Benthic foraminifera-based reconstruction of the first Mediterranean-Atlantic exchange in the early Pliocene Gulf of Cadiz

Abstract

Upper Miocene to lower Pliocene sediment cores from Integrated Ocean Drilling Program (IODP) Hole U1387C (IODP Expedition 339) have been studied. The main goal of this study is to reconstruct initial Mediterranean-Atlantic water exchange after the opening of the Gibraltar Strait in the early Pliocene. Benthic foraminiferal assemblages are the focus of the palaeoceanographic analyses of this work. A distinct faunal turnover indicates a considerable change of the depositional environment at the Miocene/Pliocene boundary. Foraminiferal composition suggests high rates of organic carbon flux and poorly oxygenated sea-floor environment for the basal, late Messinian interval. In contrast, earliest Pliocene assemblages point to periodic advection of warm, better ventilated and likely Mediterranean-sourced waters to the studied site. Parallel periodic changes in Siphonina tubulosa, Globocassidulina subglobosa, and Planulina ariminensis and previously established sedimentological and geochemical records (Zr/Al, δ18O) suggest a rather sluggish Mediterranean Outflow between ~5.3 and 5.2Ma. Benthic foraminifera, along with sedimentological and geochemical data thus provide the earliest indications of Mediterranean-Atlantic water exchange following the opening of the Gibraltar Strait. Sandy layers become frequent towards the top of the interval. These sediments are rich in shelf foraminifera, reflecting episodes of turbidite deposition possibly caused by tectonic adjustments related to the opening of the Gibraltar Strait. The allochthonous assemblages frequently contain Cibicides lobatulus and C. refulgens, species that have been considered indicators of Mediterranean Outflow Water (MOW) in previous studies. Our results warrant for caution when applying these species as MOW proxy in the fossil record if there is evidence for downslope transport.