CLIMATIC CHANGES IN THE LATE FRASNIAN: TRIGGER OF THE FRASNIAN-FAMENNIAN CRISIS

The Late Devonian faunal crisis represents one of the big five Phanerozoic mass extinction events with shallow-water tropical faunas being severely decimated. Besides a bolide impact or anoxia, climatic warming as well as climatic cooling were discussed as potential trigger of the extinction event. However, no high-resolution palaeotemperature data were yet available that could support the idea of a climatic change.

Carbon isotope records for inorganic and total organic carbon as well as for specific organic biomarkers show two +3‰ excursions in the late Frasnian. These positive shifts in d¹³C coincide with the deposition of the Late Frasnian Kellwasser Horizons and are explained by a 10% increase in the organic carbon burial rate. A drawdown of atmospheric pCO2 and climatic cooling were inferred as consequence of the enhanced organic carbon burial. In order to reconstruct the palaeotemperature history for the Middle and Late Devonian we measured the oxygen isotopic composition of conodont apatite. By assuming a d¹⁸O value of -1‰ for Devonian sea water (ice-free world), the oxygen isotope ratios of Eifelian and Givetian conodonts give average temperatures of 22 to 25° C for low-latitude surface waters. A prominent increase in surface water temperature is observed in the middle Frasnian with calculated temperatures rising to 32° C. Conodonts from the Frasnian-Famennian transition show two positive d¹⁸O excursions with maximum amplitudes of +1 to +1.5‰ that parallel the +3‰ shifts in d¹³C. The positive shifts in d¹⁸O are indicative for two cooling pulses of 5 to 7° C during the latest Frasnian. Early Famennian palaeotemperatures are in the range of 30 to 33° C, comparable to those measured for the Frasnian.

The documented climatic changes are interpreted to have had a severe impact on the Frasnian low-latitude shallow water ecosystems. Tropical surface water temperature seem to have been at a maximum level during the late Frasnian and early Famennian. These very warm tropical surface water temperatures as well as the superimposed cooling pulses during the latest Frasnian may have been highly stressful for tropical shallow water faunas and may have culminated in low origination rates of new species that seem to be one of the major factors of the decline in diversity during the late Frasnian