Comparison of organic and inorganic carbon isotope patterns across the Frasnian–Famennian boundary

The carbon isotopic composition of organic matter is reported for two Frasnian–Famennian boundary sections from the Rheinische Schiefergebirge (Germany). During the late Frasnian δ13Corg values commonly range from −27.5‰ to −26‰. Highest δ13Corg values around −24‰ are encountered immediately above and below the Kellwasser horizons. All samples from the Kellwasser horizons are depleted in δ13Corg by −2.5‰ to −4.7‰. This depletion is explained by a higher proportion of biomass derived from isotopically depleted chemoautotrophic bacteria. A comparison of the positive δ13C excursions shows that the δ13Corg shifts precede the δ13Ccarb shifts by some 105 years. Consequently, Δδ13C values document a shift towards lower values below the Upper Kellwasser horizon. A contribution of terrestrial organic matter, as well as a thermal alteration of the δ13Corg signals, do not provide compelling explanations for the earlier δ13Corg shift. Increases in primary productivity or carbon burial rate will change the CO2(aq) content of surface waters, affect the carbon isotope fractionation during photosynthetic carbon uptake and thus enrich organic carbon in 13C. However, both processes cannot account for the earlier onset of the δ13Corg shift, because the δ13Ccarb record should document the resulting 13C enrichment in surface waters as well. A reduction in the calcium carbonate precipitation rate, due to the global extinction of the reef ecosystem, may have played an important role in reducing the CO2(aq) content of surface waters without changing the isotopic composition of surface waters. Whether this process or other factors, such as a rise in sea-surface temperature or a change in the phytoplankton association, account for the earlier onset of the positive shift in the δ13Corg curve must be determined by further studies that may be able to confirm the global significance of the reported δ13Corg pattern in Frasnian–Famennian boundary sections from other parts of the globe.