WHAT DO SEDIMENTARY SULFIDE δ34S VALUES ACTUALLY RECORD? IN SITU SEM-SIMS ANALYSIS OF FRAMBOIDS AND OTHER SULFIDE PHASES IN THE MISSISSIPPIAN-DEVONIAN BAKKEN FORMATION

Sulfide δ³⁴S values in sediments are a powerful tool for studying microbial activity, the global sulfur cycle, and redox conditions in Earth’s past. In particular, formation of pyrite framboids in sediments is rapid and can be closely tied to microbial metabolisms and the δ³⁴S value of seawater sulfate, making framboids powerful potential indicators of past environmental conditions in their own right. However, there has been much uncertainty regarding the processes governing framboid development and structure, and interpreting pyrite δ³⁴S data on the basis of bulk sampling requires assumptions regarding sample homogeneity and relative timing of sulfide formation. Microanalytical techniques provide an avenue for more nuanced and systematic understanding of these questions on a sample-by-sample basis.

Combined in situ SEM imaging and SIMS analysis (≤10 µm spots) of pyrite from 5 drillcores intersecting Devonian to Mississippian-aged Bakken Formation shales in the central Williston Basin shows a bimodal distribution of δ³⁴S values amongst several petrographically distinct types of sulfide growth. Framboidal pyrites fall in the -45 to -25‰ range and may exceed 20 µm in diameter, requiring strong open-system H₂S behavior at the sea floor or in the shallow sediment. Based on δ³⁴S analyses on framboids we can conclude that: 1) Framboids grew primarily through simultaneous microcrystal growth and not via progressive precipitation along the framboid’s rim, as is typical of most crystallographic features, and 2) Variability in δ³⁴S indicates different durations or chemical environments of formation, even amongst adjacent framboids. In contrast with the framboids, coarser pyrites have δ³⁴S values above -20‰, are predominantly replacive and are found associated with skeletal carbonates, silicified microfossils, and isolated dolomite crystals. The bimodal distribution indicates either a) a large change in δ³⁴S_(fluid) or Δ³⁴S_{(sulfate-sulfide)} in the sediment, possibly associated with a systematic change in microbial activity during early burial, or b) two temporally isolated periods of growth, one in the sediment during or shortly after deposition, and one at >100°C during thermochemical sulfate reduction. Integrated geochemistry and petrographic observations support thermochemical sulfate reduction.