Continental Weathering and Climate
Continental weathering and erosion are amongst the primary processes responsible for the evolution of the landscape and exert a major control on the transport of material from the continents to the oceans, and on the cycles of many elements at the Earth’s surface. Over geological (million year) timescales weathering is the principal feedback regulating atmospheric levels of the greenhouse gas carbon dioxide (CO2), thereby controlling the Earth’s climate. This operates in two ways: First, the chemical weathering of silicate minerals converts atmospheric CO2 to hydrogen carbonate ions, which, along with metal ions, are transported to the oceans where the CO2 is fixed by formation of carbonate minerals by organisms. Second, the production of organic carbon during photosynthesis on land, and its burial in oceanic and floodplain sediments.
Research in this area is focused on understanding the processes of weathering and erosion, preserved in the chemistry of soils, rivers and groundwaters. Much of the silicate weathering and organic carbon production occurs in soils, while rivers (and groundwaters) play a central role, exporting metal ions, and the solid and dissolved carbon mobilised by weathering and erosion. Together, these processes act to negate CO2 input to the atmosphere by volcanic and metamorphic degassing and by the weathering and oxidation of fossil organic carbon from sedimentary rocks.
Soil ecosystem development is regulated by the supply of nutrients from rock weathering and the dissolution of silicate minerals, the decomposition of organic matter, and atmospheric deposition. Over the past twenty years the radiogenic isotopes systems (Sr, Nd and Pb) has been used with considerable success to trace these sources, their variation in time, and across different climate regimes and geographical rock types. The stable isotopes of Li, Mg, Si, Sr and Fe also offer a means of tracing the sources, and mechanisms of transfer and release of elements in soils. Our recent research has been using both radiogenic and stable isotopes to trace the mechanisms of soils formation and the inorganic and biological controls on element behaviour.
Rivers represent an ‘average’ composition of the products of weathering (both waters and suspended sediment), and their isotope and elemental composition yields information on the net effects of weathering on the chemistry of material delivered to the oceans. Recent work has included the study of rivers draining young basaltic (Iceland, Azores and Costa Rica) and ancient shield terrains (Greenland. This research has also included the study of groundwaters (Great Artesian Basin, Iceland) and Lakes, which reflect a combination of local input and biogeochemical cycling within the lake itself. The effects of glacial weathering and transport, and the influence of aeolian transport and deposition are also active areas of our research.
The chemical signal from continental weathering may be significantly modified during the mixing of freshwater and seawater in estuaries. Recent work in this area has focused on the role of weathering of riverine particulates in estuarine-shelf environments, which occurs through dissolution prior to sedimentary burial, by divalent metal exchange for Na on clay mineral surfaces and through secondary mineral formation. This work indicates that the elemental flux from particulate dissolution may be significant and that the processes controlling weathering in seawater are very different to those dictating the composition of riverine dissolved species on land.