Global and local climatic cycles recorded by vegetation in the Mediterranean region during the Early-Middle Pleistocene transition.
Résumé
Earth climate was affected by a relatively progressive cooling during the Pliocene and the Pleistocene. This cooling trend was constituted by successive climate cycles. Cycles of 41 kyr-long were recognized during the early Pleistocene and were forced by the obliquity orbital parameter. During the Middle Pleistocene, cycles of 100 kyr-long were forced by the eccentricity (precession modulator). The shift from the “41 kyr world” to the “100 kyr world” was a period of increased cooling called the Mid Pleistocene Transition (MPT: 1.200 to 0.500 Ma). The marine isotopic stages (MIS) characterized the increasing duration of climatic cycles associated with a reinforced precession influence during the MPT. However, the oxygen isotopic proxy shows weak precessional control in marine Pleistocene sediments. As the Mediterranean region is under the influence of both obliquity and precession and because the heat-transfer from low to high latitudes is almost entirely supported by atmospheric transport, vegetation from Mediterranean region is expected to record climatic conditions, which experienced this atmospheric transport. Pollen analysis has been performed on five sections: Site ODP 976 (Alborean Sea), Santa Lucia and Montalbano Jonico (South Italy), Tsampika (Rhodes, Greece) and Shamb (Armenia). These sections are placed along a 4,000 km long transect from the Western Mediterranean region to Little Caucasus. Calibrations of these marine and continental sections were provided by integration of results from magnetostratigraphic and radiochronological dates as well as foraminiferal and nannofossil biostratigraphic events. More than 100,000 pollen grains were counted and permitted the construction of pollen diagrams, which were correlated with foraminiferal oxygen isotopic measurements provided by the same samples or close sections. Palynology was used to reconstruct vegetation modifications, which show the same cyclic vegetation change pattern related to climatic cycles that occurred during the Early-Middle Pleistocene (1.400-0.700 Ma). “Long-term” vegetation successions were controlled by climatic cycles and related to climate response to obliquity forcing. A similar vegetation dynamics with shorter and longer-term durations were forced by climate responses to precession and its modulator (the eccentricity), respectively. As influences of both parameters were superimposed in the vegetation, before and during the Mid-Pleistocene Transition, the expected shift from obliquity to 100 ka long-cycles related to eccentricity is not observed.