| Co-Authors: | Brumsack, Hans-Jürgen, ICBM, brumsack@icbm.de; Kuypers, Marcel, Max-Plank-Institut für Marine Mikrobiologie (MPI-MM), Celsiusstraße 1, 28359 Bremen, Germany, mkuypers@mpi-bremen.de; Böttcher, Michael E., MPI-MM, mboettch@mpi-bremen.de |
| Abstract: | The mid- to Late Cretaceous "greenhouse" world was characterised by elevated temperatures and atmospheric CO2, ice-free poles, equable climates, and generally sluggish circulation. The Cretaceous stratigraphic record is punctuated within several important organic-rich shale intervals representing quasi-global "Oceanic Anoxic Events" (OAEs). One of the most spectacular expressions of these OAEs is the Livello Bonarelli of central Italy, deposited during the Cenomanian-Turonian boundary (OAE2) at around 93.5 Ma. In the Umbrian-Marche Basin in Italy, the Livello Bonarelli consists of approximately 1 m of pyrite-rich black shales interspersed with radiolarian sandstone layers, and contrasts sharply with the extensive over- and underlying siliceous limestones. The duration of OAE2 deposition is assumed to be between 100 and 500 kyrs.
For this study, 42 Livello Bonarelli samples (average interval ~2 cm) were taken from the Gubbio Core, drilled from the Contessa Quarry. Powdered samples were analysed for Ctot, Corg, Stot, and several major- and minor elements using X-ray fluorescence (XRF) on fused borate disks. These sediments, characterised by high Corg (up to 23%), S, and SiO2 contents (up to 95%) consist of a simple two component mixing system ("average shale"-SiO2). Major element concentrations are low, except for Si and P, owing to a dilution effect by high levels of excess silica (52% on average). As Al-normalised ratios for heavy elements such as Rb, Zr, taken as proxies for higher energy environments, are close to "average shale" values, it is postulated that the excess SiO2 represents biogenic opal rather than quartz.
Most elements, when compared to associated Al2O3 data, plot along "average shale" lines. Elements such as Ti, Mg, K, Rb, Zr, show positive relationships with Al2O3, pointing to homogeneous source area material. Most minor elements (As, Co, Cr, Cu, Mo, Ni, Pb, Sr, U, V, Y, Zn), many of them redox-sensitive or sulphide-residing, are strongly enriched in the black shales, based on Al-normalised ratios, documenting the absence of oxygen and potential availability of hydrogen sulfide in the water column, while high Zn concentrations hint at elevated underwater volcanism during this time interval. Lower than "average shale" MnO concentrations (0.01% compared to 0.11%, with corresponding Mn/Al ratios of 34.5 and 96.4 respectively), are indicative of suboxic bottom waters conditions through which more soluble Mn2+ (reduced in the sediments) is exported via an oxygen minimum zone in an open marine environment during or shortly after deposition. Ba is also present in very high concentrations (up to >3%, with a weighed average of 0.98% for the entire Livello Bonarelli), and is possibly indicative of high paleoproductivity in this area. This is supported by the high P concentrations, which hint at nutrient availability at this site. |