@article {holt_tropical_2023,
	title = {A Tropical Cocktail of Organic Matter Sources: Variability in Supraglacial and Glacier Outflow Dissolved Organic Matter Composition and Age Across the Ecuadorian Andes},
	journal = {Journal of Geophysical Research: Biogeosciences},
	volume = {128},
	number = {5},
	year = {2023},
	note = {_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2022JG007188},
	month = {05/2023},
	pages = {e2022JG007188},
	abstract = {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{\textperthousand} 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.},
	keywords = {cryosphere, dissolved organic carbon, Dissolved organic matter, Ecuador, FT-ICR MS, glaciers, δ13C-DOC, Δ14C-DOC},
	issn = {2169-8961},
	doi = {10.1029/2022JG007188},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2022JG007188},
	author = {Holt, Amy D. and Kellerman, Anne M. and Battin, Tom. I. and McKenna, Amy M. and Hood, Eran and Andino, Patricio and Crespo-P{\'e}rez, Ver{\'o}nica and Peter, Hannes and Sch{\"o}n, Martina and De Staercke, Vincent and Styllas, Michail and Tolosano, Matteo and Spencer, Robert G. M.}
}
@article {2969,
	title = {Heterogeneous Patterns of Aged Organic Carbon Export Driven by Hydrologic Flow Paths, Soil Texture, Fire, and Thaw in Discontinuous Permafrost Headwaters},
	journal = {Global Biogeochemical Cycles},
	volume = {36},
	year = {2022},
	month = {Jan-04-2022},
	abstract = {Climate change is thawing and potentially mobilizing vast quantities of organic carbon (OC) previously stored for millennia in permafrost soils of northern circumpolar landscapes. Climate-driven increases in fire and thermokarst may play a key role in OC mobilization by thawing permafrost and promoting transport of OC. Yet, the extent of OC mobilization and mechanisms controlling terrestrial-aquatic transfer are unclear. We demonstrate that hydrologic transport of soil dissolved OC (DOC) from the active layer and thawing permafrost to headwater streams is extremely heterogeneous and regulated by the interactions of soils, seasonal thaw, fire, and thermokarst. Repeated sampling of streams in eight headwater catchments of interior Alaska showed that the mean age of DOC for each stream ranges widely from modern to \~{}2,000 years B.P. Together, an endmember mixing model and nonlinear, generalized additive models demonstrated that Δ14C-DOC signature (and mean age) increased from spring to fall, and was proportional to hydrologic contributions from a solute-rich water source, related to presumed deeper flow paths found predominantly in silty catchments. This relationship was correlated with and mediated by catchment properties. Mean DOC ages were older in catchments with >50\% burned area, indicating that fire is also an important explanatory variable. These observations underscore the high heterogeneity in aged C export and difficulty of extrapolating estimates of permafrost-derived DOC export from watersheds to larger scales. Our results provide the foundation for developing a conceptual model of permafrost DOC export necessary for advancing understanding and prediction of land-water C exchange in changing boreal landscapes. In high latitude environments, soils that have been frozen for millennia are thawing, releasing organic carbon (OC). Thawing and export of OC to downstream aquatic ecosystems is a potential biogeochemical feedback that may accelerate climate warming if large amounts of ancient OC are transformed and released to the atmosphere as greenhouse gases. The magnitude and timing of ancient OC thaw and mobilization are not well defined, so predicting these patterns at local to global scales is challenging. Using a suite of diverse headwater catchments in the discontinuous permafrost zone of Alaska, USA, we identify the main controls on the mobilization of ancient OC from thawing landscapes into adjacent streams. Our surveys show that ancient OC export depends on the complex interaction between fire history, soil type and thawing characteristics, and seasonal warming. We find that all of these factors play a role, resulting in highly heterogeneous release of ancient OC to headwater streams. Aged dissolved organic carbon (DOC) in headwater streams in interior Alaska is heterogeneous across permafrost landscapes indicating the interplay of multiple drivers Aged DOC concentrations increase from spring to fall and are highest in catchments with >50\% burned extent and dominated by silty uplands Aged DOC is proportional to stream solute loads, indicating that deeper flow paths deliver permafrost C to streams Aged dissolved organic carbon (DOC) in headwater streams in interior Alaska is heterogeneous across permafrost landscapes indicating the interplay of multiple drivers Aged DOC concentrations increase from spring to fall and are highest in catchments with >50\% burned extent and dominated by silty uplands Aged DOC is proportional to stream solute loads, indicating that deeper flow paths deliver permafrost C to streams},
	issn = {0886-6236},
	doi = {10.1029/2021GB007242},
	url = {https://onlinelibrary.wiley.com/doi/10.1029/2021GB007242},
	author = {Koch, Joshua C. and Bogard, Matthew J. and Butman, David E. and Finlay, Kerri and Ebel, Brian and James, Jason and Johnston, Sarah Ellen and Jorgenson, M. Torre and Pastick, Neal J. and Spencer, Robert G. M. and Striegl, Robert and Walvoord, Michelle and Wickland, Kimberly P.}
}
@article {2933,
	title = {Limited Presence of Permafrost Dissolved Organic Matter in the Kolyma River, Siberia Revealed by Ramped Oxidation},
	journal = {Journal of Geophysical Research: Biogeosciences},
	volume = {126},
	year = {2021},
	month = {Jan-07-2021},
	issn = {2169-8953},
	doi = {10.1029/2020JG005977},
	url = {https://www.webofscience.com/wos/woscc/full-record/WOS:000677821700019?AlertId=4d48b20a-7d27-4fa2-a6a8-37f0daa89864\&SID=8EgwPROzuFaD4cTggN2},
	author = {Rogers, Jennifer A. and Galy, Valier and Kellerman, Anne M. and Chanton, Jeffrey P. and Zimov, Nikita and Spencer, Robert G. M.}
}
@article {2732,
	title = {Groundwater as a major source of dissolved organic matter to Arctic coastal waters},
	journal = {Nature Communications},
	volume = {11},
	year = {2020},
	month = {Jan-12-2020},
	abstract = {Groundwater is projected to become an increasing source of freshwater and nutrients to the Arctic Ocean as permafrost thaws, yet few studies have quantified groundwater inputs to Arctic coastal waters under contemporary conditions. New measurements along the Alaska Beaufort Sea coast show that dissolved organic carbon and nitrogen (DOC and DON) concentrations in supra-permafrost groundwater (SPGW) near the land-sea interface are up to two orders of magnitude higher than in rivers. This dissolved organic matter (DOM) is sourced from readily leachable organic matter in surface soils and deeper centuries-to millennia-old soils that extend into thawing permafrost. SPGW delivers approximately 400{\textendash}2100 m3 of freshwater, 14{\textendash}71 kg of DOC, and 1{\textendash}4 kg of DON to the coastal ocean per km of shoreline per day during late summer. These substantial fluxes are expected to increase as massive stocks of frozen organic matter in permafrost are liberated in a warming Arctic.},
	doi = {10.1038/s41467-020-15250-8},
	url = {http://www.nature.com/articles/s41467-020-15250-8http://www.nature.com/articles/s41467-020-15250-8.pdfhttp://www.nature.com/articles/s41467-020-15250-8.pdfhttp://www.nature.com/articles/s41467-020-15250-8},
	author = {Connolly, Craig T. and Cardenas, M. Bayani and Burkart, Greta A. and Spencer, Robert G. M. and McClelland, James W.}
}
@article {2722,
	title = {The Ephemeral Signature of Permafrost Carbon in an Arctic Fluvial Network},
	journal = {Journal of Geophysical Research: Biogeosciences},
	volume = {123},
	year = {2018},
	month = {Jul-05-2018},
	pages = {1475 - 1485},
	abstract = {Arctic fluvial networks process, outgas, and transport significant quantities of terrestrial organic carbon (C), particularly dissolved organic carbon (DOC). The proportion of permafrost C in these fluxes, however, is poorly constrained. A primary obstacle to the quantification of permafrost-derived DOC is that it is rapidly respired without leaving a unique tracer of its presence. In this study, we investigated the production of bacterial respiratory carbon dioxide (CO2; measured as dissolved inorganic carbon; DIC) during maximum late-summer thaw in sites spanning a fluvial network (Kolyma Basin, Siberia) to assess whether the biodegradation of permafrost DOC could be detected by the presence of a persistent aged (14C-depleted) signature on the DIC pool. Using Keeling plot interpretation of DIC produced in bioincubations of river water, we show that bacteria respire varying sources of DOC moving downstream through the fluvial network. Respiration of permafrost (production of aged CO2) was only detected in heavily permafrost thaw influenced sites. In nonpermafrost thaw impacted sites, ambient DIC was modern (14C-enriched), but rather than precluding the respiration of permafrost OC upstream, we suggest that 14C-depleted DIC is overwhelmed by modern DIC. Investigation of dissolved organic matter composition via Fourier transform ion cyclotron resonance mass spectrometry highlighted that elevated levels of aliphatic and nitrogen-containing compounds were associated with the production of aged DIC, providing molecular-level insight as to why permafrost-derived dissolved organic matter is rapidly respired. Overall, results from this study demonstrate the difficulty of tracing inputs of a highly reactive substrate to systems with diverse organic matter sources.},
	issn = {2169-8953},
	doi = {10.1029/2017JG004311},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2017JG004311},
	author = {Drake, Travis W. and Guillemette, Fran{\c c}ois and Hemingway, Jordon D. and Chanton, Jeffery P. and Podgorski, David C. and Zimov, Nikita S. and Spencer, Robert G. M.}
}
@article {2826,
	title = {Unifying Concepts Linking Dissolved Organic Matter Composition to Persistence in Aquatic EcosystemsUnifying Concepts Linking Dissolved Organic Matter Composition to Persistence in Aquatic Ecosystems},
	journal = {Environmental Science \& Technology},
	volume = {52},
	year = {2018},
	month = {Jun-03-2018},
	pages = {2538 - 2548},
	abstract = {The link between composition and reactivity of dissolved organic matter (DOM) is central to understanding the role aquatic systems play in the global carbon cycle; yet, unifying concepts driving molecular composition have yet to be established. We characterized 37 DOM isolates from diverse aquatic ecosystems, including their stable and radiocarbon isotopes (δ13C-dissolved organic carbon (DOC) and Δ14C-DOC), optical properties (absorbance and fluorescence), and molecular composition (ultrahigh resolution mass spectrometry). Isolates encompassed end-members of allochthonous and autochthonous DOM from sites across the United States, the Pacific Ocean, and Antarctic lakes. Modern Δ14C-DOC and optical properties reflecting increased aromaticity, such as carbon specific UV absorbance at 254 nm (SUVA254), were directly related to polyphenolic and polycyclic aromatic compounds, whereas enriched δ13C-DOC and optical properties reflecting autochthonous end-members were positively correlated to more aliphatic compounds. Furthermore, the two sets of autochthonous end-members (Pacific Ocean and Antarctic lakes) exhibited distinct molecular composition due to differences in extent of degradation. Across all sites and end-members studied, we find a consistent shift in composition with aging, highlighting the persistence of certain biomolecules concurrent with degradation time.},
	issn = {0013-936X},
	doi = {10.1021/acs.est.7b0551310.1021/acs.est.7b05513.s001},
	url = {https://pubs.acs.org/doi/10.1021/acs.est.7b05513},
	author = {Kellerman, Anne M. and Guillemette, Fran{\c c}ois and Podgorski, David C. and Aiken, George R. and Butler, Kenna D. and Spencer, Robert G. M.}
}
@article {2458,
	title = {Anthropogenic aerosols as a source of ancient dissolved organic matter in glaciers},
	journal = {Nature Geoscience},
	volume = {5},
	year = {2012},
	month = {Jul-02-2013},
	pages = {198 - 201},
	abstract = {Glacier-derived dissolved organic matter represents a quantitatively significant source of ancient, yet highly bioavailable carbon to downstream ecosystems1. This finding runs counter to logical perceptions of age{\textendash}reactivity relationships, in which the least reactive material withstands degradation the longest and is therefore the oldest2. The remnants of ancient peatlands and forests overrun by glaciers have been invoked as the source of this organic matter1, 3, 4. Here, we examine the radiocarbon age and chemical composition of dissolved organic matter in snow, glacier surface water, ice and glacier outflow samples from Alaska to determine the origin of the organic matter. Low levels of compounds derived from vascular plants indicate that the organic matter does not originate from forests or peatlands. Instead, we show that the organic matter on the surface of the glaciers is radiocarbon depleted, consistent with an anthropogenic aerosol source. Fluorescence spectrophotometry measurements reveal the presence of protein-like compounds of microbial or aerosol origin. In addition, ultrahigh-resolution mass spectrometry measurements document the presence of combustion products found in anthropogenic aerosols. Based on the presence of these compounds, we suggest that aerosols derived from fossil fuel burning are a source of pre-aged organic matter to glacier surfaces. Furthermore, we show that the molecular signature of the organic matter is conserved in snow, glacier water and outflow, suggesting that the anthropogenic carbon is exported relatively unchanged in glacier outflows.},
	keywords = {Atmospheric chemistry, Biogeochemistry, Cryospheric science},
	issn = {1752-0894},
	doi = {10.1038/ngeo1403},
	url = {http://www.nature.com/doifinder/10.1038/ngeo1403},
	author = {Stubbins, Aron and Hood, Eran and Raymond, Peter A. and Aiken, George R. and Sleighter, Rachel L. and Hernes, Peter J. and Butman, David and Hatcher, Patrick G. and Striegl, Robert G. and Schuster, Paul and Abdulla, Hussain A. N. and Vermilyea, Andrew W. and Scott, Durelle T. and Spencer, Robert G. M.}
}
