Modelling the Effects of Non-Steady State Transport Dynamics on the Sulfur and Oxygen Isotope Composition of Sulfate in Sedimentary Pore Fluids

Angus Fotherby,Gilad Antler,Alexandra V Turchyn,Jennifer L Druhan,Harold J Bradbury,Xiaole Sun

doi:10.3389/feart.2020.587085

Abstract

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters.

Highlights

Microbial sulfate reduction (MSR), where sulfate is respired in the absence of oxygen by microbial communities, is a key reaction in the global biogeochemical sulfur cycle and is understood to have been an important microbial metabolism over the course of Earth’s history (Jørgensen, 1982; Garrels and Lerman, 1984; Kasten and Jørgensen, 2000)
We have presented the results of a 1D isotope-enabled reactive transport model to demonstrate the effects of transport and nonsteady state behavior on the sulfur and oxygen isotopic compositions in sediments undergoing MSR
In particular we explored the effects of these phenomena on the δ34S and δ18O cross plot for extracellular sulfate and assessed its utility under conditions of variable transport and non-steady state dynamics

Summary

Introduction

Microbial sulfate reduction (MSR), where sulfate is respired in the absence of oxygen by microbial communities, is a key reaction in the global biogeochemical sulfur cycle and is understood to have been an important microbial metabolism over the course of Earth’s history (Jørgensen, 1982; Garrels and Lerman, 1984; Kasten and Jørgensen, 2000). Microbial communities performing MSR oxidize a significant fraction of organic carbon in modern marine sedimentary environments; estimated to be between 30 and 50% of all organic carbon deposited on the sea floor (Bowles et al, 2014; Egger et al, 2018). These same communities oxidize most methane produced in sediment through the Modelling Isotope Composition During MSR anaerobic oxidation of methane (AOM) (Niewöhner et al, 1998; Reeburgh, 2007; Egger et al, 2018). There has been decades of research into understanding the various controls on MSR, including how changes in environmental conditions influence the overall rate of MSR and the sulfur and oxygen isotope ratio fractionation during sulfate consumption. Looking for geochemical tools that may elucidate MSR metabolism has been a goal of the community for decades (Rees, 1973; Canfield, 2001; Farquhar et al, 2003; Bruchert, 2004; Brunner et al, 2005; Bradley et al, 2011; Leavitt et al, 2013; Wing and Halevy, 2014; Leavitt et al, 2015; Bradley et al, 2016)

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