Environmental Dynamics of Dissolved Black Carbon in the Amazon River

Dissolve black carbon (DBC) is an important component in the global carbon cycle and constitutes a significant portion of dissolved organic carbon (DOC) in aquatic systems. While global fluxes of DBC may be well understood, little is known about systematic processing of this carbon pool in fluvial systems. Similar to DOC, DBC composition may change as it moves throughout a river continuum before it is eventually deposited into the ocean. This is especially important for large river systems that are major sources of DOC to the ocean and may have significant impacts on ocean biogeochemistry and carbon cycling. To better understand variations in DBC dynamics throughout a large fluvial system, DBC was quantified using the benzene polycarboxylic acid method (BPCA) in three major tributaries of the Amazon River, each with varying biogeochemical characteristics. Principal component analysis of the BPCA abundances was used to assess the DBC compositional differences between sampling locations. In some rivers, light availability appeared to influence both DBC quantity and quality. Higher concentrations of DBC characterized by a larger, more aromatic DBC pool was found in the Rio Negro, a black water river with high levels of chromophoric dissolved organic matter and low light penetration. In the Rio Tapajós, a clear water river with higher light penetration, lower DBC concentrations characterized by higher abundances of the less polycondensed DBC pool provided evidence of photodecomposition under such conditions. The Rio Madeira, characterized as a white water river with high suspended sediment yields and high mineral/clay content, had the lowest DBC concentrations and the least polycondensed DBC content, suggesting a preferential adsorption of the more highly polycondensed DBC components onto clay particles.