SINGLE ORGANIC MICROFOSSILS CARBON ISOTOPES DURING THE LATE DEVONIAN BIOTIC CRISIS, INSIGHTS INTO ECOLOGY AND CARBON CYCLE DYNAMICS

Organic carbon isotope analyses of single microfossils can provide a window into short-term environmental variability and reveal ecological data about enigmatic organic fossil groups. Here we apply a new technique for the δ¹³C analysis of single organic walled microfossils (OWM) to samples taken from three sites in the Appalachian Basin (AB) of New York through the Late Devonian Biotic Crisis. Our data provide new insights into the nature of the Frasnian- Famennian carbon cycle in the AB, and also provide constraints on the paleoecology of enigmatic organic microfossils ubiquitous in Paleozoic shale successions.

The δ¹³C of single leiospherid (smooth), acanthomorphic (spinose) and chitinozoan OWM range from -32 to -17‰, but average -25‰ across all samples and are consistent with organic matter of autochthonous origins. We observe no difference between the δ¹³C_OWM of leiospheres and acanthomorphic acritarchs. This indicates that our data are an ecological signal, not a taxonomic one. Leiospheres and acanthomorphs are clearly sampling the same pool of carbon, had similar metabolisms and inhabited the same ecospace. Further, in accordance with morphological similarities between leiospherids and acathomorphs and modern algae, their δ¹³C values are consistent with oxygenic photoautotrophic metabolisms. By contrast, chitinozoans are ¹³C-depleted compared to both leiospheres and acanthomorphs, which indicates that the difference is being driven by metabolic, biosynthetic or carbon assimilatory pathways, or that chitinozoans are sampling a different carbon pool than acritarchs. We also observe a consistent 3 to 5‰ offset between δ¹³C_OWM and δ¹³C_bulk that we attribute to a δ¹³C gradient in the AB water column where OWM utilized relatively ¹³C-enriched dissolved inorganic carbon near the surface. Thus, the organisms producing the balance of the total organic carbon were assimilating ¹³C-depleted C sources, either respired CO₂ or byproducts of fermentation. In this context, the deposition of Kellwasser black shales likely reflects a combination of an external influx of nutrients, shallowing of remineralization as a result of warming, and increased organic preservation. We also observe a systematic decrease in both δ¹³C_OWM and δ¹³C_bulk of 3‰ from shoreward to open ocean facies. This signal may reflect the effect of ¹³C-enriched, weathering-derived DIC from riverine sources in this relatively enclosed epeiric seaway, a model that is consistent an estuarine mode of circulation that has been proposed for the AB.