CARBON ISOTOPE GEOCHEMISTRY OF THE FRASNIAN-FAMENNIAN TRANSITION

Abstract

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 (#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 #p. As such, δ13C of primary organic carbon and δ13Ccarb will exhibit parallel excursions. The data imply that carbon isotopes and the relationship between #p and [CO2] will not be effective to study changes in pCO2 levels during the Devonian since CO2 concentrations were too high.