Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System : The Argentine Continental Margin
The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation, and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope—a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory—conditions that result in low organoclastic sulfate reduction rates (SRR). Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulﬁde is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulﬁde and elemental sulfur accumulate above and below this zone. Iron monosulﬁde precipitation is driven by the reaction of low amounts of hydrogen sulﬁde with ferrous iron and is in competition with the oxidation of sulﬁde by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulﬁde are bordered by two distinct peaks in total organic sulfur (TOS). Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulﬁde zone, potentially linked to the presence of polysulﬁdes formed by reaction between dissolved sulﬁde and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulﬁde-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulﬁde phases meters below the SMT demonstrates that in sulﬁde-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths.