SQUID–SIMS is a useful approach to uncover primary signals in the Archean sulfur cycle

Abstract

Many aspects of Earth's early sulfur cycle, from the origin of massanomalous fractionations to the degree of biological participation, remain poorly understood-in part due to complications from postdepositional diagenetic and metamorphic processes. Using a combination of scanning high-resolution magnetic superconducting quantum interference device (SQUID) microscopy and secondary ion mass spectrometry (SIMS) of sulfur isotopes ( 32 S, 33 S, and 34 S), we examined drill core samples from slope and basinal environments adjacent to a major Late Archean (∼2.6-2.5 Ga) marine carbonate platform from South Africa. Coupled with petrography, these techniques can untangle the complex history of mineralization in samples containing diverse sulfur-bearing phases. We focused on pyrite nodules, precipitated in shallow sediments. These textures record systematic spatial differences in both mass-dependent and mass-anomalous sulfur-isotopic composition over length scales of even a few hundred microns. Petrography and magnetic imaging demonstrate that mass-anomalous fractionations were acquired before burial and compaction, but also show evidence of postdepositional alteration 500 million y after deposition. Using magnetic imaging to screen for primary phases, we observed large spatial gradients in Δ 33 S (>4‰) in nodules, pointing to substantial environmental heterogeneity and dynamic mixing of sulfur pools on geologically rapid timescales. In other nodules, large systematic radial δ 34 S gradients (>20‰) were observed, from low values near their centers increasing to high values near their rims. These fractionations support hypotheses that microbial sulfate reduction was an important metabolism in organic-rich Archean environments-even in an Archean ocean basin dominated by iron chemistry. mass independent fractionation | MIF | metamorphism | metasomatism P atterns emerging from multiple sulfur-isotope ratios in sedimentary rocks have provided powerful insights into the behavior of the Archean (>2.5 Ga) sulfur cycle and the timing of the rise of atmospheric oxygen (1). Sulfur-isotope data from Archean sedimentary successions show distinct differences from those contained in younger successions-in rocks older than ∼2.32 Ga, S-bearing minerals display large deviations in isotope ratios from predicted mass-fractionation relationships, captured by the statistic (1,4,5). If this interpretation is correct, photochemical and geochemical models can use stratigraphic sulfur-isotope data to provide quantitative constraints on the oxidative power of the Archean atmosphere (e.g., O 2 mixing ratio << 10 −5 ; ref. 6), continental crust growth, volatile fluxes from the solid earth (7), seawater chemistry, and the degree of biological cycling (8).In addition to the nature of the MAF that is so conspicuous in Archean strata, questions remain about what role, if any, biology played in the early sulfur cycle. Comparative molecular biology suggests that both sulfide-based photosynthesis and dissimilatory sulfate reduction (DSR) were important ear...

Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

“…Such zones may have two isotopically distinct populations of pyrite and TOS, corresponding to the upper and lower SMT fringe. Techniques such as sulfur isotope analyses using SIMS have the potential to reveal such variations and clustering of isotope signatures that remain undetected in a bulk sample (Xiao et al, 2010;Bontognali et al, 2012;Farquhar et al, 2013;Fischer et al, 2014).…”

Timescales of Oxygenation Following the Evolution of Oxygenic Photosynthesis

“…Alternatively, these signals could largely be the result of alteration of sedimentary strata by postdepositional fluids. While chemical imaging approaches exist for evaluating the impacts of post-depositional alteration (e.g., Johnson et al 2013a;Fischer et al 2014;Stefurak et al 2015), a critical issue going forward will be constructing analytical frameworks that are able to carefully consider and avoid false positive detections of oxygenic Cyanobacteria in the early sedimentary record, particularly as geochemical techniques for O 2 detection continue to become more sophisticated and more sensitive.…”

Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration

“…Near thermodynamic fractionations appear to be accessible at extremely low sulfate levels (<10 μM), as long as the average respiration rate of a sulfate-reducing population is low enough (79). This feature may neatly unite two conflicting views of S cycling on the Archean earth: large intrasample variability in δ 34 S values (80,81) in the face of low marine sulfate concentrations (6,82).…”