The Thermal Reactivity and Molecular Diversity of Particulate Organic Carbon in the Amazon River Mainstem

Abstract

The Amazon River mobilizes one of the largest fluxes of particulate organic carbon (POC) fromland to coastal ocean sediments, playing an important role in the long‐term sequestration of biospheric organiccarbon in the ocean. Ramped oxidation (RPO) analyses of suspended sediments collected from the AmazonRiver mainstem, Solimões River, Madeira River, and Tapajós River presented an opportunity to parse riverinePOC by thermal reactivity, extract the activation energy distributions of specific biomolecular pools in thesesamples, and characterize the molecular diversity of POC across the floodplain. The thermal reactivity dataimply that POC from the Amazon River basin spans a wide but relatively homogenous activation energy rangeacross samples, suggesting that the degradation history of the organic carbon comprising riverine suspendedparticles is relatively constant across depths within the mainstem and different tributary locations. Couplingactivation energy distributions to stable and radiocarbon isotopic analyses shows that ca. 85% of mainstem POCderives from a range of partially degraded terrestrial sources, likely organic matter from mineral soil horizons,and that a similar range of soil sources influences the biomolecular diversity in tributary samples. In agreementwith earlier assessments, ca. 10% of the riverine POC flux is fresh vegetation and up to 5% of it is petrogenicorganic matter. Expanded RPO analyses of samples across the Amazon river‐to‐ocean continuum wouldprovide an opportunity to track the fate of these different organic matter pools downstream that is uniquelydifferent from, but complementary to, past compound‐specific and bulk analyses of riverine POC.