The role of orbital forcing in the early-middle Eocene carbon cycle: a marine perspective

In the last decades global concentration of atmospheric greenhouse gases (GHGs) have produced irreversible effects on our climate system (Intergovernmental Panel for Climate Change - IPCC - 2014). One issue of particular concern is the rising of the carbon dioxide (CO2) atmospheric concentration, which overcame the threshold of 400 parts per million (ppm), modifying the terrestrial radiative forcing. The most recent IPCC report (2014) considers diverse future scenarios by the end of the 21th century, characterized by different hypothesis of magnitude and emission rates of GHGs, defined as Representative Concentration Pathways (RCPs). Scenarios without additional efforts to limit the emissions lead to a RCP8.5 pathway (IPCC, 2014), which is characterized by atmospheric CO2 concentration of ~1000 ppm. Forecasting the magnitude of the global warming is one of the big scientific challenges, above all because all these pathways are characterized by a huge uncertainties due to the complexity of the climatic system. Geological record can be used as a key that can lead to a better understanding of the impact of extreme global warming on the ocean-atmosphere coupled system and the relationship between carbon cycle and climate in worlds characterized by GHG concentrations very similar to that forecast by different RCP scenarios. Accordingly, certain time intervals in the past provide a unique opportunity to assess climate model because they capture the behavior of Earth’s climate system under pCO2 concentrations that likely could be reached in the future. In particular, some time intervals and events are representative of mean global conditions similar to the more extreme scenarios predicted for the end of the century, being characterized by levels equal or higher than 1000 ppm. The most recent geological time period characterized by this GHGs concentration is represented by the early Eocene and the so called hyperthermal events, occurring between ~56 and 51 million years ago (Ma). Hyperthermal events are linked to massive perturbations of the global carbon cycle related to the input of isotopically light carbon into the exogenic carbon pool. These extreme events are recorded globally as negative Carbon Isotopic Excursions (CIEs) and are associated with concomitant sediments depleted in calcium carbonate (CaCO3) in deep-sea and shallow marine successions as a response to the sudden rising of the Carbonate Compensation Depth (CCD ) and the acceleration of the global hydrological cycle, thus runoff, respectively. The most pronounced of these events is the Paleocene-Eocene Thermal Maximum (PETM), at ~56 Ma. Beside the PETM, at least other two hyperthermal events, Eocene Thermal Maximum (ETM) 2 (also known as ELMO or H1) and ETM3 (also known as K or X event), at ~54.1 and ~52.8 Ma, respectively, occurred during the early Eocene. However, the associated CIEs are smaller and also deep marine carbonate dissolution is less pronounced compared to the PETM. Recently, similar events of comparable isotopic excursions have been discovered in the global bulk and benthic isotopic records. In particular, the Early Eocene Climatic Optimum (EECO), ~51 Ma, seems to be the maximal expression of these transient events. The EECO recorded the highest temperature of the Cenozoic. Based on the available data the EECO is characterized by mean atmospheric CO2 levels which well approach the RCP8.5 scenario. The interpretation of the EECO is an issue of particular interest because of its potential relevance for understanding the dynamics of this climatic system. Although the hypothesis of successive releases of huge amounts of carbon as a mechanism for the emplacements of hyperthermals is widely accepted by the scientific community, a debate still exists on the possible sources. Several authors highlight the importance of the orbital forcing as a trigger mechanism for hyperthermal events. Indeed, an astronomical forcing signature on the succession of hyperthermals allows to develop a cyclostratigraphy at a very high-resolution that permits to correlate each event to their respective counterparts of the Pacific and Atlantic Oceans, by designing a global observation system. Furthermore, cyclostratigraphic approach permits to obtain an astronomical time scale which allows to verify the phase relationships between the orbital forcing and the sedimentary/geochemical response. The accuracy of the time scale is important for a better estimate of the emission rates of GHGs, in order to provides a crucial elements for the design of climate (paleo)models. Moreover, the noted amplitude of the astronomical forcing allows to analyze the response of the system to a measurable forcing. The main aim of this project was to contribute to the characterization of hyperthermal events across the early-middle Eocene time interval, defining a precise cyclochronological scheme of these events accompanied with the development of an accurate astronomically calibrated age model.