Strong local, not global, controls on marine pyrite sulfur isotopes

Abstract

Understanding variation in the sulfur isotopic composition of sedimentary pyrite (δ34Spyr) is motivated by the key role of sulfur biogeochemistry in regulating Earth’s surface oxidation state. Until recently, the impact of local depositional conditions on δ34Spyr has remained underappreciated, and stratigraphic variations in δ34Spyr were interpreted mostly to reflect global changes in biogeochemical cycling. We present two coeval δ34Spyr records from shelf and basin settings in a single sedimentary system. Despite their proximity and contemporaneous deposition, these two records preserve radically different geochemical signals. Swings of ~65‰ in shelf δ34Spyr track short-term variations in local sedimentation and are completely absent from the abyssal record. In contrast, a long-term ~30‰ decrease in abyssal δ34Spyr reflects regional changes in ocean circulation and/or sustained pyrite formation. These results highlight strong local controls on δ34Spyr, calling for reevaluation of the current practice of using δ34Spyr stratigraphic variations to infer global changes in Earth’s surface environment.

Geochemical records reveal protracted and differential marine redox change associated with Late Ordovician climate and mass extinctions

“…Important output fluxes for sulfur are the burial of sulfate-evaporites (0.83 × 10 18 mol/ Myr), which has a minor isotopic fractionation, and sedimentary pyrite with a flux of 0.67 × 10 18 mol/Myr (Burke et al, 2018). Pyrite formation via microbial sulfate reduction (MSR) records up to a −70‰ sulfur isotope fractionation between sulfate and the product sulfide, (approximated by Δ 34 S; Δ 34 S = δ 34 S SO4 -δ 34 S H2S ) and thus leaves residual sulfate isotopically heavier, compared to isotopically light sulfide (Lang et al, 2020;Pasquier et al, 2021). This process occurs in anaerobic environments and is dependent on the availability of labile organic matter, reactive iron, and sulfate (Gomes & Hurtgen, 2015;Sim, 2019).…”

“…Since marine sulfate throughout the Phanerozoic had a significantly longer residence (10 5 -10 7 yrs) time than inter-ocean mixing timescales (10 3 yrs) and thus is homogenous throughout ocean basins, δ 34 S CAS values are generally representative of the global seawater reservoir. Pyrite sulfur (δ 34 S pyr ) isotopes, in contrast, are best used as a local proxy for MSR activity and the associated factors that control the magnitude of fractionation, such as rates of sulfate reduction, iron availability for pyrite formation, and interplays between open and closed system dynamics (Lang et al, 2020;Pasquier et al, 2021).…”

“…Within the Monitor Range section, most of the highest I/(Ca + Mg) values occur alongside the heaviest δ 34 S CAS values, and the lowest I/(Ca + Mg) values occur with the most negative δ 34 S CAS values (Figure 2). Other studies have suggested that sedimentation rates may be the most important factor for modulating MSR and subsequent control on fractionation factors between porewater sulfate and sulfide due to the increased disconnection between porewater and water column sulfate (Pasquier et al, 2021). Sea-level reconstructions and previous biostratigraphic studies of the Monitor Range section indicate that sedimentation rates were mostly stable throughout the late Katian at this locality, which would limit the extent of fluctuations in sulfur isotope fractionation between sulfate and sulfide due to a more constant diffusive length between porewaters and water column sulfate.…”

The Formation of Highly Positive δ34S Values in Late Devonian Mudstones: Microscale Analysis of Pyrite (δ34S) and Barite (δ34S, δ18O) in the Canol Formation (Selwyn Basin, Canada)

“…More recently, studies have shown how sulfur isotope variability may instead be controlled by sedimentary facies and diagenetic processes (Magnall et al, 2016;Pasquier et al, 2017;Marin-Carbonne et al, 2018;Bryant et al, 2019;Richardson et al, 2019;Bryant et al, 2020;Pasquier et al, 2021). For example, the progressive modification of pore fluid sulfate by MSR during diagenesis can result in strong isotopic gradients and a range of δ 34 S pyrite values, depending on the location of pyrite formation in the sediment (Canfield, 2001b;Canfield et al, 2010).…”

Geochemical Records Reveal Protracted and Differential Marine Redox Change Associated With Late Ordovician Climate and Mass Extinctions

“…Important output fluxes for sulfur are the burial of sulfate‐evaporites (0.83 × 10 18 mol/Myr), which has a minor isotopic fractionation, and sedimentary pyrite with a flux of 0.67 × 10 18 mol/Myr (Burke et al., 2018). Pyrite formation via microbial sulfate reduction (MSR) records up to a −70‰ sulfur isotope fractionation between sulfate and the product sulfide, (approximated by Δ 34 S; Δ 34 S = δ 34 S SO4 – δ 34 S H2S ) and thus leaves residual sulfate isotopically heavier, compared to isotopically light sulfide (Lang et al., 2020; Pasquier et al., 2021). This process occurs in anaerobic environments and is dependent on the availability of labile organic matter, reactive iron, and sulfate (Gomes & Hurtgen, 2015; Sim, 2019).…”