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)

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 (similar to 5 m and similar to 15 m-thick at Sogno and Gajum, respectively) is subdivided into three lithostratigraphic intervals: a lower part, with minimum CaCO3 (5-10 %) and TOC (similar to 0.2-0.3 %) values; a central part with a progressive increase in TOC up to similar to 1.4 %, and an upper part characterized by the highest TOC up to similar 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 delta C-13(carb) negative excursion of similar to 3 parts per thousand at Sogno and similar to 6 parts per thousand at Gajum, and a delta C-13(org) negative excursion of similar to 7 parts per thousand 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 delta C-13 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.