Sulfur isotope systematics of a euxinic, low-sulfate lake: Evaluating the importance of the reservoir effect in modern and ancient oceans

Abstract

The sulfur (S) isotope difference between sedimentary sulfate and sulfi de phases preserved in sedimentary rocks (Δ 34 S) has been utilized to reconstruct marine sulfate concentrations and inferentially the redox evolution of Earth's surface. These interpretations are largely based on experimental studies that indicate that microbial sulfate reduction is accompanied by a substantial kinetic isotope effect (up to 66‰), but only at sulfate concentrations >~200 µM. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (~100-350 μM) and fi nd that the calculated kinetic isotope effect associated with microbial sulfate reduction (ε 34 S) is relatively large (~23.5‰), but preserved Δ 34 S values are considerably smaller (4.7‰-9.9‰). Δ 34 S values in this system are controlled by the fraction of the sulfate reservoir that is consumed during sulfate reduction and the location of pyrite formation. This reservoir effect strongly infl uences the S isotope composition of sulfi de preserved in the rock record such that Δ 34 S values increase as a function of sulfate levels, even when sulfate concentrations are >200 µM and the kinetic isotope effect is expressed. These fi ndings have important implications for reconstructing the chemical evolution of the ocean-atmosphere system throughout Earth history-not just for the Precambrian.

Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records

“…(<10&) in Archean sedimentary pyrites was due to the suppression of biologically imparted S isotope fractionation at low sulfate levels (>200 lM; Habicht et al, 2002), subsequent work has shown that MSR can impart substantial S isotope fractionations even at very low sulfate concentrations (Nakagawa et al, 2012;Gomes and Hurtgen, 2013;Crowe et al, 2014). Yet, it is still likely that Archean sulfate levels were low.…”

“…The mean e SR value at Lake Matano was determined by a reactive-diffusion model (Crowe et al, 2014). For Lake McCarrons (Gomes and Hurtgen, 2013) and Lake Fukami-ike (Nakagawa et al, 2012), a Rayleigh fractionation model was used to calculate e SR (after Mariotti et al, 1981). There was not sufficient water column d 34 S data to calculate e SR for Effingham Inlet and Tyro Basin.…”

“…1;Gomes and Hurtgen, 2013). That is, when the size of the sulfate reservoir is small, it is more likely that a significant fraction of the sulfate reservoir will be consumed by MSR such that d 34 S of the pooled product, sulfide, will evolve to values that are close to the initial d 34 S of the sulfate reservoir.…”

“…Thus, D 34 S values do not always equal the in situ S isotope fractionation due to MSR (i.e., e SR ) because d 34 S of sulfate reflects the S isotope composition surface water sulfate and d 34 S of pyrite captures the averaged S isotope composition of sulfide deep in the water column and/or sediment. In low sulfate systems, this S isotope difference, D 34 S, is typically lower than e SR (e.g., Gomes and Hurtgen, 2013) because the size of the sulfate reservoir and transport dynamics directly affect the magnitude of D 34 S. It is possible to take advantage of this reservoir effect when MSR occurs in a partially closed system to make interpretations about past sulfate levels ( Fig. 1).…”

Sulfur Isotope Effects of Dissimilatory Sulfite Reductase

“…In laboratory experiments and natural marine and lacustrine systems, volumetric sulfate reduction rates scale primarily as a function of the availability of sulfate relative to common electron donors like organic carbon Goldhaber and Kaplan, 1975;Sim et al, 2011b;Leavitt et al, 2013). Indeed, sulfate can be non-limiting even in environments with as little as µM sulfate (Nakagawa et al, 2012;Gomes and Hurtgen, 2013;Crowe et al, 2014;Bradley et al, 2015), assuming organic matter is more limiting to allow a fractionation to occur (Wing and Halevy, 2014;Bradley et al, 2015). Constrained whole cell (in vivo) laboratory experiments demonstrate that when electron donors are limiting, the magnitude of fractionation between sulfate and sulfide ( 34 ε) carries a nonlinear inverse relationship with cell-specific sulfate reduction rates (Harrison and Thode, 1958;Kaplan and Rittenberg, 1964;Sim et al, 2011b;Leavitt et al, 2013).…”