VALIDITY OF BIOLOGICAL INDICATORS OF SEA-LEVEL VARIATIONS IN A BRADYSEISMIC ENVIRONMENT

Fixed littoral plants and invertebrates (notably Vermetid Gastropods), the skeletons of which results in littoral bioconstructions of some e xtent, 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 se nsitivity for small scale events such as local earthquakes which leave few or no trace in the sedimentary record. It is nevertheless sometim es difficult to use in the case of rapid submersion of the shoreline o r of rapid up and down relative sea level movements. Taking advantage of the well known sequence of bradyseismic events in the Phlegraean Fi elds of Pozzuoli, near Naples in Italy, we attempted to estimate the a ccuracy 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 bio logical complexes of Vermetid tubes were put into evidence at the alti tudes of 2,38 m and 0,87 m. above present (1995-96) mean sea level cor responding to standstills anterior to 1969 and between 1973 and 1982. The rapidity of the bradyseismic rise of the substratum, with velociti es from 45 to 85 cm per year during the crises, was clearly apparent f rom the total absence of biological midlittoral erosion on the tubes. Our results are consistent with those obtained by levelling, except fo r the short vertical oscillation during the bradyseismic crisis of 198 2-1984, which was too shore to be clearly registered by plant or anima l growth. In the same manner, the recent submersion between 1985 and 1 990 was too rapid to be registered by marine organisms. In conclusion, the limits of the biological method are linked to the minimum rime re quired by organisms for completing a recognizable bioconstruction or b ioerosion pattern. Episodes about 5-10 years long can be however detec ted if the remains have not been carried underwater by a downward move ment of the substrate.