SULFUR ISOTOPIC (δ34S) EVIDENCE FOR MARINE ANOXIA AND LINKS TO LOWER ORDOVICIAN (TREMADOCIAN) TRILOBITE EXTINCTIONS

The late Cambrian to Early Ordovician is well known for the occurrence of trilobite extinctions and in some cases associated positive carbon isotope excursions (δ¹³C). This relationship is interpreted to record the expansion of anoxia onto the shelf that created conditions that initiated extinctions and caused δ¹³C excursions via increased organic burial. The largest of these excursions occurred during the late Cambrian where globally correlative δ¹³C excursions and in some cases sulfur (δ³⁴S) excursions of varying magnitude immediately follow an extinction event. This relationship is consistent with the notion that anoxic conditions were global, but it remains unclear if anoxia persisted for younger Ordovician 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.