Effect of forest and savanna vegetation on the carbon‐isotope composition of sediments from the Sanaga River, Cameroon

Abstract

The carbon‐isotope composition (δ13C) of organic matter from modern sediments in the Sanaga River basin, Cameroon, shows a progressive downriver decrease in δ13C values. Sediments in tributaries draining montane grassland regions have consistent δ13C values of ∼−19‰, while rivers draining savanna regions have δ13C values of around −22‰. After the river passes through extensively forested regions, the 13C values of the sediments closest to the mouth of the river decrease to −25‰. Samples collected from rivers with entirely forested catchment had low δ13C values (from −27 to −29‰) consistent with a pure C3 vegetation source. The downriver carbon‐isotope trends are consistent with the hypothesis that observed variations are due to changes in the relative proportions of C3 and C4 carbon in the sediments, which are derived from terrestrial vegetation in the river catchment.Some sediments from the Mbam River, the principal tributary of the Sanaga River, have extremely high δ13C values (up to −16.7‰). These high values may be the result of severe erosion in the densely populated and intensively farmed Bamileke region in the headwaters of the Mbam.An underrepresentation of C4‐derived carbon in the upper reaches of the Sanaga is possibly due to the damming of the Djerem River at Mbakaou. Since the dam was constructed <25 yr ago, this suggests that the δ13C value of river sediments responds rapidly to changes in the basin.