Tracing Riverine Particulate Black Carbon Sources in Xijiang River Basin: Insight from Stable Isotopic Composition and Bayesian Mixing Model.

Rivers transport abundant terrestrial carbon into the ocean, constituting a fundamental channel between terrestrial carbon pools and oceanic carbon pools. The black carbon (BC) derived from biomass and fossil fuel combustion is an important component of the riverine organic carbon flux. A recent study estimated that approximately 17 ~ 37 Tg C of BC was delivered in suspended particle phase by rivers per year. The particulate black carbon (PBC) in river systems has rarely been investigated and its controlling factors have remained largely unknown. The stable isotopic compositions of PBC in Xijiang River during the wet season are reported in this study. We found that the PBC/particulate organic carbon (POC) ratio in Xijiang River was slightly higher than that of other rivers, which may be a result of the mobility difference between POC and PBC, aerosol BC input and riverine biogenic effect. We found that the isotopic compositions of PBC depleted C compared with those of POC and dissolved organic carbon (DOC). This divergence may be derived from the fractionation during soil organic matter production and biomass burning or fossil fuel combustion BC particles input with different isotopic compositions. The MixSIAR model indicated that most of the PBC in the study area was derived from fossil fuel combustion (~80%), the contribution of C4 plants burning was limited. Our result highlights that in the watershed without wildfire impact, the aeolian transport and deposition of the particles from fuel oil, coal combustion, and vehicle exhaust could significantly affect the BC flux in rivers.