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  • 1
    Online Resource
    Online Resource
    Howat Receives 2007 Cryosphere Young Investigator Award
    American Geophysical Union (AGU) ; 2008
    In:  Eos, Transactions American Geophysical Union Vol. 89, No. 10 ( 2008-03-04), p. 100-100
    Details
    In: Eos, Transactions American Geophysical Union, American Geophysical Union (AGU), Vol. 89, No. 10 ( 2008-03-04), p. 100-100
    Abstract: I have had the great pleasure of knowing Ian M. Howat over the past seven and a half years and to benefit from his great intellect, rock‐solid work ethics, and collegiality. I have been always impressed by his ability to stay focused on science while collecting field data in Antarctica and Iceland, or during long periods of computational work at UCSC. It is terrific to now see Ian receive the 2007 AGU Cryosphere Young Investigator Award for his major contribution to understanding recent, rapid changes in ice discharge from Greenland outlet glaciers. In 2005, Ian, then still a graduate student, was the first scientist to document that major outlet glaciers in SE Greenland experienced dramatic acceleration and thinning in the first half of this decade. His findings represent a timely addition to the scientific effort aimed at evaluating the stability of the Greenland ice sheet and its potential contribution to near‐future sea level rise. Ian's research results drew the attention of national and international media. They also helped motivate a number of subsequent scientific studies of Greenland outlet glaciers. In the process of doing his groundbreaking work, Ian made a significant technological improvement in the feature‐ tracking software used to calculate glacier flow velocities from satellite images.
    Type of Medium: Online Resource
    ISSN: 0096-3941 , 2324-9250
    URL: Article
    DOI: 10.1029/2008EO100010
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2008
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  • 2
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    Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site
    Copernicus GmbH ; 2021
    In:  The Cryosphere Vol. 15, No. 8 ( 2021-08-27), p. 4117-4133
    Details
    In: The Cryosphere, Copernicus GmbH, Vol. 15, No. 8 ( 2021-08-27), p. 4117-4133
    Abstract: Abstract. The crystal orientation fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect bulk anisotropic fabric patterns by exploiting the birefringence of ice crystals at radar frequencies, with the assumption that one of the crystallographic axes is aligned in the vertical direction. In this study, we conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations near the Western Antarctic Ice Sheet (WAIS) Divide ice core site. From these measurements, we are able to quantify the azimuthal fabric asymmetry at this site to a depth of 1400 m at a bulk-averaged resolution of up to 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide ice core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to identify and conclude that the fabric orientation is depth-invariant to at least 1400 m, equivalent to 6700 years BP (years before 1950) and aligns closely with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk fabric asymmetry and orientation compares well with much more laborious sample-based COF measurements from thin ice sections. Because it is the bulk-averaged fabric that ultimately influences ice flow, polarimetric radar methods provide an opportunity for its accurate and widespread mapping and its incorporation into ice flow models.
    Type of Medium: Online Resource
    ISSN: 1994-0424
    URL: Article
    URL: Article
    DOI: 10.5194/tc-15-4117-2021
    DOI: 10.5194/tc-15-4117-2021-supplement
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
    detail.hit.zdb_id: 2393169-3
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  • 3
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    Late Quaternary glaciation of the Upper Soča River Region (Southern Julian Alps, NW Slovenia)
    Elsevier BV ; 2004
    In:  Sedimentary Geology Vol. 165, No. 3-4 ( 2004-3), p. 265-283
    Details
    In: Sedimentary Geology, Elsevier BV, Vol. 165, No. 3-4 ( 2004-3), p. 265-283
    Type of Medium: Online Resource
    ISSN: 0037-0738
    URL: Article
    DOI: 10.1016/j.sedgeo.2003.11.011
    RVK:
    RA 1000
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2004
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  • 4
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    High geothermal heat flux measured below the West Antarctic Ice Sheet
    American Association for the Advancement of Science (AAAS) ; 2015
    In:  Science Advances Vol. 1, No. 6 ( 2015-07-03)
    Details
    In: Science Advances, American Association for the Advancement of Science (AAAS), Vol. 1, No. 6 ( 2015-07-03)
    Abstract: The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m 2 , significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m 2 . The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.
    Type of Medium: Online Resource
    ISSN: 2375-2548
    URL: Article
    DOI: 10.1126/sciadv.1500093
    Language: English
    Publisher: American Association for the Advancement of Science (AAAS)
    Publication Date: 2015
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  • 5
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    A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics
    International Glaciological Society ; 2003
    In:  Journal of Glaciology Vol. 49, No. 165 ( 2003), p. 240-256
    Details
    In: Journal of Glaciology, International Glaciological Society, Vol. 49, No. 165 ( 2003), p. 240-256
    Abstract: Mega-scale glacial lineations (MSGLs) are longitudinally aligned corrugations (ridge–groove structures 6–100 km long) in sediment produced subglacially. They are indicators of fast flow and a common signature of ice-stream beds. We develop a qualitative theory that accounts for their formation, and use numerical modelling, and observations of ice-stream beds to provide supporting evidence. Ice in contact with a rough (scale of 10–10 3 m) bedrock surface will mimic the form of the bed. Because of flow acceleration and convergence in ice-stream onset zones, the ice-base roughness elements experience transverse strain, transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards. Where such keels slide across a soft sedimentary bed, they plough through the sediments, carving elongate grooves, and deforming material up into intervening ridges. This explains MSGLs and has important implications for ice-stream mechanics. Groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream. Keels may provide basal drag in the force budget of ice streams, thereby playing a role in flow regulation and stability. We speculate that groove ploughing permits significant ice-stream widening, thus facilitating high-magnitude ice discharge.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.3189/172756503781830719
    Language: English
    Publisher: International Glaciological Society
    Publication Date: 2003
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  • 6
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    Roughness of a subglacial conduit under Hansbreen, Svalbard
    Cambridge University Press (CUP) ; 2017
    In:  Journal of Glaciology Vol. 63, No. 239 ( 2017-06), p. 423-435
    Details
    In: Journal of Glaciology, Cambridge University Press (CUP), Vol. 63, No. 239 ( 2017-06), p. 423-435
    Abstract: Hydraulic roughness exerts an important but poorly understood control on water pressure in subglacial conduits. Where relative roughness values are 〈 5%, hydraulic roughness can be related to relative roughness using empirically-derived equations such as the Colebrook–White equation. General relationships between hydraulic roughness and relative roughness do not exist for relative roughness 〉 5%. Here we report the first quantitative assessment of roughness heights and hydraulic diameters in a subglacial conduit. We measured roughness heights in a 125 m long section of a subglacial conduit using structure-from-motion to produce a digital surface model, and hand-measurements of the b -axis of rocks. We found roughness heights from 0.07 to 0.22 m and cross-sectional areas of 1–2 m 2 , resulting in relative roughness of 3–12% and 〉 5% for most locations. A simple geometric model of varying conduit diameter shows that when the conduit is small relative roughness is 〉 30% and has large variability. Our results suggest that parameterizations of conduit hydraulic roughness in subglacial hydrological models will remain challenging until hydraulic diameters exceed roughness heights by a factor of 20, or the conduit radius is 〉 1 m for the roughness elements observed here.
    Type of Medium: Online Resource
    ISSN: 0022-1430 , 1727-5652
    URL: Article
    DOI: 10.1017/jog.2016.134
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 2017
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  • 7
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    Mechanism for the subglacial formation of cryogenic brines
    Cambridge University Press (CUP) ; 2023
    In:  Annals of Glaciology
    Details
    In: Annals of Glaciology, Cambridge University Press (CUP)
    Abstract: Cryogenic brines are under-studied, despite the fact that they may contain information about past ice-sheet behavior. Cryogenic brines form through cryoconcentration of seawater, although the specific setting and mechanism of formation have been debated. Previous conceptual models of brine formation require seawater isolation from the ocean in a closed basin experiencing freezing. We propose instead that they may form in pore spaces of marine sediments subjected to repeat cycles of ice-sheet advance and retreat. During periods of basal freezing, cryoconcentration produces hypersaline brines which experience downward flow driven by unstable density stratification. Our advection-diffusion model of porewater chemistry evolution successfully recreates the porewater chemistry of two deep Antarctic cores containing cryogenic brines (AND-1B and AND-2A), suggesting that cryogenic brines can be formed through the repeated isolation and cryoconcentration of marine waters within subglacial sediment pore spaces of modern and past ice sheets.
    Type of Medium: Online Resource
    ISSN: 0260-3055 , 1727-5644
    URL: Article
    DOI: 10.1017/aog.2023.28
    Language: English
    Publisher: Cambridge University Press (CUP)
    Publication Date: 2023
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  • 8
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    Ocean Stratification and Low Melt Rates at the Ross Ice Shelf Grounding Zone
    American Geophysical Union (AGU) ; 2018
    In:  Journal of Geophysical Research: Oceans Vol. 123, No. 10 ( 2018-10), p. 7438-7452
    Details
    In: Journal of Geophysical Research: Oceans, American Geophysical Union (AGU), Vol. 123, No. 10 ( 2018-10), p. 7438-7452
    Abstract: The ocean cavity near the grounding zone of the Ross Ice Shelf is vertically stratified with a boundary layer freshened by ice melting A tidally mixed zone and tidal currents are absent from this 10‐m‐thick ocean cavity in the flexure zone The observed stratification and low current velocities result in low melt rates (7 cm/year)
    Type of Medium: Online Resource
    ISSN: 2169-9275 , 2169-9291
    URL: Article
    DOI: 10.1029/2018JC013987
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2018
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    detail.hit.zdb_id: 161667-5
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  • 9
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    Thermal legacy of a large paleolake in Taylor Valley, East Antarctica, as evidenced by an airborne electromagnetic survey
    Copernicus GmbH ; 2021
    In:  The Cryosphere Vol. 15, No. 8 ( 2021-08-03), p. 3577-3593
    Details
    In: The Cryosphere, Copernicus GmbH, Vol. 15, No. 8 ( 2021-08-03), p. 3577-3593
    Abstract: Abstract. Previous studies of the lakes of the McMurdo Dry Valleys have attempted to constrain lake level history, and results suggest the lakes have undergone hundreds of meters of lake level change within the last 20 000 years. Past studies have utilized the interpretation of geologic deposits, lake chemistry, and ice sheet history to deduce lake level history; however a substantial amount of disagreement remains between the findings, indicating a need for further investigation using new techniques. This study utilizes a regional airborne resistivity survey to provide novel insight into the paleohydrology of the region. Mean resistivity maps revealed an extensive brine beneath the Lake Fryxell basin, which is interpreted as a legacy groundwater signal from higher lake levels in the past. Resistivity data suggest that active permafrost formation has been ongoing since the onset of lake drainage and that as recently as 1500–4000 years BP, lake levels were over 60 m higher than present. This coincides with a warmer-than-modern paleoclimate throughout the Holocene inferred by the nearby Taylor Dome ice core record. Our results indicate Mid to Late Holocene lake level high stands, which runs counter to previous research finding a colder and drier era with little hydrologic activity throughout the last 5000 years.
    Type of Medium: Online Resource
    ISSN: 1994-0424
    URL: Article
    DOI: 10.5194/tc-15-3577-2021
    Language: English
    Publisher: Copernicus GmbH
    Publication Date: 2021
    detail.hit.zdb_id: 2393169-3
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  • 10
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    A ploughing model for the origin of weak tills beneath ice streams: a qualitative treatment
    Elsevier BV ; 2001
    In:  Quaternary International Vol. 86, No. 1 ( 2001-12), p. 59-70
    Details
    In: Quaternary International, Elsevier BV, Vol. 86, No. 1 ( 2001-12), p. 59-70
    Type of Medium: Online Resource
    ISSN: 1040-6182
    URL: Article
    DOI: 10.1016/S1040-6182(01)00050-7
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2001
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    detail.hit.zdb_id: 1077692-8
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