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 δ¹³C value of river sediments responds rapidly to changes in the basin.