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Riverine export of aged carbon driven by flow path depth and residence time Rebecca T Barnes, David Ellison Butman, Henry Wilson, and Peter A. Raymond Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04717 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018
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Environmental Science & Technology
Riverine export of aged carbon driven by flow path depth and residence time
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Rebecca T. Barnes1*, David E. Butman2, Henry F. Wilson3, Peter A. Raymond4
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Environmental Program, Colorado College, Colorado Springs, CO 80903, USA
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School of Environmental and Forest Sciences, University of Washington, Seattle, WA 98195,
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USA
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Canada
Brandon Research Centre, Agriculture and Agri-Food Canada, Brandon, Manitoba R7A 5Y3,
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*corresponding author: rbarnes@coloradocollege.edu
School of Forestry & Environmental Sciences, Yale University, New Haven, CT 06115, USA
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Abstract
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The flux of terrestrial C to rivers has increased relative to pre-industrial levels, a fraction of
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which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the
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ocean, or (bio)chemically processed and released as CO2. Disturbance changes land cover and
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hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of
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aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major
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ions from nineteen rivers draining the coterminous U.S. and Arctic. DOC optics indicated that
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the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC
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tended to consist of smaller, microbial degradation products. Aged DOC exports, observed
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regularly in arid basins and during base flow in arctic rivers, are associated with higher
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proportion of mineral weathering products, suggesting deeper flows paths. These patterns also
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indicate potential for production of microbial byproducts as DOC ages in soil and water with
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longer periods of time between production and transport. Thus, changes in hydrology associated
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with landscape alteration (e.g. tilling or shifting climates) that can result in deeper flow paths or
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longer residence times will likely lead to a greater proportion of aged carbon in riverine exports.
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Environmental Science & Technology
Introduction To predict how the carbon (C) cycle will respond to global change drivers, we need to
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understand how organic matter pools at the land-water interface will shift in response to drivers
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such as altered precipitation and increased agricultural development1. The terrestrial landscape
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exports approximately 2.7 Pg C yr-1 to aquatic systems2, 3, a value that has increased by ~0.1 –
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0.2 Pg C yr-1 due to anthropogenic activities3. Inland waters both process and transport organic
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matter, storing C in sediment4 and releasing it to the atmosphere as CO2 and CH42, 5-7. The fate of
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organic carbon within inland waters is largely determined by its chemical composition and the
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hydrology of the system. For example, the chemical composition of dissolved organic matter will
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determine, in part, how susceptible it is to microbial metabolism8 and UV oxidation9. Further,
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studies have illustrated that the majority of annual dissolved organic C (DOC) export occurs
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during high flow events, in both temperate10, tropical11, and arctic12 systems. During these
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periods of high flow, it is likely that shorter transit times lead to increased export of less
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processed DOC to the coastal ocean.
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Stream DOC is dominated by terrestrial sources in most systems13 and thus if the age and
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nature of the DOC varies between systems or through time it follows that DOC source regions
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(or processing) are different or shifting. The majority of this DOC is modern in age, reflecting its
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dominant sources: terrestrial vegetation and surface soils14. However, rivers also export aged
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DOC, organic carbon that is stored in terrestrial sinks, including: shales15, peatlands16,
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permafrost17, as well as non-terrestrial sources such as precipitation18, 19 and petroleum products
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ranging from soaps to motor oil20. A recent synthesis illustrated that human landscape
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disturbance, i.e. urban and agricultural development, is associated with the export of aged carbon
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from streams and rivers21. Further, several studies have documented relationships between flow
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conditions and DOC age; though some systems export older carbon during baseflow17 and others
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during high flow22, 23. This aged carbon, once thought recalcitrant, is biologically available
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within aquatic systems 24-26, fueling CO2 production and evasion. Given that shifts in
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precipitation are expected to be a dominant driver in future riverine C fluxes27, 28, it is critical that
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we understand how this changing hydrology will affect C source areas, export and, cycling.
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Changes in hydrology are likely to affect flow paths and thus the nature and amount of
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carbon exported. Kaiser and Kalbitz29 describe a conceptual model that explains the vertical
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profile observations of soil organic matter (SOM) characteristics. Several studies have
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documented an increase in the age of SOM with depth30 which cannot be explained by the simple
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leaching of organic matter decomposition products in surface horizons to depth31. In addition to
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the physio-chemical stripping of dissolved organic matter (DOM) components (fractional
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sorption and co-precipitation32), DOM undergoes microbial processing during transport29. Thus
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given that exported DOM reflects both the sources and cycling of SOM33, riverine chemistry
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should reflect differences in flow path depth. For example, surficial and shallow flow paths will
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export large amounts of modern, plant-derived DOM, while deeper flows paths are more likely
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to have an aged, microbial DOM signature.
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Deeper portions of the soil column will likely have a greater proportion of organic matter
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protected via organo-mineral associations, which are key to retaining C within soils34, 35 as
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organo-mineral associations (cation bridging, ligand exchange, cation-anion exchange, hydrogen
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bonds, Van der Waal forces, etc.) are more important in predicting stability and turnover of SOM
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than the molecular structure of the OM35, 36. Tipping et al.37 found that SOM associated with
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minerals had an average residence time of 100-200 years, while SOM not associated with
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minerals turned over in 20-30 years. As such, it follows that older DOC should be exported with
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relatively greater mineral weathering products (e.g. base cations, Si). In addition, many microbial
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byproducts are non-aromatic and recalcitrant38 and when in the deeper portion of the soil more
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likely protected from further decomposition, increasing the likelihood that these molecules will
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age in place. Weathering and pedogenic processes stratify soils structure into distinct vertical
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horizons, that create large ranges of physical/chemical environments experiences by microbes.
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Overtime, changes in soil mineralogy and the release of metal cations like Fe and Al, produce
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binding sites on the remaining organo-mineral complexes that can control the stabilization of soil
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organic matter, as well as alter the quality and quantity of DOM. The alteration of soil
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mineralogy and the release of Fe and Al over time produce binding sites on mineral surfaces and
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organo-mineral complexes that not only stabilize SOM but alter the quality and quantity of DOM
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remaining in soil39-41. For example, reducing conditions in soil can result in the export of Fe2+
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and DOC to streams42. As water moves through this portion of the soil it flushes both particulates
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and dissolved constituents into ground and/or surface waters, where additional OM may be
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released from particles43.
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Given that multiple factors of global change (climate, land cover and land use, etc.)
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directly and indirectly shift flow paths, and thus the nature of carbon exports, we examine the
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spatiotemporal variability in the stoichiometry of DOC and mineral weathering products (base
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cations) within large river basins draining the coterminous United States and Arctic. Given the
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conceptual model of Kaiser and Kalbitz29, we hypothesize that more labile, older DOC will be
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exported with relatively greater amounts of weathering products (i.e. a low DOC to base cation
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ratio). These exports will be associated with more mineral rich and/or deeper flow paths and
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longer residence times associated with arid climates44, temporal changes in water routing over
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the hydrograph (e.g. baseflow17), or alteration to the landscape (e.g. agricultural liming45).
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Materials & Methods To examine these questions, we examined the riverine export of DOC to coastal waters
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from thirteen major watersheds of the U.S.44 and the six largest Arctic watersheds that comprise
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the Arctic Great Rivers Observatory (Arctic-GRO). The U.S. watersheds are monitored as part of
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the National Stream Quality Accounting Network (NASQAN) program and in 2009 additional
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DOC quality and age were measured on 6-11 of the monthly samples. This one year of data,
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describing the coterminous U.S. watersheds, allows us to examine how large spatial variability
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(different climates, ecosystems, levels of development) shifts dissolved weathering product and
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organic carbon fluxes. The Arctic-GRO data (2003-2016) provides a way to examine six systems
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with strong seasonal shifts in hydrology (i.e. freshet vs. baseflow) draining relatively organic rich
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soils.
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The U.S. watershed data describe samples collected across the 2009 hydrograph from 13
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rivers that drain 79% of the land area and make up 90% of the freshwater discharge in the
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coterminous U.S.44. Watersheds range in size from 2.9 million km2 (Mississippi) to 29,973 km2
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(Potomac) and encompass a range of climate regimes with average precipitation ranging from
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>1400 mm yr-1 in southeastern U.S. watersheds to less than 350 mm yr-1 in the high plains of the
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Colorado River basin. As such, water yield varies significantly across these systems from
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monthly lows of 1 mm yr-1 in the Rio Grande (Texas) to highs of 2615 mm yr-1 in the Altamaha
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River (Georgia). Additional information (e.g. land use, population density) about these sites is
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available in Tables 1a-b in Butman et al.44
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The six Arctic watersheds (Yenisey, Lena, Ob’, Mackenzie, Yukon, Kolyma) comprise more than 50% of the land area draining to the Arctic Ocean, suggesting that differences in
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permafrost extent, microbial community, soil characteristics, and bedrock weathering amongst
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the basins should be reflected in this dataset. Five of the six basins’ headwaters are within boreal
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forest, draining a tundra dominated landscape before emptying into the Arctic Ocean. In contrast,
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the Yukon River follows a different trajectory, with its headwaters in tundra, moving south into a
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forest dominated landscape. Continuous permafrost coverage varies significantly between
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systems with the Kolyma almost entirely underlain with permafrost (99%) and the Ob’ having
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just 1% of its area occupied by permafrost46. In all six systems, water and constituent fluxes
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associated with organic matter are greatest during the spring freshet12. There is significant
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variability in the strength of the seasonality in discharge and DOC fluxes, with 50% or greater of
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annual DOC flux occurring during this ~two month period of time in the Yenisey, Lena,
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Kolyma, and Yukon while less than 30% of the annual total occurred during this period of time
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in the Ob’ and the Mackenzie46. The average water yield at the time of sampling ranges from 210
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± 120 mm yr-1 (Mackenzie) to 460 ± 460 mm yr-1 (Lena) with relatively larger seasonal
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variability than the NASQAN rivers. The fourteen years of sampling across the hydrograph in
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these systems, provides a robust way to compare spatial variation amongst the catchments as
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well as how seasonal shifts in hydrology affect the nature and concentration of dissolved carbon
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and base cation exports.
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Sampling protocols were similar between projects allowing us to compare the data.
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Sampling of these large rivers consisted of integrated grab samples occurring at gauged sites or
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near gauging stations (in most cases maintained by the USGS), providing concomitant discharge
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and water quality measurements. The NASQAN and majority of Arctic (2002-2011) samples
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represent a composite of depth integrated samples from multiple points across the channel, while
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more recent sampling efforts in the Arctic (2012-2016) represent surface samples from multiple
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points across the channel.
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Sample processing for carbon constituents was similar (all samples filtered through 0.7
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µm pre-combusted GF/F filters) and for the most part conducted in the same group of labs at the
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USGS, Yale University, and Woods Hole. There were a few differences in the processing or
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handling of samples. While all samples were filtered through the same filters, the NASQAN
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samples were shipped unfiltered on ice within 24 hours of sampling and immediately filtered
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upon arrival, while the Arctic samples were filtered in the field and then shipped. The largest
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difference in carbon constituent analysis is the measurement of DOC concentrations. DOC
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concentration for NASQAN rivers was measured using the persulfate wet oxidation on an OI
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Analytical Model 700 TOC Analyzer47, while Arctic samples were analyzed using a Shimadzu
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TOC analyzer12. All carbon quality measurements were made consistently across the two studies.
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Briefly, radiocarbon isotope (∆14C-DOC) measurements determined the average age of DOC
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exported and all samples were processed using established methods14, 48, which involve oxidizing
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the DOC with UV light, converting it to CO2 which was then trapped and cryogenically purified.
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The sample was then sent to the National Ocean Sciences Accelerator Mass Spectrometry
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(NOSAMS) for isotopic analysis. The specific UV absorbance at 254 nm (SUVA254) was used as
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a proxy for the aromaticity of dissolved organic matter, therefore sample absorbance at 254 nm
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was measured using a UV-Vis Spectrometer and then normalized to DOC concentrations and
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reported in L/(mg C*m) 49. Organic acid fractions were chromatographically separated using
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columns packed with Amberlite™ XAD-8 and XAD-4 resins47. These operationally defined
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dissolved organic matter fractions: larger molecular weight hydrophobic organic acid (HPOA),
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smaller molecular weight hydrophilic molecules (HPI), and transphilic acids (TPIA)47 can
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provide information about carbon quality for reactivity, bacterial mineralization, and
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photodegradation. The sum of the four major base cations (Ca2+, Na+, K+, Mg2+), as reported in
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meq L-1, was used as a proxy for mineral soil weathering. In the Arctic river systems, water
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isotopic (δ18O-H2O) measurements were used as proxy for shifts in flow paths, given systematic
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seasonal shifts of the system (i.e. freshet always has more depleted δ18O values as compared to
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later season base flow samples)50. For a more detailed field and lab protocol descriptions
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associated with Arctic-GRO please see: carbon12, major ion chemistry51, and water isotope
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analyses50. Carbon sampling protocols for the NASQAN sites are given in Butman et al.44 and
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major ions and discharge measurements were downloaded from the USGS National Water
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Inventory System (www.usgs.gov/nwis).
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All statistical analyses were conducted in R (3.3.2, 2016 R Foundation for Statistical
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Modeling) and variables were transformed to meet requirements of normality. For example, the
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ratio of DOC to the sum of base cations was transformed using log10. Statistical relationships
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were considered significant at p