Quantifying carbonate weathering with novel tracers

The carbon cycle, Earth's climate and geology are intimately coupled on a range of time-scales. On million year time-scales, atmospheric carbon dioxide, a key greenhouse gas, has maintained the surface of the Earth at equable temperatures. Key negative climatic feedbacks have prevented a runaway greenhouse as on Venus. This long-term geological feedback is generally accepted to result from the temperature-sensitivity of organic carbon burial and silicate chemical weathering but this remains controversial. Critically it is generally assumed that silicate weathering only plays a role in the climate feedback on million year time-scales, but the observation that secondary carbonate precipitating in some river systems removes 70% of the Ca that was originally dissolved suggests that part of the silicate weathering cycle may operate with a much shorter timescale. This project will use a suite of novel isotopic tracers to quantify processes associated with carbonate weathering.

The observation that secondary carbonate precipitating in some river systems removes 70% of the calcium that was originally in solution suggests that the traditional view of the silicate weathering cycle may be accelerated to decadal or centennial time-scales. Quantification and understanding of the process of secondary carbonate deposition is pivotal to modelling the silicate weathering climate feedback. In recent years it has become possible to measure an array of stable isotope systems such as Ca, Mg, and Sr, all of which are sensitive to carbonate precipitation but have yet to be applied to carbonate weathering systems. In this project, these will be developed as sensitive tracers of secondary carbonate formation, in an small catchment in southern France where it is possible to easily isolate the key processes acting during weathering.

The student will conduct fieldwork in southern France collecting an array of samples from an iconic location where abundant secondary carbonate is precipitating. In the labs in Cambridge (s)he will measure stable Ca, Mg, and Sr isotopes using state-of-the-art mass spectrometry in river and spring waters and secondary carbonates. Isotope fractionation factors and elemental distribution coefficients for carbonate precipitation will be determined. Ca, Mg and Sr have contrasting elemental and isotopic sensitivities to carbonate precipitation. The impact on the carbon cycle will be assessed through the development of reactive transport models for carbonate weathering for chemical concentrations and isotopic compositions.