KILLER OCEANS OF THE EARLY TRIASSIC

The biotic recovery from the end-Permian mass extinctions was unusually long and spanned the entire Early Triassic Period (~ 8 m.y.). Three main hypotheses have been proposed to explain the long duration of the recovery: suppression of metazoans due to prolonged environmental stress (perhaps related to the extinctions), increased difficulty of recovery due to the severity of the mass extinctions, and a taphonomic bias during the interval in question.

Slope-basin carbonates of the Union Wash Formation (Spathian, eastern CA) contain seafloor precipitated fans and shallow shelf facies of the Virgin Limestone Member of the Moenkopi Formation (Spathian, southern NV) contain large subtidal stromatolites, both consistent with the unusual ocean chemistry hypothesis. New strontium isotope data from the fan-bearing unit in the Union Wash Formation and the stromatolite bearing interval in the Virgin Limestone demonstrates their strong temporal equivalence.

In addition, the Union Wash Formation records a d³⁴S excursion in carbonate associated sulfate (CAS) from +5 per mil below the fan-bearing unit to +25 per mil within the fan-bearing unit (versus CDT). Trace sulfate concentrations decrease with the rise of d³⁴S. In nearshore facies of the Virgin Limestone sulfur isotopic values reach +30 in both bedded gypsum (measured via traditional means) and intercalated carbonates (measured via CAS); the covariance of the d³⁴S in gypsum and limestone demonstrates the veracity of the CAS method. The d³⁴S excursion reported here mirrors that found in Lower Triassic marine evaporites around the world and is the largest sulfur isotope excursion of the Phanerozoic.

Such large d³⁴S excursions, in concert with widespread stromatolites and seafloor fans, would lend support to the role of environmental stress in the prolonged recovery. The results are consistent with a stratified ocean in which bacterial sulfate reduction in an anoxic water column would drive sulfur isotopes to heavier d³⁴S values and consume sulfate. Byproducts of bacterial sulfate reduction include HCO₃^- and HS^-. Increased levels of resultant CO₂ and H₂S could prove lethal for metazoa. As suggested for the end-Permian mass extinctions, the introduction of waters rich in CO₂ and H₂S to the shallow marine ecosystem may have also suppressed the biotic recovery.