EVIDENCE FOR PERTURBATION OF THE CARBON CYCLE IN THE MIDDLE FRASNIAN PUNCTATA CONODONT ZONE

New carbon isotopic data on 257 fabric-retentive and non-luminescent brachiopods ‘secondary' layers of Ardennes (Belgium) and Holy Cross Mountains (Poland) from late Emsian to late Frasnian are proposed. Belgian and Polish d13C curves show a similar secular variation. Nine main excursions are observed in both areas. The episodes of eustatic sea-level rise are correlated with the positive d13C shifts. The negative d13C shifts are more gradual than the positive ones and correspond to the main eustatic sea-level falls. This work also highlights an abrupt and high-amplitude d13C excursion in the punctata conodont Zone (Middle Frasnian). Published information from Moravia and China suggests that this negative excursion, jointly with preceding large-scale positive shift, should be used as a global chemostratigraphic marker. The d13C fluctuations of the punctata Zone are among the highest d13C oscillations known in the Phanerozoic. Causation scenarios for this “punctata event” may be related to the Alamo Impact Event. Another explanation could be intense and rapid sea-level changes at this period, recently deciphered in the Ardennes. Although the punctata conodont Zone coincides with a regional reefal crisis in Europe, the Early to Middle Frasnian interval is marked by very low extinction intensity in the world ocean. So, despite at least one proved impact event, and the possible onset of a global warming in the punctata Zone, the d13C excursion is not connected with a major breakdown of carbonate production. This lends support to the hypothesis that principal extrinsic environmental stimuli such as impacts and climate change may not have a major effect on biota. Further detailed multidisciplinary works are however necessary to improve our knowledge of the mechanisms involved in the punctata d13C excursion.