Description: 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. 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. 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.

Description: An abrupt transition from oxic to anoxic-sulfidic (euxinic) marine bottom waters occurred in the Cariaco Basin in response to increasing productivity resulting from the late Pleistocene post-glacial rise in sea level and corresponding increase in surface-water nutrient availability. The microlaminated sediments of the euxinic interval, which span the last not, vert, similar14.5 ky, suggest a predominance of water-column (syngenetic) pyrite formation based on (1) high pyrite sulfur (Spy) concentrations in the surficial sediment layers, (2) values for degree-of-pyritization (DOP) that generally do not increase appreciably with increasing burial, (3) ratios of total iron (FeT) to Al that are elevated above the continental baseline recorded in the underlying oxic sediments, and (4) Spy isotope trends that largely mimic the d34SHS- of the modern water column. Intermediate DOP values in the microlaminated deposits and FeT/Al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of Fe-bearing siliciclastic accumulation. As predicted from the relative rates of siliciclastic delivery, FeT/Al and DOP data lie between end-member values observed in the modern Black Sea. As viewed broadly, FeT/Al and DOP relationships in euxinic sediments reflect the balance between syngenetic Fe scavenging and temporal and spatial gradients in siliciclastic input. Pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon (Corg). However, the upper 80 cm of the Fe-rich, Corg-poor, bioturbated sediment show evidence for a strong diffusional HS- overprint from the overlying, Fe-limited euxinic marine environment. This post-glacial transition manifests in pyrite overprints that are strongly 34S-depleted relative to those in restricted, presently euxinic marine settings elsewhere in the world, such as the Black Sea, where the sedimentary sequence spanning the last glacial-interglacial transition begins with a shift from freshwater to Corg-poor oxic marine deposition and thus dominantly sulfate diffusion. Trends for Mo/Al ratios in the microlaminated sediments suggest that Mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits. Organic matter plays a role by enhancing HS- production and/or by providing a substrate for Mo scavenging. Significant Mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite HS- -rich pore waters as recorded in the heavy iron sulfide overprint. We have not, however, proven that high sulfide concentrations within the water column are required for enhanced Mo sequestration in sediments.

Description: Leg 165 of the Ocean Drilling Program afforded a unique opportunity to investigate organic and inorganic geochemistry across a wide gradient of sediment compositions and corresponding chemical pathways. The solid fractions at Sites 998, 999, 1000, and 1001 reveal varying proportions of reactive carbonate species, a labile volcanic ash fraction occurring in discrete layers and as a dispersed component, and detrital fluxes that derive from continental weathering. The relative proportions and reactivities of these end-members strongly dictate the character of the diagenetic profiles observed during the pore-water work of Leg 165. In addition, alteration of the well-characterized basaltic basement at Site 1001 has provided a strong signal that is reflected in many of the dissolved components. The relative effects of basement alteration and diagenesis within the sediment column are discussed in terms of downcore relationships for dissolved calcium and magnesium. With the exception of Site 1002 in the Cariaco Basin, the sediments encountered during Leg 165 were uniformly deficient in organic carbon (typically 〈0.1 wt%). Consequently, rates of organic oxidation were generally low and dominated by suboxic pathways with subordinate levels of bacterial sulfate reduction and methanogenesis. The low rates of organic remineralization are supported by modeled rates of sulfate reduction. Site 1000 provided an exception to the generally low levels of microbially mediated redox cycling. At this site the sediment is slightly more enriched in organic phases, and externally derived thermogenic hydrocarbons appear to aid in driving enhanced levels of redox diagenesis at great depths below the seafloor. The entrapment of these volatiles corresponds with a permeability seal defined by a pronounced Miocene minimum in calcium carbonate concentration recognized throughout the basin and with a dramatic downcore increase in the magnitude of limestone lithification. The latter has been tentatively linked to increases in alkalinity associated with microbial oxidation of organic matter and gaseous hydrocarbons. Recognition and quantification of previously unconstrained large volumes and frequencies of Eocene and Miocene silicic volcanic ash within the Caribbean Basin is one of the major findings of Leg 165. High frequencies of volcanic ash layers manifest as varied but often dominant controls on pore-water chemistry. Sulfur isotope results are presented that speak to secondary metal and sulfur enrichments observed in ash layers sampled during Leg 165. Ultimately, a better mechanistic understanding of these processes and the extent to which they have varied spatially and temporally may bear on the global mass balances for a range of major and minor dissolved components of seawater.