Heterogeneous CO2 and CH4 content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

Accelerated melting of the Greenland Ice Sheet has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of Greenland Ice Sheet meltwater on Arctic greenhouse gas budgets. We evaluate subglacial discharge from the Greenland Ice Sheet for carbon dioxide (CO2) and methane (CH4) concentrations and δ13C values and use geochemical models to evaluate subglacial CH4 and CO2 sources and sinks. We compare discharge from southwest (a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the Russell Glacier) and southern Greenland (Kiattut Sermiat). Meltwater CH4 concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat. In contrast, meltwaters from southwest sites are supersaturated, even though oxidation reduces CH4 concentrations by up to 50 % during periods of low discharge. CO2 concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO2 is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO2 sources result in meltwaters that are either sources or sinks of atmospheric CO2. At the sub-Isunnguata site, the predominant source of CO2 is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO2 sources maintain atmospheric CO2 concentrations at Russell but not at Kiattut Sermiat, where CO2 is undersaturated. These results highlight a previously unrecognized degree of heterogeneity in greenhouse gas dynamics under the Greenland Ice Sheet. Future work should constrain the extent and controls of heterogeneity to improve our understanding of the impact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as well as the role of continental ice sheets in greenhouse gas variations over glacial–interglacial timescales.

Abstract. Accelerated melting of the Greenland Ice Sheet has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of Greenland Ice Sheet meltwater on Arctic greenhouse gas budgets. We evaluate subglacial discharge from the Greenland Ice Sheet for carbon dioxide (CO 2 ) and methane (CH 4 ) concentrations and δ 13 C values and use geochemical models to evaluate subglacial CH 4 and CO 2 sources and sinks. We compare discharge from southwest (a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the Russell Glacier) and southern Greenland (Kiattut Sermiat). Meltwater CH 4 concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat. In contrast, meltwaters from southwest sites are supersaturated, even though oxidation reduces CH 4 concentrations by up to 50 % during periods of low discharge. CO 2 concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO 2 is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO 2 sources result in meltwaters that are either sources or sinks of atmospheric CO 2 . At the sub-Isunnguata site, the predominant source of CO 2 is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO 2 sources maintain atmospheric CO 2 concentrations at Russell but not at Kiattut Sermiat, where CO 2 is undersaturated. These results highlight a previously unrecognized degree of heterogeneity in greenhouse gas dynamics under the Greenland Ice Sheet. Future work should constrain the extent and controls of heterogeneity to improve our understanding of the impact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as well as the role of continental ice sheets in greenhouse gas variations over glacial-interglacial timescales.

Introduction
Glaciers play an important role in global chemical cycles due to the production of fine-grained sediments that participate in carbonate and silicate mineral weathering reactions (Table 1), which are the principal sink of atmospheric CO 2 over geologic timescales (Berner et al., 1983;Walker et al., 1981). Variations in the weathering intensity of comminuted sediments may contribute to glacial-interglacial atmospheric CO 2 variations as sediments are alternately covered by ice and exposed following ice retreat. However, the importance of CO 2 consumption by mineral weathering is poorly understood, including effects from the advance and retreat of continental ice sheets (Ludwig et al., 1999). Recent evaluations of carbon budgets in proglacial environments indicate that mineral weathering results in net sequestration of atmospheric CO 2 , suggesting that proglacial systems are underrecognized as Arctic CO 2 sinks (St Pierre et al., 2019); however, alternate processes could lead to the production of greenhouse gases in glacial systems. For instance, CH 4 production in anaerobic subglacial environments driven by the remineralization of organic matter (OM) contained in soils and forests covered during glacial margin fluctuations has been suggested as a potential carbon feedback to drive warming (Sharp et al., 1999;. Because the global warming potential of CH 4 is 25 times greater than CO 2 , even limited subglacial methanogenesis has the potential to strongly impact the greenhouse gas composition of glacial meltwater. Combined inorganic and organic subglacial processes may therefore produce glacial meltwater that is a source or sink of greenhouse gas. While the net impact of these processes on modern carbon fluxes is poorly constrained, determining these impacts will improve modern carbon budgets as well as depictions of how fluxes may have evolved during the advance and retreat of continental ice sheets. In subglacial environments where remineralization is limited by low OM availability, the major element solute load of glacial meltwater is typically dominated by products of mineral weathering reactions (Tranter, 2005). The extent of mineral weathering in subglacial environments depends in part on the availability of acids to drive reactions, namely sulfuric and carbonic acids (Table 1). Sulfuric acid is derived from the oxidation of reduced sulfur species, which largely occur as iron sulfide minerals including pyrite (Tranter, 2005). Sulfide oxidation may occur abiotically; however, the kinetics of microbially mediated sulfide oxidation is several orders of magnitude faster and may lead to local depletion of oxygen given a sufficient supply of sulfide minerals (Sharp et al., 1999). In contrast, carbonic acid may be derived from multiple external or in situ sources of CO 2 to the system. The dominant external source is supraglacial meltwater that flows to the subglacial system through moulins following equilibration with atmospheric CO 2 (Fig. 1). Unlike proglacial environments where free exchange between water and the atmosphere may resupply CO 2 consumed by weathering, subglacial environments may be partially or fully isolated from the atmosphere, limiting further atmospheric CO 2 invasion and thus the extent of mineral weathering with carbonic acid. However, additional atmospheric CO 2 may be delivered in open portions of the subglacial environment though exchange in fractures or moulins along subglacial flow paths or in partially air filled conduits, allowing a much greater magnitude of carbonic acid mineral weathering (Graly et al., 2017). CO 2 may also be derived from in situ sources, such as gaseous CO 2 contained in ice bubbles of basal ice or fluid inclusions in rocks that release volatiles (including CO 2 ) following mechanical grinding (Macdonald et al., 2018). When OM is available, its remineralization also generates CO 2 (and potentially CH 4 ) along with nutrients, but low OM availability in many subglacial systems limits remineralization as a CO 2 source (Fig. 1).
The role of subglacial carbon processes may play an increasingly important role in modern Arctic carbon budgets as disproportionate warming increases glacial meltwater and sediment fluxes to the ocean, particularly from the Green- Figure 1. Conceptual diagram of subglacial sources and sinks of CO 2 and CH 4 . Arrows indicate the direction of fluxes. Boxes represent processes, and sources of gases to subglacial meltwaters are indicated by blue text, while sinks of gases to subglacial meltwater are indicated by red text. Gas bubbles, mechanical grinding, and OM remineralization are grouped because all are CO 2 and CH 4 sources.
land Ice Sheet . The is the last remaining ice sheet in the Northern Hemisphere following collapse of other ice sheets since the Last Glacial Maximum (∼ 20 ka). It has been losing mass at increasing rates that averaged 286 ± 20 Gt/yr between 2010-2018, representing a 6fold increase since the 1980s (Mouginot et al., 2019). While mineral weathering significantly modifies the chemical composition of Greenland Ice Sheet subglacial discharge (e.g., Hindshaw et al., 2014;Deuerling et al., 2018;Urra et al., 2019) and should consume CO 2 similar to other glacial and proglacial environments, the recent identification of microbially driven reactions (including methanogenesis) in subglacial environments of the Greenland Ice Sheet indicates that organic processes may also play a role (Dieser et al., 2014;Lamarche-Gagnon et al., 2019). The relative importance of subglacial greenhouse gas sinks (CO 2 consumption through mineral weathering) and sources (such as OM remineralization) determines the greenhouse gas composition of subglacial discharge, which may then serve as a source or a sink of atmospheric greenhouse gases. Constraining the relative impacts and variability of these processes underneath the Greenland Ice Sheet will provide important information regarding the current and future impact of Greenland Ice Sheet loss on Arctic carbon budgets, as well the role of continental ice sheets on carbon cycle feedbacks.
To evaluate the net impact of carbon processes on the greenhouse gas composition of subglacial discharge of the Greenland Ice Sheet, we compare water chemistry, dissolved CO 2 and CH 4 concentrations, and gas stable isotopic compositions between three subglacial discharge sites draining land-terminating glaciers of the Greenland Ice Sheet over the melt seasons of 2017 and 2018 (Fig. 2). We employ mass balance models utilizing the concentrations of major cations and anions to determine the magnitude of the impact on CO 2 concentrations from mineral weathering reactions (Table 1). These results are combined with measured gas concentrations and δ 13 C to determine the relative importance of mineral weathering compared to OM remineralization on the CH 4 and CO 2 content of subglacial discharge. We also assess the temporal and spatial variability of these processes under the Greenland Ice Sheet to improve our understanding of carbon cycling in Greenland subglacial environments and the implications of Greenland Ice Sheet mass loss on Arctic carbon budgets.

Study locations
Our three subglacial discharge locations are located in southwest ( Fig. 2a The short stretch of river downstream of the confluence of the Akuliarusiarsuup Kuua and the main stem of the Qinnguata Kuussua near the town of Kangerlussuaq is also known as the Watson River (Fig. 2b). The majority of drainage from the Isunnguata Glacier drains to the northern Isortoq River (Fig. 2); however, a sub-catchment of the Isunnguata drains to a stream that directly feeds the Akuliarusiarsuup Kua, which we refer to as the Northern Tributary, while the Akuliarusiarsuup Kuua refers to river that flows between the outlet of the Russell Glacier and the confluence with the Qinnguata Kuussua (Fig. 2b) (Hawkings et al., 2016). Underlying lithologies differ between sites. Southwest Qinnguata Kuussua sites are located near the boundary between the Archean craton to the south and the southern Nagssugtoqidian Orogen to the north (Henriksen et al., 2009). The Archean block is composed of granites and granulite facies orthogneisses that were intruded by mafic dikes during Paleoproterozoic rifting. These rocks were deformed and modified during subsequent continent-to-continent collision in the Paleoproterozoic to create the amphibolite facies gneisses of the southern Nagssugtoqidian Orogen (van Gool et al., 2002). Kiattut Sermiat lies within the Paleoproterozoic Ketilidian fold belt (Henriksen et al., 2009). Lithologies in this region include the Julianehåb granite and associated basic intrusions and the sedimentary and volcanic rocks of the Mesoproterozoic Gardar Province that include a suite of alkaline igneous rocks and basaltic dikes with interbedded sandstones (Kalsbeek and Taylor, 1985;Upton et al., 2003).
Previous studies have characterized chemical weathering reactions in subglacial discharge to the Akuliarusiarsuup Kua and Qinnguata Kuussua (Deuerling et al., 2018;Hasholt et al., 2018;Yde et al., 2014), in the Kuusuaq that drains Kiattut Sermiat (Hawkings et al., 2016), and in comparison be- tween these two sites (Urra et al., 2019). There has been extensive work regarding ice sheet dynamics and hydrology in the southeast glaciers draining to the Akuliarusiarsuup Kua and Qinnguata Kuussua (Van As et al., 2017, 2018Lindbäck et al., 2015) as well as southern Kuusuaq catchments (Warren and Glasser, 1992;Winsor et al., 2014). Subglacial permafrost has been identified near the sub-Isunnguata site (Ruskeeniemi et al., 2018) and mostly likely formed during Holocene fluctuations in the ice sheet margin. While a similar Holocene ice retreat and re-advance may have occurred in southern Greenland (Larsen et al., 2016), it is unknown whether this retreat led to the organic deposits below the ice sheet.

Sample collection
We collected water samples from subglacial discharge sites in spring and fall of 2017 and the summer of 2018 to observe seasonal variations in water chemistry. To minimize atmospheric influence, samples were collected as close as possible to the glacier front where subglacial waters emerge, which was less than 10 m for the sub-Isunnguata site and approximately 100 m for the Russell Glacier site. Subglacial discharge from Kiattut Sermiat site flowed through a glacial meltwater lake prior to arriving at the sampling location, which was approximately 1.1 km from the glacial outlet ( Fig. 2) and therefore may experience a more interaction with atmospheric gases during transport from the subglacial discharge site to our sampling location. We collected water samples by pumping water through a 0.5 cm flexible PVC tube that was placed in flowing water as far as possible from shore (approximately 1-2 m). A YSI Pro Plus sensor that was calibrated daily was installed in an overflow cup filled from the bottom to measure specific conductivity (Sp.C), temperature, pH, dissolved oxygen, and oxidation-reduction potential (ORP). These parameters were monitored until stable, between about 10 and 30 min, after which samples were collected and preserved in the field according to the solute to be measured after being filtered through a 0.45 µm tracemetal grade Geotech high-capacity disposable canister filter. Samples for cations and anions were collected in HDPE bot-tles; cation samples were preserved with Optima-grade ultrapure nitric acid (pH < 2), while no preservative was added to anion samples. Dissolved inorganic carbon (DIC) samples were filtered through 0.2 µm filters directly to the bottom of 20 mL Qorpak glass vials and allowed to overflow until sealed tightly with no headspace.
Gas samples were collected in duplicate via headspace extractions according to methods outlined in Repo et al. (2007) and Pain et al. (2019b). Unfiltered water was pumped into the bottom of 500 mL bottles until they overflowed. Bottles were immediately capped with rubber stoppers fitted with two three-way inlet valves. A total of 60 mL of water was extracted from one inlet and replaced with 60 mL of atmospheric air (for spring and fall 2017 sampling trips) or ultrapure N 2 gas in a gas bag (summer 2018 sampling trip). Bottles were shaken for 2 min to equilibrate headspace gas with water, and headspace gas was extracted and immediately injected into 60 mL glass serum bottles that had been evacuated immediately prior to sample introduction. Samples were stored at room temperature until analysis, which occurred within 1 week of collection. Measured headspace concentrations were converted to dissolved concentrations using methods outlined in Pain et al. (2019b). When atmospheric air was used for headspace extractions, atmosphere samples were collected in tandem and analyzed to correct the calculated dissolved CO 2 and CH 4 concentrations and isotopic compositions for atmospheric CO 2 and CH 4 . This correction altered CH 4 concentrations by up to 22 % for one sample from the Russell Glacier, though less than 5 % for all other samples, and resulted in a correction of δ 13 C-CH 4 of up to 1.3 ‰. For CO 2 , the correction altered concentrations by up to 15 % for one sample collected at Kiattut Sermiat, though less than 10 % for all other samples, and resulted in a correction of δ 13 C-CO 2 of up to 0.4 ‰.
In samples collected in fall 2017 and summer 2018, alkalinity was measured in the field laboratory within 3 d of collection by titration with 0.01 N HCl using the Gran method. Because alkalinity measurements were not available for the spring 2017 sampling trip, we estimate alkalinity with PHREEQC modeling and the phreeqc.dat database (Parkhurst, 1997) using major cations and anions, pH, temperature, and DIC concentrations as model inputs.

Laboratory analysis
Gas samples were analyzed for CO 2 and CH 4 concentrations as well as δ 13 C-CO 2 and δ 13 C-CH 4 on a Picarro G2201-i cavity ring-down spectrometer in the field within a few days of collection. Carbon isotopic compositions are reported in reference to Vienna Pee Dee Belemnite (VPDB). Check standards of known CO 2 and CH 4 concentrations and isotopic compositions were measured during each sample run and were accurate within 10 %. Anion and cation concentrations were measured on an automated Dionex ICS-2100 and ICS-1600 ion chromatograph, respectively. Error on replicate analyses was less than 5 %. DIC concentrations were measured on a UIC (coulometrics) 5011 CO 2 coulometer coupled with an AutoMate Preparation Device. Samples were acidified, and the evolved CO 2 was carried through a silver nitrate scrubber to the coulometer where total C was measured. Accuracy was calculated to be ±0.1 mg/L based on measurement of check standards.

Methane modeling
To assess CH 4 sources and sinks, we calculate ε c , or the carbon isotopic fractionation factor between CO 2 and CH 4 as defined in Whiticar (1999): Values of ε c reflect methanogenesis pathways (acetoclastic or CO 2 reduction) as well as the extent of oxidation. Values of ε c between approximately 40 ‰ and 55 ‰ are produced for CH 4 generated via acetoclastic methanogenesis, while CO 2 reduction produces values between approximately 55 ‰ and 90 ‰. Lower values (ε c between 5 and 30) result when CH 4 oxidation predominates. Modern atmospheric input without additional alteration of CO 2 or CH 4 isotopic systematics results in a ε c value of approximately 40 (Whiticar, 1999). We calculated CH 4 oxidation using the isotopic method outlined in Mahieu et al. (2008) and Preuss et al. (2013). The fraction of oxidized methane (f ox ) in an open system is given by where δ E is the measured δ 13 C-CH 4 value for each water sample, δ P is δ 13 C-CH 4 of produced methane, α ox is the oxidation fractionation factor, and α trans is a fractionation factor resulting from diffusive transportation of CH 4 . While the exact value of δ P is unknown, diagenetic alteration of δ 13 C-CH 4 values through oxidation or transport only enrich δ 13 C-CH 4 signatures; therefore, the value of δ P is taken as the most depleted δ 13 C-CH 4 signature assuming it is the least impacted by diagenetic alteration. Literature-reported values for α ox range between 1.003 and 1.049. We calculate the fraction of oxidized methane with the largest fraction factor (α ox = 1.049; Mahieu et al., 2008), which yields the minimum amount of CH 4 oxidation required to explain the observed variations in δ 13 CH 4 and thus is a conservative estimate for CH 4 oxidation, and actual oxidation ratios may be higher. Literature-reported values for α trans vary from 1 for advection-dominated systems to 1.0178 for diffusiondominated porous media (de Visscher et al., 2004;Mahieu et al., 2008;Preuss et al., 2013). We assume that transport is advection dominated and thus assume α trans = 1; however, diffusive transport of CH 4 may result in fractionation of CH 4 in the subglacial environment and lead to relatively lower estimates of f ox . Because hydrologic connectivity between sub-glacial methanogenic meltwater pockets and drainage features is not well described, the relative importance of advective compared to diffusive CH 4 transport within the subglacial drainage system is not well understood; however, it is presumed to be an advection-dominated system in which expanding drainage networks access and drain methanogenic meltwater pockets throughout the melt season.

Mineral weathering and carbonate modeling
We used major cation and anion concentrations and alkalinity to partition solutes into the four mineral weathering reactions in Table 1 after correcting solute concentrations for marine aerosol deposition using measured chloride concentrations and standard seawater element ratios. The mass balance model followed the methods of Deuerling et al. (2019).
After apportioning solutes to mineral weathering reactions, we used the stoichiometries of reactions to calculate the impact of each reaction on dissolved CO 2 concentrations (Table 1). The mineral weathering model apportions solutes to reactions in Table 1 based on the ratios of Ca/Na and Mg/Na in silicate minerals in stream bed load samples, which were taken to be 0.54 and 0.38, respectively, for sub-Isunnguata and Russell Glacier samples (Deuerling et al., 2019;Hindshaw et al., 2014;Wimpenny et al., 2010Wimpenny et al., , 2011 and 0.39 and 0.27, respectively, for Kiattut Sermiat samples (Da Prat and Martin, 2019). Because mineral weathering reactions may both add and remove CO 2 , we discuss both the net impact of mineral weathering on CO 2 concentrations (Net CO 2-MW ), which may have a positive or negative value, as well as the total impact of mineral weathering on CO 2 concentrations (Total CO 2-MW ), where changes in the concentrations of CO 2 are defined by their absolute values. To discuss the relative importance of individual reactions, we define proportional contributions of each reaction as follows: We combine measured CO 2 concentrations with Net CO 2-MW in order to determine the magnitude of CO 2 production or consumption in the subglacial environment due to processes besides mineral weathering. This analysis assumes that the concentration of CO 2 measured at the subglacial outlet is equivalent to the net change in CO 2 due to mineral weathering plus the sum of all other subglacial CO 2 sources and sinks. We refer to the sum of all other subglacial CO 2 sources and sinks as CO 2-total , which represents the amount of CO 2 that must have been supplied to the subglacial environment to balance the mineral weathering CO 2 sink: The sources of CO 2 to CO 2-total may be evaluated through the use of Keeling plots, which are constructed as the inverse of CO 2 concentrations ([CO 2 ] −1 ) versus stable isotopic composition (δ 13 C-CO 2 ). If variations in the concentration and isotopic composition of CO 2 arise from the mixing of two CO 2 reservoirs with constant isotopic compositions and concentrations (Keeling, 1958), a linear relationship is expected between [CO 2 ] −1 and δ 13 C-CO 2 . The y intercept of a regression between these variables represents the isotopic composition of the high-CO 2 end-member. Because measured CO 2 concentrations include both subglacial CO 2 sources and sinks, which may include considerable consumption through mineral weathering reactions, the magnitude of the total subglacial CO 2 source is taken as CO 2-total . We therefore construct Keeling plots between [CO 2-total ] −1 and measured δ 13 C-CO 2 values because while mineral weathering impacts the concentration of CO 2 , its isotopic composition is not appreciably altered (Myrttinen et al., 2012) compared to the range of isotopic compositions of potential CO 2 end-members, namely OM remineralization, atmospheric CO 2 , and lithogenic CO 2 sources due to mechanical grinding (Fig. 1).

Discharge relationships
We evaluate the relationship between subglacial CH 4 and CO 2 dynamics and glacial meltwater river discharge records collected downstream of the sub-Isunnguata and upstream of the Russell sampling sites. Proglacial river discharge was collected in the Akuliarusiarsuup Kuua (AK) River at the AK4 site, 2 km downstream of the sub-Isunnguata sampling site (Fig. 2b). The river discharge dataset is an updated and extended version of Rennermalm et al. (2012) using reference and regression models to correct Solinst level logger drift in water stage (Solinst, 2017) and a total of 57 discharge measurements to convert continuous water stage to discharge. The standard uncertainty (i.e., the 68th percent confidence interval or 1 standard deviation) was determined to be 17 % using methods and recommendations provided by Herschy (1999), ISO Guide 98-3 (2008), and WMO (2010). Because diurnal fluctuations in river discharge can be large, and differing water travel times from subglacial outlet sites to the discharge monitoring site induces a lag between maximum daily discharge at subglacial discharge sites and the AK4 site outlet, we compare subglacial CH 4 and CO 2 concentrations to average daily discharge, calculated as the average of hourly discharge estimates over the days on which subglacial discharge water samples were collected. We use the AK4 site discharge records for evaluating concentrationdischarge relationships for both sub-Isunnguata and Russell sites. Although this site is upstream of the Russell Glacier, its close proximity to the Russell Glacier suggests it is more likely to reflect local melting patterns similar to those that would be controlling discharge dynamics at the Russell than discharge records collected at the Watson River outlet. While Watson River discharge records are also available through PROMICE , which includes some contributions from the Russell Glacier, the Watson River includes discharge from the Akuliarusiarsuup Kuua (draining sub-Isunnguata, Russell, and Leverett catchments) as well as the much larger Qinnguata Kuussua catchment, and therefore Watson River discharge records are not likely to be representative of the temporal changes in the magnitude and variability of discharge from the much smaller Russell Glacier catchment.

Temporal variability in water chemistry and gas concentrations
Chemical parameters differ spatially between subglacial discharge sites as well as temporally through the 2017-2018 melt seasons. Comparing the means and standard deviations of water samples collected throughout 2017 and 2018, specific conductivity (Sp.C; Fig. 3a) is typically highest at Kiattut Sermiat (26 ± 8 µS/cm), followed by Russell (22 ± 5 µS/cm) and sub-Isunnguata sites (13 ± 9 µS/cm; Fig. 3a). All sites show variability over time, with lowest values occurring in the summer for sub-Isunnguata and Russell, while Sp.C drops continuously with days of the year for Kiattut Sermiat. Sites differ in pH, and average values at Kiattut Sermiat (8.2 ± 0.4) are higher than both Russell (7.2 ± 0.2) and sub-Isunnguata (6.6 ± 0.6; Fig. 3b), and while values vary over time, no consistent trend is identified between sites. The saturation of dissolved oxygen (D.O.) with respect to atmospheric concentrations is similar between sites, though sub-Isunnguata (98 ± 8 %) values fall below Russell (115 ± 16 %) and Kiattut Sermiat (117 ± 11 %) during all sampling times and exhibit undersaturation in the mid-summer samples, while Russell and Kiattut Sermiat are consistently supersaturated (Fig. 3c).

Methane oxidation and relationship with discharge
Values of ε c are similar over time for sub-Isunnguata (38 ± 10 ‰) and Russell (38 ± 9 ‰) and are relatively higher in the summer sampling period, while Kiattut Sermiat ε c values are higher on average (42 ± 13 ‰) with lowest values in the summer (Fig. 4a). Estimates of f ox are similar between sub-Isunnguata (17 ± 15 %), Russell (23 ± 15 %), and Kiattut Sermiat sites (25 ± 22 %; Fig. 4b). However, f ox values are higher in the spring and fall sampling times compared to summer for sub-Isunnguata and Russell and approach 50 % in the spring, while Kiattut Sermiat values decrease throughout the melt season. CH 4 concentrations, δ 13 C-CH 4 values, and f ox are weakly negatively correlated to average daily discharge for both sub-Isunnguata and Russell sites (Fig. 5a, b and d), while ε c is weakly positively correlated with discharge for both sub-Isunnguata and Russell (Fig. 5c).
For both sub-Isunnguata and Russell sites, average daily discharge is negatively correlated with CO 2 concentrations (Fig. 8a), while it is positively correlated with δ 13 C-CO 2 (Fig. 8b).
Keeling plots between [CO 2-total ] −1 and δ 13 C-CO 2 for each site indicate no linear relationship for Russell or Kiattut Sermiat samples; however, a strong linear correlation is observed for sub-Isunnguata (r 2 = 0.99; p < 0.001) samples  with the removal of one outlier, which also had the lowest CO 2-total value (Fig. 9).

Discussion
We observe orders of magnitude variability in dissolved CH 4 and CO 2 concentrations in subglacial discharge of the Greenland Ice Sheet, indicating significant differences in the magnitudes of the sources and sinks of these gases across time and space. Supersaturation of both CO 2 and CH 4 with respect to atmospheric concentrations indicates that sub-Isunnguata discharge is a source of both gases to the atmosphere, the neighboring Russell Glacier discharges water that is a source of CH 4 but near equilibrium with respect to CO 2 , and Kiattut Sermiat in southern Greenland is a sink of atmospheric CO 2 but near equilibrium with respect to CH 4 ( Fig. 3e, g). Because CH 4 dynamics may be largely microbially driven, while CO 2 dynamics include microbial as well as abiotic mineral weathering processes, we first discuss CH 4 dynamics including a comparison of concentrations, isotopic compositions, and extent of oxidation between sites and over the melt season. We then discuss CO 2 concentrations, impacts of mineral weathering reactions (Table 1), and an assessment of subglacial CO 2 sources, including OM remineralization. These assessments will contribute to our understanding of the variability and controls of CH 4 and CO 2 concentrations in subglacial discharge from the Greenland Ice Sheet and may improve predictions of the impact of future ice melt on Arctic carbon budgets.

Sources and sinks of CH 4
Differences in CH 4 concentrations and relationships with discharge between sites imply heterogeneity in both the extent and controls of subglacial methanogenesis under the  (Fig. 2b). However, CH 4 concentrations are near atmospheric equilibrium for the Kiattut Sermiat site (Fig. 3e). Because methanogenesis is an anaerobic OM remineralization pathway, it is more likely to occur in subglacial environments isolated from atmospheric O 2 sources. Widespread observations of methanogenesis in glacial meltwater of southwest Greenland from this and other studies (Christiansen and Jørgensen, 2018;Dieser et al., 2014;Lamarche-Gagnon et al., 2019), and limited observations of CH 4 in subglacial discharge in southern Greenland, suggests heterogeneity in subglacial conditions that support methanogenesis. Methanogenesis fueled by organic material overridden during ice sheet growth has been suggested as a potential climate feedback over glacial interglacial timescales  and may contribute to observed variations in CH 4 concentrations. Subglacial methane concentrations may additionally be controlled by hydrologic factors as the subglacial hydrological network develops throughout the melt season and channelization of meltwater conduits increases subglacial drainage efficiency (Andrews et al., 2015;Cowton et al., 2013). Drainage efficiency impacts both subglacial water residence time as well the transport of aerobic supraglacial meltwater to the ice bed. Both residence time and oxygen delivery may impact subglacial redox status and methanogenesis potential and favor methanogenesis when oxygen supply rates are low compared to OM remineralization rates. This condition is most likely to be met in distributed subglacial systems that are hydrologically isolated with limited inputs from aerobic supraglacial meltwater. Such a hydrologic control on methanogenesis is supported by the negative correlation between CH 4 concentrations and average daily discharge at both sites (Fig. 5a). This correlation would suggest that either CH 4 production occurs predominantly during periods of low discharge and greater residence time or higher discharge results in the dilution of a CH 4 -laden subglacial water source. While both mechanisms would lead to a similar relationship between discharge and CH 4 concentrations, they carry different implications for subglacial methanogenesis. If limited by residence time, a hydrologic link between glacial hydrology and subglacial biogeochemistry would be established because supraglacial discharges deliver terminal electron acceptors to the ice bed and would limit methanogenesis. If predominantly controlled by dilution, however, active methanogenesis would not be required -only the existence of meltwater pockets containing CH 4 that may or may not have been recently produced. Further discussion of these mechanisms is outside the scope of this study. While we have limited data to make further inferences about hydrologic controls of methanogenesis, the presence of several outliers at the sub-Isunnguata site in particular (Fig. 5a) highlights the possibility for additional controls including stochastic drainage events or heterogeneity in subglacial CH 4 concentrations that result in variability in the relationship between concentra-tion and discharge, as was observed in Lamarche-Gagnon et al. (2019).
Our results suggest heterogeneity in the extent of methanogenesis between outlet glaciers but homogeneity of the microbial methanogenesis pathway as well as CH 4 oxidation dynamics between sites. Methanogenesis pathways may be evaluated by δ 13 C-CH 4 as well as ε c values because they impart distinct δ 13 C signatures to CH 4 and CO 2 (Whiticar and Schoell, 1986). Dieser et al. (2014) measured a microbial δ 13 C-CH 4 production signal at the Russell Glacier with values between −63 ‰ and −64 ‰, which was interpreted to reflect a possible combination of CH 4 produced through both acetoclastic and CO 2 reduction pathways. The most depleted δ 13 C-CH 4 value measured at the sub-Isunnguata in this study was −62.7 ‰, close to values measured by Dieser et al. (2014) (Fig. 3f) and similar to values reported by Lamarche-Gagnon et al. (2019) for the Leverett Glacier. The similar isotopic ratio between our samples and that measured in active methanogenic communities could indicate that similar methanogenesis pathways occur across this region or that the δ 13 C-CH 4 of stored subglacial CH 4 has not been fractionated by oxidation or transport in the peak melt season when we observe these depleted δ 13 C-CH 4 values.
While the exact contributions from each methanogenesis pathway cannot be inferred from isotopic information alone, the range of ε c values at outlet glaciers are consistent with predominantly acetoclastic methanogenesis during the peak melt season (Fig. 4a). However, ε c values fall below the expected range from acetoclastic methanogenesis during the early and late melt seasons and may result from variations in the extent of subglacial CH 4 oxidation. Seasonal variation in CH 4 oxidation is supported by consistency between ε c and f ox values, which both indicate the greatest impact of oxidation (approaching 50 %) in the early melt season compared to peak melt season (Fig. 4a, b), with additional evidence of elevated CH 4 oxidation in the late melt season at both sub-Isunnguata and Russell sites. Because our water sampling locations were slightly downstream of glacial discharge outlets, there is also the possibility that outgassing in between the outlet and our sampling location reduced dissolved CH 4 concentrations and led to more enriched isotopic compositions of remaining dissolved CH 4 . It is likely that some outgassing did occur; however, it is unlikely that the extent of outgassing between the glacial outlet and our sampling location would vary significantly between sampling times, and thus outgassing would not fully explain temporal differences in concentration, δ 13 C-CH 4 , f ox , or ε c . While our measured gas concentrations and isotopic compositions likely reflect some modification of CH 4 and CO 2 isotopic compositions due to outgassing, the differences over time are more likely due to changes in subglacial CH 4 dynamics than outgassing.
The extent of CH 4 oxidation may be controlled by multiple factors including oxygen availability, subglacial residences time, and subglacial hydrology, similar to methanogenesis. A hydrologic control of CH 4 oxidation is supported by relationships between f ox and ε c with average daily discharge (collected at site AK4; Fig. 2b) at both sub-Isunnguata and Russell sites: f ox is negatively correlated with discharge for both sites (Fig. 5b) while ε c is positively correlated with discharge (Fig. 5c). While weak, these correlations suggest that CH 4 oxidation is greatest during periods of low flow, which may be associated with longer residence times to allow subglacial CH 4 oxidation; however, this relationship could also result from differences in CH 4 sources throughout the melt season as the subglacial drainage network expands. Assuming the former, the delivery of oxygen to the subsurface by supraglacial melting does not appear to be a limiting factor in subglacial CH 4 oxidation, which should increase f ox as more oxygenated supraglacial water is delivered to the subglacial system. Instead, the observed greater CH 4 oxidation during periods of low discharge may reflect mixing of methane-rich subglacial meltwater pockets and aerobic subglacial meltwater leading to CH 4 oxidation. Longer transit times during periods of low flow may allow more subglacial methane oxidation to occur than during peak discharge, when the development of channelized flow paths reduces meltwater residence time in the subglacial environment.
Our results indicate a high degree of heterogeneity in subglacial methanogenesis under the Greenland Ice Sheet, as well as a significant impact of CH 4 oxidation, which serves to reduce atmospheric CH 4 fluxes. Given the observed spatial and temporal heterogeneity of CH 4 concentrations and processes, further investigation of the spatial variability in outlet glacier CH 4 concentrations is needed to determine the impact of Greenland Ice Sheet loss on Arctic and global CH 4 budgets, while a better understanding of the controls of these differences will improve models of how CH 4 fluxes from subglacial discharge will change with continued warming.

Sources and sinks of CO 2
Dissolved CO 2 concentrations in subglacial discharge are consistently supersaturated with respect to atmospheric concentrations at the sub-Isunnguata site, near atmospheric equilibrium at the Russell Glacier, and undersaturated at Kiattut Sermiat, indicating that glacial meltwater from the Greenland Ice Sheet can serve as either a source or sink of CO 2 to the atmosphere. Similar to CH 4 , differences in dissolved CO 2 dynamics (Fig. 3g) imply variability in carbon processes under the Greenland Ice Sheet. We first discuss potential subglacial CO 2 sources, including OM remineralization, followed by a discussion of CO 2 consumption due to mineral weathering.

Subglacial CO 2 sources
There are many potential sources of CO 2 in the subglacial environment including dissolution of atmospheric gases in air-filled conduits or fractures in ice, CO 2 contained in ice bubbles ( Fig. 1; Anklin et al., 1995;Graly et al., 2017), mechanical grinding and volatilization of fluid inclusions in bedrock (Macdonald et al., 2018), and OM remineralization. While previous studies have indicated that additional atmospheric CO 2 input through fractures and air-filled conduits may supply sufficient CO 2 to drive mineral weathering observed in many subglacial environments, including several sites in Greenland (Graly et al., 2017), CO 2 is also a product of OM remineralization, which is believed to account for CH 4 concentrations elevated above atmospheric equilibrium at the two southwest sites in this study. Both CO 2 and CH 4 exhibit negative correlations with average daily discharge for both sub-Isunnguata and Russell sites and could suggest a common OM remineralization source (Fig. 8a). While the magnitude of this source and its relative importance compared to other subglacial CO 2 sources is currently unknown, differing sources of carbonic acid for mineral weathering reactions carry different implications for subglacial CO 2 budgets. For instance, carbonic acid weathering driven by invasion of atmospheric CO 2 would represent a sink of atmospheric CO 2 , but carbonic acid weathering driven by OM remineralization would instead serve to consume CO 2 from in situ sources and limit the potential for subglacial meltwater to be an atmospheric CO 2 source once discharged from the glacier. Determining the sources of carbonic acid to subglacial weathering reactions is therefore critical to understand the controls of mineral weathering in subglacial environments and its role in atmospheric CO 2 sequestration.
Comparisons between measured δ 13 C-CO 2 in subglacial discharge samples and likely δ 13 C-CO 2 values of CO 2 sources indicate that CO 2 sources differ between sites, with OM remineralization as the most important CO 2 source at the sub-Isunnguata but likely not the predominant or sole source at Russell or Kiattut Sermiat glaciers. Keeling plots of [CO 2-total ] −1 versus δ 13 C-CO 2 indicate that CO 2-total may be represented by a two-end-member mixing model for sub- Figure 9. Keeling plot indicating correlations between the magnitude of CO 2-total and δ 13 C-CO 2 and for sub-Isunnguata (sub-IS), Russell (RU), and Kiattut Sermiat (KS) samples. Asterisk denotes the outlier not included in the regression between CO 2-total and δ 13 C-CO 2 for sub-Isunnguata samples. The plotted regression line was constructed using the sub-IS samples only.
Isunnguata discharge but not for discharge from the Russell and Kiattut Sermiat glaciers (Fig. 9). Mixing model endmembers include a 13 C-enriched, lower-concentration CO 2 source and a 13 C-depleted, higher-concentration CO 2 source. The y intercept of the regression between [CO 2-total ] −1 versus δ 13 C-CO 2 , which represents the isotopic signature of the high-CO 2 end-member, is −27.4 ‰. This value is close to what would be expected from OM remineralization as indicated by remineralized OM in Greenlandic heath soils that produced δ 13 C-CO 2 of approximately −27‰ to −25 ‰ (Ravn et al., 2020) and thawed Alaskan permafrost soils that produced δ 13 C-CO 2 of between −20 ‰ to −30 ‰ (Mauritz et al., 2019), both of which may be similar to subglacial organic matter. The low-CO 2 end-member could reflect atmospheric CO 2 input, with an associated δ 13 C-CO 2 value of approximately −8 ‰. While the δ 13 C-CO 2 value of the lowest-CO 2-total samples in the sub-Isunnguata Keeling plot (e.g., highest [CO 2-total ] −1 not including the outlier) is slightly depleted compared to atmospheric values at −12.1 ‰, even the lowest CO 2 concentrations measured at sub-Isunnguata are supersaturated with respect to atmospheric concentrations (Fig. 3g). Consistent CO 2 supersaturation suggests that OM remineralization contributes CO 2 even for low-CO 2 -concentration samples and isotopically depletes the subglacial CO 2 reservoir.
While δ 13 C-CO 2 values of Russell and Kiattut Sermiat samples are within the range of sub-Isunnguata samples, suggesting possible contributions of CO 2-atm and CO 2-OM , scatter in the Keeling plots indicates variability in the CO 2 concentration and/or isotopic composition of end-members, or significant contributions of at least one other major subglacial CO 2 source. We address both possibilities here. While atmospheric CO 2 concentrations and δ 13 C values should be relatively invariable, CO 2-OM may vary both in concentration and isotopic composition, depending on variability in the quantity and composition of organic deposits as well as remineralization rates. For instance, if remineralization largely occurs in hydrologically isolated subglacial meltwater pockets, some variability in the concentration and δ 13 C-CO 2 of CO 2-OM is likely. While no data yet exist to characterize the variability in subglacial OM reservoirs, variability in either concentration or isotopic composition of CO 2-OM could plausibly result in the scatter shown in Fig. 9. Additional subglacial CO 2 sources from ice bubbles, or lithogenic CO 2 liberated by mechanical grinding, would be expected to enrich rather than deplete the δ 13 C-CO 2 values of the samples relative to modern atmospheric δ 13 C-CO 2 values. Ice bubbles contain gaseous CO 2 at concentrations and isotopic compositions reflecting atmospheric conditions during ice formation. While heterogeneity may result from gas bubbles recording changes in atmospheric CO 2 , variability in δ 13 C-CO 2 of gas bubble CO 2 should be only a few per mill, which is small compared to the variation observed in Russell and Kiattut Sermiat samples (e.g., Tipple et al., 2010;Fig. 9). Gas bubble CO 2 should also be 13 C-enriched compared to modern atmospheric CO 2 due to fossil fuel contributions and thus would be unlikely to cause the observed depletion of 13 C in the subglacial water.
Recent work has also highlighted the potential for subglacial mechanical grinding to result in CO 2 production through the volatilization of bedrock fluid inclusions (Macdonald et al., 2018). While this process was found to produce sufficient CO 2 to drive approximately 20 % of mineral weathering in Svalbard subglacial environments, the expected isotopic composition of lithogenic CO 2 is more 13 Cenriched than our measured δ 13 C-CO 2 values. For instance, estimates of δ 13 C for bulk hydrocarbons in fluid inclusions in the Ilímaussaq alkaline complex of South Greenland have values of −4.5 ± 1.5 ‰ (Madsen, 2001), which is close to the δ 13 C-CO 2 of CO 2 in fluid inclusions in the Bamble granulite sector of South Norway (∼ −6 ‰; Newton et al., 1980). Because mechanical grinding should not fractionate the δ 13 C-CO 2 values (Lüders et al., 2012), our low δ 13 C-CO 2 values suggest this source is small relative to other sources.
One additional source or sink of CO 2 to some of our samples is atmospheric exchange as water flows from the subglacial outlet site to our sampling sites. However, atmospheric CO 2 exchange after discharge would have the same impact on Keeling plots as atmospheric CO 2 exchange prior to discharge. Incorporation of an atmospheric source between the outlet and sample site would be most likely at Kiattut Sermiat, where CO 2 concentrations are undersaturated with respect to atmospheric concentrations, which would promote invasion of atmospheric CO 2 . However, the measured δ 13 C-CO 2 values are more 13 C-depleted than modern atmospheric CO 2 and are not consistent with atmospheric CO 2 as the sole or dominant source of CO 2 to these samples (Fig. 9).
While more information is needed to determine all possible sources of CO 2 to Russell and Kiattut Sermiat samples, δ 13 C-CO 2 values of samples from both sites imply mixing between a 13 C-depleted CO 2 source, such as OM remineralization, and one or more 13 C-enriched CO 2 sources, such as atmospheric or lithogenic CO 2 . Similar to CH 4 , concentrations and isotopic compositions of gases may be impacted by atmospheric exchange between the glacial outlet and our sampling locations, which would alter dissolved CO 2 concentrations and δ 13 C-CO 2 compositions to values more similar to atmospheric values. Therefore, we are unable to distinguish the impacts of the atmospheric exchange that occurs prior to discharge from the exchange that occurs between discharge and our sampling locations. However, the impact of this exchange should be relatively constant between sampling times and sampling locations; therefore, outgassing would not account for temporal or spatial variability in CO 2 concentrations or isotopic compositions between sites.

Subglacial CO 2 sink: mineral weathering reactions
Mineral weathering leads to net CO 2 consumption in all subglacial discharge samples (Fig. 6), and thus the measured CO 2 concentrations at glacial outlets represent only a fraction of the total CO 2 that would have been present in the absence of mineral weathering reactions (CO 2-total ; Eq. 10). Net consumption occurs because the CO 2 source from Carb SA is ubiquitously low compared to sinks from either Carb CA or Sil CA (Fig. 6b). The range in Net CO 2-MW is similar between subglacial discharge sites (between 10-150 µM; Fig. 6a), but average values increase from Kiattut Sermiat to Russell to sub-Isunnguata, likely reflecting the relative weatherability of alkaline igneous rocks, granulite facies gneisses, and amphibolite facies gneisses. Kiattut Sermiat is characterized by a relatively high proportion of Carb CA compared to sub-Isunnguata and Russell sites, which may arise from the presence of trace carbonates in abundant and readily weatherable basaltic intrusions as has been implicated in other studies (Urra et al., 2019). The relatively greater influence of carbonate dissolution compared to silicate dissolution on Total CO 2-MW at Kiattut Sermiat may also relate to more rapid dissolution kinetics of carbonates, which allow carbonate dissolution to have a large influence on major cation and anion load even when carbonates are only present in trace amounts (Deuerling et al., 2019;Tranter, 2005). At sub-Isunnguata and Russell sites, Sil CA has a greater influence than Carb CA on Total CO 2-MW , which could result from either a lower abundance of trace carbonates to participate in weathering reactions or relatively longer subglacial residence times that would allow a greater accumulation of silicate weathering products.
Despite the impact of Carb CA on Total CO 2-MW at Kiattut Sermiat compared to sub-Isunnguata and Russell sites, Carb SA is notably lower at Kiattut Sermiat than other sites and suggests a limited role for sulfuric acid weathering that may relate to subglacial sulfide oxidation dynamics. Lower abundances of sulfide minerals in the subglacial environment may limit the production of sulfuric acid and could result from differences in lithology between sites, the depletion of sulfide minerals due to prior weathering (Graly et al., 2014), or weathering occurring in anoxic environments that limit the oxidation of sulfide to sulfuric acid (Deuerling et al., 2019). The kinetics of sulfide oxidation may also significantly differ between sites depending on the relative contributions of abiotic compared to microbially mediated sulfide oxidation, as microbially mediated sulfide oxidation is several orders of magnitude faster than abiotic sulfide oxidation (Boyd et al., 2014;Harrold et al., 2016). Rapid microbially mediated sulfide oxidation has been implicated in the development of anaerobic conditions, which could also support subglacial methanogenesis (Sharp et al., 1999;Stibal et al., 2012;Wadham et al., 2010). Observations of higher CH 4 concentrations as well as higher contributions of Carb SA at sub-Isunnguata and Russell compared to Kiattut Sermiat may therefore be linked to subglacial microbial activity, which is known to vary based on factors such as the presence of organic and fine-grained rock flour to serve as growth substrates, insulation from fluctuations in temperature, and delivery of nutrients and organic matter from supraglacial sources (Sharp et al., 1999). If microbially driven, our results suggest possible linkages between microbial processes and subglacial mineral weathering regimes, with significant impacts to both CH 4 and CO 2 dynamics due to the role of Carb SA as a CO 2 source (Table 1).
Whether controlled by geochemical, microbial, or mechanical processes, the relationships between CO 2 concentrations and δ 13 C-CO 2 and average daily discharge are similar between sub-Isunnguata and Russell sites (Fig. 8). These similarities suggest that underlying controls in carbonate chemistry may be consistent between sites despite the heterogeneity in measured dissolved CO 2 concentrations. For both sub-Isunnguata and Russell sites, mineral weathering reactions consume CO 2 , which implies contributions from in situ CO 2 sources (such as atmospheric CO 2 invasion or OM remineralization) to produce measured CO 2 concentrations. The different CO 2 concentrations observed between sites therefore appear to result from the strength of in situ CO 2 sources relative to CO 2-MW , both of which impart the greatest chemical change at times of low discharge and high subglacial residence time. At Kiattut Sermiat, where measured CO 2 concentrations are lowest, the magnitude of in situ sources is insufficient to maintain atmospheric equilibrium, leading subglacial discharge to be a sink of atmospheric CO 2 , while CO 2-total maintains close to atmospheric equilibrium concentrations at the Russell Glacier. At sub-Isunnguata, however, OM remineralization produces more CO 2 than is consumed by mineral weathering and causes meltwater to be a source of CO 2 to the atmosphere. This finding implies that subglacial mineral weathering serves to partially or fully consume CO 2 produced from in situ sources under the Greenland Ice Sheet but does not always serve to directly consume modern atmospheric CO 2 .

Conclusions
Subglacial reactions impact the concentrations of CO 2 and CH 4 in subglacial discharge of the Greenland Ice Sheet, and differences in the relative magnitudes of microbial and geochemical processes result in a high degree of previously unrecognized heterogeneity between glacial discharge sites of the Greenland Ice Sheet. Our results imply a significant role of OM remineralization in driving this heterogeneity and leading to CO 2 and CH 4 supersaturation at the sub-Isunnguata site and CH 4 supersaturation at the Russell site. Heterogeneity may result in significant uncertainty in total greenhouse gas flux estimates from subglacial systems of the Greenland Ice Sheet, which will be an increasingly important carbon flux as the Arctic warms in the coming decades. While heterogeneous, the uncertainty in greenhouse gas fluxes from Greenland Ice Sheet meltwater may be reduced by a better understanding of the controls and variability of the weathering reactions and microbial processes driving heterogeneous gas concentrations. Such a processbased understanding could also improve estimates of the impact of greenhouse gas variability associated with the growth and retreat of continental ice sheets over glacial-interglacial timescales. Subglacial OM remineralization further implies the existence of links between subglacial OM deposits and export of other biogeochemical solutes from the Greenland Ice Sheet, including nutrients as well as redox-sensitive elements. While the export of nutrients from the Greenland Ice Sheet has been the focus of numerous studies (Bhatia et al., 2013;Hawkings et al., 2016;Lawson et al., 2014), little is currently known regarding the role of OM sources in governing these exports. Given the variability in greenhouse gas concentrations observed in this study, constraining the extent of heterogeneity in outlet glaciers of the Greenland Ice Sheet as well as the biogeochemical, hydrologic, and geologic controls of this heterogeneity will be important for upscaling atmospheric fluxes as well as efforts to predict impacts of ice loss on carbon and nutrient budgets due to current and future melting of the Greenland Ice Sheet.
Author contributions. JBM and EEM participated in conceptualization, data collection, data interpretation, reviewing and editing the manuscript, and acquired the funding for this project. AR provided the discharge data that were used in analyses of dischargeconcentration relationships. SR participated in data collection and early interpretation and presentation of results. AJP participated in conceptualization, data collection, analysis, and interpretation, and took the lead on writing the manuscript with contributions from JBM, EEM, and AR.