A Critical Look at the Combined Use of Sulfur and Oxygen Isotopes to Study Microbial Metabolisms in Methane-Rich Environments

Abstract

Separating the contributions of anaerobic oxidation of methane and organoclastic sulfate reduction in the overall sedimentary sulfur cycle of marine sediments has benefited from advances in isotope biogeochemistry. Particularly, the coupling of sulfur and oxygen isotopes measured in the residual sulfate pool (δ18OSO4 vs. δ34SSO4). Yet, some important questions remain. Recent works have observed patterns that are inconsistent with previous interpretations. We differentiate the contributions of oxygen and sulfur isotopes to separating the anaerobic oxidation of methane and organoclastic sulfate reduction into three phases; first evidence from conventional high methane vs. low methane sites suggests a clear relationship between oxygen and sulfur isotopes in porewater and the metabolic process taking place. Second, evidence from pure cultures and organic matter rich sites with low levels of methane suggest the signatures of both processes overlap and cannot be differentiated. Third, we take a critical look at the use of oxygen and sulfur isotopes to differentiate metabolic processes (anaerobic oxidation of methane vs. organoclastic sulfate reduction). We identify that it is essential to develop a better understanding of the oxygen kinetic isotope effect, the degree of isotope exchange with sulfur intermediates as well as establishing their relationships with the cell-specific metabolic rates if we are to develop this proxy into a reliable tool to study the sulfur cycle in marine sediments and the geological record.

The Sedimentary Carbon-Sulfur-Iron Interplay – A Lesson From East Anglian Salt Marsh Sediments

“…From the trajectories in the δ 18 O SO4 vs. δ 34 S SO4 space we deduce that in the iron-rich sediment, a high percentage of the sulfate that is reduced is ultimately re-oxidized ("sulfate recycling"-blue line, Figure 1; Böttcher et al, , 2005Aller et al, 2010;Riedinger et al, 2010;Mills et al, 2016). The δ 18 O SO4 vs. δ 34 S SO4 trajectory in the sulfiderich sediment in turn indicates high rates of sulfate reduction with only a negligible portion of sulfate reoxidized ("net sulfate reduction" -red line, Figure 1; Antler et al, 2014Antler et al, , 2015Antler and Pellerin, 2018).…”

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

“…The measurement of the sulfur and oxygen isotope compositions of sulfate (δ 34 S and δ 18 O, measured as the ratio of the heavy to light isotope and reported in "permil" units relative to international standards; VCDT and VSMOW, respectively) has been a powerful tool for exploring the reversibility of the various intracellular enzymes, or "mechanism" of MSR both in the laboratory and the in the natural environment (e.g., Böttcher et al, 1998;Böttcher et al, 1999;Böttcher et al, 2000;Brunner et al, 2005;Knöller et al, 2006;Brunner et al, 2012;Antler et al, 2017;Antler and Pellerin, 2018). Each step in MSR partitions the sulfur and oxygen isotopes of the sulfate and intermediate valence state sulfur species, preferentially taking up the light isotopes ( 32 S and 16 O) over the heavier isotopes ( 34 S and 18 O-note we are ignoring the rarer, heavier, stable isotopes of sulfur and oxygen 36 S, 33 S, and 17 O, although past work has investigated the information provided by minor isotopes during MSR (Farquhar et al, 2003;Johnston et al, 2007;Pellerin et al, 2015).…”

“…The converse is true when the SALP is high. Thus, the SALP has been be used to explore the influence of various environmental conditions on the overall mechanism of MSR (Böttcher et al, 1998;Böttcher et al, 1999;Aharon and Fu, 2000;Böttcher et al, 2000;Aharon and Fu, 2003;Antler et al, 2013;Mills et al, 2016;Gomes and Johnston, 2017;Antler and Pellerin, 2018). The SALP has been shown to be related to the rate of sulfate reduction, the kinetic sulfur and oxygen isotope fractionation factors associated with the uptake of sulfate in the cell, and the degree of reversibility of the intracellular steps involved in MSR (Brunner et al, 2005;Wortmann et al, 2007;Brunner et al, 2012;Antler et al, 2013;Antler et al, 2017); all of these parameters are influenced by environmental conditions including the amount of sulfate, the type and availability of the electron donor and the temperature/pressure.…”

The benthic foraminiferal δ34S records flux and timing of paleo methane emissions

“…The validity of the sulphate δ 18 O-δ 34 S slope to investigate microbial metabolism in sediments has recently been questioned (Antler and Pellerin 48 ), but it can still provide important information regarding the biochemistry of sediments when used in a site-specific approach and in combination with other proxies (Antler and Pellerin 48 ). For instance, our data demonstrate the possibility to reconstruct the migration of the SMTZ and infer the flux (i.e., diffusive vs. advective) of paleo CH 4 emissions combining the benthic foraminiferal δ 13 C record with the slope in δ 18 O-δ 34 S. In geological records, the possibility to distinguishing between CH 4 advection and diffusion (or degradation of organic matter) can provide new insight regarding the impact of marine CH 4 to the carbon cycle (cf.…”