Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 9 (2016): 687-690, doi:10.1038/ngeo2778. ; The age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds. While elevated ages in fluvially-transported organic matter are usually explained by erosion of soils and sediments deposits it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and rapid carbon turnover. Here we present compound-specific radiocarbon data of terrigenous organic fractions from a sedimentary archive offshore the Congo River in conjunction with molecular markers for methane-producing land cover reflecting wetland extent. We find that the Congo River has been discharging aged organic matter for several thousand years with apparently increasing ages from the Mid- to the Late Holocene. This suggests that aged organic matter in modern samples is concealed by radiocarbon from atmospheric nuclear weapons testing. By comparison to indicators for past rainfall changes we detect a systematic control of organic matter sequestration and release by continental hydrology mediating temporary carbon storage in wetlands. As aridification also leads to exposure and rapid remineralization of large amounts of previously stored labile organic matter we infer that this process may cause a profound direct climate feedback currently underestimated in carbon cycle assessments. ; This study was supported by the Deutsche Forschungsgemeinschaft (grants SCHN 621/3-3, RU 458/29-3, GR 1845/2-3, SCHE 903/1), the US National Science Foundation (grant OCE-0137005), a Starting Grant from the European Research Council (ERC) awarded to HMT for project AMOPROX (No. 258734) and grants ICM-NC120066 and FONDAP15110009 to RDP-H. This work was supported by the DFG Research Center/Cluster of Excellence ‘The Ocean in the Earth System’ at MARUM – Center for Marine Environmental Sciences, University of Bremen. ; 2017-02-15