Précision des mesures de variation relative verticale du niveau marin à partir d'indicateurs biologiques le cas des soulèvements bradysismiques de Pouzzoles, Italie du sud (1969-1972 et 1982-1984)

Fixed littoral plants and invertebrates (notably Vermetid Gastropods), the skeletons of which results in littoral bioconstructions of some extent, or in distinctive bioerosive patterns, have been used for many years as biological indicators of the variation of relative sea level in Holocene and late Pleistocene times. This method displays a fair sensitivity for small scale events such as local earthquakes which leave few or no trace in the sedimentary record. It is nevertheless sometimes difficult to use in the case of rapid submersion of the shoreline or of rapid up and down relative sea level movements. Taking advantage of the well known sequence of bradyseismic events in the Phlegraean Fields of Pozzuoli, near Naples in Italy, we attempted to estimate the accuracy and reliability of the biological measurements, comparing them to the high precision levelling surveys of the Osservatorio Vesuviano which bear on more than 35 years. A biological survey was done on the cliffs of volcanic tuffa of Rione Terra (Pozzuoli). Two clear cut biological complexes of Vermetid tubes were put into evidence at the altitudes of 2,38 m and 0,87 m above present (1995-96) mean sea level corresponding to standstills anterior to 1969 and between 1973 and 1982. The rapidity of the bradyseismic rise of the substratum, with velocities from 45 to 85 cm per year during the crises, was clearly apparent from the total absence of biological midlittoral erosion on the tubes. Our results are consistent with those obtained by levelling, except for the short vertical oscillation during the bradyseismic crisis of 1982-1984, which was too short to be clearly registered by plant or animal growth. In the same manner, the recent submersion between 1985 and 1990 was too rapid to be registered by marine organisms. In conclusion, the limits of the biological method are linked to the minimum time required by organisms for completing a recognizable bioconstruction or bioerosion pattern. Episodes about 5-10 years long can be however detected if the remains have not been carried underwater by a downward movement of the substrate.