Using the boron isotope-pH proxy to investigate CO 2 -climate coupling in the geological past

• Main Author: Brown, Rachel
• Format: Thesis
• Language: English
• Published: University of Southampton 2024
• Subjects: Ice Sheet
• Online Access: https://eprints.soton.ac.uk/486342/; https://eprints.soton.ac.uk/486342/1/2023_11_10_Thesis_export.pdf; https://eprints.soton.ac.uk/486342/2/Final_thesis_submission_Examination_Mx_Rachel_Brown.pdf

Over the last 7 million years, the Earth has undergone major long-term cooling culminating in the development of continental scale ice sheets in the northern hemisphere around 2.5 million years ago. This cooling is believed to be associated with a reduction in the potent green house gas carbon dioxide (CO 2 ). It is well documented that anthropogenic global warming and climate change is driven by humanity’s emissions of CO 2 (and other greenhouse gasses) from fossil fuel burning, cement making and deforestation. However, various feedbacks in the Earth’s climate system (i.e., ice sheet albedo, water vapour, etc.) introduce a large amount of uncertainty regarding the expected warming for a given future emission scenario. Studying the relationship between natural CO 2 change and climate in the past enables us to investigate the role of these feedbacks and is a vital way in which we can narrow the uncertainty in the predictions of the magnitude of future climate change. To this end, in this thesis, the boron isotopic composition ( δ 11 B) of planktic foraminifera are used to reconstruct CO 2 across two periods of dramatic global climate change: the Late Miocene around 6 million years ago, and the Plio-Pleistocene Transition that occurred 2.6 million years ago. In both cases the records presented are at the highest temporal resolution to date, they thus provide unique insights into the role of CO 2 in determining the thermal evolution of the Earth and reveal valuable constraints on climate sensitivity and its dependence on background climate state. Although the δ 11 B-(pH) CO 2 proxy is emerging as a powerful tool, the applicability of the existing δ 11 B-pH calibration for G. ruber to deep time, and hence its reliability as a CO 2 proxy, has recently been questioned. The first aim of this thesis was therefore to investigate the “vital effects” of G. ruber using a combination of carbon, oxygen, and boron isotope ( δ 13 C, δ 18 O and δ 11 B) data from across a range of test size fractions. These data show that there ...