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

David A Hodell,Jennifer L Druhan,Gilad Antler,Harold J Bradbury,Adam Bateson,Mervyn Greaves,Alexandra V Turchyn,Angus Fotherby,Duygu S Sevilgen

doi:10.3389/feart.2021.652960

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.

Highlights

Organic carbon oxidation in marine sediments is a key process in the global carbon and oxygen cycles as it mitigates the burial of reduced forms of carbon (Froelich et al, 1979; Berner, 1989; Kump and Arthur, 1999; Diester-Haass et al, 2009; Kump et al, 2011)
We propose that the cross plot of δ13CDIC vs. δ34SSO4 holds significant information about the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins
The major controls on the initial decrease in δ13CDIC relative to δ34S are related to the early microbial diagenetic reactions as well as to the amount and δ13C of seawater DIC, which can be visualized in the upper left

Summary

Introduction

Organic carbon oxidation in marine sediments is a key process in the global carbon and oxygen cycles as it mitigates the burial of reduced forms of carbon (Froelich et al, 1979; Berner, 1989; Kump and Arthur, 1999; Diester-Haass et al, 2009; Kump et al, 2011). In the absence of oxygen, anaerobic remineralization of organic carbon continues for many hundreds of meters below the sediment-water interface provided there are suitable alternative electron acceptors present (Froelich et al, 1979; Kasten et al, 2003). This anaerobic remineralization of organic carbon is important because when these electron acceptors are depleted, any remaining organicallyderived material may be converted into methane through methanogenesis (Claypool and Kaplan, 1974; Whiticar and Faber, 1986; Whiticar, 1999; Sivan et al, 2007). Exploring the controls on the remineralization of organic carbon is vital to resolving the carbon budget in dynamic sedimentary systems along continental margins

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