Two-phased collapse of the shallow-water carbonate factory during the late Pliensbachian–Toarcian driven by changing climate and enhanced continental weathering in the Northwestern Gondwana Margin

Abstract

The end Pliensbachian–Toarcian is characterized by several carbon-cycle perturbations and faunal turnovers (e.g., ammonites and foraminifera), which are most likely triggered by pulses of the Karoo-Ferrar-Chon Aike large igneous province. The majority of information about these events is based on detailed studies of sites deposited in deep-water settings, which leaves vast uncertainties about the expression of, and response to, these events in shallow-marine ecosystems. Here, we present a comprehensive assessment of paleoclimatic impacts on neritic depositional environments from the latest Pliensbachian through the middle Toarcian in the central High Atlas Basin, Morocco, and compare those with changes observed in coeval neritic environments within the western Tethyan realm. A total of four new stratigraphic sections were investigated in the southern part of central High Atlas Basin and these new sections are synthesized with six previously published sections, distributed over eight localities. Correlations between sections are based on biostratigraphy, chemostratigraphy and lithostratigraphy. In Morocco, two episodes of carbonate factory shutdown are observed, spanning the Pliensbachian/Toarcian boundary and the Polymorphum/Levisoni transition. Each carbonate factory collapse correlates to well-documented environmental disturbances during the latest Pliensbachian–middle Toarcian interval, including the Toarcian Oceanic Anoxic Event (T-OAE). Moreover, each episode of carbonate factory shutdown coincides with an interval characterized by a significant increase of coarse siliciclastic input in the basin, further demonstrating the link between global warming, increased continental weathering, and ecosystem turnovers. Furthermore, these two episodes of carbonate factory shutdown are each followed by episodes of renewed carbonate production, showing the resilience of the neritic carbonate factory in this region. The first recovery interval, occurring during the late Polymorphum Zone, is associated with a mixed siliciclastic‑carbonate system. The second episode of carbonate recovery quickly follows the shutdown associated with the onset of the T-OAE. It is associated with an abiotic-dominated carbonate production mode, resulting in an elevated ooid production. A full recovery of biotic carbonate production only occurs in the late stage of the T-OAE. Although biotic turnover occurs at both events, from a shallow-marine perspective, the major biotic and abiotic crisis occurred at the Pliensbachian/Toarcian boundary and not during the T-OAE. This is in contrast to the deep-marine record, where the T-OAE is often inferred to be the most significant event. An enhanced hydrological cycle and the subsequent increase of continental nutrient shedding might have triggered the most severe changes of the carbonate productivity at the Pliensbachian/Toarcian transition; whereas, ocean acidification and increased storm activity likely played a significant role at the onset of the T-OAE.