Chemical Weathering

Chemical weathering is the term used to describe changes in the chemical composition of rocks, often when they are exposed to water. Elements within the rocks are leached out of the original, primary minerals and can either remain in situ and form new, secondary minerals or they are transported away with water into rivers. Therefore, by analysing stream water chemistry and soil (containing secondary minerals) we can gain information on how much chemical weathering is occuring.

Why do we want to know how much chemical weathering is happening?

One of the most common chemicals to attack rocks is carbonic acid, which is formed when atmospheric carbon dioxide (CO2) is dissolved in water. This is why the pH of rain is acidic. When rocks weather, they are effectively removing CO2 from the atmosphere. This CO2 reaches the ocean in the form of bicarbonate (HCO3-) where it can combine with calcium (Ca) to form carbonates which are used to build coral reefs and the shells of sea creatures. Importantly, some carbonates sink to the ocean floor where they are eventually subducted, effectively removing that carbon for a very long time.

Chemical weathering is a chemical reaction and therefore changes in factors such as temperature will affect how fast the reaction proceeds. The hypothesis is that if atmospheric CO2 increases then the resultant warmer temperatures will increase weathering rates, causing a reduction in atmospheric CO2; a negative feedback.

One aim is to develop a predictive set of equations describing the response of weathering to changes in environmental factors e.g. how much does weathering increase if the average annual temperature doubles? Then weathering in the past and in the furture can be modelled and the exact role of weathering in driving climate changes can be determined. By analysing and comparing weathering rates in a wide range of present-day environments, we can gain insight into how weathering rates respond to variations in climate. This project focuses on weathering under an Arctic climate.

Are weathering rates not insignificant in the Arctic?

No, although it is cold, weathering still happens! One very important process in the arctic is frost-shattering of rocks. Freeze-thaw cycles break the rock into smaller pieces and the smaller the size of the rock fragments, the faster the rate of chemical weathering. Glaciers achieve a similar effect by grinding up the rock they pass over.

Changing climate in the Arctic

The Arctic is warming faster than any other region in the world, and it is expected that this will lead to marked environmental changes. One key factor is the depth of the active layer - the layer of permafrost which melts every year. As this will increase the amount of rock in contact with water, it can be expected that this will increase weathering rates.

Weathering releases nutrients into the environment which can be taken up by vegetation and can also influence productivity in the sea. Both catchments contain black shale which is rich in organic carbon. The organic carbon contained in shale represents carbon which was previously buried underwater. As the shale rock weathers, some of this carbon may be oxidised and release CO2 back into the atmosphere or it may be transported as particles to the ocean where it may be buried again and so begin another cycle.