OldCO2NewArchives · CO2 reconstruction over the last 100 Myr from novel geological archives

CO2 exerts a major control on Earth’s environment, including ocean acidity and global climate. Human carbon emissions have elevated CO2 levels to above 400 ppm, substantially higher than at any time in the 800,000 year ice core record. If we want to understand how Earth’s environment and climate will respond to a high CO2 world, we need to look deeper into the geological past. This project provides a novel way to reconstruct ocean pH and atmospheric CO2 levels over the last 100 Myr. This will allow us to fathom the fundamental mechanisms governing Earth’s environmental evolution, and improve predictions of environmental response to CO2 change in the future. Atmospheric CO2 and ocean pH are closely coupled, because CO2 is acidic and is readily exchanged between the ocean and atmosphere. If ocean pH is known, we can place strong constraints on atmospheric CO2. Thanks to recent developments in geochemistry, it is possible to reconstruct changes in ocean pH using the boron isotope composition (d11B) of fossil shells. The well-studied systematics of this method and its underlying thermodynamic framework provide confidence in its application to the geological record. However calculation of pH from carbonate d11B requires knowledge of the boron isotope composition of past seawater d11Bsw. Here I propose novel strategies and techniques with new or underutilized archives (evaporites, shallow carbonates, and infaunal foraminifera) to constrain this crucial parameter. With d11BSW constrained, new d11B records from benthic foraminifera will provide a 100 Myr record of ocean pH. This benchmark reconstruction will be used to test key hypotheses on major environmental change in the geological record, and to constrain atmospheric CO2 using a state-of-the-art biogeochemical model. These paired data and modelling outcomes will provide a major step forward in our understanding of the fundamental processes regulating Earth’s climate and long-term habitability.