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We investigate the dissolved delta Si-30 of the Congo River, the world's second largest riverine source of Si to the ocean. Small tributaries rich in dissolved organic carbon running through wetlands ("Black Rivers") exhibit the lowest delta Si-30 ever measured in running surface waters (+0.02 +/- 0.15 parts per thousand), whilst the main branch and largest tributaries have higher values (+ 0.98 +/- 0.13 parts per thousand, well within the average of what has been measured so far. Our data suggest that the contribution of Black Rivers to the total discharge of the basin is 22 +/- 10% and that delta Si-30 is mostly controlled by weathering intensity rather than fluxes. We propose both a mass and Si-isotopic balance model, which suggest that the distribution of Si in the particulate and/or dissolved components in Congo rivers results mainly from mixing between two types of weathering regimes: one where clays are formed and remain stable, and one where they are dissolved under the action of organic matter. Citation: Cardinal, D., J. Gaillardet, H. J. Hughes, S. Opfergelt, and L. Andre (2010), Contrasting silicon isotope signatures in rivers from the Congo Basin and the specific behaviour of organic-rich waters, Geophys. Res. Lett., 37, L12403, doi: 10.1029/ 2010GL043413.