Sourcing and long-range transport of particulate organic matter in river bedload: Río Bermejo, Argentina

Abstract

Fluvial transport of organic carbon from the terrestrial biosphere to the oceans is an important term in the global carbon cycle. Traditionally, the long-term burial flux of fluvial particulate organic carbon (POC) is estimated using river suspended sediment flux; however, organic carbon can also travel in river bedload as coarse particulate organic matter (POM Bed). Estimates of fluvial POC export to the ocean are highly uncertain because few studies document POM bedsources, flux, and evolution during long-range fluvial transport from uplands to ocean basins. This knowledge gap limits our ability to determine the global terrestrial organic carbon burial flux. In this study we investigate the flux, sources, and transformations of POM Bedduring fluvial transport over a ∼1300km long reach of the Río Bermejo, Argentina, which has no tributary inputs. To constrain sourcing of POM Bed, we analyzed the composition and stable hydrogen and carbon isotope ratios ( δ2H, δ13C) of plant wax biomarkers from POM Bedat six locations along the Río Bermejo and compared this to samples of suspended sediment, soil, leaf litter, and floating organic debris (POM float) from both the lowland and headwater river system. Across all samples, we found no discernible differences in n-alkane average chain length or nC29 δ13C, indicting a common origin for all sampled POM Bed. Leaf litter and POM floatnC29 δ2H values decrease with elevation, making it a useful proxy for POM Bedsource elevation. Biomarker δ2H values suggest that POM Bedis a mix of distally derived headwater and locally recruited floodplain sources at all sampling locations. These results indicate that POM Bedcan be preserved during transport through lowland rivers for hundreds of kilometers. However, the POM Bedflux decreases with increasing transport distance, suggesting mechanical comminution of these coarse organic particles and progressive transfer into the suspended load. Our provisional estimates suggest that the carbon flux from POM Bedcomprises less than 1 % of the suspended load POC flux in the Río Bermejo. While this represents a small portion of the river POC flux, this coarse, high-density material likely has a higher probability of deposition and burial in sedimentary basins, potentially allowing it to be more effective in long-term CO 2drawdown relative to fine suspended particles. Because the rate and ratio of POM Bedtransport versus comminution likely vary across tectonic and climatic settings, additional research is needed to determine the importance of POM Bedin the global carbon cycle.