Evidence for local carbon‐cycle perturbations superimposed on the Toarcian carbon isotope excursion

Abstract

Sudden and large-magnitude changes in global carbon-cycle perturbations along with carbon isotope excursions (CIEs) seem to be recurrent phenomena during greenhouse periods in Earth's history (Jenkyns, 2010). One of the considerably negative CIEs in Earth's history occurred during the Early Jurassic, near-contemporaneously with the Toarcian Oceanic Anoxic Event (OAE at ~183 Ma; Jenkyns, 1988). The Toarcian OAE was a time of severe environmental change that was linked to elevated continental weathering rates (Cohen,

Restricted Oxygen‐Deficient Basins on the Northern European Epicontinental Shelf Across the Toarcian Carbon Isotope Excursion Interval

“…These shale samples contain a considerable component of land plant‐derived OM supported by low HI value (<400 mg HC/g TOC) and high oxygen index values (up to 100 mg CO 2 /g TOC) (cf. Frimmel et al., 2004; Wang et al., 2020). In contrast, from 986 to 600 cm of this section, the OM in T‐CIE fine‐grained organic‐rich sedimentary rocks (Figure 2) was primarily of marine origin (Frimmel et al., 2004; Wang et al., 2020) and thus likely reflect N biogeochemical processes in marine environments during the T‐CIE.…”

“…This sedimentary section belongs to the well‐recognized lower Toarcian Posidonia Shales that are enriched in high abundance of well‐preserved OM and are widely distributed over Europe (Bour et al., 2007; Fantasia et al., 2018; Montero‐Serrano et al., 2015; Song et al., 2017). A lot of previous work thus has been performed on the Posidonia Shales as case studies to understand the early Toarcian carbon cycling, continental weathering, marine deoxygenation and mass extinction (e.g., Montero‐Serrano et al., 2015; Röhl et al., 2001; van Acken et al., 2019; Wang et al., 2020). This Dotternhausen section has been also extensively studied and been well characterized in terms of lithostratigraphy and biostratigraphy (Röhl et al., 2001; Röhl & Schmid‐Röhl, 2005).…”

“…Röhl et al., 2001). In addition, three lithologic units for the Dotternhausen section have been defined in the light of the distinct abundances of the OM and siliciclastic component and different fabric types (Röhl et al., 2001; van Acken et al., 2019; Wang et al., 2020). They are bioturbated light‐gray (organic‐lean) shales from the depth of 1,180–980 cm, laminated fine‐grained organic‐rich shales and marlstones from 980–550 cm, and bituminous light‐gray (organic‐lean) shales from 550 to 0 cm, respectively (Röhl et al., 2001).…”

“…These organic‐rich sedimentary rocks were not metamorphosed and only experienced diagenesis in the oil window which is reflected by their persistently high hydrogen index(HI) values (580 mg hydrocarbon (HC)/g TOC on average) and their oil shale character (cf. Frimmel et al., 2004; Röhl et al., 2001; Wang et al., 2020). In the Dotternhausen section, there are three limestone beds, including the Unterer Stein, Steinplatte and Oberer Stein at the depth of ∼800, ∼680 and ∼610 cm, respectively (Figure 2).…”

“…This is because a large dissolved marine N reservoir is required to record the isotopic fingerprints of its redox‐driven cycle in co‐deposited sediments (Stüeken et al., 2016). In contrast, if hydrographic water‐mass restriction represents the main depositional settings of T‐CIE sedimentary rocks, quantitative denitrification and/or anammox is most likely to occur as a result of strongly redox‐stratified water column (Schwark & Frimmel, 2004; Wang et al., 2020). Biomass deposited under such conditions is expected to preserve the δ 15 N value of the main input source that is, N 2 fixation (Stüeken et al., 2016).…”