Controls on 20th century ice shelf dynamics in northeast Greenland.

Overview

Over the past three decades satellite observations over Greenland and Antarctica have revealed that marine terminating glaciers and ice shelves have been thinning at an accelerating rate in response to both increased air and ocean temperatures (The Imbie Team, 2018, 2020; Fig. 1). Thinner ice shelves are less able to buttress inland ice, leading to grounding line retreat, increased ice sheet thinning and ultimately, sea-level rise. This process, known as the marine ice sheet instability, has the potential to drive rapid and irreversible ice sheet collapse. Indeed, some ice sheet models indicate that these processes are already underway in Antarctica (e.g. Joughin et al., 2014). In Greenland, several studies have linked recent ice shelf thinning/loss and accelerated ice flow to the incursion of warm Atlantic water (Holland et al., 2008; Straneo et al., 2013). However, understanding the nature and rate of this recent grounding line response to ocean forcing (and other perturbations) requires knowledge of how these processes have evolved over recent centuries. In Antarctica, some changes are known to have started prior to satellite observations (Smith et al., 2017; Fig. 1c), highlighting the need to understand the recent (centennial) history in order to understand the interplay between changing ocean properties and ice sheet dynamics. Such knowledge is essential if we are to accurately predict future ice sheet behaviour as well as the ocean feedbacks in the North Atlantic region.

This project will explore the twentieth century evolution of the 79N ice shelf, which currently buttresses the Northeast Greenland ice stream (NEGIS) (Fig. 2). The NEGIS drains the northeast sector of the Greenland ice Sheet (GrIS) and contains approximately 1.2 m sea-level equivalent (sle). Its future stability is pivotal not only to future mass balance of the GrIS but also the freshwater flux to the northeast Atlantic and specifically, to the North Atlantic Deep Water overturning circulation. Starting in the early 2000s, the ice shelves that front NEGIS (Zachariae Isstrom and 79N) have started to destabilise (Fig. 2b), but while ZI has disintegrated (see Mouginot et al., 2015 and references therein), 79N has remained relatively stable. Some modelling studies suggest that the relative stability of 79N could continue over the next century (Choi et al., 2017), but recent oceanographic observations have shown that ocean heat flux and melt rates maybe increasing (Schaffer et al., 2020). Thus, there is great concern that 79N will be the next ice shelf to disintegrate and if this occurs, it will result in a substantial increase in ice discharge to the ocean.

Methodology

This project will use existing sediment gravity, box and lake sediment cores collected from beneath and adjacent to the 79N ice shelf (Figs. 2 and 3) to reconstruct ice shelf history and Atlantic Water circulation over the last ~200 years. The available material was collected in 2016 and 2017 as part of the NERC project (Greenland in a Warmer Climate) in collaboration with the Alfred Wegener Institute in Germany.
Specifically the student will employ a multi-proxy examination of key cores including analyses of sediment properties (physical properties, grain size), geochemical proxies (oxygen, carbon isotopes, XRF, clay mineral) and microfossil content (diatoms, foraminifera) to determine ice shelf presence/absence, changes in ice shelf thickness and water mass characteristics. Foraminiferal and oxygen and carbon stable isotope analyses will be used to investigate the variability in meltwater flux and Atlantic Water adjacent to and beneath the ice shelf.

A geochronological framework for changing ice shelf dynamics and incursions of warm water will be determined using radiometric (210Pb, 137Cs) and radiocarbon (14C) dating methods. 210Pb analysis will be critical to establish a chronology twentieth century ice shelf dynamics.
Integration of these datasets will enable the processes controlling retreat behaviour (e.g. external forcing like oceanic warming vs. internal glaciological response) to be determined.

The successful student will be trained in all aspect of multi-proxy core analyses and wherever possible spend time at key facilities (e.g. NERC radiocarbon laboratory) and partner laboratories (e.g. BAS). In addition, the student will benefit from a large and energetic Polar research communities at Durham, BAS and Newcastle.

Project Timeline

Year 1

Develop an understanding of glacier and ice shelf retreat (drivers and glaciological response) for the Greenland Ice Sheet; develop skills for sediment core analysis, identify key cores to work on and perform non-destructive analyses of selected sediment cores. Submit samples for dating and compile all available chronological and palaeoenvironmental information for northeast Greenland.

Year 2

Analyse marine/lake datasets to determine environmental setting and retreat histories (ice sheet limits and configuration, rates of retreat); produce first proxy records for environmental change from selected sediment cores.

Year 3

Finalise environmental reconstruction, establish the key drivers of change and place these in a robust chronological framework; draft publications; present outcomes to IAPETUS2 and at international conference; draft thesis.

Year 3.5

Submit thesis; finalise remaining publication manuscripts.

Training
& Skills

Techniques in sediment core analysis and palaeoenvironmental reconstruction form the core of this project. Applicants must be numerate with some previous experience of environmental geoscience/sediment core analyses being particularly beneficial. Specific training in all aspects of work will be delivered at Durham University, BAS and Newcastle together with partner institutes for specific laboratory analyses.
The student will be supported in broader skills training via PhD training programmes through the award-winning Career and Research Development (CAROD) group at Durham (thesis writing, writing for publication, presentation skills, enterprise skills etc.) and at BAS and Newcastle University. In addition, IAPETUS2 provides a wide range of training opportunities to its students.
The student will be encouraged to write papers for publication throughout the duration of the project. This will benefit their career and will enable the supervisory team to support the development in writing skills and help them navigate the publication process. The project is specifically designed to give the student a broad, cross-disciplinary skillset in ice shelf environments, marine and glacial geology that includes quantitative palaeoenvironmental analyses; training is designed to ensure that the student becomes a well-rounded scientist who is comfortable working independently and in teams.

References & further reading

Choi, Y., Morlighem, M., Rignot, E., Mouginot, J., & Wood, M. 2017. Modeling the response of Nioghalvfjerdsfjorden and Zachariae Isstrom glaciers, Greenland, to ocean forcing over the next century. Geophysical Research Letters, 44, 11,071-11,079.

Holland, D. M., Thomas, R. H., de Young, B., Ribergaard, M. H. & Lyberth, B. 2008
Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters. Nature
Geoscience 1, 659-664.

Joughin, I., Smith, B.E., Medley, B. 2014. Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica. Science 344 (6185), 735-738.

Mouginot, J., Rignot, E., Scheuchl, B., Fenty, I., Khazendar, A., Morlighem, M., Buzzi, A., Paden, J. 2015. Fast retreat of Zacharie Isstrom, northeast Greenland. Science 350, 1357-1361. DOI: 10.1126/science.aac7111.

Muschitiello, F., D’Andrea, W.J., Schmittner, A. et al. 2019. Deep-water circulation changes lead North Atlantic climate during deglaciation. Nature Communications 10.

Nagler, T., Rott, H., Hetzenecker, M., Wuite, J., Potin, P. 2015. The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations. Remote Sensing 7, 9371-9389.

Schaffer, Janin; Kanzow, Torsten; von Appen, Wilken-Jon; von Albedyll, Luisa; Arndt, Jan Erik; Roberts, David H. 2020. Bathymetry constrains ocean heat supply to Greenland’s largest glacier tongue. Nature Geoscience, 13(3), 227-231.

The IMBIE Team. 2018. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature. 558, pp. 219-222.

The IMBIE Team. 2020. Mass balance of the Greenland Ice Sheet from 1992 to 2018. Nature. 579(7798), pp. 233-239.

Smith, J.A., Andersen, T.J., Shortt, M., Truffer, M., Stanton, T.P., Bindschadler, R., Dutrieux, P., Jenkins, A., Hillenbrand, C.-D., Ehrmann, W., Corr, H.F.J., Farley, N., Crowhurst, S., Vaughan, D.G. 2017. Sub-ice-shelf sediments record history of 20th Century retreat of Pine Island Glacier. Nature 540, doi:10.1038/nature20136.

Straneo, F., and Heimbach, P. 2013. North Atlantic warming and the retreat of Greenland’s outlet glaciers. Nature, 504(7478), 36-43. doi: 10.1038/nature12854

Further Information

Prof David Roberts
D.H.Roberts@durham.ac.uk
Dr James Smith
Jaas@bas.ac.uk

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