Isotopic Evidence for Degradation of Particulate Black Carbon in the Ocean.

Black carbon (BC) has emerged as an integral part of the global carbon (C) cycle, constituting 12% ± 5% of the organic C pool in rivers and soils, with the potential to generate negative climate feedback. However, its ability to sequester C depends on the recalcitrant nature of BC in the environment, which is under debate. Using CTO‐375 method and by measuring concentrations and isotopic compositions of particulate BC (δ13CPBC), we explore the transformation of particulate black carbon (PBC) along the atmosphere‐river‐ocean continuum. Significantly high δ13CPBC in the ocean compared to rivers and atmospheric particulate matter indicates (a) degradation of PBC, potentially through photodegradation and leaching, and/or (b) availability of an enriched source other than fluvial or aeolian inputs. This evidence for degradation of PBC in aquatic systems warrants rethinking on its C sequestration potential and role in aquatic C biogeochemistry and further raises concerns regarding the use of sedimentary BC as a paleoenvironmental proxy. Plain Language Summary: Particulate black carbon (PBC) forms ∼12% ± 5% of the particulate organic carbon pool in aquatic systems. Black carbon has been considered highly recalcitrant and therefore a potent sink for carbon. Paleo‐environmental and biogeochemical studies have been conducted assuming BC as highly stable and its isotopic composition to remain unaltered after formation. This study explores the variability in stable isotopic composition of PBC in the riverine, atmospheric, and oceanic systems and provides evidence contrary to the general belief of the refractory nature of BC, which warrants revisiting and rethinking of the BC dynamics in aquatic systems. Key Points: Particulate black carbon (PBC) concentration decreases along the river ocean continuumStable carbon isotopic composition of the oceanic PBC pool is higher compared to the riverine and atmospheric poolsIsotopic evidence suggests degradation of PBC with the preferential loss of lighter carbon [ABSTRACT FROM AUTHOR]