Balancing the Fluvial Organic Carbon Budget: Integrating Sediment Transit Time and Channel Mobility into the Equation

Abstract:

Rivers are primary pathways for terrestrial organic carbon (OC) fluxes between mountain ranges and ocean basins, and a key link in the global carbon cycle. During fluvial transit, OC may be oxidized and emitted to the atmosphere as CO₂, or escape oxidation via burial in sedimentary deposits. The balance between oxidation and burial determines whether fluvial OC transfer results in a net atmospheric OC source or sink, but the factors regulating this balance are insufficiently constrained. To tackle this knowledge gap, we quantified the effects of fluvial transit on particulate OC (POC) fluxes along the lowland channel of the Rio Bermejo, Argentina, which has no tributaries for nearly 1300 km. By comparing radiocarbon data and sediment transit time estimates, we determined that sediment transit velocity regulates the magnitude of fluvial POC turnover, while mineral stabilization slows the rate of turnover. We integrated these findings into a fluvial OC turnover model, and estimated that the Rio Bermejo annual POC export flux is two orders of magnitude greater than its oxidation flux, making the lowland river a net OC sink. We applied this model to other large river systems where data are available, which shows that many rivers may be net OC sinks under modern conditions, due to lateral influxes from channel migration into OC-rich floodplains. Our results show that sediment transit time and lateral channel mobility are first order controls on whether a river system is a net atmospheric OC source or sink, but additional variables modulate the fluvial carbon budget, which are linked in complex ways. With the role of sediment transit time constrained, we can explore the feedbacks among these variables and their external drivers, to quantify the net effect of source to sink transit on the global carbon cycle.