Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin

The age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds. Although elevated ages in fluvially transported organic matter are usually explained by erosion of soils and sedimentary deposits(1,2), 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(3-7.) 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 that is at present underestimated in carbon cycle assessments.