500 million years of ocean pH and atmospheric CO2

forthcoming

CO2 is an important greenhouse gas and building block for life. It plays a major role in regulating Earth's environment, maintaining planetary habitability, and influencing both ocean acidity and global climate. Anthropogenic carbon emissions have raised CO2 levels to their current concentration of 420 ppm, last seen 3 million years ago. Is this high? Was Pre-Industrial CO2 low? If we want to place CO2 changes in context and understand how Earth’s environment and climate will respond to changing CO2, we need to look into Earth’s geological past. My project provides a novel way to reconstruct ocean pH and atmospheric CO2 over the last 500 million years, from the dawn of complex multi-cellular life. This will permit us to quantify pH and CO2 during key periods of Earth's history, providing fundamental insights into CO2’s past and future role in driving climate, environment and life on our planet. The CO2 molecule is acidic and freely exchanges between the ocean and atmosphere, resulting in a close coupling of ocean pH and atmospheric CO2. If ocean pH is known, atmospheric CO2 can be reconstructed. Recent advances in geochemistry make it possible to constrain pH from the boron isotope composition (δ11B) of fossil shells. The principles of this pH-meter are well-understood and rooted in the laws of thermodynamics. However, to calculate pH from fossil δ11B, two parameters are needed: (i) knowledge of boron isotope composition of seawater (δ11Bsw) and (ii) organisms’ biological imprint on δ11B (‘vital effects’). Here, I propose novel approaches to reconstruct past δ11Bsw from fluid inclusions and apatite, and to calibrate vital effects on ‘living fossils’. Armed with this knowledge, and aided by advanced robotics and computation, I will create an iconic 500-million-year record of ocean pH and atmospheric CO2 from fossil brachiopods. This will provide a step change in our understanding of CO2 and its role in Earth’s climate and habitability, past, present and future.