Title: Oysters indicators of climate change - case study: The palaeoenvironmental distribution of late Miocene oysters in the northwestern Algerian basins

Abstract

Understanding the various interactions of benthic organisms with their biotopes is a valuable tool for reconstructing the various parameters of marine palaeoenvironments (Videt et Néraudeau, 2002). Among the pteriomorphic bivalves, with calcitic shells, the ostreidae, due to the excellent preservation of their shells and the ubiquitous distribution of this group of mollusc, allowed to consider them among the most useful benthic marine organisms in the interpretation of palaeoenvironments (Videt et Néraudeau, 2002; Satour 2012) The main aim of this work is to specify the Tortonian-Messinian oyster environments in the three Neogene Algerian basins (Lower Chelif, Tafna, M’sirda). This study was made on the basis of shell material collected from eight sections, during several field trip missions (2002, 2003, 2004, 2005 and 2008). The distribution of oysters in the tortonian-messinian studied deposits is highlighted, taking into consideration the main parameters prevalent the establishment of oyster levels: sedimentary dynamics and basin tectonics (Videt et Néraudeau 2002). References on upper Miocene and current oysters in the Mediterranean region (Lecointre et Ranson 1961; Ranson 1941, 1949; Jiménez et al., 1991; Freneix et al. 1988..) allowed better consideration of the oyster palaeoenvironments interpretation of the neogene basins. The adopted bathymetric method is that of Pérès and Picard (1964). New data have been brought about the late Miocene oyster palaeoenvironmental distribution in the northwestern Algerian basins. The analysis of ecological features of these oysters has shown the recurrent occurrence of four species of oysterids (Ostrea lamellosa offreti, Crassostrea gryphoides, Neopycondonte navicularis and Hyotissa hyotis). Alone or in association, they characterize deep environments of large calm circalittoral mud flats (Neopycnodonte navicularis), infralittoral environments with strong hydrodynamics (Crassostrea gryphoides), shallow environments (infralittoral-mediolittoral) with significant detrital inputs (Ostrea lamellosa offreti) and reef to peri-reef contexts (Hyotissa hyotis). Tortonian oyster emphasizes the refreshment trend of the late Tortonian. The onset of the Tortonian cooling climate is marked by the implementation of a slightly displaced shell accumulation, containing two oysters: Ostrea lamellosa offreti (Kilian, 1889) and Hyotissa hyotis (Linnaeus, 1758). Crassostrea gryphoides (Schlotheim, 1913) confirms the cooling climate trend, with the reduced size of shells. Furthermore, the Messinian climate tendency is related to the diatomite extension and the 1st coral cycle. The large, deep circalittoral mud flats are succeeded by marly diatomites con fined environments. Climate conditions are warmer or even subtropical and characterize a peri-reefal to reefal environments.