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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. 

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ⁴¹K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ⁴¹K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ⁴¹K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ⁴¹K record of paleo-seawater to infer continental weathering intensity through Earth’s history.