Devon Island Ice Cap Research Articles

Glacial secrets uncovered: Revealing the modes of survival of metabolically active microbial communities entrapped in polar glacial ice

Glacial secrets uncovered: Revealing the modes of survival of metabolically active microbial communities entrapped in polar glacial ice

Modelling intra-annual dynamics of a major marine-terminating Arctic glacier

ABSTRACT Significant intra-annual variability in flow rates of tidewater-terminating Arctic glaciers has been observed in recent years. These changes may result from oceanic and/or atmospheric forcing through (1) perturbations at the terminus, such as enhanced submarine melt and changes in sea-ice buttressing, or (2) increased surface melt, in response to atmospheric warming, reaching the bed and promoting glacier slip. We examine the influence of these processes on Belcher Glacier, a large fast-flowing tidewater outlet of the Devon Island ice cap in the Canadian Arctic. A hydrologically-coupled higher-order ice flow model is used to estimate changes in glacier flow speed as a result of changes in sea-ice buttressing and hydrologically-driven melt-season dynamics. Daily run-off from five sub-catchments over the 2008 and 2009 melt seasons provides meltwater forcing for the model simulations. Model results are compared with remotely-sensed and in situ ice-surface velocity measurements. Sea-ice effects are found to have a minor influence on glacier flow speed relative to that of meltwater drainage, which is clearly implicated in short-term velocity variations during the melt season. We find that threshold drainage is essential in determining the timing of these short-lived accelerations.

Atmospheric deposition of current use pesticides in the Arctic: Snow core records from the Devon Island Ice Cap, Nunavut, Canada

Current use pesticides (CUPs) have been detected in the Arctic, even though there are no direct sources and their long range atmospheric transport potential is generally lower than that of legacy pesticides. Data on the deposition of CUPs in the Arctic are required to assess the impact of their global usage and emission. In this study, selected CUPs were measured in the layers of a snow pit sampled on the Devon Ice Cap, Nunavut, Canada. The oldest sampled layers correspond to deposition from the early 1990s. Dacthal and endosulfan sulfate were most frequently detected, with peak deposition fluxes of 1.0 and 0.4 pg cm(-2) per year. While endosulfan sulfate was more abundant than its parent compounds in most years, endosulfan (sum of α and β isomers) was predominant in 2003 and 2006, which together with air mass backward trajectories suggests a possible origin from ongoing use in Eurasia. The interannual variation in CUP deposition fluxes could not be explained with annual variations in the extent of air mass origin over agricultural lands, suggesting that other factors, such as the interannual variation in pesticide use, play a role in affecting the long range transport of CUPs to the Arctic. The very high variability in the concentrations of CUPs in the horizontal layers of Arctic ice caps is most plausibly explained by the highly episodic nature of long range atmospheric transport and deposition. While this strong influence of rare events limits the suitability of ice caps as reliable records of historical trends in Arctic contaminant deposition with annual resolution, the presence of concentration peaks in the ice record is proof of the possibility of such transport and deposition.

A New Record of Holocene Sea Levels from the Northwestern Gulf of Mexico Based on Paleo Algal Mats

A series of cores (box and piston) were collected at 2 key locations in Lancaster Sound (cores 2004-804-009 BC and PC) and Barrow Strait (cores 2005-804-004 BC and PC) in the Canadian Arctic Archipelago to document the evolution of sea-surface conditions in the main axis of the Northwest Passage during the Holocene time period. Reconstruction of sea-surface parameters (summer temperature and salinity, duration of sea-ice cover) were estimated based on transfer functions using dinoflagellate cyst assemblages as proxy indicators. The chronology of these cores is based on calibrated AMS-14C dates, 210Pb analyses, and correlations between paleomagnetic secular variations of the geomagnetic field and a predicted spherical harmonic model of the geomagnetic field (CALS7 K.2). Our age models for both cores indicate that 009 PC spans the last 11,100 cal BP, while 004 PC encompasses the last 10,800 cal BP. Calculated sedimentation rates are in the range of 43–140 cm/kyr for core 009 PC and 15–118 cm/kyr for core 004 PC, allowing for a millennial time scale resolution in each core. Our results indicate relatively harsh conditions in Lancaster Sound between 10,800 and 9000 cal BP (summer temperatures 2 °C cooler than at present), which we associate with the presence of active ice-streams in northernmost Baffin Bay and a weak West Greenland Current. This is followed by a warming trend (up to 3 °C higher than present) that took place between ∼8500 and 5500 cal BP, which we associate with the Holocene thermal maximum and to a large scale atmospheric pattern such as the Arctic Oscillation operating at the millennial time scale. This is concomitant with an increase in the relative abundance of phototrophic dinoflagellate cyst taxa. A gradual cooling of sea-surface temperature and increased sea-ice follow this from 5500 cal BP until the modern period. In Barrow Strait, harsh sea-surface conditions prevailed from 11,100 to 5500 cal BP, with summer temperatures as low as 4 °C cooler than at present. The warming trend occurred later in this region (between 5500 and ∼1000 cal BP), after which a gradual cooling is observed until the modern period. The apparent shift, or opposite warming trends, between Lancaster Sound and Barrow Strait after 5500 cal BP could be the result of a change in atmospheric circulation patterns related to a possible shift in the AO mode (from AO+ to AO−). Comparison of ice core δ18O record from Devon Island ice cap and the reconstructed sea-surface temperature from core 009 PC suggests a strong atmospheric-oceanic coupling throughout the Holocene in this area.

Holocene paleoceanography of the northwest passage, Canadian Arctic Archipelago

A series of cores (box and piston) were collected at 2 key locations in Lancaster Sound (cores 2004-804-009 BC and PC) and Barrow Strait (cores 2005-804-004 BC and PC) in the Canadian Arctic Archipelago to document the evolution of sea-surface conditions in the main axis of the Northwest Passage during the Holocene time period. Reconstruction of sea-surface parameters (summer temperature and salinity, duration of sea-ice cover) were estimated based on transfer functions using dinoflagellate cyst assemblages as proxy indicators. The chronology of these cores is based on calibrated AMS- 14C dates, 210Pb analyses, and correlations between paleomagnetic secular variations of the geomagnetic field and a predicted spherical harmonic model of the geomagnetic field (CALS7 K.2). Our age models for both cores indicate that 009 PC spans the last 11,100 cal BP, while 004 PC encompasses the last 10,800 cal BP. Calculated sedimentation rates are in the range of 43–140 cm/kyr for core 009 PC and 15–118 cm/kyr for core 004 PC, allowing for a millennial time scale resolution in each core. Our results indicate relatively harsh conditions in Lancaster Sound between 10,800 and 9000 cal BP (summer temperatures 2 °C cooler than at present), which we associate with the presence of active ice-streams in northernmost Baffin Bay and a weak West Greenland Current. This is followed by a warming trend (up to 3 °C higher than present) that took place between ∼8500 and 5500 cal BP, which we associate with the Holocene thermal maximum and to a large scale atmospheric pattern such as the Arctic Oscillation operating at the millennial time scale. This is concomitant with an increase in the relative abundance of phototrophic dinoflagellate cyst taxa. A gradual cooling of sea-surface temperature and increased sea-ice follow this from 5500 cal BP until the modern period. In Barrow Strait, harsh sea-surface conditions prevailed from 11,100 to 5500 cal BP, with summer temperatures as low as 4 °C cooler than at present. The warming trend occurred later in this region (between 5500 and ∼1000 cal BP), after which a gradual cooling is observed until the modern period. The apparent shift, or opposite warming trends, between Lancaster Sound and Barrow Strait after 5500 cal BP could be the result of a change in atmospheric circulation patterns related to a possible shift in the AO mode (from AO + to AO −). Comparison of ice core δ 18O record from Devon Island ice cap and the reconstructed sea-surface temperature from core 009 PC suggests a strong atmospheric-oceanic coupling throughout the Holocene in this area.

Forty-seven Years of Research on the Devon Island Ice Cap, Arctic Canada

The Devon Island ice cap has been the subject of scientific study for almost half a century, beginning with the first mass balance measurements in 1961. Research on the ice cap was the first to investigate (1) the role of meltwater in seasonal ice-velocity variations on a polythermal Arctic ice cap, (2) the use of air temperature rather than net radiation as a proxy for the energy driving surface melt, and (3) the influence of the changing frequency of specific synoptic weather configurations on glacier melt and mass balance. Other research has included investigations of ice cap geometry, flow dynamics, and mass balance; ice core analyses for records of past climate and contaminant deposition; and studies of changes in ice cap area and volume and their relationship to surface mass balance and ice dynamics. Current research includes ground studies connected to efforts to calibrate and validate the radar altimeter that will be carried by the European Space Agency’s (ESA) CryoSat2 satellite, and a major collaborative Canadian International Polar Year (IPY) project focused on the Belcher Glacier, on the northeast side of the ice cap, that examines hydrodynamics of large tidewater glaciers. This paper summarizes our current knowledge of the Devon Island ice cap and identifies some of the outstanding questions that continue to limit our understanding of climate-ice cap interactions in Arctic regions.

Trends and Sources of Perchlorate in Arctic Snow

Samples from the Devon Island ice cap (Nunavut, Canada) were used to calculate the annual input of atmospheric formed perchlorate. Depth samples collected in the spring of 2006 were dated between 1996 and 2005. An optimized ion chromatography tandem mass spectrometry (IC-MS/MS) method with direct injection allowed detection of perchlorate in all analyzed samples. Concentrations varied between 1 and 18 ng L(-1), showed seasonality, and were correlated with the total ozone levels from the area. A significant correlation was observed between chloride and perchlorate only for data sets corresponding to peak perchlorate concentrations. Data available suggests that perchlorate from the Arctic snow was formed in the atmosphere following two different mechanisms. Stratospheric chlorine radicals reacted with ozone year around, producing concentrations of perchlorate correlated with the total ozone level. The second pathway was specific to the summer months, when the amounts of perchlorate were correlated with the chloride concentrations, suggesting a possible tropospheric formation. Analysis of a deep ice core sample confirmed that perchlorate was present in precipitation at similar concentration more than 2000 years ago. Perchlorate ion represents a sink for the stratospheric chlorine, being removed via precipitation. The estimated amount of perchlorate that reached the Arctic in 2005 was 41-86 t.

Ice Surface Topography Digital Elevation Model by Interferometric SAR Method

Research on the issue of climate change and its influence on ice sheets calls for increasing use of methods based on remote measurements and monitoring. Work on the Devon Island Ice Cap in Canada demonstrated the value of interferometric synthetic aperture radar. Analyses of two image pairs yielded a digital elevation model that could be used for evaluations of ice mass dimensions and characteristics. This method was found to produce metric accuracy with a resolution of 100 m geocoded DEM and with an accuracy of 5.43 m compared to field-derived GPS measurements.

Recent changes in thickness of the Devon Island ice cap, Canada

Long‐term rates of thickness change were derived at several spatial scales using a variety of methods for most of the Devon Island ice cap, Nunavut, Canada. Basin‐wide thickness change calculations were derived for the accumulation zones of all major drainage basins as the area‐averaged volume difference between balance and InSar fluxes at the altitude of the long‐term equilibrium line (ELA). Thickness changes for ablation zones were derived as a function of the surface mass balance, flux across the EL and calving flux, averaged across the ablation areas. Average rates of thickness change are near zero in the accumulation zones of the northern and southwestern basins but reach −0.23 ± 0.11 m a−1 w.e. in the southeast basin due to dynamic thinning. Thickness changes were also estimated along five major outlet glaciers as a function of flux divergence and net surface mass balance and along the Belcher Glacier by comparing elevation values derived from 1960s aerial photography with those derived from 2005 NASA Airborne Topographic Mapper (ATM) surveys. Ice dynamics have had a significant influence on the pattern of thickness change of all outlet glaciers examined in this study. Volume changes derived from the basin‐wide values indicate a net loss of −76.8 ± 7 km3 water equivalent from the main portion of the ice cap from 1960 to 1999, contributing 0.21 ± 0.02 mm to global sea level over this time. This value is ∼44% greater than previous estimates of volume change based on volume‐area scaling methods and surface mass balance alone.

A 15,800‐year record of atmospheric lead deposition on the Devon Island Ice Cap, Nunavut, Canada: Natural and anthropogenic enrichments, isotopic composition, and predominant sources

Using appropriate clean methods for handling and preparation, 57 ice samples from the Devon Island Ice Cap, representing the period 134 to 15,800 years before present (BP), were analyzed for Pb, Sc, and Pb isotopes (206Pb, 207Pb, 208Pb) using ICP‐SMS. The greatest Pb concentrations were found in samples dating from the Younger Dryas. Despite the large range in Pb concentrations (from 2.2 to 181 pg g−1), the Pb concentrations were proportional to those of Sc until 3100 BP when the Pb/Sc ratio exceeded by a factor of 2 the natural “background” value (Pb/Sc = 6.3 ± 1.8) for the first time. The uniform ratio of Pb to Sc until 3100 BP is consistent with the hypothesis that soil dust particles derived from physical and chemical weathering dominate the inputs of Pb to the atmosphere, with the magnitude of these sources climate‐dependent. Isotopic analyses of Pb further support this paradigm, with the average ratio of 206Pb/207Pb (1.230) and 208Pb/206Pb (2.059) well within the range given for the Upper Continental Crust (UCC). The shift to higher Pb/Sc ratios and lower 206Pb/207Pb values starting at 3100 BP is consistent with historical records and other archival evidence of the onset of atmospheric Pb contamination caused by Pb mining and smelting in the Iberian Peninsula. Since that time, the Devon Island ice core records several other episodes of notable atmospheric Pb contamination, including those dating from Roman and medieval times, as well as the industrial period. The Pb, Sc, and Pb isotope data presented here represent the first chemical and isotopic record of the natural, “background” atmospheric inputs to the Canadian Arctic, against which modern values may be compared.

CO2 isotopes as tracers of firn air diffusion and age in an Arctic ice cap with summer melting, Devon Island, Canada

Firn air and ice have been sampled and analyzed for trace gases (CO2, N2O, CH4, and CO) and isotopes (14C, 13C, and 18O of CO2; 3H of ice) at 3 m intervals from the surface to the depth of closure at 60 m on the Devon Island Ice Cap, a low‐elevation permanent glacier in the Canadian Arctic Islands, to investigate firn diffusion and the effects of summer melting. The 14CO2 profile from the permeable firn includes the 1963 thermonuclear peak at a depth of 53.9 ± 1.5 m. The twofold increase and rapid decay that characterize the recent atmospheric history for 14CO2 provide a robust atmospheric scenario that is used with a firn air diffusion model to inversely construct the firn diffusivity profile. The results show a permeable but essentially nondiffusive zone from 50 to 60 m depth. A firn‐ice age profile was produced from density measurements, and accumulation rates were calibrated with the depth of the 1963 thermonuclear 3H peak. The average ages for CO2 in the sampled firn air profile were determined by a new method based on the rate of 18O exchange between CO2 and the ice matrix. Calibrated with the 1963 peak for thermonuclear 14CO2, a 21.2‐year reaction halftime is calculated for exchange taking place at the firn temperature of −22.8°C on Devon. This gives an average age of 54.9 (+6.0/−12.0) years for firn air at 60 m depth in 140‐year‐old ice. Thus CO2 has a mean age 85 years younger than associated ice at the point of occlusion. The measured in firn air provides no indication of alteration by summer melting, which is attributed to a high degree of convective and diffusive flushing of the upper firn as shown by diffusion modeling. This suggests that ice sheets with summer melt layers can reliably preserve atmospheric trace gas signals.

An ultra-clean firn core from the Devon Island Ice Cap, Nunavut, Canada, retrieved using a titanium drill specially designed for trace element studies

An electromechanical drill with titanium barrels was used to recover a 63.7 m long firn core from Devon Island Ice Cap, Nunavut, Canada, representing 155 years of precipitation. The core was processed and analysed at the Geological Survey of Canada by following strict clean procedures for measurements of Pb and Cd at concentrations at or below the pg g(-1) level. This paper describes the effectiveness of the titanium drill with respect to contamination during ice core retrieval and evaluates sample-processing procedures in laboratories. The results demonstrate that: (1) ice cores retrieved with this titanium drill are of excellent quality with metal contamination one to four orders of magnitude less than those retrieved with conventional drills; (2) the core cleaning and sampling protocols used were effective, contamination-free, and adequate for analysis of the metals (Pb and Cd) at low pg g(-1) levels; and (3) results from 489 firn core samples analysed in this study are comparable with published data from other sites in the Arctic, Greenland and the Antarctic.

Form and flow of the Devon Island Ice Cap, Canadian Arctic

In this study, 3370 km of 100 MHz ice‐penetrating radar data were acquired from Devon Ice Cap, Arctic Canada. Bed returns were obtained from >90% of flight tracks. Mean crossing point errors in ice surface elevation and ice thickness were 7–8 m. Digital elevation models of ice cap surface and bed elevation, and ice thickness, were produced and can be used as boundary conditions in numerical modeling. Devon Ice Cap, including 1960 km2 of contiguous stagnant ice to its west, is 14,010 km2. The ice cap proper is 12,050 km2. Its largest drainage basin is 2630 km2. The ice cap crest has a maximum measured elevation of 1921 m. Maximum recorded ice thickness is 880 m. Ice cap volume is 3980 km3 (about 10 mm sea level equivalent). The bed, 8% of which lies below sea level, is an upland plateau dissected by steep‐sided valleys that control the locations of the major outlet glaciers which dominate ice cap drainage. About 73 km, 4%, of the ice cap margin ends in tidewater. The margin is not floating. Icebergs of <100 m were observed offshore. Only a single outlet glacier showed signs of past surge activity. All major outlet glaciers along the eastern ice cap margin have retreated 1–3 km since 1960. Synthetic aperture radar‐interferometic velocity structure shows slow, undifferentiated flow predominating in the west and center, with fast‐flowing outlet glaciers and intervening slow‐flowing ridges typical elsewhere. Outlet glacier velocities are 7–10 times higher than in areas of undifferentiated flow. This velocity structure, of fast‐flowing units within slower‐flowing ice, appears typical of many large (103 km2) Arctic ice caps.

Lacustrine Sedimentary Environments in High Arctic Proglacial Bear Lake, Devon Island, Nunavut, Canada

Combined catchment, subbottom acoustic, and sedimentary studies of high arctic, proglacial Bear Lake, Devon Island, were carried out to evaluate the role lacustrine processes and Holocene catchment evolution have had on the sedimentary record. In the proximal basin, bottom deposits up to 60 m thick are generated in part by turbid underflows associated with peak meltwater flow from the Devon Island Ice Cap. These underflows produce rhythmically laminated structures that are likely varves. In shallower locations, accumulation is slower and results from homopycnal distribution of fine suspended sediment throughout the proximal basin, resulting in a simple varve couplet. The distal basin is isolated from glacial meltwater by a shallow sill, although some fine-grained detrital carbonate transported by the glacial meltwater is deposited in both basins. In most locations in the distal basin Holocene sediments are less than 10 m thick. They are composed primarily of massive clay and some carbonate interrupted by irregular graded carbonate units produced by sporadic heavy summer rainfall and sediment transport from small plateau tributaries. Eolian sedimentation is also important throughout the lake, especially when high accumulations melt through the ice in the proximal basin, producing isolated grains and layers of coarse sand in the sedimentary record. Holocene ice margin changes have influenced the sedimentary record substantially, particularly during the mid-Holocene when the ice cap is inferred to have retreated from the catchment.

Lacustrine Sedimentary Environments in High Arctic Proglacial Bear Lake, Devon Island, Nunavut, Canada

Combined catchment, subbottom acoustic, and sedimentary studies of high arctic, proglacial Bear Lake, Devon Island, were carried out to evaluate the role lacustrine processes and Holocene catchment evolution have had on the sedimentary record. In the proximal basin, bottom deposits up to 60 m thick are generated in part by turbid underflows associated with peak meltwater flow from the Devon Island Ice Cap. These underflows produce rhythmically laminated structures that are likely varves. In shallower locations, accumulation is slower and results from homopycnal distribution of fine suspended sediment throughout the proximal basin, resulting in a simple varve couplet. The distal basin is isolated from glacial meltwater by a shallow sill, although some fine-grained detrital carbonate transported by the glacial meltwater is deposited in both basins. In most locations in the distal basin Holocene sediments are less than 10 m thick. They are composed primarily of massive clay and some carbonate interrupted by irregular graded carbonate units produced by sporadic heavy summer rainfall and sediment transport from small plateau tributaries. Eolian sedimentation is also important throughout the lake, especially when high accumulations melt through the ice in the proximal basin, producing isolated grains and layers of coarse sand in the sedimentary record. Holocene ice margin changes have influenced the sedimentary record substantially, particularly during the mid-Holocene when the ice cap is inferred to have retreated from the catchment.

A high resolution proxy-climate record from an arctic lake with annually-laminated sediments on Devon Island, Nunavut, Canada

Sediments from a 3 ha lake (75 °34.34′N, 89 °18.55′W) from the coastal region of northern Devon Island, Nunavut, Canada, contain discrete laminations in the deepest part of the basin. The laminations are varves as indicated by the correspondence between counts and thickness measurements of the couplets and 210Pb dating. A 14 cm core representing 150 years of sedimentation contained laminated couplets consisting of a lighter inorganic layer with a higher percentage of calcium and magnesium, alternating with fine darker bands, typically more cohesive, and comprising higher proportions of silica and carbon. A reddish oxidation zone with higher iron and aluminum frequently separates the laminations. The dark layer represents a biogenic component deposited in summer and is made cohesive by bacterial filaments among the other particles. The light inorganic layer represents clastic deposition from allochthonous sources. Deposition rates were relatively consistent through the core with an increase in varve thickness in the 1950s. Diatom concentrations in the sediments increased by two orders of magnitude in this century, with major increases in the 1920s and 1950s. The increase in varve thickness and diatom abundance coincides with an increase in summer melt percentage in an ice core from the Devon Island Ice Cap (Koerner, 1977). The relatively high sedimentation rate (0.15 cm yr-1) coupled with the consistency of deposition makes this lake a significant indicator for recent climate changes of the Canadian Arctic Archipelago.

Signal and noise in four ice-core records from the Agassiz Ice Cap, Ellesmere Island, Canada: details of the last millennium for stable isotopes, melt and solid conductivity

Four ice cores and two deep pit/auger sequences from the top of the Agassiz Ice Cap have been variously analysed for δ(O18), ECM (solid conductivity) and ice melt-layer stratigraphy. The high- resolution data are presented on time scales covering about the last 1000 years. The 8 time series are compared and the noise examined in terms of snow-drifting and wind-scouring of winter snow from the higher exposed parts of flow lines. The highly scoured 8 records reflect a mixture of summer-biased conditions and the amount of winter-scoured snow. Sites away from the ridges and domes are not scoured. The signal part of the scoured and unscoured sites is obtained by stacking (averaging normalized series). The stacked unscoured series of 8 and accumulation are compared to similar stacked series from central Greenland for the last century of annual averages. The comparison is surprisingly good suggesting that at these resolutions low-noise 8 series have some coherence over fairly large distances. The percentage melt series are presented and compared to those from the Devon Island Ice Cap.

The Effects of Wind on δ(18O) and Accumulation Give an Inferred Record of Seasonal δ Amplitude From the Agassiz Ice Cap, Ellesmere Island, Canada

Wind plays an important role in determining accumulation and δ(18O) on some ice caps. Three surface-to-bed cores spaced about 1 km apart have been taken on a flow line of the Agassiz Ice Cap, Ellesmere Island. The A84 core comes from the top of a local dome. The A79 core is 1200 m down the flow line, but very close to the ridge through the local dome. The A77 core is 1100m from A79 and well away from the ridge. The ridge causes wind turbulence, which removes or scours the soft winter snow from the A84 and A79 sites. No snow is scoured from the A77 site. Because of scour the retained accumulation and average δ(l8O) are different. The accumulations are 17.5, 11.5, 9.7 cm/a (ice equivalent) at A77, A79 and A84 respectively and the corresponding surface δs are –30.40, -27.90 and –27.05‰. The core records were dated by annual layer thicknesses and by identification of electrical conductivity measurement (ECM) acid peaks. With the three cores accurately aligned we examine the (δA84-δA77) and (δA84-δA79) time series. Significant variations in these difference series are interpreted as being caused by changes in the seasonal δ amplitude, which is then explained by changes in sea-ice cover. A seasonal δ amplitude series independently obtained from the Devon Island ice cap δ noise record is consistent with that from the Agassiz Ice Cap sites.

The Effects of Wind on δ(18O) and Accumulation Give an Inferred Record of Seasonal δ Amplitude From the Agassiz Ice Cap, Ellesmere Island, Canada

Wind plays an important role in determining accumulation and δ(18O) on some ice caps. Three surface-to-bed cores spaced about 1 km apart have been taken on a flow line of the Agassiz Ice Cap, Ellesmere Island. The A84 core comes from the top of a local dome. The A79 core is 1200 m down the flow line, but very close to the ridge through the local dome. The A77 core is 1100m from A79 and well away from the ridge. The ridge causes wind turbulence, which removes or scours the soft winter snow from the A84 and A79 sites. No snow is scoured from the A77 site. Because of scour the retained accumulation and average δ(l8O) are different. The accumulations are 17.5, 11.5, 9.7 cm/a (ice equivalent) at A77, A79 and A84 respectively and the corresponding surface δs are –30.40, -27.90 and –27.05‰. The core records were dated by annual layer thicknesses and by identification of electrical conductivity measurement (ECM) acid peaks. With the three cores accurately aligned we examine the (δA84-δA77) and (δA84-δA79) time series. Significant variations in these difference series are interpreted as being caused by changes in the seasonal δ amplitude, which is then explained by changes in sea-ice cover. A seasonal δ amplitude series independently obtained from the Devon Island ice cap δ noise record is consistent with that from the Agassiz Ice Cap sites.

Application of a Flow Model to the Ice-divide Region of Devon Island Ice Cap, Canada

AbstractThe steady-state flow model of Reeh (1988) is applied to a flow line that starts at the highest point of the Devon Island ice cap, follows the surface crest for 7.6 km, and then runs down the slope for a further 3.7 km. The effects of bedrock undulations, divergence of the flow lines, the variation of temperature with depth, and a basal layer of “soft” ice-age ice are taken into account. A flow law with n = 3 and a value of A close to that of Paterson (1981) is used. Longitudinal stress variations are neglected so that shear stress is calculated by the usual formula. It is estimated that these calculated values may be in error by at most 30%. Depth profiles of effective shear stress, and of the components of velocity and normal strain-rate, are presented at selected points along the flow line. These illustrate the large variations that occur near an ice divide and over bedrock undulations of amplitude comparable with the mean ice thickness. The model gives good predictions of the surface profile and of longitudinal and transverse surface strain-rates measured at ten points along the flow line. Predicted depth profiles of horizontal and vertical velocity components are compared with those measured in a bore hole. Comparison is limited by the fact that the model works in ice equivalent, whereas about 20% of the ice column consists of firn with different rheological properties from ice. The vertical velocity prediction is good. However, the model does not reproduce well the shape of the horizontal velocity profile, although measured and calculated fluxes differ only slightly. Predicted annual-layer thicknesses are within 15% of the measured ones in the upper half of the ice column, which consists of ice deposited in the last 1000 years. Predicted thicknesses in older ice are too small and the discrepancy increases with depth. This might indicate increased precipitation or, more likely, a thinner ice cap in the climatic optimum. However, it could also result from the fact that the layer of “soft” ice has been thinning continuously since the end of the ice age, so that the ice cap has never been in a steady state.