Palaeoclimatology, stratigraphy and biotic responses in the middle Eocene

Abstract

The Middle Eocene (37 to 49 million years ago, Ma) was characterised by warmer global temperatures and higher atmospheric carbon dioxide (pCO2) levels than today with only
small/non-existant icesheets. Because predicted pCO2 levels for the coming century have not been seen on Earth since at least ~40 Ma thus, the primary objective of this thesis is to improve our understanding of the nature, timing and consequences of carbon perturbations in the Middle Eocene between 39 and 43 Ma.

In Chapter 3, a new (instrument specific) methodology for measuring Mg/Ca in foraminiferal calcite is developed to enable the simultaneous measurement of additional trace elements indicative of foraminiferal test contamination. This new methodology enables Mg/Ca data to be screened more efficiently for contamination and increases confidence in palaeoceanographic reconstructions based on the Mg/Ca palaeotemperature proxy.

In Chapter 4, new foraminiferal stable isotope records (~5 kyr resolution) from Demerara Rise in the equatorial Atlantic are generated to test the hypothesis that the onset of continental ice sheets in the Northern Hemisphere occurred at ~41.6 Ma in the Middle Eocene, 30 million years earlier than previously thought. The new data herein, indicate that if continental ice sheets were present, they were small and easily accommodated on Antarctica with no need to invoke storage of ice in the Northern Hemisphere.

The dearth of appropriate Middle Eocene sedimentary sections on which to work means that a well calibrated timescale for this interval remains to be produced. In Chapters 5 and 6, this problem is addressed using Middle Eocene sediments recovered from the Blake Nose plateau in the western North Atlantic. A new high resolution magnetic stratigraphy and new quantitative foraminiferal biostratigraphic counts were developed between 39.5 and 42.0 Ma, which allows re-assessment and refinement of previous magnetostratigraphic and biostratigraphic interpretations. This provides excellent age control for these sediments and new calibrations to the Geomagnetic Polarity Time Scale (GPTS).

In Chapter 7, the first high-resolution quantitative planktic foraminiferal assemblage counts were developed for the global warming event the Middle Eocene Climatic Optimum (MECO) between ~39.5 and 41.5 Ma. New biotic records show that the MECO was accompanied by significant biotic shifts that suggest a shift from warm, oligotrophic surface waters to warmer, more productive surface waters during the MECO.

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