STRATIGRAPHIC, MICROFOSSIL, AND GEOCHEMICAL ANALYSIS OF THE NEOPROTEROZOIC UINTA MOUNTAIN GROUP, UTAH: EVIDENCE OF BIOTIC CHANGE DRIVEN BY EUTROPHICATION?

Neoproterozoic microfossil diversity studies yield evidence for a relatively sudden biotic change prior to low-latitude glaciation. In an event interpreted as a mass extinction of eukaryotic phytoplankton followed by bacterial dominance, diverse assemblages of complex acritarchs are replaced by less diverse assemblages consisting of simple leiosphaerid acritarchs and bacteria. Data from the Chuar Group of the Grand Canyon (~742-770 Ma) suggest this biotic change was caused by eutrophication rather than the effects of low-latitude glaciation. Evidence includes a positive δ¹³C_org excursion followed by rising total organic carbon values (indicative of increasing primary productivity) and subsequent iron speciation values that suggest sustained water column anoxia. Here we present new data suggesting that this eutrophication event may also be recorded in organic-rich shale units of the Hades Pass quartzite and the Red Pine Shale of Utah’s Neoproterozoic Uinta Mountain Group (~742-770 Ma). In these strata, a biotic change from higher-diversity leiosphaerid-dominated assemblages to lower-diversity bacterial assemblages is associated with rising total organic carbon values following a positive δ¹³C_org excursion, suggesting increased primary productivity spurred by an influx of nutrients. This biotic turnover coincides with an interval of elevated Fe/Al ratios, interpreted as a period of persistent water column anoxia which may have resulted from increased primary productivity and eutrophication. Together, the Uinta Mountain and Chuar Group data suggest that biotic turnover was driven by eutrophication of surface waters that was at least regional in extent. Additionally, these data support recent models of punctuated global ocean anoxia during mid-late Neoproterozoic time.