Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: Retaining carbon in the lithosphere

About 15x1015 t of carbon is stored in rocks as fossil organic matter. This is almost 400 times the amount of carbon present in the atmosphere and oceans. The balance between the growth of this geological reservoir through burial of newly photosynthesised organic matter, and its decrease through oxidation of fossil organic carbon (OCfossil) plays a crucial role in the long-term evolution of atmospheric CO2, and thus global climate. Mountain building exposes OCfossil in exhumed sedimentary rocks. Oxidation of this material releases carbon dioxide from long-term geological storage to the atmosphere. However, OCfossil can be mobilised from hillslopes by mass wasting and transferred to the particulate load of rivers. In large fluvial systems it is thought to be oxidised in transit, but in short, steep rivers that drain mountain islands, OCfossil may escape oxidation and re-enter geological storage due to rapid fluvial transfer to the ocean. In these settings, the rates of OCfossil transfer and their controls remain poorly constrained. Here we quantify the erosion of OCfossil from the Taiwan mountain belt, combining measurements of water discharge and suspended sediment with quantification of particulate organic carbon load and its source in 11 rivers. Annual OCfossil yields in Taiwan vary from 12±1 to 246±22 tC km-2 yr-1, and represent significant natural transfers of carbon. They are controlled by the high physical erosion rates that accompany rapid crustal shortening and frequent typhoon impact. Efficient transfer of this material ensures that <15% of the OCfossil exhumed in Taiwan is lost to weathering and that 1.3±0.1x106 tC yr-1 is delivered to the ocean. Our findings suggest that erosion of coastal mountain ranges may force efficient transfer and re-accumulation of OCfossil in marine sediments, further enhancing the role of mountain building in the long-term storage of carbon in the lithosphere.