Abstract
Abstract The chemostratigraphic record of the Toarcian Oceanic Anoxic Event (T-OAE) is generally well documented in epicontinental basins; however, the duration of anoxia and recovery remains poorly constrained in the Tethys Ocean owing to limited stratigraphic resolution of oceanic sections with hiatuses. In this paper, we describe a relatively continuous hemipelagic record in the Central Western Carpathians of Slovakia (Skladana Skala section), which contains the Pliensbachian/Toarcian boundary, the T-OAE, and the subsequent recovery phase. In our section, the Pliensbachian Spinatum Zone is represented by a ~ 7 m thick alternation of spotted, spiculitic marly limestones and limestones of the Allgau Formation that contain moderately diverse trace fossils including abundant Zoophycos, Lamellaeichnus and Teichichnus. In the uppermost part of the Spinatum Zone, a ~ 0.8‰ decline in δ13Ccarb values marks the carbon isotope excursion (CIE) at the Pliensbachian/Toarcian boundary. This isotope anomaly is overlain by a ~ 3 m-thick interval of spotted marly limestones and marls, which contains a positive CIE that represents the base of the T-OAE. The T-OAE interval comprises a 60 cm-thick pyritic black shale interval that lacks bioturbation and shows a negative CIE (δ13Corg values drop by 4–5‰), coincident with an increase in TOC values (from 0.3% to 2–3%), and a drop in CaCO3 values (from 50 to 80% to 3–7%). The black shale is overlain by ~8 m thick marls with rare intercalations of marly limestones, which show ammonites of the Exaratum Subzone in the lowermost part (~1 m) succeeded by Serpentinum Zone beds. This unit exhibits a second positive CIE (δ13Corg increases by 2.5‰ and δ13Ccarb increases by 1.5‰), significantly more negative δ18O values relative to the pre-T-OAE interval, and a low diversity of trace fossils dominated by Chondrites and extremely thin Zoophycos. Sponge spicules re-appear in the uppermost 6.5 m-thick interval of the section formed by the alternation of spotted marls and limestones, with ammonites of the Falciferum Subzone in the lower part and of the Bifrons Zone in the uppermost parts; however, diversity of trace fossils remains low. Based on these findings, we argue that the 60 cm-thick pyritic black shale that represents the T-OAE is a strongly condensed succession because it includes the onset of the negative CIE and the subsequent rebound, and thereby encompasses the full duration of the anoxic event. Anoxia in basins open to the oceanic Tethys thus coincided not only with reduced carbonate production but also with reduced siliciclastic supply, generating very thin black shale deposits. In contrast to current-swept swells where black shales associated with hardgrounds are thin or missing owing to winnowing, carbonate starvation was induced by a calcification crisis coincident with the T-OAE and siliciclastic starvation was probably induced by rapid sea-level rise. Although bottom waters were not anoxic in the aftermath of T-OAE, bioturbation remained restricted during the Falciferum Subzone in basinal settings, indicating that environmental stress induced by limited ventilation and/or high bottom-water temperatures were persistent. The reduced diversity of trace fossils and the re-appearance of sponge spicules close to the lower/middle Toarcian boundary indicate that the ecological recovery in basins open to the oceanic Tethys was delayed, a pattern similar to the epicontinental seas that experienced anoxia.
Highlights
Two major carbon cycle perturbations related to CO2 emissions from the Karoo-Ferrar large igneous province and/or thermal metamorphosis of coal sediments in the Karoo Basin (e.g. McElwain et al, 2005; Svensen et al, 2007) are detected at the Pliensbachian-Toarcian boundary (PlTo) (Littler et al, 2010) and during the Toarcian Oceanic Anoxic Event (T-OAE, Jenkyns, 1988; Jenkyns Event in Müller et al, 2017)
We argue that the 60 cm-thick pyritic black shale that represents the T-OAE is a strongly condensed succession because it includes the onset of the negative carbon isotope excursion (CIE) and the subsequent rebound, and thereby encom passes the full duration of the anoxic event
In contrast to current-swept swells where black shales associated with hardgrounds are thin or missing owing to winnowing, carbonate starvation was induced by a calcification crisis coincident with the T-OAE and siliciclastic starvation was probably induced by rapid sea-level rise
Summary
Two major carbon cycle perturbations related to CO2 emissions from the Karoo-Ferrar large igneous province and/or thermal metamorphosis of coal sediments in the Karoo Basin (e.g. McElwain et al, 2005; Svensen et al, 2007) are detected at the Pliensbachian-Toarcian boundary (PlTo) (Littler et al, 2010) and during the Toarcian Oceanic Anoxic Event (T-OAE, Jenkyns, 1988; Jenkyns Event in Müller et al, 2017). The dynamics of the TOAE and the ecosystem recovery in deeper Tethyan oceanic environ ments not directly attached to the European shelf remain poorly docu mented or unknown These environments were differentiated during the Pliensbachian and Toarcian into current-swept structural highs with condensed sedimentation (nodular limestones and hardgrounds) and more sheltered basins with less condensed marly hemipelagic deposits (Fig. 1). Understanding the nature and distribu tion of these refugia during the T-OAE remains limited as the extent and duration of anoxia and the extent of bottom-water warming (i.e., factors that affected ecosystem dynamics and extinctions of benthic fauna during the T-OAE in shelf environments; García-Joral et al, 2011; Gomez and Goy, 2011; Baeza-Carratala, 2013; Them et al, 2018; Ull mann et al, 2020) in these deeper oceanic environments is poorly understood
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