Palaeoenvironmental turnover across the Cenomanian-Turonian transition in Oued Bahloul, Tunisia: foraminifera and geochemical proxies

The integrated analysis of foraminiferal assemblages, geochemical proxies, and stable isotopes in the Oued Bahloul section (Tunisia) allowed us to reconstruct the environmental turnover across the Cenomanian-Turonian boundary. An increase in palaeoproductivity proxies (P/Ti, U/Al, Sr/Al) and in delta C-13 values, and a decrease in foraminiferal diversity and delta O-18 values mark the beginning of the Oceanic Anoxic Event 2 (OAE2) at the Rotalipora cushmani and Whiteinella archaeocretacea biozones boundary. Eutrophic conditions at the seafloor and in the water column are evidenced by high proportions of buliminids and the replacement.of planktic oligotrophic specialist Rotalipora by eutrophic opportunist Hedbergella. The enrichment in organic matter and redox sensitive elements, together with the abundance of low-oxygen tolerant benthic foraminifera, indicate dysoxic conditions in the deep-water column and at the seafloor (higher U-aut than Mo-aut). Among planktic foraminifera, deep- and intermediate-dwellers disappear (Rotalipora and Globigerinelloides), and surface-dwellers proliferate (Hedbergella). The persistency of the poorly oxygenated conditions during the W. archaeocretacea Biozone locally produced euxinic conditions where Mo-EF and Mo-aut reach high values, diversity presents minimum values, and benthic foraminifera temporarily disappear. The maximum percentage of heterohelicids indicates a stratified water column with a well-developed oxygen minimum zone. Improved oxygen conditions returned in the upper part of the W. archaeocretacea Biozone and Helvetoglobotruncana helvetica Biozone, with a slow recovery of foraminiferal assemblages, decrease in eutrophic genera (Heterohelix) and increase in mesotrophic genera (Whiteinella). A gradual increase in delta O-18 values suggests decreased temperatures in surface waters. The OAE2 has been attributed to global temperature changes and palaeoceanographic reorganization. The poor mixing of surface and deep waters and enhanced primary productivity related to global warming - associated with increasing continental weathering and nutrient runoff may have favored the eutrophication of the ocean and the expansion of the oxygen minimum zone. (C) 2014 Elsevier B.V. All rights reserved.