Ice sheets matter for the global carbon cycle

Microbial oases in the ice: A state-of-the-art review on cryoconite holes as diversity hotspots and their scientific connotations

Cryoconite holes, small meltwater pools on the surface of glaciers and ice sheets, represent extremely cold ecosystems teeming with diverse microbial life. Cryoconite holes exhibit greater susceptibility to the impacts of climate change, underlining the imperative nature of investigating microbial communities as an essential module of polar and alpine ecosystem monitoring efforts. Microbes in cryoconite holes play a critical role in nutrient cycling and can produce bioactive compounds, holding promise for industrial and pharmaceutical innovation. Understanding microbial diversity in these delicate ecosystems is essential for effective conservation strategies. Therefore, this review discusses the microbial diversity in these extreme environments, aiming to unveil the complexity of their microbial communities. The current study envisages that cryoconite holes as distinctive ecosystems encompass a multitude of taxonomically diverse and functionally adaptable microorganisms that exhibit a rich microbial diversity and possess intricate ecological functions. By investigating microbial diversity and ecological functions of cryoconite holes, this study aims to contribute valuable insights into the broader field of environmental microbiology and enhance further understanding of these ecosystems. This review seeks to provide a holistic overview regarding the formation, evolution, characterization, and molecular adaptations of cryoconite holes. Furthermore, future research directions and challenges underlining the need for long-term monitoring, and ethical considerations in preserving these pristine environments are also provided. Addressing these challenges and resolutely pursuing future research directions promises to enrich our comprehension of microbial diversity within cryoconite holes, revealing the broader ecological and biogeochemical implications. The inferences derived from the present study will provide researchers, ecologists, and policymakers with a profound understanding of the significance and utility of cryoconite holes in unveiling the microbial diversity and its potential applications.

CH4 emissions from runoff water of Alaskan mountain glaciers

Recent studies have observed high methane concentrations in runoff water and the ambient air at various glacier sites, including the Greenland Ice Sheet, the glacier forefield in Svalbard, and the ice cap in Iceland. This study extends these findings to smaller mountain glaciers in Alaska. Methane and carbon dioxide concentrations in the ambient air near the meltwater outlet, fluxes of these gases at the surface of runoff water and riverbank sediments, and dissolved methane content in the runoff water were measured at four glaciers. Three of the four glaciers showed conspicuous signals of methane emissions from runoff water, with the Castner Glacier terminus exhibiting a methane concentration three times higher than background levels, along with elevated dissolved methane levels in the runoff water. This study marks the detection of significant methane emissions from small mountain glacier runoff, contributing to the understanding that mountain glaciers also release methane into the atmosphere.

Sediment discharge from Greenland's marine-terminating glaciers is linked with surface melt

Sediment discharged from the Greenland Ice Sheet delivers nutrients to marine ecosystems around Greenland and shapes seafloor habitats. Current estimates of the total sediment flux are constrained by observations from land-terminating glaciers only. Addressing this gap, our study presents a budget derived from observations at 30 marine-margin locations. Analyzing sediment cores from nine glaciated fjords, we assess spatial deposition since 1950. A significant correlation is established between mass accumulation rates, normalized by surface runoff, and distance down-fjord. This enables calculating annual sediment flux at any fjord point based on nearby marine-terminating outlet glacier melt data. Findings reveal a total annual sediment flux of 1.324 + /- 0.79 Gt yr-1 over the period 2010-2020 from all marine-terminating glaciers to the fjords. These estimates are valuable for studies aiming to understand the basal ice sheet conditions and for studies predicting ecosystem changes in Greenland's fjords and offshore areas as the ice sheet melts and sediment discharge increase.

The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling

Submarine turbidity currents form the largest sediment accumulations on Earth, raising the question of their role in global carbon cycles. It was previously inferred that terrestrial organic carbon was primarily incinerated on shelves and that most turbidity current systems are presently inactive. Turbidity currents were thus not considered in global carbon cycles, and the burial efficiency of global terrestrial organic carbon was considered low to moderate (∼10-44%). However, recent work has shown that burial of terrestrial organic carbon by turbidity currents is highly efficient (>60-100%) in a range of settings and that flows occur more frequently than once thought, although they were far more active at sea-level lowstands. This leads to revised global estimates for mass flux (∼62-90 Mt C/year) and burial efficiency (∼31-45%) of terrestrial organic carbon in marine sediments. Greatly increased burial fluxes during sea-level lowstands are also likely underestimated; thus, organic carbon cycling by turbidity currents could play a role in long-term changes in atmospheric CO2 and climate.

Viruses under the Antarctic Ice Shelf are active and potentially involved in global nutrient cycles

Viruses play an important role in the marine ecosystem. However, our comprehension of viruses inhabiting the dark ocean, and in particular, under the Antarctic Ice Shelves, remains limited. Here, we mine single-cell genomic, transcriptomic, and metagenomic data to uncover the viral diversity, biogeography, activity, and their role as metabolic facilitators of microbes beneath the Ross Ice Shelf. This is the largest Antarctic ice shelf with a major impact on global carbon cycle. The viral community found in the cavity under the ice shelf mainly comprises endemic viruses adapted to polar and mesopelagic environments. The low abundance of genes related to lysogenic lifestyle (<3%) does not support a predominance of the Piggyback-the-Winner hypothesis, consistent with a low-productivity habitat. Our results indicate a viral community actively infecting key ammonium and sulfur-oxidizing chemolithoautotrophs (e.g. Nitrosopumilus spp, Thioglobus spp.), supporting a "kill-the-winner" dynamic. Based on genome analysis, these viruses carry specific auxiliary metabolic genes potentially involved in nitrogen, sulfur, and phosphorus acquisition. Altogether, the viruses under Antarctic ice shelves are putatively involved in programming the metabolism of ecologically relevant microbes that maintain primary production in these chemosynthetically-driven ecosystems, which have a major role in global nutrient cycles.

Methylotrophic Communities Associated with a Greenland Ice Sheet Methane Release Hotspot

Subglacial environments provide conditions suitable for the microbial production of methane, an important greenhouse gas, which can be released from beneath the ice as a result of glacial melting. High gaseous methane emissions have recently been discovered at Russell Glacier, an outlet of the southwestern margin of the Greenland Ice Sheet, acting not only as a potential climate amplifier but also as a substrate for methane consuming microorganisms. Here, we describe the composition of the microbial assemblage exported in meltwater from the methane release hotspot at Russell Glacier and its changes over the melt season and as it travels downstream. We found that a substantial part (relative abundance 27.2% across the whole dataset) of the exported assemblage was made up of methylotrophs and that the relative abundance of methylotrophs increased as the melt season progressed, likely due to the seasonal development of the glacial drainage system. The methylotrophs were dominated by representatives of type I methanotrophs from the Gammaproteobacteria; however, their relative abundance decreased with increasing distance from the ice margin at the expense of type II methanotrophs and/or methylotrophs from the Alphaproteobacteria and Betaproteobacteria. Our results show that subglacial methane release hotspot sites can be colonized by microorganisms that can potentially reduce methane emissions.

Metagenome-assembled genomes from High Arctic glaciers highlight the vulnerability of glacier-associated microbiota and their activities to habitat loss

The rapid warming of the Arctic is threatening the demise of its glaciers and their associated ecosystems. Therefore, there is an urgent need to explore and understand the diversity of genomes resident within glacial ecosystems endangered by human-induced climate change. In this study we use genome-resolved metagenomics to explore the taxonomic and functional diversity of different habitats within glacier-occupied catchments. Comparing different habitats within such catchments offers a natural experiment for understanding the effects of changing habitat extent or even loss upon Arctic microbiota. Through binning and annotation of metagenome-assembled genomes (MAGs) we describe the spatial differences in taxon distribution and their implications for glacier-associated biogeochemical cycling. Multiple taxa associated with carbon cycling included organisms with the potential for carbon monoxide oxidation. Meanwhile, nitrogen fixation was mediated by a single taxon, although diverse taxa contribute to other nitrogen conversions. Genes for sulphur oxidation were prevalent within MAGs implying the potential capacity for sulphur cycling. Finally, we focused on cyanobacterial MAGs, and those within cryoconite, a biodiverse microbe-mineral granular aggregate responsible for darkening glacier surfaces. Although the metagenome-assembled genome of Phormidesmis priestleyi, the cyanobacterium responsible for forming Arctic cryoconite was represented with high coverage, evidence for the biosynthesis of multiple vitamins and co-factors was absent from its MAG. Our results indicate the potential for cross-feeding to sustain P. priestleyi within granular cryoconite. Taken together, genome-resolved metagenomics reveals the vulnerability of glacier-associated microbiota to the deletion of glacial habitats through the rapid warming of the Arctic.

Large contributions of petrogenic and aged soil-derived organic carbon to Arctic fjord sediments in Svalbard

Svalbard fjords are recognized as hotspots for organic carbon (OC) burial and storage due to their high sedimentation rates, which effectively trap terrestrial sediments and inhibit extensive OC remineralization. In this study, we investigated surface sediments (n = 48) from eight Svalbard fjords, along with bedrock (n = 17), soil (n = 28), and plant (n = 12) samples, to identify the sources of sedimentary OC in these fjords using geochemical parameters. All examined surface sediments from the fjords showed a depletion in 14Corg (- 666.9 ± 240.3‰), indicating that recently fixed terrestrial and marine biomass alone cannot account for the entire sedimentary OC pool. Conventional bulk indicators such as Norg/TOC ratio and δ13Corg were insufficient for fully determining the sources of sedimentary OC. Therefore, we employed a four-end-member approach, using Δ14Corg, δ13Corg, and lignin phenols to assess the relative contributions of petrogenic, soil-derived, plant-derived, and marine OC to the sedimentary OC pool. The analyzed fjord sediments consisted, on average, of 59.0 ± 28.1% petrogenic OC, 16.8 ± 12.1% soil-derived OC, 2.5 ± 2.2% plant-derived OC, and 21.8 ± 18.5% marine OC. This approach highlights the substantial contributions of petrogenic and aged soil-derived OC to present-day sedimentary OC in Svalbard fjords. Considering predicted global warming, accelerated inputs of petrogenic and soil-derived OC into fjords due to rapid glacier retreat may significantly impact the active carbon cycle and potentially contribute to CO2 emissions to the atmosphere, depending on burial efficiency.

Trajectories of freshwater microbial genomics and greenhouse gas saturation upon glacial retreat

Due to climate warming, ice sheets around the world are losing mass, contributing to changes across terrestrial landscapes on decadal time spans. However, landscape repercussions on climate are poorly constrained mostly due to limited knowledge on microbial responses to deglaciation. Here, we reveal the genomic succession from chemolithotrophy to photo- and heterotrophy and increases in methane supersaturation in freshwater lakes upon glacial retreat. Arctic lakes at Svalbard also revealed strong microbial signatures form nutrient fertilization by birds. Although methanotrophs were present and increased along lake chronosequences, methane consumption rates were low even in supersaturated systems. Nitrous oxide oversaturation and genomic information suggest active nitrogen cycling across the entire deglaciated landscape, and in the high Arctic, increasing bird populations serve as major modulators at many sites. Our findings show diverse microbial succession patterns, and trajectories in carbon and nitrogen cycle processes representing a positive feedback loop of deglaciation on climate warming.

Glacial Water: A Dynamic Microbial Medium

Microbial communities and nutrient dynamics in glaciers and ice sheets continuously change as the hydrological conditions within and on the ice change. Glaciers and ice sheets can be considered bioreactors as microbiomes transform nutrients that enter these icy systems and alter the meltwater chemistry. Global warming is increasing meltwater discharge, affecting nutrient and cell export, and altering proglacial systems. In this review, we integrate the current understanding of glacial hydrology, microbial activity, and nutrient and carbon dynamics to highlight their interdependence and variability on daily and seasonal time scales, as well as their impact on proglacial environments.

Active and dormant microorganisms on glacier surfaces

Glacier and ice sheet surfaces host diverse communities of microorganisms whose activity (or inactivity) influences biogeochemical cycles and ice melting. Supraglacial microbes endure various environmental extremes including resource scarcity, frequent temperature fluctuations above and below the freezing point of water, and high UV irradiance during summer followed by months of total darkness during winter. One strategy that enables microbial life to persist through environmental extremes is dormancy, which despite being prevalent among microbial communities in natural settings, has not been directly measured and quantified in glacier surface ecosystems. Here, we use a combination of metabarcoding and metatranscriptomic analyses, as well as cell-specific activity (BONCAT) incubations to assess the diversity and activity of microbial communities from glacial surfaces in Iceland and Greenland. We also present a new ecological model for glacier microorganisms and simulate physiological state-changes in the glacial microbial community under idealized (i) freezing, (ii) thawing, and (iii) freeze-thaw conditions. We show that a high proportion (>50%) of bacterial cells are translationally active in-situ on snow and ice surfaces, with Actinomycetota, Pseudomonadota, and Planctomycetota dominating the total and active community compositions, and that glacier microorganisms, even when frozen, could resume translational activity within 24 h after thawing. Our data suggest that glacial microorganisms respond rapidly to dynamic and changing conditions typical of their natural environment. We deduce that the biology and biogeochemistry of glacier surfaces are shaped by processes occurring over short (i.e., daily) timescales, and thus are susceptible to change following the expected alterations to the melt-regime of glaciers driven by climate change. A better understanding of the activity of microorganisms on glacier surfaces is critical in addressing the growing concern of climate change in Polar regions, as well as for their use as analogues to life in potentially habitable icy worlds.

Glacial meltwater determines the balance between autotrophic and heterotrophic processes in a Greenland fjord

Global warming accelerates melting of glaciers and increases the supply of meltwater and associated inorganic particles, nutrients, and organic matter to adjacent coastal seas, but the ecosystem impact is poorly resolved and quantified. When meltwater is delivered by glacial rivers, the potential impact could be a reduction in light and nutrient availability for primary producers while supplying allochthonous carbon for heterotrophic processes, thereby tipping the net community metabolism toward heterotrophy. To test this hypothesis, we determined physical and biogeochemical parameters along a 110-km fjord transect in NE Greenland fjord, impacted by glacial meltwater from the Greenland Ice Sheet. The meltwater is delivered from glacier-fed river outlets in the inner parts of the fjord, creating a gradient in salinity and turbidity. The planktonic primary production was low, 20-45 mg C m^-2 d^-1, in the more turbid inner half of the fjord, increasing 10-fold to around 350 mg C m^-2 d^-1 in the shelf waters outside the fjord. Plankton community metabolism was measured at three stations, which displayed a transition from net heterotrophy in the inner fjord to net autotrophy in the coastal shelf waters. Respiration was significantly correlated to turbidity, with a 10-fold increase in the inner turbid part of the fjord. We estimated the changes in meltwater input and sea ice coverage in the area for the last 60 y. The long-term trend and the observed effects demonstrated the importance of freshwater runoff as a key driver of coastal ecosystem change in the Arctic with potential negative consequences for coastal productivity.

Following the flow-Microbial ecology in surface- and groundwaters in the glacial forefield of a rapidly retreating glacier in Iceland

The retreat of glaciers in response to climate change has major impacts on the hydrology and ecosystems of glacier forefield catchments. Microbes are key players in ecosystem functionality, supporting the supply of ecosystem services that glacier systems provide. The interaction between surface and groundwaters in glacier forefields has only recently gained much attention, and how these interactions influence the microbiology is still unclear. Here, we identify the microbial communities in groundwater from shallow (<15 m deep) boreholes in a glacial forefield floodplain ('sandur') aquifer at different distances from the rapidly retreating Virkisjökull glacier, Iceland, and with varying hydraulic connectivity with the glacial meltwater river that flows over the sandur. Groundwater communities are shown to differ from those in nearby glacial and non-glacial surface water communities. Groundwater-meltwater interactions and groundwater flow dynamics affect the microbial community structure, leading to different microbial communities at different sampling points in the glacier forefield. Groundwater communities differ from those in nearby glacial and non-glacial surface waters. Functional potential for microbial nitrogen and methane cycling was detected, although the functional gene copy numbers of specific groups were low.

Investigation of water-soluble organic constituents and their spatio-temporal heterogeneity over the Tibetan Plateau

Investigating the migration and transformation of carbonaceous and nitrogenous matter in the cryosphere areas is crucial for understanding global biogeochemical cycle and earth's climate system. However, water-soluble organic constituents and their transformation in multiple water bodies are barely investigated. Water-soluble organic carbon (WSOC) and organic nitrogen (WSON), and particulate black carbon (PBC) in multiple types of water bodies in eastern Tibetan Plateau (TP) cryosphere for the first time have been systematically investigated. Statistical results exhibited that from south to north and from east to west of this region, WSOC concentrations in alpine river runoff were gradually elevated. WSOC and nitrogenous matter in the alpine river runoff and precipitation in the glacier region presented distinct seasonal variations. WSON was the dominant component (63.4%) of water-soluble total nitrogen in precipitation over high-altitude southeastern TP cryosphere. Water-soluble carbonaceous matter dominated the carbon cycle in the TP cryosphere, but particulate carbonaceous matter in the alpine river runoff had a small fraction of the cryospheric carbon cycle. Analysis of optical properties illustrated that PBC had a much stronger light absorption ability (MAC-_PBC: 2.28 ± 0.37 m² g^-1) than WSOC in the alpine river runoff (0.41 ± 0.26 m² g^-1). Ionic composition was dominated by SO₄^2-, NO₃^-, and NH₄⁺ (average: 45.13 ± 3.75%) in the snow of glaciers, implying important contribution of (fossil fuel) combustion sources over this region. The results of this study have essential implications for understanding the carbon and nitrogen cycles in high altitude cryosphere regions of the world. Future work should be performed based on more robust in-situ observations and measurements from multiple environmental medium over the cryosphere areas, to ensure ecological protection and high-quality development of the high mountain Asia.

Diatoms and Their Microbiomes in Complex and Changing Polar Oceans

Diatoms, a key group of polar marine microbes, support highly productive ocean ecosystems. Like all life on earth, diatoms do not live in isolation, and they are therefore under constant biotic and abiotic pressures which directly influence their evolution through natural selection. Despite their importance in polar ecosystems, polar diatoms are understudied compared to temperate species. The observed rapid change in the polar climate, especially warming, has created increased research interest to discover the underlying causes and potential consequences on single species to entire ecosystems. Next-Generation Sequencing (NGS) technologies have greatly expanded our knowledge by revealing the molecular underpinnings of physiological adaptations to polar environmental conditions. Their genomes, transcriptomes, and proteomes together with the first eukaryotic meta-omics data of surface ocean polar microbiomes reflect the environmental pressures through adaptive responses such as the expansion of protein families over time as a consequence of selection. Polar regions and their microbiomes are inherently connected to climate cycles and their feedback loops. An integrated understanding built on “omics” resources centered around diatoms as key primary producers will enable us to reveal unifying concepts of microbial co-evolution and adaptation in polar oceans. This knowledge, which aims to relate past environmental changes to specific adaptations, will be required to improve climate prediction models for polar ecosystems because it provides a unifying framework of how interacting and co-evolving biological communities might respond to future environmental change.

Attribution of Space-Time Variability in Global-Ocean Dissolved Inorganic Carbon

The inventory and variability of oceanic dissolved inorganic carbon (DIC) is driven by the interplay of physical, chemical, and biological processes. Quantifying the spatiotemporal variability of these drivers is crucial for a mechanistic understanding of the ocean carbon sink and its future trajectory. Here, we use the Estimating the Circulation and Climate of the Ocean-Darwin ocean biogeochemistry state estimate to generate a global-ocean, data-constrained DIC budget and investigate how spatial and seasonal-to-interannual variability in three-dimensional circulation, air-sea CO₂ flux, and biological processes have modulated the ocean sink for 1995-2018. Our results demonstrate substantial compensation between budget terms, resulting in distinct upper-ocean carbon regimes. For example, boundary current regions have strong contributions from vertical diffusion while equatorial regions exhibit compensation between upwelling and biological processes. When integrated across the full ocean depth, the 24-year DIC mass increase of 64 Pg C (2.7 Pg C year^-1) primarily tracks the anthropogenic CO₂ growth rate, with biological processes providing a small contribution of 2% (1.4 Pg C). In the upper 100 m, which stores roughly 13% (8.1 Pg C) of the global increase, we find that circulation provides the largest DIC gain (6.3 Pg C year^-1) and biological processes are the largest loss (8.6 Pg C year^-1). Interannual variability is dominated by vertical advection in equatorial regions, with the 1997-1998 El Niño-Southern Oscillation causing the largest year-to-year change in upper-ocean DIC (2.1 Pg C). Our results provide a novel, data-constrained framework for an improved mechanistic understanding of natural and anthropogenic perturbations to the ocean sink.

Robust Estimation and Forecasting of Climate Change Using Score-Driven Ice-Age Models

We use data on the following climate variables for the period of the last 798 thousand years: global ice volume (Icet), atmospheric carbon dioxide level (CO2,t), and Antarctic land surface temperature (Tempt). Those variables are cyclical and are driven by the following strongly exogenous orbital variables: eccentricity of the Earth’s orbit, obliquity, and precession of the equinox. We introduce score-driven ice-age models which use robust filters of the conditional mean and variance, generalizing the updating mechanism and solving the misspecification of a recent climate–econometric model (benchmark ice-age model). The score-driven models control for omitted exogenous variables and extreme events, using more general dynamic structures and heteroskedasticity. We find that the score-driven models improve the performance of the benchmark ice-age model. We provide out-of-sample forecasts of the climate variables for the last 100 thousand years. We show that during the last 10–15 thousand years of the forecasting period, for which humanity influenced the Earth’s climate, (i) the forecasts of Icet are above the observed Icet, (ii) the forecasts of CO2,t level are below the observed CO2,t, and (iii) the forecasts of Tempt are below the observed Tempt. The forecasts for the benchmark ice-age model are reinforced by the score-driven models.

Characteristics of dissolved organic carbon and nitrogen in precipitation in the northern Tibetan Plateau

Dissolved organic carbon (DOC) and nitrogen (N) play essential roles in global C and N cycles. To address the possible role of DOC and N in precipitation and enrich the related global database, the characteristics of DOC and N in precipitation were investigated in a typical remote permafrost region (upper Heihe River Basin) of the northern Tibetan Plateau (TP) from February 2019 to March 2020. The results demonstrated that the average DOC and total dissolved N (TDN) concentrations in the precipitation were 1.41 ± 1.09 μg mL^-1 and 0.84 ± 0.48 μg mL^-1, respectively, with relatively lower concentrations in the summer. The annual DOC and TDN fluxes were estimated to be 6.42 kg ha^-1 yr^-1 and 3.39 kg ha^-1 yr^-1, respectively, indicating that precipitation was a significant factor in C and N deposition. The light-absorbing properties of precipitation DOC from the SUVA₂₅₄ and spectral slope revealed that precipitation DOC containing more aromatic components and lower molecular weights mostly was present during the summer; the mass cross-section (at the wavelength of 365 nm) ranged 0.26-1.84 m² g^-1, suggesting the potential impact of DOC on climatic forcing in the area. The principal component analysis combined with air mass backward trajectories indicated that the air masses from west Siberia, Central Asia, and northwestern China most significantly influenced the precipitation C and N in the study area. The WRF-Chem simulations and aerosol vertical distributions further illustrated the air mass transport pathways, demonstrating that dust and anthropogenic emissions could be transported over the studied area by westerlies and monsoonal winds. In the study basin, the precipitation deposition of DOC and N contributed largely to the riverine DOC and N exportation during the summer and had potential ecological effects. These results highlight the importance of DOC and N deposition from precipitation in the northern TP.

The 79°N Glacier cavity modulates subglacial iron export to the NE Greenland Shelf

Approximately half of the freshwater discharged from the Greenland and Antarctic Ice Sheets enters the ocean subsurface as a result of basal ice melt, or runoff draining via the grounding line of a deep ice shelf or marine-terminating glacier. Around Antarctica and parts of northern Greenland, this freshwater then experiences prolonged residence times in large cavities beneath floating ice tongues. Due to the inaccessibility of these cavities, it is unclear how they moderate the freshwater associated supply of nutrients such as iron (Fe) to the ocean. Here, we show that subglacial dissolved Fe export from Nioghalvfjerdsbrae (the '79°N Glacier') is decoupled from particulate inputs including freshwater Fe supply, likely due to the prolonged ~162-day residence time of Atlantic water beneath Greenland's largest floating ice-tongue. Our findings indicate that the overturning rate and particle-dissolved phase exchanges in ice cavities exert a dominant control on subglacial nutrient supply to shelf regions.

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.

Enhanced trace element mobilization by Earth's ice sheets

Trace elements sustain biological productivity, yet the significance of trace element mobilization and export in subglacial runoff from ice sheets is poorly constrained at present. Here, we present size-fractionated (0.02, 0.22, and 0.45 µm) concentrations of trace elements in subglacial waters from the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Concentrations of immobile trace elements (e.g., Al, Fe, Ti) far exceed global riverine and open ocean mean values and highlight the importance of subglacial aluminosilicate mineral weathering and lack of retention of these species in sediments. Concentrations are higher from the AIS than the GrIS, highlighting the geochemical consequences of prolonged water residence times and hydrological isolation that characterize the former. The enrichment of trace elements (e.g., Co, Fe, Mn, and Zn) in subglacial meltwaters compared with seawater and typical riverine systems, together with the likely sensitivity to future ice sheet melting, suggests that their export in glacial runoff is likely to be important for biological productivity. For example, our dissolved Fe concentration (20,900 nM) and associated flux values (1.4 Gmol y-1) from AIS to the Fe-deplete Southern Ocean exceed most previous estimates by an order of magnitude. The ultimate fate of these micronutrients will depend on the reactivity of the dominant colloidal size fraction (likely controlled by nanoparticulate Al and Fe oxyhydroxide minerals) and estuarine processing. We contend that ice sheets create highly geochemically reactive particulates in subglacial environments, which play a key role in trace elemental cycles, with potentially important consequences for global carbon cycling.

The Influence of Glacial Cover on Riverine Silicon and Iron Exports in Chilean Patagonia

Glaciated environments have been highlighted as important sources of bioavailable nutrients, with inputs of glacial meltwater potentially influencing productivity in downstream ecosystems. However, it is currently unclear how riverine nutrient concentrations vary across a spectrum of glacial cover, making it challenging to accurately predict how terrestrial fluxes will change with continued glacial retreat. Using 40 rivers in Chilean Patagonia as a unique natural laboratory, we investigate how glacial cover affects riverine Si and Fe concentrations, and infer how exports of these bioessential nutrients may change in the future. Dissolved Si (as silicic acid) and soluble Fe (<0.02 μm) concentrations were relatively low in glacier-fed rivers, whereas concentrations of colloidal-nanoparticulate (0.02-0.45 μm) Si and Fe increased significantly as a function of glacial cover. These colloidal-nanoparticulate phases were predominately composed of aluminosilicates and Fe-oxyhydroxides, highlighting the need for size-fractionated analyses and further research to quantify the lability of colloidal-nanoparticulate species. We also demonstrate the importance of reactive particulate (>0.45 μm) phases of both Si and Fe, which are not typically accounted for in terrestrial nutrient budgets but can dominate riverine exports. Dissolved Si and soluble Fe yield estimates showed no trend with glacial cover, suggesting no significant change in total exports with continued glacial retreat. However, yields of colloidal-nanoparticulate and reactive sediment-bound Si and Fe were an order of magnitude greater in highly glaciated catchments and showed significant positive correlations with glacial cover. As such, regional-scale exports of these phases are likely to decrease as glacial cover disappears across Chilean Patagonia, with potential implications for downstream ecosystems.

Seasonal variability of net sea-air CO2 fluxes in a coastal region of the northern Antarctic Peninsula

We show an annual overview of the sea-air CO2 exchanges and primary drivers in the Gerlache Strait, a hotspot for climate change that is ecologically important in the northern Antarctic Peninsula. In autumn and winter, episodic upwelling events increase the remineralized carbon in the sea surface, leading the region to act as a moderate or strong CO2 source to the atmosphere of up to 40 mmol m-2 day-1. During summer and late spring, photosynthesis decreases the CO2 partial pressure in the surface seawater, enhancing ocean CO2 uptake, which reaches values higher than - 40 mmol m-2 day-1. Thus, autumn/winter CO2 outgassing is nearly balanced by an only 4-month period of intense ocean CO2 ingassing during summer/spring. Hence, the estimated annual net sea-air CO2 flux from 2002 to 2017 was 1.24 ± 4.33 mmol m-2 day-1, opposing the common CO2 sink behaviour observed in other coastal regions around Antarctica. The main drivers of changes in the surface CO2 system in this region were total dissolved inorganic carbon and total alkalinity, revealing dominant influences of both physical and biological processes. These findings demonstrate the importance of Antarctica coastal zones as summer carbon sinks and emphasize the need to better understand local/regional seasonal sensitivity to the net CO2 flux effect on the Southern Ocean carbon cycle, especially considering the impacts caused by climate change.

Using CO2 as an Oxidant in the Catalytic Pyrolysis of Peat Moss from the North Polar Region

As global warming and climate change become perceived as significant, the release of greenhouse gases (GHGs) stored in the earth's polar regions is considered a matter of concern. Here, we focused on exploiting GHGs to address potential global warming challenges in the north polar regions. In particular, we used CO₂ as a soft oxidant to recover energy as syngas (CO and H₂) and to produce biochars from pyrolysis of peat moss. CO₂ expedited homogeneous reaction with volatile matters from peat moss pyrolysis, and the mechanistic CO₂ role resulted in the conversion of CO₂ and peat moss to CO at ≥530 °C. Steel slag waste was then used as an ex situ catalyst to increase reaction kinetics, addressing the issue of the role of CO₂ being limited to ≥530 °C, with the result where substantial H₂ and CO formation was achieved at a milder temperature. The porosity of biochars, a solid peat moss pyrolysis product, was modified in the presence of CO₂, with a significant improvement in CO₂ adsorption capacity compared to those achieved by N₂ pyrolysis. Therefore, CO₂ has the potential to serve as an initial feedstock in sustainable biomass-to-energy applications and biochar production, mitigating atmospheric carbon concentrations.