The Amazon River mobilizes one of the largest fluxes of particulate organic carbon (POC) from land to coastal ocean sediments, playing an important role in the long-term sequestration of biospheric organic carbon in the ocean. Ramped oxidation (RPO) analyses of suspended sediments collected from the Amazon River mainstem, Solimões River, Madeira River and Tapajós River presented an opportunity to parse riverine POC by thermal reactivity, extract activation energy distributions associated with the biomolecules in these samples, and characterize the molecular diversity of POC across the floodplain. The RPO data show that POC from the Amazon River Basin spans a wide but constant activation energy range across samples, suggesting that the degradation state of POC is relatively constant across tributary locations and depths within the mainstem. Coupling the thermal reactivity data to stable and radiocarbon isotopic analyses shows that ca. 85% of mainstem POC derives from a range of partially degraded terrestrial sources, likely organic matter from mineral soil horizons. In agreement with earlier assessments of POC across the Amazon River Basin, ca. 10% of the riverine POC flux is fresh vegetation and up to 5% of it is petrogenic organic matter. We hypothesize that the higher activation energy POC pools are more likely to survive the journey from the river basin to the coastal Atlantic Ocean, and end with the argument that expanded RPO analyses of samples across the Amazon river-to-ocean continuum would provide an opportunity to test such hypotheses linking POC composition to long-term fate.

The Amazon River mobilizes organic carbon across one of the world’s largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems like the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles to Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kilograms per second) between the rising and falling stages of Amazon River mainstem. The C/N ratio, stable and radiocarbon signatures of bulk POC are less variable within the cross-section at Óbidos, and suggest that riverine POC in the Amazon River is predominantly soil-derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n-alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross-sectional surveys at Óbidos highlight the importance of depth-specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river.