Chitinozoan bio-event in the Frasnian-Famennian boundary beds at La Serre (Montagne Noire, Southern France)

Abstract

Chitinozoans and other palynomorphs have been investigated in Frasnian-Famennian boundary beds from trench C at La Serre (Montagne Noire, Southern France). This section is located some 30 km from the Coumiac quarry containing the Global Stratotype Section and Point (GSSP) for the base of the Famennian. At La Serre, the Frasnian-Famennian (F-F) boundary falls within interbedded anoxic dark shales and limestones. This lithology proves to be more suitable for palynological investigations than the reddish nodular limestones of the F-F stratotype in the Coumiac section, which are virtually devoid of organic matter. At La Serre, the uppermost meter of Frasnian deposits, corresponding to the Upper Kellwasser Event level, contains only a few chitinozoans but other palynomorphs, especially tasmanaceans ( Maranhites) proliferate in some horizons. Conversely, the most basal Famennian bed yields an exceptional number of chitinozoans (up to 19,000 specimens per gram of rock), a few tracheids, but virtually no tasmanaceans. The absence—or the extremely low numbers—of miospores and acritarchs in these F-F boundary beds seems related to the distal position of the deposition site. We have compared the fluctuation of the chitinozoan abundance both with the abundance of the other palynomorphs, and with that of the conodonts present in the same samples. These data, collated with sedimentological and palaeoecological information document the relationships between the anomalous abundance of the chitinozoans and the latest Frasnian-earliest Famennian mass extinction. It is concluded that the exceptional chitinozoan concentration in the most basal Famennian bed at La Serre is not related to a sorting process generated by turbidity currents or by a temporary increase in the hydrodynamic energy at the water-sediment interface. It corresponds to the conjunction of a high production of chitinozoans, a very low rate of destruction of the vesicles and minimal dilution of the deposit by terrestrial mineral input. The environmental factors leading to such an exceptionally high record of chitinozoans are discussed with a particular attention to consequences of the late Frasnian mass extinction.