Sourcing and Long-Range Transport of Particulate Organic Matter in River Bedload: Rio Bermejo, Argentina

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 _bed sources, 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 _Bed during fluvial transport over a ~1300 km long reach of the Rio Bermejo, Argentina, which has no tributary inputs. To constrain sourcing of POM _Bed, we analysed the composition and stable hydrogen and carbon isotope ratios (δ ²H, δ ¹³C) of plant wax biomarkers from POM _Bed at six locations along the Rio 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 nC ₂₉ δ ¹³C values, indicating a common origin for all sampled POM _Bed. Leaf litter and POM _floatnC ₂₉ δ ²H values decrease with elevation, making it a useful proxy for POM _Bed source elevation. Biomarker δ ²H values suggest that POM _Bed is a mix of distally-derived headwater and locally-recruited floodplain sources at all sampling locations. These results indicate that POM _Bed can be preserved during transport through lowland rivers for hundreds of kilometres. However, the POM _Bed flux 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 _Bed comprises less than 1 percent of the suspended load POC flux in the Rio Bermejo. While this represents a small portion of the river POC flux, this coarse and 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 ₂ drawdown relative to fine suspended particles. Because the rate and ratio of POM _Bed transport versus comminution likely varies across tectonic and climatic settings, additional research is needed to determine the importance of POM _Bed in the global carbon cycle.