Influence of permafrost, rock and ice glaciers on chemistry of high-elevation ponds (NW Italian Alps)

The influence of atmospheric deposition and climate change driven catchment internal processes on the recovery from acidification of high-altitude Alpine lakes

The impact of atmospheric deposition and environmental factors on catchment processes and water chemistry of 20 high-altitude Alpine lakes in Southern Switzerland was investigated over four decades. Through the analysis of input-output budgets of sulphur (S), nitrogen (N), base cations and alkalinity significant trends emerged. Notably, S and N input concentrations significantly declined since the 1980s, by approximately 78 % and 22 %, respectively, with N primarily declining after 2000. Recovery from acidification was slightly delayed, likely due to the increased release of S, possibly originating from legacy S pools, alongside the simultaneous reduction in leaching of base cations from exchange sites. Catchments heavily impacted by thawing cryospheric features increasingly released S and base cations due to enhanced weathering processes, with hardly any impact on the recovery process, as evidenced by the balanced releases of S and base cations. N output concentrations followed the decrease of N input concentrations, while the relative N retention in the catchments remained relatively stable. Recently, both input concentrations of S and N have stabilised, while output concentrations of base cations began to increase across all catchments. The trend likely arises from the stabilisation of S and N input concentrations and/or the ongoing increase in weathering rates induced by climate change. Consequently, there was a consistent rise in alkalinity output concentrations even after the stabilisation of the S and N input concentrations. Ion ratio analysis suggests that carbonation primarily drives weathering processes in catchment areas unaffected by thawing cryosphere, while in areas impacted by thawing cryosphere, sulphide oxidation (or sulphate dissolution) is the dominant process. Further recovery depends on future N deposition and the effects of climate change.

Hydrochemistry dynamics in a glacierized headwater catchment of Lhasa River, Tibetan Plateau

Mountain glaciers are essential for supplying water resources that sustain downstream communities and livelihoods, yet the hydrogeochemical dynamics at glacier terminals and the impact of glacier retreat on downstream water chemistry are not fully understood. This study addresses this by conducting comprehensive observations and analysis of water chemistry at refined spatial and temporal resolutions in the Lhasa River Valley Glacier No. 1 (LRVG-1) catchment, a vital source of drinking and irrigation water for the local population on the Tibetan Plateau. Our findings reveal a weakly alkaline water environment within this glacierized basin, with HCO3- and Ca2+ as the dominant anions and cations, respectively, resulting in a hydrochemical pattern classified as HCO3--Ca2+ type. Solute concentrations increase along the glacier meltwater pathway, influenced by water-rock interaction, dilution, and diverse sources. The cations are predominantly from carbonate weathering, constituting 72.86 % of the total cations, followed by sulfide oxidation (11.08 %), glacier meltwater inputs (8.13 %), and silicate weathering (7.93 %). The contribution of cations from glacier meltwater diminishes as they travel along the glacier meltwater flow pathway. Our study indicates the localized yet significant impact of glacier meltwater on hydrochemistry, particularly in the vicinity of the glacier terminus. We recommend considering glacial meltwater and the entire glacier watershed as a continuum, essential for understanding the cumulative effects of glacier melt and human activities on water quality. This perspective is crucial for predicting future river chemistry trajectories in high-mountain basins and informing policy-making for water quality conservation across the Tibetan Plateau.

Changes in air temperature, but not in precipitation, determine long-term trends in water chemistry of high mountain lakes of the Alps with and without rock glacier influence

Climate change has strongly affected lakes around the world, but the relative effects of warmer air temperatures and changing precipitation on the water chemistry of alpine systems are not well understood. Here we tested the effect of monthly and seasonal climate on the water chemistry of six high mountain lakes located in the Alps. From 1982 to 2020, water samples were collected annually from different depths during the autumn mixing. We observed a simultaneous increase in electrical conductivity, ionic content, and pH with air temperature. In lakes with rock glacier influence, the increase in conductivity, ionic content, and especially in sulfate was even more pronounced, but accompanied by a strong decrease in pH. These differences are attributed to the direct influence of acidic meltwater from active rock glaciers in catchments with acidic bedrock. We then analyzed changes in lake chemistry, taking into account seasonal trends in air temperature and precipitation, using redundancy analysis. Temperature increase significantly affected water chemistry in five of the six lakes, especially at times of ice breakup. Increasing warming explained 17% to 32% of the changes in electrical conductivity, alkalinity, pH, major ions, and nitrogen. In contrast, precipitation had little effect on the changes of those parameters. Nevertheless, late spring snowfall and high snowfall in early fall, which result in prolonged ice cover, had a dampening effect on the impact of climate warming on lake chemistry. Our results confirm that climate warming remains a major driver of chemical changes in alpine lakes, but provide new evidence that late spring temperatures are the most important triggers.

Chemical weathering in glacial catchment acting as a net carbon source

Over geological time scales, continental silicate weathering is considered as a critical carbon sink that regulates long-term climate feedback. By contrast, recent studies indicate that sulfide oxidation during weathering can be as a potential carbon source. However, whether chemical weathering in glacial conditions characterized by extreme erosion is a net carbon sink or source remains elusive. Here, we present the seasonal carbon cycle processes in a typical glacier catchment, via high-resolution (weekly) river water sampling during the whole 2017 in the Laohugou river, northeastern Tibetan Plateau. Our seasonal result shows that the release of CO2 by sulfide oxidation during the monsoon period can be much faster than CO2 consumption through weathering of silicate rocks, with maximum of ∼26 times. Extending to global glacial basins, we observed a consistent pattern that inorganic carbon releases in alpine glaciers are faster than atmospheric CO2 consumption. We propose that weathering in global glacial environment acts as a significant carbon source, and thus affects climate feedback.

Influence of permafrost and hydrogeology on seasonal and spatial variations in water chemistry of an alpine river in the northeastern Qinghai-Tibet Plateau, China

Known as the third pole of the world, the Qinghai-Tibet Plateau has been experiencing rapid permafrost warming and thawing over the last few decades. However, the impact of permafrost distribution and hydrogeology on river hydrochemistry in alpine areas remains unclear. This study conducted four sampling campaigns to reveal the temporal and spatial variations in and factors driving river hydrochemistry in the upper reaches of the Heihe River, the northeastern Qinghai-Tibet Plateau. We found that the concentrations of major ions and total dissolved solids (TDS) in river water showed substantial seasonal variations; the concentrations were generally lower during the initial thawing and thawed periods than during the initial freezing period. However, solute fluxes during the thawed period were much higher than those during the frozen period. The concentrations of major ions and TDS gradually decreased to a minimum from the permafrost meander (PM) section to the seasonal frost meander (SFM) section and then increased the seasonal frost canyon section. Using the revised forward model, we found that river solutes were contributed by carbonate weathering (mean 38.9%) > sulfide oxidation (22.9%) > evaporite dissolution (20.2%) > atmospheric precipitation (8.7%) > silicate weathering (5.0%) > glacial meltwater (4.3%). The higher TDS, Na⁺, Cl^-, Ca²⁺, Mg²⁺, and SO₄^2- concentrations in the PM section reflected the influence of freeze-out fractionation. The concentrations of major ions and TDS were lowest in the SFM section, indicating that the riparian porous aquifer was essential in regulating river hydrochemistry, thus reducing its spatiotemporal variations in the alpine area. In the mountain glacier-hillslope-riparian porous aquifer-river system, the river was mainly recharged by groundwater with insufficient water-rock interactions due to the rapid flow owing to the high elevation difference and high permeability of the riparian quaternary porous aquifers. Our findings provide insights into the construction of hydrogeochemical models in alpine areas and are practically important for the scientific management of water resources in the Qinghai-Tibet Plateau.

Seasonal variations in the optical characteristics of dissolved organic matter in glacial pond water

Large amounts of dissolved organic matter (DOM) are stored in mountain glaciers. However, few researches have analysed the optical characteristics of DOM in surface waters fed by mountain glaciers and their seasonal variations. In a pond fed by a glacier we observed simultaneous decreases in the dissolved organic carbon, and increases in both absorbance at 254 nm and specific absorption coefficient (SUVA₂₅₄) during the ice-free season 2015. This behaviour differs from the typical behaviour of lake/pond water in summer, and from the trends observed in a nearby pond not fed by a glacier. The trends of DOM properties, main ions and water stable isotopes at the glacier-fed pond could be attributed to transient modifications of the subglacial hydrological system. Flushing of previously isolated pools of subglacially stored water, containing terrestrial DOM derived from glacially-overridden soil and vegetation, would be driven by intense rainfall events during the melting season. These findings suggest that heavy rainfall events during the melting season have the capability to transiently modify the characteristics of DOM in a glacial pond. These events may be further exacerbated in the future, as summer rainfall events in the Alps are predicted to increase due to global warming.

Modeling Surface Processes on Debris-Covered Glaciers: A Review with Reference to the High Mountain Asia

Surface processes on debris-covered glaciers are governed by a variety of controlling factors including climate, debris load, water bodies, and topography. Currently, we have not achieved a general consensus on the role of supraglacial processes in regulating climate–glacier sensitivity in High Mountain Asia, which is mainly due to a lack of an integrated understanding of glacier surface dynamics as a function of debris properties, mass movement, and ponding. Therefore, further investigations on supraglacial processes is needed in order to provide more accurate assessments of the hydrological cycle, water resources, and natural hazards in the region. Given the scarcity of long-term in situ data and the difficulty of conducting fieldwork on these glaciers, many numerical models have been developed by recent studies. This review summarizes our current knowledge of surface processes on debris-covered glaciers with an emphasis on the related modeling efforts. We present an integrated view on how numerical modeling provide insights into glacier surface ablation, supraglacial debris transport, morphological variation, pond dynamics, and ice-cliff evolution. We also highlight the remote sensing approaches that facilitate modeling, and discuss the limitations of existing models regarding their capabilities to address coupled processes on debris-covered glaciers and suggest research directions.

Alpine headwaters emerging from glaciers and rock glaciers host different bacterial communities: Ecological implications for the future

Mountain glacier shrinkage represents a major effect of the current global warming and 80-100% of the Alpine glaciers are predicted to vanish within the next few decades. As the thawing rate of mountain permafrost ice is much lower than for glacier ice, a shift from glacial to periglacial dynamics is predicted for Alpine landscapes during the 21st century. Despite the growing literature on the impacts of deglaciation on Alpine hydrology and ecosystems, chemical and biological features of waters emerging from Alpine rock glaciers (i.e. permafrost landforms composed by a mixture of ice and debris) have been poorly investigated so far, and knowledge on microbial biodiversity of headwaters is still sparse. A set of glacier-, rock glacier- and groundwater/precipitation-fed streams was investigated in the Italian Central Alps in late summer 2016, aiming at exploring bacterial community composition and diversity in epilithic and surface sediment biofilm and at verifying the hypothesis that rock glacier-fed headwaters represent peculiar ecosystems from both a chemical and biological point of view. Rock glacier-fed waters showed high values of electrical conductivity and trace elements related to their bedrock lithology, and their highly diverse bacterial assemblages significantly differed from those detected in glacier-fed streams. Bacterial taxonomic composition appeared to be mainly related to water and substrate type, as well as to water chemistry, the latter including concentrations of nutrients and trace metals. The results of this study confirm the chemical and biological peculiarity of rock glacier-fed waters compared to glacial waters, and suggest a potential driving role of thawing permafrost in modulating future ecological traits of Alpine headwaters within the context of progressing deglaciation.

Decadal trends in water chemistry of Alpine lakes in calcareous catchments driven by climate change

High mountain lakes are considered sensitive indicators of the effects of natural and anthropogenic drivers, including atmospheric deposition and climate change. In this study, we assess long-term trends in the chemistry of a group of high altitude lakes in the Western Alps, Italy, lying in bedrock with a relevant presence of basic, soluble rocks. An in-depth investigation was performed on two key-sites (Lakes Boden Inferiore and Superiore) for which continuous chemical data are available for a period of 30 years. A group of 10 additional lakes in the same area was also considered; these lakes were sampled at the end of the ice-free period during irregular surveys in the period 1980-2017. Water samples were analysed for the main chemical variables, including pH, electrical conductivity, major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, Cl^-, SO₄^2-, NO₃^-) and algal nutrients (phosphorus and nitrogen compounds, reactive silica). A steep increase in conductivity and ion concentrations was detected at the key-sites: conductivity increased from 40-45 to 60-70 µS cm^-1 over the period 1984-2017; sulphate concentrations more than doubled over the same period (from 50-60 to 120-180 µeq L^-1) and base cations increased from 400-500 to 600-750 µeq L^-1. An increase in the solute content was also detected in the survey lakes (average conductivity from 39 ± 20 to 57 ± 23 µS cm^-1). The analysis of meteorological data revealed a significant increase of air temperature (0.019 °C y^-1 over the period 1950-2017), mainly in spring and summer (0.033 °C y^-1), and a decrease of snow cover depth and duration. Meteo-climatic drivers were identified as the responsible for the chemical changes occurred in the lakes. Climate-driven effects on weathering rates were mainly indirect and occurred by affecting the flow paths of water at both surface and subsurface level. Cryosphere modification (reduced snow cover and permafrost thawing) also played a role.

Headwaters’ Isotopic Signature as a Tracer of Stream Origins and Climatic Anomalies: Evidence from the Italian Alps in Summer 2018

Glaciers are shrinking due to global warming, resulting in a diminishing contribution of ice and snowmelt to headwaters and subsequent consequences to freshwater ecosystems. Within this context, we tested whether water-stable isotopes are spatio-temporal tracers of (i) water in high altitude periglacial environments, being the isotopic signature of surface water inherited from the snow/icemelt, groundwater, and rainfall; and (ii) regional (year-specific) meteorological conditions, being the isotopic signature of precipitations affected by air temperature, humidity and aqueous vapour origin, ascribing stable isotopes to the list of “essential climate variables″ (ECVs). To this end, we investigated the ionic and isotopic composition (δ18O and δ2H) of six high-altitude streams and one pond in the Italian Alps (Noce and Sarca basins) during the ablation season in 2018. Differences between habitat types (pond, kryal, rhithral, krenal) were detected. More negative values of δ18O and δ2H were recorded in the kryal and glacio-rhithral sites, dominated by ice and snowmelt, in early summer. Less negative values were recorded in these sites in late summer, as well as in the krenal sites, which were dominated by groundwater and rainfall inputs. The isotopic results also show that the complex alpine orography influences air masses and moisture, ultimately resulting in isotopic differences in the precipitations of neighboring but distinct catchments (Sarca and Noce basins). On average, less negative values were recorded in the Sarca basin, characterized by a higher contribution of precipitation of Mediterranean origin. In general, isotopic results of the entire water population appeared to be strongly influenced by the regional climatic anomaly of 2018, which was anomalously warm. Therefore, the study will provide additional information for the climate change debate, proposing water isotopes as ECVs for assessing change in a warmer future.