IAP-25-093

Measuring the isotopic fingerprint of Greenland Ice Sheet melt

​​Recent decades have seen an acceleration in the melting of the Greenland ice sheet (GIS) with implications for global sea level rise and the stability of ocean circulation. Rates of ice sheet melt into the ocean are a crucial measure of GIS stability, while downstream, the oceanic pathways of meltwater are important for the stratification and stability of ocean currents.

​Meltwaters from the Greenland Ice Sheet can be observed through the specific geochemical signatures they impart in ocean water (see diagram). Evaporation and precipitation result in isotopic fractionation of the H2O molecule, such that high-latitude precipitation, glacial ice, and its meltwaters, have a distinct isotopic fingerprint. Meanwhile, freezing and melting of sea ice only impacts the ocean’s salinity, with minimal change in isotopic ratios. Consequently, observations of water isotopes, together with salinity, can be used to distinguish between processes such as sea ice formation/melt and glacial melt/refreezing, allowing the abundance of ice sheet meltwater to be detected and quantified.

​Observations of this unique isotopic fingerprint of meltwater are extremely sparse, owing to laborious and costly measurement requirements. Historically, water isotopes have been measured by sample collection during research cruises, followed by onshore laboratory analysis using mass spectrometers. Recent advances in spectroscopy, however, have allowed sample analysis onboard ships and, more recently, integration into a ship’s underway system, so that the isotopic composition of the seawater can be measured continuously along the ship track.

​This project will leverage these advances in isotope spectrometry to map the pathways of glacial meltwater from the Kangerlussuaq Fjord in East Greenland – a key region of ice sheet mass loss – and quantify its change over time. Combining novel measurements collected during two research cruises with ocean circulation theory and numerical simulations, the student will investigate the processes controlling the movement of meltwaters from the fjord to the shelf seas, and beyond.

​In this PhD, recent advances in infrared spectroscopy will be exploited to advance our capacity to observe ice sheet meltwater in the ocean via in situ measurement of its isotopic fingerprint. The PhD candidate will develop a water isotope measurement system for deployment during research cruises to the East Greenland shelf seas. The dataset will be analysed to assess the pathways of meltwater as they exit the Kangerlussuaq Fjord, and to understand the processes determining those pathways. This observational work will be complemented by numerical modelling to better understand the processes of meltwater exchange between the fjord and the shelf seas, and to optimise future observing networks.

​The project will involve three primary phases:

​Phase 1: The student will become familiar with the measurement of water isotopes using a cavity ring-down spectrometer, including with a continuous water sampler, in a lab setting in St Andrews and during local field trips (e.g. to Loch Etive). The student will take part in one or more research cruises to the East Greenland shelf to collect water isotope measurements using both bottle samples and the ship’s underway system.

​Phase 2: The student will analyse the results from the cruise(s), discerning the meltwater content across the shelf sea region. They will combine their novel measurements with an inventory of historical measurements to see if there are any detectable patterns or trends in the meltwater amount or pathways.

​Phase 3: Depending on progress, the student may interrogate the understanding derived from observations using an isotope-enabled numerical ocean circulation model. They will investigate the processes controlling the dispersion of meltwater in the shelf seas and determine whether it has changed in the past or will change in the future.

​With these three phases, this student project will greatly advance our understanding of meltwater exchange in the region of Kangerlussuaq Fjord, and more widely comprehend the processes by which meltwater from the GIS is transported out into the ocean, impacting ocean circulation and climate.

​The project will be carried out in collaboration with the British Antarctic Survey, who will provide extensive expertise in water isotope sampling and interpretation, polar oceanography, and Greenland meltwater pathways. There will be opportunities for interactions with BAS collaborators throughout the project, including visits to the facility in Cambridge.

Year 1

​​The student will carry out a literature review on both Greenland glacial meltwater and its dispersion in the marine environment, and in the use of water isotopes to interrogate meltwater pathways. They will be trained in the measurement of water isotopes using mass spectrometers and cavity ring-down spectrometers, and they will prepare for deploying a measurement system on the upcoming cruise.​

Year 2

​​The student will participate in a research cruise to the mouth of Kangerlussuaq Fjord in Autumn 2027​, collecting water isotope samples and implementing sampling of the ship’s underway system. They will quality check and analyse their data, alongside any existing isotope measurements from the region, and investigate meltwater pathways.

Year 3

​​The candidate may return on a second research cruise to the same location and deploy the same, or an improved, sampling strategy. They will assimilate the new observations into the analysis. If time permits, they will run some numerical simulations in an isotope-enabled model and investigate controls on meltwater pathways, and possible changes under future scenarios.​

Year 3.5

Write up results and compile thesis for submission.

​​The candidate will be trained in analytical chemistry, data management and processing, coding, modelling, and scientific writing and presentations.​ Additionally, they will gain extensive understanding of ocean circulation, ocean-cryosphere interactions, and the impact of the ocean circulation on climate. They will have the opportunity to attend conferences and workshops related to these themes and to present their scientific results.