STABLE ISOTOPE AND CARBONATE ASSOCIATED SULFATE RECORD OF A LACUSTRINE STROMATOLITE, GREEN RIVER FORMATION (EOCENE)

Stable isotope (¹³C and ¹⁸O) and Carbonate Associate Sulfate (CAS- trace sulfate bound into the carbonate crystal lattice at the time of precipitation) data was collected from an ~8 cm thick stromatolite found near the Tipton-Wilkins Peak Member contact of the Green River Formation, Boar’s Tusk locality, Wyoming. The stromatolite, deposited in a shallow lacustrine setting in the Early Eocene (~51 Ma), is composed of two alternating microstructures: a micritic microfacies and a calcite crystal fan microfacies. Facies were microdrilled along lamina in order to construct an isotopic and CAS framework during deposition of the stromatolite. The calcite fans record a lighter d13C and d18O isotopic signature and a lower abundance of CAS versus the micritic facies. Furthermore, the CAS variability along individual calcite fan layers is negligible, whereas the variability in the micrite layers is high. In general, evaporation causes ¹⁸O and SO₄^2- enrichment, assuming SO₄^2- behaves conservatively during evaporation. The isotopic and elemental patterns suggest the calcite fan layers formed with less evaporative influence (lighter d18O, less sulfate) whereas the micrite formed with more evaporative influence (heavier d18O, more sulfate). On the one hand, the isotopic and elemental abundances may indicate strong lake level influence, with a relatively high lake level during the formation of the calcite fan layers and evaporative events associated with the micritic layers. On the other hand, the calcite fan layers, which were precipitated in place, may represent ambient lake chemistry, but the micrite may be allochthonous, originating from the extensive evaporitic mudflats thought to exist during the deposition of paleolake Gosiute and trapped within the stromatolite during stromatolite morphogenesis. The strong variability in CAS argues for the latter, but either hypothesis is interesting with respect to the environments of the ancient lake system.