AN EARLY ORDOVICIAN MASS EXTINCTION AND THE PERSISTENCE OF ANOXIA

Frequent mass extinction events ("biomere" boundaries) characterize the middle to late Cambrian and extend into the Early Ordovician (Tremadocian Stage). Cambrian biomere extinction events are associated with the onset of global positive carbon isotope excursions in marine carbonate and some studies have indicated that this pattern may persist into the Tremadocian. We examined carbon isotope stratigraphy across a Lower Ordovician extinction event at the base of the North American Stairsian Stage (middle Tremadocian) at two localities in Nevada (Shingle Pass) and Utah (Ibex), spanning the House-Parker Spring and House-Fillmore formational contacts, respectively. Our results, when combined with published data from west Texas (Taylor et al., 2004) and elsewhere around the world (Argentina and South Korea), demonstrate a continuation of the pattern in which biomere boundary extinctions are linked to the onset of global positive carbon isotope excursions. Although the magnitude and duration of the positive excursion at the base Stairsian Stage is at least a factor of two smaller than the late Cambrian SPICE event (base Paibian Stage), the evidence as a whole suggests the persistence of oceanic anoxia into at least the middle part of the Tremadocian Stage of the Early Ordovician. Anoxia is inferred for the base Stairsian as a causal link betweenthe observed positive carbon isotope excursion, which was likely produced by enhanced organic matter burial, and shallow marine extinctions of trilobite and conodont faunas. As both the evidence for carbon isotope excursions and extinction events appear to wane in the Tremadocian it may be inferred that the progressive oxygenation of the oceans reached a threshold in the post-Tremadocian oceans, and contributed to the lowering of extinction rates at that time prior to the onset of the Great Ordovician Biodiversification Event.