Conference:
Scientific Workshop ” What happened in southern Romania and Black sea when the Mediterranean Sea desiccated 5.6 Ma ago”, Bucarest, Romania, 29 February – 1 March 2008, Abstract volume, pages 18 -19.
Authors:
Mihaela Carmen Melinte-Dobrinescu, Jean-Pierre Suc, Georges Clauzon, Speranta-Maria Popescu, M. Namik Çagatay.
Abstract:
Calcareous nannoplankton represents a group of marine algae very sensitive to environmental fluctuations. Therefore, the paleobiogeographical changes which took place in European areas (i.e., Mediterranean, Paratethys) across the Miocene/Pliocene are mirrored by the calcareous nannofloral variability. Outside the Mediterranean area, the Miocene/Pliocene boundary (i.e., the Messinian/Zanclean boundary – MZB) is well constrained based on calcareous nannoplankton events, such as the last occurrence (LO) of Discoaster quinqueramus at 5.537 Ma (Backman and Raffi, 1997- calibrated in the Atlantic region), followed by the first occurrence (FO) of Ceratolithus acutus at 5.37 Ma (Backman and Raffi, 1997, also calibrated in the Atlantic region), which is only 40 kyrs earlier than the GSSP of the MZB (placed at 5.332 Ma, Van Couvering et al., 2000), and by the LO of Triquetrorhabdulus rugosus, 5.23 Ma in Mediterranean and Atlantic (Castradori, 1998). Within the Mediterranean and Eastern Paratethys areas a continuous marine record is not available because the Messinian Salinity Crisis, characterized by a gap of sediments on the margins corresponding to the peak of the desiccation of the basins (5.60-5.48 Ma: Clauzon et al., 1996; Clauzon et al., 2005; Popescu et al., in press). In addition, some nannofossils are rare or even absent (e.g. Discoaster quinqueramus). In general, the distribution of calcareous nannofloras in the basal Pliocene of Mediterranean and Paratethys areas is poorly known, possibly because of local scarcity of oceanic marker species.
This contribution presents the results of calcareous nannofossil investigations carried out on the Intepe and nearby sections (located at the western opening of the Dardanelle Strait, NW Turkey) which belong to the regional AlçıtepeFormation, widely distributed in the Marmara and Gulf of Saros area and classically considered as representing the entire Messinian Stage (Çağatay et al., 2006). This formationis characterized by a brackish water Paratethyan fauna (Mactra sp., Paradacna abichi, Dreissena spp., etc.), similar to the “Lago Mare”facies associated with a marine Mediterranean fauna (Ostrea, Cardium spp.). A brackish environment dominated this time-interval, but several marine influxes are to be assumed, based also on nannofloras recorded by us. Calcareous nannoplankton assemblages of the lower half of the Intepe section belongs to the NN11b Calcareous Nannoplankton Subzone, dominated by long-ranging and diagenetical resistant taxa such as Reticulofenestra spp., Dictyococcites spp., Sphenolithus moriformis. But, in some intervals, nannofossils related to an open marine environment (Discoaster spp. and Sphenolithus spp.) also occur. Above, an interval with almost monospecific assemblages (Braarudosphaera bigelowii), indicating strong salinity variations, was observed. Around the base and top of this interval, blooms of the calcareous dinoflagellate genus Thoracosphaera (which mirrored unstable marine conditions) were also identified. This latter interval is topped by a lignite (Gillet et al., 1978), above which a marine open environment is restored according to prevalence of Discoaster and Sphenolithus within the nannoflora that belongs to the Calcareous Nannoplankton Subzone NN12b (i.e. earliest Zanclean) according to the regular presence of Ceratolithus acutus. The lignite is continuously overlain by red (oxidized) clays (20-30 cm thick) because of an emersion event that we necessarily refer to the peak of the Messinian Salinity Crisis. This gap in sedimentation caused by the desiccation of the Aegean Sea within this apparently continuous section is also strongly supported by the discovery in the area of deep Messinian fluvial valleys (cutting sediments of the NN11b Nannoplankton Subzone) and infilled by Gilbert-type fan delta deposits, the marine bottomset of which belong to the NN12b Nannopklankton Subzone.
Accordingly, the region of the Marmara Sea was so severely affected by the Messinian Salinity Crisis as the other peri-Mediterranean areas, that also contributes to discard a marine gateway in this area during the Lago Mare events (Clauzon et al., 2005; Popescu, 2006; Popescu et al., in press).