SULFUR ISOTOPE HETEROGENEITY IN FRAMBOIDAL PYRITES FROM A STRATIFIED EUXINIC LAKE

Sulfur isotope measurements of pyrite and calcite-associated sulfate in sedimentary rocks are used to interpret the evolution of the global sulfur cycle and the history of ocean redox change. However, recent work suggests that local depositional and diagenetic processes can obscure relevant environmental information recorded in δ³⁴S. One difficulty in interpreting sedimentary δ³⁴S_pyrite records may result from the presence of both syngenetic and diagenetic sources in a traditional bulk δ³⁴S measurement. In this study, we examine the relationship between pyrite framboid diameter and δ³⁴S of framboids in a modern euxinic analog to examine the extent to which sedimentary δ³⁴S_pyrite reflects water column versus sedimentary sulfur cycling. We used density separation techniques to isolate pyrites from surface sediments collected from depths at and below the chemocline in meromictic Fayetteville Green Lake (FGL), NY. We measured framboid diameter via SEM and δ³⁴S of individual framboids via SIMS. Framboidal pyrites isolated from a single sample have a range of up to 40 permil in δ³⁴S with no clear relationship to framboid diameter. This is greater than the range of δ³⁴S reported in bulk pyrite and sulfide samples from FGL, but similar to other high-resolution studies of pyrite. We suggest that that these measurements capture both pyrites formed in the water column and in the sediments. Isotopically enriched pyrites may reflect closed system behavior, either within particles in the water column or within the sediments. This high-resolution S isotope approach may improve our understanding of pyrite formation and preservation in FGL as well as in euxinic intervals of the rock record.