Oceanic and Sedimentary Microbial Sulfur Cycling Controlled by Local Organic Matter Flux During the Ediacaran Shuram Excursion in the Three Gorges Area, South China

Abstract

The increased difference in the sulfur isotopic compositions of sedimentary sulfate (carbonate-associated sulfate: CAS) and sulfide (chromium-reducible sulfur: CRS) during the Ediacaran Shuram excursion is attributed to increased oceanic sulfate concentration in association with the oxidation of the global ocean and atmosphere. However, recent studies on the isotopic composition of pyrites have revealed that CRS in sediments has diverse origins of pyrites. These pyrites are formed either in the water column/shallow sediments, where the system is open with respect to sulfate, or in deep sediments, where the system is closed with respect to sulfate. The δ³⁴S value of sulfate in the open system is equal to that of seawater; on the contrary, the δ³⁴S value of sulfate in the closed system is higher than that of seawater. Therefore, obtaining the isotopic composition of pyrites formed in an open system, which most likely retain microbial sulfur isotope fractionation, is essential to reconstruct the paleo-oceanic sulfur cycle. In this study, we carried out multiple sulfur isotope analyses of CRS and mechanically separated pyrite grains (>100 μm) using a fluorination method, in addition to secondary ion mass spectrometry (SIMS) analyses of in situ δ³⁴S values of pyrite grains in drill core samples of Member 3 of the Ediacaran Doushantuo Formation in the Three Gorges area, South China. The isotope fractionation of microbial sulfate reduction (MSR) in the limestone layers of the upper part of Member 3 was calculated to be ³⁴ε = 55.7‰ and ³³λ = 0.5129 from the δ³⁴S and Δ³³S' values of medium-sized pyrite grains ranging from 100 to 300 μm and the average δ³⁴S and Δ³³S' values of CAS. Model calculations revealed that the influence of sulfur disproportionation on the δ³⁴S values of these medium-sized pyrite grains was insignificant. In contrast, within the dolostone layers of the middle part of Member 3, isotope fractionation was determined to be ³⁴ε = 47.5‰. The ³⁴ε value in the middle part of Member 3 was calculated from the average δ³⁴S values of the rim of medium-sized pyrite grains and the average δ³⁴S values of CAS. This observation revealed an increase in microbial sulfur isotope fractionation during the Shuram excursion at the drill core site. Furthermore, our investigation revealed correlations between δ³⁴S_CRS values and CRS concentrations and between CRS and TOC concentrations, implying that organic matter load to sediments controlled the δ³⁴S_CRS values rather than oceanic sulfate concentrations. However, these CRS and TOC concentrations are local parameters that can change only at the kilometer scale with local redox conditions and the intensity of primary production. Therefore, the decreasing δ³⁴S_CRS values likely resulted from local redox conditions and not from a global increase in the oceanic sulfate concentration.