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CO2 reconstruction over the last 100 Myr from novel geological archives

Periodic Reporting for period 3 - OldCO2NewArchives (CO2 reconstruction over the last 100 Myr from novel geological archives)

Período documentado: 2022-02-01 hasta 2023-07-31

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 provides novel insight to this question by transforming the state of the art in our knowledge of how CO2 has changed through Earth's history. Specifically, by reconstructing CO2 levels over the last 100 Million years, we can examine what Earth's climate looks like at a given level of CO2. For instance, the last time 2021 levels of 414 ppm were experienced was 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 20 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 is to produce the most reliable reconstruction of how CO2 has changed over the past 100 Million years, using a method involving the isotopes of boron. This requires us to figure out new ways to reconstruct seawater chemistry, so we are making pioneering developments in the analysis of ancient seawater trapped in salt crystals. We are also examining the fingerprints of seawater changes that are left in different kinds of shells and developing novel mathematical frameworks in which we can robustly present these data.
The seawater data is 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.

Ultimately our data will show how CO2 has transformed our planet's climate in the past and provide context for its critical role in our planet's future.
The initial phase of this project focussed on securing the right equipment, people, and materials.

Key equipment acquisition includes a state of the art triple quadrupole Inductively-Coupled Plasma Mass Spectrometer (QQQ-ICPMS), capable of measuring the vast majority of elements across the periodic table to high precision and at very low levels. This has been fully set up and is being used to analyse ancient shells and brine-inclusions in salt crystals. The other key instrument is a custom-built laser ablation system with a bespoke cryostage. This allows us to analyse material at extremely fine spatial resolution - as small as 20 microns - allow us to investigate the variability of chemical composition within samples, which provides interesting constraints on seawater chemistry not achievable by analysing the bulk material alone. The cryostage provides novel new capabilities, by allowing us to freeze samples (including brines with freezing points as low as -80C) and thus effectively ablate samples that are not solid at room temperature. Full installation of this equipment has been delayed due to the COVID-19 pandemic, but the laser system is now up and running (with a first manuscript submitted) and the cryostage has recently been installed and is currently being tested.

A multi-talented project team has been assembled, including 4 postdoctoral scholars and a technician. Their expertise spans salt geochemistry, paleoceanography, novel geochemical analysis, and numerical modelling. We have also used paid student interns (in particular over the summer) to help up the throughput and efficiency of preparation work in the lab. Note that there were several delays in assembling the postdoctoral team due to the pandemic.

We have secured the key materials required to make this project a success. Critically, we have established a collaboration that secures access to a unique collection of ancient salt samples, which are well-characterised and will help ensure the success of this component of the work. We have also collected samples of salt and water from modern evaporitic environments and obtained lab-grown salts for ground-truthing studies. As well as salts, we have obtained a large sample set of ancient shell materials, mainly from sediment cores, which will be used to make long-term records of ocean pH and atmospheric CO2. At present we have over 600 samples in hand and this sample set continues to grow.

Having secured the necessary components, we are now obtaining exciting new results.

To stay at the cutting edge of understanding in this field of geochemistry, we are continuing to probe the fundamentals of our methods to provide improved analytical precession and insight. Publications in this area include the first detailed computation of boron's behaviour in the presence of fluorine, two inter laboratory studies on boron isotopes, and experimental work on the incorporation of boron and trace metals into carbonate material.

We are also applying these techniques to examine past changes in Earth's environment. Notable publications include the most complete estimates of CO2 over the last 66 million years, based on compilation and re-calculation of boron isotope and alkenone data. This paper is currently the 5th most downloaded paper in Annual Reviews of Earth and Planetary Sciences (across all years) and also generated significant public interest (see section below). We have also contributed to the first high-resolution reconstruction of pH and CO2 change across the KPg extinction, again generating significant scientific impact and public interest, and I have edited a journal special issue on past carbon cycle change. There are now also several very exciting manuscripts in preparation and in review, which address other aspects of environmental change in Earth's history, and we have presented some of these findings (virtually) at international conferences and in invited departmental seminars.
An array of exciting work is now underway, with expected results that will progress beyond the state of the art.

Our work on salt geochemistry is yielding extremely promising initial data and we are eagerly anticipating the ability to analyse these samples with our cryo-stage laser once
the current testing is complete. We are also very excited about developments in boron isotope analysis in carbonates by laser ablation, an area where we are also pushing innovation. These data will yield novel records of the chemical evolution of seawater.

In anticipation of these results we have developed a novel numerical framework to examine long-term changes in seawater chemistry. This is proving more widely applicable than initially expected, so we are using it to re-evaluate a variety of different types of change in seawater. Linked to this, we are also developing new numerical routines to make seawater CO2 calculations, allowing more accurate determination of past CO2 with better defined uncertainty.

Work on reconstruction of pH and CO2 in different intervals of geological time is also continuing apace. Records of pH and CO2 change over major extinction events, climate transitions, orbital-scale variability, and over climate's long-term evolution are all underway. Some of this work is providing the best available evidence for long-standing hypotheses, while other data challenge existing ideas.

Our results to date have also substantially progressed public interest and understanding. The paper on the KPg extinction was widely picked up by the world's media, including an extensive piece in the New York Times. The paper on CO2 over the last 66 Million years was also widely picked up, leading to interviews with several news organisations, including a BBC TV news piece. It has also formed the basis for extensive public outreach work and has led to an invitation to attend COP26.
CO2 levels over the last 66 Million years, adapted and simplified from Rae et al. 2021, AREPS