Multiple microscale stable isotope signatures record metabolic processes in ancient deep subsurface barite-pyrite-calcite assemblages

Abstract. Fracture-coating pyrite from deep within the Fennoscandian Shield records the largest ³⁴S-enrichment observed on Earth to date (δ³⁴S = +147 ‰) – likely the result of late-stage Rayleigh distillation during closed-system microbial sulfate reduction (MSR). This implies even heavier sulfur isotope values for the complementary sulfate reservoir during pyrite formation, possibly recorded in coeval sulfate minerals such as barite [BaSO₄]. However, barite has been poorly explored as an archive of ancient deep subsurface biosignatures. Here, we compiled published microscale δ³⁴S_pyrite, δ¹⁸O_calcite, and δ¹³C_calcite data with new secondary ion mass spectrometry (SIMS) δ³⁴S_barite and δ¹⁸O_barite analyses from two localities on the Fennoscandian Shield (Forsmark and Laxemar/Äspö, Southeastern Sweden), aiming to constrain how barite records the history of microbial processes. Comparison between the δ³⁴S_pyrite range across both localities (−53.9 to +131.7 ‰) with the δ³⁴S_barite range (+7.6 to +52.0 ‰) demonstrates that the sulfate reservoir corresponding to extremely ³⁴S-enriched pyrite is not recorded in barite. We identified two groups of barite distinguished by their distinct δ¹⁸O/δ³⁴S trends, which is proposed to record different MSR-related processes based on cogenetic δ¹³C_calcite data. Although there is overlap between the metabolic processes recorded in both groups, the steeper trending Group 1 was dominantly associated with sulfate-dependent anaerobic oxidation of methane (AOM), whereas the shallower trending Group 2 was dominated by organoclastic sulfate reduction (OSR). The different trends likely resulted from an interplay of MSR pathways (AOM vs. OSR), as well as variations in sulfate reduction rates (SRR) and fractionation-related isotope enrichment (³⁴ε), attributed to paleoenvironmental ratios of sulfate to electron donor abundances. Lower sulfate/electron donor ratios favored methanogenesis and AOM at lower SRR, whereas higher ratios inhibited methanogenesis and favored OSR at higher SRR. A preservation bias against extremely ³⁴S-enriched barite due to undersaturation at high degrees of Rayleigh distillation likely explains its absence in the deep subsurface. Our study highlights the need for microscale multiple stable isotope signatures in fracture-hosted mineral assemblages to understand metabolic processes in the ancient deep biosphere, while stressing that these records are strongly affected by local hydrogeochemical conditions.