4.1 Chemical characteristics of precipitation and supraglacial waters

To better understand the chemistry of the proglacial stream emerging from the snout of Dokriani glacier basin, we first examined the precipitation chemistry as snowfall, rainfall and supraglacial waters. It represents deposition of atmospheric pollutants and suspended dust particles in the snow. The deposition chemistry was assessed using snow samples collected through snowpit profiling. We reassessed the ionic variability of two snowpit profiles collected at an altitude of 4350 and 4364 m a.s.l. during the study period of 2013–2015 (Sundriyal and others, Reference Sundriyal2018). The observed K⁺/Na⁺ (0.42), Mg²⁺/Na⁺ (0.27) and Ca²⁺/Na⁺ (3.66) ratios in the snowpit samples were significantly higher than sea salts ratios i.e. K⁺/Na⁺ (0.02), Mg²⁺/Na⁺ (0.11) and Ca²⁺/Na⁺ (0.02). This suggests that the ionic contribution from terrestrial sources is higher than atmospheric deposition at Dokriani glacier basin. However, the concentration of SO₄²⁻ ion was similar to Ca²⁺ which could be correlated with the dissolution of carbonate dust in supraglacial melt waters during melting (Tranter and others, Reference Tranter2002; Stachnik and others, Reference Stachnik2016). Earlier observations also support the fact that the contribution of Na⁺, K⁺, Ca²⁺ and Mg²⁺ is principally controlled by various other factors like weathering, and industrial or biological missions (Nijampurkar and others, Reference Nijampurkar, Sarin and Rao1993; Singh and Ramanathan, Reference Singh and Ramanathan2017; Singh and others, Reference Singh, Keshari and Ramanathan2020). Furthermore, it was also observed that the ratio between the different ions like Cl⁻/Na⁺, K⁺/Na⁺, Mg²⁺/Na⁺ and Ca²⁺/Na⁺ are elevated in the snow pit compared with those in surface snow. It was likely to be explained by the elution effect of snow melting during the peak ablation season. Generally, most of the ions are concentrated on the surface and most of the salts concentrate on the snow surface which easily gets eluted by percolating meltwater. Further, the chemical trends of wet deposition were assessed through ionic concentration of rainfall samples which suggest the variability of major anions (HCO₃⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻ > F⁻) and cation (Ca²⁺ > Na⁺ > Mg²⁺ > K⁺) follow the similar trends as stream chemistries of the region. Here the HCO₃⁻ and Ca²⁺ was found to be the most abundant anion and cation in the rainfall respectively.

4.2 Temporal trends of water chemistry at the hydrometric station

To better understand the seasonal variation and quantify the prominent change in the meltwater chemistry at the Dokriani glacier basin, different ablation periods i.e. early ablation, peak ablation and late ablation were studied (Fig. 3). The variabilities observed are presented below:

Figure 3. Seasonal concentration of ions representing different ablation at Dokriani Glacier basin. Datasets represent the studies done in years 1992, 1994 (taken from Hasnain and Thayyen, Reference Hasnain and Thayyen1996, Reference Hasnain and Thayyen1999 respectively); and years 2015 (taken from Tiwari and others, Reference Tiwari2018). Rest datasets were generated in the present study. Datasets are presented in supplementary tables S7 and S8.

4.2.1 Early ablation season

Early ablation chemistries of Dokriani glacier basin were first accessed through piper plots (Fig. 4). The results suggest a shift in ionic chemistries, for example, early ablation datasets show that the chemistries of the years 1994 changed from Ca-Mg-HCO₃ type to Ca-Mg-Cl-SO₄ type. We observed the dominance of alkali metals (Na⁺ + K⁺) and HCO₃⁻ composition in the meltwater during the years 1992 and 1994 respectively. However during the years 2015–2018, alkaline earths metals (Ca²⁺ + Mg²⁺) and SO₄²⁻ were dominant. The early ablation chemistries of years 2015–2018 suggests that the physical parameters recorded from the hydrological site range as follows: pH 5.3 to 7.4, EC from 43.8 to 170 μS cm⁻¹, whereas Na⁺ and Cl⁻ show the range 3.8–54.70 and 2.5–13.2 μE L⁻¹ respectively, and SO₄²⁻ shows the highest concentration among anions i.e. from 66.1–433.69 μE L⁻¹. Examined order of variability for ions was as follows: SO₄²⁻ > NO₃⁻ > HCO₃⁻ > F⁻ > Cl⁻ for anion and Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ for cation during the early ablation season. The pH value (<7) of the early ablation period suggests a slightly acidic nature of the meltwater stream; however, with in the range of other central and western Himalayan glaciers (Hasnain and Thayyen, Reference Hasnain and Thayyen1999; Singh and Ramanathan, Reference Singh and Ramanathan2017). The average concentration and std dev. of the datasets are presented in supplementary table S7. Among the major anions, SO₄²⁻ constitutes up to 80% of TZ⁻ (total anions) concentration and Ca²⁺ a major cation adds up to 76% of TZ⁺ (total cations) concentration. At the start of June month, the concentrations of ions decreased slightly, much likely due to the dilution effect caused by heavy rainfall events. While as the year progressed a slight change (drop) in concentrations for all ions was observed throughout the ablation season. This might be linked with the dilution effect of discharge, however, due to the absence of daily discharge data it is difficult to link with variability of chemical weathering patterns.

Figure 4. Piper plot of the water samples collected for different ablation seasons between the years 1992 and 2018. For data sources please read Fig. 3 caption. Color symbols reflect the water type and study period. The main rock types by carbonic and sulfuric acid were taken from Spence and Telmer (Reference Spence and Telmer2005).

4.3 Influence of discharge over ionic chemistries over time: c-Q relationships

To assess the c-Q relationships of major ions released from the glacier basin, all the datasets collected in present study and from published estimates from Dokriani glacier basin have been combined (Fig. 5). This was done as the datasets in the present study is collected for different ablation periods and accurately represents the characteristics of different seasons from Dokriani glacier system. We observed that the c-Q relationships for early, peak and late ablation periods were significantly distinctive (Fig. 5). The early to peak ablation period has witnessed an increase in the average discharge from 2.5 to 5 m³ s⁻¹ much likely due to increased rainfall and its attendant runoff. This pattern shows a gradual decrease during the late ablation season commencing from the start of late ablation period i.e. September month. Although, the overall trend of c-Q relationship was very weak (>0.2) for all ions, as we observed a decreasing trend initially followed by an increase in discharge values, much likely resulting through the dilution effect. Similarly, we observed some variability for higher discharge values as well particularly for Ca²⁺ ion. For example, correlations for Ca²⁺ ion were observed negative for peak ablation periods, positive for late ablation periods and no significant correlation for early ablation periods. We observed a clear negative slopes of c-Q relationships for peak ablation periods suggesting dilution effect, while a near zero slopes for different early and late ablation periods (e.g. SO₄²⁻, Ca²⁺ + Mg²⁺ and Na⁺ + K⁺) suggest the system might behave similar to chemostatic behavior of ions. We clarify that the chemostatic behavior of ions should theoretically falls under the slope of −0.2 to 0.2, and except for Na⁺ + K⁺ none fall under this criterion. However, we observed that the trends of peak ablation period datasets are not as identical as early and late ablation periods. For example, the trends of Ca⁺² + Mg²⁺ with discharge during early and late ablation periods suggest the concentration of ions has increased with respect to discharge. However, during early ablation periods this rate (increasing) was slower compared to the late ablation period. This is much likely due to the slower melting rates during the late ablation period. However, during peak ablation concentration of Ca²⁺ + Mg²⁺ ions first decreased as discharge increases and then we observed a increasing trend, however, the rates were faster than the early and late ablation periods. We explain this variability with higher discharge generated through high rainfall and subsequent fast glacier melting during the peak ablation period. The trends of other ions i.e. Na⁺ + K⁺, SO₄²⁻ and HCO₃⁻ with discharge has almost followed the trend similar to Ca²⁺ + Mg²⁺ with discharge. Overall, we conclude that the ionic release during early and late ablation periods was controlled by the geochemical reactions operating in the glacial/subglacial zones while the discharge has little or second order effect. While during peak ablation periods we could not be able to draw any conclusions due to a large variability observed in the datapoints.

Figure 5. Concentration of conservative ions plotted against discharge variability during different ablation periods at Dokriani glacier basin. Datapoints having corresponding discharge values are presented here between the years 1994 and 2018. Solutes with negative slopes refer to dilution and will have their lowest concentrations at high flows, and thus will exhibit increasing concentrations during hydrograph recession. Because this concentration increase is usually less than proportional to the decrease in discharge, power-law cQ slopes are rarely steeper than −1. Solutes with slopes near zero refer as chemostatic which do not vary systematically with discharge. Solutes with positive slopes refer mobilization which exhibit higher concentrations at high flows, and decreasing concentrations during hydrograph recession. Power-law slopes steeper than 1 indicate that concentrations change more than proportionally to discharge.

4.4 Lithological controls of chemical weathering trends in glacial environment

Lithological controls over ionic release were characterized by the availability of H⁺ ion that interacted with rock/mineral surface and results in reactions presented below. The reactions (Eqns (1)–(7)) suggests the possible origin of ions in meltwater could through following reactions i.e. dissolution of silicate and carbonate minerals, organic carbon oxidation, sulfide oxidation reaction coupled with carbonate (SOCD)/feldspar dissolution (SOSW) and carbonation of carbonate. The reactions presented here have disproportionate effects on the ionic release and results in a variable change in ionic ratios and compositions. We presented the stoichiometry of ions through scatter plot and linear regressions (Figs 6–8) generated through the reactions listed below, but in different proportions. For example, the SOCD reaction (Eqn (1)) generates the SO₄²⁻ ion at 1 : 2 ratio with HCO₃⁻ and Ca²⁺ + Mg²⁺, therefore the gradients (as dash lines at Figs 6–8) were presented as a reflection of its stoichiometry. Similarly, the carbonation of carbonates (Eqn (3)) and carbonation of feldspars reaction (Eqn (4)) has slope coefficient of 1 for Ca²⁺ + Mg²⁺ vs HCO₃⁻ respectively. Plots of HCO₃⁻ vs SO₄²⁻ in Figure 6 suggests the gradients of ~1 and 2 and also in Ca²⁺ + Mg²⁺ vs SO₄²⁻ reflects the coupling of SOCD reaction via oxic or anoxic mechanism (Tranter and others, Reference Tranter2002; Wadham and others, Reference Wadham2010a; Reference Wadham2010b). Further, the gradients of <1 in HCO₃⁻ vs SO₄²⁻ ionic plot suggests the process of H⁺ generation during oxidation of sulfide minerals that might be used in dissolution reactions of silicate and carbonate minerals occurring simultaneously. Since the Dokriani Glacier is a well-developed valley system with well-defined subglacial drainage networks, gradients of 1 and 2 are close to the predictive frameworks. We noticed that a very low and intermediate gradients of HCO₃⁻ vs SO₄²⁻ in the late ablation periods, which reflect the rapid flushing of ions as surface melt inputs were mostly absent during late ablation periods.

Figure 6. Association of ions for Dokriani Glacier system. Figure present the trends of ionic release resulting from geochemical reactions (Eqns (1)–(7)) operating at the glacial/subglacial system of the Dokriani glacier basin. Data sources are detailed in Fig. 3 caption. Dash lines refer to the release of ions presented in Eqns (1) and (2). For details please refer to section 4.4.

Sulfide oxidation coupled to carbonate dissolution (SOCD)

(1)$$\eqalign{& 4{\rm Fe}{\rm S}_2 + 16{\rm Ca}( {{\rm Mg}} ) {\rm C}{\rm O}_3 + 15{\rm O}_2 + 14{\rm H}_2{\rm O}\to \cr & \quad 16{\rm C}{\rm a}^{2 + } + 16{\rm M}{\rm g}^{2 + } + 16{\rm HC}{\rm O}_3^-{ + } 8{\rm S}{\rm O}_4^{2-} + 4{\rm Fe}( {{\rm OH}} ) _3} $$

Sulfide oxidation coupled to silicate weathering (SOSW)

(2)$$\eqalignb{& 4{\rm Fe}{\rm S}_2 + 16{\rm NaKAlS}{\rm i}_3{\rm O}_8 + 15{\rm O}_2 + 86{\rm H}_2{\rm O}\to \cr & \quad 16{\rm N}{\rm a}^ + { + } 16{\rm K}^ + { + } 8{\rm S}{\rm O}_4^{2-} + 4{\rm A}{\rm l}_4{\rm S}{\rm i}_4{\rm O}_{10}( {{\rm OH}} ) _8 + 32{\rm H}_4{\rm Si}{\rm O}_4 \cr& \quad + 4{\rm Fe}( {{\rm OH}} ) _3} $$

Carbonation of carbonate

(3)$${\rm Ca}( {{\rm Mg}} ) {\rm C}{\rm O}_3 + {\rm C}{\rm O}_2 + {\rm H}_2{\rm O}\to {\rm C}{\rm a}^{2 + } + {\rm M}{\rm g}^{2 + } + 2{\rm HC}{\rm O}_3^- $$

Carbonation of feldspar (albite/microcline-orthoclase)

(4)$$\eqalign{& 2( {{\rm Na}, \;{\rm K}} ) {\rm AlS}{\rm i}_2{\rm O}_8 + 2{\rm C}{\rm O}_2 + 9{\rm H}_2{\rm O}\to \cr & \quad 2( {{\rm N}{\rm a}^ + , \;{\rm K}^ + } ) + 2{\rm HC}{\rm O}_3^-{ + } {\rm A}{\rm l}_2{\rm S}{\rm i}_2{\rm O}_5 + 4{\rm H}_4{\rm Si}{\rm O}_4} $$

Dissolution of gypsum (an example of efflorescent salts)

(5)$${\rm CaS}{\rm O}_4.2{\rm H}_2{\rm O}\to {\rm C}{\rm a}^{2 + } + {\rm S}{\rm O}_4^{2-} + 2{\rm H}_2{\rm O}$$

Simple hydrolysis of carbonate

(6)$${\rm Ca}( {{\rm Mg}} ) {\rm C}{\rm O}_3 + {\rm H}_2{\rm O}\to {\rm C}{\rm a}^{2 + } + {\rm H}_2{\rm O}\to {\rm H}^ + { + } {\rm HC}{\rm O}_3^- $$

Oxidation of organic carbon

(7)$${\rm C}_{{\rm org}} + {\rm O}_2 + {\rm H}_2{\rm O}\to {\rm C}{\rm O}_2 + {\rm H}_2{\rm O}\to {\rm H}^ + { + } {\rm HC}{\rm O}_3^- $$

Further, Figs 7 and 8 assess the dominance of SOCD reaction in ionic release for different ablation periods and compare with other Himalayan glaciers. The gradient represents products of chemical reactions explained above. The SOCD reaction has slope coefficients of 2.0 to 1.0 for ion associations with total cations (TZ⁺) vs SO₄²⁻ and TZ⁺ vs HCO₃⁻ respectively (Eqns (3) and (4)) (Tranter and others, Reference Tranter2002; Wadham and others, Reference Wadham2010a). We noticed that the sulfuric mineral weathering (most probably SOSW, Eqn (2)) is not occurring in concurrence with silicate weathering as all the data sets falling between organic matter oxidation and SDC line in 1 : 1 equliline proportions for the whole ablation season (both in Figs 7 and 8). SOSD reaction results in the increase of SO₄²⁻ concentrations leading to rise the slope of TZ⁺ vs SO₄²⁻ and fall in the slope of TZ⁺ vs HCO₃⁻. Tranter and others (Reference Tranter2002) suggested that this intercept predicts the initial stage of weathering when water comes in contact with rock for the very first time in early ablation seasons and release one Ca⁺² and HCO₃⁻ ion in meltwater through the simple hydrolysis reaction (Eqn (6)). However, the higher intercept indicates the production of supplementary H⁺ ions formed by other reactions such as the oxidation of organic carbon present in glacial substrate soils (Eqn (7)). In the case of simple hydrolysis through weathering silicate minerals, pH increases but the intercept didn't change. Further, the assumption that if Ca⁺² and Mg²⁺ was derived mainly from the carbonate weathering then the proportions of SO₄²⁻ and HCO₃⁻ (Figs 7 and 8) release in meltwater indicates that weathering reactions of sulpate bearing mineral are operational in the meltwater. A decent slope of intercept between TZ⁺ vs SO₄²⁻ and TZ⁺ vs HCO₃⁻ (r ² = 0.75 and r ² = 0.31) in Figure 7 confirms that the sulfuric acid mediated reaction has occurred prominently in the subglacial environments of the Dokriani glacier basin. The bivariate plots of total cations (Ca+ Mg + Na + K) at y-axis plotted against most domination anions of SO₄⁻² and HCO₃⁻ represent the products of carbonate, silicate and sulfuric mineral dissolution process. The observations presented for seasonal variability at Dokriani glacier basin (Fig. 7) shows that the cation release in meltwater is weakly correlated with the SOSD and silicate dissolution through the carbonic acid process. Therefore, the SOSD and SDC mineral weathering effect in the ionic release is very less likely. Further, in bottom two figures (in Fig. 7) suggests the total cationic release from carbonate weathering reactions are dominant and a clear control of SOCD reaction in ionic release in the subglacial reactions is visible. Seasonal effects (mainly discharge) do not seem to have a significant effect on SOCD reaction. Yet some Ca²⁺ and Mg²⁺ ions have been originating along with HCO₃⁻ which reflects the possibility of simple hydrolysis reaction occurrence in the meltwater drainage from Himalayan glaciers. The results obtained by the seasonal ionic evolution at Dokriani glacier basin are consistent with other Himalayan glaciers as well (Fig. 8).

Figure 7. Plots of total cations TZ⁺ vs sulfate and bicarbonate ion concentrations for seasonal evolution of ions from Dokriani glacier basin. Here ‘*’ symbol symbolizes the ions corrected for precipitation inputs. The solid line represents orthogonal regression lines for each ablation season with the theoretical slopes (dashed and red arrows). These lines were based on ideal reactions shown in the main text. Data sources are detailed in Fig. 3 caption.

Figure 8. Plots of total cations TZ⁺ vs sulfate and bicarbonate ion concentrations from different glacier system of the Himalaya. Here ‘*’ symbol symbolizes the ions corrected for precipitation inputs. The solid line represents orthogonal regression lines for each ablation season with the theoretical slopes (dashed and red arrows). These lines were based on ideal reactions shown in the main text. Data sources are detailed in Fig. 3 caption.

Overall it has been observed in the Himalayan glaciers that Ca²⁺ and Mg²⁺ is the dominant anion and HCO₃⁻ and SO₄⁻² has been the dominant cation in the glacier systems. The trends of seasonal ions release from the Dokriani glacier basin (Fig. 7) and other parts of the Himalaya (Fig. 8) implies that among geochemical reactions listed in Eqns (1)–(7), two reactions i.e. SOCD and carbonate mineral dissolution dominates. The oxidation of FeS₂ could be a key reaction in the subglacial environment having active role in chemical weathering. We observed that the seasonal pattern of ionic release from Dokriani Glacier (Fig. 7) is similar through years i.e. no such change in the reaction patters form 1992 to 2018. Only change we observed in piper plot that the water become more acidic through time. We surmise that since the glacier has retreated significantly (approximately 1 kilometer) in last 28 years and left more fresh mineral surface to react. The high ratio of silicate weathering also explains the presence of active layer on the top of the mineral surfaces which consists of the loose mineral surface, the majority of data suggests that the carbonate phases leached out after that only silicate mineral remain to chemically weather. The nature of weathering reactions observed here indicates the dominance of Ca²⁺, SO₄²⁻ and HCO₃⁻ ions in the meltwater chemistries, along with SOCD reaction therefore, the control of specific minerals in lithology over ionic release is evidenced. Therefore, we contend that the bedrock lithology has a second-order effect over ionic release. The first order control is the chemical weathering reaction typess which operates in the glacial substrats. This assertion could be verified by the fact that Himalayan glaciers are composed of silicate rocks yet the carbonate weathering is dominant. This is true for other glaciers and ice sheets as well and it is much likely due to the rapid kinetics of the carbonate mineral dissolution process worldwide.

4.5 Climatic control over chemical weathering trends

Dokriani glacier basin is an ideal landscape to isolate the effects of climate-induced changes in glacier and chemical weathering trends because it has been studied on long-term basis using field methods and modeling aspects. We used the recent study (Azam and Srivastava, Reference Azam and Srivastava2020) integrating the field-validated modeling efforts of climate trends and change in the glacial state of Dokriani glacier basin to compare the long-term (1980–2020) chemical weathering trends of the Dokriani Glacier. Our results from the ionic chemistries at the Dokriani glacier basin (Fig. 3) suggest no significant change in concentrations, ionic dominance saturation indices of calcite, pCO₂ levels and SMF fractions (Fig. S2). This could be correlated to near steady-state conditions of the Dokriani glacier between 1992 and 1997 and moderate mass wasting rates between 2007 and 2018 (Azam and Srivastava, Reference Azam and Srivastava2020). The reason behind more acidic nature of the glacier suggested by the piper plot (Fig. 4), could be correlated to increased mass wasting rates of the Dokriani glacier basin. Corroborated to present study, the increased chemical weathering rates are also observed in previous studies (Li and others, Reference Li2022 and references therein). Authors suggested the cation denudation rates has increased upto three times in last two decades. Having positive correlation with temperature, chemical weathering rates are supposed to get higher through time (Li and others, Reference Li2022). Yet the overall glacier-wide mass balance was negative in the last four decades, there are no significant changes were observed in mass balance and discharge trends (Fig. S3). Moving further, we applied change point analysis (by using the Past software version 4.11), to detect changes in time series events. The results detect no change in annual, summer mass balance, ELA and AAR yet we observed a distinct trend in a few datasets as winter mass balance has decreased slightly after 2000, however, a slight gain in summer mass balance and discharge is observed (Fig. S4). This change in mass balance trends and discharge has slight impact over the acidic nature and increased ionic release from the Dokriani glacier basin. However, these assertions so not fully explain the control of climate induced mass wasting rates from glacier in question. We contend that similar to lithology, climatic impacts also have second or third-order effects over ionic release. The chemical reactions seem to be controlling the ionic release pattern in the Himalayan glacial basins. Yet more studies are required to ascertain this assumption. Our results highlight a fine balance between weathering mechanisms, and underlying processes linked with climate lithology and glacial intrinsic parameters, yet some accurate assessment of the net effects of weathering budgets on atmospheric chemistry is required further.