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

Abstract

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.

The effects of organic matter and anaerobic oxidation of methane on the microbial sulfate reduction in cold seeps

“…This reaction forms H 2 S with lower 34 S isotope values, resulting in the residual sulfate with higher 34 S isotope values (Canfield, 2001;Böttcher et al, 2006;Deusner et al, 2014). In addition, previous studies also noted a relationship between the magnitude of the sulfur isotope fractionation and the sulfate reduction rate (Aharon and Fu, 2000;Stam et al, 2011;Gilad et al, 2013;Bradbury et al, 2021;Chen et al, 2022). In all these studies, higher sulfur isotope fractionation corresponded to slower sulfate reduction rates.…”

Assessing Sedimentary Boundary Layer Calcium Carbonate Precipitation and Dissolution Using the Calcium Isotopic Composition of Pore Fluids

“…To explore how the balance of carbonate mineral precipitation and recrystallization influences changes in calcium concentrations and pore fluid δ 44 Ca, we use the reactive transport modeling (RTM) software, CrunchTope for AQ700 (Druhan et al, 2013;Druhan et al, 2014;Steefel et al, 2015;Fantle and Ridgwell, 2020). The RTM is set up similar to previous models of sulfate reduction, carbonate mineral recrystallization, and precipitation in marine sediments (Steefel et al, 2014;Huber et al, 2017;Fantle and Ridgwell, 2020;Bradbury et al, 2021). The complete modeling methods for the RTM, including the governing equations, are derived from the work of Bradbury et al (2021).…”

“…The RTM is set up similar to previous models of sulfate reduction, carbonate mineral recrystallization, and precipitation in marine sediments (Steefel et al, 2014;Huber et al, 2017;Fantle and Ridgwell, 2020;Bradbury et al, 2021). The complete modeling methods for the RTM, including the governing equations, are derived from the work of Bradbury et al (2021). We use CrunchTope to simulate advection, diffusion, and reaction over a 10 m pore fluid column with a coexisting solid phase, a seawater Dirichlet upper boundary condition, and a Neumann or dC/dx 0 lower boundary condition.…”

“…The system starts from the initial conditions listed in Supplementary Table S1 and is allowed to run until the fluid concentrations and isotopic composition are no longer changing with time, which is assumed to be the steady state. Microbial sulfate reduction is modeled with formaldehyde (CH 2 O) representing the bulk composition of organic matter (Meister, 2013;Meister, 2014;Meister et al, 2019) using a dual Monod rate law as detailed in Bradbury et al (2021). The sulfate δ 34 S profile is fit with a sulfur isotope fractionation factor of α 0.9955, which is common in deep sea sediments.…”

Changes in North Atlantic deep-water oxygenation across the Middle Pleistocene Transition

“…Low organic carbon concentrations (<1%) at Site U1385 leads to relatively low rates of organic carbon oxidation and a deep sulfate-methane transition at ~50 meters below the seafloor (mbsf ) (27). The Iberian Margin near-surface sediment column demonstrates low rates of sulfate reduction in the upper tens of centimeters (28), suggesting that Δδ 13 C at Site U1385 is governed predominantly by the amount of aerobic respiration controlled by deep-water [O2]. Following recommendations of Jacobel et al (26), we compare the paleo-[O2] estimates derived from δ 13 Ccib-aff with other redox/oxygenation proxies: U/Ca, U/Mn and C26OH/(C26OH + C29).…”