Dr Jeremy Ely
Department of Geography University of Sheffield Sheffield
UK
Email: j.ely@sheffield.ac.uk
To whom it may concern,
Re: Thesis Defence, Helen Elizabeth Dulfer.
Please find below my report on “ Reconstructing the central sector of the Cordilleran Ice Sheet” by Helen Dulfer.
Thank you for inviting me to review this thesis, which I have thoroughly enjoyed. The thesis is well written and extremely interesting. I judge it to be of a very high standard, and it is my opinion that the thesis demonstrates that the candidate fulfils the requirements of a PhD.
Thesis quality Scientific Value
Palaeo-ice sheets serve as important analogues for improving our understanding of the role of ice sheets in the climate system. To unlock these records of ice sheet change, so we might better understand how our current ice sheets will change in our warming world, we need to reconstruct palaeo-ice sheets from the fragmentary evidence they leave behind.
The subject of this thesis is the Cordilleran Ice Sheet, Western Canada, which has traditionally been poorly constrained. One reason for this is the complex setting. The neighbouring Laurentide Ice Sheet, which covered most of the rest of Canada, has a rich history of study. However, this sister ice sheet sat on a flat bed. The mountainous topography over which the Cordilleran Ice Sheet sits
makes reconstructing the past geometry a challenge. Rather than being a two-dimensional problem, solved by a line on a map, one must consider a complicated three-dimensional topography, that alters through time. Helen has tackled this complicated problem, and filled in important details constraining its behaviour. This thesis therefore represents a big step forward in solving a complex problem.
The bulk of this thesis are the 4 papers, each of which represents a novel contribution to
knowledge. The first three papers focus on the Cordilleran Ice Sheet. Helen has demonstrated her observational skills, through generating a comprehensive map of glacial landforms (paper 1). Newly collected cosmogenic nuclide data is analysed in paper 2. This paper highlights the
three-dimensionality of the problem Helen tackled. The emergence of the Rockies from beneath the ice sheet is more tightly constrained than before, and plausibly reconstructed. In paper 3, the two pieces of the puzzle, the landforms and the dates, are brought together. This paper in particular highlights Helen's high level of analytical skills. This is a complex and large area to reconstruct, and a reasonable answer is derived. It is my opinion that the reconstruction that has been made will only be refined, and not disproved, with further data collection.
Throughout the first three papers, Helen demonstrates a firm understanding of the field.
Particularly impressive is the acknowledgement of uncertainty when dealing with data. This demonstrates a necessary degree of pragmatism to arrive at an answer. If a particular piece of data is an outlier, or difficult to assign an age or origin, it is flagged and classified as such. This honesty in reporting data and interpretations is commendable and essential. This makes the science highly reproducible and robust.
Paper 4 is a truly monumental effort, expanding the scope of the thesis to the rest of the Cordilleran Ice Sheet, and linking this with the rest of Canada and parts of the USA. The authors donʼt just reconstruct the ice sheet once, but three times over, depicting its potential maximum, optimum and minimum extent through time. This paper is likely to be a landmark paper, and I can see this being of use to multiple disciplines (Quaternary Science, ice sheet modelling, archeology,
palaeobotany to name a few).
Quality of writing
The writing of the thesis is of high quality throughout. The discussion is an interesting overview of the four papers, and draws out some . My only criticism is that some wider implications of the work could have been discussed. To me, this was an opportunity for more speculation and discussion, so
it would have been interesting to hear Helenʼs thoughts on the place of this work within the wider discipline. Some of my questions below reflect this.
Points of discussion
The quality of the thesis, like all good science, has prompted some more thoughts and deliberation.
These are outlined in reference to the specific papers below. I look forward to discussing these with Helen.
Paper 1
● Figure 7 highlights some subglacial ribs. What do you think controls their distribution? And did you use these in the reconstruction? Do they provide a further constraint on ice
conditions?
● You are rightly cautious with your interpretation and categorisation of landforms. Especially when you assign an age interpretation to moraines and meltwater channels. On page 424 of paper 1, you assigned 20% of your moraines an unknown origin. How much would the picture of changed, or the reconstruction become easier, if you could have used this extra data?
● How representative of the behaviour and geomorphology of the rest of the Cordilleran do you think your study site is?
Paper 2
● This question is with particular reference to Figure 5. You have some really tightly clustered ages from Mount Morfee. The dates from Mount Spieker are more difficult to interpret.
Looking at Figure 5b, why did you combine these ages? They look almost like separate populations. Sample RM18-05, the youngest, appears the most plausible. Perhaps this one lacks inheritance.
● How much do you trust the timing of the model from Seguinot? This is difficult for a model to get right, given the uncertainty of climate forcing.
● In the model, the surface slope on the Eastern edge of the ice sheet is steeper than the West.
Is this how you envisage the ice sheet? Is there any evidence to support this from the dating or geomorphology?
Paper 3
● To form lineations, most, if not all, glacial geomorphologists agree you need warm-based ice. You make the interesting observation of the “oldest” flowsets of lineations being those at higher elevations. If these relate to the local glacial maximum, does this mean that the ice sheet was warm-based towards itsʼ centre? This is likely difficult on higher elevations, given that thinner ice is less isolated from the surface and the pressure melting point will be higher under thinner ice. Does this provide a constraint on ice dynamics or climate in some manner?
● Related, I like the idea of ageing flowsets based on their preservation. What features were you looking for in particular to assign different ages?
● The readvances shown in Figure 8, with the meltwater channels cutting through moraines, is this a climatic signal, or a glacio-dynamic one? Has anyone dated a moraine that has this evidence of readvance?
● In section 6, you list a number of avenues for future work, which of these would you pursue first and why?
Paper 4
● You demonstrate that the advance and maximum extent of the Cordilleran ice sheet is out of sync with the Laurentide (Figure 5, and in the discussion). This seems like an interesting conundrum to solve. What is your hunch as to what caused this?
● Why does the rest of the Laurentide reach an asynchronous maximum extent?
● When you were making your three reconstructions (optimum, maximum and minimum), did you consider the age ranges and error on dates? Might this make the windows of uncertainty larger than your currently have?
● The ice sheet complex seems reach a maximum extent at around 25 ka (Figure 7), with very little change until 17 ka. During this time, there are quite a lot of climate fluctuations (Figure 7a). How do you reconcile this changing climate with a static ice sheet extent?
Discussion
● How useful of an analogue for Greenland is the Cordellerian? And what might we learn about the future of Greenland from your study?
● Do you think your data might help constrain ice sheet models? But conversely, how much did the model you reference help you build your reconstruction?
● If money and time were no object, what would you focus on doing next?
Concluding remarks
I would like to congratulate Helen on an outstanding thesis and several novel contributions to the literature. I would also like to thank the committee for inviting me to review the thesis.
Yours sincerely,
Dr Jeremy Ely (PhD)
