Mediterranean Late Cenozoic Stable Isotope Record: Stratigraphic and Paleoclimatic Implications

Abstract

Ocean evolution during the past 100 million years (m.y.) has involved the replacement of warm Cretaceous oceans and seas by cold Neogene oceans, the closure of the Tethys, a mqjor circum-tropical ocean, and the development of a circum-polar ocean and a northsouth axis linking the Arctic with a southern ocean. The response of the Tethys (then the Mesogean and subsequently the Mediterranean) to this global climatic evolution would likely have been a specific one. Information on this climatic history is obtained by comparing the evolution of geochemical parameters, as recorded in deep-sea sediments, in the Tethys-or Mediterranean-and in the Atlantic. Among others, carbon and oxygen isotopes in marine carbonates have proved to be an indispensable tool for climatic reconstruction. Additional characteristic isotopic events have been identified that are synchronous on a world-wide scale and can serve as stratigraphic markers. These isotopic events are generally linked with paleoclimatic or hydrologic changes. The olgO variations are controlled by temperature changes and/or ice formation on land and at the poles; among the important factors affecting the distribution of 0J3C of the total dissolved inorganic carbon in the world ocean is the apparent oxygen utilization (related to the progressive oxidation of organic matter). In deep waters, longer residence time favors the addition of 0 J3C-depleted carbon to the dissolved bicarbonate by the oxidation of organic matter and accounts for the relation between the residence time of deep water masses and the 0J3C values of the ~C02 (the greater the residence time, the lower the OJ3C of the ~C02)' In the Mediterranean, oxygen isotope stratigraphy has proved to be valid for the Quaternary, at least for sediments as old as 650,000 years. In this sea, oxygen and carbon isotopic events also can be correlated with those recorded in pen oceans for periods older than the Quaternary and thus serve as stratigraphic markers. The specific isotopic response of Mediterranean carbonates to global climatic events (rapid oscillations, higher amplitude of the signal) is related to tectonic processes that shaped the geometry of the basin which, in turn, controlled the hydrography of the sea. Progressive closing of the connections with open oceans (mainly the Atlantic) and hydrodynamic changes have induced the periodic occurrence of rhythmic (diatomites) or widespread anaerobic (sapropels) deposits since the late Cenozoic. Faunal and isotopic analysis across these levels show that fresh-water input forming a low salinity surface layer was responsible for episodic local to regional stagnant phases, stratification of water masses, and oxygen depletion in bottom sediments. 0J3C changes recorded by deep calcite clearly indicate that from late Burdigalian to early Pliocene time, the residence time of the waters increased and the Mediterranean Basin was not a source of deep waters. It seems unlikely that during that time the flux of Mediterranean waters was sufficiently high to have been of influence on the formation of North Atlantic Deep Water.