Carbon isotope geochemistry of the Frasnian–Famennian transition

• Published in: Palaeogeography, palaeoclimatology, palaeoecology Vol. 181; no. 1; pp. 91 - 109
• Main Authors: Joachimski, M.M, Pancost, R.D, Freeman, K.H, Ostertag-Henning, C, Buggisch, W
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.06.2002
• Subjects: carbon isotopes; Late Devonian; mass extinction; photosynthetic fractionation; mass extinction; Late Devonian; carbon isotopes; photosynthetic fractionation
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/S0031-0182(01)00474-6

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 δ 13C. The positive shifts in δ 13C 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 CO 2. TOC δ 13C values do not exhibit a larger shift than that recorded by inorganic carbon δ 13C values; this is unexpected since a decrease in the dissolved CO 2 concentration should result in lower carbon isotope fractionation during photosynthesis ( ε p). δ 13C 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 CO 2 concentrations during the Devonian resulted in maximum photosynthetic fractionation. Any change in the CO 2 concentration would thus not affect ε p. As such, δ 13C of primary organic carbon and δ 13C carb will exhibit parallel excursions. The data imply that carbon isotopes and the relationship between ε p and [CO 2] will not be effective to study changes in p CO 2 levels during the Devonian since CO 2 concentrations were too high.