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

Chemical weathering is the process by which rocks dissolve in rainwater, which is naturally acidic. This is because atmospheric carbon dioxide dissolves in rain to form carbonic acid, and the rainwater interacts with rocks making them dissolve. The dissolved carbon dioxide becomes trapped in river and seawater, as bicarbonate (present in all natural waters such as mineral water for example), where it resides stably for thousands, or tens of thousands of years, and is then stored permanently in a mineral form as calcium carbonate (like limescale) and deposited as limestone in the oceans.

Rock dissolution or chemical weathering is a major process in the global carbon cycle and it is thought that this terrestrial chemical weathering of rocks, and subsequent burial of carbon as calcium carbonate, acts as the feedback which has controlled the carbon cycle and thus climate over Earth history.

The carbon fluxes associated with chemical weathering are commonly estimated from river chemistry, assuming that the river composition can be matched to the type of rock dissolving. This is a simplification because chemical reactions mean that a river doesn't simply have the same chemical composition as a rock which dissolves. 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.

We have generated a suite of preliminary data that 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.

We have developed a new tool kit that can address the significance of cation exchange. Our tools are 1) isotope geochemistry, that can trace the rapid chemical reactions, 2) nuclear magnetic resonance that can characterise the mineral surfaces where exchange is occurring and 3) X-ray diffraction that is sensitive to the specific compositions of exchangeable sites in minerals.

We have planned a series of experimental studies to quantify the processes in well constrained controlled examples, coupled to a study on the largest rivers in the world (on an archive collection of samples) to determine the global importance of the problem.