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 (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.

Description: Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] März et al (2008) GCA, 72, 3703-3717.

Description: The partitioning of Fe in sediments and soils has traditionally been studied by applying sequential leaching methods. These are based on reductive dissolution and exploit differences in dissolution rates between different reactive Fe (oxyhydr)oxide minerals. We used lab-made ferrihydrite, goethite, hematite and magnetite spiked with 58Fe and leached two-mineral mixtures with both phases abundant in excess of the methods dissolution capacity. Leaching was performed with 1) hydroxylamine-HCl and 2) Na-dithionite as the reactive agent. Following Poulton & Canfield (2005) [1], the first dissolution is designed to selectively leach the most reactive Fe-phases, ferrihydrite and lepidocrocite, whereas the second dissolution is designed to leach goethite and hematite. Magnetite would then be dissolved in a third dissolution step with oxalic acid. First results show that the hydroxylamine-HCl method for ferrihydrite dissolves only insignificant amounts of goethite and hematite. However, magnetite-Fe constitutes about 10% of the total dissolved Fe. The Na-dithionite dissolved Fe from goethite-magnetite and hematite-magnetite mixtures contain about 30% of magnetite-Fe. We applied selective sequential leaching and Fe isotope analysis to fine-grained marine sediments from a depocenter in the North Sea, which contain abundant reactive Fe (oxyhydr)oxides and show evidence for Fe sulfide formation within the upper 10 cm. Fe isotopes of the hydroxylamine-HCl leach targeting ferrihydrite shows a downcore increase of !56Fe typical for sediments undergoing microbial reductive Fe dissolution, whereas Fe isotopes of the Na-dithionite leach (goethite and hematite) and oxalic acid leach (magnetite) are identical and show no downcore variation in !56Fe. This means, that only the most reactive Fe phases participate in the Fe redox cycle in this location. The similar isotopic composition of goethite + hematite and magnetite suggests a detrital source, which is not utilized possibly due to the abundant ferrihydrite and lepidocrocite present. [1] Poulton & Canfield (2005), Chemical Geology 214, 209– 221

Description: The glacier melt of the Western Antarctic Peninsula and its surrounding islands influences biogeochemical processes in the water column and the marine sediment by changing the flux of mineral particles and nutrients (e.g. Fe) into the ocean. Sediment and pore water samples were collected at King George Island (South Shetland Islands) to unravel how the vicinity of ice-covered and -uncovered terrestrial environment affects redox zonation and diagenetic processes in the coastal sediments. The post-depositional dissolution of Fe-minerals and the stable Fe isotope signatures of pore water and specific Fe minerals were of special interest since changing Fe supplies - as reactive particles via melting icebergs or meltwater streams or dissolved via diffusion from the sediment into the bottom water - might not only impact local biogeochemical cycles but most likely also impact productivity in the Southern Ocean. Sediment cores of up to 45 cm length were retrieved in Potter Cove, Marian Cove, and Maxwell Bay. In vicinity to the glaciers the sediments showed an extended redox zonation. The post-oxic zone with Fe2+ concentrations of up to 300 μM ranged from 1 to 25 cm depth. Most probably, microbial activity in sediments close to the glaciers is sluggish due to low input of organic matter (OM). More condensed redox zones prevailed in troughs where OM from terrestrial or marine sources accumulates and in vicinity to research stations. The upward directed diffusive Fe2+ fluxes as inferred from pore water profiles range between 0 and ~1050 μM m-2 d-1. However, the correlation to the intensity of diagenesis is not straightforward. Fe isotopes of specific minerals were used to assess the intensity of Fe cycling. With ongoing Fe-oxide dissolution, the residual Fe pool becomes enriched in 56Fe, whereas dissolved Fe and secondary Fe-oxides become enriched in 54Fe. Thus, easily reducible Fe oxides show lowest !56Fe values at the top of the sediment column. We suggest that the retreat of the glaciers indirectly results in higher OM fluxes to shelf areas fueling diagenetic processes/nutrient recycling.