δ13C composition of sediments from the Sanaga River, Cameroon

Abstract

The Sanaga River basin in Cameroon represents an ideal system for investigating the relationship between the carbon-isotope composition of organic matter in river sediments, and the type of vegetation in the river catchment. The headwaters of the river and its tributaries are located on a savannah-covered plateau in central Cameroon, with abundant Ca grasses (813 C = _ 12°/oo). The river then flows through transitional forest /savannah regions and finally through tropical rainforest (comprising only C3 vegetation with 813 C = _ 29%o) before discharging into the Gulf of Guinea. 8t3C values in the headwaters of the river are as high as 19%0 consistent with the presence of abundant C4 carbon derived from extensive savannah and montane regions. As the Sanaga River descends into the zone of transitional vegetation and into the forested regions, the 8~3C value of carbon in the samples decreases progressively to -26.9%0 due to the addit ion of forest-derived C 3 carbon. Below its confluence with the Mbam River, the ~13C value of the sediments increase to ~ 24%o, due to the addition of organic carbon with a ~3C value of -23%o derived from the Mbam River, which has extensive savanna and agricultural regions in its catchment. The 8~3C value again decreases downstream of the confluence to around -25%o at the mouth of the river. The trends in the ~3C data can be duplicated using a simple box model which assumes that the observed ~3C values are the result of mixing of carbon derived from C3and C4dominated vegetation types within the catchment. This suggests that the carbon-isotope composit ion of riverine organic carbon can provide information on catchment vegetation, and that ancient sediments can provide information on changes in catchment vegetation in the past.