Rivers are the major carriers of dissolved black carbon (DBC) from land to ocean; the sources of DBC during its continuous transformation and cycling in the ocean, however, are not well characterized. Here, we present new carbon isotope data for DBC in four large and two small mountainous rivers, the Yangtze and Yellow river estuaries, the East China Sea and the North Pacific Ocean. We found that the carbon isotope signatures of DBC are relatively homogeneous, and the DBC 14C ages in rivers are predominantly young and increase during continuous transport and cycling in the ocean. The results of charcoal leaching experiments indicate that DBC is released from charcoal and degraded by bacteria. Our study suggests that riverine DBC is labile and respired during transport and mixing into the ocean and that residual DBC is cycled and aged on the same time scales as bulk DOC in the ocean.

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The oceans represent a significant sink for atmospheric CO 2 , acting as the largest pool of exchangeable carbon in the world. Globally, approximately 39,000 GtC of total carbon is stored in the ocean, up to 95% of which is in the form of dissolved inorganic carbon (DIC; Key et al., 2004;Schuur et al., 2016). Each year, approximately 30%-40% of anthropogenically produced CO 2 is absorbed by the ocean through air-sea exchange and dissolved as DIC in the ocean (Gruber et al., 2009;Lemke et al., 2007). The variability in DIC in the ocean is controlled not only by air-sea exchange (Tsunogai, 2000;Winn et al., 1998) but also by changes in ocean circulation and biological activities (Gruber, 2011;Tsurushima et al., 2002;Wakita et al., 2010). The distribution and cycling of DIC in the ocean therefore play crucial roles in the global carbon cycle and in climate change (Key et al., 2004;Valsala et al., 2012;Yasunaka et al., 2014).Radiocarbon ( 14 C) natural abundances have been used in studies of marine carbon cycling to determine the sources, residence times, transformations, and interactions of both organic and inorganic carbon reservoirs (Bau-