DOI: 10.5880/gfz.4.6.2023.005
title: Dataset to: Sourcing and Long-Range Transport of Particulate Organic Matter in River Bedload: Rio Bermejo, Argentina

description: The data package encompasses field data of clastic and organic sediment, river width and flow velocities of six river transects along the Rio Bermejo, Argentina. The laboratory data entails long-chain n-alkanes and d2H and d13C values of organic matter (soil, deposited sediment, suspended sediment (published by Repasch et al., 2020), leaf litter, floating organic matter, and bedload organic matter from the Rio Bermejo catchment. It further contains the bedload organic matter and estimated bedload organic carbon fluxes of six river transects along the Rio Bermejo. 
 
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-distance 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, indicting a common origin for all sampled POMBed. 
 
We define three potential POMBed sources: Coarse organic debris we sampled at distinct elevations in the catchment: floodplain leaf litter, headwater leaf litter, and headwater POMfloat. We aim to understand the mixing range of the widely spread POMBed. We determine the range of a possible POMBed mixing signal of the sources within the geochemical parameters, and in addition, determine potential missing POMBed sources, using a mixing-space model developed by (Smith et al., 2013). 
 
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, 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.
description: The field data was collected using ADCP measurements for river width and flow velocity, and a Helley-Smith Sample for the bed material. Laboratory analysis included solid phase extraction (SPE) and analysis with gas chromatography and mass spectrometry. Analysis was conducted with R 4.1.2 GUI 1.77 High Sierra build (8007).