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Quantifying the link between weathering and past CO2 levels

Periodic Reporting for period 4 - CONTROLPASTCO2 (Quantifying the link between weathering and past CO2 levels)

Reporting period: 2021-08-01 to 2022-12-31

Climate warming or cooling episodes (especially in the long term) are primarily caused by the abundance of atmospheric carbon (largely as carbon dioxide). Given the large variations observed over the billions of years of Earth history, a big question is why the climate has always maintained itself within the relatively narrow bands necessary for life to continue. In other words, what regulates climate?
Chemical weathering of continental silicate rocks is thought to be the dominant control of the long-term carbon cycle: silicate weathering removes atmospheric CO2, following which it is transported to the oceans by rivers as bicarbonate, and sequestered for long time periods as marine carbonate. However, while weathering is the critical controlling mechanism, we do not know exactly what controls weathering (and hence why weathering would act to stabilise climate), nor how quickly weathering and climate interact.
This is the gaol of this project: to examine past periods of climatic warming and cooling, and to determine how weathering and CO2 drawdown changed and interacted with climate. This is important, because it is a key part of the carbon cycle – without which it is difficult to fully quantify how the climate system operates, or what will happen as the climate warms.
The objectives of the project are therefore to study both global and local weathering from climate change periods during the last 60 million years. This will yield rates of climate stabilisation – critical knowledge for climate projections. To do this, we are conducting isotopic analyses of different rocks that record past water chemistry, and will then also put our findings into climate models to fully integrate the controls on carbon with the climate.
1. Examination of climate stabilisation mechanisms across the end-Ordovician cooling, one of the most significant mass extinctions in Earth history. The data and climate model clearly show that weathering was controlled by the decreasing temperatures, removing less CO2 and stabilising the cooling climate. This represents the first evidence from the geological record that the “weathering thermostat” operates to stabilise the climate, and represents one of the key outputs of the project actions.

2. Examination of weathering mechanisms across the last few glacial-interglacial transitions. These climatic changes represent rapid warming and cooling episodes, and our data show that weathering responded directly to temperature, providing further evidence for the operation of the weathering thermostat.

3. Examination of other climate change periods are on-going (the warming periods Palaeocene-Eocene Thermal Maximum, Middle Miocene Climatic Optimum and the cooling period Oi-1).

4. Laboratory weathering experiments to examine the precise behaviour of our isotopic proxies of choice during weathering. These experiments involve studying the interaction of different rocks, minerals and water, and provide an ability to quantify weathering behaviour. Some of these experiments have been published, while others are still on-going.
1. Precision of analyses on small sample sizes: our method development in the project so far has allowed us to analyse smaller sample sizes to a greater precision than ever before for lithium isotopes. This is an important development, because often only small samples are available.
2. Experiments: we have developed laboratory weathering experiments to a greater extent than ever before. These now give us quantitative fractionation factors and weathering behaviour – which is an important step for this project.
3. Short timescale weathering changes: until this project, there had been no studies of weathering and carbon drawdown across rapid climate change events. We are pushing the boundaries and what is possible with analysis, interpretation and modelling to gain better and more accurate data.

By the end of the project, we expect to have weathering and carbon drawdown data across several climate change events: the Palaeo-Eocene Thermal Maximum, Middle Miocene Climatic Optimum and Middle Eocene Climatic Optimum warming events, and the Oi-1 cooling event. These events are all driven by CO2 abundance in the atmosphere.
We will also have similar localised data from caves for the recent glacial-interglacial events, which will help us understand how weathering responds to temperature and glaciation.
Finally, by the end of the project, we will have added all our new data into climate models, to directly determine the effect of weathering on the climate.