EUXINIA SEEN THROUGH PYRITE FRAMBOIDS IN TRAPPER CREEK, IDAHO, LEADING UP TO THE END-PERMIAN EXTINCTION

The end-Permian mass extinction, caused by eruption of the Siberian Traps Large Igneous Province, led to the extinction of 96% of all living species, making it the greatest mass extinction in Earth’s history. The Late Permian, including the interval leading up to the extinction, featured a major global sea level lowstand, leading to little deposition in marine shelf environments. Therefore, there are very few rock formations accessible today that show continuous sedimentation throughout the Late Permian and into the Triassic, with none from the continental United States.

The Phosphoria Formation, which spans parts of Wyoming, Montana, and Idaho, is recognized as a unique sedimentary unit because of the phosphatogenesis that occurred there during the Early to Middle Permian. The outcrop of the Phosphoria Formation at Trapper Creek, in southern Idaho, contains over one hundred meters of chert strata overlying a volcanic ash dated to approximately the Guadalupian-Lopingian (i.e., Middle-Late Permian) boundary, making it possibly the only place in the continental United States with continuous deposition through the Late Permian and into the Triassic. If the Trapper Creek section spans the extinction, it preserves a record of environmental conditions in the interval leading up to the Permian-Triassic boundary.

Anoxia and euxinia are understood to have played a key role in the end-Permian mass extinction, and both can be used as indicators to recognize the earliest signs of the impending crisis. Anoxic waters are those severely depleted in oxygen, while euxinic waters also contain high levels of reduced sulfur due to anaerobic bacterial sulfate reduction. Sulfide in euxinic waters can react with dissolved iron to form pyrite framboids. Additionally, the color and organic content of chert samples may indicate changes in oxygenation. Dark colors reflecting high organic content are indicative of anoxia, while orange bands indicate oxidized iron, and yellow may indicate concentrations of sulfur. For this project, we assessed twenty thin sections from the Trapper Creek section, to search for the presence of framboids and other indicators of environmental stressors leading up to the end-Permian extinction.