PNNL

Abstract

Riverine dissolved organic carbon (DOC) contains charcoal byproducts, termed black carbon (BC). As a large and slow-cycling component of the global carbon cycle, it is important to constrain the sources and fate of this elusive carbon pool during land-ocean transfer. Substantial uncertainties in regional and global-scale BC budgets persist to determine the significance of BC as a sink of atmospheric CO2, due to poor constraints on its fluvial dynamics. The Amazon River is a significant part of global BC cycling because it exports an order of magnitude more DOC, and thus dissolved BC (DBC), than any other river on Earth. We report the first spatially resolved DBC quantity and radiocarbon (?14C) measurements, paired with molecular-level characterization of dissolved organic matter from the Amazon River and tributaries during low discharge. The proportion of BC-like polycyclic aromatic structures decreased downstream, but marked spatially variability in abundance and ?14C values of DBC molecular markers imply dynamic sources and cycling in a manner that is incongruent with bulk DOC. We estimate a DBC flux from the Amazon River of 1.9-2.7 Tg DBC yr-1 and find that the Amazon exports predominately young DBC to the ocean. This contrasts the old age of DBC in the ocean, suggesting that loss processes of modern DBC in the coastal plume must be important.