Periodic Reporting for period 5 - CONTROLPASTCO2 (Quantifying the link between weathering and past CO2 levels)
Berichtszeitraum: 2023-01-01 bis 2023-12-31
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.
The project therefore analysed multiple climate change events, using both marine and continental rocks, using different novel isotopic tracers. We determined that, as originally hypothesised, weathering is indeed temperature dependent. Thus, during climate warming weathering speeds up and removes more CO2 (hence stabilising climate), while during cooling the opposite occurs. However, the detail is more nuanced than that, because weathering also produces clays, which hinder CO2 uptake by weathering. Thus, the efficiency of the CO2 removal by weathering strongly depends on the weathering regime, and the abundance of clay. For example, a similar increase in atmospheric CO2 led to warming periods at the Palaeocene-Eoecene Thermal Maximum, and the Middle Eocene Climate Optimum. However, the latter was several times longer than the former. Our work shows that this was because of a greater abundance of clay during the latter, which shielded the rock from weathering.
Our research also shows that, when the climate warms, while weathering increases, erosion (the physical transport of material) increases even more. This is because the hydrological cycle accelerates, causing more extreme events and flooding. This is exactly the phenomenon we are seeing in the present day as the climate warms.
Overall, as well as data generation, we have developed an Earth System model that incorporates these nuances of weathering into its carbon cycle, which has significant implications going forward.
The tracers we developed during this work have now also been applied to negative emissions technologies, which artificially remove CO2 from the atmosphere. This is also a critical development of this project.
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.
The project therefore analysed multiple climate change events, using both marine and continental rocks, using different novel isotopic tracers. We determined that, as originally hypothesised, weathering is indeed temperature dependent. Thus, during climate warming weathering speeds up and removes more CO2 (hence stabilising climate), while during cooling the opposite occurs. However, the detail is more nuanced than that, because weathering also produces clays, which hinder CO2 uptake by weathering. Thus, the efficiency of the CO2 removal by weathering strongly depends on the weathering regime, and the abundance of clay. For example, a similar increase in atmospheric CO2 led to warming periods at the Palaeocene-Eoecene Thermal Maximum, and the Middle Eocene Climate Optimum. However, the latter was several times longer than the former. Our work shows that this was because of a greater abundance of clay during the latter, which shielded the rock from weathering.
Our research also shows that, when the climate warms, while weathering increases, erosion (the physical transport of material) increases even more. This is because the hydrological cycle accelerates, causing more extreme events and flooding. This is exactly the phenomenon we are seeing in the present day as the climate warms.
Overall, as well as data generation, we have developed an Earth System model that incorporates these nuances of weathering into its carbon cycle, which has significant implications going forward.
The tracers we developed during this work have now also been applied to negative emissions technologies, which artificially remove CO2 from the atmosphere. This is also a critical development of this project.
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 warming periods (the Palaeocene-Eocene Thermal Maximum, Middle Eocene Climate Optimum, Earth Eocene Climate Optimum, Middle Miocene Climatic Optimum) shows that weathering also increases, but that the rate of clay formation during weathering is also very important. Several of these studies have now been published, while some are still being written up.
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.
5. A compilation of the data generated for the above points led to the realisation that the background Li isotope ratio of the oceans showed a step-change in the Palaeozoic. This led to a now published study that recreated the ocean lithium chemistry over the past 3 billion years, and which shows that the fundamentals of how the carbon cycle operates were very different before and after plants colonised the continents. Not due to photosynthesis, but in how plants changed weathering and clay formation.
6. The use of the tracers developed to understand natural CO2 removal in artificial (negative emissions) CO2 removal. This may be the development of this project most relevant to future society.
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 warming periods (the Palaeocene-Eocene Thermal Maximum, Middle Eocene Climate Optimum, Earth Eocene Climate Optimum, Middle Miocene Climatic Optimum) shows that weathering also increases, but that the rate of clay formation during weathering is also very important. Several of these studies have now been published, while some are still being written up.
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.
5. A compilation of the data generated for the above points led to the realisation that the background Li isotope ratio of the oceans showed a step-change in the Palaeozoic. This led to a now published study that recreated the ocean lithium chemistry over the past 3 billion years, and which shows that the fundamentals of how the carbon cycle operates were very different before and after plants colonised the continents. Not due to photosynthesis, but in how plants changed weathering and clay formation.
6. The use of the tracers developed to understand natural CO2 removal in artificial (negative emissions) CO2 removal. This may be the development of this project most relevant to future society.
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 have pushed the boundaries and what is possible with analysis, interpretation and modelling to gain better and more accurate data, and how can analyse climate change periods that are only a few 10s of kya long.
4. The development of local, and not just global, weathering archives. In particular speleothems and detrital silicates - in both cases our papers on the first on these topics, and we have continued to exploit these archives.
5. A new Earth System model that can incorporate all the newly-discovered weathering controls into the carbon cycle.
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 have pushed the boundaries and what is possible with analysis, interpretation and modelling to gain better and more accurate data, and how can analyse climate change periods that are only a few 10s of kya long.
4. The development of local, and not just global, weathering archives. In particular speleothems and detrital silicates - in both cases our papers on the first on these topics, and we have continued to exploit these archives.
5. A new Earth System model that can incorporate all the newly-discovered weathering controls into the carbon cycle.