[ { "id": "authors:hnzkp-dws78", "collection": "authors", "collection_id": "hnzkp-dws78", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20130822-134141566", "type": "article", "title": "Kamb Ice Stream flow history and surge potential", "author": [ { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" } ], "abstract": "A basal zone, tens of meters thick, of debris-laden ice was observed in Kamb Ice Stream, West Antarctica, using a video camera lowered into boreholes made by hot-water drilling. The debris content varies, sometimes abruptly, forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction. In most parts, the concentration of debris is low, a few percent by weight, with particles, often mud clots, dispersed in a matrix of clear ice. The nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of Kamb Ice Stream including processes leading up to the termination of its streaming behavior and possible reactivation.", "doi": "10.3189/2013AoG63A535", "issn": "0260-3055", "publisher": "International Glaciological Society", "publication": "Annals of Glaciology", "publication_date": "2013-07-01", "series_number": "63", "volume": "54", "issue": "63", "pages": "287-298" }, { "id": "authors:jyqjd-tpx72", "collection": "authors", "collection_id": "jyqjd-tpx72", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20090910-103645334", "type": "article", "title": "Bacteria beneath the West Antarctic Ice Sheet", "author": [ { "family_name": "Lanoil", "given_name": "Brian D.", "clpid": "Lanoil-Brian-D" }, { "family_name": "Skidmore", "given_name": "Mark", "clpid": "Skidmore-Mark" }, { "family_name": "Priscu", "given_name": "John C.", "clpid": "Priscu-John-C" }, { "family_name": "Han", "given_name": "Sukkyun", "clpid": "Han-Sukkyun" }, { "family_name": "Foo", "given_name": "Wilson", "clpid": "Foo-Wilson" }, { "family_name": "Vogel", "given_name": "Stefan W.", "clpid": "Vogel-Stefan-W" }, { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-Slawek" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" } ], "abstract": "Subglacial environments, particularly those that lie beneath polar ice sheets, are beginning to be recognized as an important part of Earth's biosphere. However, except for indirect indications of microbial assemblages in subglacial Lake Vostok, Antarctica, no sub-ice sheet environments have been shown to support microbial ecosystems. Here we report 16S rRNA gene and isolate diversity in sediments collected from beneath the Kamb Ice Stream, West Antarctic Ice Sheet and stored for 15 months at 4\u00b0C. This is the first report of microbes in samples from the sediment environment beneath the Antarctic Ice Sheet. The cells were abundant (-10^7 cells g^(\u22121)) but displayed low diversity (only five phylotypes), likely as a result of enrichment during storage. Isolates were cold tolerant and the 16S rRNA gene diversity was a simplified version of that found in subglacial alpine and Arctic sediments and water. Although in situ cell abundance and the extent of wet sediments beneath the Antarctic ice sheet can only be roughly extrapolated on the basis of this sample, it is clear that the subglacial ecosystem contains a significant and previously unrecognized pool of microbial cells and associated organic carbon that could potentially have significant implications for global geochemical processes.", "doi": "10.1111/j.1462-2920.2008.01831.x", "issn": "1462-2912", "publisher": "Blackwell Publishing", "publication": "Environmental Microbiology", "publication_date": "2008-12-23", "series_number": "3", "volume": "11", "issue": "3", "pages": "609-615" }, { "id": "authors:s4nyx-yrz48", "collection": "authors", "collection_id": "s4nyx-yrz48", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110107-142059992", "type": "article", "title": "Subglacial conditions during and after stoppage of an Antarctic Ice Stream: Is reactivation imminent?", "author": [ { "family_name": "Vogel", "given_name": "S. W.", "clpid": "Vogel-S-W" }, { "family_name": "Tulaczyk", "given_name": "S.", "clpid": "Tulaczyk-S" }, { "family_name": "Kamb", "given_name": "B.", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "H.", "clpid": "Engelhardt-H-F" }, { "family_name": "Carsey", "given_name": "F. D.", "clpid": "Carsey-F-D" }, { "family_name": "Behar", "given_name": "A. E.", "clpid": "Behar-A-E" }, { "family_name": "Lane", "given_name": "A. L.", "clpid": "Lane-A-L" }, { "family_name": "Joughin", "given_name": "I.", "clpid": "Joughin-I" } ], "abstract": "Borehole observations from the base of the West-Antarctic Ice Sheet (WAIS) reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of Kamb Ice Stream (KIS). This ice layer has formed over a time of several thousand years by freeze-on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location. Analysis of samples and videos sequences from boreholes drilled to the bottom of KIS confirms that KIS-stoppage was due to basal freeze-on and that relubrication of the ice stream is well underway. These results further suggest that ice stream cyclicity may be shorter than expected (1000s of years) and that a restart of KIS may be imminent within decades to centuries.", "doi": "10.1029/2005GL022563", "issn": "0094-8276", "publisher": "American Geophysical Union", "publication": "Geophysical Research Letters", "publication_date": "2005-07-20", "series_number": "14", "volume": "32", "issue": "14", "pages": "Art. No. L14502" }, { "id": "authors:mjc6n-4g606", "collection": "authors", "collection_id": "mjc6n-4g606", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20190911-132613633", "type": "article", "title": "A borehole camera system for imaging the deep interior of ice sheets", "author": [ { "family_name": "Carsey", "given_name": "Frank", "clpid": "Carsey-F" }, { "family_name": "Behar", "given_name": "Alberto", "clpid": "Behar-A" }, { "family_name": "Lane", "given_name": "A. Lonne", "clpid": "Lane-A-L" }, { "family_name": "Realmuto", "given_name": "Vince", "clpid": "Realmuto-V" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" } ], "abstract": "The design and first deployment is described for the Jet Propulsion Laboratory\u2013California Institute of Technology ice borehole camera system for acquisition of down-looking and side-looking images in a borehole made by a hot-water drill. The objective of the system is to acquire images in support of studies of the basal dynamics and thermodynamics of West Antarctic ice streams. A few sample images, obtained during the 2000/01 Antarctic field season, are shown from the basal layers of Ice Stream C.", "doi": "10.3189/172756502781831124", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "2002", "series_number": "163", "volume": "48", "issue": "163", "pages": "622-628" }, { "id": "authors:kfj3y-w3s89", "collection": "authors", "collection_id": "kfj3y-w3s89", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110107-150648203", "type": "article", "title": "Estimates of effective stress beneath a modern West Antarctic ice stream from till preconsolidation and void ratio", "author": [ { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-S" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann F.", "clpid": "Engelhardt-H-F" } ], "abstract": "Preconsolidation stress recorded in subglacial sediments provides important information about subglacial effective stresses. It is commonly used to reconstruct past effective stresses from sediments left after ice retreat. In this article, we use properties of sub-ice-stream till samples to estimate effective stresses beneath a modern West Antarctic ice stream. Two previous estimates of sub-ice-stream effective stress were derived for the Upstream B (UpB) area of Ice Stream B from shear wave velocities (50 \u00b1 40 kPa, Blankenship et al 1987) and borehole water level measurements (63 \u00b1 24 kPa, Engelhardt & Kamb 1997). However, geotechnical tests performed on samples of the UpB till have shown that if subjected to effective stress of 50\u201363 kPa this till would have significantly lower porosity (~0.32\u20130.35) and higher strength (~22\u201328 kPa) than it apparently has in situ (~0.4 and ~2kPa). We derive new estimates of sub-ice-stream effective stress using: (1) Casagrande's construction applied to the results of six confined uniaxial tests, and (2) a combination of void-ratio data for 51 till samples and 3 experimentally constrained equations describing compressibility of the UpB till under normal consolidation, overconsolidation and in the critical state. Casagrande's method yields an upper bound on effective stress of 25 kPa for four till samples and values of 13, and 4.4kPa for two other samples. The void-ratio approach gives 11.7 \u00b1 2.6 (normal consolidation), 18.3 \u00b1 4.4 (overconsolidation) and 2.0 \u00b1 0.8 kPa (critical state). These new, lower estimates of effective stress are consistent with the low till strength that has been independently measured and inferred from recent theoretical ice-stream models. Our interpretation of data on till void ratio in terms of sub-ice-stream effective stress means that we can qualitatively evaluate the nature of the vertical distribution of this stress in the UpB till layer. We infer that in the sampled top 3 m of till the effective-stress distribution is non-hydrostatic, probably close to lithostatic. The results may be useful in future modeling of ice-stream behavior and may aid efforts to delineate paleo-ice streams based on their geologic record.", "doi": "10.1111/j.1502-3885.2001.tb01216.x", "issn": "0300-9483", "publisher": "Wiley-Blackwell", "publication": "Boreas", "publication_date": "2001-06", "series_number": "2", "volume": "30", "issue": "2", "pages": "101-114" }, { "id": "authors:60hvh-fjt86", "collection": "authors", "collection_id": "60hvh-fjt86", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110107-142827273", "type": "article", "title": "A hot-water ice-coring drill", "author": [ { "family_name": "Engelhardt", "given_name": "H.", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "B.", "clpid": "Kamb-B" }, { "family_name": "Bolsey", "given_name": "R.", "clpid": "Bolsey-R" } ], "abstract": "A new method of ice-core drilling uses an annulus of hot-water jets to melt out a cylindrical ice core. This lightweight device used in combination with a fast hot-water drill can quickly obtain ice cores from any depth.", "doi": "10.3189/172756500781832873", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "2000-03", "series_number": "153", "volume": "46", "issue": "153", "pages": "341-345" }, { "id": "authors:pbdcj-0bw11", "collection": "authors", "collection_id": "pbdcj-0bw11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140522-090055760", "type": "article", "title": "Basal mechanics of Ice Stream B, west Antarctica: 1. Till mechanics", "author": [ { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-S" }, { "family_name": "Kamb", "given_name": "W. Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann F.", "clpid": "Engelhardt-H-F" } ], "abstract": "Data from laboratory geotechnical tests on till recovered from beneath Ice Stream B, West Antarctica, at the Upstream B camp (hereinafter the UpB till) show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate. These data support use of a Coulomb-plastic rheology in modeling of ice stream behavior and subglacial till deformation. Our testing program combined triaxial, ring shear, and confined uniaxial tests to investigate till strength and compressibility. Results show that the UpB till follows closely Coulomb's equation in which shear strength is a linear function of normal effective stress (apparent cohesion near zero and internal friction angle \u03d5 equal to 24\u00b0). Till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress. In general, the behavior of the UpB till is consistent with other experimental evidence regarding mechanical behavior of granular materials. Based on our laboratory results we formulate the Compressible-Coulomb-Plastic till model in which there are three interrelated, primary state variables: shear strength, void ratio, and normal effective stress. This model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings. These simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics: (1) occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records, and (2) distributed till deformation to depths of 0.1\u20131.0 m beneath the ice base. Our laboratory and modeling results substantiate application of the Compressible-Coulomb-Plastic model in simulations of the motion of Ice Stream B over its weak till bed.", "doi": "10.1029/1999JB900329", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "2000-01-10", "series_number": "B1", "volume": "105", "issue": "B1", "pages": "463-481" }, { "id": "authors:vtw4s-hw530", "collection": "authors", "collection_id": "vtw4s-hw530", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140522-073224522", "type": "article", "title": "Basal mechanics of Ice Stream B, west Antarctica: 2. Undrained plastic bed model", "author": [ { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-S" }, { "family_name": "Kamb", "given_name": "W. Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann F.", "clpid": "Engelhardt-H-F" } ], "abstract": "Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non-linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes (\u0117 = \u2202e/\u2202t) on the basal melting rate m_r. Since till strength \u03c4_b{e} and ice stream velocity U_b{\u03c4_b} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage \u0117 = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating m_r{ U_b \u03c4_b}. This is the \"weak till\" condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress \u03c4_b < (n + 1)^(\u22121) \u03c4_d. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities (\"ice sheet\" mode) and one with a weak bed and fast ice velocities (\"ice-stream\" mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming.", "doi": "10.1029/1999JB900328", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "2000-01-10", "series_number": "B1", "volume": "105", "issue": "B1", "pages": "483-494" }, { "id": "authors:xyd6r-afm14", "collection": "authors", "collection_id": "xyd6r-afm14", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20110107-152638407", "type": "article", "title": "Basal sliding of Ice Stream B, West Antarctica", "author": [ { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" } ], "abstract": "A \"tethered stake\" apparatus is used to measure basal sliding in a borehole on Ice Stream B, West Antarctica, about 300 km upstream (east) from its: grounding line near the head of the Ross Ice Shelf A metal stake, emplaced at the top of a layer of unfrozen till underlying the ice, is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding. The measured sliding motion includes any actual slip across the ice-till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface. This 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment, based on features of the record of apparent sliding. Alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm. In any case it is small compared to the seismically inferred till thickness of 9 m. Measured apparent sliding averages 69% of the total motion of 1.2 m d^(-1) over 26 days of observation if a 3.5 day period of slow apparent sliding (8% of the total motion) is included in the average. The occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c. 80% and c. 8% of the total motion, on a time-scale of a few days. However, it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole. If the slow period is therefore omitted, the indicated average basal sliding rate is 83 % of the total motion. In either case, basal sliding predominates as the cause of the rapid ice-stream motion. In the last 2 days of observation the average apparent sliding rate reached 1.17 m d^(-1), essentially 100% of the motion of the ice stream. If till deformation contributes significantly to the ice-stream motion, the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer. These observations, if applicable to the West Antarctic ice sheet in general, pose complications in modeling the rapid ice-streaming motion.", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "1998-12", "series_number": "147", "volume": "44", "issue": "147", "pages": "223-230" }, { "id": "authors:jtsa8-whv31", "collection": "authors", "collection_id": "jtsa8-whv31", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140522-095331661", "type": "article", "title": "Pleistocene Collapse of the West Antarctic Ice Sheet", "author": [ { "family_name": "Scherer", "given_name": "Reed P.", "clpid": "Scherer-R-P" }, { "family_name": "Aldahan", "given_name": "Ala", "clpid": "Aldahan-A" }, { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-S" }, { "family_name": "Possnert", "given_name": "G\u00f6ran", "clpid": "Possnert-G" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" } ], "abstract": "Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B contain Quaternary diatoms and up to 108 atoms of beryllium-10 per gram. Other samples contain no Quaternary diatoms and only background levels of beryllium-10 (less than 10^6 atoms per gram). The occurrence of young diatoms and high concentrations of beryllium-10 beneath grounded ice indicates that the Ross Embayment was an open marine environment after a late Pleistocene collapse of the marine ice sheet.", "doi": "10.1126/science.281.5373.82", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1998-07-03", "series_number": "5373", "volume": "281", "issue": "5373", "pages": "82-85" }, { "id": "authors:9jend-ky920", "collection": "authors", "collection_id": "9jend-ky920", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140527-084602192", "type": "article", "title": "Sedimentary processes at the base of a West Antarctic ice stream: Constraints from textural and compositional properties of subglacial debris", "author": [ { "family_name": "Tulaczyk", "given_name": "Slawek", "clpid": "Tulaczyk-S" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Scherer", "given_name": "Reed P.", "clpid": "Scherer-R-P" }, { "family_name": "Engelhardt", "given_name": "Hermann F.", "clpid": "Engelhardt-H-F" } ], "abstract": "Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, sub-glacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.", "doi": "10.2110/jsr.68.487", "issn": "1527-1404", "publisher": "Society for Sedimentary Geology", "publication": "Journal of Sedimentary Research", "publication_date": "1998-05", "series_number": "3", "volume": "68", "issue": "3", "pages": "487-496" }, { "id": "authors:q4ajb-dm645", "collection": "authors", "collection_id": "q4ajb-dm645", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140522-095645772", "type": "article", "title": "Basal hydraulic system of a West Antarctic ice stream: constraints from borehole observations", "author": [ { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" } ], "abstract": "Pressure and tracer measurements in boreholes drilled to the bottom of Ice Stream B, West Antarctica, are used to obtain information about the basal water conduit\nsystem in which high water pressures are developed. These high pressures presumably make possible the rapid movement of the ice stream. Pressure in the system is indicated by\nthe borehole water level once connection to the conduit system is made. On initial connection, here also called \"breakthrough\" to the basal water system, the water level drops in a few minutes to an initial depth in the range 96- 117 m below the surface. These water levels are near but mostly somewhat deeper than the flotation level of about 100m depth (water level at which basal water pressure and ice overburden pressure are equal), which is calculated\nfrom depth-density profiles and is measured in one borehole. The conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed; the thickness of the gap \u03b4 has to be about 2 mm to account for the water-level drop on breakthrough, and about 4 mm to fit the results of a salt-tracer experiment indicating downstream transport at a speed of 7.5 mm s^-1. The above gap-conduit model is, however, ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole, and also by the large hole-to-hole variation in measured basal pressure, which if present in a gap-conduit system with \u03b4 = 2 or 4 mm would result in unacceptably large local water fluxes. An alternative model that avoids\nthe e objections, called the \"gap opening\" model, involves opening a gap as injection proceeds: starting with a thin film, the injection of water under pressure lifts the ice mass a round the borehole, creating a gap 3 or 4 mm wide at the ice/bed interface. Evaluated quantitatively, the gap-opening model accounts for the volume of water that the basal watery system accepts on breakthrough, which obviates the gap-conduit model. In order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits, of which the most promising\ntype is the \"canal\" conduit proposed theoretically by Walder and Fowler (1994): flat, low conduits incised into the till, ~0.1 m deep and perhaps ~1 m wide, with a flat ice roof The basal water-pressure data suggest that the canals are spaced ~50-300 m apart, much closer than R-tunnel would be. The deepest observed water level, 117 m, i the most likely to reflect the actual water pressure in the canals, corresponding to a basal effective pressure\nof 1.6 bar. In this interpretation, the shallower water levels are affected by loss of hydraulic head in the narrow passageway(s; that connect along the bed from borehole\nto canal (s). Once a borehole has frozen up and any passageways connecting with canals have become closed, a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till, given enough time for pressure equilibration.\nThis pressure varies considerably with time, over the equivalent water-level range from 100 to 113 m. Basal pressure sensors 500 m apart report uncorrelated variations, whereas sensors in boreholes 25 m apart report mostly (but not entirely) well-correlated variations,\nof unknown origin. In part of the record, remarkable anticorrelated variations are interspersed with positively correlated one, and there are rare, abrupt excursions to\nextreme water levels as deep as 125m and as shallow as 74 m. A diurnal pressure fluctuation, intermittently observed, may possibly be caused by the ocean tide in the Ross Sea.\nThe lack of any observed variation in ice-stream motion, when large percentagewise variations in basal effective pressure were occurring according to our data, suggests that the observed pressure variations are sufficiently local, and so randomly variable from place to place, that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed.", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "1997", "series_number": "144", "volume": "43", "issue": "144", "pages": "207-230" }, { "id": "authors:1ehvx-v0f67", "collection": "authors", "collection_id": "1ehvx-v0f67", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140701-103209347", "type": "article", "title": "Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 2. Interpretation", "author": [ { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Fahnestock", "given_name": "Mark", "clpid": "Fahnestock-Mark" }, { "family_name": "Humphrey", "given_name": "Neil", "clpid": "Humphrey-Neil" }, { "family_name": "Meier", "given_name": "Mark", "clpid": "Meier-Mark" }, { "family_name": "Stone", "given_name": "Dan", "clpid": "Stone-Dan" } ], "abstract": "The data presented in part 1 of this paper (Meier et al., this issue) are here used to assess the role of water input/output, water storage, and basal water pressure in the rapid movement of Columbia Glacier, Alaska. Consistently high basal water pressures, mostly in the range from 300 kPa below to 100 kPa above the ice overburden pressure, are responsible in an overall way for the high glacier flow velocities (3.5\u20139 m d^\u22121), which are due mainly to rapid basal sliding caused by the high water pressure. Diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site. The water pressure fluctuation tracks the time-integrated water input (less a steady withdrawal), as expected for the diurnally fluctuating storage of water in the glacier far from the terminus. At the downglacier site, the situation is more complex. Diurnal peaks in water level, which are directly related to intraglacial water storage as well as to basal water pressure, are shifted forward in time by 4 hours, probably as a result of the effect of diurnal fluctuation in water output from the glacier, which affects the local water storage fluctuations near the terminus. Because of the forward shift in the basal water pressure peaks, which at the downglacier site lead the velocity peaks by 6 hours, a mechanical connection between water pressure and sliding there would have to involve a 6-hour (quarter period) delay. However, the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single, consistent control mechanism at both sites. The velocity variations in nondiurnal \"speed-up events\" caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier. It appears that small variations in water pressure (\u2264100 kPa) sufficient to produce the observed velocity variations (15\u201330%) are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity. At the achieved level of observation the clearest (though not complication free) control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed.", "doi": "10.1029/94JB00467", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1994-08-10", "series_number": "B8", "volume": "99", "issue": "B8", "pages": "15231-15244" }, { "id": "authors:tnma1-m9r30", "collection": "authors", "collection_id": "tnma1-m9r30", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140523-100401162", "type": "article", "title": "Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 1. Observations", "author": [ { "family_name": "Meier", "given_name": "Mark", "clpid": "Meier-Mark" }, { "family_name": "Lundstrom", "given_name": "Scott", "clpid": "Lundstrom-Scott" }, { "family_name": "Stone", "given_name": "Dan", "clpid": "Stone-Dan" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Humphrey", "given_name": "Neil", "clpid": "Humphrey-Neil" }, { "family_name": "Dunlap", "given_name": "William W.", "clpid": "Dunlap-W-W" }, { "family_name": "Fahnestock", "given_name": "Mark", "clpid": "Fahnestock-Mark" }, { "family_name": "Krimmel", "given_name": "Robert M.", "clpid": "Krimmel-R-M" }, { "family_name": "Walters", "given_name": "Roy", "orcid": "0000-0002-1835-6078", "clpid": "Walters-Roy" } ], "abstract": "Measurements of glacier flow velocity and basal water pressure at two sites on Columbia Glacier, Alaska, are combined with meteorological and hydrologic data to provide an observational basis for assessing the role of water storage and basal water pressure in the rapid movement of this large glacier. During the period from July 5 to August 31, 1987, coordinated observations were made of glacier surface motion and of water level in five boreholes drilled to (or in one case near to) the glacier bed at two sites, 5 and 12 km from the terminus. Glacier velocities increased downglacier in this reach from about 4 m d^\u22121 to about 7 m d^\u22121. Three types of time variation in velocity and other variables were revealed: (1) Diurnal fluctuation in water input/output, borehole water level, and ice velocity (fluctuation amplitude 5 to 8%); (2) Speed-up events in glacier motion (15\u201330% speed up), lasting about 3 days, and occurring at times of enhanced input of water, in some cases from rain and in others from ice ablation enhanced by strong, warm winds; (3) \"Extra-slowdown\" events, in which, after a speed-up event, the ice velocity decreased in about 3 days to a level consistently lower than that prior to the speed-up event. All of the time variations in velocity were due, directly or indirectly, to variations in water input to the glacier. The role of basal water in causing the observed glacier motions is interpreted by Kamb et al. (this issue).", "doi": "10.1029/94JB00237", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1994-08-10", "series_number": "B8", "volume": "99", "issue": "B8", "pages": "15219-15229" }, { "id": "authors:h5kxk-ssg07", "collection": "authors", "collection_id": "h5kxk-ssg07", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140523-092026792", "type": "article", "title": "Satellite Radar Interferometry for Monitoring Ice Sheet Motion: Application to an Antarctic Ice Stream", "author": [ { "family_name": "Goldstein", "given_name": "Richard M.", "clpid": "Goldstein-R-M" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Frolich", "given_name": "Richard M.", "clpid": "Frolich-R-M" } ], "abstract": "Satellite radar interferometry (SRI) provides a sensitive means of monitoring the flow velocities and grounding-line positions of ice streams, which are indicators of response of the ice sheets to climatic change or internal instability. The detection limit is about 1.5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction. The grounding line, detected by tidal motions where the ice goes afloat, can be mapped at a resolution of \u223c0.5 kilometer. The SRI velocities and grounding line of the Rutford Ice Stream, Antarctica, agree fairly well with earlier ground-based data. The combined use of SRI and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior.", "doi": "10.1126/science.262.5139.1525", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1993-12-03", "series_number": "5139", "volume": "262", "issue": "5139", "pages": "1525-1530" }, { "id": "authors:vy9gj-cx431", "collection": "authors", "collection_id": "vy9gj-cx431", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140523-094900349", "type": "article", "title": "Characteristics of the Bed of the Lower Columbia Glacier, Alaska", "author": [ { "family_name": "Humphrey", "given_name": "Neil", "clpid": "Humphrey-N" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Fahnestock", "given_name": "Mark", "clpid": "Fahnestock-M" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" } ], "abstract": "An unplanned, but unique, experiment has given an in situ measurement of the strength of deforming subglacial till under the central region of a major valley glacier. We report on both planned and unplanned borehole investigations of the subglacial shear zone of Columbia Glacier, southeast Alaska. Basal samples, coring and down-hole water samples show that the fiord-filling lower reach of the glacier is underlain by a thin, \u223c 7-cm, veneer of rock debris. Fluidized debris intruded at least a meter up the borehole. At a higher site, 13 km from the terminus and above the fiord, probing, samples, and the bending of a drill stem, which was stuck in the basal zone for 5 days, showed that the basal till layer was \u223c 65 cm thick. Horizontal velocity of the till decreased monotonically downward from the ice/till interface. Till at the interface moved with the ice velocity. Plastic deformation of the drill stem gave an estimate of the strength of the basal till, which is normally described as a viscoplastic material. If the till is assumed to be either perfectly plastic or Newtonian viscous, then the strengths are as follows; the plastic yield strength of the till was 5.5\u00d710^3 Pa (0.055 bar) with an upper bound of 1.3 \u00d710^4 Pa (0.13 bar), while the nominal viscosity was of the order of 2\u00d710^8 Pa s (2\u00d710^9 poise), with an upper bound of 5\u00d710^8 Pa s. In neither case is the till \"strength\" enough to supply the bulk basal shear stress to resist the glacier flow.", "doi": "10.1029/92JB01869", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research B", "publication_date": "1993-01-10", "series_number": "B1", "volume": "98", "issue": "B1", "pages": "837-846" }, { "id": "authors:hdzd9-csc86", "collection": "authors", "collection_id": "hdzd9-csc86", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140611-095003658", "type": "article", "title": "Physical Conditions at the Base of a Fast Moving Antarctic Ice Stream", "author": [ { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Humphrey", "given_name": "Neil", "clpid": "Humphrey-N" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Fahnestock", "given_name": "Mark", "clpid": "Fahnestock-M" } ], "abstract": "Boreholes drilled to the bottom of ice stream B in the West Antarctic Ice Sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1.6 bars of the ice overburden pressure. These conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick. The mechanics of ice streaming plays a role in the response of the ice sheet to climatic change.", "doi": "10.1126/science.248.4951.57", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1990-04-06", "series_number": "4951", "volume": "248", "issue": "4951", "pages": "57-59" }, { "id": "authors:trgr6-pah07", "collection": "authors", "collection_id": "trgr6-pah07", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140912-140642167", "type": "article", "title": "Waves of accelerated motion in a glacier approaching surge: the mini-surges of Variegated Glacier, Alaska, U.S.A.", "author": [ { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" } ], "abstract": "Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-81 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These \"mini-surges\" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down-glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-11 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal outflow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The\nmini-surge wave propagation is controlled by the propagation\nof the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longitudinal gradient (up-glacier increase) in hydraulic\nconductivity of the basal water system in response to the\nup-glacier increase of the basal water pressure in the mini-surge wave. The mini-surge waves are initiated in a\nsuccession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial (?) reservoirs is released suddenly into the basal water system immediately below, generating a pressure rise that propagates down-stream from there. Relationships of the mini-surges to the main surge are seen in the role of high basal water pressure in causing the rapid glacier motion in both phenomena, in the pulse- propagation features of both, and in the high outflow turbidity associated with both. The mini-surges of Variegated Glacier have a strong resemblance to movement and uplift events observed in Unteraargletscher and Findelengletscher, Switzerland. This bears on the question whether the mini-surges are a particular characteristic of surge-type glaciers prior to surge.", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "1987", "series_number": "113", "volume": "33", "issue": "113", "pages": "27-46" }, { "id": "authors:4fneh-q4762", "collection": "authors", "collection_id": "4fneh-q4762", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140908-130056304", "type": "article", "title": "Glacier Surge Mechanism: 1982-1983 Surge of Variegated Glacier, Alaska", "author": [ { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" }, { "family_name": "Raymond", "given_name": "C. F.", "clpid": "Raymond-C-F" }, { "family_name": "Harrison", "given_name": "W. D.", "clpid": "Harrison-W-D" }, { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Echelmeyer", "given_name": "K. A.", "clpid": "Echelmeyer-K-A" }, { "family_name": "Humphrey", "given_name": "N.", "clpid": "Humphrey-N" }, { "family_name": "Brugman", "given_name": "M. M.", "clpid": "Brugman-M-M" }, { "family_name": "Pfeffer", "given_name": "T.", "clpid": "Pfeffer-T" } ], "abstract": "The hundredfold speedup in glacier motion in a surge of the kind the kind that took place in Variegated Glacier in 1982-1983 is caused by the buildup of high water pressure in the basal passageway system, which is made possible by a fundamental and pervasive change in the geometry and water-transport characteristics of this system. The behavior of the glacier in surge has many remarkable features, which can provide clues to a detailed theory of the surging process. The surge mechanism is akin to a proposed mechanism of overthrust faulting.", "doi": "10.1126/science.227.4686.469", "issn": "0036-8075", "publisher": "American Association for the Advancement of Science", "publication": "Science", "publication_date": "1985-02-01", "series_number": "4686", "volume": "227", "issue": "4686", "pages": "469-479" }, { "id": "authors:tpwp6-6zk10", "collection": "authors", "collection_id": "tpwp6-6zk10", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140819-135437662", "type": "article", "title": "Structure of ice IV, a metastable highpressure phase", "author": [ { "family_name": "Engelhardt", "given_name": "Hermann", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "Barclay", "clpid": "Kamb-B" } ], "abstract": "Ice IV, made metastably at pressures of about 4 to 5.5 kb, has a structure based on a rhombohedral unit cell of dimensions a_R = 760\u00b11 pm, \u03b1 = 70.1\u00b10.2\u00b0, space group R3\u0304c, as observed by x\u2010ray diffraction at 1 atm, 110 K. The cell contains 12 water molecules of type 1, in general position, plus 4 of type 2, with O(2) in a special position on the threefold axis. The calculated density at 1 atm, 110 K is 1.272\u00b10.005 g\u2009cm^(\u22123). Every molecule is linked by asymmetric H bonds to four others, the bonds forming a new type of tetrahedrally\u2010connected network. Molecules of type 1 are linked by O(1)\u22c5\u22c5\u22c5O(1\u2032) bonds into puckered six\u2010rings of 3 symmetry, through the center of each of which passes an O(2)\u22c5\u22c5\u22c5O(2\u2032) bond between a pair of type\u20102 molecules, along the threefold axis. The six\u2010rings are linked laterally by type\u20102 molecules to form puckered sheets that are topologically similar to such sheets in ice I, but are connected to one another in a very different and novel way. One quarter of the intersheet bonds connect not directly between adjacent sheets but remotely, from one sheet to the second nearest sheet, through holes in the intervening sheet. These remote connections are the O(2)\u22c5\u22c5\u22c5O(2\u2032) bonds, passing through the O(1)\u2010type six\u2010rings. The sheets are stacked in a sequence based on ice Ic, modified by reversal of the puckering to form the remote connections and by internal distortion of the sheets to complete the remaining intersheet bonds. Of the four nonequivalent H bonded O\u22c5\u22c5\u22c5O distance in the structure, two (279 and 281\u00b11pm) are only moderately lengthened relative to the bonds in ice I (275 pm), whereas the O(1)\u22c5\u22c5\u22c5O(1\u2032) bond (288\u00b11pm) and O(2)\u22c5\u22c5\u22c5O(2\u2032) bond (292\u00b11pm) are lengthened extraordinarily. This is caused by repulsion between O(1) and O(2) at nonbonded distances of 314 and 329 pm in the molecular cluster consisting of the O(1)\u2010type six\u2010ring threaded by the O(2)\u22c5\u22c5\u22c5O(2\u2032) bond. The mean O\u22c5\u22c5\u22c5O bond distance of 283.3 pm, which is high relative to other ice structures except ice VII/VIII, reflects similarly the accommodation of a relatively large number (3.75 on average) of nonbonded neighbors around each molecule at relatively short distances of 310\u2013330 pm. Bond bending in ice IV, as measured by deviation of the O\u22c5\u22c5\u22c5O\u22c5\u22c5\u22c5O bond angles from 109.5\u00b0, is relatively low compared to most other dense ice structures. All H bonds in ice IV except O(1)\u22c5\u22c5\u22c5O(1\u2032) are required to be proton\u2010disordered by constraints of space\u2010group symmetry. The x\u2010ray structure\u2010factor data indicate that O(1)\u22c5\u22c5\u22c5O(1\u2032) is probably also proton\u2010disordered. Ice IV is the only ice phase other than ice I and Ic to remain proton\u2010disordered on quenching to 77 K. The increased internal energy of ice IV relative to ice V, amounting to about 0.23 kJ\u2009mole^(\u22121), which underlies the metastability of ice IV in relation to ice V, can be explained structurally as a result of extra overlap and bond\u2010stretching energy in ice IV, partially compensated by extra bond\u2010bending energy in ice V. The structural relation between ice IV and ice I offers a possible explanation for the reduced barrier to nucleation of ice IV, as compared to ice V, in crystallizing from liquid water.", "doi": "10.1063/1.442040", "issn": "0021-9606", "publisher": "American Institute of Physics", "publication": "Journal of Chemical Physics", "publication_date": "1981-12-15", "series_number": "12", "volume": "75", "issue": "12", "pages": "5887-5899" }, { "id": "authors:3m7ke-yat50", "collection": "authors", "collection_id": "3m7ke-yat50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181116-112255845", "type": "article", "title": "The Ice-Rock Interface and Basal Sliding Process as Revealed by Direct Observation in Bore Holes and Tunnels", "author": [ { "family_name": "Kamb", "given_name": "B.", "clpid": "Kamb-B" }, { "family_name": "Engelhardt", "given_name": "H. F.", "clpid": "Engelhardt-H-F" }, { "family_name": "Harrison", "given_name": "W. D.", "clpid": "Harrison-W-D" } ], "abstract": "The glacier bed, where basal sliding occurs, was reached by cable-tool drilling and sand-pump bailing in seven bore holes in Blue Glacier, Olympic National Park, Washington. Basal sliding velocities measured by bore-hole photography and confirmed by inclinometry are unexpectedly low, ranging from 0.3 to 3.0 cm/day and averaging 1.0 cm/day. This is much less than about half the surface velocity of 15 cm/day, which was the sliding-rate expected from earlier deformation measurements in bore holes made by thermal drilling alone.", "doi": "10.1017/S0022143000030124", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "1979", "series_number": "89", "volume": "23", "issue": "89", "pages": "416-419" }, { "id": "authors:e35s7-pgc32", "collection": "authors", "collection_id": "e35s7-pgc32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20181119-113107131", "type": "article", "title": "Observation of Basal Sliding of Variegated Glacier, Alaska", "author": [ { "family_name": "Engelhardt", "given_name": "H.", "clpid": "Engelhardt-H-F" }, { "family_name": "Kamb", "given_name": "B.", "clpid": "Kamb-B" }, { "family_name": "Raymond", "given_name": "C. F.", "clpid": "Raymond-C-F" }, { "family_name": "Harrison", "given_name": "W. D.", "clpid": "Harrison-W-D" } ], "abstract": "Variegated Glacier is a surge-type glacier in the St Elias mountain range in Alaska. The interval between surges is about 20 years; the last one occurred in 1964 to 1965. This glacier has been studied extensively since 1973 (Bindschadler and others, 1977). Thus far, measurements of ice velocities have been restricted to the surface. They have been analyzed using geophysically measured ice depths, in order to estimate ice velocities in the ice mass and at the base (Bindschadler and others, 1978). From 1973 to 1977 the distribution of annual ice velocities along most of the length of the glacier can be explained primarily by internal deformation without major contribution from sliding at the base. However, the variation of surface velocity with time gives definite indication that sliding occurs in summer and that the average summer rate is increasing progressively from summer to summer and that in a zone 5 to 7 km below the head of the glacier the summer-to-summer increase in inferred sliding rate is especially rapid. This is a notably distinguishing feature, which is probably indicative of a build-up toward the next surge. In order to obtain direct information about sliding-rates and water pressures at the base in this zone, a bore hole was drilled to the bottom of the glacier about 6 km below the glacier head. Observations in the hole started in June 1978 and were continued until 31 July 1978. The hole connected to an englacial water system at a depth of 204 m whereupon the water level dropped gradually to about 100 m below the surface. The last 6 m above-the base at 356 m could be drilled only by means of a cable tool because of the presence of debris-rich ice. Upon reaching the bottom, the water level increased rapidly to the firn water table at about 8 m below surface. Large variations in water level of about 200 m occurred during the following period of observation of 35 d. Major events such as audible icequakes, heavy rainfalls, and a period of unusually high ablation were associated with abrupt increases of water level up to the firn water table. High water pressure at the bottom drove a flow of muddy and sandy water upward in the hole. Consequently high freezing rates in the lower 150 m of the hole produced a very rough bore-hole wall covered with ledges, coral-reef-like features, grooves, and pockets filled with sand. Near the bottom, embedded rocks stuck out of the bore-hole wall. These features were recognized by bore-hole television. The bore-hole bottom consisted of sand which continuously proliferated and washed into the hole. Attempts to remove this sand by means of a sand pump failed, the bailed-out sand being replaced immediately. From bore-hole inclinometry an internal deformation of the ice mass of 0.22 m d^(\u22121) was obtained. Together with average surface velocity of 0.47 m d^(\u22121) we get a sliding velocity of 0.25 m d^(\u22121), averaged over the time of observation. This result confirms the sliding velocities inferred from surface velocity measurements. It also lies on the exponential trend line of increasing summer-to-summer velocities showing a doubling of sliding velocities about every two years (Bindschadler and others, unpublished). This strongly indicates that the next surge is likely to occur in the early eighties. Input of water from the surface probably will play a role in triggering the surge.", "doi": "10.1017/S0022143000030033", "issn": "0022-1430", "publisher": "International Glaciological Society", "publication": "Journal of Glaciology", "publication_date": "1979", "series_number": "89", "volume": "23", "issue": "89", "pages": "406-407" } ]