100 Myr CO2
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 (d11Bsw), 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 d11BSWconstrained, 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.
New Cenozoic CO2 compilation
Our new paper in Annual Reviews of Earth and Planetary Sciences compiles and re-evaluates CO2 reconstructions over the last 66 million years from marine archives - boron isotopes in foraminifera and carbon isotopes in alkenones. This provides the most coherent picture of Cenozoic CO2 change to date and places some interesting constraints on long-term climate sensitivity and mechanisms of CO2 change. It also sets the scene for the rest of the work underway in our ERC project! The data are available here.
Ocean acidification at the KPg extinction
New paper in PNAS, led by Michael Henehan, provides the first evidence for rapid ocean acidification at the KPg boundary. We also resolve the "Strangelove Ocean" paradox - showing that minimal surface to deep carbon isotope gradients doesn't require collapse of the biological pump, when 3D ocean circulation is taken into account. Given the link between acidification and extinction in the past, we need to worry about what CO2-driven acidification will do to marine life in the future.