Denudation rates on cratonic landscapes: comparison between suspended and dissolved fluxes, and Be-10 analysis in the Nyong and Sanaga River basins, south Cameroon

摘要

South Cameroon is located in a tropical and tectonically quiescent region, with landscapes characterized by thick highly weathered regolith, indicative of the long-term predominance of chemical weathering over erosion. Currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources. We analysed two of the main south Cameroon rivers: the Nyong River and Sanaga River. The Sanaga catchment undergoes a contrasted tropical climate from sub-humid mountainous and humid climate and is impacted by deforestation, agriculture, damming, mining and urbanization, especially in the Mbam sub-basin, draining the highly populated volcanic highlands. By contrast, the Nyong catchment, only under humid tropical climate, is preserved from anthropogenic disturbance with low population except in the region of Yaounde (Mefou sub-basin). Moreover the Nyong basin is dam-free and less impacted by agriculture and logging. We explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment-averaged erosion and denudation datasets. The first one consists of an 11-year long gauging dataset, while the second one comes from cosmogenic radionuclides [CRNs, here beryllium-10 (Be-10)] from sand sampled in the river mainstreams (timescale of tens to hundreds of thousands of years). Modern fluxes estimated from gauging data range from 5 to 100m/Ma (10 to 200t/km(2)/yr); our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60% and 40%, respectively, of the total denudation. Beryllium-10 denudation rates and sediment fluxes range from 4.8 to 40.3m/Ma or 13 to 109t/km(2)/yr, respectively, after correction for quartz enrichment. These fluxes are slightly less than the modern fluxes observed in Cameroon and other stable tropical areas. The highest Be-10-derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact. Copyright (c) 2016 John Wiley & Sons, Ltd.