Influence of sulfate reduction rates on the Phanerozoic sulfur isotope record

Abstract

Phanerozoic levels of atmospheric oxygen relate to the burial histories of organic carbon and pyrite sulfur. The sulfur cycle remains poorly constrained, however, leading to concomitant uncertainties in O 2 budgets. Here we present experiments linking the magnitude of fractionations of the multiple sulfur isotopes to the rate of microbial sulfate reduction. The data demonstrate that such fractionations are controlled by the availability of electron donor (organic matter), rather than by the concentration of electron acceptor (sulfate), an environmental constraint that varies among sedimentary burial environments. By coupling these results with a sediment biogeochemical model of pyrite burial, we find a strong relationship between observed sulfur isotope fractionations over the last 200 Ma and the areal extent of shallow seafloor environments. We interpret this as a global dependency of the rate of microbial sulfate reduction on the availability of organic-rich sea-floor settings. However, fractionation during the early/mid-Paleozoic fails to correlate with shelf area. We suggest that this decoupling reflects a shallower paleoredox boundary, primarily confined to the water column in the early Phanerozoic. The transition between these two states begins during the Carboniferous and concludes approximately around the Triassic-Jurassic boundary, indicating a prolonged response to a Carboniferous rise in O 2 . Together, these results lay the foundation for decoupling changes in sulfate reduction rates from the global average record of pyrite burial, highlighting how the local nature of sedimentary processes affects global records. This distinction greatly refines our understanding of the S cycle and its relationship to the history of atmospheric oxygen.Phanerozoic oxygen | sulfate-reducing bacteria T he marine sedimentary sulfur isotope record encodes information on the chemical and biological composition of Earth's ancient oceans and atmosphere (1, 2). However, our interpretation of the isotopic composition of S-bearing minerals is only as robust as our understanding of the mechanisms that impart a fractionation. Fortunately, decades of research identify microbial sulfate reduction (MSR) as the key catalyst of the marine S cycle, both setting the S cycle in motion and dominating the massdependent fractionation preserved within the geological record (1, 3, 4). Despite the large range of S-isotope variability observed in biological studies (4-6), attempts to calibrate the fractionations associated with MSR are less mechanistically definitive (7, 8) than analogous processes influencing the carbon cycle (9, 10). What is required is a means to predict S isotope signatures as a function of the physiological response to environmental conditions (e.g., reduction-oxidation potential).Microbial sulfate reduction couples the oxidation of organic matter or molecular hydrogen to the production of sulfide, setting in motion a cascade of reactions that come to define the biogeochemical S cycle. In modern marine sediments, sulfide i...

Pleistocene sediment offloading and the global sulfur cycle

“…This yields δ 34 S pyrite value of −18 ‰, which is in agreement with previous estimates (Strauss, 1997;Seal, 2006;Leavitt et al, 2013). This implies the average offset ( pyr ) between δ 34 S seawater and δ 34 S pyrite of −40 ‰ (VCDT) which is similar to the Cenozoic average (Leavitt et al, 2013).…”

“…This gives pyr of −50 ‰ for the most severe glaciations (lowest shelf extent) and 35 ‰ for the maximum shelf inundation, which is in line with the Leavitt et al (2013) pyr range of estimates for the past 200 Myr. To test the impact of these temporal variations of pyr , we also run our model with "fixed" pyr of −40 ‰ (our steady state value).…”

“…This implies the average offset ( pyr ) between δ 34 S seawater and δ 34 S pyrite of −40 ‰ (VCDT) which is similar to the Cenozoic average (Leavitt et al, 2013).…”

“…The sulfur isotope fractionation is negatively correlated with sedimentation rate (e.g., Goldhaber and Kaplan, 1975), and the recent work by Leavitt et al (2013) has identified a negative correlation between δ 34 S pyrite value and shelf area. Since the shelf area has changed dramatically across glacial-interglacial cycles, it is likely that the offset ( pyr ) between δ 34 S seawater and δ 34 S pyrite values has also changed.…”

Rapid organic matter sulfurization in sinking particles from the Cariaco Basin water column

“…Polysulfides are in rapid isotopic equilibrium with S 0 and sulfide (Rickard and Luther, 2007;Amrani and Aizenshtat, 2004b), and a primary control on their δ 34 S value is the isotopic fractionation associated with dissimilatory sulfate reduction. The scale of this fractionation varies with sulfate reduction rate and substrate availability, with faster metabolic rates and higher energetic yields generally producing smaller fractionations (e.g., Sim et al, 2011a;Leavitt et al, 2013). Given abundant sulfate in the water column, sulfate reduction rate should be primarily controlled by OM lability, which decreases with water depth.…”