Tracking past grounding line retreat from onshore rates of ice sheet thinning

Overview

Satellite measurements and direct observations demonstrate that the ice sheets in Greenland and Antarctica are losing mass (Shepherd et al., 2018). Recent IPCC predictions suggest that under moderate emissions scenarios they will contribute to a total sea-level rise of >60 cm by 2100, with a worst-case scenario of more than 1 m of sea-level rise by this time. The measureable expressions of current ice sheet mass loss are; 1) retreat of grounding lines, the point where an ice sheet begins to float as it flows into the ocean, 2) acceleration of major marine terminating ice streams, and 3) thinning of ice sheets inland of the grounding line.

Contemporary observations demonstrate that these changes are mechanistically linked (e.g. Miles et al., 2020) however it is not clear what the longer term context of these links are. This limits our ability to make accurate predictions of the future contribution of ice sheets to sea level change. Specifically, there is still limited understanding of how grounding line retreat over centuries to millennia is reflected by simultaneous (or not) changes in ice sheet thickness. In other words, we wish to ask the question: can changing rates of ice sheet thinning be directly and quantitatively linked to rates of grounding line retreat? This is important because in key areas of Antarctica such as the Weddell Sea it is possible to reconstruct past changes in ice thickness but records of past grounding line change are severely restricted by thick ice shelves.
Satellite measurements and direct observations demonstrate that the ice sheets in Greenland and Antarctica are losing mass (Shepherd et al., 2018). Recent IPCC predictions suggest that under moderate emissions scenarios they will contribute to a total sea-level rise of >60 cm by 2100, with a worst-case scenario of more than 1 m of sea-level rise by this time. The measureable expressions of current ice sheet mass loss are; 1) retreat of grounding lines, the point where an ice sheet begins to float as it flows into the ocean, 2) acceleration of major marine terminating ice streams, and 3) thinning of ice sheets inland of the grounding line.

Contemporary observations demonstrate that these changes are mechanistically linked (e.g. Miles et al., 2020) however it is not clear what the longer term context of these links are. This limits our ability to make centennial-scale predictions of the future contribution of ice sheets to sea level change. Specifically, there is still limited understanding of how grounding line retreat over centuries to millennia is reflected by simultaneous (or not) changes in ice sheet thickness. In other words, we wish to ask the question: can changing rates of ice sheet thinning be directly and quantitatively linked to rates of grounding line retreat? This is important because in key areas of Antarctica such as the Weddell Sea it is possible to reconstruct past changes in ice thickness but records of past grounding line change are severely restricted by thick ice shelves.

This project aims to test if past changes in ice sheet thickness can be used as a proxy for grounding line retreat. To do so the student will produce new records of ice thickness change from two locations. Firstly, Northwest Scotland; here the new record can be integrated with extensive legacy data (Bradwell et al., 2019) to explore the mechanistic links between grounding line retreat and ice thickness change. Secondly, the insights from this site will be extended a new record of ice thickness change to be produced from the Weddell Sea sector of Antarctica.

Methodology

This project will generate new records of ice thickness change using cosmogenic nuclide exposure dating. The student will undertake fieldwork in NW Scotland to collect samples from one or more vertical transects on the margins of the former Minch Ice Stream. Analysis of these samples will constrain the timing and rates of ice sheet thinning. This new record will be integrated with both freely available bathymetric data and sedimentological and seismic data collected during recent research cruises as part of a completed NERC project (BRITICE-CHRONO). The student will use this data to determine the nature and timing of grounding line retreat. The overall aim is to test if periods of rapid grounding line retreat correspond to periods of ice sheet thinning. Depending on student interest there is also the potential to use a flow-line model to see how various parameters (e.g. bed strength, trough width) control the links between ice sheet thickness and grounding line retreat.

The student will also work on new samples to be collected from Antarctica as part of a NERC funded project. In this location there is scarce data relating to past grounding line retreat so the new record will represent a first order constraint on said retreat. This record will be compared to existing models of Antarctic deglaciation to see what pattern of grounding line retreat accords with the new data. Again, depending on student interest, there is opportunity to explore controls using a flow-line model.

Project Timeline

Year 1

Develop understanding of ice sheet retreat (drivers, feedbacks etc). Undertake fieldwork in Northwest Scotland. Develop skills in marine geophysical and geological data analysis. Begin process of sample preparation.

Year 2

Complete sample preparation and analysis; generating records of ice thickness change. Integrate marine and terrestrial data from NW Scotland. Potentially set up flow-line model(s). Develop writing skills.

Year 3

Compare new Antarctic data to existing ice sheet model output. Synthesise findings. Present outcomes to IAPETUS2 and at international conference. Begin drafting of results.

Year 3.5

Submission of thesis and research paper(s).

Training
& Skills

The student will gain experience of a wide range of research methods including fieldwork, sample collection and preparation for cosmogenic nuclide analysis, marine geological and geophysical data analysis, and, potentially, numerical flow-line modelling. The supervisory team covers the wide breadth of methods to be applied and can provide hands-on training in all aspects of the project including at Durham and Newcastle Universities and through lab visits to partner organisations for specific laboratory analyses in cosmogenic nuclides. The student will be supported to attend international training opportunities (e.g. Karthaus summer school on ‘ice and climate dynamics’) and appropriate conferences. The student will be a member of the Sea Level, Ice and Climate Research Cluster in Geography at Durham (https://www.dur.ac.uk/geography/slic/).

Broader transferable skills (e.g. communicating science, thesis writing, writing for publication, presentation skills) will be developed through various training events at Durham University offered by IAPETUS as well as through Durham’s award winning Career and Research Development (CAROD) group. At all stages the students writing skills will be developed and supported and they will be encouraged to lead papers outlining the project results. The project and supervisory team is designed to give the student broad, multi-disciplinary training including quantitative skills to ensure they have a range of applicable and transferable skills.

Students will also be encouraged to engage with Scientific Committee on Antarctic Research (SCAR) activities which involve wider international collaboration and discussion and will be supported in doing so by the project supervisors who are active in this area.

References & further reading

Bradwell, T., et al., 2019. Ice-stream demise dynamically conditioned by trough shape and bed strength. Science advances, 5(4), eaau1380.

Miles, B.W., et al., 2021. Recent acceleration of Denman Glacier (1972–2017), East Antarctica, driven by grounding line retreat and changes in ice tongue configuration. The Cryosphere, 15(2), 663-676.

Siegert, M.J., et al., 2019. Major ice sheet change in the Weddell Sea sector of West Antarctica over the last 5,000 years. Reviews of Geophysics, 57(4), 1197-1223.

Shepherd, A. et al., 2018. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature 558, 219-222.

Small, D., et al., 2019. Antarctic ice sheet palaeo-thinning rates from vertical transects of cosmogenic exposure ages. Quaternary Science Reviews, 206, 65-80.

Further Information

Please contact David Small (david.p.small@durham.ac.uk) for further information.

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