Measuring and modelling the Raymond Effect for to infer low strain-rate ice rheology

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Physics

Abstract

The rate at which ice flows into the sea from the large ice-sheets directly affects sea-level. The forces which drive this flow are controlled by the increasingly well-known geometry of the the ice-sheets, but the resistance to flow depends upon the viscous properties of ice. Ice has the peculiar property that the the viscosity depends upon the rate at which the ice is deforming. This sensitivity is usually described with the Glen index. Recent theoretical studies have shown that our knowledge of the Glen index is not sufficiently well known to (i) accurately predict very basic outcomes of marine ice-sheet change during glacial cycles; and (ii) predict the spatial dimensions of surface response in ice-streams to a better accuracy than current satellite measurements. The Glen index can be measured in the laboratory or the field. Laboratory measurements diverge from field measurements, and are very difficult to make at the low strain-rates observed in the field. In many field measurements it is difficult to characterise the stresses very well and to know how the provenance of the ice has affected measurements. We will go to divide locations where the stress field can be characterised well and the provenance is very well constrained. Radar layers provide markers within the ice, and their vertical displacement over relatively short time periods can be measured using interferometric phase-sensitive radar techniques. This will provide instantaneous vertical velocity fields and strain-rate fields in the upper third to a half of the ice field. GPS techniques will also be used to measure surface strain-rates, which can be compared with the vertical strain-rates derived from the radar. Measurements will be made at GRIP and NEEM (Greenland), sites for ice-core drilling at intervals of one year. We have carried out a proof-of-concept study making measurements with a time interval of a fortnight at NGRIP (a 3000m thickness of ice). Here we were able to measure vertical strain-rates of less than 10^-4/yr with 30% accuracy. Time intervals of a year will improve the accuracy considerably, likely below 5%. At divide locations the velocity field is especially sensitive to the Glen index, and this is particularly the case in the upper part of the ice. We will use full-system modelling to determine the Glen index which best fits the data, and thereby measure the Glen index in the field in a well-controlled location. These are essentially plane-flow experiments, where flow is two-dimensional. The viscosity of ice is dependent on three-dimensional flow effects. It is not feasible to measure carry out the procedure for three-dimensional flows, but rheological models can be tested for three dimensional flows using layering. Triple junctions, where three divide ridges meet, are similarly well-controlled locations. We have extensive radar layer measurements from two triple junctions in Antarctica (Berkner Island and Flecher Ice Rise), and will use these to constrain the flow field and determine their consistency of the our rheological measurements.
 
Description Understanding how ice deforms under applied forces is important to predicting global sea-level rise. Laboratory measurements cannot be performed over the appropriate (long) timescales. We have developed a geophysical technique that (i) uses radar-sounding to measure ice velocity in situ; and (ii) uses mathematical modelling to determine quantitatively those viscous properties of ice that best match the observations.
Exploitation Route The method can be applied to other divides to infer ice rheology and most recent disturbance dates.
Sectors Environment

 
Title Ice-penetrating radar measurements of englacial velocity 
Description Much post-glacial retreat occurring in the Northern Hemisphere over the most recent 20000 years has been investigated by surveying the emergent sub-glacial landforms. Using modern analytical techniques, this can provide geological dating. Such techniques are problematic to apply in Antarctica, owing to the presence of the two largest ice-shelves, the Ross Ice Shelf and the Ronne-Filchner Ice Shelf - they overlie the main post-glacial retreat areas. These are the largest ice shelves on Earth and possibly in the Solar System. In the past two decades, radar-detected englacial folds in ice-rises (isolated islands of grounded ice) have been used to date the retreat of the main Antarctic ice-sheet, since the fold development is adequately described by theory and is a stable process insensitive to small disturbances. This theory predicts ice vertical velocities over the whole thickness of the ice, which is several hundred metres in ice-rises. BAS has developed a radar-based technique to measure these velocities, in order to constrain the theoretical predictions. The technique uses two accurately co-located ground-based radar surveys, separated in time by several months or years. The methodology has proven successful in measuring velocities, and has been used in several ice rises located within the Ross and Ronne-Filchner ice-shelves. These velocities have been used to date the retreat of the Antarctic ice-sheet. 
Type Of Material Improvements to research infrastructure 
Year Produced 2010 
Provided To Others? Yes  
Impact More accurate large-scale ice-sheet retreat dates, and quantitative estimates of the consequent changes in ice-rise geometry. The work has been published in two papers (i) Gillet-Chaulet, F., R. C. A. Hindmarsh, H. F. J. Corr, E. C. King, and A. Jenkins (2011), "In-situ quantification of ice rheology and direct measurement of the Raymond Effect at Summit, Greenland using a phase-sensitive radar", Geophys. Res. Lett., 38, L24503, doi:10.1029/2011GL049843 (ii) Kingslake, J., R.C.A. Hindmarsh, G. Aðalgeirsdóttir, H. Conway, H. F. J. Corr, F. Gillet-Chaulet, C. Martín, E. C. King, R. Mulvaney and H.D. Pritchard, (2014), "Full-depth englacial vertical ice-sheet velocities measured using phase-sensitive radar", J. Geophys. Res., 119, doi:10.1029/2014JF003275 
 
Title Velocity profiles - Adelaide, Berkner, Fletcher, Roosevelt 
Description Vertical velocity data from within a few kilometres of four ice divides sitting within ice rises. 
Type Of Material Database/Collection of data 
Year Produced 2014 
Provided To Others? Yes  
Impact Provides data that can be used to invert for (i) ice rheology; and (ii) the formation date of the ice-rise. 
 
Description Workshop on "Ice-Rises"; Tromsö 2014 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The workshop was on the subject of "Ice Rises" - these are islands of grounded ice, surrounded by open sea or ice shelves, situated on the continental shelf ringing the main Antarctic ice-sheets. Ice-rises can be used for and in many applications. The workshop was 'networking' for interested workers from many disciplines. The organisers of the workshop were Kenichi Matsuoka (NPI Tromsö) and Richard Hindmarsh (BAS). A review paper Matsuoka et al. 2015 appeared - its DOI number is 10.1016/j.earscirev.2015.09.004
Year(s) Of Engagement Activity 2014