100 Myr CO2

Overview

CO₂ is invoked to drive some of the most profound changes in Earth’s history, from long-term climate evolution between greenhouse and ice-house worlds, to mass extinctions due to rapid perturbations to the carbon cycle. However CO₂reconstruction beyond the reach of the ice core record remains challenging, limiting our understanding of Earth’s past and insights into its future. This ERC-funded project aims to transform our ability to reconstruct ocean chemistry and atmospheric CO₂ over the last 100 Million years, using novel archives and approaches, and use these new records to better understand CO₂’s role in major environmental change.

This will be achieved by developing new methodologies for the robust application of the boron isotope (d¹¹B) pH proxy, coupled with Earth System modelling to reconstruct atmospheric CO₂. Substantial progress has been made in the use of the d¹¹B proxy, but a few key uncertainties currently limit the accuracy and precision with which this method can be applied. The most critical is knowledge of the boron isotope composition of seawater (d¹¹B_sw), which is required to convert measurements of d¹¹B in carbonates into pH. Secondary constraints on seawater carbonate and major element chemistry are also required for accurate CO₂system determination. Finally, the scope of d¹¹B-based pH reconstructions also remains limited simply by the lack of long-term d¹¹Brecords. This project will address these challenges, transforming our understanding of pH, CO₂, and carbon cycle change over the past 100 Myr.

To reconstruct the boron isotope composition of seawater we will develop a variety of independent new approaches. The first focuses on the composition of evaporites, which have boron isotope compositions close to that of seawater, and have been shown to provide valuable constraints on the evolution of the ocean’s major element composition. However several processes may alter the composition of evaporite forming brines away from that of seawater, and new approaches and analytical techniques are required to constrain these. This work will first ground truth these methods, using lab experiments and work in modern evaporitic settings, before applying them to ancient evaporite deposits.

We will also use detailed geochemical measurements on foraminifera and corals to constrain d¹¹Bsw and ocean pH, taking advantage of limits on pH imposed by under-appreciated biomineralisation pathways and ecological niches.

With d¹¹B_SWconstrained, d¹¹Brecords can be converted to pH and used to constrain CO₂. We will create detailed high-quality long-term d¹¹Bdata from a variety of locations and foram species, and pair this with trace element data and Earth system modelling, to reconstruct the evolution of the ocean-atmosphere CO₂system.