Dating and modelling fast ice-sheet grounding-line retreat over the last 4000 years in the SW Weddell Sea, Antarctica

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

The threat the West Antarctic ice-sheet poses to sea-level is largely through retreat of the grounding line, the line that separates floating ice, whose effects on sea-level are negligible, and grounded ice, which can affect sea-level. A concern is that theoretical models suggest grounding-line retreat may be unstable.

Geological evidence for the grounding line retreat under the Antarctic ice shelves, where the most substantial retreat took place, is scarce owing to poor accessibility. We will use a new technique that dates ice-flow changes from stratigraphic anomalies in the small ice-rises lying within the Ronne ice shelf. This will provide information about retreat of the grounding line there. Firstly, an established technique which looks at anticline geometry will be applied. A new feature is the application of a technique (the BAS-developed pRES) for the direct measurement of deformation in the ice, which will allow us to quantitatively constrain the development of the anticlines, rather than having to make assumptions about ice viscosity. Our project will therefore investigate both the retreat and rheology (flow) of the Antarctic ice-sheet.

We will visit several ice divides in the SW Ronne area - the Korff, Henry, Skytrain and Fowler - to apply these techniques and date the most recent flow changes associated with the passage of the grounding line past the ice-rise. These will give information about the retreat of the Ronne grounding line between Berkner Island and the Ellsworth Mountains. There are many indications that this has happened in the past 4000 years, representing some of the most recent response of the Antarctic ice-sheet to deglaciation following the last glacial maximum.

Measurements of ice rheology have general relevance to ice-sheet modelling. 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 largely well-known geometry of the the ice-sheets, but the resistance to flow depends upon the viscous (rheological) properties of ice. Ice has the property that the the viscosity depends upon the rate at which the ice is deforming. This sensitivity is usually described with the Glen index. 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 sufficiently well to quantify the viscosity.

By going to divide locations, where the stress field can be characterised and ice provenance is well constrained, we will be able to measure the Glen index. The technique is based on the fact that 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. These provide instantaneous strain-rate fields in the upper third to a half of the ice field. The technique has been proven at Summit in Greenland.

Once we have obtained the data, we will have an idea of how quickly the grounding line retreated. We will then run ice-sheet models, to see what kind of situations could have caused this. Grounding line retreat rates are ultimately fixed by several variables; how the ocean pumps warm water under shelves, how sea-level is rising, how the land surface recovers as ice unloads, and how ice softens as it warms. The mathematical model will be able to assess the influence of all of these factors. Our team has also collected a lot of other data about ice-sheet extent in the area, and we will be able to see if the model can predict ice-sheet geometry that matches all of these.

At the end of the project, with our new dates for grounding line retreat and our model that represents the past, we will have a much clearer idea of how and why the ice-sheet retreated as it did in the Weddell Sea area.

Planned Impact

We have identified three target groups whom we feel should or may want to learn more about our activities.

a) General Public: High-interest scientific papers will be communicated using the BAS media section. In addition, we will attempt direct engagement with sectors of the public with novel approaches that provide accessible background to our research activities and the considerations that surround and direct them.

i) Ice-rises in Antarctic exploration. We intend to commission an article for Planet Earth written by an Antarctic expert to outline the role of ice-rises in Antarctic exploration from Amundsen to the present-day, to publicise our work amongst the public through a more human-interest angle.
ii) Hands-on ice-sheet modelling. We believe that there is a sufficiently large world-wide technical audience to benefit from web-based simple ice-sheet modelling exercises. Our intention is to put Matlab-based programs on the web, so that interested people can use them to understand aspects of ice-sheet modelling.

b) Policy Makers: We will use the strong links within BAS to BIS and DEC to communicate our strongest and most policy-relevant findings.

i) Rates of grounding-line retreat and consequent rates of sea-level rise. Our results regarding maximum rate of sustained grounding-line retreat will be of interest to policy makers, those making decisions about flood defences, and insurance companies.

c) Interdisciplinary Impact: Promulgate by targeted attendance and conferences and workshops:

i) Raymond effect dating with geochronologists
ii) Rheological measurements and inversions with geodynamicsts and mechanicians
iii) Knowledge of Antarctic grounding-line movement data with Antarctic geologists
iv) Increased knowledge of grounding-line dynamics with glaciologists
v) Further field studies of ice-rises with Antarctic glaciologists

Publications

10 25 50
 
Description The West Antarctic Ice-Sheet (WAIS) has retreated since the Last Glacial Maximum 20000 years ago. While the mechanism controlling this retreat has become clearer over the past decade, the timing has not. We have used two types of radar data to help clarify this; a lower frequency 'surveying' radar, which shows up flow-induced structures within the ice, and a higher frequency 'point-measurement' radar, which can show vertical ice velocities. We have used these two radars together to date the retreat of the main WAIS grounding line (where the ice begins to float) past several grounded ice rises in the Weddell Sea area. The dates are all in the past few thousand years, compared with the end of the last glaciation fifteen to twenty thousand years ago. These retreat dates obtained from glaciological surveying of ice-rises are crucial information for understanding the post-glacial WAIS retreat.
Exploitation Route The research has dated the retreat of the grounding line past four ice-rises. There are over one hundred such features surrounding Antarctica, so the technique can be applied to many, if not all of these.
Sectors Electronics,Environment,Government, Democracy and Justice,Security and Diplomacy,Other

 
Title Determining contribution of internal instabilities and external forcing to ice-stream formation and geometry 
Description Ice streams were identified in the Antarctic ice-sheet in the 1970s, and seen to have existed in the former Northern Hemisphere ice-sheets in the 1980s. They are between 20km and 100km wide and 100-300 km long. It is clear that ice-sheet thermodynamics can cause these streams to arise spontaneously within certain parameter ranges (accumulation rate, geothermal flux, bed slipperiness), and it is also very clear that external forcing can determine where the streams form. We (me and Gisela Hiess) are running ice-sheet models to evaluate the respective role of internal instabilities and external forcing. So far, neither has been shown to have a dominating effect. 
Type Of Material Improvements to research infrastructure 
Year Produced 2012 
Provided To Others? Yes  
Impact Studies in the BRITICE project have shown that ice-streams existed in locations surrounding Ireland and Great Britain, and that certain aspects of their dynamics remain unexplained. We have shown that internal instabilities may have played a role in the dynamics of such streams. 
 
Title Effect of glacio-isostatic movement on depth of ice at the grounding-line 
Description The grounding-line (GL) of a marine ice-sheet is the location where the ice has thinned with flow to reach the flotation thickness. Therefore, the GL indicates where an ice-shelf commences. Ice flux across the GL increases with the thickness of the ice, and recent theoretical work indicates that flux is proportional to the thickness to a power between four and five. These considerations led to the idea that a GL on a reverse slope (sloping inwards) is dynamically unstable, i.e. sensitive to the operation of feedbacks, since a retreat of the GL would lead to greater ice flux and further retreat. In contrast, an advance would lead to lower ice flux and further advance. An early discovery in glacial geology was the association of uplift of the earth surface with the retreat of large ice-sheets, owing to the unloading. For marine ice-sheets, such uplift means a further source of change in the ice-thickness at the grounding-line. Our discovery was that such shallowing can turn the unstable retreat of a marine ice-sheet into an unstable advance. This was used to explain some anomalous measurements of uplift from Antarctica, undertaken by recently installed continuous GPS stations. The anomalous results are either very slow rates of uplift, or in one case, downwards motion. The cause of these was attributed to reloading by ice, due to plausibly unstable GL readvance in the Weddell Sea sector of the the West Antarctic ice-sheet over the past 4000 years. 
Type Of Material Improvements to research infrastructure 
Provided To Others? No  
Impact Publication of two papers (i)"Thomas, I.D., M.A. King, M.J. Bentley, P.L. Whitehouse, N.T. Penna, S.D.P. Williams, R.E.M. Riva, D.A. Lavallee, P.J. Clarke, E.C. King, R.C.A. Hindmarsh, H. Koivula, (2011), "Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations", Geophys. Res. Lett.,38, L22302, doi:10.1029/2011GL049277" (ii) "Bradley, S.L., R.C.A. Hindmarsh, P.L. Whitehouse, M.J. Bentley and M.A. King, (2015), Low post-glacial rebound rates in the Weddell Sea due to Late Holocene ice-sheet readvance, Earth Planet. Sci. Lett., 413, 79-89, doi:10.1016/j.epsl.2014.12.039" 
 
Title Generation of large internal plumes in ice-sheets that emanate from the base 
Description In the past decade ice-penetrating radars have identified large plumes within ice-sheets that emanate from the bed. These plumes are up to 1 km thick and 20 km long. There are three separate theories regarding their origin. We (Carlos Martin BAS, Gwendolyn Leysinger Vieli and Martin Luethi Zurich University) are interested in the idea that they are caused by freeze-on in locations where water is flowing uphill. My personal contribution is providing software to solve the advection equation in flowing ice, in order to understand how the geometry of the plumes evolves. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? Yes  
Impact Publication in 2018 in Nature Communications lead authored by Leysinger Vieli. 
 
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 Modelling ice-sheet grounding-line migration 
Description In the 1970s several papers were published pointing out that if ice-flux across the grounding-line (the origin of the floating ice-shelf) increased with ice thickness, then grounding-lines situated on locations where the bed deepened up-glacier (has a 'reverse slope'), the situation was potentially unstable. For example, a small retreat of the grounding line would increase ice-flux, causing the ice-sheet to lose volume and induce further retreat of the grounding line. In contrast, a small advance would reduce ice-flux, allowing the ice-sheet to gain volume and result in more advance. These ideas had to wait until 2007 before they were given a rigorous mathematical footing by Schoof. Schoof gave a formula for flux as a function of thickness, which allowed the testing of numerical models. Two model intercomparisons have been carried out, both lead-authored by Frank Pattyn, with DOI's 10.5194/tc-6-573-2012 and 10.3189/2013JoG12J129. Richard Hindmarsh participated in these intercomparisons. We now have greatly increased confidence in the ability of numerical models to accurately compute grounding-line migration. 
Type Of Material Improvements to research infrastructure 
Year Produced 2012 
Provided To Others? Yes  
Impact Greatly increased confidence in the numerical simulations of grounding line motion. 
 
Title Polarimetric phase sensitive radar from Korff Ice Rise, West Antarctica, 2014 
Description In 2014 polarimetric phase sensitive radar data were collected at Korff Ice Rise, West Antarctica, with the aim of studying fabric within the ice column and ice bed properties. Data were collected at sites within 700m of one another along the axis of the ice divide. The radar data were collected by rotating the antenna through 180 deg to allow reconstruction of the azimuthal variation in power and phase. This study is part of the British Antarctic Survey programme Polar Science for Planet Earth. All data were collected with the support of the British Antarctic Survey. The ApRES fieldwork were funded by Natural Environmental Research Council grant NE/J008087/1, led by Richard Hindmarsh. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Questions about ice dynamics formulated on the basis of newly acquired data 
Description The large group of earth scientists involved in investigating the palaeo-Antarctic Ice-Sheet (p-AIS) and palaeo-British-Irish Ice-Sheet (p-BIIS) has quite naturally led them to think about ice dynamical explanations for their observations. Nearly all of their conjectures and questions are well thought-through, which ultimately create questions for ice dynamicists and modellers to answer. We are improving and amending models to focus on these questions. 
Type Of Material Computer model/algorithm 
Year Produced 2015 
Provided To Others? Yes  
Impact There are several different groups working on the p-AIS and p-BIIS, all aiming to make use of the new data. These groups talk with each other, but are all interested in different aspects of the behaviour of the ice-sheet. 
 
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. 
 
Title Velocity profiles from ice-rises. - Korff, Henry, Skytrain, Fowler, Fletcher 
Description Vertical velocity profiles can be measured with two accurately co-located radar surveys separated by a suitable period of time - in these cases, one year. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact These velocity profiles can be used with radar surveys of sub-surface structure to date the formation time of the ice-rises. The ice-rises are about 100 km by 30 km in horizontal dimension, up to 600 m thick. 
 
Description BRITICE 
Organisation University of Sheffield
Country United Kingdom 
Sector Academic/University 
PI Contribution BRITICE is a collaborative effort amongst many UK institutions, gaining and analysing data relevant to the time-dependent coverage of the British and Irish Ice Sheets over the North Sea and Norwegian Sea. These data are anticipated to be able to constrain ice-sheet models, coupled with models of other components of the Earth System. Hindmarsh (BAS) will supervise the running of these models.
Collaborator Contribution BRITICE are gathering and analysing the data from both geophysical cruises and land-based studies.
Impact None so far - the data have been gathered and but require further analysis before the modelling can commence. The two cruises funded have been carried out.
Start Year 2013
 
Description Durham LGM Antarctica 
Organisation Durham University
Department Department of Geography
Country United Kingdom 
Sector Academic/University 
PI Contribution BAS have organised the data gathering, geophysical surveying of ice-rises by radar-techniques. We can use these data to date the retreat of the main Antarctic grounding line. These ice-rises are located within the Ronne Ice Shelf.
Collaborator Contribution The project is ongoing - Durham have committed to using the data to model the behaviour of the Antarctic ice-sheet since the Last Glacial Maximum.
Impact BAS have completed the geophysical surveys. The data are due to be shared; once this occurs, Durham will use the data, in conjunction with other data they have acquired and models, to constrain the behaviour of the West Antarctic ice-sheet in the Weddell Sea area. BAS will contribute by collaborating on the organisation of the modelling studies.
Start Year 2013
 
Title BASISM - the BAS Ice Sheet Model 
Description BASISM has been developed over the past fifteen years, and has achieved successes in formal model intercomparison projects (MIPs), as well as being used to settle some long-standing problems. 
Type Of Technology Software 
Year Produced 2013 
Impact This software (and its brethren) have been used in MIPs to validate theoretical advances in grounding-line motion/stability and ice-stream formation. 
 
Description Modelling panel for BRITICE, for groups both internal and external to the main BRITICE project 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact The new data acquired by BRITICE from the North, Norwegian and Celtic Seas shows very clearly that the British-Irish Ice Sheet (BIIS) was a marine ice-sheet. The stability of such ice sheets is a major concern, since Antarctica is a marine ice-sheet and its observed retreat threatens to raise global sea-level by between 0.5m and 2m by 2100 CE. The quantity of new data points means the the BIIS is the most well-described marine ice-sheet, and already these data are illuminating issues of marine ice-sheet stability.
Year(s) Of Engagement Activity 2014,2015,2016,2017,2018
 
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