LATE MIOCENE GLOBOROTALIID EVENT-STRATIGRAPHY AND BIOGEOGRAPHY IN THENE-ATLANTIC AND MEDITERRANEAN

The quantitative study of keeled and unkeeled globorotaliids in severa l Late Miocene sites of the NE Atlantic, the North Betic and South Rif ian Gateways and the Western Mediterranean enabled us to recognize a s equence of planktic foraminiferal (PF) events useful for a high resolu tion correlation of the Mediterranean with the global ocean for the ti me interval immediately prior to the salinity crisis. PF-Event 1 (6.7 Ma, end of Subchron 7n1) is defined by the sudden disappearance or red uction of Globorotalia menardii group I (sinistral) in most of the NE Atlantic and Mediterranean. This is probably due to a southward migrat ion of these forms to more subtropical latitudes. PF-Event 2 (6.56 Ma; Subchron 6r2) is defined by the abundant occurrence of Globorotalia m enardii group II (dextral). Between PF-Events 1 and 2, the temperate w aters of the NE Atlantic Ocean and Mediterranean Sea were almost compl etely lacking in keeled globorotaliids. The most prominent change in t he assemblages, the replacement of the Globorotalia menardii group II by the Globorotalia miotumida group (PF-Event 3; 6.4 Ma, Subchron 6n) may be correlated with the Tortonian/Messinian boundary and is related to a widespread second southward migration of northern temperate faun as replacing the subtropical ones as the North Atlantic latitudinal th ermal gradients increased. This event also coincides with the beginnin g of the Mediterranean crisis recorded by isotope fluctuations in surf ace waters and immediately followed by the development of widespread a noxic conditions in the deep and intermediate waters during the deposi tion of the Tripoli Formation. PF-Event 4 (first abundant occurrence o f dextral specimens in the Neogloboquadrina acostaensis group; approxi mately between 5.6 and 5.5 Ma, Subchron 5r1) and PF-Event 5 (reduction or disappearance of the Globorotalia miotumida group; approximately b etween 5.6 and 5.5 Ma, Subchron 5r1) predate the Lower Evaporites and PF-Event 6 (First abundant occurrence of Globorotalia margaritae s.s.; Subchron 5n1) is contemporary to their deposition. The coiling direct ion of unkeeled globorotaliids, highly variable in the first part of t he Late Tortonian, became almost 100% dextral (PF-Event A) a short tim e after PF-Event 2. This group maintained this pattern until the Late Messinian with the exception of a short interval with a peak of sinist ral forms just at the Tortonian/Messinian boundary. On comparing these events with the carbon isotope records of different sites of the Atla ntic Ocean we recognized similar trends between the events. Maximum de ltaC-13 values in planktic and benthic foraminifera coincide with PF-E vent 1, they decrease between PF-Events 1 and 2 and increase again imm ediately after PF-Event 2. PF-Event 3 coincides with the beginning of the global Late Miocene carbon shift.