Geological respiration of a mountain belt revealed by the trace element rhenium

Abstract

Oxidation of rock-derived, petrogenic, organic carbon (OCpetro) during weathering of sedimentary rocks is a major source of carbon dioxide (CO2) to the atmosphere. This geological respiration is thought to be enhanced by physical erosion, suggesting that mountain belts could release large amounts of CO2 to counter the CO2 sequestration achieved by the erosion, riverine transfer and oceanic burial of organic carbon from the terrestrial biosphere. However, OCpetro oxidation rates in mountain belts have not been quantified. Here we use rhenium (Re) as a proxy to track OCpetro oxidation in mountain river catchments of Taiwan, where existing measurements of physical erosion rate allow the controls on OCpetro oxidation to be assessed. Re has been shown to be closely associated with OCpetro in rocks and following oxidation during chemical weathering forms a soluble oxyanion (View the MathML source) which contributes to the dissolved load of rivers. Soils on meta-sedimentary rocks in Taiwan show that Re loss is coupled to OCpetro loss during weathering, confirming previous observations from soil profiles on sedimentary rocks elsewhere. In Taiwan rivers, dissolved Re flux increases with the catchment-average sediment yield, suggesting that physical erosion rate is a major control on OCpetro oxidation. Based on our current understanding of Re mobility during weathering, the dissolved Re flux can be used to quantify an upper bound on the OCpetro oxidation rate and the associated CO2 transfer. The estimated CO2 release from this mountain belt by OCpetro oxidation does not negate estimates of CO2 sequestration by burial of biospheric OC offshore. The findings are compared to OC transfers estimated for the Himalaya, where OCpetro oxidation in the mountain belt remains unconstrained. Together, these cases suggest that mountain building in the tropics can result in a net sink of OC which sequesters atmospheric CO2.