Sulfur isotope's signal of nanopyrites enclosed in 2.7 Ga stromatolitic organic remains reveal microbial sulfate reduction

Abstract

Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7-Gyr-old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ S values of about 84‰ (from -33.7‰ to +50.4‰). The recognition that a large δ S range of 80‰ is found in individual carbonaceous-rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF-S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…Analysis of fragments of hydrothermal pyrite and marcasite crystals, used as S‐isotopic standards for our other SIMS experiments, resulted in little inter‐ (Figures S1 and S2, supporting information) or intra‐fragment (Figures S3 and S4, supporting information) variability in the δ 34 S value (i.e., the standard deviation of measurements between or within fragments was always smaller than the average standard error associated with those inter‐ or intra‐grain measurements). Therefore, the method is suitable for the detection of the potentially large variations in δ 34 S values that may exist within or between sedimentary iron sulfide grains …”

“…In contrast, a bimodal pyrite δ 34 S distribution might indicate two distinct sulfide sources, e.g., from successive exposure to distinct sulfide‐bearing fluids. Pyrites from a single source may also have a range of δ 34 S values, representing their continuous formation in the presence of an evolving sulfide reservoir . In diffusively limited environments such as marine sediments, pore water sulfate can be drawn down by microbial sulfate reduction (MSR).…”

“…Progress has been made using spot analyses by secondary ion mass spectrometry (SIMS) or laser ablation (LA)‐MC‐ICP‐MS, to make spatially resolved δ 34 S measurements. Some have already attributed detectable intra‐sample pyrite δ 34 S variability to temporal changes in the S‐isotopic composition of the fluids from which the pyrites precipitated . The method described here, scanning ion imaging by SIMS, is designed to address several of the limitations of currently available methods for micro‐scale measurement of δ 34 S values in sedimentary pyrite.…”

“…Pyrites from a single source may also have a range of δ 34 S values, representing their continuous formation in the presence of an evolving sulfide reservoir. 23,24 In diffusively limited environments such as marine sediments, pore water sulfate can be drawn down by microbial sulfate reduction (MSR). Sulfate reducers have a strong preference for the lighter isotopes of S in sulfate, 25,26 which leads to increasingly 34 S-enriched sulfate and sulfide with depth in the sediment.…”

“…Some have already attributed detectable intra-sample pyrite δ 34 S variability to temporal changes in the S-isotopic composition of the fluids from which the pyrites precipitated. 24,28 The method described here, scanning ion imaging by SIMS, is designed to address several of the limitations of currently available methods for micro-scale measurement of δ 34 S values in sedimentary pyrite. First, by rastering over grains, scanning ion imaging can generate a continuous record of isotope variations, 29 one that can be interrogated at variable spatial resolution after data collection.…”