Exploratory Radioisotope Measurements Suggest That In-Stream Erosion Represents the Main Sediment Source in a Pristine, Tropical Rainforest in Costa Rica

Volcanic, humid tropical landscapes are characterized by short-term geomorphic transformations due to volcanism and seismic activity, landslides, and other frequent mass movements. These landscape-forming processes are amplified by high temperatures, high annual precipitation rates, and intense rainstorms. The latter can result in significant surface runoff and sediment mobilization, even under pristine rainforest cover. However, knowledge about sediment sources and the magnitude of the associated erosion and accumulation rates remains limited in these systems. This study explores the use of radioisotopes (U-235, Bi-214, Pb-214, total Pb-210, and K-40) and of the fallout radionuclide (FRN) Cs-137 to address that knowledge gap in a pristine, tropical rainforest catchment in northern Costa Rica. We analyzed FRN and radioisotope activities from two reference soil profiles and compared them with those of 17 superficial soil samples collected on two hillslopes and of three streambed sediment samples. Modeled hillslope erosion and accumulation rates ranged from 6 t ha−1 year−1 erosion to 6.7 t ha−1 year−1 deposition with up to ± 60% uncertainty reflecting spatially variable interception of rainfall inputs. Preliminary sediment fingerprinting results suggested that deeper soil material, likely originating from in-stream bank erosion and channel incision, was the dominant source of stream sediment (79 ± 19%), whereas superficial soil present on the hillslopes only contributed 22 ± 18% to the stream mixture. Our exploratory work highlights the potential importance of channel erosion processes in the sediment yield of steep rainforest catchments, even when hillslopes and streams have a strong hydrological connection.