A quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment

Christoffersen, Poul; Tulaczyk, S. M.; Carsey, F.; Behar, A.

doi:10.1029/2005jf000363

Cited by 40 publications

(65 citation statements)

References 85 publications

(168 reference statements)

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“…Clean ice would form where the water supply is equal to the freezing rate but progressive restriction in the supply of sub-glacial water led to the formation of laminated and subsequently solid basal ice facies by freeze-on and regelation into sub-glacial sediments. The succession of basal ice at Kuannersuit Glacier from S M to S L is very similar to the outcome of Christoffersen et al (2006) model indicating that it was formed in a setting with sub-freezing temperatures by supercooling of pore water by ice-water interfacial processes and with an increasingly suppressed meltwater supply. In the beginning S L facies was formed with restricted meltwater available but in the end when water supplies were exhausted S M facies was formed by regelation into sub-glacial sediments.…”

Section: Discussionsupporting

confidence: 68%

Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island, west Greenland

Larsen

Kronborg

Yde

et al. 2010

Earth Surf Processes Landf

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Kuannersuit Glacier, a valley glacier on Disko Island in west Greenland, experienced a major surge from 1995 to 1998 where the glacier advanced 10·5 km and produced a ~65 m thick stacked sequence of debris-rich basal ice and meteoric glacier ice. The aim of this study is to describe the tectonic evolution of large englacial thrusts and the processes of basal ice formation using a multiproxy approach including structural glaciology, stable isotope composition (δ 18 O and δD), sedimentology and ground-penetrating radar. We argue that the major debris layers that can be traced in the terminal zone represent englacial thrusts that were formed early during the surge. Thrust overthrow was at least 200-300 m and this lead to a 30 m thick repetition of basal ice at the ice margin. It is assumed that the englacial thrusting was initiated at the transition between warm ice from the interior and the cold snout. The basal debris-rich ice was mainly formed after the thrusting phase. Two sub-facies of stratifi ed basal ice have been identifi ed; a lower massive ice facies (S M ) composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies (S L ) consisting of millimetre thick lamina of alternating debris-poor and debris-rich ice. We interpret the stratifi ed basal ice as a continuum formed mainly by freeze-on processes and localized regelation. First laminated basal ice is formed and as meltwater is depleted more sediment is entrained and fi nally the glacier freezes to the base and massive diamict is frozen-on. The increased ability to entrain sediments may partly be associated with higher basal freezing rates enhanced by loss of frictional heat from cessation of fast fl ow and conductive cooling through a thin heavily crevassed ice during the fi nal phase of the glacier surge. The dispersed basal ice facies (D) was mainly formed by secondary processes where fi ne-grained sediment is mobilized in the vein system of ice. Our results have important implications for understanding the signifi cance of basal ice formation and englacial thrusting beneath fast-fl owing glaciers and it provides new information about the development of landforms during a glacier surge.

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Section: Discussionsupporting

confidence: 68%

Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island, west Greenland

Larsen

Kronborg

Yde

et al. 2010

Earth Surf Processes Landf

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“…Dilute debris likely exists in a basal ice layer~10 m thick, frozen onto the base of the ice stream (15) from a through-going water system. Debris concentrations are poorly known in this layer but are probably on the order of 3% or less, based on limiting values reported in (24) for presumably comparable ice beneath the neighboring Kamb Ice Stream. The grounding-zone melt rate in this area has been estimated as~0.05 m a -1 (25), based on modeling that assumed deeper water than what is locally present, which likely overestimates the heat transport and melt rate.…”

mentioning

confidence: 96%

Discovery of Till Deposition at the Grounding Line of Whillans Ice Stream

Anandakrishnan

Catania

Alley

et al. 2007

Science

181

101

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We report on the discovery of a grounding-line sedimentary wedge ("till delta") deposited by Whillans Ice Stream, West Antarctica. Our observation is that grounding-line deposition serves to thicken the ice and stabilize the position of the grounding line. The ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium. Thus, the grounding line will tend to remain in the same location despite changes in sea level (until sea level rises enough to overcome the excess thickness that is due to the wedge). Further, our observation demonstrates the occurrence of rapid subglacial erosion, sediment transport by distributed subglacial till deformation, and grounding-line sedimentation, which have important implications for ice dynamics, numerical modeling of ice flow, and interpretation of the sedimentation record.

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“…The dynamics of ice sheets and their climatic feedback and future contribution to sea level rise still remains highly uncertain (Church et al, 2013). Establishing more accurate and constrained models of ice sheet behaviour has therefore become an important scientific challenge.…”

Section: Introductionmentioning

confidence: 99%

“…It is, according to the following description (Knight, 1997), "a rheological control on ice sheet dynamics; an indicator of subglacial conditions and processes; a limit to the downward extension of climate record from deep ice core(s)". Because its physical and chemical characteristics are representative of the different processes leading to its formation, a multi-parametric study of the BIL offers the opportunity to infer the former thermal, rheological and environmental conditions prevailing during its formation and re-veals the processes acting at the ice-bedrock interface (Hubbard and Sharp, 1995;Alley et al, 1998;Lawson et al, 1998;Christoffersen and Tulaczyk, 2003;Christoffersen et al, 2006;Cook et al, 2007;Hubbard et al, 2009). These inferences allow the establishment of better constrained initial and boundary conditions required for ice sheet modelling and bound the validity of paleoclimatic data interpretation.…”

Section: Introductionmentioning

confidence: 99%

A comprehensive interpretation of the NEEM basal ice build-up using a multi-parametric approach

et al. 2016

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Abstract. Basal ice is a common expression to describe bottom ice layers of glaciers, ice caps and ice sheets in which the ice is primarily conditioned by processes operating at the bed. It is chemically and/or physically distinct from the ice above and can be characterized by a component of basally derived sediments. The study of basal ice properties provides a rare opportunity to improve our understanding of subglacial environments and processes and to refine ice sheet behaviour modelling. Here, we present and discuss the results of water stable isotopes (δ 18 O and δD), ice fabrics, debris weight/size distribution and gas content of the basal part of the NEEM (North Greenland Eemian Ice Drilling Project) ice core. Below a depth of 2533.85 m, almost 10 m of basal debris-rich material was retrieved from the borehole, and regular occurrence of frozen sediments with only interstitial ice lenses in the bottom 5 m suggest that the ice-bedrock interface was reached. The sequence is composed of an alternation of three visually contrasting types of ice: clear ice with specks (very small amounts) of particulate inclusions, stratified debrisrich layers and ice containing dispersed debris. The use of water stable isotope signatures (δ 18 O and δD), together with other parameters, allows discrimination between the different types of ice and to unravel the processes involved in their formation and transformation. The basal debris-rich material presents δ 18 O values [−39.9 ‰; −34.4 ‰] within the range of the above last 300 m of unaltered meteoric ice [−44.9 ‰; −30.6 ‰] spanning a glacial-interglacial range of values. This rules out the hypothesis of a basal ice layer originating from pre-ice sheet ice overridden by the growing ice sheet, as previously suggested e.g. in the case of GRIP (Greenland Ice Core Project). We show that clear basal ice with specks corresponds to altered meteoric glacial ice where some of the climatic signal could have been preserved. However, the stratified debris-rich layers and the ice containing dispersed debris layers respectively express an "open" or "closed" system melting/refreezing signature, somewhat blurred by mixing processes in the upper part of the sequence. Climatic reconstruction is therefore prohibited from these ice types. We propose a first interpretative framework for the build-up of the NEEM basal ice sequence, based on the origin of the various ice types.

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A quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment

Cited by 40 publications

References 85 publications

Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island, west Greenland

Debris entrainment by basal freeze‐on and thrusting during the 1995–1998 surge of Kuannersuit Glacier on Disko Island, west Greenland

Discovery of Till Deposition at the Grounding Line of Whillans Ice Stream

A comprehensive interpretation of the NEEM basal ice build-up using a multi-parametric approach

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