Carbon isotope geochemistry of the Frasnian-Famennian transition

Carbon isotope records for inorganic and total organic carbon (TOC) as well as for individual organic biomarkers show two positive excursions in the Late Frasnian with comparable shape and amplitude in δ¹³C. The positive shifts in δ¹³C can be correlated with the deposition of the Kellwasser horizons. The inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved CO₂. TOC δ¹³C values do not exhibit a larger shift than that recorded by inorganic carbon δ¹³C values; this is unexpected since a decrease in the dissolved CO₂ concentration should result in lower carbon isotope fractionation during photosynthesis (ε_p). δ¹³C values of presumed algal biomarkers (low-molecular-weight n-alkanes, pristane, phytane), although offset by ∼2.5‰, exhibit essentially identical records as TOC, and confirm this unexpected result. It is proposed that high atmospheric and oceanic CO₂ concentrations during the Devonian resulted in maximum photosynthetic fractionation. Any change in the CO₂ concentration would thus not affect ε_p. As such, δ¹³C of primary organic carbon and δ¹³C_carb will exhibit parallel excursions. The data imply that carbon isotopes and the relationship between ε_p and [CO₂] will not be effective to study changes in p_CO2 levels during the Devonian since CO₂ concentrations were too high.