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.