River Organic Carbon Fluxes Modulated by Hydrodynamic Sorting of Particulate Organic Matter

• Published in: Geophysical research letters 2022-02, Vol.49 (3), p.n/a
• Main Authors: Repasch, Marisa, Scheingross, Joel S., Hovius, Niels, Vieth‐Hillebrand, Andrea, Mueller, Carsten W., Höschen, Carmen, Szupiany, Ricardo N., Sachse, Dirk
• Format: Article
• Language: English
• Subjects: Atmosphere; Carbon; Carbon cycle; Carbon dioxide; Carbon dioxide atmospheric concentrations; Carbon dioxide concentration; carbon fluxes; compound-specific stable isotopes; Detritus; Downstream; Downstream effects; Fluvial sediments; Fluxes; Fragments; GEOSCIENCES; hydrodynamic sorting; Hydrodynamics; Marine plants; Mass spectrometry; Mass spectroscopy; Minerals; Mountains; NanoSIMS; Ocean basins; Oceans; Organic carbon; Organic matter; Oxidation; Oxidation resistance; Particle settling; Particle sorting; Particulate organic carbon; Particulate organic matter; Probability theory; River water; Rivers; Secondary ion mass spectrometry; Sediment; sediment transport; Sedimentary basins; Settling rate; Settling velocity; Water analysis
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL096343

Rivers regulate the global carbon cycle by transferring particulate organic carbon (POC) from terrestrial landscapes to marine sedimentary basins, but the processes controlling the amount and composition of fluvially exported POC are poorly understood. We propose that hydrodynamic sorting processes modify POC fluxes during fluvial transit. We test this hypothesis by studying POC transported along a ∼1,200 km reach of the Rio Bermejo, Argentina. Nanoscale secondary ion mass spectrometry revealed that POC was either fine, mineral‐associated organic matter, or coarse discrete organic particles. Mineral‐associated POC is more resistant to oxidation and has a lower particle settling velocity than discrete POC. Consequently, hydraulic sorting and downstream fining amplify the proportion of fine, mineral‐associated POC from ∼55% to ∼78% over 1,220 km of downstream transit. This suggests that mineral‐associated POC has a greater probability of export and preservation in marine basins than plant detritus, which may be oxidized to CO2 during transit. Plain Language Summary The carbon cycle regulates Earth's climate by determining the amount of carbon in our atmosphere. Rivers are important for the carbon cycle because they deliver large amounts of organic carbon from mountains to the oceans. If this carbon is quickly buried in ocean sediment, atmospheric CO2 concentrations decrease. In order to predict the fluxes of carbon to the oceans, we need to understand the processes controlling organic carbon transport from source to sink. In a 1,300‐km‐long river in Argentina, we measured the amount of sediment and carbon in river water at multiple stations along the source‐to‐sink pathway. Using geochemical methods, we identified two types of organic carbon: (a) plant fragments, which are easily oxidized to CO2, and (b) organic carbon attached to minerals, which is resistant to oxidation. We calculated the mass fluxes of these two types of organic carbon and found that export of mineral‐protected carbon is greater than the export of plant fragments. This suggests that most riverine organic carbon may be preserved upon delivery to the ocean. Our data show that we need to differentiate between plant fragments and mineral‐attached organic carbon to predict the amount of organic carbon moving between rivers, the oceans, and the atmosphere. Key Points In lowland meandering rivers, downstream particle size fining and hydraulic sorting can modulate particulate organic carbon (POC)