CO2 reconstruction over the last 100 Myr from novel geological archives

Informations projet

OldCO2NewArchives

N° de convention de subvention: 805246

DOI

10.3030/805246

Projet clôturé

Date de signature de la CE 17 Octobre 2018

Date de début 1 Février 2019

Date de fin 31 Octobre 2024

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 996 784,00

Contribution de l’UE

€ 1 996 784,00

Coordonné par

THE UNIVERSITY COURT OF THE UNIVERSITY OF ST ANDREWS
United Kingdom

Periodic Reporting for period 4 - OldCO2NewArchives (CO2 reconstruction over the last 100 Myr from novel geological archives)

Période du rapport: 2023-08-01 au 2024-10-31

Résumé du contexte et des objectifs généraux du projet

Understanding the impact of CO2 on Earth's climate is one of society's most critical scientific goals. In the years since this project was designed, this has become ever more apparent, with the impact of climate change increasingly felt by society in heatwaves, fires, and floods. The solutions to the climate crisis are also ever-more discussed: what level of atmospheric CO2 guards against the worst climate impacts?

This project provided novel insight on the role of CO2 in driving major climate change by transforming the state of the art in our knowledge of how CO2 has changed through Earth's history. Specifically, by reconstructing CO2 levels in the geological past, we examined what Earth's climate looks like at different levels of CO2. For instance, the last time modern levels of >400 ppm were experienced was around 4 Million years ago, a time with no ice on Greenland and forests of beech trees in parts of Antarctica; globally, sea level was around 10 m higher. This shows us, starkly and without the need for future projections or models, where we are headed if we allow CO2 to remain at today's levels. And if emissions continue to grow, CO2 may climb to levels not seen since the Eocene around 50 Million years ago, a time when alligators roamed the Arctic.

The overall objective of this project was to create more reliable reconstruction of how CO2 has changed over the past 100 Million years, using a method involving the isotopes of boron. This required us to figure out new ways to reconstruct seawater chemistry, making pioneering developments in the analysis of ancient seawater trapped in salt crystals. We also examined the fingerprints of seawater changes left in different kinds of shells and developed novel mathematical frameworks to perform robust calculations using these data.
The seawater chemistry estimates are then combined with measurements of boron isotopes in other kinds of shells, which track the pH of the ancient ocean and the CO2 content of the atmosphere.

Our reconstructions show how CO2 has transformed our planet's climate in the past and provide context for its critical role in our planet's future.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

Technical
This project was underpinned by development of several pieces of novel equipment and new techniques. Key equipment included a state of the art Mass Spectrometer (QQQ-ICPMS), capable of measuring the vast majority of elements across the periodic table to high precision and at very low levels. New methods allowed us to analyse elements not previously possible by earlier generations of such instruments, unlocking new kinds of analysis on ancient shells and brine-inclusions in salt crystals. The other key instrument is a custom-built laser ablation system that allows us to analyse material at extremely fine spatial resolution with a cryostage which allows us to freeze and ablate brine inclusions in salts. These were complemented by additional technical developments on the cleaning and purification of samples in our clean chemistry lab, substantially improving sample throughput and analytical reproducibility.

Objective 1: Seawater chemistry
Salt crystals grow from seawater and the brine inclusions trapped within them record seawater’s chemistry at the time of growth. Detailed study of modern salt samples has allowed us to understand the processes by which boron is incorporated into salts and their inclusions and we have compared these results to geochemical modelling experiments to help substantiate these findings. We find that halite inclusion chemistry closely tracks seawater, allowing us to then use analysis of ancient salt crystals to reconstruct the chemistry of ancient oceans. This has been complemented with novel constraints based on corals and benthic forams. All of these constraints are then combined in a newly developed statistical model to obtain the best constraints on the evolution of seawater chemistry over the last 100 million years and beyond.

Objective 2: pH and CO2
To reconstruct ocean pH and atmospheric CO2 we combined the seawater chemistry constraints above with complementary measurements on foraminifera taken from deep sea sediment cores. We generated ~700 measurements on foraminifera to create an paralleled dataset tracking the evolution of ocean-atmosphere CO2 chemistry. We also produced data on fossil brachiopod shells, and compiled and reprocessed all existing measurements using new calculation routines. This allowed us to reconstruct the pH and CO2 of the ocean and the atmosphere over key intervals of geological time.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

Our analytical breakthroughs have transformed the ability of the community to generate large high quality boron isotope and trace elemental datasets. Our pumped columns and a batch chemistry approach has improved the speed of sample purification by a factor of 3, while simultaneously reducing lab based contamination. Our laser ablation and QQQ-ICPMS development has unlocked fluid inclusion analysis and also the measurement of several novel, hard to analyse elements in fossil shells.

The ability to directly analyse halite fluid inclusions has led to cleaner, more robust data sets then in previous work. Our results followed predictions based on simple models of boron evolution during sea water evaporation and incorporation in evaporite minerals. Our constraints on the evolution of the boron isotope composition of sea water have allowed us to rule out some previous estimates of CO2 which had proved challenging for the community to interpret. The statistical framework for these estimates that we have developed has allowed us, for the first time, to produce continuous estimates of pH evolution with accurately quantified uncertainties. Furthermore, this framework is flexible, allowing us to continually update our constraints on seawater evolution as we generate new data. It has also proved invaluable in other applications, including providing robust continuous interpolations of the secular evolution of various other isotope systems and the elemental composition of sea water.

Our reconstructions of pH and CO2 have substantially transformed the state-of-the-art. In addition to tripling the data density of such reconstructions over the Cenozoic, we have also created frameworks which allow data both new and published to be processed consistently, giving reconstructions of carbonate chemistry with well quantified uncertainties. This has led to a series of high profile publications which show, much more clearly than was possible prior to this proposal, a compelling link between long term CO2 and climate change. They also demonstrate that major carbon release acidifies the ocean and has impacted marine life during mass extinction events; that marine calcifying organisms are both affected by and can affect change in ocean chemistry; that long term – and orbital scale – CO2 decline led to major ice ages; under the current CO2 levels or unprecedented in the last three million years.

In addition to widespread scientific dissemination of these findings, through conference presentations and published papers, our results have also lead to substantial engagement with the media and the public. To enable this I undertook 70 different public engagement activities over the course of this project, including attendance at COP26. This work has also had substantial positive impact on my scientific development and career, and the careers of my project team.

CO2 levels over the last 66 Million years, adapted and simplified from Rae et al. 2021, AREPS