The oxidative weathering of organic matter and its carbon dioxide emissions: Insight from the trace elements rhenium and molybdenum

Abstract

Throughout geological history, the exposure of sedimentary rocks to chemical weathering at Earth’s surface has profoundly affected the geochemistry of the atmosphere, rivers and oceans. Oxidative weathering reactions can release redox-sensitive trace elements, and the concentration and isotope composition of these elements in geological materials may provide an insight into the environmental conditions and processes occurring both today and over Earth’s history. To constrain how Earth’s geological carbon cycle operates, and the influence it has on global climate change, a better understanding of the controls on oxidative weathering is required. The oxidation of organic carbon in sedimentary rocks (petrogenic organic carbon, OCpetro) releases carbon dioxide (CO2) from long-term storage in the lithosphere, and consumes atmospheric O2. Alongside volcanism, the oxidative weathering of OCpetro is the main source of CO2 to the atmosphere over millions of years. However, OCpetro oxidation is poorly understood, both in terms of the rate at which it releases CO2 and the factors that interact to drive the reaction. Trace metals associated with organic matter in rocks, such as rhenium (Re) and molybdenum (Mo), can be released to the dissolved load of rivers during oxidative weathering. Quantifying these element fluxes has the potential, therefore, to provide insight into the oxidative weathering processes involved. Here, rates of OCpetro oxidation rates are quantified in rapidly eroding mountain river catchments in the western Southern Alps, New Zealand, and the Mackenzie River Basin, north west Canada. Physical erosion is found to be a first order control on the oxidative weathering fluxes, but catchments dominated by valley glaciers and exposed to frost-shattering processes experience a further two to three times elevation in CO2 emissions relative to catchments with less glacial cover. The oxidative weathering processes are also found to fractionate metal isotopes (e.g. Mo) in the Critical Zone, which places an important control on the Mo isotope composition of both continental runoff and the World’s oceans.