Paleoceanographic and paleoclimatic reconstruction of the late Albian – early Turonian time interval: from an unstable to a stable ocean

Earth during the Cretaceous (145.5-65.5 My) experienced long-lasting periods of warm climate with temperatures much higher than today. The continuous development of Large Igneous Provinces and the global rearrangement of plate boundaries led to an increase in atmospheric levels of greenhouse gasses (CO2 and possibly CH4), and times of restricted ocean circulation. Global warming and the abnormal increase in CO2 changed the equilibria of the carbon cycle thus altering the hydrologic cycle, nutrients distribution, upwelling intensity and deep water formation. The geological record of the late Albian to early Turonian (~104-92 My) time interval represents a particularly intriguing natural laboratory to understand the physical, chemical and biological processes that acted during a period of alternated phases of relative equilibrium and carbon cycle perturbations. The late Albian to early Turonian time interval is characterized by three major geochemical anomalies corresponding to positive excursions of the stable carbon isotope profile: the Oceanic Anoxic Event 1d (OAE1d), the Mid-Cenomanian Event (MCE) and the Oceanic Anoxic Event 2 (OAE2). This study, combining sedimentological, geochemical (C and O isotopes, inorganic and organic geochemistry) and high-resolution cyclostratigraphic data, provides an integrated investigation of paleoenvironmental and climatic dynamics accompanying these major events. The interplay of surface water changes and sea bottom processes are here discussed with the purpose of better defining how water masses react during highly stressed climatic/environmental conditions. Five pelagic Tethyan key-sections, four from the Umbria-Marche Basin (Furlo, Contessa, Le Brecce, Monte Petrano – central Italy) and one from the Belluno Basin (Cismon – northern Italy), were chosen as natural archives of the past oceanographic and environmental conditions. The extremely detailed field- to microscopic-scale sedimentological description of the five sections allowed a better comprehension of the physical processes that acted on the sea-floor during the deposition of the studied sequences. Peculiar sedimentary structures and facies indicate that settling of biogenic particles wasn’t the only physical process controlling pelagic sedimentation. Waning and waxing cycles of bottom currents actively redistributed sediments, forming specific facies. In order to account for this process, a new depositional model for calcareous pelagic contourites was established. New high-resolution oxygen, carbonate- and organic-carbon isotope record were produced for Monte Petrano and Cismon sections. High-resolution δ13C record exhibits three positive excursions in correspondence of OAE1d, MCE and OAE2. The detailed correlation with published and unpublished isotopic data from the other studied sections better defines the basin-scale stratigraphic framework. viii Abstract Detailed correlations of the δ13C profiles through the OAE2 interval revealed the presence of a significant hiatus at the top of the Bonarelli Level in all studied sections. Even if the amount of missing sequence slightly differs from site to site, the hiatus affects both the Umbria-Marche and Belluno basins. Important lithological changes are associated with the observed carbon isotopic excursions (CIEs). Few centimeter thick black shale layers are associated with the Pialli Event (OAE1d) and the impressive Bonarelli Level is the lithologic expression of the OAE2. The MCE corresponds, instead, to more subtle and intriguing conundrum. Starting from this event up to the Bonarelli Level the studied successions are characterized by the lithologic alternation of organic-rich shales and black chert bands with whitish limestones. A stratigraphic vertical variation of the identified facies was reconstructed and linked to the major geologic events. The interplay of the paleoceanographic processes controlling the sediment composition combined with current-driven sediment distribution resulted in characteristic cyclic patterns. Recurrent stacking patterns, from layer- to lamina-scale, are the result of alternated phases of slackish conditions with times of enhanced circulation. Sedimentological evidence above the Pialli Level (OAE1d) and above the Bonarelli Level (OAE2) indicates that intense bottom-current activity characterized the recovery phases from these major climatic/oceanographic perturbations. In particular, the diffused sedimentary structures indicative of particularly intense bottom-current traction above the Bonarelli Level suggest that the observed hiatus might have been produced by major erosions and reworking of the sediments on the sea-floor during the recovery from anoxic conditions. Major and trace element geochemical data were produced for Cismon and Monte Petrano sections in order to better understand the paleoenvironmental changes that characterized these two adjacent basins. Element/Al ratios of Ti, Mg, K, Rb, and Zr are fairly constant, indicating a mostly homogeneous source area. Ba and P are present in high concentrations in both sections indicating relatively highproductivity conditions in both areas. The Bonarelli Level is severely depleted in Mn, suggesting oxygen-depleted bottom water conditions in an open marine environment in both the Umbria-Marche and Belluno Basins, while the Cenomanian rhythmic black levels seem to have been deposited in a much more restricted setting. At Cismon, with the exception of the Bonarelli Levels, the depositional environment was dysoxic without ever reaching truly anoxic conditions. However, redox-sensitive elements from both analyzed sections indicate that the MCE marked the onset of more unstable conditions, with alternate times of dysoxic/anoxic and well-oxygenated bottom-waters. Zn and Cu concentrations rise up to high values in the interval between the MCE and the Bonarelli Level, presumably indicating hydrothermal activity. ix Abstract The application of cyclostratigraphic techniques to all the studied sections from the Umbria-Marche Basin shows that primary productivity variations, represented by the cyclic alternation of carbonate and siliceous lithologies, reflect the climatic, oceanographic and depositional response to orbital cycles. In particular, sediment production and deposition were controlled by short eccentricity (about 100ky) and obliquity (about 40ky) forcing. The applied probabilistic approach allows the determination of a proper sedimentation rate model for the studied sections. The results show a general increase in the sedimentation rate after the MCE and a progressive reduction of sedimentation rates in the OAE2 interval. Spectral analyses of mm-scale sedimentological logs of the Bonarelli Level at Furlo and Contessa indicate very low sedimentation rates for the OAE2, and confirm the presence of hiatuses. According to the sedimentation rate model, and deriving an age of 94My for the beginning of the OAE2, the OAE1d started at about 102.4My and lasted for about 1.2My, while the age of the MCE is about 97My and lasted about 200ky. Highly detailed sedimentological characterization was central to fully comprehend the pelagic sedimentation in Western Tethys during times of stable and perturbed conditions. Based on bio-chemoand cyclostratigraphy, facies evolution was precisely dated and event duration was estimated. This multi-proxy approach was proved vital to characterize an apparently homogenous and monotonous sedimentary environment that was indeed extremely dynamic. The vertical repetitive stacking of facies belonging to contrasting depositional suites (settled and tractive) and to opposite oxygenation regimes (oxic vs. anoxic) suggests that the environmental resilience of the pelagic system recovers from perturbations through various instability stages, such as sediment erosion, winnowing and redistribution by bottom currents. This kind of adaptive behavior can be considered an autocyclic process (the recovery), forced by allo-cyclic factors (the perturbations). This study provides an important step towards a more comprehensive understanding of the mechanisms at the origin of ‘mid-Cretaceous’ paleoceanographic events, including feedbacks to trigger anoxia-dysoxia and re-establish normal conditions at various time scales. The multi-proxy approach used here to decipher the geological record demonstrates that the onset, transition and recovery from anoxia didn’t happen instantaneously but was the result of a continuous subtle variation of processes in an extremely dynamic setting.