Bridging the Faraoni and Selli oceanic anoxic events: short and repetitive dys- and anaerobic episodes during the late Hauterivian to early Aptian in the central Tethys

Abstract

A detailed stratigraphical and geochemical analysis was performed on the upper part of the Maiolica Formation outcropping in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). In these localities, the Maiolica Formation consists of well-bedded, partly siliceous, pelagic, micritic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-metal contents (RSTE: Mo, U, Co, V and As) were measured. Higher densities of organic-rich layers were identified in the uppermost Hauterivian, lower Barremian and the Barremian-Aptian boundary intervals, whereas the upper Barremian interval and the interval immediately following the Barremian-Aptian boundary interval are characterized by lower densities of organic-rich layers. TOC contents, RSTE pattern and C<sub>org</sub>:P<sub>tot</sub> ratios indicate that most layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. <br><br> Correlations are possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal northwest German Basin, and with the facies and drowning pattern in the evolution of the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This brief episode is followed by further episodes of dysaerobic conditions in the Tethys and the northwest German Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and northwest German Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterized by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were likely the cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning

The Late Hauterivian Faraoni ‘Oceanic Anoxic Event’: an update

“…Fiume Bosso). As proposed by Baudin et al [2002] and Föllmi et al [2012], this would be the result of the development of local anoxic pockets within a larger dysoxic bottom-water masses. By comparing the enrichment factors of U and Mo, Tribovillard et al [2012] also suggest that depositional conditions were dominantly suboxic and anoxia developed at some depth below the sediment-water interface.…”

“…In the Lombardian basin, a Faraoni level equivalent was recognized in the southern Swiss Alps (Breggia Gorge section [Bersezio et al, 2002;Föllmi et al, 2012]), whereas its record in the Capriolo section (Italian Alps ) is less conclusive with regards to the geochemical tracers. Indeed, there are no levels within the upper Hauterivian interval of the Capriolo section that are particularly enriched in organic-carbon or P (relative to C org ), nor an ammonite-rich bed equivalent to bed D of the type-area.…”

“…According to the initial definition of OAEs [Schlanger and Jenkyns, 1976;Jenkyns, 1980] flooding of landmasses and creation of shelf seas may have enhanced productivity, in turn expanding the O 2 -minimum zone and drowning the carbonate platforms surrounding pelagic basins where dysoxia/anoxia were developed [Jenkyns, 1991;Föllmi et al, 1994;Skelton and Gili, 2012]. However, several recent data suggest that the drowning of carbonate platforms pre-dates the OAEs [Najaro et al, 2011;Huck et al, 2011;Léonide et al, 2012;Föllmi et al, 2012] or may have different responses depending of their latitude [Huck et al, 2010]. By contrast, some carbonate platforms survived to OAEs (i.e.…”

“…As beds A, C, E and G are concerned (figs 4 and 6), their organic content range between 0.5 and 25% in the Umbria-Marche basin [Cecca et al, 1994a;Coccioni et al, 1998;Baudin et al, 2002], between 2.5 and 11% in the Trento plateau Lukedener and Grunert, 2013] and between 1.5 and 12% in the Lombardian basin [Bersezio et al, 2002;Föllmi et al, 2012], while the Faraoni level is less enriched in organic matter in the Vocontian basin (0.5 to 2.7% Godet et al, 2008]), in the Ultrahelvetic basin (0.3 to 1.5%; ) and in the Subbetic basin (0.5 to 3.8% [Sauvage et al, 2013], and unpublished data).…”

Palaeoenvironmental evolution of the Southern Alps across the Faraoni Level equivalent: new data from the Trento Plateau (Upper Hauterivian, Dolomites, N. Italy)

“…The observed change coincides with the Faraoni anoxic event, one of the first distinctive perturbations of the carbon cycle during the early Cretaceous. In the sedimentary record, this short-lived oxygen-deficient event is reflected in the widespread deposition of organic-rich black shales throughout the western Tethys realm (Cecca et al 1994a, b;Cecca et al 1996;Faraoni et al 1996;Baudin et al 1997;Coccioni et al 1998;Baudin et al 1999;Erba et al 1999;Bersezio et al 2002;Baudin et al 2002;Bellanaca et al 2002;Busnardo et al 2003;Baudin 2005;Company et al 2005;Coccioni et al 2006;Bodin et al 2007;Bodin et al 2009;Tremolada et al 2009;Föllmi et al 2011). In the isotopic records, the Faraoni event is expressed as the culmination of a minor long-term increase in δ 13 C, thereby contrasting with major carbon isotope excursions related to other Cretaceous anoxic events (Föllmi et al 1994;Erba et al 1999;Schootbrugge et al 2000;Company et al 2005;Godet et al 2006).…”