Sedimentary controls on pyrite sulfur isotopes in euxinic settings

Pyrite is widely used to reconstruct ancient euxinic (anoxic and sulfidic) conditions, yet whether its sulfur isotope composition ( δ 34 S) preserves primary water-column signals remains debated. Here, we show combined bulk and microscale δ 34 S data from pyrite grains collected across the Black Sea water-column and deep-basin sediments. Water-column-derived pyrite consists exclusively of framboidal grains with narrow size and δ 34 S ranges, consistent with formation from local aqueous sulfide and minimal alteration during nucleation and settling. Upon sedimentation, however, these grains undergo progressive textural and isotopic modification. With increasing burial, framboids are supplemented by larger, more 34 S-enriched recrystallized grains formed during dissimilatory sulfate reduction in diffusion-limited sedimentary microenvironments. Only ≈17% of pyrite grains that match WC framboids in size, texture, and δ 34 S signature retain a water-column-consistent isotopic composition in the uppermost sediment, declining to ≤4% at depth. Sulfide profiles and gene abundances further indicate that most water-column sulfide diffuses upward from sediments. Consequently, δ 34 S records from black shales likely integrate complex diagenetic histories rather than reflect direct snapshots of WC chemistry.