LESSONS FROM THE CARBON ISOTOPIC COMPOSITION OF SINGLE ORGANIC WALLED MICROFOSSILS

The legacy of Andrew Knoll and his many students and collaborators rests largely in their unique insights about the Proterozoic, particularly toward the microfossil record as well as the dynamics of the deep time carbon cycle. Here we combine these two areas through the analysis of single organic walled microfossils for their carbon isotopic composition using nano-EA-IRMS. Resolving questions about the ecology of organic walled microfossils (OWM) in the Proterozoic and Paleozoic has largely relied on taxonomy of the fossils themselves coupled with the geochemistry of the host sediments. However, direct geochemical analyses of OWM are increasingly common and will sharpen our understanding of OWM-producing organisms, the conditions under which they evolved, and controversies regarding the carbon cycle to which they contributed.

Our work thus far has focused on analyses of well-preserved microfossils from late Devonian of NY, the Tonian Chuar Group and the Mesoproterozoic Velkerri Formation. In general, OWM δ¹³C analyses of a single taxon from one sample have a broad δ¹³C range, in some cases more than 15‰, which underscores the environmental and C-cycle variability contained within the ~kiloyears preserved in one sample compared to the days reflected in the lifetime of one single OWM. In samples where OWM diversity allows for analysis of multiple taxa we observe δ¹³C differences between OWM types, suggesting ecological differences. In the Late Devonian rocks and the Roper Group average OWM δ¹³C is higher than bulk rock δ¹³C by 2 to 10‰, regardless of species. The ¹³C-enrichment of OWM may be the result of biosynthetic processes (fossil size, wall composition or carbon concentration mechanisms) or related to a shallower water habitat and utilization of ¹³C-enriched carbon substrates. However, the Chuar Group provides an alternative perspective; the OWM are almost exclusively ¹³C-depleted relative to bulk rock δ¹³C, that can be as high as -14‰. In the case of the Chuar Group the origins of the disparities between the bulk rock and OWM δ¹³C values may be the result of unique OWM ecologies, ¹³C-enriched detrital OM or processes yet to be revealed. While still in its initial stages, this work has revealed new insights about OWM and their ecosystems, and perhaps more importantly, it is breaking organic carbon into its physically resolvable components to determine where the bulk rock δ¹³C_org signal resides, and what it means. Thus, this approach has wide utility in addressing other long-standing questions regarding carbon cycle dynamics in deep time.