Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin

Fly ash-the residuum of coal burning-contains a considerable amount of fossilized particulate organic carbon (FOC_ash) that remains after high-temperature combustion. Fly ash leaks into natural environments and participates in the contemporary carbon cycle, but its reactivity and flux remained poorly understood. We characterized FOC_ash in the Chang Jiang (Yangtze River) basin, China, and quantified the riverine FOC_ash fluxes. Using Raman spectral analysis, ramped pyrolysis oxidation, and chemical oxidation, we found that FOC_ash is highly recalcitrant and unreactive, whereas shale-derived FOC (FOC_rock) was much more labile and easily oxidized. By combining mass balance calculations and other estimates of fly ash input to rivers, we estimated that the flux of FOC_ash carried by the Chang Jiang was 0.21 to 0.42 Mt C⋅y^-1 in 2007 to 2008-an amount equivalent to 37 to 72% of the total riverine FOC export. We attributed such high flux to the combination of increasing coal combustion that enhances FOC_ash production and the massive construction of dams in the basin that reduces the flux of FOC_rock eroded from upstream mountainous areas. Using global ash data, a first-order estimate suggests that FOC_ash makes up to 16% of the present-day global riverine FOC flux to the oceans. This reflects a substantial impact of anthropogenic activities on the fluxes and burial of fossil organic carbon that has been made less reactive than the rocks from which it was derived.