Presence Of Meltwater Research Articles

Ice sheet decline and rising atmospheric CO2 control AMOC sensitivity to deglacial meltwater discharge

The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO 2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO 2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge. • Climate model sensitivity experiments are performed using state-of-the-art ice sheet and freshwater reconstructions • Declining Northern Hemisphere ice sheets increase the sensitivity of the AMOC to North Atlantic meltwater discharge • Deglacial rise in atmospheric CO 2 concentration decreases the sensitivity of the AMOC to North Atlantic meltwater discharge • Both effects provide a complementary perspective to existing explanations for abrupt AMOC transitions

Seasonal Cold-Wave Propagation Into the Near-Surface Ice of Debris-Covered Khumbu Glacier, Nepal

Meltwater from high-elevation debris-covered glaciers—particularly those located in the greater Himalaya and Andes—shapes the water supply of major rivers and nourishes substantial terrestrial, estuarine, and marine habitats (Kraaijenbrink et al., 2017; Immerzeel et al., 2020). However, the relative inaccessibility and high elevation of such glaciers results in a paucity of data relating to their fundamental physical properties and processes, limiting the information available to constrain and evaluate numerical models of their behaviour and project future change. Knowledge of the subsurface properties of such glaciers is particularly deficient because it is largely obscured to satellite and airborne remote sensing; englacial investigations therefore commonly require direct access (Miles et al., 2020). Of the physical properties of glaciers, ice temperature exerts an important control over glaciological processes, such as glacier motion, and their modelled behaviour. For example, ice viscosity is sensitive to temperature such that, under the same stress, ice approaching the melting point deforms 5–10 times more rapidly than it would at −10°C (Deeley and Woodward, 1908; Cuffey and Paterson, 2010). Basal motion depends on lubrication facilitated by the presence of meltwater at the ice-bed interface and/or within the pore space of a subglacial sediment layer. Measurements of near-surface ice temperatures are important for modelling the surface energy balance and projecting the future mass-balance response of glaciers to anticipated climate change. This is especially the case for glaciers with a thick supraglacial debris layer that insulates the underlying ice (according to debris layer thickness and lithology), reducing ablation and potentially extending glacier longevity (Nicholson and Benn, 2006; Nicholson and Benn, 2013; Anderson and Anderson, 2016). Here, we present a one-year time series of near-surface ice temperatures, measured between 1.5 and 7.0 m below the ice surface, in a borehole drilled by hot water into the debris-covered tongue of Khumbu Glacier, Nepal.

Abrupt Bølling‐Allerød Warming Simulated under Gradual Forcing of the Last Deglaciation

AbstractDuring the last deglaciation, a major global warming trend was punctuated by abrupt climate changes, likely related to Atlantic meridional overturning circulation (AMOC). One problem is that an abrupt increase in the AMOC during the Bølling‐Allerød (BA) transition occurred when the melting of Northern Hemisphere ice sheets was significant, which tended to weaken the AMOC. Here, from transient simulations of the last deglaciation using an atmosphere‐ocean general circulation model, we show that an abrupt increase in the AMOC during the BA transition could occur without reduction in glacial meltwater. The abrupt increase in the AMOC accompanied abrupt warming in Greenland and sea ice retreat in the North Atlantic, consistent with proxies and previous modeling studies. The results imply that abrupt BA warming during the middle stage of the last deglaciation was a response to gradual warming under the presence of meltwater from continental ice sheets.

Patterns of suspended particulate matter across the continental margin in the Canadian Beaufort Sea during summer

Abstract. The particulate beam attenuation coefficient at 660 nm, cp(660), was measured in conjunction with properties of suspended particle assemblages in August 2009 within the Canadian Beaufort Sea continental margin, a region heavily influenced by freshwater and sediment discharge from the Mackenzie River, but also by sea ice melt. The mass concentration of suspended particulate matter (SPM) ranged from 0.04 to 140 g m−3, its composition varied from mineral to organic dominated, and the median particle diameter determined over the range 0.7–120 µm varied from 0.78 to 9.45 µm, with the fraction of particles <1 µm in surface waters reflecting the degree influenced by river water. Despite this range in particle characteristics, a strong relationship between SPM and cp(660) was found and used to determine SPM distributions across the shelf based on measurements of cp(660) taken during summer seasons of 2004, 2008, and 2009. SPM spatial patterns on the stratified shelf reflected the vertically sheared two-layer estuarine circulation and SPM sources (i.e., fluvial inputs, bottom resuspension, and biological productivity). Along-shelf winds generated lateral Ekman flows, isopycnal movements, and upwelling or downwelling at the shelf break. Cross-shelf transects measured during three summers illustrate how sea ice meltwater affects river plume extent, while the presence of meltwater on the shelf was associated with enhanced near-bottom SPM during return flow of upwelled Pacific-origin water. SPM decreased sharply past the shelf break with further transport of particulate matter occurring near the bottom and in interleaving nepheloid layers. These findings expand our knowledge of particle distributions in the Beaufort Sea controlled by river discharge, sea ice, and wind, each of which is sensitive to weather and climate variations.

Subglacial hydrology as a control on emergence,scale, and spacing of ice streams

AbstractObservations have long associated ice streams with the presence of meltwater at the bed. More recently, theoretical models have been able to reproduce ice stream behavior as a consequence of the coupled dynamics of ice and subglacial meltwater. In this paper we analyze the properties of ice streams that form in a coupled model of ice flow and subglacial hydrology. We see that there is a natural length scale defining ice stream separation and width. This arises as a result of the balance between effective pressure gradients driving meltwater away from ice streams and the enhanced water production in the streams due to the fast ice flow. We further discuss how the model interacts with topography, and we show that small perturbations to a uniform bed have a strong effect on where ice streams emerge in the model. However, in many cases ice streams then evolve to be closer to the dimensions defined by the natural length scale of the unperturbed system. The nondimensional parameter that defines this length scale is therefore of fundamental importance in the model.

Avalanching glacier instabilities: Review on processes and early warning perspectives

AbstractAvalanching glacier instabilities are gravity‐driven rupture phenomena that might cause major disasters, especially when they are at the origin of a chain of processes. Reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action. Recently, considerable efforts in monitoring, analyzing, and modeling such phenomena have led to significant advances in destabilization process understanding, improving early warning perspectives. The purpose of this paper is to review the recent progress in this domain. Three different types of instabilities can be identified depending on the thermal properties of the ice/bed interface. If cold (1), the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity. A prediction of the final break off is possible using these precursory signs. For the two other types, water plays a key role in the development of the instability. If the ice/bed interface is partly temperate (2), the presence of meltwater may reduce the basal resistance, which promotes the instability. No clear and easily detectable precursory signs are known in this case, and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime. The last type of instability (3) concerns steep temperate glacier tongues switching for several days/weeks during the melting season into a so‐called “active phase” followed in rare cases by a major break‐off event. Although the prediction of such events is still far from being achievable, critical conditions promoting the final instability can be identified.

Excess ground ice of condensation–diffusion origin in University Valley, Dry Valleys of Antarctica: Evidence from isotope geochemistry and numerical modeling

This study investigates the origin and age of ground ice in the uppermost 1m of permafrost in University Valley, one of the upper valleys in the McMurdo Dry Valleys of Antarctica. In contrast to other regions in the MDV, mean daily air and soil temperatures at the coring sites are always below 0°C, which allows for unique cryogenic processes to occur. In the two cores that were analyzed, excess ground ice was measured throughout, ranging between 23% and 85%. Isotope geochemical trends in the ice-rich permafrost indicate that the ground in Core 5 (65cm long) and the uppermost 52cm of Core 7 originated from condensation–diffusion of water vapor; whereas the ground ice between 57–90cm in Core 7 originated from freezing of liquid water. Using numerical modeling, we show that the excess ground ice of condensation–diffusion origin formed by the long-term thermal contraction–expansion of the cryotic sediments, which allowed for the ice content to exceed pore-filling capacity. Absolute age estimates of the sandy-loam sediments based on Optically Stimulated Luminescence dating indicate that soils have been accreting at the site for at least the last 170±36kayears, and this places an upper limit to the age of the ground ice. Absolute soil ages allowed us to link the change in ground ice origin in Core 7, which took place around 152±12kayears, with shifts in climate conditions since marine isotope stage 5e interglacial period. Our findings offer a new process of ground ice emplacement in sediments in cold–dry environments and allow an alternative explanation regarding the enigmatic origin of excess ground ice identified by Mars Odyssey and Phoenix in the northern martian plain, which is that overfilled pore ice can form by vapor deposition and repeated thermal cycling without the presence of melt water.

Coupling the flow of ice, water, and sediment in a glacial landscape evolution model

The processes of subglacial erosion depend not only on the sliding motion of glaciers over bedrock but also on the presence of meltwater and sediment in the subglacial environment. In particular, theoretical models for subglacial quarrying and abrasion, as well as hypothesized erosion thresholds associated with subglacial sediment transport, include both positive and negative effects of subglacial water and sediment on the rate of erosion.In order to incorporate the existing theoretical models for subglacial erosion by quarrying and abrasion in a long-term glacial landscape evolution model, we here present a coupled computational framework for simulating the simultaneous flow processes of ice, water, and sediment. We supplement a higher order ice sheet model with simple long-term models for glacial hydrology and subglacial sediment transport, as well as fluvial and hillslope-related erosion processes.An important strength of the modeling framework presented relates to the morphological detail of the landscapes produced, which facilitates direct comparison with existing landforms. This improves the connection between glacial landscape evolution models, geomorphological observations, and the existing knowledge of the physical processes that operate under glaciers. We demonstrate with model examples, how increased basal meltwater pressure and transport-limited subglacial erosion lead to both positive and negative feedbacks related to glacial erosion and the formation of overdeepenings.

Hydrographic changes in the eastern subpolar North Atlantic during the last deglaciation

Millennial-scale climate fluctuations of the last deglaciation have been tied to abrupt changes in the Atlantic Meridional Overturning Circulation (MOC). A key to understanding mechanisms of MOC collapse and recovery is the documentation of upper ocean hydrographic changes in the vicinity of North Atlantic deep convection sites. Here we present new high-resolution ocean temperature and δ 18O sw records spanning the last deglaciation from an eastern subpolar North Atlantic site that lies along the flow path of the North Atlantic Current, approaching deep convection sites in the Labrador and Greenland-Iceland-Norwegian (GIN) Seas. High-resolution temperature and δ 18O sw records from subpolar Site 980 help track the movement of the subpolar/subtropical front associated with temperature and Atlantic MOC changes throughout the last deglaciation. Distinct δ 18O sw minima during Heinrich 1 (H1) and the Younger Dryas (YD) correspond with peaks in ice-rafted debris and periods of reduced Atlantic MOC, indicating the presence of melt water in this region that could have contributed to MOC reductions during these intervals. Increased tropical and subtropical δ 18O sw during these periods of apparent freshening in the subpolar North Atlantic suggest a buildup of salt at low latitudes that served as a negative feedback on reduced Atlantic MOC.

The death mask of the antarctic ice sheet: Comparison of glacial geomorphic features across the continental shelf

Multibeam swath bathymetry data, seismic lines, and shallow cores were collected seaward of all of the major drainage outlets of the Antarctic Ice Sheet from the Pennell Coast of North Victoria Land to the northwestern Weddell Sea. The results show that during the last glacial maximum, the ice sheet extended onto the outer shelf in all areas, to the shelf break in many. Swath bathymetry and deep-tow sidescan sonar data from the continental shelf also record the geomorphology left by the retreating ice sheet. Glacial troughs occur offshore of all major glacial outlets. In each drainage area except for parts of the Ross Sea, the inner shelf is characterized by acoustic basement interpreted as crystalline substrate. The geomorphology of these inner shelf areas consists of erosional features such as grooves that show that ice flow tended to follow the structural grain of the bedrock. Outer shelf areas are floored by sedimentary substrate where the direction of ice flow was more directly offshore and depositional features characterize the seafloor. In these areas the signature of the grounded ice consists of till deposits and large-scale geomorphic features, mega-scale glacial lineations. Drumlins occur within the region of contact between crystalline and sedimentary substrates. Meltwater channels also have been imaged in both inner and outer shelf settings. While the presence of meltwater across the shelf has long been suspected, we present here the first multibeam image of a meltwater channel on the outer continental shelf of Antarctica. At the shelf break gullies are in all areas where we know ice reached the shelf break. We note the remarkable similarity in form of mega-scale glacial lineations observed on the outer shelf over sedimentary substrate across all areas surveyed, typically measuring 200–600 m crest to crest. This suggests that the shape of these landforms is governed by the same process in each drainage area and that process was occurring under a large area of the ice sheet. The erosional and transitional features show much variability in form and distribution.

Reconstruction of past temperatures for Arctic glaciers subjected to intense subsurface melting

AbstractMany glaciers are subject to melting due to high summer air temperatures. Here, the presence of meltwater in the subsurface layers of the glacier bulk, and its subsequent percolation and refreezing are implemented in the calibration of a paleothermometer. Accounting for the melt feature index and the measured oxygen-isotope ratio allows for calibration of the paleothermometer and comparison of different climatic proxies. The results of reconstructions agree with previous reconstructions at the depth of attenuation of the seasonal climate signals, which supports the validity of the paleothermometer calibration. The sensitivity of the reconstruction to variations of the model parameters was also studied. It was found that most likely snow–firn sequence and temperature fields were subjected to significant change due to current warming. Temperature changes in the snow–firn thickness of Akademii Nauk (Severnaya Zemlya, Russian High Arctic) and Austfonna (Svalbard) ice caps exceed by ∼6˚C the average Arctic temperature anomalies for the last 150 years. The reconstruction of the past surface temperatures and the parameters of the subsurface heat source due to refreezing of meltwater lead to the conclusion that meltwater spreads inside two to four annual layers for Akademii Nauk and Austfonna ice caps, respectively.

The effect of postdepositional process on the chemical profiles of snow pits in the percolation zone

Two snow pits, 3 and 2.3 m in depth, were successively sampled at the same spot (4230 m a.s.l.) from Glacier no. 1 at the head of Ürümqi River, Tien Shan, China, on June 4 and 17, 1996, respectively. All samples were analyzed for major anions (SO 4 2−, NO 3 − and Cl −) and oxygen isotope ratios ( δ 18O). Similar mean Cl − concentrations are observed for the two snow pits, but the NO 3 − and, to a less degree, SO 4 2− concentrations increased considerably. The increase in NO 3 − can be attributed by anthropogenic emission, biomass burning, and soil emission resulted from the use of nitrogen fertilizers, while the increase in SO 4 2− is suggested due to the chemical reactions of pyrite and carbonate in presence of meltwater. Moreover, all the major anion profiles were further modified by elution process, which leaches most of the ions from the uppermost part of the snow/firn to the bottom. The correlation coefficients among the major anions can also change owing to the postdepositional process. Change of the δ 18O profiles as meltwater percolating through the snow/firn is less significant as compared with that of ionic profiles, consequently, causing a phasic discrepancy among the stable isotopic and major ionic profiles. Therefore, the ice core records are not only affected by the climatic and environmental conditions during precipitation, but also affected by the postdepositional process.

The Utilization of Spaceborne Microwave Radiometers for Monitoring Snowpack Properties

Snow accumulation and depletion at specific locations can be monitored from space by observing related variations in microwave brightness temperatures. Using vertically and horizontally polarized brightness temperatures from the Nimbus 6 Electrically Scanning Microwave Radiometer, a discriminant function can be used to separate snow from no snow areas and map snowcovered area on a continental basis. For dry snow conditions on the Canadian high plains significant relationships between snow depth or water equivalent and microwave brightness temperature were developed which could permit remote determination of these snow properties after acquisition of a wider range of data. The presence of melt water in the snowpack causes a marked increase in brightness temperature which can be used to predict snowpack priming and timing of runoff. As the resolutions of satellite microwave sensors improve the application of these results to snow hydrology problems should increase.

Ice Avalanches

Ice Avalanches

Ice Avalanches

Ice Avalanches

Observations of Arctic Sea Ice Dynamics Using the Earth Resources Technology Satellite (ERTS-1)

This study shows that ERTS-l, launched by the U.S. National Aeronautics and Space Administration in July 1972, can be used to make synoptic observations of dynamic changes in arctic sea ice .... Each data swath of ERTS-l is 185 kilometres wide at the surface and is repeated once every 18 days. ... Because of a near-polar orbit, this sidelap increases to over 80 per cent at arctic latitudes. As a consequence of this large sidelap, the tracking of individual ice features for periods up to 5 or 6 days is permitted. The ERTS-1 Multispectral Scanner Subsystem (MSS) records data by simultaneously scanning across the satellite track in 4 spectral bands. The wavelength limits of the 4 bands are: green (0.5-0.6 µm), red (0.6-0.7 µm), and two near infrared bands (0.7-0.8 µm and 0.8-1.1 µm). The nominal spatial resolution for all 4 bands is 80 metres. A standard ERTS-1 photographic format has an image scale of nearly 1:1,000,000, which is convenient for direct comparison with available maps. Hendriksen Strait, the passage between Amund Ringnes Island and Cornwall Island at about 77°45' N and 95°00' W constituted the study area; it is in the Queen Elizabeth Islands of Arctic Canada, and is one of the most enticing and promising areas of recent oil and gas strikes. ... during a 6-day period from 23 to 28 August, ERTS-1 provided 5 days of coverage of Hendriksen Strait. Various types of sea ice can be identified in the ERTS-l imagery (0.6-0.7 µm band) .... In such a sequence of observations, a number of sea ice changes with time are detectable. Changes in position of individual ice floes can easily be translated to velocity. A number of ice floes were tracked over the 5-day period and their velocities calculated. The average velocity for ice floe movement was 8.5 kilometres/day .... ERTS-l can be used to study ice floe morphology and dynamics in the Beaufort Sea at time scales of several days to months. Repeated observations of individual ice floes such as those available here will also allow calculation of the ablation of the ice mass, in this case the decrease of surface area with time. ... The ice cover, i.e. the relative amount of sea ice present in a given area, is important for shipping purposes and air-sea interaction processes. The ice cover in Hendriksen Strait on 23 August was approximately 7/10, decreasing to 2/10 by 28 August. This type of sea ice change is easily observed from ERTS-1 .... In regard to the delineation of navigation routes through sea ice, the detection of active melting on the surface of the ice would indicate areas likely to be ice free in the near future. ERTS-1 has the ability to do this through observations of reflectance variability both temporally and spectrally. The second method of observing the sea ice reflectance change is provided by comparing two separate images made at the same time but in different spectral bands. ... Because the absorption of solar radiation by water is much greater in the near infrared than in the visible portion of the spectrum, the lower reflectance is again probably due to the presence of melt-water on the surface of the ice. Thus sea ice with water on it, even in very thin layers, will show a considerable difference in reflectance between these spectral bands of observation. ... Temporal and spectral observations of reflectance variations afforded by ERTS-l thus make it possible to locate areas of sea ice that are in varying stages of melting and breakup. The results presented here demonstrate that for high latitudes, ERTS-1 will provide overlapping coverage on sequential days that will allow observation of dynamic changes in the polar regions. In addition, route planning for shipping in the Arctic should benefit from frequent observations of sea ice movement and reflectance variations of the type obtainable from ERTS-l. Projected further, sea ice observations from ERTS-l over a period of years in the Arctic Islands should aid in the placement of offshore oil-drilling structures. Reflectance measurements over this period will also increase our understanding of the heat balance in the polar regions. Finally, ERTS-l observations of the amount of ice cover versus the amount of open water will be important in determining boundary conditions for future use in models of the global heat balance.