Microscale C-O-S isotope signatures record microbial sulfate reduction in ancient deep subsurface pyrite–calcite–barite assemblages
- 1Friedrich-Alexander-Universität Erlangen-Nürnberg (Erlangen, Germany)
- 2Linnaeus University (Vaxjo, Sweden)
Abstract
The deep biosphere harbors diverse microbial communities amounting to a substantial fraction of Earth's biomass for much of its history. Analyses of fracture-hosted minerals (e.g., pyrite, calcite) and their C-O-S isotope signatures are beginning to establish a fossil record of subsurface microbial communities and their relationship with major geological events. Fracture-coating pyrite from deep within the Fennoscandian Shield records the largest ³⁴ S-enrichment observed on Earth to date ( δ ³⁴ S = +147 ‰) – likely the result of late-stage Rayleigh distillation during closed-system microbial sulfate reduction (MSR). This implies even heavier S isotope values for the complementary sulfate reservoir during pyrite formation, possibly recorded in coeval sulfate minerals such as barite [BaSO ₄ ]. However, barite has been poorly explored as an archive of ancient deep subsurface biosignatures, particularly in samples with extreme ³⁴ S-enrichment in pyrite. Here, we compiled published microscale δ ¹³ C calcite , δ ¹⁸ O calcite , and δ ³⁴ S pyrite data with new secondary ion mass spectrometry (SIMS) δ ¹⁸ O barite and δ ³⁴ S barite analyses from two localities on the Fennoscandian Shield (Forsmark and Laxemar, Southeastern Sweden), aiming to constrain the record of microbial processes in barite. We identified two groups of barites distinguished by their distinct δ18O/δ34S trends. Comparison of these trends with δ ¹³ C values of cogenetic calcite reveals variable associations with sulfate-dependent anaerobic oxidation of methane (AOM) and organotrophic sulfate reduction (OSR). Strongly ¹³ C-depleted calcite ( δ ¹³ C calcite < −40 ‰), indicating a relationship with AOM, almost exclusively occurs in the steeper-trending Group 1. Evidence for barite affected by OSR ( δ ¹³ C calcite > −30 ‰) occurs in both groups and dominates the shallower-trending Group 2. The different trends likely resulted from an interplay of MSR pathways (AOM vs. OSR) and variations in sulfate reduction rates (SRR), which we attribute to the relative abundances of sulfate and electron donors in the parent fluid of barite. Lower sulfate and electron donor abundances likely permitted methanogenesis and AOM at lower SRR, resulting in a steeper δ18O/δ34S trend in Group 1 barite. Conversely, higher sulfate and electron donor abundances inhibited methanogenesis and AOM, favoring OSR at a higher SRR, which is reflected by the shallower δ18O/δ34S trend in Group 2 barite. Comparison between the δ ³⁴ S pyrite range (−53.9 ‰ to +131.7 ‰) and the δ ³⁴ S barite range (+7.6 ‰ to +52.0 ‰) across both localities demonstrates that the sulfate reservoir corresponding to extremely ³⁴ S-enriched pyrite is not recorded in barite. The discrepancy between barite and pyrite could reflect their formation in separate microenvironments or barite undersaturation at high degrees of Rayleigh distillation. Our study highlights the need for coupled microscale C-O-S analyses in fracture-hosted mineral assemblages to understand metabolic processes in the ancient deep biosphere, while stressing that these records are affected by local hydrogeochemical conditions.
Versions
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BibTeX
@article{runge2026microscale,
title = {{Microscale C-O-S isotope signatures record microbial sulfate reduction in ancient deep subsurface pyrite–calcite–barite assemblages}},
author = {Eric Runge and Vanessa Fichtner and Mikael Tillberg and Martin Whitehouse and Henrik Drake},
journal = {Biogeosciences},
year = {2026},
month = nov,
doi = {10.5194/bg-23-7893-2026},
url = {https://bg.copernicus.org/articles/23/7893/2026/},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Microscale C-O-S isotope signatures record microbial sulfate reduction in ancient deep subsurface pyrite–calcite–barite assemblages
AU - Eric Runge
AU - Vanessa Fichtner
AU - Mikael Tillberg
AU - Martin Whitehouse
AU - Henrik Drake
PY - 2026
DA - 2026/11/06
JO - Biogeosciences
DO - 10.5194/bg-23-7893-2026
UR - https://bg.copernicus.org/articles/23/7893/2026/
AB - The deep biosphere harbors diverse microbial communities amounting to a substantial fraction of Earth's biomass for much of its history. Analyses of fracture-hosted minerals (e.g., pyrite, calcite) and their C-O-S isotope signatures are beginning to establish a fossil record of subsurface microbial communities and their relationship with major geological events. Fracture-coating pyrite from deep within the Fennoscandian Shield records the largest ³⁴ S-enrichment observed on Earth to date ( δ ³⁴ S = +147 ‰) – likely the result of late-stage Rayleigh distillation during closed-system microbial sulfate reduction (MSR). This implies even heavier S isotope values for the complementary sulfate reservoir during pyrite formation, possibly recorded in coeval sulfate minerals such as barite [BaSO ₄ ]. However, barite has been poorly explored as an archive of ancient deep subsurface biosignatures, particularly in samples with extreme ³⁴ S-enrichment in pyrite. Here, we compiled published microscale δ ¹³ C calcite , δ ¹⁸ O calcite , and δ ³⁴ S pyrite data with new secondary ion mass spectrometry (SIMS) δ ¹⁸ O barite and δ ³⁴ S barite analyses from two localities on the Fennoscandian Shield (Forsmark and Laxemar, Southeastern Sweden), aiming to constrain the record of microbial processes in barite. We identified two groups of barites distinguished by their distinct δ18O/δ34S trends. Comparison of these trends with δ ¹³ C values of cogenetic calcite reveals variable associations with sulfate-dependent anaerobic oxidation of methane (AOM) and organotrophic sulfate reduction (OSR). Strongly ¹³ C-depleted calcite ( δ ¹³ C calcite < −40 ‰), indicating a relationship with AOM, almost exclusively occurs in the steeper-trending Group 1. Evidence for barite affected by OSR ( δ ¹³ C calcite > −30 ‰) occurs in both groups and dominates the shallower-trending Group 2. The different trends likely resulted from an interplay of MSR pathways (AOM vs. OSR) and variations in sulfate reduction rates (SRR), which we attribute to the relative abundances of sulfate and electron donors in the parent fluid of barite. Lower sulfate and electron donor abundances likely permitted methanogenesis and AOM at lower SRR, resulting in a steeper δ18O/δ34S trend in Group 1 barite. Conversely, higher sulfate and electron donor abundances inhibited methanogenesis and AOM, favoring OSR at a higher SRR, which is reflected by the shallower δ18O/δ34S trend in Group 2 barite. Comparison between the δ ³⁴ S pyrite range (−53.9 ‰ to +131.7 ‰) and the δ ³⁴ S barite range (+7.6 ‰ to +52.0 ‰) across both localities demonstrates that the sulfate reservoir corresponding to extremely ³⁴ S-enriched pyrite is not recorded in barite. The discrepancy between barite and pyrite could reflect their formation in separate microenvironments or barite undersaturation at high degrees of Rayleigh distillation. Our study highlights the need for coupled microscale C-O-S analyses in fracture-hosted mineral assemblages to understand metabolic processes in the ancient deep biosphere, while stressing that these records are affected by local hydrogeochemical conditions.
SN - 1726-4189
ER -