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)

Abstract

The early Toarcian Oceanic Anoxic Event (T-OAE) was associated with major climatic changes involving profound effects on the global carbon cycle. In this study, we present new carbon-and oxygenisotope, CaCO 3 and total organic carbon (TOC) records from two cores (Sogno and Gajum Cores) that recovered pelagic successions from north-western Tethys. A palaeobathymetry of about 1000 and 1500 m water depth is tentatively reconstructed for the Gajum and Sogno sites, respectively. The investigated sections thereby represent some of the deepest records of the T-OAE in the western Tethys. During the early Toarcian, sedimentation in the Lombardy Basin (Southern Alps, northern Italy) was characterized by the deposition of the Fish Level (Livello a Pesci), a dark grey to black marly claystone with low CaCO 3 content and relatively high TOC content. In the two cores, the Fish Level (~5 m and ~15 m-thick at Sogno and Gajum, respectively) is subdivided into three lithostratigraphic intervals: a lower part, with minimum CaCO 3 (5-10 %) and TOC (~0.2-0.3 %) values; a central part with a progressive increase in TOC up to ~1.4 %, and an upper part characterized by the highest TOC up to ~2.5 %. Within the Fish Level a lower grey interval and an upper black interval are defined based on lithological features. Carbon-isotope chemostratigraphy resolves a δ 13 C carb negative excursion of ~3 ‰ at Sogno and ~6 ‰ at Gajum, and a δ 13 C org negative excursion of ~7 ‰ at both locations. This global carbon cycle anomaly, named the 'Jenkyns Event', is here subdivided into a lower J1 and an upper J2 segment. As highlighted by lithostratigraphic evidence, nannofossil biostratigraphy and chemostratigraphic correlations, a hiatus elides part of the succession below the Fish Level in the Gajum Core, although without compromising the completeness of the Fish Level itself. High-resolution δ 13 C data indicate that the base of the Fish Level is synchronous, but the top diachronous at the two coring sites. The same synchroneity of the base and diachroneity of the top of the black shale interval is identified in the Umbria-Marche Basin, suggesting that the duration of anoxia was not identical over very modest to relatively long distances.

Schizosphaerella size and abundance variations across the Toarcian Oceanic Anoxic Event in the Sogno Core (Lombardy Basin, Southern Alps)

“…(Toarcian-earliest Bajocian) was formalized by Gaetani and Poliani (1978). Within the lower part of the Sogno Fm., the ~5 meter-thick interval of dark grey-to-black marly claystones is named Fish Level (Gaetani and Poliani, 1978;Gaetani and Erba, 1990;Casellato and Erba, 2015;Erba et al, 2022). In recent times, above the Upper Pliensbachian Domaro Limestone Fm., the Lower Toarcian portion of the Sogno Fm.…”

“…Kälin and Bernoulli, 1984;Claps et al, 1995;Mattioli, 1997;Erba, 2004;Casellato and Erba, 2015;Peti and Tibault, 2017) as to produce a "schizosphaerellite" (Erba et al 2019a). In this work, we focus on Schizosphaerella across the T-OAE interval recovered in the Sogno Core that consists of pelagic, well-dated Erba et al, 2022) limestones, marlstones and black shales from the Lombardy Basin (Figure 1).…”

“…Yet, they did not provide a definition of the beginning and end of the Jenkyns Event, thus impeding its unequivocal identification for correlations at regional, supra-regional and global scale. Here, for our investigation, we applied the definition by Erba et al (2022) that proposed naming as Jenkyns Event only the δ 13 C negative anomaly within the T-OAE. Indeed, relatively detailed chemostratigraphic records through the T-OAE interval document that the Jenkyns Event comprises a lower part marked by a sharp decrease of the δ 13 C curve followed by minimum values (isotopic segment Jenkyns 1 = J1), and an upper part corresponding to gradual recovery to pre-anomaly δ 13 C values (isotopic segment Jenkyns 2 = J2) (Figure 2a).…”

“…The specific objective of this investigation is the quantification of changes in size and abundance of the micrite-forming schizosphaerellids to derive their biocalcification tempo and mode in response to the Jenkyns Event (J1 and J2) perturbation and also relative to the Fish Level black shale interval in the Sogno Core. The pelagic nature of the Sogno section (Gaetani and Poliani, 1978;Jenkyns and Clayton, 1986;Gaetani and Erba, 1990;Casellato and Erba, 2015;Erba et al, 2019b;Erba et al, 2022), deposited at ~ 1500 m water depth (Erba et al, 2022), is crucial for excluding the influence of shallow-water derived micrite and, thus, assessing the implications of Schizosphaerella biocalcification changes -in terms of abundance and size -for the pelagic carbonate sedimentation. Nannofossil biostratigraphy from .…”

Dinosaur extinctions related to the Jenkyns Event (early Toarcian, Jurassic)

“…The early Toarcian was characterized by an important environmental change called the Jenkyns Event (e.g., Müller et al, 2017;Reolid et al, 2020Reolid et al, , 2021aErba et al, 2022), one of the most signifi cant hyperthermal events of the Mesozoic (e.g., García Joral et al, 2011;Korte & Hesselbo, 2011;Suan et al, 2011;Danise et al, 2013;Baghli et al, 2020;Storm et al, 2020;Ruebsam et al, 2020aRuebsam et al, , 2020b. TEX 86 palaeothermometry proxies applied to NW Tethys sediments suggest a warming of 5ºC during the Pliensbachian-Toarcian transition and a peak of 10ºC warming during the Jenkyns Event (Ruebsam et al, 2020b).…”