Carbon dioxide drawdown by Devonian lavas

Lower Devonian volcanic rocks in the northern British Isles, especially Scotland, show extensive evidence for contemporaneous subaerial weathering. Basalt and andesite lavas were altered to red iron oxides, commonly accompanied by calcite. Measurement of carbonate contents in 104 samples over a region of 100,000 km2 show an average of 13% calcite. Weighted for outcrop thickness, this represents an estimated 7.3×1016 moles CO2, extracted from surface waters and ultimately the atmosphere. The time frame for this drawdown is difficult to constrain, but complete weathering of a one-metre unit over 1000 years would involve CO2 consumption comparable with the highest rates determined in modern basaltic watersheds. These data demonstrate that volcanic activity can be a major sink, as well as a source for CO2, and provide a data set for modelling of CO2 flux during episodes of volcanic activity in the geological record. The high capacity of the Devonian lavas for CO2 drawdown emphasises the potential of basalts for CO2 sequestration.