Carbonate‐Associated Sulfate Does Not Record Superheavy δ ³⁴ S in Deep Scandinavian Bedrock Fractures: Implications for Pyrite Formation in the Deep Biosphere

Abstract In deep bedrock fractures of the Fennoscandian shield, pyrite with “superheavy” δ 34 S values of up to 147‰ exists together with calcite. The extreme variability and 34 S‐enrichment in pyrite indicate that microbial sulfate reduction (MSR) evolved to almost complete sulfate exhaustion in closed‐system microenvironments. The bulk δ 34 S pyrite of up to 81‰ implies a sulfate source with unusually high δ 34 S, although such a source has not yet been identified. Here, we aim to decipher this source by analyzing carbonate‐associated sulfate (CAS) of calcite related to superheavy pyrite. Our δ 34 S CAS values reveal low to moderate 34 S enrichment, with δ 34 S CAS between 2.3 and 28.2‰, which is far below the bulk δ 34 S pyrite of up to 81‰ indicating a decoupled sulfate pool for CAS and pyrite origin. The extreme δ 34 S fractionation recorded by pyrite implies very slow MSR rates, which typically promote calcite dissolution rather than precipitation. Calcite precipitation was thus inhibited during superheavy pyrite formation. Instead, the calcite shows signs for anaerobic oxidation of methane coupled to sulfate reduction (AOM‐SR), reflected by 13 C‐depletion and moderate 34 S‐enrichment. The AOM‐SR process reduces sulfate concentration, in turn providing conditions suitable for the formation of superheavy pyrite. We propose that a limited supply of substrates (sulfate and organic matter) and Fe 2+ , their episodic input into the deep fracture system together with low diffusion rates of fracture fluids enabled a balance and completion of microbial methanogenesis, AOM‐SR, and MSR. Extreme isotope signatures may thus represent diagnostic features of episodic, energy‐limited deep biosphere ecosystems, with implications for understanding subsurface microbial ecology and biogeochemical cycling.