Thomas Tanner successfully defended PhD thesis

Before embarking on his PhD journey, Thomas Tanner has already carried out his Bachelor's and Master's programme at ETH Zürich. Thomas is also a passionate educator and has completed the High-school teaching degree in Geography.

Thesis summary:

Throughout Earth's history, there have been periods of major glaciations in the Northern and Southern Hemispheres. Geological records from the Quaternary period show glacial-interglacial cycles with changes in sea level of up to 120 meters. Ice core records spanning the past 800,000 years reveal that atmospheric carbon dioxide (pCO2) levels have oscillated between 180 and 280 ppm, closely tracking these ice volume shifts in the Northern Hemisphere. For periods preceding ice core records, various proxies have been used to estimate past pCO2 levels, with increasing uncertainties as we go further back in time. However, there is a consensus that today's atmospheric pCO2 levels, now at around 420 ppm, 50% higher than pre-industrial levels, resemble those from about 3 million years ago during the mid-Pliocene epoch, when the Arctic was much warmer, large-scale ice sheets were limited, and sea levels were over 20 meters higher.

This doctoral thesis aims to reconstruct past atmospheric pCO2 levels during two specific periods in Earth's history, the Late Miocene (~5 to 8 Ma) and the Oligocene-Miocene Transition (~23 Ma), characterized by significant changes in ice volume and global temperature. We utilize the carbon isotopic fractionation during photosynthesis (Ep) from sedimentary alkenones is a well-established paleoceanographic proxy to investigate the role of pCO2 variations during these periods and their connections to temperature fluctuations, ice volume shifts and sea level variations, particularly concerning the Antarctic Ice Sheet's stability. Since the orbital “pacemakers” of the global climate are assumed to have remained relatively consistent over the past tens of millions of years, understanding the forcing and feedback mechanisms in the climate system can provide insight into a potential warm-climate analogue for our future world.
The first part of this thesis focuses on the Late Miocene and explores the apparent discrepancy between reported cooling trends and the absence of a decline in atmospheric pCO2. We present the first proxy record suggesting a connection between atmospheric pCO2 levels and significant changes in global climate and ecology (Chapter 1). To reinforce these findings, we combine alkenone-Ep and foraminiferal-δ11B-based pCO2 proxies to gain further insight into the evolution of the carbon system. Therefore, we first present a technical study showing that organic extraction procedures do not affect stable isotope or trace element chemistry in foraminiferal carbonates and that these different laboratory approaches can be combined (Chapter 2). Subsequently, we present updated pCO2 estimations from a combined proxy approach and discuss non-CO2 factors, such as algal growth rate, cell size and light, that may influence the Ep-signal (Chapter 3).
The second part of this thesis (Chapter 4 and 5) focuses on reported changes in Antarctic Ice Volume over the Oligocene-Miocene Glaciation. Although benthic oxygen records imply similar glacial-interglacial cycles as observed during the past 800’000 years, no existing proxy record has a high enough resolution to show conclusive evidence of a coupling between atmospheric pCO2 and ice volume change. Here, we present two remarkably similar trends of alkenone-Ep, indicating a strong radiative forcing on the size and volume of the Antarctic Ice Sheet during the glaciation and the subsequent deglaciation. Importantly, these two records are from two entirely different environments, reducing the influence of non-CO2 factors on the combined Ep-signal. Additionally, we report a substantial cooling of the Southern Ocean as Antarctic Ice Sheets expanded at the end of the Oligocene and identify obliquity and eccentricity-paced signals in records reflecting either local hydrological changes or global climatic shifts.