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.
