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.

Unravelling the complexity of the carbon cycle and how climatic feedbacks operate at different time-scales is of global socio-economic importance. Fresh water is at the heart of NERC science, and tracing the quality of water is of fundamental importance to UK and international communities. All processes which control the chemistry of natural waters are fundamental to the management of water resources. This proposal has one such process at its centre, cation exchange. In addition we are working with rivers that drain a series of GCRF countries, providing fundamental information about the stability of the nutrient pool in flood plains and the quality of water and its controlling factors in these countries. River engineers and managers will use the quantitative water and sediment data arising from our research. Environmental policy makers will use the water and sediment geochemical fluxes to inform management of the carbon cycle, rivers and floodplains.

For the above environmental reasons, the outcomes of our research (system knowledge, cation exchange capacities, fraction of matter transported in the exchange pool, water chemistry and sediment concentrations) will have direct impact as these provide the detailed data and framework necessary to quantify baseline water quality, sediment loads and storage, and elemental cycling behaviour.

Who will benefit?
Important beneficiaries of the research outlined below are engineers, river managers, environmental policy makers, schools and the public, and the developing nations' skills base.

For example, in Myanmar one of our partners is the the Department of Water and Improvement of Rivers (DWIR) where our in-going work on sediment fluxes and water chemistry is of pivotal importance to engineering and flood risk management. The supply of nutrients from this suspended sediment via the cation exchange process on flood plain sediment is also of importance to agriculture.

A large part of the world's population lives in coastal areas and depends on the water supply of coastal aquifers. The freshening of aquifers, accompanied by sequential elution of the saltwater (seawater) cations from the sediment's exchange complex is a known phenomenon, precisely the processes that are targeted in the present proposal.

Understanding the fundamentals of how contaminants are transported in the environment is intimately linked to cation exchange and is of obvious societal and economic interest.

The IAEA is the process of establishing a Global Network on Isotopes in Rivers (GNIR) to monitor continental environmental change and PI Tipper and PP Hilton have acted as consultants on this. The isotopic data that we will generate will directly feed into the GNIR database.

The wider community will benefit via an outreach programme which includes making a film describing the field and lab methods. A new technology-led teaching pedagogy is emerging, and the development of new interactive learning experiences are at the forefront of primary, secondary and higher education. Outreach videos will be developed into e-learning experiences. Through the combination of video footage and 360-degree photos with new technology such as Adobe Captivate, the Mackenzie fieldwork expedition will be transformed into 'virtual fieldwork', where students can vicariously experience the environment and work undertaken to generate data about the carbon cycle. Using the virtual reality setting on mobile phones, students can use headsets to view the 360 degree footage, which will be overlain with clickable items such as information blurbs, audio content, videos and quizzes.