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Runoff from rapidly melting mountain glaciers is a dominant source of riverine organic carbon in many high‐latitude and high‐elevation regions. Glacier dissolved organic carbon is highly bioavailable, and its composition likely reflects internal (e.g., autotrophic production) and external (i.e., atmospheric deposition) sources. However, the balance of these sources across Earth's glaciers is poorly understood, despite implications for the mineralization and assimilation of glacier organic carbon within recipient ecosystems. We assessed the molecular‐level composition of dissolved organic matter from 136 mountain glacier outflows from 11 regions covering six continents using ultrahigh resolution 21 T mass spectrometry. We found substantial diversity in organic matter composition with coherent and predictable (80% accuracy) regional patterns. Employing stable and radiocarbon isotopic analyses, we demonstrate that these patterns are inherently linked to atmospheric deposition and in situ production. In remote regions like Greenland and New Zealand, the glacier organic matter pool appears to be dominated by in situ production. However, downwind of industrial centers (e.g., Alaska and Nepal), fossil fuel combustion byproducts likely underpin organic matter composition, resulting in older and more aromatic material being exported downstream. These findings highlight that the glacier carbon cycle is spatially distinct, with ramifications for predicting the dynamics and fate of glacier organic carbon concurrent with continued retreat and anthropogenic perturbation. Plain Language Summary: The controls on glacier organic matter composition globally remain poorly constrained, despite the importance of this material for downstream carbon cycling, and freshwater and marine ecosystems. We present the first systematic global analysis of glacier dissolved organic matter and demonstrate that its composition is largely determined by the relative balance of organic material derived from in situ production versus atmospheric deposition. As a result, globally, glaciers can be considered a diverse organic matter pool, exporting regionally distinct material to recipient ecosystems. Glaciers relatively dominated by atmospheric‐derived organics export older and more stable material with ramifications for the fate of glacier‐derived carbon in recipient ecosystems. Ultimately, this work highlights glaciers globally as having a dynamic role in carbon cycling. Key Points: Glacier dissolved organic matter is diverse but with an underlying compositional uniformity suggesting universal controls on compositionComposition appears driven by the relative balance of in situ production versus atmospheric deposition derived organic matterGlacier dissolved organic matter composition is predictable by region and linked to source [ABSTRACT FROM AUTHOR]

The biogeochemistry of rapidly retreating Andean glaciers is poorly understood, and Ecuadorian glacier dissolved organic matter (DOM) composition is unknown. This study examined molecular composition and carbon isotopes of DOM from supraglacial and outflow streams (n = 5 and 14, respectively) across five ice capped volcanoes in Ecuador. Compositional metrics were paired with streamwater isotope analyses (δ18O) to assess if outflow DOM composition was associated with regional precipitation gradients and thus an atmospheric origin of glacier DOM. Ecuadorian glacier outflows exported ancient, biolabile dissolved organic carbon (DOC), and DOM contained a high relative abundance (RA) of aliphatic and peptide‐like compounds (≥27%RA). Outflows were consistently more depleted in Δ14C‐DOC (i.e., older) compared to supraglacial streams (mean −195.2 and −61.3‰ respectively), perhaps due to integration of spatially heterogenous and variably aged DOM pools across the supraglacial environment, or incorporation of aged subglacial OM as runoff was routed to the outflow. Across Ecuador, Δ14C‐DOC enrichment was associated with decreased aromaticity of DOM, due to increased contributions of organic matter (OM) from microbial processes or atmospheric deposition of recently fixed and subsequently degraded OM (e.g., biomass burning byproducts). There was a regional gradient between glacier outflow DOM composition and streamwater δ18O, suggesting covariation between regional precipitation gradients and the DOM exported from glacier outflows. Ultimately, this highlights that atmospheric deposition may exert a control on glacier outflow DOM composition, suggesting regional air circulation patterns and precipitation sources in part determine the origins and quality of OM exported from glacier environments. Plain Language Summary: The composition of Ecuadorian glacier dissolved organic matter (DOM) is unknown, despite its potential importance for downstream carbon cycling. Here, regional variability in the source, composition, and age of DOM in glacier surface and outflow streams was explored to identify drivers of variability in DOM composition across Ecuadorian glaciers. Ecuadorian glacier outflows exported aged material (2,730–910 yBP), that was available for microbial metabolism, and hence was associated with a DOM pool enriched in simple, energy‐rich compounds. Nonetheless, regionally, dissolved organic carbon (DOC) in glacier outflows was older than in studied surface streams, perhaps indicating integration of compositionally diverse surface sources, or potentially aged organic matter (OM) from beneath the glaciers, in transit to the outflow. Across Ecuador, the age and molecular composition of DOM covaried, demonstrating changing contributions of compositional diverse and variably aged OM sources to the glaciers. Regionally, a fraction of this variation was likely driven by atmospheric deposition to glacier surfaces. Given the rapid retreat of tropical glaciers and dearth of DOM compositional studies, future work must further constrain the origins of atmospheric deposition, since it may impact the quality of DOM exported to downstream ecosystems, potentially impacting its fate. Key Points: Dissolved organic matter (DOM) in surface streams was younger in carbon age than in outflows, despite similarity in molecular compositionRegionally, DOM aromaticity increased with older carbon ages, due to lower relative contributions of modern, aliphatic‐rich DOM to runoffTrends between water isotopes and DOM composition suggest that atmospheric deposition may be a control on outflow DOM composition [ABSTRACT FROM AUTHOR]