Ecological adaptation of marine floras and faunas across the Early Jurassic Toarcian Oceanic Anoxic Event – A case study from northern Germany

Abstract

We present new, high resolution bio- and chemostratigraphic data (calcareous nannofossils, ammonites, δ13Corg) of the lower Toarcian from an outcrop (Hondelage) in northern Germany. The three data sets, which cover lithostratigraphically the uppermost Amaltheenton-Formation (upper Pliensbachian) and the lower part of the Posidonienschiefer-Formation (lower Toarcian), allow a calibration of the different stratigraphic schemes in ultra high resolution. They further shed light on the ecologic conditions controlling the assemblage composition and evolutionary patterns of calcareous nannofossils, ammonites and gastropods.The distinctive negative δ13Corg excursion (negative CIE, Jenkyns Event), which is part of the early Toarcian Oceanic Anoxic Event (T-OAE), falls lithostratigraphically into the lower part of the laminated black shales of the Posidonienschiefer facies. The lower part of the Jenkyns Event is marked by the first occurrences of the calcareous nannofossil species Carinolithus superbus crassus and Diductius constans. These two events are biostratigraphically assigned to the Tiltoniceras antiquum ammonite Biohorizon (uppermost Dactylioceras tenuicostatum ammonite Zone). The top of the Jenkyns Event has been assigned to the Cleviceras exaratum ammonite Biohorizon (Harpoceras serpentinum ammonite Zone) of the lower Toarcian. The last occurrence of the nannofossil species Biscutum grande lies in the middle part of the Jenkyns Event.Calcareous nannofossils, ammonite faunas and gastropods are discussed with respect to the environmental and climatic changes, which occurred across the Pliensbachian/Toarcian boundary interval and throughout the Jenkyns Event. The extreme impoverished nannofossil and ammonite assemblages of the uppermost Pliensbachian are interpreted in the context of a cooling, primarily affecting the calcareous shelled primary producers. The onset of the Jenkyns Event in the lower Toarcian post-dates this cooling and is related to a subsequent warming. This climatic shift caused a bloom of diverse opportunistic nannofossil and ammonite taxa in the lower Toarcian. Nannofossil abundance patterns were controlled by an increased nutrient availability. The rapid evolution of the ammonite genera Tiltoniceras, Eleganticeras and Cleviceras (family Hildoceratidae), which started in the lower part of the Jenkyns Event, is potentially related to their adaptation to a shallow water dwelling habitat. The mass occurrence of phylogenetically first holoplanktonic gastropods in the lower part of the Jenkyns Event is interpreted in the context of impoverished ocean water oxygenation, which in turn triggered the evolution of a new ecological strategy.