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 delta(13)C(org) values commonly range from -27.5 parts per thousand to -26 parts per thousand. Highest delta(13)C(org) values around -24 parts per thousand are encountered immediately above and below the Kellwasser horizons. All samples from the Kellwasser horizons are depleted in delta(13)C(org) by -2.5 parts per thousand to -4.7 parts per thousand. This depletion is explained by a higher proportion of biomass derived from isotopically depleted chemoautotrophic bacteria. A comparison of the positive delta(13)C excursions shows that the delta(13)C(org) shifts precede the delta(13)C(carb) shifts by some 10(5) years. Consequently, Delta delta(13)C 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 delta(13)C(org) signals, do not provide compelling explanations for the earlier delta(13)C(org) 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 C-13. However, both processes cannot account for the earlier onset of the delta(13)C(org) shift, because the delta(13)C(carb) record should document the resulting C-13 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 Tor the earlier onset of the positive shift in the delta(13)C(org) curve must be determined by further studies that may be able to confirm the global significance of the reported delta(13)C(org) pattern in Frasnian-Famennian boundary sections from other parts of the globe. (C) 1997 Elsevier Science B.V.