Carbon cycle dynamics and ecology revealed by the carbon isotopic composition of single organic microfossils during the Late Devonian Biotic Crisis

We apply a new approach for the δ¹³C analysis of single organic-walled microfossils (OWM) to three sites in the Appalachian Basin of New York (AB) that span the Late Devonian Biotic Crisis (LDBC). Our data provide new insights into the nature of the Frasnian–Famennian carbon cycle in the AB and also provide possible constraints on the paleoecology of enigmatic OWM ubiquitous in Paleozoic shale successions. The carbon isotope compositions of OWM are consistent with normal marine organic matter of autochthonous origins and range from −32 to −17‰, but average −25‰ across all samples and are consistently ¹³C-enriched compared to bulk sediments (δ¹³C_bulk) by ~0–10‰. We observe no difference between the δ¹³C_OWM of leiospheres (smooth-walled) and acanthomorphic (spinose) acritarch OWM, indicating that our data are driven by ecological rather than taxonomic signals. We hypothesize that the offset between δ¹³C_OWM and δ¹³C_bulk is in part due to a large 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, including but not limited to respired organic carbon or byproducts of fermentation. We also observe a systematic decrease in both δ¹³C_OWM and δ¹³C_bulk of 3‰ from shoreward to open-ocean facies that may reflect the effect of ¹³C-enriched dissolved inorganic carbon (DIC) derived from riverine sources in the relatively enclosed AB. The hypothesized steep carbon isotope gradient in the AB could be due to a strong biological pump; this in turn may have contributed to low oxygen bottom water conditions during the LDBC. This is the first time single-microfossil δ¹³C_org analyses of eukaryotes have been directly compared to bulk δ¹³C_org in the deep-time fossil record.