Muddying the waters: cation exchange processes as a major control on weathering fluxes.

Estimates of carbon consumption by silicate weathering could be wrong by up to 80% because they assume that cations behave in a conservative way during dissolution and transport. One suite of chemical reactions are referred to as cation exchange reactions. They occur rapidly as a chemical equilibrium develops between charged mineral surfaces and a water. One of the most important mineral groups which have charged surfaces are clays. These rapid reactions are well studied in soils and aquifers, but the scientific community working on river chemistry has largely neglected these reactions. Our work shows that once the cation exchange process is taken into account it changes significantly the chemistry of natural waters and the total amount of carbon consumption through chemical weathering, with major implications for climate feedbacks.

This project will determine the impact of cation exchange between clay minerals and river waters on estimates of chemical weathering fluxes and thus on the feedback which moderates climate on long timescales. The main objective will be achieved by a combination of laboratory experimental studies, and paired measurements of water chemistries and suspended load depth-profiles on major rivers allowing re-evaluation of silicate chemical weathering fluxes. The student will determine the magnitude of the release of cations from the suspended load exchange pool when rivers enter the ocean and Na replaces Ca, Mg and Sr on exchange sites. This will be done in two ways, firstly with a set of controlled experiments, and secondly in the field setting of the Mackenzie River in the Arctic Circle. The student will sample paired suspended sediment and water across the salinity gradient in the estuary of the Mackenzie River as part of a dedicated expedition. We will trace the effects of cation exchange using a suite of isotope ratio tracers such as Mg, Li and Sr isotope ratios.