A Tropical Cocktail of Organic Matter Sources: Variability in Supraglacial and Glacier Outflow Dissolved Organic Matter Composition and Age Across the Ecuadorian Andes

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 (delta O-18) 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 Delta C-14-DOC (i.e., older) compared to supraglacial streams (mean -195.2 and -61.3 parts per thousand 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, Delta C-14-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 delta O-18, 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.