The Toarcian oceanic anoxic event in the Western Saharan Atlas, Algeria (North African paleomargin); role of anoxia and productivity

We used an integrated analysis of benthic foraminiferal assemblages, geochemical proxies (paleoproductivity, redox, and detrital proxies), and C and O stable isotopes to characterize the Toarcian oceanic anoxic event along the North African paleomargin, more precisely in the Saharan Atlas (Algeria). Special emphasis was placed on time intervals previous to the anoxic event and the recovery of normal conditions after this event. The Fisher's alpha diversity of the foraminiferal assemblages decreased with respect to the end of the Pliensbachian in the polymorphum Zone, with lower values of shallow infaunal forms and an increase in opportunists (Reinholdella, Lenticulina, and Eoguttulina). This indicates fluctuations in the oxygenation degree of the infaunal microhabitat in the polymorphum Zone despite the fact that redox proxies do not show any fluctuations, indicating that dysoxic conditions were not reached. The lower part of the levisoni Zone reflects an abrupt paleoenvironmental change, evidenced by the extinction of foraminifera, enrichment in redox-sensitive elements, increase in detrital and paleoproductivity proxies, and fluctuations of delta (super 13) C and delta (super 18) O. An anoxic or strongly dysoxic event happened by the polymorphum-levisoni zone boundary in the Western Saharan Atlas, coincident with increasing eolian detrital input and paleoproductivity. The upper part of the levisoni Zone represents a return to normal oxic conditions, and the slow recovery of benthic foraminiferal assemblages dominated by opportunistic forms (Lenticulina, Eoguttulina, and Reinholdella) of low diversity. The Toarcian oceanic anoxic event occurred in a generalized transgression that may have favored water stratification and confinement of bottom waters in the subbasins developed in the Atlasic domain. This research demonstrates the potential of the integration of data from foraminiferal assemblages and a wide range of geochemical proxies for understanding anoxic events during the Mesozoic.