Meltwater Delivery Research Articles

Upper-flow regime bedforms in a subglacial triangular-shaped landform (murtoo), Late Pleistocene, SW Finland: Implications for flow dynamics and sediment transport in (semi-)distributed subglacial meltwater drainage systems

We know less about subglacial meltwater flow properties in distributed inefficient and semi-efficient systems in comparison to those of ice marginal eskers and proglacial environments. While previous studies have indicated the overall common presence of upper-flow-regime (UFR) bedforms in glacigenic settings, facies expressions of subglacial meltwater flows remain poorly documented. Three ca. 3 m deep and up to 70 m long trenches excavated across a triangle-shaped subglacial landform called a murtoo in a Lateglacial to Holocene meltwater route in SW Finland provide a detailed window into sedimentary structures presumably formed ca. 40–50 km away from the coeval subaqueous margin of the Fennoscandian Ice Sheet (FIS). The aim of this paper is to document small-scale bedforms, which formed subglacially by meltwater flow and to characterize the proximal and central parts of the studied murtoo during its early evolutionary phase. We defined seven main facies types that characterize the depositional processes of the unit. Overall, the studied deposits reflect increasing meltwater delivery through time and are characterized by abrupt lateral changes in sedimentary structures and grain size. While the initial deposits are dominated by massive and horizontally laminated silt with sand lenses interpreted as lower-flow-regime deposits, the latter sediments are characterized by sinusoidal stratification, sigmoidal cross-stratification and scours with backsets or chaotic fill interpreted as deposits of antidunes, humpback dunes, chutes-and-pools and cyclic steps of the upper-flow regime. The upper-flow-regime bedforms developed on a 1 m high and 15 m long bed slope and are associated with the formation of a short-lived enlarged water-filled cavity or pond, where supercritical density flows allowed for the deposition of upper-flow regime bedforms. The final coarse-grained murtoo head-bar development, characterized by planar-cross stratified gravel and pebbly sand, indicates avalanche processes that were controlled by grain size.Our results confirm that the core of the murtoo is depositional and meltwater processes played a key role in its deposition. Despite the subglacial setting with a subaqueous ice-sheet margin, the meltwater flow was not permanently characterized by pipe-flow conditions. Overall, the findings contribute to the understanding of semi-distributed subglacial meltwater systems during the retreat of a continental ice sheet (FIS) in a rapidly warming climate.

Evolution of global snow drought characteristics from 1850 to 2100

Seasonal snow is an integral part of the global water supply and storage system. Snow droughts impact ecological, agricultural, and urban systems by altering the amount and timing of meltwater delivery. These droughts are characterized by a lack of on-the-ground snow (snow water equivalent, SWE) that can be caused by low total precipitation (dry drought) or low proportion of precipitation falling as snowfall (warm drought), often combined with an early melt. The standardized SWE index (SWEI) ranks the current status of SWE for a given location compared to a baseline condition and identifies the existence, but not the cause, of snow drought. In this work, we use estimates of SWE, temperature, and precipitation from nine coupled model intercomparison project phase 6 (CMIP6) models to quantify the frequency, severity, and type of snow droughts globally for historical and future scenarios. Compared to a historical baseline (1850–1900) total snow drought frequency more than doubles under socio-economic pathway (SSP)2-4.5 and SSP5-8.5; all of the increase in snow drought frequency comes from an increase in warm droughts. The probability distribution of future SWEI in major snowy basins around the world are likely to be centered on more negative values, which corresponds to more severe drought and, with only moderate changes in distribution spread, more frequent drought. CMIP6 simulations pinpoint snow drought as an emerging global threat to water resources and highlight the need to explore higher resolution future models that better capture complex mountain topography, wildland fires, and snow-forest interactions.

Shallow Fracture Buffers High Elevation Runoff in Northwest Greenland

AbstractThe expansion of refrozen ice slabs in Greenland's firn may enhance meltwater runoff and increase surface mass loss. However, the impermeability of ice slabs and the pathways for meltwater export from these regions remain poorly characterized. Here, we present ice‐penetrating radar observations of extensive meltwater infiltration and refreezing beneath ice slabs in Northwest Greenland. We show that these buried ice complexes form where supraglacial streams or lakes drain through surface crevasses into relict firn beneath the ice slabs. This suggests that the firn can continue to buffer mass loss from surface meltwater runoff and limit meltwater delivery to the ice sheet bed even after ice slabs have formed. Therefore, a significant time lag may exist between the initial formation of ice slabs and the onset of complete surface runoff and seasonal meltwater drainage to the subglacial system in interior regions of the ice sheet.

Late erosion pulse triggered by rapid melt in the cold‐based interior of the last Fennoscandian Ice Sheet, an example from Rogen

AbstractIce sheet interiors are conventionally regarded as non‐erosive. Yet subglacial conditions may be transformed during deglaciation by the arrival of large volumes of meltwater at the ice sheet bed. The development of a dynamic meltwater drainage system and the onset of basal sliding have potential to increase erosion rates in bedrock and sediment. Here, we examine the impact of late deglacial thawing on the Rogen plateau, located near the former ice divide of the Fennoscandian Ice Sheet. We provide new maps of glacial and glacifluvial landforms which we combine with existing data on Quaternary sediments and landforms. Cross‐cutting and overlapping relations allow for an event sequence to be established of the deglaciation period. In the Early Holocene (< 11 ka), an ice lobe onset zone developed at the Rogen plateau. In places where meltwater reached the bed and where pressures rose to overpressure, it caused fracture dilation in horizontally bedded sandstones and rock brecciation. The onset of sliding and application of drag resulted in the mobilization of bedrock sheets. The establishment of meltwater corridors led to fluidization of sediments at the bed, dissection and modification of ribbed moraines and formation of murtoos and hummock corridors. During final stagnation of the ice sheet, meltwater drained through channels forming axial eskers. Bedrock erosion during deglaciation reached depths up to 4 m, and in conjunction with some recycling of till, generated 317 km2 of boulder cover. The average erosion depths by removal and reworking of sediment are 0.9–1.1 m across areas below 900 m elevation. This study shows that when the cold‐based interiors of ice sheets become briefly activated by large subglacial meltwater delivery late in deglaciation, there can be significant reworking and erosion of rock and sediment.

A ∼14 000-year record of environmental change from Lake Simcoe, Canada

While the evolution of the Laurentian Great Lakes has been researched intensively, fewer studies have focused on reconstructing the history of Lake Simcoe, the fourth largest lake wholly within southern Ontario. This study uses proxy data, including ostracod valves, contained in lake sediments to reconstruct a ∼14 000-year history of paleoenvironmental change in the Lake Simcoe basin. First, oxygen-isotope compositions of ostracod valves (δ18Ovalve) are used to estimate the oxygen-isotope composition of ancient lake water, δ18Olake water. In Lake Simcoe, δ18Olake water is insensitive to changes in lake level but sensitive to shifts in temperature, capturing periods such as the early Holocene warming. This δ18Olake water record demonstrates that the delivery of meltwater from Lake Algonquin to Lake Simcoe began to slow at ∼12 050 cal yr BP, supporting previous research based on geophysical data. It also suggests that an isolated pulse of 16O-rich water from Early Lake Mattawa may have occurred from ∼10 699 to 10 591 cal yr BP. Next, stable carbon-isotope compositions of ostracod valves (δ13Cvalve) are used to estimate the stable carbon-isotope composition of dissolved inorganic carbon (δ13CDIC) in Lake Simcoe. These data, combined with ostracod assemblages, grain size, magnetic susceptibility (MS) and sediment mineralogy, support our interpretation of temperature-sensitive variation in δ18Olake water and provide further insights into in-lake conditions. A pronounced increase in accumulation rate, and subtler increases in grain size and detrital carbonate contents, between ∼8300 and ∼8000 cal yr BP may reflect the abrupt transition from cold and dry to wet and warm conditions resulting from the collapse of the Laurentide Ice Sheet (LIS) at ∼8200 cal yr BP. In summary, this study refines our understanding of the evolution of Lake Simcoe and provides a holistic picture of environmental change over the last ∼14 000 years.

Threshold response to melt drives large-scale bed weakening in Greenland.

Ice speeds in Greenland are largely set by basal motion1, which is modulated by meltwater delivery to the ice base2-4. Evidence suggests that increasing melt rates enhance the subglacial drainage network's capacity to evacuate basal water, increasing bed friction and causing the ice to slow5-10. This limits the potential of melt forcing to increase mass loss as temperatures increase11. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20-40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales.

Flood activity revealed millennial-scale climatic changes during the late Holocene in the Qaidam Basin, northeastern Tibetan Plateau

Flood activity in arid region is not only crucial process for evolution of rivers and terminal salt lakes, but also sensitive indicator of paleoclimatic changes. Here, slackwater paleoflood sediments of the Nalinggele River, the largest river in the Qaidam Basin, were studied with optically stimulated luminescence dating and grain size analysis to reconstruct flood events, mechanisms and their climatic implications. Seven slackwater deposit layers which show a weak hydrodynamic sedimentary environment, were dated to three main flood stages at ca. 3.6–3.4 ka, 2.2 ka and 1.0 ka. These stages correspond to similar millennial-scale flood activities in the basin bordering Kunlun Mountains, millennial-scale humid intervals in the eastern Qaidam Basin, glacial melting events on the Tibetan Plateau, and as well warm intervals between cold ice-rafted debris events in the North Atlantic, which may triggered by and the intensified solar activities. These flood stages were attributed to increased precipitation from the enhanced Asian Summer Monsoon and increased glacial meltwater delivery during the late Holocene. The findings suggest that a climate on the northeastern Tibtean Plateau that was fluctuant and with humid periods during the late Holocene, and not undergoing a constant drying trend as previously understood.

Supraglacial streamflow and meteorological drivers from southwest Greenland

Abstract. Greenland ice sheet surface runoff is drained through supraglacial stream networks. This evacuation influences surface mass balance as well as ice dynamics. However, in situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discrete discharge measurements and continuous water level measurements for 62 d spanning the majority of of the melt season (13 June to 13 August) in 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedentedly long record of supraglacial discharge that captures both diurnal variability and changes over the melt season. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting. However, during high-melt episodes, the contribution of shortwave radiation to melt energy is reduced by ∼40 % (from 1.13 to 0.73 proportion). Instead, the relative contribution of longwave radiation, sensible heat fluxes, and latent heat fluxes to overall melt increases by ∼24 %, 6 %, and 10 % (proportion increased from −0.32 to −0.08, 0.28 to 0.34, and −0.04 to 0.06) respectively. Our data also identify that the timing of daily maximum discharge during clear-sky days shifts from 16:00 local time (i.e., 2 h 45 min after solar noon) in late June to 14:00 in late July and then rapidly returns to 16:00 in early August. The change in the timing of daily maximum discharge could be attributed to the expansion and contraction of the stream network, caused by skin temperatures that likely fell below freezing at night. The abrupt shift, in early August, in the timing of daily maximum discharge coincides with a drop in air temperature, a drop in the amount of water temporarily stored in weathering crust, and a decreasing covariance between stream velocity and discharge. Further work is needed to investigate if these results can be transferable to larger catchments and uncover if rapid shifts in the timing of peak discharge are widespread across Greenland supraglacial streams and thus have an impact on meltwater delivery to the subglacial system and ice dynamics.

An inventory of supraglacial lakes and channels across the West Antarctic Ice Sheet

Abstract. Quantifying the extent and distribution of supraglacial hydrology, i.e. lakes and streams, is important for understanding the mass balance of the Antarctic ice sheet and its consequent contribution to global sea-level rise. The existence of meltwater on the ice surface has the potential to affect ice shelf stability and grounded ice flow through hydrofracturing and the associated delivery of meltwater to the bed. In this study, we systematically map all observable supraglacial lakes and streams in West Antarctica by applying a semi-automated Dual-NDWI (normalised difference water index) approach to >2000 images acquired by the Sentinel-2 and Landsat-8 satellites during January 2017. We use a K-means clustering method to partition water into lakes and streams, which is important for understanding the dynamics and inter-connectivity of the hydrological system. When compared to a manually delineated reference dataset on three Antarctic test sites, our approach achieves average values for sensitivity (85.3 % and 77.6 %), specificity (99.1 % and 99.7 %) and accuracy (98.7 % and 98.3 %) for Sentinel-2 and Landsat-8 acquisitions, respectively. We identified 10 478 supraglacial features (10 223 lakes and 255 channels) on the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula (AP), with a combined area of 119.4 km2 (114.7 km2 lakes, 4.7 km2 channels). We found 27.3 % of feature area on grounded ice and 54.9 % on floating ice shelves. In total, 17.8 % of feature area crossed the grounding line. A recent expansion in satellite data provision made new continental-scale inventories such as these, the first produced for WAIS and AP, possible. The inventories provide a baseline for future studies and a benchmark to monitor the development of Antarctica's surface hydrology in a warming world and thus enhance our capability to predict the collapse of ice shelves in the future. The dataset is available at https://doi.org/10.5281/zenodo.5642755 (Corr et al., 2021).

A 5000-year lacustrine sediment oxygen isotope record of late Holocene climate change in Newfoundland, Canada

The spatiotemporal patterns and underlying causes of climate variations spanning the mid-to late-Holocene in the North Atlantic region are largely unsettled. Here, we present a decadally resolved carbonate oxygen isotope (δ18O) record spanning the last ∼5000 calibrated years before present (cal yr BP) from Norman's Pond, a small, hydrologically open lake located in west-central Newfoundland, Canada. Stable isotope data from regional lakes, rivers, and precipitation samples indicate the δ18O record primarily reflects changes in the integrated, annual δ18O of precipitation, which is largely controlled by local atmospheric temperature and to a lesser extent, changes in the seasonality of precipitation. The record exhibits a general trend of decreasing δ18O values over the last ∼5000 cal yr BP that is consistent with gradually declining Boreal summer insolation and long-term cooling. Large decadal to centennial timescale fluctuations in δ18O values occurred during the last millennium, with persistent low δ18O values during the early Medieval Climate Anomaly (MCA, 950 to 1250 CE) and more variable and low values across the Little Ice Age (LIA, 1450 to 1850 CE). Relatively low δ18O values during the early MCA at ∼950 cal yr BP (∼1000 CE) suggest colder temperatures in Newfoundland and/or an increase in cold-season or decrease in warm-season precipitation across the island. MCA δ18O values are lower than those of the LIA, revealing that climate conditions in Newfoundland were variable and differed from the broader North Atlantic region. For example, the shift to very low δ18O during the early MCA is coincident with a transition to colder sea surface temperatures (SSTs) in the Labrador Sea region, which was potentially triggered by melting of the Greenland Ice Sheet and alpine glaciers and ice caps in/around Baffin Bay, and subsequent delivery of cold waters further south via the Labrador Current. The early MCA interval of inferred cooling is coincident with Norse settlement at L'Anse aux Meadows in northern Newfoundland at ∼929 cal yr BP (∼1021 CE). Subsequent low and variable δ18O during the LIA suggests regional shifts in the seasonality of precipitation and/or cooling, consistent with other terrestrial climate records from the region. Proxy evidence from Norman's Pond therefore reinforces the idea of substantial spatiotemporal variability in climatic change in the broader North Atlantic region since at least the mid-Holocene, and in particular during the MCA, when cooler conditions prevailed in Newfoundland as a result of lower offshore sea surface temperatures that were likely driven by greater meltwater delivery via the Labrador current.

Tight benthic-pelagic coupling drives seasonal and interannual changes in iron‑sulfur cycling in Arctic fjord sediments (Kongsfjorden, Svalbard)

Glaciated fjords are dynamic systems dominated by seasonal events such as spring phytoplankton blooms and pulses of glacial sediment-bearing meltwater delivery. These fjords are also characterized by strong spatial gradients in environmental factors such as sedimentation rate and primary productivity from the glacier-influenced head to the marine-influenced mouth. Such seasonal variations and spatial gradients, combined with the ongoing influence of climate change, generate non-steady state conditions, which have a strong impact on the mineralization of organic carbon in the fjord sediments and the flux of nutrients from the seabed. In order to investigate the role of fjord seasonal events and variability on diagenetic cycling of iron (Fe) and sulfur (S), we sampled Kongsfjorden (Svalbard, 79°N) in the spring, mid-summer, and late summer. We investigated sediment structure and biogeochemistry, conducted laboratory experiments to determine reaction rates, and compared these findings to water column productivity and turbidity. We found that rapid sedimentation near the glacial input buried algal matter-rich layers that fueled sub-surface peaks in mineralization rates over multi-year timescales. Sulfate reduction rates were limited by organic carbon availability and competition with Fe-reducers, while Fe reduction was controlled by the availability of reactive Fe(III) oxides. Pore water Fe 2+ concentrations were influenced by sulfur cycling pathways and abiotic reactions such as carbonate precipitation and potentially reverse weathering. Seasonal changes in sedimentation and organic carbon supply caused lower sulfate reduction and sulfide production rates in spring, driving generally higher spring fluxes of Fe 2+ from the sediment. The results of this study reveal the potential for an increased benthic source of nutrients such as Fe with continued benthic remineralization over winter in Kongsfjorden. Interannual changes in primary productivity, which are likely to intensify with global warming, and shifts in glacial sediment delivery have immediate impacts on the benthic cycling of Fe and S in this tightly coupled system, with a long term trend likely toward decreased benthic Fe fluxes. With the glacial retreat and changes in productivity predicted due to climate change, glaciated fjords such as Kongsfjorden may become a less efficient carbon sink by burying less terrestrial and marine-sourced organic matter in the deep sediments. • Investigation of seasonal Fe-S-C cycling in Arctic fjord sediments and water column. • Results show benthic respiration and increased benthic Fe 2+ flux over winter. • Findings suggest that fjord sediments respond rapidly to water column changes. • With glacial retreat, fjords may produce less benthic Fe and sequester less carbon.

Patterns and Drivers of Extracellular Enzyme Activity in New Zealand Glacier-Fed Streams.

Glacier-fed streams (GFSs) exhibit near-freezing temperatures, variable flows, and often high turbidities. Currently, the rapid shrinkage of mountain glaciers is altering the delivery of meltwater, solutes, and particulate matter to GFSs, with unknown consequences for their ecology. Benthic biofilms dominate microbial life in GFSs, and play a major role in their biogeochemical cycling. Mineralization is likely an important process for microbes to meet elemental budgets in these systems due to commonly oligotrophic conditions, and extracellular enzymes retained within the biofilm enable the degradation of organic matter and acquisition of carbon (C), nitrogen (N), and phosphorus (P). The measurement and comparison of these extracellular enzyme activities (EEA) can in turn provide insight into microbial elemental acquisition effort relative to environmental availability. To better understand how benthic biofilm communities meet resource demands, and how this might shift as glaciers vanish under climate change, we investigated biofilm EEA in 20 GFSs varying in glacier influence from New Zealand’s Southern Alps. Using turbidity and distance to the glacier snout normalized for glacier size as proxies for glacier influence, we found that bacterial abundance (BA), chlorophyll a (Chl a), extracellular polymeric substances (EPS), and total EEA per gram of sediment increased with decreasing glacier influence. Yet, when normalized by BA, EPS decreased with decreasing glacier influence, Chl a still increased, and there was no relationship with total EEA. Based on EEA ratios, we found that the majority of GFS microbial communities were N-limited, with a few streams of different underlying bedrock geology exhibiting P-limitation. Cell-specific C-acquiring EEA was positively related to the ratio of Chl a to BA, presumably reflecting the utilization of algal exudates. Meanwhile, cell-specific N-acquiring EEA were positively correlated with the concentration of dissolved inorganic nitrogen (DIN), and both N- and P-acquiring EEA increased with greater cell-specific EPS. Overall, our results reveal greater glacier influence to be negatively related to GFS biofilm biomass parameters, and generally associated with greater microbial N demand. These results help to illuminate the ecology of GFS biofilms, along with their biogeochemical response to a shifting habitat template with ongoing climate change.

First freshwater microcrustacean record in the bottom sediments of arctic ponds in Bellsund area (SW Spitsbergen)

First finds of small freshwater Crustacea in southern Bellsund (SW Spitsbergen) are presented. The remains of seven species of ostracods and three species of cladocerans were recovered from the bottom sediments of seven ponds in the region. The most abundant Arctocypris fuhrmanni Petkovski, Scharf and Keyser, 2016 was accompanied by the most widespread Candona rectangulata Alm, 1914, Limnocythere liporeticulata Delorme, 1968 and Tonnacypris glacialis (Sars, 1890) which are endemic to Arctic regions. New to the area Tonnacypris cf. mazepovae (Van der Meeren, Khand and Martens, 2009) was also encountered, being the first record of this species outside Northern Mongolia. Cladocerans were represented by the remains of Chydorus sphaericus (Müller, 1776), Daphnia pulex group Leydig, 1860 and Bosmina longirostris (Müller, 1785) for the first time noted in the bottom sediments in Spitsbergen.Significant differences in crustacean distribution occurred both between and within studied ponds presumably connected with the duration of active vegetation period and/or melt water delivery. The physical and chemical properties of the hostwater and/or some post-depositional processes could also affect the microcrustacean faunal distribution.

Simulation of Preferential Flow in Snow With a 2‐D Non‐Equilibrium Richards Model and Evaluation Against Laboratory Data

AbstractRecent studies of water flow through dry porous media have shown progress in simulating preferential flow propagation. However, current methods applied to snowpacks have neglected the dynamic nature of the capillary pressure, such as conditions for capillary pressure overshoot, resulting in a rather limited representation of the water flow patterns through snowpacks observed in laboratory and field experiments. Indeed, previous snowmelt models using a water entry pressure to simulate preferential flow paths do not work for natural snowpack conditions where snow densities are less than 380 kg m−3. Because preferential flow in snowpacks greatly alters the flow velocity and the timing of delivery of meltwater to the base of a snowpack early in the melt season, a better understanding of this process would aid hydrological predictions. This study presents a 2‐D water flow through snow model that solves the non‐equilibrium Richards equation. This model, coupled with random perturbations of snow properties, can represent realistic preferential flow patterns. Using 1‐D laboratory data, two model parameters were linked to snow properties and model boundary conditions. Parameterizations of these model parameters were evaluated against 2‐D snowpack observations from a laboratory experiment, and the resulting model sensitivity to varying inputs and boundary conditions was calculated. The model advances both the physical understanding of and ability to simulate water flow through snowpacks and can be used in the future to parameterize 1‐D snowmelt models to incorporate flow variations due to preferential flow path formation.

Ice-marginal lake hydrology and the seasonal dynamical evolution of Kennicott Glacier, Alaska

AbstractGlacier basal motion is responsible for the majority of ice flux on fast-flowing glaciers, enables rapid changes in glacier motion and provides the means by which glaciers shape alpine landscapes. In an effort to enhance our understanding of basal motion, we investigate the evolution of glacier velocity and ice-marginal lake stage on Kennicott Glacier, Alaska, during the spring–summer transition, a time when subglacial drainage is undergoing rapid change. A complicated record of > 50 m fill-and-drain sequences on a hydraulically-connected ice-marginal lake likely reflects the punctuated establishment of efficient subglacial drainage as the melt season begins. The rate of change of lake stage generally correlates with diurnal velocity maxima, both in timing and magnitude. At the seasonal scale, the up-glacier progression of enhanced summer basal motion promotes uniformity of daily glacier velocity fluctuations throughout the 10 km study reach, and results in diurnal velocity patterns suggesting increasingly efficient meltwater delivery to and drainage from the subglacial channel system. Our findings suggest the potential of using an ice-marginal lake as a proxy for subglacial water pressure, and show how widespread basal motion affects bulk glacier behavior.

Surface and subsurface hydrology of debris-covered Khumbu Glacier, Nepal, revealed by dye tracing

While the supraglacial hydrology of debris-covered glaciers is relatively well studied, almost nothing is known about how water is transported beneath the glacier surface. Here, we report the results of sixteen fluorescent dye tracing experiments conducted in April–May 2018 over the lowermost 7 km of the high-elevation, debris-covered Khumbu Glacier, Nepal, to characterise the glacier's surface and subsurface drainage system. Dye breakthroughs indicated a likely highly sinuous and channelised subsurface hydrological system draining water from the upper part of the ablation area. This flowpath was distinct from the linked chain of supraglacial ponds present along much of the glacier's lower ablation area, through which water flow was extremely slow (∼0.003 ms−1), likely reflecting the study's timing during the pre-monsoon period. Subsurface drainage pathways emerged at the glacier surface close to the terminus, and flowed into small near-surface englacial reservoirs that typically delayed meltwater transit by several hours. We observed rapid pathway changes resulting from surface collapse, indicating a further distinctive aspect of the drainage of debris-covered glaciers. We conclude that the surface and subsurface drainage of Khumbu Glacier is both distinctive and dynamic, and argue that further investigation is needed to refine the characterisation and test its regional applicability to better understand future Himalayan debris-covered glacier meltwater delivery to downstream areas.

Biogeochemical cycling and ecological thresholds in a High Arctic lake (Svalbard)

Lakes are a dominant feature of the Arctic landscape and a focal point of regional and global biogeochemical cycling. We collected a sediment core from a High Arctic Lake in southwestern Svalbard for multiproxy paleolimnological analysis. The aim was to find linkages between the terrestrial and aquatic environments in the context of climate change to understand centennial-long Arctic biogeochemical cycling and environmental dynamics. Two significant thresholds in elemental cycling were found based on sediment physical and biogeochemical proxies that were associated with the end of the cold Little Ice Age and the recent warming. We found major shifts in diatom, chironomid and cladoceran communities and their functionality that coincided with increased summer temperatures since the 1950s. We also discovered paleoecological evidence that point toward expanded bird (Little Auk) colonies in the catchment alongside climate warming. Apparently, climate-driven increase in glacier melt water delivery as well as a prolonged snow- and ice-free period have increased the transport of mineral matter from the catchment, causing significant water turbidity and disappearance of several planktonic diatoms and clear-water chironomids. We also found sedimentary accumulation of microplastic particles following the increase in Little Auk populations suggesting that seabirds potentially act as biovectors for plastic contamination. Our study demonstrates the diverse nature of climate-driven changes in the Arctic lacustrine environment with increased inorganic input from the more exposed catchment, larger nutrient delivery from the increased bird colonies at the surrounding mountain summits and subsequent alterations in aquatic communities.

Surface Meltwater Impounded by Seasonal Englacial Storage in West Greenland

AbstractThe delivery of surface meltwater through englacial drainage systems to the bed of the Greenland Ice Sheet modulates ice flow through basal lubrication. Recent studies in Southeast Greenland have identified a perennial firn aquifer; however, there are few observations quantifying the input or residence time of water within the englacial system and it remains unknown whether water can be stored within solid ice. Using hourly stationary radar measurements, we present observations of englacial and episodic subglacial water in the ablation zone of Store Glacier in West Greenland. We find significant storage of meltwater in solid ice damaged by crevasses extending down to 48 m below the ice surface during the summer, which is released or refrozen during winter. This is a significant hydrological component newly observed in the ablation zone of Greenland that could delay the delivery of meltwater to the bed, changing the ice dynamic response to surface meltwater.

Near‐surface hydraulic conductivity of northern hemisphere glaciers

AbstractThe hydrology of near‐surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near‐surface glacial weathering crust, we describe the design and operation of a capacitance‐based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high‐resolution record of near‐surface water‐level fluctuations. Piezometers were employed at 10 northern hemisphere glaciers, and through the application of standard bail–recharge techniques, we derive hydraulic conductivity (K) values from 0.003 to 3.519 m day−1, with a mean of 0.185 ± 0.019 m day−1. These results are comparable to those obtained in other discrete studies of glacier near‐surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short‐wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near‐surface ice hydrology than hitherto appreciated and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. Not only does this have implications for the accuracy of predictive meltwater run‐off models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, impurities, microbes, and nutrients to downstream ecosystems. Because continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions, and consequently, detailing weathering crust hydraulics represents a research priority because the flow path it represents remains poorly constrained.

The role of meltwater in high-latitude trough-mouth fan development: The Disko Trough-Mouth Fan, West Greenland

The Disko Trough-Mouth Fan (TMF) is a major submarine sediment fan located along the central west Greenland continental margin offshore of Disko Trough. The location of the TMF at the mouth of a prominent cross-shelf trough indicates that it is a product of repeated glacigenic sediment delivery from former fast-flowing outlets of the Greenland Ice Sheet, including an ancestral Jakobshavn Isbrae, which expanded to the shelf edge during successive glacial cycles. This study focuses on the uppermost part of the fan stratigraphy and analyses multibeam swath bathymetry and sub-bottom profiler records, supplemented by a series of vibrocores up to 6 m in length. The swath bathymetry data show that the surface of the fan is prominently gullied and channelled with channels extending downslope from a series of shelf-edge incising gullies. Sub-bottom profiles from across- and down-fan show that the fan sediments are often acoustically stratified. Glacigenic-debris flows (GDFs) were recovered in sediment cores from the uppermost slope but they are absent in cores from elsewhere on the fan. Instead, glacimarine lithofacies in the Disko TMF are dominated by turbidites, hemipelagic sediments and IRD. The gullied and channelled surface of the fan implies erosion at the base of dense, sediment-laden, turbidity currents related to the delivery of meltwater and sediment from an ice sheet grounded at the shelf edge. Such meltwater-related fans have been documented previously on mid-latitude, glacier-influenced margins, but they have rarely been described from high-latitude settings. Although GDFs are often regarded as the building blocks of TMFs, the morphology and sedimentary architecture of the uppermost, Late Quaternary part of the Disko TMF indicates that it represents a clear example of a fan in which sediment delivery is strongly influenced by meltwater. This implies that there is a spectrum of TMFs on glaciated continental margins that reflects the relative dominance of meltwater processes vs. GDFs. It highlights the variability in fan morphology and mechanisms of sediment delivery on high-latitude TMFs and shows that the classic Polar North Atlantic model of GDF dominated fans is but one of a number of styles for such large-scale, high-latitude glacimarine sedimentary depocentres.