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)

Elisabetta Erba, Liyenne Cavalheiro, Alexander J. Dickson, Giulia Faucher, Gabriele Gambacorta*, Hugh C. Jenkyns, Thomas Wagner

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)
11 Downloads (Pure)

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 oxygen-isotope, CaCO3 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 CaCO3 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 CaCO3 (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 δ13 Ccarb negative excursion of ~3 ‰ at Sogno and ~6 ‰ at Gajum, and a δ13 Corg 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 δ13C 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.

Original languageEnglish
Pages (from-to)451-477
Number of pages27
JournalNewsletters on Stratigraphy
Volume55
Issue number4
Early online date18 Feb 2022
DOIs
Publication statusPublished - 2 Sep 2022

Keywords

  • black shales
  • C-and O-stable isotopes
  • chemostratigraphy
  • T-OAE
  • Toarcian

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

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