CARBON AND SULFUR ISOTOPE STRATIGRAPHY OF THE LATE IBEXIAN (FLOIAN) TO EARLY WHITEROCKIAN (DARRIWILIAN) OF THE POGONIP GROUP, GREAT BASIN, USA

The late Ibexian (Floian) to the Whiterockian (Dapingian-Darriwilian) is an important time in the fossil record as it marks one of the first major pulses of faunal radiation of the Great Ordovician Biodiversification Event. The Ibexian is interpreted to be a time of cooling of shallow seas and increases in oceanic oxygen. The δ¹³C and δ³⁴S records of marine carbonate can provide a test of these hypotheses. Results show δ¹³C increased gradually from -2.0‰ to -0.5‰ (VPDB) during the Floian. Preliminary carbonate associated sulfate δ³⁴S values are between 24‰ to 32‰ (VCDT). Literature values are generally in this range, but only data from Argentina and Newfoundland (Thompson and Kah, 2012) show high-resolution trends during this time.

An increase in δ¹³C and δ³⁴S during the late Ibexian would be consistent with an increase in the rate of organic carbon and pyrite sulfur burial. Burial of organic matter increases the amount of free oxygen and removes atmospheric CO₂. Organic matter in sediment fuels sulfate-reducing bacteria that convert seawater sulfate into sedimentary pyrite, which also results in oxygenation when pyrite burial is maintained.