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) 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 in the Chang Jiang (Yangtze River) basin, China, and quantified the riverine FOC fluxes. Using Raman spectral analysis, ramped pyrolysis oxidation, and chemical oxidation, we found that FOC is highly recalcitrant and unreactive, whereas shale-derived FOC (FOC) 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 carried by the Chang Jiang was 0.21 to 0.42 Mt C⋅y 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 production and the massive construction of dams in the basin that reduces the flux of FOC eroded from upstream mountainous areas. Using global ash data, a first-order estimate suggests that FOC 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.