Characterizing ancient seep environments by in-situ sulfur isotope composition of authigenic pyrite

Authigenic pyrite commonly coexists with authigenic carbonate forming at marine methane seeps. Both pyrite and carbonate minerals are by-products of sulfate-driven anaerobic oxidation of methane (SD-AOM), the dominant biogeochemical process at seeps. The sulfur isotopic composition of pyrite has been extensively used to study the sulfur cycle and seepage activity in modern seep environments. However, to what extent δ³⁴S_py values can aid the interpretation of ancient seep environments is poorly known. To improve on this knowledge gap, secondary ion mass spectroscopy (SIMS) was applied to investigate the sulfur isotope patterns of different types of pyrite (framboidal and euhedral) on a microscale from seep deposits of Beauvoisin (Jurassic, France) and Whiskey Creek (Eocene, USA), which is supplemented by the sulfur isotope measurement of bulk pyrite (chromium reducible sulfur; CRS). Framboids of the Beauvoisin seep deposits show low δ³⁴S_py values (−40.1 ‰ to −23.9 ‰), those of the Whiskey Creek deposit exhibit a wider range of values (−40.7 ‰ to 9.9 ‰). Most of the euhedral pyrite from both sites is typified by higher δ³⁴S_py values (4.4 ‰ to 5.7 ‰ for Beauvoisin; 0.4 ‰ to 3.6 ‰ for Whiskey Creek). The δ³⁴S_CRS values of Beauvoisin deposits (−23.0 ‰ to 1.80 ‰) are also lower than those of Whiskey Creek (−10.8 ‰ to 2.9 ‰). The large variations in δ³⁴S_py values, reflecting different types of pyrite, suggest that pyrite formed episodically over a prolonged period of seepage during early diagenesis. Overall, high rates of sulfate reduction and high replenishment of the pore water pool of sulfate with seawater sulfate at active methane seeps controlled the crystal habit and the sulfur isotope composition of the studied pyrite. Higher δ³⁴S_py values of euhedral pyrite record progressively sulfate-limited conditions where sulfate consumption by SD-AOM exceeded sulfate replenishment – as observed for later-stage pyrite extracted from modern seepage-affected sediment with δ³⁴S_py values higher than 100 ‰. The lack of such extremely high δ³⁴S_py values within the studied seep limestones is probably not only controlled by the degree of replenishment of seawater sulfate but also by the engulfment of pyrite by authigenic seep carbonate, impeding the formation of later stages of pyrite. Overall, the identified sulfur isotope patterns of pyrite preserved in the Beauvoisin and Whiskey Creek seep deposits resemble observations made on pyrite from modern seeps, confirming that the sulfur isotopic composition of pyrite can serve as a potent tool for reconstructing the biogeochemical cycling of sulfur in ancient seep environments.