Orogenesis as a carbon dioxide source or sink? New insights from the organic carbon cycle of Taiwan

Robert G. Hilton
Department of Geography, Durham University, Durham, UK, r.g.hilton@durham.ac.uk

Mountain building can promote the sequestration of atmospheric carbon dioxide (CO2) by the erosion of organic matter from the terrestrial biosphere (OCbiosphere), and its transport in sediment-laden rivers to sedimentary deposits. However, erosion and exhumation can expose rock-derived organic carbon to chemical weathering. Oxidation of this petrogenic organic carbon (OCpetro) is a source of CO2 which is very poorly constrained. Here, the OC budget of a mountain belt is quantified to better understand whether erosion in mountain belts results in a net source, or sink of CO2.
The erosion, transport and offshore fate of OCbiosphere has been tracked through mountain river systems in Taiwan, using a suite of elemental (C, %), stable isotope (δ13C) and radiocarbon (Δ14C) measurements. During floods at high suspended sediment concentrations, OCbiosphere and OCpetro is transferred rapidly to the deep ocean. Marine sediments show evidence for efficient, long-term preservation of OCbiosphere. Large amounts of terrestrial OC and sediment are also delivered to the surface ocean, dispersing OC over a larger area. Marine sediments sourced by this delivery mechanism have C, δ13C and Δ14C values which suggest >70% of the terrestrial OC is preserved offshore.
To quantify OCpetro oxidation rates in mountain catchments, the trace element Rhenium (Re) is used as a proxy. Existing measurements of physical erosion in Taiwan rate also allow the controls on OCpetro oxidation to be assessed. Re is associated with OCpetro in rocks and following oxidation during chemical weathering forms a soluble anion which contributes to the dissolved load of rivers. Soils in Taiwan confirm that Re loss is coupled to OCpetro loss during weathering, and so the dissolved Re flux is used to provide a first order estimate of the corresponding release of CO2 in river catchments. OCpetro oxidation increases with erosion rate. It is likely that the OCpetro oxidation rates estimated from dissolved Re flux are an upper bound. Nevertheless, the estimated CO2 release from Taiwan by OCpetro oxidation does not negate the estimates of CO2 sequestration by burial of OCbiosphere offshore. The findings from Taiwan 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.