Early precipitated micropyrite in microbialites: A time capsule of microbial sulfur cycling

Abstract

Microbialites are organosedimentary rocks that have occurred throughout the Earth's history. The relationships between diverse microbial metabolic activities and isotopic signatures in biominerals forming within these microbialites are key to understanding modern biogeochemical cycles, but also for accurate interpretation of the geologic record. Here, we performed detailed mineralogical investigations coupled with NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry) analyses of pyrite S isotopes in mineralising microbial mats from two different environments, a hypersaline lagoon (Cayo Coco, Cuba) and a volcanic alkaline crater lake (Atexcac, Mexico). Both microbialite samples contain two distinct pyrite morphologies: framboids and euhedral micropyrites, which display distinct ranges of δ 34 S values 1 . Considering the sulfate-sulfur isotopic compositions associated with both environments, micropyrites display a remarkably narrow range of Δ pyr (i.e. Δ pyr ≡ δ 34 S SO 4 − δ 34 S pyr ) between 56 and 62 ‰. These measured Δ pyr values agree with sulfate-sulfide equilibrium fractionation, as observed in natural settings characterised by low microbial sulfate reduction respiration rates. Moreover, the distribution of S isotope compositions recorded in the studied micropyrites suggests that sulfide oxidation also occurred at the microbialite scale. These results highlight the potential of micropyrites to capture signatures of microbial sulfur cycling and show that S isotope composition in pyrites record primarily the local micro-environments induced by the microbialite.

Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios

“…Our results demonstrate that pyrite is a sensitive recorder of the diagenetic evolution of pore water sulfide in marine sediments, as suggested by previous studies ( 1 – 5 , 29 , 30 , 36 ). That the magnitude of intergrain isotopic variability in modern marine sedimentary pyrite populations can be so large (Fig.…”

“…3A). This calculated e mic approaches the expected equilibrium fractionation between sulfate and sulfide at the temperature of the studied sediments (35), which is much larger than reconstructions of e mic that are based on prior bulk sampling work ( 14) but consistent with SIMS-based findings from other localities (36). Despite the profound environmental changes across glacial-interglacial regimes, nearly constant e mic values indicate no substantive change in the activity of sulfatereducing microbes.…”

Evaluating the multiple sulfur isotope signature of Eoarchean rocks from the Isua Supracrustal Belt (Southwest‐Greenland) by MC‐ICP‐MS: Volcanic nutrient sources for early life

“…While some initial studies using this method reported sample size requirements of 100 nmol to 5 μmol sulfur (Ono et al., 2006; Paris, Fehrenbacher, et al., 2014) more recent workers have been able to push this limit down to 20 nmol (Au Yang et al., 2016). Alternatively, an ion microprobe can be used for measuring sulfur isotopes of geological samples in situ (Mojzsis et al., 2003; Whitehouse et al., 2005), and this technique has even been optimized for finely disseminated sulfides in S‐poor rock samples (Bryant et al., 2019; Marin‐Carbonne et al., 2022). These in situ techniques are vital for understanding multiple generations of sulfides that can exist in Archean rocks, and for distinguishing fluid‐rock interactions and diagenetic and recrystallization histories (Mojzsis et al., 2003; Whitehouse et al., 2005).…”