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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) fluxesNano‐scale secondary ion mass spectrometry and biomarker isotope data show that mineral‐associated and discrete POC differ in composition and particle settling velocityDownstream particle size fining enhances fluvial export of recalcitrant, mineral‐associated POC that is more resistant to oxidation [ABSTRACT FROM AUTHOR]

The impact of the diverse processes responsible for the distribution of suspended sediments in the wide, shallow, and microtidal Río de la Plata estuary and the adjacent shelf is studied by means of a set of process-oriented numerical simulations. Model results show that the large width and the geometry of the estuary play a major role in the sedimentation processes. The widening and deepening of the estuary drives a significant reduction in current speeds at (i) the confluence of the tributaries and (ii) downstream Barra del Indio Shoal. Thus, sediment deposition enhances downstream those areas. Even though tides are of small amplitude in the study area, they have a significant impact on lateral mixing and resuspension of the bottom sediments. Resuspension augments the concentration of fine sediments in the layers close to the bottom, but tidal energy is not enough to lift them to the surface. Winds (which can be quite strong over this area) enhance horizontal mixing, smoothing the pattern produced by tides. Wind waves increase the concentration of sediment by vertical mixing, and their effect is most evident along the southern coast where wind waves lift the sediments resuspended by tides to the surface. The estuarine circulations associated to the bottom salinity front acts retaining sediments upstream the Barra del Indio Shoal, where estuarine currents and flocculation play an important role in sediment deposition. [ABSTRACT FROM AUTHOR]