Coring the sedimentary expression of the early Toarcian Oceanic Anoxic Event: new stratigraphic records from the Tethys Ocean

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) interval was cored at Colle di Sogno and Gajum in the Lombardy Basin (Southern Alps, northern Italy). The Sogno and Gajum cores recovered 26.83 and 31.18 stratigraphic metres, respectively, of pelagic sediments consisting of marly limestones, marlstone, marly claystone, and black shale. Drilling at both sites resulted in 100 % recovery of unweathered material. The pelagic succession comprises a relatively expanded black shale interval of 4.98 m in the Sogno core and 15.35 m in the Gajum core, with lower and upper boundaries without evidence of hiatuses. The Sogno and Gajum cores can be considered reference sections for the pelagic lower Toarcian interval of the western Tethys and will provide high-resolution micropaleontological, inorganic and organic geochemical, isotopic multiproxy data. Integrated stratigraphy and cyclostratigraphy are predicted to result in estimates of durations and rates to model the ecosystem resilience to the extreme perturbations of the T-OAE and gain a better understanding of current global changes and help provide better projections of future scenarios.Published by Copernicus Publications on behalf of the IODP and the ICDP.

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 Sogno Core (45°47'20.5" N, 9°28'30.0" E) (Erba et al 2019b) was drilled next to the outcropping type-section of the Sogno Formation (Gaetani and Poliani 1978), along the road SP 179 on the northern slope of Monte Brughetto (Gaetani and Poliani 1978, Jenkyns and Clayton 1986, Gaetani and Erba 1990, Hinnov et al 2000, Muttoni et al 2005, Channell et al 2010, Casellato and Erba 2015 (Fig. 1A).…”

“…The cored Fish Level (sensu Gaetani and Poliani 1978) consists of ~5 m of dark grey to black marly claystones. In particular, following the lithostratigraphy of Erba et al (2019b), within the Fish Level three intervals are distinguished from bottom to top: an interval consisting of grey to very dark grey and dark red clayey marlstones (Unit 7); grey to very dark grey clayey marlstones with reddish to greyish spots (Unit 6); black shales characterized by well-developed lamination and pyrite nodules (Unit 5).…”

“…The Gajum Core (45° 51' 3.2" N, 9° 17' 19.5" E) (Erba et al 2019b) was drilled close to "Fonte Gajum" located east of Canzo (CO), in a lateral incision of the Ravella Valley next to the trail named Via delle Alpi (Fig. 1B).…”

“…The Fish Level in the Gajum Core is more expanded than in the Sogno Core, with a thickness of about 16 metres versus 5 metres, respectively. Following the lithostratigraphy of Erba et al (2019b), three units are distinguished within the Fish Level, from bottom to top: an interval consisting of dark grey to very dark grey to black marly claystones, with evident laminations and local faint bioturbated patches (Unit 6); dark to very dark grey to black clayey marlstones interrupted by four intervals of dusky red limy cherts with a few levels characterized by green-coated limy-chert nodules (Unit 5); dark to very dark grey to black marly claystones with evident lamination and common pyrite nodules (Unit 4).…”

“…1). Excellent recovery of the Sogno and Gajum Cores (Erba et al 2019b) provided high-quality material for this study. The estimated palaeowater depth of the selected pelagic sites ranges from 1000-2000 m (Gaetani and Poliani 1978), thereby representing some of the deepest records known of the T-OAE and potentially offering new insights into the dynamics of the Tethyan deep carbonate system under environmentally perturbed conditions.…”