Redox element record shows that environmental perturbations associated with the T-OAE were of longer duration than the carbon isotope record suggests – the Aubach section, SW Germany

Abstract

The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) with its associated carbon-isotope excursion (CIE) was possibly one of the most pronounced periods of widespread oxygen deficiency in the Mesozoic ocean. The event has been extensively studied in order to understand the processes triggering the environmental perturbations and the extreme oxygen depletion in many marine basins. However, comparatively little focus has been placed on the end of the positive CIE and the stratigraphic coherent end of anoxiceuxinic conditions. In the present study, we constrain the stratigraphic extent of anoxic-euxinic conditions and define the termination of the positive CIE in the Swabo-Franconian Basin covering the Lower Toarcian strata using carbon-isotope ratios, organic matter pyrolysis and redox-sensitive element concentrations of outcrop samples from the Aubach section. Bulk organic carbon-isotope values, corrected for changes in type of organic matter using the Hydrogen Index (HI), suggest that the amplitude of the negative CIE in organic matter is as little as 3.3-3.5 ‰, in contrast to 4.5 ‰ change in δ 13 C carb in the same section. Enrichment in redox-sensitive proxies (V/Al and DOP-T) and %TOC suggest that environmental perturbations associated with the T-OAE continued until the upper falciferum Zone in the Aubach section. This indicates that anoxic-euxinic conditions terminated in the same stratigraphic interval in which δ 13 C values return to steady, light values at ~-28 ‰ (termination of positive CIE). This synchronism in the return to normal marine conditions is also observed in the southern Paris Basin, but not in the Cleveland Basin.

Carbon- and oxygen-isotope signature of the Toarcian Oceanic Anoxic Event: insights from two Tethyan pelagic sequences (Gajum and Sogno Cores – Lombardy Basin, northern Italy)

“…Recently, Hougård et al (2021) documented the T-OAE anomalies in the Swabo-Franconian Basin (SW Germany) and discussed the extension of anoxia relative to carbon-isotope chemostratigraphy. Results were compared to similar datasets from the Paris Basin, the Cleveland Basin and the Swiss Jura to assess synchroneity/diachroneity of the termination of anoxia in different parts of the seaway connecting the Tethys to the Boreal Oceans.…”

“…Results were compared to similar datasets from the Paris Basin, the Cleveland Basin and the Swiss Jura to assess synchroneity/diachroneity of the termination of anoxia in different parts of the seaway connecting the Tethys to the Boreal Oceans. Hougård et al (2021) raised the critical issue of the definition of the T-OAE that hitherto has not been systematically applied, thereby introducing artefacts and/or misunderstandings in correlations and modelling at supra-regional scale.…”

“…The T-OAE, originally recognized due to the globally distributed record of a coeval lower Toarcian black shale (Jenkyns 1985), is associated with a negative carbonate and organic-carbon isotope anomaly measured both in terrestrial (lacustrine), shallow-and deep-marine archives, including sediments from the ancestral Pacific Ocean (Jenkyns and Clayton 1986, Hesselbo et al 2000, Hesselbo et al 2007, Schouten et al 2000, Röhl et al 2001, Jenkyns et al 2001, Jenkyns et al 2002, McElwain et al 2002, Kemp et al 2005, Emmanuel et al 2006, van Breugel et al 2006, Sabatino et al 2009, Al-Suwaidi et al 2010, Caruthers et al 2011, Hesselbo and Pieńkowski 2011, Kafousia et al 2011, Kafousia et al 2014, Izumi et al 2012, Trabucho-Alexandre et al 2012, Reolid 2014, Xu et al 2017, Them et al 2017, Fantasia et al 2018, Ikeda et al 2018, Filatova et al 2020, Ruebsam and Al-Husseini 2020, Remirez and Algeo 2020, Hougård et al 2021.…”

“…Fig.9. On the left, the schematic δ 13 C reference curve for the latest Pliensbachian-Toarcian time interval is reported (modified afterRuebsam and Al-Husseini (2020), with minor modifications byHougård et al 2021). The stratigraphic extent of the T-OAE is indicated with a light red rectangle, while the proposed interval of the Jenkyns Event is indicated by a yellow band.…”

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

“…The Posidonia Shale including the Toarcian CIE and the T-OAE in the Southwest German Basin has been described in great detail (e.g., Schouten et al 2000 ; Röhl et al 2001 ; Frimmel et al 2004 ; Röhl and Schmid-Röhl 2005 ; van de Schootbrugge et al 2005 ; Bour et al 2007 , Schwark and Frimmel 2004 ; Suan et al 2008 , 2015 ; Hougård et al 2021 ). The following description of the Posidonia Shale in the Southwest German Basin is based on the Dormettingen section (van Acken et al 2019 ; Galasso et al 2021 ; Fig.…”