ISO-THERM: Isotopic testing of Earth's weathering thermostat

Weathering is a chemical reaction which dissolves rocks in rainwater and over long timescales removes carbon dioxide (CO2) from the atmosphere. Both theories and experiments predict that rocks will dissolve faster with warmer temperatures, potentially making chemical weathering the key process that has stabilised Earth's climate over millions of years. Conceptually, if atmospheric CO2 levels increase, the greenhouse effect would lead to warmer temperatures and more weathering, thereby removing CO2 and cooling climate. Hence, weathering can provide a climate "thermostat", preventing big swings in climate and maintaining a habitable planet.

However, weathering in the real world is more complex than in the laboratory, and evidence for how this climate "thermostat" operates is lacking. We simply do not know how sensitive weathering is to climate, either locally or globally, and therefore we do not know how well this thermostat works. Indeed, we do not even know if weathering is the most important control on the earth's climate, as some scientists have proposed alternative controls such as seafloor alteration, biological carbon cycling, and sulphuric acid weathering of limestone. Our poor understanding of weathering represents a major gap in our understanding of the global carbon cycle, and a significant challenge for modelling past and future climate change.

To test the weathering "thermostat" on Earth, this project will reconstruct how weathering has changed in the past using a programme of geological detective work. Although there have been interesting clues to date, the evidence has been circumstantial and often unreliable. The problem is that records of past ocean chemistry have indicated weathering changes, but we have not had reliable forensics to tie these changes to the continental regions where the weathering occurred.

Fortunately, two discoveries from my previous investigations lead to a way forward in this case. First, the distinct composition of the lead (Pb) atoms in continental rocks provides a geological "fingerprint" that is transferred by chemical weathering via rivers into the ocean. Second, sediments formed in the ocean are witnesses to this "fossil seawater" composition. Therefore, by analysing ocean sediments of different ages, a detailed timeline of weathering changes will be reconstructed, and comparison to those continental Pb fingerprints will reveal the weathering culprits. Measuring another element, lithium (Li), will provide corroborating evidence on the weathering environment, revealing how the weathering was carried out and what controlled it.

Together, this new evidence will reveal the controls of climate and mountain uplift on the weathering of different rock types in different regions. Computer modelling will then be used, in combination with evidence of past changes in climate and CO2, to determine the strength of the weathering "thermostat". This result is crucial for addressing the question of how a habitable climate is maintained on Earth. Furthermore, this information will improve climate models, because predicting Earth's future climate evolution in response to anthropogenic carbon emissions relies on an understanding of how, and how quickly, weathering will respond to these changes.

This project is addressing fundamental science relating to the evolution of the Earth's carbon cycle, and especially the operation of feedbacks between tectonics, weathering and climate. Such research is of global significance for both establishing natural processes of climate variability and for predicting future changes. In particular, continental weathering will represent the ultimate major sink for anthropogenic carbon emissions, and how quickly weathering responds to a changing climate will affect our future climate evolution. It is therefore important to constrain the sensitivity of weathering to climate, which is at present poorly known.

Enhanced weathering reactions may also represent a viable means for future geoengineering to accelerate carbon dioxide removal from the atmosphere for human benefit. Therefore, understanding the timescales of weathering-related carbon dioxide drawdown, and how to accelerate these, could be of major societal relevance. In addition, nutrient supply changes related to future weathering fluxes will impact upon marine and terrestrial ecosystems, biological productivity and diversity.

More broadly, the question of how and why climate has changed in the past has puzzled scientists for decades and represents a fascinating detective story. My research aims to provide the most direct evidence yet to solve this question. There is an excellent opportunity for outreach activities to highlight research on global change, including explaining the carbon cycle, and comparing and contrasting mechanisms and rates of change for perturbations of natural and anthropogenic origin. An interesting analogy (albeit with some differences) can be drawn between weathering changes in geological history and the future response to our anthropogenic carbon release experiment.

School children: For junior school age groups, the broad topics of the formation of mountains, global climate changes and the history of ice sheets can be explained, and also readily demonstrated with models and visualisations. For secondary school age groups, a detective story starting from the idea of the Himalayan "uplift hypothesis" will be a good way to introduce students not only to climate science, but also to geology and isotope chemistry, and to STEM subjects more generally.

Wider public: The public stands to benefit by gaining an awareness of earth systems, climate and global change. In part, this can be achieved through the narrative of the role of the Himalayas in our natural climate evolution, and by contrasting that with current and future scenarios.

Wider geoscience community: Educating colleagues and practitioners with a geoscience background, but who lack a specialised understanding of climate change, paleoclimate and geochemistry, may be readily achieved through this project, given its interdisciplinary nature (e.g. links with earth surface processes, sedimentology, solid earth processes, modelling).

Politicians and decision-makers: The scientific concepts involved in this project are highly relevant to political decisions relating to climatic and global change. There is an interesting analogy between past climate and weathering changes, and the Earth's future response to anthropogenic emissions, including potential mitigation. Constraining the nature of the climate-weathering feedback has specific relevance for predicting our climatic future.