Mediterranean Quaternary interglacial molluscan assemblages: Palaeobiogeographical and palaeoceanographical responses to climate change

Abstract

The analysis of several central to eastern Mediterranean shallow-water deposits, mostly related to Posidonia oceanica palaeocommunities, sheds light on the presence of thermophilic molluscan assemblages throughout the Mediterranean Pleistocene. The taxonomic composition of these assemblages is totally different from that of the well-known Senegalese molluscan fauna and they play an important role in understanding some of the hydroclimatic conditions during Quaternary interglacial episodes in the central-eastern Mediterranean basin. Two of these assemblages from the Kyllini–Trypiti sequence (NW Peloponnesus) can be referred to the Upper Pleistocene interglacial marine isotope (sub)stage (MIS) 5e and 5a, on the basis of U/Th dating of corals (Cladocora coespitosa). Other assemblages belong to Lower Pleistocene (mainly Emilian substage, possible MIS 37), late Lower (Sicilian substages)–early Middle Pleistocene and late Middle–Upper Pleistocene (MIS 7 and MIS 5e). As a whole, these assemblages consist of 15 species of palaeoclimatic and stratigraphic value. Among these, Craspedochiton altavillensis (Polyplacophora), Jujubinus? bullula, Ersilia aliceae, Haedropleura bucciniformis, Strioterebrum basteroti (Gastropoda), and Plicatula mytilina (Bivalvia) are (Euro)Mediterranean strictly endemic extinct species, whereas Rissoina decussata, Niso terebellum, Kyllinia parentalis, Strioterebrum grayi, Terebra corrugata, Terebra reticularis (Gastropoda), Anadara sp., Chama placentina, and Corbula revoluta (Bivalvia) can be regarded as closely related to recent tropical West African taxa or as species living in tropical waters. The latter stock is useful for reconstructing palaeoclimatic conditions, suggesting winter sea surface temperatures 4–6°C and 2–4°C warmer than today in western Sicily and the easternmost part of the Mediterranean Sea, respectively. In contrast with Pleistocene glacial settings, detected warm episodes are characterized by a lower degree of seasonality, with a difference between summer and winter sea surface temperatures of 6–7°C vs. 10–11°C recorded at present in the Mediterranean. The molluscan assemblages also indicate that central to eastern Mediterranean salinity was lower than today, suggesting that a different hydrological regime, triggered by increase in rainfall/runoff, was manifested during warmer Pleistocene periods in that area. The distribution of some of the most meaningful taxa studied here indicates that the Mediterranean Sea was divided into two palaeobioprovinces, a cooler western and a warmer eastern one, with a boundary zone possibly located near the present Sicilian–Tunisian Strait.The Anadara sp.–Jujubinus? bullula association is a meaningful ecobiostratigraphical marker for recognizing warm Lower Pleistocene–Upper Pleistocene events. The youngest occurrence of this association has been detected in MIS 5a, in the uppermost part of the Kyllini–Trypiti sequence. Craspedochiton altavillensis, J.? bullula, Ersilia aliceae, Anadara sp., Chama placentina and Plicatula mytilina became extinct during the MIS 5a. Kyllinia parentalis and Terebra corrugata became extinct during MIS 5e. The disappearance of Niso terebellum took place during the Emilian substage (Lower Pleistocene). The gastropod Nassarius musivus musivus, previously considered as becoming extinct at the top of the Gelasian stage, has been recorded in the eastern Mediterranean deposits younger than MIS 5a and older than MIS 3.