Interpreting multiple sulfur isotope signals in modern anoxic sediments using a full diagenetic model (California-Mexico margin: Alfonso Basin)

Abstract

Recent studies targeting the metabolic, physiological, and biochemical controls of sulfur isotope fractionation in microbial systems have drawn linkages between results from culture experiments and the sulfur isotope signatures observed in natural environments. Several of those studies have used newer techniques to explore the minor isotope (³³S and ³⁶S) variability in those systems, and also have attempted to place them in an ecophysiological context. Sparingly few have incorporated this newfound understanding of minor isotope behavior into natural systems (sediment pore waters, water columns) and none of them have refined existing isotope-dependent reaction-transport models to explicitly include ³³S. In this study, we construct a three-isotope (³²S, ³³S, and ³⁴S) reaction-transport model of pore water sulfate for a well-characterized sedimentary system within the California-Mexico Margin (Alfonso Basin). An additional goal is placing recent laboratory culture work into a natural, physical context. The model first reproduces the measured bulk geochemical characteristics of the pore water profiles of [SO₄²⁻], [CH₄], dissolved inorganic carbon ([DIC]), and [Ca²⁺]—and predicts bulk (non-isotope-specific but depth-dependent) rates of sulfate reduction. Next, the model uses those depth-dependent bulk rates, in combination with empirically calibrated fractionation factors, to explain the minor isotope characteristics (δ³⁴S and Δ³³S values) of the 0 to 40 cm pore water SO₄²⁻. The down core, isotopic evolution of pore water sulfate requires a large fractionation associated with sulfate reduction (³⁴ε_SR = 70 ± 5‰) that appears to be independent of bulk rate, but in line with low temperature thermodynamic predictions. The minor isotope characteristics (³³λ_SR ∼ 0.5130) are also independent of rate and fall within the range expected from microbial calibrations, but differ from minor isotope predictions of thermodynamic equilibrium. The high value of ³⁴ε_SR raises key questions in relating the physiological state of marine microorganisms relative to their laboratory counterparts, as well as point toward exceedingly low metabolic rates in natural marine sediments.