SULFUR ISOTOPIC (δ34S) EVIDENCE FOR MARINE ANOXIA AND LINKS TO LOWER ORDOVICIAN (TREMADOCIAN) TRILOBITE EXTINCTIONS
Here we present new δ34S data measured from carbonate-associated sulfate from a Lower Ordovician carbonate succession in the Ibex area, UT and compare with two previously recognized trilobite extinctions. The older extinction occurs near the base of the North American Stairsian Stage where δ34S values increase from 28‰ ~50 m below the extinction horizon, are depleted by ~15‰ just below the extinction, and reach a peak at 48‰ near the same interval with a previously recognized ~2‰ positive δ13C excursion. Both excursions begin near a relative sea level lowstand within the uppermost House Limestone and continue to rise throughout lithologies that vary from bioturbated mudstone to intraclastic rudstone. δ13C returns to –1‰ before δ34S returns to 33‰ where trends remain throughout the Stairsian. The younger extinction occurs near the base of the Tulean Stage, but isotope trends decouple at this interval. Near the base Tulean extinction δ13C decreases from –1 to –2.3‰, whereas δ34S increases prior to the extinction interval from 35 to 43‰ before returning to 32‰, similar to the older excursion. The cause of this decoupling remains unclear and if the timing represents local effects or global trends, but the link between the end of the extinctions and positive δ34S excursions is consistent with the view that anoxic conditions persisted throughout the Early Ordovician. The frequency and size of isotope excursions diminished throughout the Early Ordovician and end prior to some of the first pulses of biodiversification, suggesting that changes in ocean oxygenation permitted the Ordovician radiation.