Wildfires and incomplete combustion of fossil fuel produce large amounts of black carbon. Black carbon production and transport are essential components of the carbon cycle. Constraining estimates of black carbon exported from land to ocean is critical, given ongoing changes in land use and climate, which affect fire occurrence and black carbon dynamics. Here, we present an inventory of the concentration and radiocarbon content (Δ 14C) of particulate black carbon for 18 rivers around the globe. We find that particulate black carbon accounts for about 15.8 ± 0.9% of river particulate organic carbon, and that fluxes of particulate black carbon co-vary with river-suspended sediment, indicating that particulate black carbon export is primarily controlled by erosion. River particulate black carbon is not exclusively from modern sources but is also aged in intermediate terrestrial carbon pools in several high-latitude rivers, with ages of up to 17,000 14C years. The flux-weighted 14C average age of particulate black carbon exported to oceans is 3,700 ± 400 14C years. We estimate that the annual global flux of particulate black carbon to the ocean is 0.017 to 0.037 Pg, accounting for 4 to 32% of the annually produced black carbon. When buried in marine sediments, particulate black carbon is sequestered to form a long-term sink for CO2.

Supplementary Information End-member decompositionFollowing the approach of Gaillardet et al. (1999), we use a series of linear equations to solve for the proportions of sodium in the river that are attributed to evaporite, carbonate, and silicate weathering. Table 1 in the main text specifies the end-member molar ratios and their associated uncertainty. In order to propagate this uncertainty in end-member values through the calculation, we solve the linear equations for each river 10,000 times using a random sampling of weathering end-member values assuming a Gaussian distribution. For all of the calculations in the main text, the median and standard deviation of these Monte Carlo simulations were used as the solution and uncertainty for each river. For all rivers, using the median of the Monte Carlo simulations gives proportions attributed to each end-member that sum to 1 (± 0.05).Although many of the rivers have distributions of Monte Carlo solutions that are roughly Gaussian for all three end-members (e.g. Supplementary Figure 1), there are a total of 12 of the 31 rivers whose solution distributions for either the evaporite or silicate (or both) end-members are not Gaussian and they have a long tail (e.g. Supplementary Figure 2). This is likely related to the fact that the end-member values for evaporites and silicates have overlapping ranges. In contrast the carbonate end-member values are more distinct resulting in Monte Carlo solution distributions for the carbonate end-member that are typically roughly Gaussian. These 12 rivers are: Brahmaputra, Fraser, Ganges, Kaoping, Kolyma, Lena, Maipo, Mekong, Orinoco, Red, Yangtze, and Yukon rivers.We have tested the influence that these poorly constrained rivers with long tailed distributions have on the results from this study. If we use the modes instead of the medians for those rivers that do not have near-Gaussian Monte Carlo distributions, the total fluxes reported in the main text change by <0.1 Tmol/y, well within the 0.2 Tmol/y uncertainty. However, it does make a slight difference for the proportions of sulfate attributed to each of the weathering end-members (and excess sulfate) for those 12 rivers. This difference influences where these rivers plot on Figure 6 (main text), and is illustrated in Supplementary Figure 3. Although these 12 rivers plot slightly differently in this figure, the major conclusions remain valid, namely that the rivers plot to the left of the 1:1 line and are farther from the line with increasing excess sulfate values, indicating that the excess sulfate likely has a low δ 34 S value. References Gaillardet, J., Dupré, B., Louvat, P., Allegre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chemical Geology 159, 3-30.

No abstract

23The 14 C content of dissolved organic carbon (DOC) in rivers, lakes, and other non-saline waters 24 can provide valuable information on carbon cycling dynamics in the environment. DOC is 25 typically prepared for 14 C analysis by accelerator mass spectrometry (AMS) either by UV 26 oxidation or by freeze-drying and sealed tube combustion. We present here a new method for the 27 rapid analysis of 14 C of DOC using wet chemical oxidation (WCO) and automated headspace 28 sampling of CO2. The approach is an adaption of recently developed methods using aqueous 29 persulfate oxidant to determine the δ 13 C of DOC in non-saline water samples and the 14 C content 30 of volatile organic acids. One advantage of the current method over UV oxidation is higher 31 throughput: 22 samples and 10 processing standards can be prepared in one day and analyzed in a 32 second day, allowing a full suite of 14 C processing standards and blanks to be run in conjunction 33 with samples.