pH-DEPENDENT SULFUR ISOTOPE PARTITIONING OF H2S IN SEAWATER: RELATIONS TO SOFT TISSUE TAPHONOMY AND FOSSILIZATION

Neoproterozoic and Cambrian fossilized embryos and eggs are rare, but integral to the understanding of major features of animal evolution, although the biogeochemical pathways by which preservation and mineralization of these embryos and other soft-bodied tissues occurs is poorly understood. Surface waters of Late Precambrian oceans are thought to have been well oxygenated and mixed, but euxinic and sulfidic conditions may have been prevalent in oceanic basins with low circulation. Previous laboratory experiments have demonstrated that whereas autolysis by internal enzymes will destroy marine embryos in a few hours, reducing and anaerobic conditions block this process for months, preserved tissues and providing a substrate for biofilm forming microbes. A likely Precambrian-Cambrian marine source of anaerobic and reducing conditions would have been massive H₂S releases from oceanic overturn of euxinic deeper ocean waters, upwelling toxic waters to shallow shelf settings that harbored early eukaryotic life.

In order to expose marine invertebrate embryos to specific early Earth environmental conditions, a series of experiments was conducted to determine hydrogen sulfide concentrations ([H₂S]) in artificial seawater over a range of pH’s. H₂S was trapped as CdS or ZnS precipitate and analyzed for sulfur isotopes (δ³⁴S). Our preliminary δ³⁴S data show values for H₂S increase from +1.3‰ to +3.1‰ compared to tank H₂S from pH 4.5 to 6.5 and then abruptly fall to tank values at pH 7.5. These large enrichments of ³⁴S in the pH increment between 6.5 and 7.0 correspond to the pH range at which HS^- and H₂S occur at equal concentrations in seawater. At higher pH values the HS^- ion is dominant and at lower pH values H₂S is the dominant sulfide species in solution. This surprisingly large isotopic shift is inferred to result from changing proportions of H₂S and HS^- near pH 6.5. Now that the relationship between pH of seawater and [H₂S] is established, future experiments with embryos will target more modest pH drops and lower [H₂S]. As previous embryo experiments were with [H₂S] only observed in fluids associated with mid-ocean ridge hydrothermal vents, and its unlikely that Neoproterozoic–Cambrian continental shelf settings would have experienced such high [H₂S].