Anoxic vs dysoxic events reflected in sediment geochemistry during the Cenomanian–Turonian Boundary Event (Cretaceous) in the Umbria–Marche Basin of central Italy

One of the most spectacular expressions of global “Oceanic Anoxic Events” (OAEs) of the Cretaceous is the Livello Bonarelli of central Italy, deposited during the Cenomanian–Turonian Boundary Event (CTBE or OAE2) at around 93.5 Ma. At Furlo in the Umbria–Marche Basin in Italy, the Bonarelli consists of approximately 1 m of Corg-rich shales interbedded with radiolarian-rich layers, and contrasts sharply with the extensive over- and underlying siliceous limestones. Several minor Corg-rich intervals (herein termed black levels; average thickness = 4.5 cm) are interbedded within the Scaglia Bianca beneath the Bonarelli. X-ray fluorescence and ICP-MS analyses show that these sediments consist of different mixing systems with three end-members: 1) SiO2–CaCO3 (Scaglia Bianca); 2) “average shale”–CaCO3 (black levels); and 3) “average shale”–SiO2 (Bonarelli). Given the sharp lithologic contacts and the little geochemical mixing between the groups, these are thought to represent rapid switching events between sedimentary modes. Major element concentrations are generally low, except for Si and P, owing to dilution by silica (up to 96%) and carbonates. As Al-normalised ratios for elements such as Ti or Zr, taken as proxies for higher energy environments, are close to average shale values, excess SiO2 is most likely linked to biogenic processes rather than quartz. Element/Al ratios of Ti, Mg, K, Rb, and Zr are fairly constant, pointing to a mostly homogeneous source area. The black levels and Bonarelli are characterised by lower Mg/Al ratios compared to the Scaglia Bianca sediments, suggesting variability in chlorite input, which could be linked with short-term sea level drops indicated by Ti/Al ratios. The Bonarelli sediments are severely depleted in Mn, which is indicative of oxygen-depleted bottom water conditions and an open marine environment, while the black levels appear to have been deposited in a much more restricted setting. A model involving partial physical isolation of the Furlo site by a sill, coupled to OMZ variations by extra-basinal forcing mechanisms is proposed to account for these conflicting data. Based on Re/Mo ratios, the Bonarelli sediments were deposited under varying oxygen-depletion levels (from suboxic to anoxic), while Re/Mo ratios in the black levels show deposition under less oxygen-deficient conditions. Most minor elements, many of them redox-sensitive or sulphide-residing, are strongly enriched in the Corg-rich sediments, further documenting the absence of oxygen and potential availability of hydrogen sulphide in the water column at times, while high Zn concentrations suggest elevated submarine hydrothermal activity during this time interval. Ba is also present in very high concentrations (up to 1.4%, with an average weighed concentration of 0.6% for the entire Bonarelli), and is probably indicative of high paleoproductivity in this area. This is further supported by high P concentrations, hinting at nutrient availability at this site.