Investigating the drivers of perturbations to the Cenozoic carbon-climate system

Abstract

Over the course of the Cenozoic the Earth system has shifted from a CO2-rich ‘Greenhouse’ climate state to a CO2-poor ‘Icehouse’ climate state. This trend is punctuated by numerous perturbations to the carbon-climate system, but the extent of the coupling between the carbon cycle and climate system, the drivers of these perturbations, and their relationship to the longer-term Cenozoic trend is still debated. In this thesis, I use biogeochemical modelling and numerical analysis to explore the key research question: ‘What were the drivers of carbon-climate system perturbations during the Cenozoic?’, with a focus on perturbations during the Eocene-Oligocene Transition and the mid-Miocene, the role of tipping points during these periods, and the long-term evolution of the ocean carbonate system.

The potential impact of the Columbia River Basalt large igneous province on the mid-Miocene Earth system is investigated using two biogeochemical box models. This modelling indicates that ‘cryptic degassing’ from intrusive and/or crust-contaminated magma of a magnitude within the estimated possible range can drive the observed carbon cycle perturbation and warming around 16.0 Ma, but cannot by itself explain other features of the mid-Miocene palaeorecords.

The hypothesised drivers of the Eocene-Oligocene Transition (EOT) carbon cycle perturbation are explored using a biogeochemical box model. The results suggest that the glacioeustatic fractionation of carbonate burial from shelf to basin can explain most of the deepening of the carbonate compensation depth (CCD) at the EOT, but that the benthic carbon isotope excursion most likely requires additional drivers.

The shelf-basin carbonate burial fractionation hypothesis is examined further in order to quantify the relationship between shelf carbonate burial extent, the CCD, and changing sea-level during the Cenozoic. This analysis confirms that carbonate burial fractionation can drive most of the CCD deepening at the EOT but is less important either before or since then, and also indicates that the sensitivity of the CCD to sea level change has significantly declined during the Cenozoic.

Palaeorecords of a number of perturbations to the carbon-climate system during the Cenozoic are analysed in search for ‘early warning signals’ (EWS) indicative of systemic instability and impending critical transitions, and the reliability of this method when applied to palaeorecords is critically explored. EWS are found prior to some (e.g. the EOT and Palaeocene-Eocene Thermal Maximum) but not all of the events, and the results and technique are judged to be moderately reliable.

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Identifiers

ORCID for David Armstrong Mckay: orcid.org/0000-0002-0020-7461

ORCID for Toby Tyrrell: orcid.org/0000-0002-1002-1716

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