Environmental conditions during the deposition of the diatomite –organic-rich marl alternation of the lower Messinian of the Lower Chelif Basin (Algeria) interpreted from microfossil assemblages and geochemistry

Abstract

The lower Messinian diatomites and organic-rich deposits of the Lower Chelif Basin (North Algeria) constitute a record of the early stages of Mediterranean restrictions, i.e. when connections with the Atlantic were limited. Geochemical proxies and microfossil assemblages from the Ouilis section allowed us to discern three members and to confirm oxygen-depleted conditions for bottom waters during alternative deposition of diatomites and organic-rich marls. The high TOC and content of redox sensitive elements as well as the absence of benthic foraminifera during the deposition of members 1 and 2 point to oxygen-depleted conditions at the bottom. Water stagnation and anoxic conditions were more intense during deposition of Member 2, when enrichments of Mo and U were the highest. U/Th and authigenic U confirm anoxic conditions in this stratigraphic interval. These environmental conditions also affected oxygenation in the water column —the composition of radiolarian assemblage is altered in conjunction with an increased relative abundance of Nassellaria, especially Stychocorys. An increase in geochemical productivity proxies (Ba/Al and TOC) and Stychocorys is related to high productivity in the upper parts of the photic zone. Moreover, high diatom content in members 1 and 2 is congruent with eutrophic conditions in the Lower Chelif Basin.A noteworthy change occurred during the deposition of Member 3, indicated by higher CO3Ca content and a decrease in TOC and redox sensitive elements, probably related to the end of seawater stagnation and re-oxygenation of the sea bottom. The record of infaunal benthic foraminifera confirms oxygen availability at the sea bottom and in the sediment pore water. Infaunal benthic foraminifera such as Bulimina, Bolivina and Uvigerina, adapted to oxygen-depleted pore waters and mesotrophic to eutrophic conditions, prevail over typical epifaunal taxa such as Anomalinoides, Cibicidoides, Hanzawaia and Heterolepa, which prefer oxic and oligotrophic environments. Therefore, the colonizers of the sea bottom after anoxic conditions were benthic foraminifera that tolerated oxygen-depleted conditions and high nutrient content. The end of seawater stagnation and the re-oxygenation of the sea bottom could be attributed to a current that mixed deep and shallow waters, favouring upwelling processes, which is congruent with the dominance of buliminids.In the upper part of Member 3, planktic forms are represented exclusively by foraminifera, while the radiolarians disappear. Planktic foraminifera are dominated by intermediate dwellers such as Orbulina, Neogloboquadrina and Globigerina, whereas deep-dweller forms such as Globorotalia are secondary. This assemblage confirms a re-oxygenation of the water column in Member 3, being congruent with upwelling processes.