New Views On "Old" Carbon In The Amazon River: Insight From The Source Of Organic Carbon Eroded From The Peruvian Andes

Key PointsEnrichment of soil and vegetation content at peak floods in Andean RiversFossil particulate organic carbon a significant presence in Andean river POCMountain rivers play a key role in the delivery of particulate organic carbon (POC) to large river systems and the ocean. Due to the extent of its drainage area and runoff, the Amazon River is one of Earth's most important biogeochemical systems. However, the source of POC eroded from the humid region of the Eastern Andes and the input of fossil POC from sedimentary rocks (POCfossil) remains poorly constrained. Here we collected suspended sediments from the Kosnipata River during flood events to better characterize Andean POC, measuring the nitrogen to organic carbon ratio (N/C), stable carbon isotopes (delta C-13(org)) and radiocarbon (Delta C-14(org)). Delta C-14(org) values ranged from -711 parts per thousand to -15 parts per thousand, and significant linear trends between Delta C-14(org,) N/C and delta C-13(org) suggested that this reflects the mixing of POCfossil with very young organic matter (Delta C-14(org) ~ 50 parts per thousand) from the terrestrial biosphere (POCnon-fossil). Using N/C and Delta C-14(org) in an end-member mixing analysis, we quantify the fraction of POCfossil (to within 0.1) and find that it contributes a constant proportion of the suspended sediment mass (0.37 +/- 0.03%) and up to 80% of total POC. In contrast, the relative contribution of POCnon-fossil was variable, being most important during the rising limb and peak discharges of flood events. The new data shed light on published measurements of "old" POC (low Delta C-14(org)) in Andean-fed tributaries of the Amazon River, with their Delta C-14(org) and delta C-13(org) values consistent with variable addition of POCfossil. The findings suggest a greater persistence of Andean POC in the lowland Amazon than previously recognized.