Astrochronology of extreme global warming events during the early Eocene greenhouse climate

Abstract

The early Eocene represents an ideal case study to analyse the impact of enhanced global warming on the ocean-atmosphere system and the relationship between carbon cycling and climate. During this time interval, the Earth’s surface experienced a long-term warming trend that culminated in a period of sustained high temperatures called the Early Eocene Climatic Optimum (EECO). The long-term temperature rise was, in turn, punctuated by a series of short-lived global warming events, so-called “hyperthermals”, of which the Paleocene/Eocene Thermal Maximum (PETM or ETM1) represents the most extreme example. These transient events were driven by fast and massive injections of 13C-depleted carbon into the ocean-atmosphere system, which led to changes in global temperatures and carbon cycle, possibly triggered by a common orbital forcing mechanism. In the geological record, the hyperthermal events are marked by prominent negative carbon and oxygen isotope excursions, recorded in marine and continental sedimentary sequences and accompanied by carbonate dissolution in the deep-ocean. Evidence from deep-sea carbonate records suggests that multiple hyperthermal events occurred from the late Paleocene throughout the early Eocene, and are recorded globally in deep-sea and land-based records. In this thesis, we present new benthic and bulk stable isotope records from a deep-ocean site (ODP Site 1263, Walvis Ridge) and a land-based section (Smirra, Umbria-Marche, Italy) encompassing the early Eocene (~49-54 Ma). Our high-resolution benthic foraminiferal carbon and oxygen isotope records of ODP Site 1263 represent the first high-resolution benthic records across the early Eocene hyperthermals and the EECO. We study the short-lived events in relation to orbital forcing and analyse the covariance of carbon and oxygen within each event, representative of the relationship between changes in deep-sea temperatures and extreme perturbations in the exogenic carbon pool. For all the global warming events, we find a coherent relationship between carbon and oxygen. Unravelling this complex climatic system strictly depends on the availability of high-quality suitable geological records and accurate age models. However, discrepancies between the astrochronological and radioisotopic dating techniques complicate the development of a robust time scale for the early Eocene (49-54 Ma). Based on the carbon isotope record of Site 1263, we develop two astronomically calibrated age model options for the early Eocene and compare our results to available age models for Site 1258. The drilling of the Smirra section provides a new detailed record of the upper Paleocene to lower Eocene for which we establish a high-resolution magneto- and bio-stratigraphic framework and carbon isotope stratigraphy. The carbon isotope record is used to build an astronomically calibrated age model for this time interval, which we compare to the ones obtained for Site 1263. Our results offer the potential to refine the geological time scale and help to close the existing “Eocene gap” in the astronomical time scale.