Deep beneath the Ice: Seismic Imaging of the Antarctic mantle

Overview

To fully understand the future of the Antarctic ice sheet we need better knowledge of what’s going on in the Earth deep beneath it.

Large masses – such as the Antarctic ice sheet – deform the Earth’s surface beneath them. As the ice sheet grows or melts, the solid Earth responds: a process known as ‘Glacial Isostatic Adjustment’ (GIA). Understanding this is central to our ability to predict how the ice sheet evolves over time, and its connections to changes in sea level and climate. For accurate GIA modeling, we need to determine the rheological properties of the crust and mantle beneath the ice sheet. This project will focus on using seismological techniques to produce a detailed image of Earth structure beneath Antarctica, providing new constraints for GIA models to improve our understanding of the dynamics of the West Antarctic Ice Sheet.

The structure of the Antarctic mantle is thought to be complex. Remnant subducted tectonic plates and potentially deep sourced mantle plumes are hypothesised to sit beneath a variable continent, with thin lithosphere in the west transitioning into an ancient thick craton in the East. However, most imaging to date has been limited to long-wave seismic tomographic models, with inherent uncertainties and limited resolution. More detailed waveform studies have focused on small-scale observations using limited numbers of seismic stations. This project will make use of all possible seismic data across Antarctica to conduct regional scale seismic waveform measurements, using over 450 seismic stations from networks deployed within the last 20 years. Observations of distant global earthquakes will be used to map out seismic discontinuities within the Antarctic mantle, to assess temperature and compositional variation. This also provides an opportunity to test recently-proposed theories relating to the thermo-chemical nature of mantle plumes, compositional variability of the mantle and changes in the nature of the lithosphere-asthenosphere boundary between cratonic and non-cratonic regions.

The candidate will assess regional variations in mantle composition, temperature, lithospheric thickness and the presence of melt which can be used to better constrain 3D GIA models. Analysis will focus on:
•Mapping local variations in globally observed transition zone discontinuities (at 410 and 660 km), to derive mantle temperature estimates.
•Looking for evidence of the poorly understood X (~300km) and mid-mantle discontinuities (~1000km) suggestive of compositional anomalies such as recycled basalt.
•Making observations of seismic conversions linked to the lithosphere-asthenosphere boundary (LAB), to assess the presence of partial melt.

This project will suit a numerate geologist or (geo-)physicist, and is primarily computational in character. While prior coding experience is not necessarily required, candidates must be willing to learn to use, adapt, and extend software written in Python and other programming languages. The candidate will also gain expertise in seismic data analysis, the handling of large datasets, inverse theory, and the structure and composition of the deep earth.

Click on an image to expand

Image Captions

Lloyd_Figure.png – “Figure 1. From Lloyd et al (2020). Depth slice and cross-sections through tomographic model highlighting seismic wave speed anomalies linked to the potential Marie Bryd Land Plume and remnant subducted material extending through the mantle transition zone.”

Antartic_stations_all.png – “Figure 2. All publicly available broadband seismic stations (457) deployed in Antarctica over the last 20 years, BEDMAP2 ice thickness and bathymetry in background.”

Methodology

Seismic data across Antarctica will be gathered from all publically available seismic databases. Recordings of distant earthquakes will be analysed for the presence of P to S and S to P converted seismic waves, caused by interactions with seismic discontinuities beneath recording stations. Data will be assessed using receiver function (RF) analysis to enhance observation of converted phases. RFs will be converted from time to depth using estimates of seismic velocity from the recent tomographic model of Lloyd et al., (2020), combined in large-scale 3D gridded data stacks (common conversion point stacks), and searched for the presence of converted phases linked to seismic discontinuities.

Ps RFs will be used to analyse deep mantle structure in the transition zone to test whether volcanism in Marie Byrd land is fed by a deep sourced mantle plume and confirm the presence of a subducted remnant beneath the Antarctic peninsula. Observations from more unusual discontinuities in the upper and the mid-mantle indicative of compositional variation will be used to test whether mantle plumes are thermo-chemical structures rich in recycled basalt, sourced from structures on the core-mantle boundary. The validity of P-S conversions will be tested with slowness stacks (to distinguish converted phases from contaminating multiples). The variability of observations with frequency will be used to determine the diffuseness of boundaries. Synthetic data modeling will be used to test the nature of the causative boundaries, which will be compared to estimates of phase transitions based on mineral physics predictions. Lower frequency Sp RFs will be used to analyse lithospheric structure, to test recent suggestions of variability in the nature of the LAB between cratonic and non-cratonic regions.

The results will be analysed in collaboration with GIA modellers from Durham’s Ice Sheets and Sea Level Research group to understand how mantle constraints can be used to improve 3D GIA models.

Project Timeline

Year 1

Literature review, gathering of seismic data set, training in seismic data processing techniques, P RF analysis of TZ discontinuities.

Year 2

Analysis, assessment and modelling to determine nature and implications of additional discontinuity observations in P RFs. The work from Years 1 & 2 should lead to at least one publication. Extension of existing software to include S RF analysis.

Year 3

Application of S RF code and mapping of LAB structure; collaborate with researchers in the Ice Sheets and Sea Levels Group to assess impact of results on GIA models; writing of further manuscript for publication.

Year 3.5

Completion of manuscript for publication and thesis writing

Training
& Skills

You will become part of the Geophysics and Geodynamics Research Groups at Durham, and the Ice Dynamics and Palaeoclimate group at the British Antarctic survey.

Training will be provided in processing, analysing and modelling seismic data as well as associated essential skills (programming, code development, and usage of high-performance computing systems). Training in a wider range of important skills (e.g. presentation skills, paper/thesis writing) will be provided by the Department of Earth Sciences at Durham University, and the student will also benefit from cross-disciplinary training provided as part of the IAPETUS2 DTP.

The student will have opportunities to work with other partners in the UK and internationally and will attend national and international scientific meetings to present results. We aim to see all students publish at least two papers in leading scientific journals during their PhD. Upon completion, the student will be well equipped for a career in academia or in a range of industries.

References & further reading

Lloyd, A. et al., (2020). Seismic Structure of the Antarctic Upper Mantle Imaged with Adjoint Tomography. Journal of Geophysical Research: Solid Earth, 125(3).
Kemp, M., Jenkins, J., Maclennan, J., & Cottaar, S. (2019). X-discontinuity and transition zone structure beneath Hawaii suggests a heterogeneous plume. Earth and Planetary Science Letters, 527, 115781.

Jenkins, J., Deuss, A., & Cottaar, S. (2017). Converted phases from sharp 1000 km depth mid-mantle heterogeneity beneath Western Europe. Earth and Planetary Science Letters, 459, 196-207.

Hopper, E., & Fischer, K. M. (2018). The changing face of the lithosphere-asthenosphere boundary: Imaging continental scale patterns in upper mantle structure across the contiguous US with Sp converted waves. Geochemistry, Geophysics, Geosystems, 19(8), 2593-2614.

Whitehouse, P. L., Gomez, N., King, M. A., & Wiens, D. A. (2019). Solid Earth change and the evolution of the Antarctic Ice Sheet. Nature communications, 10(1), 1-14.

Further Information

Please contact Jennifer Jenkins (new Durham staff member starting January 2021) for more information: earthsci.pgadmin@durham.ac.uk

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