Protracted storage of biospheric carbon in the Ganges-Brahmaputra basin

The amount of carbon stored in continental reservoirs such as soils, sediments and the biosphere greatly exceeds the amount of carbon in the atmosphere(1). As such, small variations in the residence time of organic carbon in these reservoirs can produce large variations in the atmospheric inventory of carbon dioxide. One such reservoir is the Ganges-Brahmaputra system draining the Himalayas, which represents one of the largest sources of terrestrial biospheric carbon to the ocean(2). Here, we examine the radiocarbon content of river sediments collected from the Ganges-Brahmaputra drainage basin to determine the residence time of organic carbon in this reservoir. We show that the average age of biospheric organic carbon in the drainage basin ranges from 0.5 to 17 thousand years. The radiocarbon age of plant-derived fatty acids-a proxy for labile terrestrial vegetation-ranges from just 0.05 to 1.3 thousand years. We propose that the bulk ages can be explained by the existence of a refractory, slowly cycling component of the organic carbon pool that is mixed with a younger labile pool. We estimate that this refractory component has an average age of over 15,000 years, and represents up to 20% of total biospheric carbon exported by the Ganges-Brahmaputra system. We suggest that global warming might destabilize this ancient pool of carbon, if warming stimulates microbial decomposition of organic carbon reserves.