SEPARATION AND ISOTOPIC ANALYSIS OF FRAMBOIDAL PYRITES FROM MEROMICTIC FAYETTEVILLE GREEN LAKE, NY

Changes in ocean redox throughout Earth history have been examined and evaluated using sulfur isotope measurements (δ³⁴S) of both pyrite and calcite-associated sulfate. During intervals of ocean euxinia, rapid changes in δ³⁴S of pyrites are often observed but the underlying dynamics of these changes remain incompletely understood. Difficulty in interpreting such rapid, high amplitude changes may result from the presence of both syngenetic water column pyrites and diagenetic sedimentary pyrites in traditional bulk δ³⁴S measurements. The aim of this work is to examine the relationship between framboid diameter and δ³⁴S in a modern euxinic analog to examine the extent to which sedimentary δ³⁴S_pyrite reflects water column versus sedimentary sulfide composition. Here, we measure the δ³⁴S of framboidal pyrites isolated from sediments collected from depths above, at, and below the chemocline in meromictic Fayetteville Green Lake (FGL, New York). Density separation was used to separate pyrites from other sediment components. Pyrite yields from density separation of standard samples with known pyrite abundance were improved from ~20% up to a maximum of approximately 80%. These revised separation and sample preparation methods improved our ability to measure pyrites via SEM and measure δ³⁴S of individual framboids via SIMS. 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.