We compared concentrations of organic carbon, partitioning of iron and sulfur, stable isotopic ratios of major sulfur species, and pyrite morphologies in the water column and sediments of oxic and euxinic portions of a sulfate-rich, meromictic lake (Green Lake near Fayetteville, New York) in order to examine the effects of euxinic conditions on pyrite formation and sedimentary Fe-S-C relations. About 15% of the pyrite in sediments of the euxinic basin of Green Lake forms in the water column. The bulk of pyrite in both euxinic and oxic sediments forms in the upper ∼10 cm of the sediment and is limited by availability of reactive iron. Oxic and euxinic sediments in Green Lake cannot be clearly distinguished on the basis of concentrations of organic carbon or pyrite, degree of pyritization, or sulfur isotopic ratios of pyrite, but can be distinguished by using size distributions of pyrite framboids. Therefore, Fe-S-C relations in ancient shales must be used with caution when attempting to reconstruct paleoenvironmental redox conditions.

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