Substantial export of shale‐derived particulate black carbon in the Hudson River indicated by compound‐specific stable carbon isotopes (New York, United States)

Abstract The erosion of petrogenic organic carbon links lithology, climate, and atmospheric CO 2 over geologic timescales. Recently, greater interest has been vested in identifying the dynamics of petrogenic organic carbon on modern timescales, particularly during riverine export. Petrogenic organic carbon is molecularly diverse, where labile fractions are readily oxidized to CO 2 upon entering the environment, whereas more recalcitrant fractions remain passive during transit. The condensed aromatic fraction of organic carbon—termed black carbon—is highly recalcitrant in the environment. Particulate black carbon (PBC) is derived from both petrogenic and pyrogenic sources (e.g., charcoal, soot, petroleum, and graphite). Here, we use the benzenepolycarboxylic acid method to identify and quantify the fraction of black carbon derived from regional shale lithologies in the Hudson River watershed (New York, United States). Suspended sediments collected between March 2021 and May 2022 were measured for PBC‐specific stable carbon isotopes (δ 13 C) to identify source inputs. We found that PBC‐specific δ 13 C values were higher (approximately −27‰) and similar to a wood charcoal standard at low flow but declined exponentially as flow increased, with values at peak flows (approximately −30‰) to be more consistent those observed for black carbon in regional shale endmembers. A two‐endmember mixing model (shale versus wood char) indicates shale‐derived black carbon to represent a substantial fraction of total exported PBC that increases proportionally under higher flow conditions. We estimate that shale‐derived PBC comprises a larger fraction of particulate organic carbon in the Mohawk River (59% ± 19%) compared with the Upper Hudson River (49% ± 15%), mirroring the greater extent of organic‐rich shale that underlies the Mohawk River catchment and is supported by previous work showing radiocarbon‐depleted particulate organic carbon for the catchment. Together, these results show that PBC is a conservative tracer for petrogenic organic carbon in rivers and that shale‐derived black carbon is exported most efficiently during hydrologic events.