Joyce Glacier (67∘06′ S, 50∘09′ W; 90 km2) is situated in the Garwood Valley, Antarctica. A large proglacial lake, dammed by an ice sheet grounded in the McMurdo Sound > 23 000 14C yr BP (Péwé, 1960; Hendy, 2000), is thought to have previously occupied the valley (Hendy, 2000). Joyce Glacier is cold-based, meaning that it is completely frozen to the underlying substrate. The bedrock lithology includes dolomite, granite, and metamorphic rocks. Joyce Glacier recently advanced over lake sediment (Stuiver et al., 1981), and hence the basal material is thought to contain labile OC and algal-derived organic matter of Holocene age.

Russell Glacier (67∘03′ N, 50∘10′ W; > 600 km2), situated on the west margin of the GrIS, is polythermal-based. Warm ice, with a temperature at the pressure melting point, in the interior is surrounded by a frozen layer beneath the thinner ice of the margins. Surface melting delivers supraglacial meltwater to the subglacial system from the onset of the spring thaw. The bedrock is predominantly Archaean gneiss (Escher and Watt, 1976). Basal debris contains overridden Quaternary deposits (including palaeosols) and relatively fresh organic matter (Knight et al., 2002), which was buried during the Holocene (Simpson et al., 2009).

Finsterwalderbreen (77∘28′ N, 15∘18′ E; 44 km2) is located on the southern side of Van Keulenfjorden, southern Svalbard, and is a polythermal surge-type glacier. Finsterwalderbreen last surged between 1898 and 1920 (Liestøl, 1999), which may have influenced the formation of the basal ice as has been shown, for example, at Variegated Glacier (Sharp et al., 1994). The glacier is currently retreating at a rate of 10–45 m a-1 (Wadham et al., 2007). The bedrock consists of Precambrian carbonates, sandstones, limestones, and shales (Dallmann et al., 1990). Shales exposed to water may provide a steady source of DOC (Schillawski and Petsch, 2008). The shale beneath Finsterwalderbreen contains up to 2.3 % OC (Wadham et al., 2004).

Engabreen (66∘41′ N, 13∘46′ E; 40 km2) is temperate and part of the western Svartisen ice cap, northern Norway. Engabreen bedrock consists mostly of schist and gneiss, with calcite-filled cracks (Jansson et al., 1996), and contains relatively little OC. A combination of in-washed material from the glacier surface and overridden soils of Holocene age may be the principal OC sources (Stibal et al., 2012).

Joyce Glacier basal ice samples were collected in the austral summer of 2010 from recently exposed, upthrust bands of debris-rich basal ice at the margin on the southern flank of the glacier. Basal ice was sampled where the facies were composed of frozen debris and only weakly exhibited layers that were > 1 mm thick but < 1 m thick, classified as solid banded basal ice (Hubbard et al., 2009). We assume that the basal ice was formed under cold-based conditions.

Debris-rich basal ice blocks from the Russell Glacier margin were collected in spring 2008. Samples were collected from the southern corner of the glacier where it has previous advanced into a dune and from within 1.5 m of the ice–bed contact. The basal ice samples contained subglacial sediment that had been extruded up from the glacier bed via fissures near the terminus. This comprised banded basal ice where the debris was generally restricted to narrow sediment layers and large vein networks were clearly evident. As Russell Glacier is polythermal, we assume that the basal ice was formed by a combination of regelation and cold-based processes such as basal adfreezing onto the glacier sole.

Finsterwalderbreen was sampled in autumn 2008. Basal ice blocks were collected from the terminus on the northern flank of the glacier from sections of dispersed banded basal ice (referred to as DB basal ice), within 1.5 m of the ice–bed contact, and from surface outcrops of frozen subglacial material, or thrust bands, with distinct debris layers (referred to as solid banded (SB) basal ice). It is probable that the thrust bands were formed during the most recent surge during two phases of thrusting: primary thrusting during the early surge phase in the subglacial zone between temperate ice and cold ice, and secondary thrusting during the surge termination due to ice flow compression, as envisaged for the similar polythermal surge-type Kuannersuit Glacier (Larsen et al., 2010). Finsterwalderbreen DB and SB basal ice are reported separately due to the very different mean debris concentrations (by mass): 20±27 % (DB basal ice) and 86±6 % (SB basal ice, where the debris component was much higher). As Finsterwalderbreen is polythermal, we assume that the basal ice was formed by a combination of regelation and cold-based processes. Basal ice samples from Finsterwalderbreen and Russell Glacier were collected from the same sites as the samples that were analysed in Stibal et al. (2012), and hence we use their 14C ages (Table 2).

Debris-rich basal ice samples from Engabreen were collected in autumn 2009 from an underground tunnel system excavated through bedrock beneath 210 m of sliding ice (Cohen, 2000). The basal ice stratigraphy comprises sediment-rich ice layers overlain by clean sediment-free and bubble-free ice (Jansson et al., 1996). We collected samples from sections of banded basal cryofacies from within 1.5 m of the ice–bed contact. Hot-water drilling was first implemented to create a basal cavity and the ice subsequently extracted by chain-sawing (described below). As Engabreen is temperate, we assume that the basal ice was formed primarily by regelation (Jansson et al., 1996).

Basal ice blocks (∼ 40 cm3) were collected by chain-sawing in all sample locations. The outermost ∼ 0.5 m of the ice surface was removed before the blocks were cut. The blocks were wrapped in pre-combusted foil and stored at ≤-20 ∘C, before being transported frozen to the University of Bristol and subsequently stored at ≤-20 ∘C.

Subsamples of the basal ice were prepared for analysis by chipping ∼ 15 cm3 chunks from the main block using a flame sterilized chisel. The outer ∼ 10–30 mm of the chips was removed by rinsing with ultrapure (≥ 18.2 MΩ cm) deionized water (DI) (Millipore), and the remaining ice was transferred into a pre-combusted glass beaker covered with foil. The ice was allowed to melt inside a laminar flow cabinet (Telstar Mini-H) under ambient laboratory conditions, which allowed any sediment to settle out of suspension. The ice melt was then decanted into smaller pre-combusted beakers. Ice melt was filtered through Whatman polypropylene Puradisc^™ 0.45 µm syringe filters. Water samples for subsequent OC analysis were stored in clean pre-combusted borosilicate glass bottles (thrice rinsed with the sample before storage). Five samples of filtered ice melt were taken from the ∼ 15 cm3 chunks cut out of the Joyce Glacier basal ice block, the Finsterwalderbreen DB ice block, and the Finsterwalderbreen SB ice block. Slightly larger volumes of ice melt permitted six samples of filtered ice melt to be collected from the Engabreen basal ice chunk, and seven from the Russell Glacier chunk. DI procedural blanks (n=5) were subject to identical processing as the samples from the filtration stage onwards to monitor for possible contamination during processing and storage. Sample concentrations were subsequently blank corrected (see Sect. 3.3.4).

Subsamples from each ice block were also collected for free carboxylic acid (FCA) determination. Ice was melted in an inert gas (O2-free-N2, OFN) atmosphere to limit potential contamination during the melting process (Saigne et al., 1987). The OFN gas first travelled through a hydrocarbon trap (HT200-4, Agilent) to remove any volatile OC compounds. Ice melt was filtered through Whatman polypropylene Puradisc^™ 0.45 µm syringe filters into 1.5 mL vials with PTFE caps (Chromacol). Samples were analysed within 24 h of melting to minimize losses due to the volatile nature of the FCA compounds. Procedural blanks were collected in concert.

The subglacial sediment OC content was derived from analysis of the settled particles, which were transferred from the beakers with clean, ethanol-rinsed metal spatulas and stored in sterile 0.5 L Whirl-pak bags (Nasco). Every effort was made to collect as much of the finer sediment as possible from the bottom and sides of the beakers. However, some fine sediment may have remained in the beaker and was thus excluded from the OC determinations. We were also unable to collect the fine particles that remained in suspension owing to the use of syringe filters to filter the ice melt. The total mass of this finer sediment was small compared to the mass of the settled sediment; therefore OC determinations were not unduly compromised. Sediment and filtered samples were stored in the dark at ≤-20 ∘C until analytical processing.

Basal ice debris concentrations (by mass) differed between glaciers. Finsterwalderbreen solid banded (FSB) and Russell Glacier basal ice contained the highest concentration of debris (86±7 and 55±25 %, Table 1), which are similar to percentages in GrIS banded ice (46–57 %), solid ice (61 %) (Yde et al., 2010), and debris bands (71 %) (Sugden et al., 1987). Debris concentrations in basal ice from Engabreen (37±21 %), Joyce Glacier (21±6 %), and Finsterwalderbreen dispersed banded (FDB) (20±27 %) were lower than percentages in GrIS and FSB ice.

We investigated possible correlations between DOC (and LMW-DOC) and the debris content of the basal ice, which may provide information on DOC provenance and the potential for DOC to leach from sediments into the basal ice. We acknowledge that if DOC is leached from sediments the controlling variable will be the surface area, rather than the debris concentration. However, a detailed investigation into the particle size distribution was beyond the scope of this study. We thus conducted a preliminary analysis to determine if the relationship with debris concentration differed for DOC and LMW-DOC. Significant positive associations between debris concentration and DOC were only evident in Joyce Glacier (R2=0.71, p<0.05) and Russell Glacier (R2=0.72, p<0.05) basal ice (Fig. 1a). No significant associations between LMW-DOC and debris concentrations were observed (Fig. 1b, R2<0.1, p<0.05).

The sediment OC content was low (< 0.6 %) in all basal ice samples (Table 2). Minor fractions of extractable carbohydrate (< 0.5 % of the sediment OC) were measured in Engabreen, Russell Glacier, and Finsterwalderbreen sediments. A higher carbohydrate fraction (17 % of the sediment OC) was measured at Joyce Glacier (Table 2). We use this as a proxy for lability (Biersmith and Benner, 1998; Pusceddu et al., 2009) and thus make the assumption that Joyce Glacier sediment is bioavailable. Microbial cell abundance was comparable in all samples (1–7×105 cells g-1, Table 2).

DOC abundance and composition varied between the four glaciers. The highest mean DOC concentrations were observed in basal ice from Joyce Glacier (272±99 µM C), and Engabreen (114±106 µM C), with lower concentrations in Russell Glacier basal ice (53±29 µM C), FDB (15±10 µM C) and FSB (33±33 µM C) (Table 1). The relatively large standard deviations show that subglacial DOC concentrations are highly variable, even in basal ice from the same glacier. Between five and seven replicate samples were taken from each of the ∼ 15 cm3 chunks cut out of the main ice blocks from each glacier (detailed in Sect. 3.1). The variability in the DOC concentrations suggests that there is significant spatial heterogeneity even at the level of the ∼ 15 cm3 basal ice chunks analysed from each glacier.

The composition of the subglacial DOC was investigated by spectrofluorescence and ion chromatography. The synchronous fluorescence spectra of all basal ice samples illustrated the dominance of three key fluorophores of a marine humic-like/fulvic acid type, at ca. 340, 385, and 440 nm (excitation wavelengths, Fig. 2, Table 3), and several unresolved fluorophores at longer excitation wavelengths. Protein-like peaks (∼ 279 nm excitation wavelength), indicative of tyrosine-like compounds (Ferrari and Mingazzini, 1995; Yamashita and Tanoue, 2003), were only evident in Joyce Glacier and FSB basal ice (Table 3). Ion chromatographic analyses provided a greater level of detail on the molecular composition of the DOC. LMW-DOC compounds, with concentrations > LOQ, accounted for < 3 % of the DOC in all basal ice samples. Mean LMW-DOC concentrations in Engabreen, Finsterwalderbreen, and Russell Glacier basal ice were < 420 nM C (Table 1). Mean LMW-DOC concentrations were an order of magnitude higher (4430 nM C) in Joyce Glacier basal ice. As with DOC concentrations, the variability in the LMW-DOC compound concentrations suggests high spatial heterogeneity within the basal ice.

LMW-DOC was typically dominated by FCAs (Table 1), except in Joyce Glacier samples, which are subsequently discussed. Overall, acetate was the most common analyte (Fig. 3), being present in 60 % of the samples that contained FCAs at concentrations > LOQ. Basal ice FCHO concentrations were typically < LOQ (< 4 % of the LMW-DOC, Table 1) and only detected in Joyce Glacier samples, comprising glucose (16–49 nM C) and ribose (16–19 nM C, data not shown). Joyce Glacier basal ice DOC was unique in that most (98 %) of the LMW-DOC was derived from the extremely diverse FAA pool (Fig. 4). Mean FAA concentration in Joyce Glacier basal ice (4353±2643 nM C) was an order of magnitude higher than mean FAA concentrations in Engabreen, Finsterwalderbreen, and Russell Glacier basal ice (0–51 nM C, Table 1). Some 14 FAAs were detected in Joyce Glacier basal ice, including methionine, glutamic acid, aspartic acid and cysteine, which were not observed in the other basal ice samples. Serine/proline, alanine, and valine dominated the Joyce Glacier FAA pool. FAAs accounted for 59 % of the LMW-DOC in Russell Glacier basal and FSB ice, primarily in the form of alanine and valine, respectively.

We find little evidence that the type of overridden material (i.e. the pre-entrainment sedimentary type such as lacustrine material or palaeosols) and the mean sediment OC content has a significant influence on the DOC content in basal ice. Indeed, the mean basal ice DOC concentrations (Table 1) and mean sediment OC content (Table 2) were relatively similar in all basal ice samples despite the differences in the types of overridden material. Furthermore, the fact that the highest mean DOC concentration was observed in Joyce Glacier basal ice (272 µM C) yet the corresponding sediment OC % was the lowest (0.01 %) of all four sites demonstrates the lack of a relationship between sediment OC % and basal ice DOC. This may be due to the particular section of basal sediment that was sampled as, in the case of Joyce Glacier, higher OC content has previously been observed in other Antarctic lacustrine samples, such as subglacial sediment beneath Lower Wright Glacier (0.7 % OC) (Stibal et al., 2012), and Antarctic Dry Valley lacustrine sediments containing microbial mats (∼ 9 % OC) (Squyres et al., 1991). This suggests a more diverse basal sediment matrix comprising algal mats and organic lacustrine material that mixed with sand and/or other low-OC, mineral-based material during basal ice formation beneath Joyce Glacier. However, we acknowledge that some of the difference in sediment OC (and extractable carbohydrate concentrations) may be due to the different analytical methods employed in this and previous studies. The concentrations that we present may also be conservative as our methodological approach meant that fine-sediment fractions, which may be OC-rich, remained in suspension and were not included in the OC determinations.

Key differences were, however, observed in the proportions of extractable carbohydrates (a proxy for bioavailable compounds in the basal sediment) and LMW-DOC concentrations in basal ice from the four sites. The LMW-DOC concentrations in Joyce Glacier basal ice, which were an order of magnitude higher than LMW-DOC concentrations in samples from the other three sites and predominantly due to high FAA concentrations, may have derived from the relatively large pool of potentially bioreactive extractable carbohydrates in Joyce Glacier basal sediment (17 % of the sediment OC, compared with < 0.5 % of the sediment OC in samples from Russell Glacier, Engabreen, and Finsterwalderbreen). The bioreactive OC pool in Joyce Glacier basal sediment may have been enhanced by the assimilation of proglacial algal mats into overridden material during glacial advance, which likely enriched the basal ice with lacustrine material and associated algal necromass (Pautler et al., 2012), which may include autochthonous material produced by microorganisms prior to basal ice formation. Indeed, lacustrine material is generally acknowledged as a source of reactive OC to microorganisms (Meyers and Ishiwatari, 1993). The lower extractable carbohydrate concentrations in basal sediment from Russell Glacier, Engabreen, and Finsterwalderbreen (compared with Joyce Glacier) are thought to reflect the more refractory nature of the overridden material. OC in subglacial material beneath this sampled section of Russell Glacier is thought to derive from a soil origin, based on relatively high concentrations of n-alkanoic acids, steroids, and other soil-derived functional compounds that have been identified in basal ice samples (Stibal et al., 2012). Due to this, and the relatively young age of Russell Glacier sediment OC (< 1900 14C yr BP), we expected the total and bioreactive OC concentrations to be higher than 0.44 and 0.47 % of the OC, respectively. For instance, OC content in Greenland soils range from 0.1 to 44.8 % in C horizons and peat soils (Horwath Burnham and Slettern, 2010). The low OC and extractable carbohydrate concentrations in Russell Glacier basal ice may reflect a heterogeneous sediment matrix that incorporates a lower proportion of palaeosols mixed with other low-OC, mineral-based material. However, as discussed earlier, these differences in sediment OC concentrations may be due to the conservative nature of our methodological approach that may have excluded the potentially OC-rich fine-sediment fractions. The low extractable carbohydrate concentration (0.04 % of the OC) in Finsterwalderbreen basal sediment is likely influenced by the predominance of OC from kerogen in the overridden shale bedrock (Wadham et al., 2004) that has been incorporated into the basal ice matrix. Kerogen is ancient carbon comprising stable carbon macromolecules and has limited bioreactivity. Similarly, low bioreactive OC in Engabreen basal sediment (0.17 % of the OC comprised extractable carbohydrates) is influenced by the subglacial substrate comprising overridden continental shield rock depleted in reactive OC, the limited opportunity for material from supraglacial environments to be in-washed, and the limited input of overridden palaeosols (Stibal et al., 2012). A lack of organic biomarkers (derived from algal and higher plant inputs) in Engabreen basal ice further suggests that incorporation of organic material is probably limited (Stibal et al., 2012). Alternatively, the lack of organic biomarkers may be due to debris entrainment by regelation rather than a freeze-on (adfreezing).

In summary, our data suggest that where glaciers and ice sheets override lacustrine sediments, there is an injection of particulate and dissolved bioavailable compounds into the basal ice at the glacier bed, which is less evident where the glacier overrode palaeosols or bedrock. This has implications for subglacial LMW-DOC cycling as this abiotic input of LMW-DOC (via leaching) has the potential to stimulate microbial activity in wet sediments in the subglacial environment. We go on to investigate the DOC and LMW-DOC signatures in basal ice from these contrasting subglacial environments.

The presence of LMW-DOC compounds and the similarities in the types of compounds detected in basal ice samples from the four sites may reflect common sources and pathways of transformation of DOC in subglacial environments beneath glaciers and ice sheets. The potential for interactions between basal sediment and subglacial ice melt suggests that inputs from the overridden subglacial material may represent a key contribution to basal ice DOC. The chemical composition of basal ice, including DOC compounds, should reflect characteristics of the parent water prior to being frozen (Knight, 1997), where this water might be either flowing at the base of the glacier, held in porewaters in overridden water-saturated sediment, or refrozen water from pressure melting during the regelation process. These water sources have extensive contact with the subglacial material and so have the potential to acquire dissolved compounds via biogeochemical interactions. However, these processes are highly site specific. Where there are well-developed quick-flow components and scoured bedrock channels, for instance, there will be less scope for fast-flowing waters to acquire dissolved compounds from biogeochemical interactions with the overridden material. DOC and LMW-DOC components in basal ice may also be acquired by in situ abiotic processes, e.g. by reactions, such as dissolution, in water films around debris and in liquid water veins (Mader et al., 2006). It is likely that certain organic compounds will remain associated with the debris and others will dissociate to become DOC. To fully assess whether DOC is largely terrestrially derived and leached from sediments, we would need data on the surface area of the debris and information on particle size distribution. As this was beyond the scope of this study we instead used debris concentrations for a preliminary investigation. We find that for sites where there is a bioavailable OC source in sediments (Joyce Glacier) there is a significant relationship between DOC and debris concentration (Fig. 1a). This suggests that subglacial meltwater contact with subglacial sediment beneath Joyce Glacier, which is cold-based and so has little supraglacial meltwater penetration to the glacier bed, is a major control on DOC acquisition. We find several additional lines of evidence to support the leaching of DOC from subglacial sediments, including the presence of fulvic acids that have previously been associated with terrestrial material (> 440 nm fluorescence wavelengths) in all basal ice samples (McKnight et al., 2001). The basal ice LMW-DOC compounds may also be a leached relic of the overridden material that has been preserved in the ice when frozen. However, the lack of significant association between LMW-DOC and debris concentration (Fig. 1b) is reflective of additional sources and sinks of these compounds in the basal ice layer and/or in the parent water body from which basal ice formed. The LMW-DOC signature in basal ice may also be influenced by in situ microbial production and consumption, as illustrated in earlier work that has proposed a range of microbial processes to be active in the subglacial environment, including in situ chemoautotrophic production (Bhatia et al., 2006, 2013), chemoheterotrophic oxidation of OC substrates to protein-like LMW-DOC compounds (Bhatia et al., 2010), and release of LMW-DOC from decaying cells. It is probable that subglacial microbial activity cycles LMW-DOC both before and after the formation of basal ice. For instance, microorganisms in subglacial sediment porewaters and basal meltwater flowing at the rock–water interface may actively utilize OC substrates and energy sources derived from the overridden material. Via this activity, they may also go on to produce simple LMW-DOC compounds, which may subsequently be incorporated into basal ice. The protein-like peaks that were observed in the spectrofluorescence spectra in Joyce Glacier and FSB ice (Table 3) tentatively suggest that some of the LMW-DOC is of a microbial provenance. Protein-like fluorescence is linked with recent biological activity (De Souza Sierra et al., 1994) and is associated with active FAA production during microbial metabolism (Yamashita and Tanoue, 2003). The finding that FSB samples contained larger protein-like peaks and had higher mean FCA and FAA concentrations when compared with FDB samples may be explained by the different basal ice formation processes at Finsterwalderbreen. FSB debris, sampled from surface outcrops of frozen subglacial material, or thrust bands, is expected to derive from further upglacier than FDB debris and likely formed during the most recent surge ca. 1898–1920 (Liestøl, 1969). This suggests that FSB debris may have been glacier-covered for a much longer period than FDB debris. These conditions may have led to enhanced leaching of LMW-DOC from the subglacial material and/or greater production (vs. consumption) of LMW-DOC by in situ microorganisms. It is also possible that LMW-DOC in basal ice from the polythermal and warm-based glaciers sampled in this study (Finsterwalderbreen, Russell Glacier, and Engabreen) could derive from supraglacial inputs as glacially overridden material is not the sole source of DOC in basal ice.

In this study, we were not able to categorically separate LMW-DOC derived from biotic and abiotic processes as, at a molecular level, many LMW-DOC compounds are non-specific biomarkers due to their pervasive occurrence in plants and microorganisms (Biersmith and Benner, 1998). For example, valine, a common FAA in most basal ice samples, can be synthesized in plants via several steps starting from pyruvic acid (e.g. described in Singh, 1999). Valine can also be microbially synthesized from pyruvate (Blombach et al., 2007) and produced by aerobic Gram-positive microbes (Valle et al., 2008). Similarly, glucose can be produced by photosynthesis (Kirchman et al., 2001) and chemoautotrophic bacterial activity (Jansen et al., 1982). The key point is that the presence of numerous LMW-DOC compounds in basal ice from all four glacial sites provides evidence that viable substrates for microbial growth, whether derived from a terrestrial or microbial source, are available in subglacial environments. These LMW-DOC compounds may help support microbial communities within the present-day basal ice, e.g. beneath Russell Glacier, where recent work has shown that the basal ice may be microbially active in the current frozen state (Yde et al., 2010). The microbial cell counts observed in all basal ice samples in this study (105 cells g-1, Table 2) are comparable to microbial populations (105–108 cells g-1) reported in other subglacial sediments that have been proven to be microbially active (Sharp et al., 1999; Foght et al., 2004; Kastovska et al., 2007; Yde et al., 2010; Montross et al., 2012).

The margin of Joyce Glacier rests upon ancient lake sediments and, hence, represents a case where a very labile organic matter source is overridden. This situation may have been common in past periods of glaciation, when, for example, the Pleistocene ice sheets advanced over regions with a high density of lakes, such as in northern Canada and Scandinavia (Wadham et al., 2008). Hence, the potential for LMW-DOC incorporation in Joyce Glacier basal ice and sediment may be applicable to these other types of lacustrine-based subglacial ecosystems. In addition, the abundance of LMW-DOC in Joyce Glacier suggests that overridden lacustrine material can be sequestered even if the glacier is cold-based. Contrary to traditional assumptions that drainage in cold-based glaciers is entirely supraglacial, it is possible that discrete subglacial channels exist where water is in contact with the substrate, e.g. at Longyearbreen (Yde et al., 2008). This mechanism may enable the release of DOC to downstream ecosystems. If the glacier were warm-based, then the DOC could be flushed out during the summer melt seasons and contribute to the net export of bioavailable DOC to downstream environments. DOC in glacial runoff may derive from multiple sources: terrestrial DOC derived from overridden material at the bed (Hood et al., 2009), anthropogenic aerosol deposition on the glacier surface (Stubbins et al., 2012), and biological activity in both supraglacial (Anesio et al., 2009) and subglacial (Bhatia et al., 2013) environments. The contribution from basal ice may be more significant in cold-based glacier systems, e.g. in the Antarctic Dry Valleys, where daily radiation melting of the steep ice cliffs may release solute from the debris-rich basal ice that is exposed on the cliffs. The distributed drainage system beneath temperate and polythermal glaciers may also include a constant source of water from basal ice melt and groundwater in contact with glacial till (Paterson, 1999).

The dramatic difference in the DOC composition beneath glaciers resting on different OC substrates that our data have highlighted may have implications for the rate and degree to which this overridden OC can be cycled to biogenic gases in current subglacial environments, which in turn has relevance for the global carbon cycle (Wadham et al., 2008; Stibal et al., 2012). While the DOC and LMW-DOC signatures of basal ice may arise from several confounding factors which are difficult to disentangle, identifying the abundance and composition of DOC in basal ice is an important first step to understanding LMW-DOC cycling in subglacial environments.