Previous and Current Models of Subglacial Biogeochemical Weathering

Studies at Haut Glacier d'Arolla, a warm-based alpine glacier in Switzerland, have been central to our understanding of links between subglacial hydrology, hydrochemistry, and glacier dynamics, due to intensive interdisciplinary field studies over the last two decades (e.g. Tranter et al., 1993; Brown et al., 1996a; Sharp et al., 1995; Brown, 2002; Hubbard et al., 1995, 1998). Until recently, studies at Haut Glacier d'Arolla with the preceding studies of Raiswell (1984) gave rise to a generic chemical weathering model which has been used to explain and deconvolute bulk meltwater chemistries observed at a range of alpine and polar glaciers. This model, which we will call the T-Model (Tranter et al., 1993), is based upon sequentially applied mass balance calculations, representing a budget that describes fluxes of solutes into and out of a glacier, and assigns solutes to specific sources and sinks (Brown et al., 1996a; Sharp et al., 1995; Hodson, et al., 2000) (Table 1, reactions 1 to 5). In the T-Model, subglacial solute acquisition proceeds by acid hydrolysis reactions, fuelled by protons derived from either the dissolution of atmospheric CO₂, or the oxidation of sulfide minerals by dissolved oxygen (Tranter et al., 1993).

Table 1

Equations describing the principal geochemical weathering reactions in subglacial environments proposed by Tranter et al. (2002, 2005).

However, the measurement of in situ subglacial meltwater chemistries derived from the base of boreholes drilled to the base of Haut Glacier d'Arolla have suggested problems with the T-Model which were not apparent from studies using bulk meltwater chemistry alone (Tranter et al., 1997, 2002). First, it was determined that CO₂ was not a major driving force for subglacial chemical weathering reactions, but instead there was a progression of reactions from carbonate and silicate hydrolysis through to sulfide oxidation driven by dissolved O₂. Once O₂ had been depleted and anoxic conditions were achieved, carbonate and silicate weathering was hypothesized to be driven by electrons from the reduction of Fe³⁺. Central to these new hypotheses was the discovery of viable microbial populations at the beds of alpine and polar glaciers. Microbes are abundant at the water-rock-ice interface beneath Haut Glacier d'Arolla (>10⁶ cells mL⁻¹ in some subglacial waters) (Sharp et al., 1999), as well other alpine and polar glaciers including Tsanfleuron, Switzerland (Sharp et al., 1999), Fox and Franz-Josef Glaciers, New Zealand (Foght et al., 2004), John Evans Glacier, Nunavut, Canada (Skidmore et al., 2000), and Bench Glacier, Alaska, U.S.A. (Skidmore et al., 2005).

Second, organic carbon had been previously overlooked as a source of dissolved inorganic carbon (DIC) (Sharp et al., 1999; Skidmore et al., 2000, 2005). Indeed, subglacial debris contains organic carbon which is a potential source of energy for microbes, including kerogen, bacterial necromass, and overridden organic matter. This suggests that microbially mediated reactions, utilizing the oxidation of organic carbon by O₂(aq) [as well as abiotic sulfide oxidation by O₂(aq)] would drive the bed in places to anoxia. In these anoxic ‘hot spots’ sulfide oxidation by Fe³⁺, sulfate reduction (Wadham et al., 2004), and the reduction of organic carbon (methanogenesis) was proposed to occur (Tranter et al., 2002, 2005) (Table 1).

The configuration and dynamics of the subglacial drainage system will likely control such biogeochemical gradients and the spatial dynamics of reaction processes (Tranter et al., 2005). Alpine subglacial hydrological systems broadly comprise either a distributed or a channelized system (Fig. 1). The distributed system comprises linked cavities, permeable sediments, film flow, and broad canals, and it transports delayed-flow waters. This component is fed largely by snowmelt and is the dominant mode of subglacial drainage at the end of the winter, and at low diurnal discharges. Here, access to the atmospheric gases will be limited suggesting that microbes will initially oxidize organic matter and sulfides using dissolved O₂, driving the bed towards and to anoxia. Then reactions may proceed anoxically, via Fe³⁺ reduction and methanogenesis (Tranter et al., 2002; 2005). Conversely, a hydraulically efficient dendritic channelized subglacial system develops during the melt season as surface ablation and subglacial discharges increase (Nienow et al., 1998), evolving to accommodate the increasing volume of meltwater generated during the summer months. These arterial channels expand up-glacier following the retreat of the seasonal supraglacial snowpack (Nienow et al., 1998), and drain waters which are comprised predominantly of icemelt. These channels form the dominant mode of meltwater drainage later in the ablation season, and at maximum diurnal discharges (Fig. 1). Therefore, in contrast to the distributed system, a plentiful supply of oxygenated water will promote microbially mediated organic carbon and sulfide oxidation by dissolved O₂. However, it is likely that significant microbial activity will be limited to basal sediments in the distributed system, as well as the channel marginal zones (Tranter et al., 2005). Here the residence time of water is long, and hydraulic shear stress will be low, which will promote the stable attachment of microbes to the sediment or bedrock substrate and will allow prolonged access to both solid phase and dissolved nutrients.

Figure 1

Schematic representation of the subglacial drainage system beneath Alpine glaciers (after Brown, 2002).

Additionally, these current models only crudely consider the relative importance of silicate and carbonate weathering. Specifically, Ca²⁺ is the dominant cation in bulk meltwaters in a range of glacierized alpine and polar catchments, and despite being underlain by vastly varied compositions of silicate and carbonate lithology, Ca²⁺ is thought to derive dominantly from the dissolution of trace carbonates by chemically limited weathering regimes (Brown, 2002; Tranter et al., 1993; Drever and Zobrist, 1992; White et al., 2005). However, the potential for significant silicate-derived Ca²⁺ has rarely been given any quantitative consideration (cf. Anderson et al., 1997, 2000), despite the implications to solute provenance calculations as a whole. While much of these theories were formulated from studies at Haut Glacier d'Arolla, field studies from a range of warm-based and polythermal glaciers support these assertions (Anderson et al., 1997, 2000; Anderson, 2005; Wadham et al., 2004; Skidmore et al., 2005; Hodson et al., 2004).

Alternative Approaches for Determining Subglacial Geochemical and Biogeochemical Reactions

The development of freely available computer based geochemical reaction models provides a potentially useful tool for improving current models of subglacial biogeochemical weathering in glacierized environments. These models, like the previous T-Model, work on the mass balance approach, but they allow multiple alternative weathering scenarios under different hydrological conditions to be tested simultaneously, unlike previous mass balance calculations which require mass balance equations to be solved sequentially, and ad hoc, in a spreadsheet or in a longhand mass balance matrix (Finley and Drever, 1997). They also easily allow mineral-specific weathering processes to be modeled, the thermodynamic feasibility of the reactions to be assessed, and alternative estimates to be made of the consumption and/or production of important species not easily analyzed [e.g. CH₂O(s), CO₂(g)], under differing weathering scenarios.

This paper evaluates current subglacial biogeochemical weathering models using speciation mass-balance (SMB) hydrochemical models parameterized with hydrochemical, hydrological, and mineralogical data collected from Haut Glacier d'Arolla during the melt seasons of 1999 and 2000 (Mitchell et al., 2001, 2006; Mitchell and Brown, 2007). The aim is to investigate the utility of SMB models and to provide a better quantitative understanding of these most recent subglacial biogeochemical weathering models. Specifically, we (1) present seasonal estimates on the oxidation of organic carbon, (2) quantify if organic carbon can solely provide all non-CaCO₃ DIC, (3) provide a more robust understanding of silicate weathering than has been previously considered in subglacial environments, and (4) test the thermodynamic feasibility that Fe³⁺(aq) can act as a terminal electron acceptor and oxidize sulfides under anoxic conditions. Haut Glacier d'Arolla provides an ideal case study with which to parameterize the study hydrologically and hydrochemically, and to compare and contrast with the existing studies of subglacial solute provenance and flow routing.

Discharge and Meltwater Chemistry

Supraglacial waters are very dilute and exhibit very little seasonal variation, but become heavily enriched in solute as they travel though the subglacial hydrological system, as demonstrated by the more concentrated in situ and bulk meltwaters. The median composition of supraglacial meltwater, bulk meltwater, and in situ meltwater was, respectively, for the following ions, Ca²⁺  =  0.32, 6.6, 15 mg L⁻¹; Mg²⁺  =  0.014, 0.4, 0.5 mg L⁻¹; Na⁺  =  0.034, 0.37, 0.58 mg L⁻¹; K⁺  =  0.036, 0.37, 0.35 mg L⁻¹; SO₄²⁻  =  0.075, 5.2, 9.7 mg L⁻¹; HCO₃⁻  =  1.1, 17, 33 mg L⁻¹; NO₃⁻  =  0.025, 0.4, 0.77 mg L⁻¹; Fe  =  0.083, 0.39, 0.56 mg L⁻¹; and pH  =  7.5, 8.1, 7.2. The bulk discharge magnitude and daily amplitude generally increased as the melt season progressed, reflecting variations in radiation receipts at the glacier surface and the up-glacier recession of the seasonal snowpack (Fig. 3). Four hydrological periods (P1 to P4) have been defined during the 1999 study period, based upon qualitative analysis of bulk discharge records (Fig. 3). P1 and P4 exhibit low discharges relative to P2 and P3. Calcium, Mg²⁺, K⁺, Na⁺, SO₄²⁻, and HCO₃⁻ exhibit seasonal variations, with declining concentrations during P1 as discharge magnitude and diurnal variability increased, reaching the lowest seasonal concentrations in P2 at the height of the ablation season. There is a general increase in concentrations over P3 and P4 as discharge falls towards the end of the ablation season.

Figure 3

Bulk meltwater hydrological and hydrochemical data for the 1999 ablation season at Haut Glacier d'Arolla.

Testing Viable Chemical Weathering Reactions in Subglacial Environments Using Phreeqci

We first investigated all viable reaction combinations allowing the unconstrained dissolution or precipitation of most of the main minerals available in the Haut Glacier d'Arolla catchment (Table 3) using the mean melt season concentrations of Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄²⁻, Fe, Al, and Si, and pH, for supraglacial and bulk meltwaters (Mitchell et al., 2006). This is required as a starting point to test the model, and represents net geochemical reactions occurring throughout the subglacial hydrological system on average over the melt season. Temporal variations in chemical weathering processes using the seasonal meltwater quality data will be considered subsequently in the Section COMPARISON OF MODELING-DEFINED CHANGES IN SUBGLACIAL WEATHERING PROCESSES OVER SEASONAL TIMESCALES. This produced 136 weathering scenarios, but unsurprisingly no individual weathering scenario that could account for the evolution of meltwater in the heterogeneous subglacial environment. The phase transfers of these weathering scenarios indicate dissolution of calcite, pyrite, tremolite, and O₂(aq), and precipitation of ferrihydrite, anorthite, and microcline. While such reactions are feasible based upon reaction stoichiometry, mineral saturation index (SI) calculations indicate that primary silicate minerals present in the Haut Glacier d'Arolla catchment are undersaturated in solution over the range of measured pH and bulk meltwater solutions in the system (Fig. 4). Therefore, primary silicate mineral precipitation is unlikely. More likely is the formation of clay minerals from the incongruent weathering of micas, feldspars, and other aluminosilicate minerals, as has been previously proposed (Fairchild et al., 1994; Tranter et al., 1993; Anderson et al., 1997).

Figure 4

Saturation index of minerals present in the catchment shown as a function of pH for the (a) most concentrated bulk meltwater from hydrological period (P)1, and (b) the least concentrated bulk meltwater from P3, as annotated in Figure 3.

Therefore, kaolinite and montmorillonite were chosen as two representative clays for the model; these clays commonly form from the weathering of micas and feldspars and have been observed in catchment bedrock (Tables 2 and 3). When these clays were set as precipitating phases, 47 weathering scenarios were produced. Individual weathering scenarios (WS) produced by the parameters of this model indicate montmorillonite, or kaolinite and montmorillonite, precipitation is required by the model to remove the incidence of all primary silicate precipitation (Table 4). Kaolinite-only weathering scenarios cannot remove the incidence of primary silicate precipitation. The influence of clays in the mass balance calculations indicates how aluminosilicate dissolution and clay formation are covariant. This demonstrates the model is able to account for the changes in subglacial chemistry via incongruent silicate weathering.

Table 4

Results of model testing between supraglacial and bulk meltwaters, using the mean composition of supraglacial and bulk meltwaters as input and output waters, respectively. Each line is a single weathering scenario (WS) that can account for the changes in solution chemistry between supraglacial and bulk waters. Weathering scenarios highlighted in light gray indicate the constrained model with oxygen-driven organic carbon oxidation and pyrite oxidation driven carbonate and incongruent silicate weathering. Weathering scenarios highlighted in mid gray are those which include calcite and anorthite weathering. Weathering scenarios highlighted in dark gray are those identified as being the most feasible weathering scenarios, based upon mineral dissolution rates. Units  =  mass of phase (mmol L⁻¹) transferred between input supraglacial meltwater and bulk meltwaters, with positive values showing dissolution and negative values showing precipitation.

Dissolved oxygen is consumed in all of the weathering scenarios. Oxygen-inclusive weathering scenarios always include pyrite dissolution and ferrihydrite precipitation, indicating pyrite oxidation by dissolved O₂. Pyrite is the only likely source of S since gypsum has not been recorded in the Haut Glacier d'Arolla catchment, unlike some other glacierized catchments (Skidmore et al., 2005), and thus the model cannot find any weathering scenarios unless pyrite and dissolved O₂ are consumed (Table 4). This is in agreement with current models of solute acquisition in subglacial environments, although the effect of limiting O₂ to simulate suboxic conditions will be subsequently tested (Table 1).

Organic carbon may be either consumed or completely absent from the weathering scenarios. CH₂O and calcite account for the observed DIC measured in output waters (Table 4). Non-CH₂O models always require the precipitation of a primary silicate mineral (Table 4). Conversely, CH₂O-inclusive models allow silicates and calcite to dissolve if montmorillonite precipitation, or kaolinite and montmorillonite precipitation, occurs. It is logical to assume that the net dissolution of calcite and primary silicate minerals must occur in subglacial environments to account for solute enrichment between supraglacial and bulk meltwaters. CH₂O consumption always requires dissolved O₂, indicating the simulated organic carbon oxidation is driven by electron transport to dissolved O₂ since this is the only available electron acceptor phase in the model. Organic carbon mineralization will very likely be microbially mediated, as it is an otherwise exceptionally slow abiotic process (Konhauser, 2006). Modeling based on the average seasonal bulk meltwater composition therefore suggests that carbonate and incongruent silicate weathering, driven by organic carbon oxidation and pyrite oxidation by dissolved O₂, occurs at some point in the subglacial hydrological system at Haut Glacier d'Arolla. While simple hydrolysis may account for some of the weathering, as is suggested in current subglacial biogeochemical models (Tranter et al., 2002), organic carbon oxidation and sulfide oxidation with dissolved O₂ are required at some point in the subglacial environment. Therefore, so far, PHREEQCi modeling is in agreement with current subglacial biogeochemical models (Tranter et al., 2002, 2005).

To constrain the number of weathering scenarios to produce only those that include carbonate and silicate weathering driven by organic carbon oxidation and sulfide oxidation with dissolved O₂, in the INVERSE_MODELLING toggle of PHREEQCi, primary silicates, calcite, and CH₂O were set to dissolve, and ferrihydrite and clays set to precipitate as before. These constraints significantly reduce the number of weathering scenarios from the original 47 to 18 (Table 4). All these weathering scenarios are comprised of CH₂O and sulfide oxidation by dissolved O₂ and incongruent aluminosilicate dissolution.

Only five of the main primary silicate phases (albite, biotite, muscovite, microcline, tremolite, anorthite, quartz) may be dissolved in any of the 18 individual weathering scenarios, indicating that under this model configuration the simultaneous weathering of all the main catchment minerals cannot account for the change in water composition from supraglacial to bulk meltwaters. Since the model consistently suggests organic carbon and pyrite oxidation–driven weathering, the feasibility of the 18 weathering scenarios can be given some quantitative assessment with consideration of the relative reactivity of the dissolving carbonate and silicate minerals. Glacial environments exhibit high physical weathering rates, which constantly supply freshly comminuted mineral surfaces. Under these conditions, weathering is largely chemically limited where the most reactive minerals, but not necessarily the most abundant, will be likely to provide the majority of solute (Anderson et al., 1997, 2000; White et al., 2005). Therefore, mineral-specific dissolution rates (after Lasaga et al., 1994; Langmuir, 1997) for minerals present in the Haut Glacier d'Arolla catchment were used to consider the most likely weathering scenarios. The weathering series order for carbonate and silicate minerals present in the Haut Glacier d'Arolla catchment and used in the model is anorthite ≈ calcite > biotite > tremolite > muscovite > quartz > microcline. The dissolution rate data used is derived from laboratory dissolution experiments at pH 5 and 25 °C. Lower water temperatures generally reduce weathering rates, and would therefore be expected in subglacial environments (∼0.1°C), although the effect is less pronounced for biotite (Anderson, 2005; White et al., 1999). However, these data are simply used to indicate the relative dissolution rates under fixed conditions, to provide some estimate as to the most likely weathering scenarios.

Anorthite dissolution is required in all the weathering scenarios, while calcite-free weathering scenarios are generated in 9 of the 18 scenarios (Table 4). Therefore, PHREEQCi suggests that anorthite is required to account for the enrichment of Ca between supraglacial and bulk meltwaters, as has been suggested in current models of subglacial biogeochemical weathering models (Tranter et al., 2002, 2005) but has rarely been quantified (cf. Anderson et al., 1997). Anorthite is highly reactive (2.00 * 10⁻⁹ mol m⁻² s⁻¹ [Lasaga et al., 1994]), so the model assertions seem likely. However, the absence of calcite weathering is highly unlikely even if in trace quantities in the bedrock, reflecting its high dissolution rates (8.43 * 10⁻¹⁰ mol m⁻² s⁻¹ [Lasaga et al., 1994]). Indeed, all current models of chemical weathering in glacierized and mountainous catchments indicate the dominance of carbonates as a solute source even when in trace quantities in the bedrock, owing to the chemically limited weathering regimes which operate in such environments (Anderson et al., 1997, 2000; Jacobson et al., 2002, 2003; Tranter et al., 2002; White et al., 2005). This suggests the 9 calcite and anorthite inclusive weathering scenarios (Table 4) are most feasible for the subglacial environment.

The consistent requirement of albite and anorthite dissolution in each of the weathering scenarios indicates feldspar weathering is essential for the evolution of bulk meltwater chemistry at Haut Glacier d'Arolla. Albite is the only likely major source of Na. Mica (muscovite or biotite) dissolution occurs in 7 of the 9 anorthite and calcite inclusive weathering scenarios, with K-feldspar (microcline) dissolution only occurring in 2 of the 9 weathering scenarios. The apparent dominance of biotite weathering over K-feldspar weathering appears likely as biotite dissolution rates (5.6 * 10⁻¹² mol m⁻² s⁻¹ [Lasaga et al., 1994]) are much higher than K-feldspar (3.16 * 10⁻¹³ mol m⁻² s⁻¹ [Lasaga et al., 1994]). However, muscovite dissolution rates are 3.7 times lower than that of K-feldspar (8.51 * 10⁻¹⁴ mol m⁻² s⁻¹). This suggests the 5 biotite inclusive weathering scenarios (with calcite and anorthite dissolution; WS 41, 44, 45, 46, 47; Table 4) are more likely than muscovite weathering scenarios. Indeed, the importance of biotite weathering has been previously demonstrated from glacial moraine chronosequences (Blum and Erel, 1997), and mass balance weathering schemes (Anderson et al., 2000). Tremolite exhibits dissolution rates on the same order as biotite (1.9 *10⁻¹² mol m⁻² s⁻¹ [Lasaga et al., 1994]); therefore, tremolite inclusive weathering scenarios, with combined biotite, anorthite, and calcite weathering would seem the most likely weathering scenarios, based upon mineral dissolution rates. This is achieved only in WS41. We conclude that PHREEQCi is therefore able to deconvolute silicate weathering more than can easily be achieved through traditional longhand mass balance calculations (Tranter et al., 1993; Anderson et al., 2000), owing to the ability of the numerical code to solve balance equations simultaneously, rather than sequentially. It is worth noting that the model suggests the formation of secondary phases which have notable ion exchange properties, including montmorillonite and Fe-oxyhydroxies. We cannot account for this in the SMB model, but ion exchange with such minerals may well be expected to have some influence on water chemistry (Souchez and Lorrain, 1975), such that solute ratios measured in the meltwater may not only reflect the stoichiometric release from the mineral lattice, but also ion exchange with these secondary phases (Wadham et al., 1998). However, like all previous mass balance studies, we have to assume that ion exchange is in steady state as very little is known about the ion exchange pool in aqueous systems.

Testing The Combined Influence of Ch₂o and CO₂

Current models of subglacial biogeochemical weathering suggest that atmospheric CO₂-driven weathering is relatively insignificant, but the potential for combined organic carbon oxidation and atmospheric CO₂ dissolution as a DIC source has been given little quantitative consideration (cf. Wadham et al., 2004). Some CO₂-driven weathering may be expected in turbid, well-mixed waters which are open to the atmosphere and are rapidly transported through englacial and subglacial channels, and also in connected channel marginal locations. In order to test this, CO₂(g) was added to defined phases in the model and constrained to dissolve, in conjunction with CH₂O. Only two additional weathering scenarios were generated with combined organic carbon oxidation by dissolved O₂, and CO₂(g) dissolution (WS48 and 49; Table 4). WS48 did not include calcite weathering, and is therefore unlikely to be feasible. However, WS49 did include calcite weathering, along with sulfide oxidation by dissolved O₂, and incongruent aluminosilicate dissolution, with albite, biotite, tremolite, and anorthite weathering, and is like WS41 except for the distribution of DIC between CH₂O and CO₂(g). This weathering scenario suggests that CH₂O oxidation is dominant over CO₂ dissolution at a mass ratio of ∼10:1. Therefore the results from PHRREQCi support current models of subglacial biogeochemical weathering, and indicate that even if some CO₂-driven weathering occurs, it is relatively insignificant to that driven by organic carbon oxidation.

Testing Anoxic Subglacial Weathering

It has been suggested in current subglacial biogeochemical models that bacterial catalysis of organic carbon oxidation by dissolved O₂, and sulfide oxidation by dissolved O₂, may drive subglacial environments towards anoxia (Tranter et al., 2002, 2005). Once anoxia has developed, methanogenesis has been proposed (Tranter et al., 2005). Indeed, stable isotope evidence from bulk meltwaters from a polythermal High Arctic glacier indicates that reduction of SO₄²⁻ was occurring and anoxic conditions apparent (Wadham et al., 2004). In order to test the effect of anoxia upon the weathering mechanisms suggested by PHREEQCi, O₂(aq) was omitted from the model and CH₄ was included as a potential phase to investigate whether methanogenesis could account for observed changes in DIC. The model produced 9 weathering scenarios, all of which included calcite and anorthite dissolution (Table 5). CH₂O reduced to CH₄, producing H⁺, which could account for changes in DIC between supraglacial and bulk meltwaters, and was required if no O₂(aq) was available.

Table 5

Results of model testing under anoxic conditions, between supraglacial and bulk meltwaters, and between supraglacial and in situ waters collected from the distributed system. Each line is a single weathering scenario (WS) that can account for the changes in solution chemistry between supraglacial and bulk waters or in situ waters. Units  =  mass of dissolving phases (mmol L⁻¹) transferred between input supraglacial water and bulk or in situ waters.

The SMB model was also run using supraglacial input waters evolving to in situ subglacial waters collected directly from the base of boreholes, which appeared to be representative of waters collected from the distributed system (Mitchell et al., 2006). In such environments, anoxia and methanogenesis would be most likely to occur due to poor hydraulic connectivity to oxygenated waters (Tranter et al., 2005). These waters exhibit major ion concentrations that are on average 320% the concentration of bulk meltwaters than measured in all bulk meltwaters and have low suspended sediment concentrations, suggesting these waters have had a low water velocity and a long residence time within the distributed system (Tranter et al., 1997; Mitchell et al., 2006). Using the mean composition of supraglacial meltwaters evolving to the mean composition of these waters collected from the distributed system, methanogenesis was able to account for the changes in water chemistry, producing 10 weathering scenarios, providing some quantitative evidence to support the hypothesis that methanogens operate in areas of the bed where there is poor hydraulic connectivity (Table 5) (Tranter et al., 2005). Interestingly, all these weathering scenarios required anorthite weathering, but only 5 of the 10 weathering scenarios included calcite weathering, suggesting Ca-silicate weathering may dominate over calcite as a Ca source in the distributed system. This is likely as longer water residence times would allow Ca-silicate minerals, which are in higher abundance but are generally less reactive, to provide proportionally more solute (Anderson et al., 2000; Drever and Zobrist, 1992).

The modeling therefore demonstrates that methanogenesis can account for the evolution of bulk waters, which integrates both waters from the distributed and channelized system, and also water collected directly from the distributed system. However, methanogenesis is unlikely to occur in subglacial channels or channel marginal locations due to the constant influx of oxygenated waters. Therefore, this demonstrates how viable weathering scenarios from PHREEQCi, while mathematically feasible with regard reaction stoichiometry, have to be considered within the hydrological context of the system.

The suggestion that Fe³⁺ derived from the reduction of Fe-(oxy) hydroxides or Fe-silicate minerals can act as an oxidizing agent for sulfide minerals under anoxic conditions (Tranter et al., 2002, 2005) was also tested. Recent anoxic incubation of glacial sediments has demonstrated the occurrence of microbially induced Fe reduction by the measurement of Fe²⁺(aq) (Montross, 2007). However, it remains unclear if aqueous Fe will exist sufficiently in the Fe³⁺ form under anoxic conditions, since dissolved Fe²⁺ will evidently predominate under anoxic conditions. We therefore undertook speciation modeling on the mean solution composition for in situ water collected from the distributed system, where total dissolved Fe was measured. By the nature of the model, this estimates abiotic equilibrium speciation, and will thus indicate whether Fe³⁺ in waters from the subglacial environment can be produced abiotically. The redox potential was varied from the oxic conditions of pE  =  7 used in the model thus far, down to an estimated 0 (suboxic) and −5 (anoxic) (Langmuir, 1997) (Fig. 5). Aqueous Fe²⁺(aq) increasingly dominated Fe speciation below pE 4 through to heavily reduced conditions of pE −5, reaching concentrations <8 µmoles L⁻¹. Fe(III)(aq) species [Fe(OH)²⁺, Fe(OH)³⁺, Fe(OH)⁴⁻] decreased in concentration from combined concentrations of <8 µmoles L⁻¹ under oxic conditions of pE 7 down to 0 µmoles L⁻¹ at pE 2 and below. This therefore reinforces the suggestion that abiotic processes cannot account for the generation of aqueous Fe³⁺ (Tranter et al., 2002, 2005), and suggests that microbial oxidation of Fe²⁺ to Fe³⁺ is required if Fe³⁺ is indeed an important oxidizing agent in subglacial environments. This is highly likely given that the reducing power of Fe²⁺ increases dramatically at pH values higher than 2–3, owing to the formation of ferric hydroxy and oxyhydroxy compounds. The standard redox potential of Fe³⁺/Fe²⁺ (E₀  =  +0.77 V) is relevant only under acidic conditions, whereas at pH 7, typical in in situ and bulk meltwaters, the couples Fe(OH)₃/Fe²⁺ (E′₀  =  −0.236 V) or Fe(OH)₃ + HCO₃⁻ FeCO₃ (E′₀  =  +0.200 V) prevail (Widdel et al., 1993). Fe²⁺ should thus be able to donate electrons readily, facilitating the energy requirements of subglacial Fe oxidizing microbial populations. It is also worth considering that intergranular microenvironments in subglacial sediments will exhibit different aqueous chemistries than measured in the bulk solution in in situ environments. Such differences can often account for differences between biogeochemical mechanisms measured in the laboratory and field (Geesey and Mitchell, 2008). Research into biogeochemical transformations at the micro-scale in subglacial sediments should therefore be pursued.

Figure 5

Fe(II) and Fe(III) speciation in in situ meltwaters collected from the distributed system at the base of boreholes drilled to the bed of Haut Glacier d'Arolla.

Comparison of Modeling-Defined Changes in Subglacial Weathering Processes Over Seasonal Timescales

The biogeochemical evolution of bulk meltwaters are the product of reactions occurring both in the distributed and channelized system. The modeling so far demonstrates that while anoxic conditions can account for the evolution of waters collected directly from the distributed system, anoxia is unlikely to predominate over the majority of the bed at the height of the ablation season, but rather in areas with poor hydraulic connectivity. A constant supply of oxygenated waters into the subglacial system at this time suggests there will be a net consumption of dissolved oxygen. Therefore we consider the seasonal dynamics of selected weathering scenarios from the previous modeling phase, where net subglacial weathering reactions from supraglacial to bulk meltwaters are driven by organic carbon and pyrite oxidation by dissolved O₂. The seasonal time series bulk meltwater composition from the ablation season (Fig. 3) (Mitchell et al., 2001; 2006) were run through PHREEQCi, using concurrent input supraglacial water compositions with the aforementioned phase dissolution and precipitation constraints.

Based upon mineral dissolution rates, we consider some of the apparently most feasible weathering scenarios, WS41, WS43, and WS49, identified in Sections TESTING VIABLE CHEMICAL WEATHERING REACTIONS IN SUBGLACIAL ENVIRONMENTS USING PHREEQCi and TESTING THE COMBINED INFLUENCE OF CH₂O AND CO₂; (Table 4). These weathering scenarios are chosen because they exhibit markedly different seasonal weathering dynamics to each other, while WS49 allows us to investigate seasonal changes in organic carbon oxidation versus CO₂-driven weathering. This allows us to investigate whether the modeled variations in chemical weathering are consistent with known seasonal changes in the subglacial hydrological system, and changes in meltwater routing within this system at Haut Glacier d'Arolla (Nienow et al., 1998), and other alpine glaciers which undergo the same general seasonal changes in subglacial hydrology.

Carbonate and Silicate and Weathering

The assumptions of current models of subglacial biogeochemical weathering suggest that most Ca²⁺ derives from CaCO₃ dissolution, with the rest derived from Ca-silicates (Tranter et al., 2002, 2005). Other studies in glacierized and alpine catchments (Anderson et al., 1997; 2000) have also suggested that the chemically limited weathering of trace carbonate minerals dominates Ca²⁺ sources in glacial and alpine environments underlain by igneous and metamorphic bedrock (e.g. Drever and Zobrist, 1992; Brown, 2002). Similarly, WS43 suggests the weathering of trace calcite dominates on seasonal timescales, as shown by the mass of individual dissolving phases transferred between supraglacial and bulk meltwaters (Fig. 6; Table 6). Here the proportion of Ca²⁺ derived from calcite is comparable to previous studies (e.g. Anderson et al., 2000), ranging between 70 and 100% on average over the ablation season (Table 7).

Figure 6

Mass of individual dissolving phases (mg L⁻¹) transferred between input supraglacial meltwater and bulk meltwaters, as percentage of total mass from dissolving phases (% Mass Transfer), under weathering scenarios (a) 43, (b) 41, and (c) 49 over the ablation season. Incongruent silicate dissolution  =  total mass of silicates dissolved – mass precipitated in clays.

Table 6

Median mass of dissolving phases (mg L⁻¹) transferred between input supraglacial meltwater and bulk meltwaters under modeling weathering scenarios (WS) 43, 41, and 49 over the ablation season. Mass transfer of each phase also expressed as percentage of total mass transfer.

Table 7

Average percentage of species produced or consumed by the dissolution or precipitation of individual phases between input supraglacial meltwater and bulk meltwaters under weathering scenarios (WS) 43, 41, and 49 over the ablation season.

WS43 demonstrates there is very little change in the proportion of carbonate and silicate weathering with changes in bulk meltwater discharge over the ablation season (Fig. 6). This suggests that the proportion of carbonate and silicate weathering may not be significantly affected by the evolution of the drainage system on the seasonal timescale. This seems unlikely as such large seasonal changes in subglacial water routing and water residence times should impart a large impact on the weathering of lithologies with differing geochemical reactivity (Tranter et al., 2005). Indeed, consideration of WS41 and 49 seems more plausible than WS43 with regard to seasonal changes in subglacial flow routing and mineral weathering kinetics. Here, calcite dissolution and incongruent silicate weathering provide an approximately equal mass proportion due to high transfers of anorthite, both of which provide ∼40% of Ca²⁺, and tremolite, which provides the remaining ∼20% of Ca²⁺, on average, over the ablation season (Tables 6 and 7). Unlike WS43, calcite becomes proportionally more important while anorthite, tremolite, and total incongruent silicate dissolution becomes proportionally less important as discharge increases, such that calcite and anorthite weathering is approximately equal at the height of the ablation season (Fig. 6), supplying approximately equal amounts of Ca²⁺ (Fig. 7). Calcite exhaustion is unlikely at higher discharges, and experimental investigations suggest the rate of calcite dissolution is greatest at higher suspended sediment concentrations, as generated at higher discharges (Brown et al., 1996b). Indeed, chemically limited calcite weathering should dominate and be enhanced relative to silicate weathering at higher discharges, where the most reactive but not necessarily most abundant minerals will provide the majority of solute because sediment transport times are rapid relative to mineral weathering rates. Therefore, the observed increase in the proportion of calcite weathering and decrease in the proportion of incongruent silicate dissolution with increasing discharge in WS41 and 49 is most convincing with regard to flow routing and mineral weathering kinetics, and reflects the seasonal expansion of the channelized system.

Figure 7

Ca²⁺ and HCO₃⁻ provenance under weathering scenarios (a) 43, (b) 41, and (c) 49.

The modeling suggestion that Ca-silicate weathering may predominate in the distributed system may also be apparent in other subglacial environments. This may be of particular importance to polythermal glaciers, which override bedrock containing trace amounts of calcite, and where the development of channelized systems is limited, and most waters are routed through a distributed system (Wadham et al., 1998; Hodson et al., 2000). Here, silicate minerals, which are more abundant but generally less reactive than carbonates, have sufficient time to dissolve and may ultimately provide more solute than trace reactive minerals (Drever and Zobrist, 1992).