13 Nov 2023
 | 13 Nov 2023
Status: this preprint is open for discussion.

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

Sophia Dosch, Niels Hovius, Marisa Repasch, Joel Scheingross, Jens M. Turowski, Stefanie Tofelde, Oliver Rach, and Dirk Sachse

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 (POMBed). Estimates of fluvial POC export to the ocean are highly uncertain because few studies document POMbed 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 POMBed during fluvial transport over a ~1300 km long reach of the Rio Bermejo, Argentina, which has no tributary inputs. To constrain sourcing of POMBed, we analysed the composition and stable hydrogen and carbon isotope ratios (δ2H, δ13C) of plant wax biomarkers from POMBed at six locations along the Rio Bermejo, and compared this to samples of suspended sediment, soil, leaf litter and floating organic debris (POMfloat) 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 values, indicating a common origin for all sampled POMBed. Leaf litter and POMfloat nC29 δ2H values decrease with elevation, making it a useful proxy for POMBed source elevation. Biomarker δ2H values suggest that POMBed is a mix of distally-derived headwater and locally-recruited floodplain sources at all sampling locations. These results indicate that POMBed can be preserved during transport through lowland rivers for hundreds of kilometres. However, the POMBed 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 POMBed 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 CO2 drawdown relative to fine suspended particles. Because the rate and ratio of POMBed transport versus comminution likely varies across tectonic and climatic settings, additional research is needed to determine the importance of POMBed in the global carbon cycle.

Sophia Dosch et al.

Status: open (until 25 Dec 2023)

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Sophia Dosch et al.

Sophia Dosch et al.


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Short summary
The transport of plant debris in rivers is an important part of the global carbon cycle and influences atmospheric carbon levels through time. We sampled plant debris at the bed of a lowland river and determined the sources as it is transported hundreds of kilometers. Plant debris can persist at the river bed, but mechanical breakdown reduces its amount, and it is only a small fraction compared to the suspended load. This plant debris and transport patterns need further investigations globally.