Ostracods sensitivity to reconstructing water depths and oxygen levels: A case study from the Middle-Late Eocene of the Beni Suef area (Egypt)

Abstract

Fifty-five ostracod species have been identified from 100 samples belonging to three stratigraphic sections in the Middle-Upper Eocene strata of the Beni Suef area (southern Tethys of Egypt). These ostracods have been used to subdivide the studied time interval into three zones that are correlated to the planktic foraminiferal zones E13 to E15. Based on clustering, non-metric Multidimensional Scaling (nMDS), and The Permutational MANOVA, sediment facies was found to be a significant factor controlling the ostracod fauna. Four assemblages were identified and their bio-ecological preferences were used to reconstruct paleoenvironments: 1) The upper Bartonian ostracod assemblage (El Fashn Formation) is dominated by Paracypris eskeri Bassiouni and Morsi and Novocypris eocenana Ducasse and is characterized by lower Shannon Diversity and higher ‘Dominance’ of stenohaline species with preference for lower oxygen levels and deeper neritic water depths; 2) The uppermost Bartonian (Qurn Member of Beni Suef Formation) is dominated by Paracypris eskeri Bassiouni and Morsi and Bairdia gliberti (Keij) indicating better oxygenated environmental conditions, supported by higher diversity values; 3) The lower Priabonian (Tarbul Member of Beni Suef Formation) is dominated by Novocypris eocenana Ducasse and has the lowest diversity, linked to the significant decrease of species with preference for highly oxygenated conditions; 4) The uppermost Priabonian (Maadi Formation) contains the most diverse assemblage and is characterized by a very low ‘Dominance’ and very high Shannon Index, where 26 species constitute the trophic nucleus and most of them are indicative of shallow water-depths and inner shelf settings in addition to preferences for high oxygen levels. The general increase in diversity and decrease in dominance is attributed to improved environmental conditions at shallower depths, where enhanced water mixing increased the oxygen available for biological demands. Moreover, the strong similarity of the recorded ostracod assemblages with those reported from other southern Tethyan sites reflects migration through the Atlantic and/or the trans-Sahara seaway, while the low similarity with the northern Tethys province (Europe) may be related on the one hand to their benthic nature, which may have limited their ability to move for longer distances, on the other hand to prevailing cooler waters to the north at higher latitudes. Water depth and associated environmental changes in respect to oxygen levels is interpreted to be the main factor controlling the distribution of the ostracods in the Beni Suef area.