Sedimentary, stable isotope and micropaleontological records of paleoceanographic change in the Messinian Tripoli formation (Sicily, Italy)

摘要

The Tripoli Formation (6.96-5.98 Ma) of the Central Sicilian Basin provides a good record of the paleoceanographical changes that affected the Mediterranean during the transition from slightly restricted conditions to the onset of the Messinian Salinity Crisis. The Falconara/Gibliscemi section has been selected for an integrated approach at a high resolution scale using sedimentology, stable isotopes of the carbonates and microfossils. The sedimentary succession includes 46 precession-controlled cycles resulting from the periodical increase in biosiliceous productivity (diatomites) that followed the deposition of marls and pinkish laminites, which appear as sapropel-type deposits induced by the oceanic fertilization by terrestrial nutrients during wet periods. Higher scale environmental changes are superimposed to this precession forced rhythmicity. There is a general trend of increasing basin restriction from near marine conditions at the base of the Tripoli to semi-closed settings in its uppermost part, which are the prelude of the salinity crisis. This pattern reflects the hydrological response of the Mediterranean to the progressive decrease of the Atlantic inputs and an enhanced influence of the climate on depositional conditions. However, this evolution is not linear and shows successive phases of different duration. During the first period (until 6.71 Ma), open Atlantic-Mediterranean exchanges maintained relatively stable marine conditions. The second period (6.71-6.29 Ma), marks an important step in the basin restriction with a wider range of salinity fluctuations and an increased bottom stagnation. The 6.71-Ma event, which is correlated at a Mediterranean scale, may have resulted from shallowing of the Mediterranean gateway under a tectonic control. This shallowing reduced the oceanic inputs resulting in an increased climatic constraint of the Mediterranean hydrology. During the third period (6.29-6.03 Ma) an increase of the surface water salinity resulted in stressful conditions for the marine microfauna. The 6.29-Ma change is a major step in the restriction that may be correlated with the intensification of the glaciation recorded in the Atlantic, which could have enhanced the effects of the tectonic closure. The last two cycles (48 and 49), that underlie the 'Calcare di Base', witnessed the rapid transition to a semi-closed Mediterranean setting characterized by large variations of salinity from diluted to hypersaline conditions, under a dominant climatic control, and by the nearly complete disappearance of the marine organisms. Long-trend environmental changes recognized within the Tripoli Formation resulted from a complex set of interfering factors controlling the water fluxes exchanged between the Mediterranean and the world ocean. Most of the rapid changes identified in Falconara/Gibliscemi at 7.16, 6.71 and 6.29 Ma, that closure of the Atlantic connections, although a glacio-eustatic overprint cannot be completely excluded.