The emergence of climate change as a crucial issue for society has urged the understanding of the future state of the planet within the context of increasing carbon dioxide concentrations. The ocean is the oldest ecosystem and the largest on Earth by volume and best records global changes in climate and atmospheric composition. Marine ecosystems are inextricably involved in the physical, chemical, biological processes of global change. In the near future, the ocean's uptake of CO2 is expected to rapidly decline because of surface water warming, decreasing pH (acidification), increased vertical stratification, and slowed thermohaline circulation (Solomon et al., 2009). Consequently, a very rapid “in and out” from icehouse to greenhouse state – and vice versa – urges comprehension of positive and negative feedbacks on the biosphere.

Understanding of the Earth system at timescales longer than human observations has became imperative, because anthropogenic activities are likely to increase by orders of magnitude the rates of climatic change that usually result from natural processes. The Earth's ecosystems, thus, should be scrutinized on the medium- and long-term scales using geological records of past extreme environmental disturbances that exemplify varied tempos and modes of resilience, occasionally reaching tipping points that triggered permanent modifications.

The Toarcian Oceanic Anoxic Event (T-OAE) is the oldest Mesozoic case of global anoxia with widespread deposition of organic matter-rich sediments in a variety of depositional settings from continental to shallow- and deep-marine (Jenkyns, 1985, 1988, 2010). Available evidence suggests that at ∼183 Ma the atmosphere and oceans experienced high CO2, possibly due to the degassing of lava fields in the Karoo–Ferrar large igneous province and/or from dissociation and oxidation of methane hydrates in continental-margin sediments (Jenkyns, 2010) and/or terrestrial environments (Them et al., 2017). High atmospheric carbon dioxide possibly initiated greenhouse conditions that accelerated weathering and the hydrological cycle, increasing nutrient recycling into the oceans.

Although the original definition of the T-OAE was based on the presence of a lithostratigraphic marker (Jenkyns, 1985), the development of chemostratigraphy demonstrated that the T-OAE is associated with a negative C isotopic anomaly documented in marine carbonates and organic matter as well as in terrestrial organic matter including fossil wood and specific organic compounds (Jenkyns and Clayton, 1986; Hesselbo et al., 2000, 2007; Schouten et al., 2000; Jenkyns et al., 2002; Emmanuel et al., 2006; Van Breugel et al., 2006; Al-Suwaidi et al., 2010; Caruthers et al., 2011; Izumi et al., 2012; Kafousia et al., 2014; Reolid, 2014; Xu et al., 2017; Them et al., 2017; Fantasia et al., 2018). As shown by Fantasia et al. (2018), such a negative C isotopic anomaly might have resulted from volcanogenic CO2, thermogenic methane associated with metamorphism, and dissociation of marine or terrestrial clathrates.

The T-OAE is further marked by an Os anomaly (Cohen et al., 2004; Percival et al., 2016; Them et al., 2017), biocalcification crisis (Erba, 2004; Mattioli et al., 2004; Casellato and Erba, 2015; Erba et al., 2019), increased primary productivity (Jenkyns, 2010; Erba, 2004), and ocean acidification (Erba, 2004; Trecalli et al., 2012; Casellato and Erba, 2015; Posenato et al., 2018), which occurred during an exceptional warming phase (Dera et al., 2011; Korte and Hesselbo, 2011; Gómez et al., 2016) and a major transgression (e.g. Haq et al., 1987; Hardenbol et al., 1998).

Jurassic pelagic successions in the Southern Alps have been extensively investigated for stratigraphy, sedimentology, paleontology, and geochemistry (Bernoulli and Jenkyns, 2009; Erba et al., 2019). In particular, multi- and interdisciplinary studies have demonstrated that the Jurassic pelagic successions of the Lombardy Basin represent “type-sections” of the Tethyan southern margin (Gaetani, 2010). Indeed, the Lombardy Basin is part of the relatively undeformed portion of Adria interpreted as an African “promontory” or as a microplate (Fig. 1). In the latest Triassic–earliest Jurassic a rifting phase caused the breakup of carbonate platforms into a series of “horst and graben” that exerted a physiographic control on sediment type and distribution for most of the Jurassic (Bernoulli and Jenkyns, 1974, 2009; Bosence et al., 2009; Santantonio and Carminati, 2011). As a consequence, sedimentation was differentiated with the deposition of thick complete pelagic successions in the deeper zones, while sedimentation was typically condensed and incomplete on the structural highs.

During the Jurassic the Lombardy Basin was globally a deep area between the Lugano High to the west and the Trento Plateau to the east (Fig. 1). However, the latest Triassic–earliest Jurassic rifting disentangled a number of troughs and paleohighs that are as follows, from west to east: Monte Nudo Trough, Lugano High, Generoso Trough, Corni di Canzo High, Albenza Plateau, Monte Cavallo High, Sebino Trough, Botticino High (Gaetani, 1975, 2010). In the troughs, partially resedimented Lower Jurassic marlstone–limestone sequences may reach a non-decompacted thickness of 3000 m (e.g. in the Generoso Trough), but condensation and hiatuses characterize the paleohigh sections with reddish nodular facies. Along slopes connecting structural highs to the troughs, sedimentation was marked by slumps, resedimented bodies, and, locally, megabreccias within condensed and occasionally incomplete facies (Gaetani and Erba, 1990; Gaetani, 2010) (Fig. 1). In addition to regional tectonics, the Lombardy Basin successions record global climatic and oceanographic changes, including the T-OAE (Erba et al., 2019). In fact, lower Toarcian black shales have been documented in various sections, offering the opportunity to investigate the consequences of the T-OAE global changes on marine biota in the Tethys Ocean (Erba et al., 2019).

After close investigations of section outcropping in the Lombardy Basin, the Colle di Sogno and Gajum sites were selected as the most promising locations for continuous coring of pelagic records (Gaetani and Erba, 1990; Casellato and Erba, 2015) for continuous coring. In this paper, we document coring operations and lithostratigraphic characterization of both the Sogno and Gajum cores and outline ongoing multidisciplinary research.

The T-OAE is considered a natural Earth system experiment, which allows us to (a) detect and quantify processes associated with emissions of greenhouse gases and natural atmospheric pollutants; (b) understand the role of greenhouse gases on climate dynamics and its influence on the hydrological cycle; (c) characterize changes in ocean and atmospheric chemistry and their interactions; (d) assess changes in biodiversity and dynamics of ecosystems and understand the functioning of biotic sinks; and (e) quantify biosphere–geosphere–atmosphere interactions and their timings or rates.

Analysis of past global change requires the collection of high-resolution data from continuous and ideally unweathered sequences. In surface outcrops, sedimentary rocks and particularly black shales are commonly badly degraded and, consequently, drilling is crucial to ensure the recovery of high-quality fresh cored material. In this paper we identify the T-OAE adopting the original definition by Jenkyns (1985) based on lithostratigraphy. Therefore, the T-OAE in the Lombardy Basin corresponds to the Livello a Pesci (Tintori, 1977; Gaetani and Poliani, 1978; Erba and Casellato, 2010; Erba et al., 2019). This interval has an average thickness of 0.5 to 5 m, but reaches a few tens of metres in the most expanded sections. Black shales are rarely recorded on paleohighs, whereas they are ubiquitous in deeper basins. As the T-OAE occurred at a global scale, the local lack of black shales in the Lombardy Basin is usually the result of condensation and/or stratigraphic gaps.

The Sogno drilling campaign took place in June 2013, while the Gajum core was drilled in February 2016 (Fig. 2). At both Sogno and Gajum sites, drilling operations were performed with the DELTABASE 520 Modular Hydraulic Rotary Drill. The Sogno coring was accomplished with a T2 double corer, using narrow-kerf, sawtoothed drill bits that cut a 101 mm diameter borehole and 84 mm diameter cores. The Gajum core was obtained using a modified T6 triplex corer, including a plastic liner for the best recovery, using narrow-kerf, sawtoothed drill bits that cut a 131 mm diameter borehole and 101 mm diameter cores. At Gajum, after coring, the borehole was logged using a QL40-OBI optical televiewer to obtain high-resolution images of the borehole wall, together with a total gamma radiation tool (Fig. 3).

All cores were initially described on site and a preliminary log was produced. Then, cores were packed, labelled, and put in PVC plastic boxes to prevent contamination and transported to the Department of Earth Sciences in Milan where they are archived. Here, during lab preparation, all cores were longitudinally split along the dip and divided into an archive half and a sampling half, both marked at centimetre scale. The archive half was photographed in high resolution and composite photologs were produced for each site.

The fresh material recovered with the Sogno and Gajum cores provide complete and relatively expanded pelagic records from the western Tethys Ocean. Detailed multidisciplinary investigations are in progress to collect multi-proxy records for building a high-resolution dataset prodromic for modelling the nature and significance of the T-OAE. The specific objectives are as follows:

High-resolution integrated stratigraphy. This is based on nannofossil biostratigraphy, magnetostratigraphy, chemostratigraphy, and cyclostratigraphy. Milankovitch cycles will be used to estimate durations of the T-OAE.