The sulfur geochemistry of the lacustrine Paleogene Green River Formation (Colorado, Utah, and Wyoming, USA) is unlike that of most marine and other lacustrine rocks. Distinctive chemical, isotopic, and mineralogical characteristics of the formation are pyrrhotite and marcasite, high contents of iron mineral sulfides strikingly enriched in³⁴S, cyclical trends in sulfur abundance and δ³⁴S values, and long-term evolutionary trends in δ³⁴S values. Analyses that identified and quantified these characteristics include carbonate-free abundance of organic carbon (0.13–47 wt%), total iron (0.31–13 wt%), reactive iron (>70% of total iron), total sulfur (0.02–16 wt%), acid-volatile monosulfide (S_Av), disulfide (S_Di > 70% of total sulfur), sulfate (S_SO4) and organosulfur (S_Org); isotopic composition of separated sulfur phases (δ³⁴S_Di,Av up to +49‰); and mineralogy, morphology and paragenesis of sulfide minerals.

Mineralogy, morphology, δ³⁴S_Di,Av, and δ³⁴S_Org have a distinctive relation, reflecting variable and unique depositional and early diagenetic conditions in the Green River lakes. When the lakes were brackish, dissimilatory sulfate-reducing bacteria in the sediment produced H₂S, which initially reacted with labile iron to form pyrite framboids and more gradually with organic matter to form organosulfur compounds. During a long-lived stage of saline lake water, the amount of sulfate supplied by inflow decreased and alkalinity and pH of lake waters increased substantially. Extensive bacterial sulfate reduction in the water column kept lake waters undersaturated with sulfate minerals. A very high H₂S:SO₄ ratio developed in stagnant bottom water aided by the high pH that kinetically inhibited iron sulfidization. Progressive removal of H₂S by coeval formation of iron sulfides and organosulfur compounds caused the isotopic composition of the entire dissolved sulfur reservoir to evolve to δ³⁴S values much greater than that of inflow sulfate, which is estimated to have been +20‰ A six-million-year interval within Lake Uinta cores records this evolution as well as smaller systematic changes in δ³⁴S, interpreted to reflect ~ 100,000-year lake-level cycles. When porewater was exceptionally reducing, unstable FeS phases eventually recrystallized to pyrrhotite during diagenesis. A much later reaction related to weathering altered pyrrhotite to marcasite.