Stratigraphic, Microfossil, and Geochemical Analysis of the Neoproterozoic Uinta Mountain Group, Utah: Evidence for a Eutrophication Event?
Several previous Neoproterozoic microfossil diversity studies yield evidence for arelatively sudden biotic change prior to the first well‐constrained Sturtian glaciations. In an event interpreted as a mass extinction of eukaryotic phytoplankton followed by bacterial dominance, diverse assemblages of complex acritarchs are replaced by more uniform assemblages consisting of simple leiosphaerid acritarchs and bacteria. Recent data from the Chuar Group of the Grand Canyon (770‐742 Ma) suggest this
... iotic change was caused by eutrophication rather than the direct effects of Sturtian glaciation; evidence includes total organic carbon increases indicative of increasing primary productivity followed by iron speciation values that suggest sustained water column anoxia. A new data set (this study) suggests that this same eutrophication event may be recorded in shale units of the formation of Hades Pass and the Red Pine Shale of Utah's Neoproterozoic Uinta Mountain Group (770‐742 Ma). Results of this study include a significant shift from a higher‐diversity (H'= 0.60) fauna that includes some ornamented acritarchs to a lower‐diversity (H' = 0.11) fauna dominated by smooth leiosphaerids and microfossils of a bacterial origin (Bavlinella/ Sphaerocongregus sp.). This biotic change co‐occurs with a significant increase in total iii organic carbon values that directly follows a positive carbon‐isotopic excursion, suggesting increased primary productivity that may have been the result of elevated sediment influx and nutrient availability. Both the biotic change and period of increased total organic carbon values correspond with the onset of an interval of anoxia (indicated by total iron to aluminum ratios above 0.60) and a spike in sulfur concentration. Like those reported from the Chuar Group, these biotic and geochemical changes in the upper Uinta Mountain Group are independent of changes in lithofacies , and they suggest that either a eutrophication event or direct inhibition of eukaryotes by sulfide (or perhaps both) may have [...]