The Valanginian Weissert Event on the south Tethyan margin: A dynamic paleoceanographic evolution based on the study of calcareous nannofossils

Abstract

Paleoceanographic conditions across the Valanginian Weissert oceanic anoxic event were reconstructed for the first time on the central Moroccan margin from a quantitative-based calcareous nannofossil study. Two onshore successions in the Essaouira-Agadir Basin and one offshore from DSDP Hole 416A were studied, providing a proximal-distal transect on the central Moroccan margin. The paleoceanographic conditions were reconstructed in an accurate chronostratigraphic framework. The sedimentary evolution of the Essaouira-Agadir Basin shows that sea-level variations controlled the paleogeographic context, clastic input, and nutrient availability. A thick photic zone and mesotrophic conditions favored the development of a diverse nannofossil community before the Weissert Event in the early Valanginian. In all studied successions, a collapse of the nannofossil community was recorded during the onset of the Weissert Event at the early-late Valanginian transition, and was caused by a major sea-level fall. The nannofossil community gradually recovered and reached its highest nannofossil production associated with high surface-water fertility during the Weissert Event in the late Valanginian. This eutrophication is coeval with the high and stable δ13Ccarb values of the carbon-isotope positive shift. After the Weissert Event, surface-water fertility and nannofossil production decreased but remained higher than during the early Valanginian. Additionally, the nannoconid decline started in the early Valanginian and was linked to a low sea-level. Their recovery in the early late Valanginian occurred during conditions of high sea-level and of high surface-water fertility. The central Moroccan margin is integrated into a global paleoceanographic reconstruction.