EVALUATING THE EFFECTS OF PRESERVATION ON EUKARYOTIC DIVERSITY PATTERNS OF THE LATE TONIAN CHUAR AND UINTA MOUNTAIN GROUPS OF WESTERN LAURENTIA

The Tonian Period is characterized by a rise in eukaryotic diversity and ecological complexity, however changes in both fossil preservation and depositional environment can affect the appearance of diversity patterns. Previous work ca. 770–730 Ma Chuar Group of Arizona, has revealed diverse assemblages of organic-walled microfossils (OWMs)—the remains of bacteria as well as the cells and/or cysts of eukaryotes. However, these diverse OWM assemblages disappear in the upper Chuar Group alongside the appearance of vase-shaped microfossils— the organic shells of testate amoebae. This transition has been interpreted as a biotic turnover event related to eutrophication. Although this hypothesis is supported by significant increases in total organic carbon (TOC) concentrations (up to 9.4 wt%), high TOC is also consistent with stratigraphic evidence for a transition to more distal depositional environments. Further, TOC has been shown to negatively correlate with the preservation of OWMs.

Here we present estimates of OWM preservation from the Chuar Group in stratigraphic context, as well as a second record of OWMs from the coeval Uinta Mountain Group, Utah. The lower Chuar Group is dominated by well-preserved OWM assemblages which decrease in preservational quality then disappear altogether in upper Chuar Group. The record of OWMs from the Uinta Mountain Group shows a similar pattern in diversity. Using transmitted light and scanning electron microscopy, we describe 22 species and 4 unnamed forms of OWMs from the lower-middle Uinta Mountain Group, 14 of which also occur in the lower Chuar Group. The upper Uinta Mountain Group is also characterized by facies indicative of more distal (prodeltaic) environments, lower diversity of OWMs, and the appearance of vase-shaped microfossils. However, the upper Uinta Mountain Group contains several eukaryotic species of OWMs co-occurring with vase-shaped microfossils, perhaps because the rise in TOC is more moderate (≤ 5.9 wt%) and is therefore more favorable to OWM preservation. Although the transition from diverse OWMs to abundant vase-shaped microfossils is emerging as a global signal, its appearance at the local scale can be affected by changes in depositional environment, conditions controlling preservation, or a combination of both (facies-dependent preservation).