Poul Christoffersen

Thermodynamics of basal freeze-on: predicting basal and subglacial signatures of stopped ice streams and interstream ridges

Abstract We have constructed a numerical model that simulates the response of subglacial sediments to basal freeze-on. The model is set up to emulate the basal zone of drilling sites in the Ross Sea sector of the West Antarctic ice sheet. We treat basal freeze-on at an ice–sediment interface as a thermodynamic process that couples the flow of water, heat and solutes in unfrozen subglacial sediments underlying a freezing ice base. The coupling of these flows occurs through the Clapeyron equation, which specifies the dependence of the basal freezing/melting temperature on ice pressure, water pressure, solute concentration and surface tension effects. Thermally driven water flow is induced when an ice base becomes supercooled below the pressure-melting point because ice–water surface tension inhibits ice growth in small pore spaces of fine-grained subglacial sediments. Our model results show that basal freeze-on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges. These changes are associated with specific signatures that compare with borehole observations and geophysical surveys. Water-pressure levels are reduced, and thick layers of debris-laden basal ice develop. These basal ice layers and underlying sediments contain a distinct isotopic signal. The predicted stable-isotope ratios reflect Rayleigh-type isotopic fractionation whose significance increases with increasing freezing rates. Supercooling of the ice base induces also measurable changes in the ice-temperature profile of the glacier. Till porosity represents another quantity whose evolution is influenced strongly by basal freeze-on. In particular, measurements of vertical porosity distribution beneath stopped ice streams could be used to back-calculatethe timing of the onset of basal freezing. Our model results show that the basal zone of ice streams and interstream ridges responds sensitively to changes in basal melting/freezing rates. This sensitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples. Our model results also indicate that meltwater from fast-flowing ice streams may be driven towards the freezing ice base of interstream ridges.

Glacial Sedimentary Processes and Products

Associating ice masses with the transport and deposition of sediments has long formed a central theme in glaciology and glacial geomorphology. The reason for this focus is clear, in that ice masses are responsible for much of the physical landscape which characterizes the Earths glaciated regions. This association also holds at a variety of scales, for example, from the grain-size characteristics of small-scale moraines to the structural architecture of large-scale, glacigenic sedimentary sequences in both surface and subaqueous environments.

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

The dynamic response of the Greenland Ice Sheet (GrIS) depends on feedbacks between surface meltwater delivery to the subglacial environment and ice flow. Recent work has highlighted an important role of hydrological processes in regulating the ice flow, but models have so far overlooked the mechanical effect of soft basal sediment. Here we use a three-dimensional model to investigate hydrological controls on a GrIS soft-bedded region. Our results demonstrate that weakening and strengthening of subglacial sediment, associated with the seasonal delivery of surface meltwater to the bed, modulates ice flow consistent with observations. We propose that sedimentary control on ice flow is a viable alternative to existing models of evolving hydrological systems, and find a strong link between the annual flow stability, and the frequency of high meltwater discharge events. Consequently, the observed GrIS resilience to enhanced melt could be compromised if runoff variability increases further with future climate warming.

Variable deceleration of Whillans Ice Stream, West Antarctica

The Whillans Ice Stream Ice Plain (WIP) has been slowing since at least 1963. Prior constraints on this slowdown were consistent with a constant long-term deceleration rate. New observations of ice velocity from 11 continuous and 3 seasonal GPS sites indicate the deceleration rate varies through time including on interannual time scales. Between 2009 and 2012 WIP decelerated at a rate (6.1 to 10.9 ± 2 m/yr2) that was double the multidecadal average (3.0 to 5.6 ± 2 m/yr2). To identify the causes of slowdown, we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher-order inverse model. All model results support the conclusion that the observed deceleration of WIP is caused by an increase in basal resistance to motion at a rate of 10 to 40 Pa/yr. Subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on, changes in subglacial hydrology, or increases in the area of resistant basal substrate through differential erosion. The observed variability in WIP deceleration rate suggests that dynamics in subglacial hydrology, plausibly driven by basal freeze on and/or activity of subglacial lakes, plays a key role in modulating basal resistance to ice motion in the region.

Significant groundwater contribution to Antarctic ice streams hydrologic budget

Satellite observations have revealed active hydrologic systems beneath Antarctic ice streams, but sources and sinks of water within these systems are uncertain. Here we use numerical simulations of ice streams to estimate the generation, flux, and budget of water beneath five ice streams on the Siple Coast. We estimate that 47% of the total hydrologic input (0.98 km3 yr−1) to Whillans (WIS), Mercer (MIS), and Kamb (KIS) ice streams comes from the ice sheet interior and that only 8% forms by local basal melting. The remaining 45% comes from a groundwater reservoir, an overlooked source in which depletion significantly exceeds recharge. Of the total input to Bindschadler (BIS) and MacAyeal (MacIS) ice streams (0.56 km3 yr−1), 72% comes from the interior, 19% from groundwater, and 9% from local melting. This contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes.

Large subglacial lake beneath the Laurentide Ice Sheet inferred from sedimentary sequences

Subglacial lakes identifi ed beneath the Antarctic Ice Sheet belong to a rare category of unexplored environments on Earth’s surface. The key to understanding the origin and longevity of subglacial lakes is likely contained in their sedimentary sequences. Here we explore the nature of a sedimentary succession in a deep tectonic trough identifi ed as a prime candidate for a large subglacial paleolake. The trough is the 100-km-long, 620-m-deep Christie Bay, located in the east arm of the Great Slave Lake, Canada. High-resolution seismic refl ection data and short sediment cores collected in the deep trough show a 150-m-thick sequence of fi sedimentary lake fi ll separating glacial ice-contact deposits from draped Holocene lake sediments. We interpret this sequence to consist of sediments that accumulated in a subglacial lake that covered an area larger than 130 km 2 . The inferred presence of a subglacial paleolake is supported by results from hydrologic modeling of drainage pathways beneath the Laurentide Ice Sheet during the last glacial maximum. Our data point toward the existence of a dynamic subglacial lake environment where sediments were delivered by discharge of meltwater from a subglacial water system. A core sample of the sedimentary lake fi ll in Christie Bay may elucidate whether living organisms exist in subglacial lakes.

Reactivation of Kamb Ice Stream tributaries triggers century‐scale reorganization of Siple Coast ice flow in West Antarctica

Ongoing, centennial-scale flow variability within the Ross ice streams of West Antarctica suggests that the present-day positive mass balance in this region may reverse in the future. Here we use a three-dimensional ice sheet model to simulate ice flow in this region over 250 years. The flow responds to changing basal properties, as a subglacial till layer interacts with water transported in an active subglacial hydrological system. We show that a persistent weak bed beneath the tributaries of the dormant Kamb Ice Stream is a source of internal ice flow instability, which reorganizes all ice streams in this region, leading to a reduced (positive) mass balance within decades and a net loss of ice within two centuries. This hitherto unaccounted for flow variability could raise sea level by 5 mm this century. Furthermore, better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.

Partitioning effects from ocean and atmosphere on the calving stabillity of Kangerdlugssuaq Glacier, East Greenland

Abstract We use the 7 km retreat of Kangerdlugssuaq Glacier (KG), East Greenland, to examine the mechanisms, interactions and relative significance of atmospheric forcing and ice/ocean interactions. Hydrographic data from 1991, 1993 and 2004 show that subtropical waters are common in Kangerdlugssuaq Fjord (KF), and that surface waters were warm in 2004 relative to 1991 and 1993. The main water column was nonetheless warmest in 1991. We contend that while flow of subtropical waters into fjords provides a setting in which rapid glacier retreat can occur, the triggering of retreat depends on additional environmental factors. The climatic variables standing out in our study of KG and KF are air temperature and katabatic winds. Both had strong positive anomalies during winter 2004/05, when KG retreated. We show that proglacial ice melange was absent and that fjord freeze-up did not occur until 11 April 2005, due to warm and windy conditions. We demonstrate that this setting is unusual and hypothesize that exposure to open water in winter months caused the retreat. Calculation of ice-front melt rates shows that discharge of basal meltwater, first from runoff and subsequently from frictional basal heating, should intensify the interaction between glacier and fjord.

Ice thickness and basal conditions of Vestfonna ice cap, eastern Svalbard

Pettersson, R., Christoffersen, P., Dowdeswell, J.A., Pohjola, V., Hubbard, A. and Strozzi, T., 2011. Ice thickness and basal conditions of Vestfonna Ice Cap, Eastern Svalbard. Geografiska Annaler: Series A, Physical Geography, 93, 311–322. DOI: 10.1111/j.1468‐0459.2011.00438.x Abstract We combined ground‐based pulsed radar data collected in 2008–2009 with airborne radio‐echo sounding data acquired in 1983 and 1986 over Vestfonna ice cap, Svalbard. The airborne dataset mainly covers the fast‐flowing outlet glaciers and the marginal zone, while the ground‐based data explicitly cover the interior part of the ice cap. The data presented here are thus the first complete estimate of bed topography and ice thickness. The subglacial landscape undulates with elevations between −160 and +410 m above sea level. The mean ice thickness is 186 m and the total ice area and volume are 2402 km2 and 442 ± 0.6 km3, respectively. This is a much smaller volume than those derived from empirical volume‐area scaling relationships currently used to estimate regional‐to‐global glacier volumes. This difference may depend on local conditions for Vestfonna and emphasizes the need to include more volume observations in the derivations of volume‐area scaling parameters. We also derive basal reflectivity as a proxy for thermal conditions at the bed. Basal reflectivity values suggest that fast‐flowing outlet glaciers are underlain by temperate conditions. The geometric boundaries and basal conditions for Vestfonna will be critical additions to the development of numerical models of the ice cap and to the estimation of more accurate area‐volume scaling parameters.

SPATIAL DISTRIBUTION AND CHANGE IN THE SURFACE ICE-VELOCITY FIELD OF VESTFONNA ICE CAP, NORDAUSTLANDET, SVALBARD, 1995-2010 USING GEODETIC AND SATELLITE INTERFEROMETRY DATA

Pohjola, V.A., Christoffersen, P., Kolondra, L., Moore, J.C., Pettersson, R.S., Schäfer, M., Strozzi, T. and Reijmer, C.H., 2011. Spatial distribution and change in the surface ice‐velocity field of Vestfonna ice cap, Nordaustlandet, Svalbard, 1995–2010 using geodetic and satellite interferometry data. Geografiska Annaler: Series A, Physical Geography. 93, 323–335. DOI: 10.1111/j.1468‐0459.2011.00441.x Abstract During 2007 we launched a geodetic campaign on the Svalbard ice cap Vestfonna in order to estimate the velocity field of the ice cap. This was done within the frame of the IPY project KINNVIKA. We present here the velocity measurements derived from our campaigns 2007–2010 and compare the geodetic measurements against InSAR velocity fields from satellite platforms from 1995/96 and 2008. We find the spatial distribution of ice speeds from the InSAR is in good agreement within the uncertainty limits with our geodetic measurements. We observe no clear indication of seasonal ice speed differences, but we find a speed‐up of the outlet glacier Franklinbreen between the InSAR campaigns, and speculate the outlet is having a surge phase.