The Mediterranean water body in the late Messinian: interpreting the record from marginal basins on Sicily

The stratigraphic record of climatic and palaeoceanographic changes in the Mediterranean during the late Messinian is controversial. On Sicily, sedime ntary facies show a steady transgression, suggesting that sub-basins were h ydrodynamically linked to a larger water body. Here we test this hypothesis using strontium isotope studies from Messinian materials collected from a range of sites in the Caltanissetta Basin of central Sicily. The strata inc lude a regionally regressive 'First Cycle' of early-mid-Messinian age and a younger, transgressive 'Second Cycle'. These cycles are separated by an in ter-regional unconformity which may be correlated with base-level low stand in the Mediterranean. Sr-87/Sr-86 ratios for all First Cycle gypsum fail w ithin the expected global marine composition (0.70891-0.70897). All Second Cycle analyses fall within a grouping of significantly lower values (0.7086 8-0.70878), a surprisingly tight but discrepant grouping for data collected from shells of a brackish water fauna and from gypsum. Analyses from these different Second Cycle materials are statistically indistinguishable. Thes e results indicate that the strontium isotopic composition of waters from d ifferent sub-basins on Sicily are indistinguishable regardless of salinity during the late Messinian. Therefore these basins must have mixed with a la rger, homogenized reservoir which we infer was the ancestral Mediterranean. Thus circum-Mediterranean basins may indeed chart regional palaeoceanograp hic and climatic events. By the end of Messinian times the base-level of th e Mediterranean was within the range of the world's oceans but the water-bo dy probably had a distinctly different but internally homogeneous strontium isotopic composition.