Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

Abstract

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 Riedinger et al.Non-steady State Subsurface Sulfur Cycling 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.

Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

“…Each core was extruded and subsampled immediately after collection, and pore waters were extracted from each section in a glove bag under N 2 atmosphere using CSS 5 cm Rhizon samplers attached to plastic syringes (Lyons and Berner, 1992;Dickens et al, 2007;Raiswell and Canfield, 2012;Riedinger et al, 2017). Each core segment yielded approximately 8 mL of pore waters, which were filtered at 0.2 µm, acidified with trace grade pure nitric acid (10 µL of acid per each 1 mL of sample), and stored at 4 • C (Riedinger et al, 2014(Riedinger et al, , 2017. Prior to analysis, solid-phase samples were dried, weighed, and homogenized.…”

Interplay of Subduction Tectonics, Sedimentation, and Carbon Cycling

“…Reactive iron oxide minerals, including magnetite, are thus altered into iron sulfide phase expressed in accumulation of 34 S-enriched pyrite (up to 1 wt.%). The net result is a decrease in magnetic susceptibility (Figure 3; e.g., Tarduno, 1994;Roberts et al, 1999;Riedinger et al, 2005Riedinger et al, , 2017März et al, 2008;Roberts, 2015).…”

“…We now know that not all pore water sulfate gradients are linear and that deviations from this steady state condition most often reflect recent changes in methane flux, which have been attributed to a gamut of processes including pulses in organic carbon accumulation, mass transport deposition, earthquake activity, and dynamics of deeper methane reservoirs (e.g., Egger et al, 2018;Hensen et al, 2003;Riedinger et al, 2005). We also know that AOM is accompanied by: (1) an increase in alkalinity that promotes precipitation of methane-derived authigenic carbonate (MDAC) and (2) production of H 2 S and associated sulfide mineral diagenesis that overprints the magnetic susceptibility of the sediment (e.g., Karlin & Levi, 1983;Tarduno, 1994;Roberts et al, 1999;Neretin et al, 2004;Riedinger et al, 2005Riedinger et al, , 2017März et al, 2008;Rowan et al, 2009). These minerals remain in the sediment record and offer an excellent opportunity to reconstruct the history and nature of fluid flow.…”

Vivianite formation in methane-rich deep-sea sediments from the South China Sea

“…9b). In similar systems, dissolved sulfide is limited below the S-front (Egger et al, 2016;Jørgensen et al, 2004;Riedinger et al, 2017). As vivianite is unstable in the presence of sulfide (Dijkstra et al, 2018a), its presence below ~920 cm depth supports the absence of appreciable amounts of dissolved sulfide.…”

“…As a consequence, any excess sulfide diffusing downwards from the SMTZ is being trapped as Fe monosulfides upon reaction with more readily available Fe-oxides at depth. This reaction zone, often referred to as the sulfidization front (S-front), is a common feature in Fe-rich marine sediments (Egger et al, 2016;Holmkvist et al, 2014;Jørgensen et al, 2004;Riedinger et al, 2017) and is recorded at Site 973-4 as a pronounced AVS peak at ~900cm 5 depth (Fig. 9b).…”