, Chlorobi) were identified from the marine sediment-derived enrichments. Scanning electron microscopy (SEM) and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) images showed the ability of microorganisms to colonize and concurrently reduce magnetite likely stimulated by the observed methanogenic benzoate degradation. These findings explain the possible contribution of organoclastic reduction of iron oxides to the elevated dissolved Fe2+ pool typically observed in methanic zones of rapidly accumulating coastal and continental margin sediments.

Influence of Manila clam aquaculture on rates and partitioning of organic carbon oxidation in sediment of Keunso Bay, Yellow Sea

We investigated the effects of Manila clam aquaculture on the rates and pathways of anaerobic organic carbon (OC) oxidation in highly bioturbated (HB) and poorly bioturbated (PB) sediment in Keunso Bay, Yellow Sea. Due to the labile organic matter supply via sediment reworking by Manila clams, the anaerobic OC oxidation rate in HB sediment (38.8 mmol m-2 d-1) was ~1.5 times higher than that in PB sediment (26.8 mmol m-2 d-1). Microbial Fe(III) reduction (FeR) dominated OC oxidation pathways in HB sediment, comprising 55 to 76% of anaerobic OC oxidation, whereas sulfate reduction (SR) was the dominant oxidation pathway in PB sediment, accounting for up to 92% of anaerobic OC oxidation. Despite higher anaerobic respiration rates at the HB site, concentrations of NH4+, PO43-, oxalate-extractable iron (Fe(II)(oxal)), and total reduced inorganic sulfur were 2 to 3 times lower in HB than in PB sediment. Conversely, the concentration of reactive Fe(III)(oxal) at the HB site (2243 mmol m-2) exceeded that at the PB site (1127 mmol m-2) by a factor of 2. These results indicate that bioturbation by Manila clams enhances the re-oxidation processes of reduced metabolites in the sediment, thereby prohibiting SR and promoting FeR. Overall, the results suggest that aquaculture activities of Manila clams shift the dominant OC oxidation pathways in sediment from SR to FeR, which generates relatively oxidized and less sulfidic environments.

Phosphorus dynamics associated with partitioning of organic carbon oxidation pathways in the surface sediments of the deep Ulleung Basin, East Sea

<p>As sediments play an important role as either a sink or a source of phosphorus (P) for water column, it is important to elucidate the major P fractions and behaviors (i.e., mobilization and immobilization) in the sediments to better understand P cycles in local and global scale. We investigated major P speciation associated with the partitioning of organic carbon (C<sub>org</sub>) oxidation in the sediments to elucidate the P dynamics at two contrasting sediments in the continental shelf (EB1) and rise (EC1) in the Ulleung Basin (UB), East Sea. Sulfate reduction (SR) pre-dominated C<sub>org</sub> oxidation at shelf site (EB 1), comprising % of C<sub>org</sub> oxidation, whereas Mn- and Fe-reduction combined accounted for >80% of C<sub>org</sub> oxidation in Mn-oxide and Fe-oxide-rich basin site (EC 1). Under SR-dominated condition (EB 1), H<sub>2</sub>S oxidation coupled to reductive dissolution of FeOOH to form precipitation of FeS induced the accumulation of dissolved iron and phosphate in the pore water. On the other hand, phosphate in the Mn- and Fe-oxide-rich basin sediments (EC 1) was depleted because the P released through organic matter decomposition or reductive dissolution of Fe oxide/Mn oxide was effectively adsorbed to the metal-oxides in the surface sediments. Sequential extraction of P phases revealed that Fe bound P (52-65% of total P) was the major phase in the surface sediments of both sites. Interestingly, the organic P (P<sub>org</sub>) fraction was 2.4-times higher at the basin site (12 μmol g<sup>-1</sup>) than at the shelf site (5 μmol g<sup>-1</sup>). C<sub>org</sub> : P<sub>org</sub> ratios presented as redox proxies in sediments were 644 and 191 for EB1 and EC1, respectively,. The results indicate that P<sub>org</sub> has an effective preservation relative to C<sub>org</sub> under sub-oxic conditions (EC1), whereas P<sub>org</sub> was preferentially regenerated under anoxic conditions (EB1). Overall, the dynamics of P in the UB sediments were largely regulated by the partitioning of C<sub>org</sub> oxidation pathways (i.e., sulfate reduction vs. metal reduction) and resultant interaction between Fe/Mn-S-P.</p>

Isotopic (δ34s, δ13c, δ18o) properties of the disseminated mineralization of plutonic rocks in the Dukat ore field (Northeast of Russia)

The variations in the δ34S, δ13C, and δ18O values of the disseminated sulfides and carbonate phase, which occurred in trace amounts in the plutonic rocks controlling the position of the unique Dukat Au-Ag field (Northeast of Russia), were examined. These properties were compared with similar isotopic parameters of the ore associations in the field. The δ34S values of sulfides and jarosite obtained from plutonic rocks were in a relatively narrow range (from -3.4 to +3.6‰) when compared with the range of variation of the δ34S values of sulfides obtained from the ore bodies (from 4.5 to +2.0 ‰). Pyrite sulfur obtained from the early mineralization of K-Na-leucogranite and pyrite obtained from the ore bodies were observed to have the same source. Pyrite formed during the later magmatic stages is characterized by a small amount of lighting based on the sulfur isotopic signature. The carbonate phases of the plutonic ore in the Dukat ore field are characterized by the δ13С values (from 12.8 to 8.8‰). The carbonates are split into groups according to the oxygen isotopic signature: carbonate balanced with the rock silicate matrix at high temperatures and carbonate with abnormally low δ18О values (from -0.8 to +0.9 ‰). The obtained data can be described using the model that assumes that the formation of the isotopic parameters of sulfide sulfur and carbonate carbon occurs during the process of sulfate recovery using organic carbon oxidation (TSR). Further, the calculations revealed that the observed δ34S and δ13С values in the rocks and ore associations in the Dukat field can be obtained during the abiogenic recovery of marine sulfate in a temperature range of 300 °C–450 °C. The comparison of the isotopic parameters of the rock carbonate with those of the ore association carbonate demonstrated that the surrounding/base rocks and fluid that separated during the cooling of the K-Na-leucogranite intrusion bodies, which resulted in a loss of approximately 80% CO2, could serve as the source of the carbonates of the ore bodies.