Widespread Black Carbon Deposition of Varied Origin Exported From Glaciers

Abstract Atmospheric deposition delivers carbon to glacier surfaces, including from fossil fuel and biomass combustion. Nonetheless, spatial variation in the sources of organic and black carbon deposited on glaciers is poorly understood, along with their role in driving glacier outflow dissolved organic matter (DOM) composition and fate. Here, we used bulk and compound‐specific carbon isotopic analyses to constrain the sources of dissolved organic carbon (DOC) and dissolved black carbon (DBC) in 10 glacier outflows across four regions. To understand the relationships between glacier DOM composition and sources of DOC and DBC, isotopic data were used in conjunction with ultrahigh resolution molecular‐level analyses. Globally, a substantial yet variable component of DOC was sourced from anthropogenic aerosols (12%–91%; median 50%), influencing regional DOM composition (aliphatics 26.9%–58.4% relative abundance; RA). Relatively older radiocarbon ages (i.e., larger fossil‐derived component) of glacier DOC were correlated with more 13 C depleted DOC and DBC signatures, where DOM had higher aromaticity, elevated RA of condensed aromatics, and a lower RA of aliphatic compounds. This study highlights that anthropogenic deposition is pervasive, but its extent varies spatially with ramifications for DOM composition, and thus reactivity and fate.