the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Oct 2024
EastGRIP ice core reveals the exceptional evolution of crystallographic preferred orientation throughout the Northeast Greenland Ice Stream
Abstract. A better understanding of glacial ice flow and how it is influenced by internal deformation is required to improve the projections of future sea-level rise in a warming climate. Especially large ice streams, the main contributors to solid ice discharge to the ocean, still require more observational data to be represented sufficiently in numerical ice-sheet models. The East Greenland Ice-core Project (EastGRIP) successfully drilled the first continuous deep ice core through an active ice stream, the Northeast Greenland Ice Stream (NEGIS), focusing on investigating the dynamical processes that lead to its exceptionally high velocity. Here, we show Crystallographic Preferred Orientations (CPO) data in 5–15 m depth resolution throughout 2663 m, down to bedrock, to determine the deformation regimes in this ice stream setting complemented by grain-size and borehole temperature profiles for context. A broad single-maximum CPO pattern is present in the upper 200 m caused by overlying snow and ice layers. Below, a crossed girdle CPO is observed for the first time in a deep ice core and we discuss possible formation mechanisms. Between 500 and 1230 m of depth, we observe a vertical girdle CPO indicative of along-flow extensional deformation. A complementary simple-shear component and polygonization explain the CPO between 1230 and 2500 m, a vertical girdle with horizontal maxima of varying strength. Close to bedrock, a multi-maxima CPO originates from migration recrystallisation due to high temperatures close to the pressure melting point. Ice at this depth is characterised by centimetre-large, amoeboid-shaped grains, which, together with the conductivity data from the deepest 260 m, indicates that the core contains ice from the last Eemian. A comparison with other deep ice cores from Greenland and Antarctica shows the uniquely fast development of CPO at shallow depths in the EastGRIP ice core due to its location in an area of high strain rates while the grain-size evolution with depth remains similar to less dynamic sites confirming that it is mainly governed by the varying purity of ice deposited during varying climatic conditions. We further show that the overall plug flow of NEGIS is characterised by many small-scale variations, which remain to be considered in ice-flow models.
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Status: open (until 31 Dec 2024)
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RC1: 'Comment on egusphere-2024-2653', Maurine Montagnat, 20 Nov 2024
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This paper presents a comprehensive and detailed study of the evolution of the texture (or fabric) along the EastGRIP ice core.
This paper is based on a tremendous measurement campaign that must be highlighted (more than 1200 thin sections made and measured!)
Overall the data are of very good quality, and their interpretation, for most of them, are reasonable, well argued and well documented.
A few interpretations can be questioned, as detailed further in this review, and I would appreciate the authors to consider these comments prior to publication. Some interpretations are not in good agreement with previous work what is not a problem by itself, but the discrepancies are not always well argued.
Another point concerns the citations. In view of respecting the work of former colleagues (and research ethics rules) I would suggest the authors to avoid citing only review papers (e.g. Faria et al.) but also the original work cited in the review papers that are sometime more closely related with the subject matter.
Appart from that, this paper clearly deserves publication in TC and I would like to emphasize the clarity and quality of the writing.
Specific comments :
- the authors make use of the second order orientation tensor as a proxy of the texture evolution. Please mention that this proxy is not adapted for all types of fabric, and in particular, do not discriminate some multi-maxima fabric with isotropic ones. What about the case of the crossed-girdle fabric ? How well is it represented by the 2d order orientation tensor ? What precautions should be taken when interpreting the evolution of the eigenvalues of the 2d order orientation tensor with depth ?
- In the abstract and along the text a specific « crossed girdle » fabric is mentioned as an originality of the obtained measurements. Nevertheless the specificity of this fabric does not appear so obvious. Would it be possible to find specific illustration or data treatment to make it stand out better? The 241 m depth pole figure that is shown in figure 2 could well be interpreted as a wide girdle with a low anisotropy…
- I missed some figures of microstructures that would illustrate the interpretations about the mechanisms at play, such as dynamic recrystallization. I am pretty sure a few nice microstructures could be shown, for each depth interval with specific characteristics, and at least in supplementary.
- Table 1 : illustration here of my comment about the crossed girdle. A transition from crossed-girdle to vertical-girdle is mentioned but not clearly shown elsewhere.
Anisotropy indexes could be added in this table, similar to the one shown in figure 3.
- Figure 3 : please remind somewhere how the anisotropy indexes are calculated. Is the Woodcook parameter the more adapted ? Why not use ln(a3/a2) and ln(a2/a3) that evolves in a smaller interval ?
- Figure 6 : please increase the size in the final version.
- Part 4.1 : please cite Alley et al. 1988, or Castelnau et al. 1994 for instance to illustrate the link between rotation of c-axes and deformation !
Idem, regarding the impact of recrystallization, De la Chapelle et al. 1998 or Thorsteinsson et al. 1997, provided already an overlook of recrystallization along ice core (maybe not the first ones still), please site them (or others) instead of Faria et al. (or on top of).
- Part 4.1.1 : I don’t remember in details the experiments presented in the paper by Azuma and Higashi 1985 but one should be careful when comparing deformation-induced fabrics observed along ice cores with experimental ones since, even for some of the slowest experiments made by Jacka and co-authors, the fabric results from dynamic recrystallization that takes place already above 1 % strain, and dominate from about 10 % strain… And the DRX fabrics can be way different from the deformation ones, especially in compression.
- Figure 8 : here is the illustration of the crossed-girdle that is not so obvious, although clearer that in figure 2. Could a representation of the microstructure help ? Well, I understand that it is not easy to find a clearer illustration.
- Part 4.1.2 : some of the tentative explanations lack justifications.
For instance, the comparison with quartz does not really holds since, for some temperature and deformation ranges, quartz has several slip systems of similar activity, it is not the case for ice.
About the activation of non-basal slip systems : please go back to Hondoh 2000 review work where one can read that (1) the critical resolved shear stress required to activate non-basal dislocations is way more too high regarding the level of expected stresses here, (2) it is clearly mentioned that non-basal edge dislocation segments can help activating more basal dislocations by multiplying the active basal slip planes, but can not move on long enough distances to participate to deformation. As a summary, although there exist non-basal dislocations that we observe by EBSD for instance, they are first located mainly close to grain boundary and triple junctions, where local stress can be high, in the form of subgrain boundaries but it does not mean that non-basal dislocations actively participate to deformation. As such, they participate as an accommodation mechanism that facilitates basal glide. A high enough non-basal activity is required to have non-basal glide impact on fabric.
Please go a little further in showing the limitation of such an interpretation, not to leave ambiguous information in the paper that are later re-used in other papers with no clear view on the hypotheses behind.Overall, in this part the authors could give the likelihood of each suggested mechanism.
- Part 4.1.3 :
In tension, experimental paper by Jacka and Maccagnan, 1984, could be cited ?
The effect of DRX on the strength of the tension-induced CPO could also, maybe, be mentioned ?
- Part 4.1.1:
If I’m right, the CPO transition referred to in this part is not shown in figure 2.
Similarly, line 279, figure 2 doesn’t seem to show the maxima of varying strengths in the horizontal plane. Nor does the supplementary. Did I got it right? I just see a broadening of CPO from ai data.
From line 285 I find the explanation based on dynamic recrystallization very confusing. First because I don’t see the necessity of evoking polygonization to explain the vertical girdle CPO. In particular since polygonization (one of the mechanism of rotation recrystallization) is known (not only in ice) to weaken the CPO, at least slightly, and clearly not to strengthen it…
Again, please also refer to De la Chapelle and Duval 1998 when mentioning rotation recrystallization since their modeling include some energy calculation that help interpreting the occurrence of different recrystallization mechanisms. Also in Montagnat and Duval 2000 did we link the grain size evolution with depth with the rotation recrystallization modeling.
From line 295: could DRX with various level of impurity content also be mentioned to explain these rheological differences?
- Part 4.1.5:
line 310: at the bottom of Talos Dome ice core we very likely observe some stagnant ice (Montagnat et al. 2014), if you want to find a reference to illustrate this statement.
Line 314, again Faria et al. 2014 did not initiate the dynamic recrystallization conceptual models! Please refer to the original paper (cited in Faria et al by the way).
I don’t see the interest about separating SIBM-N and SIBM-O mechanisms since, discontinuous recrystallization contains both mechanisms, nucleation and grain boundary migration (owing to the reduction in stored strain energy). On top of that, nucleation is much more likely to end up with orientations different from the parent grains than grain boundary migration. This sentence is at least unclear, but maybe also not very accurate.
- Part 4.2: line 340 “we show that assumptions valid for other,…” what assumptions are you referring to?
- Part 4.4: line 365 “no strong grain-size dependence of the dominating deformation regimes”. To my point of view it also suggests that grain growth and grain size reduction are driven similarly, what goes in favor of continuous DRX, at least in the first part of the core.
- Part 4.5: First paragraph: It would be interesting (necessary?) to discuss the relative viscoplastic anisotropy resulting from these different fabrics observed along EastGRIP. In particular, is the crossed-girdle fabric resulting into a mechanical response strongly different from an isotropic fabric, or from the slight cluster fabric that is observed just above? If no, then why bother to try to simulate such a fabric development in an ice flow model?
Lines 402-404: when discontinuous dynamic recrystallization dominates, the fabric only reflects the stress conditions, and looses the strain history, so the estimated preserved duration could be even lower in the bottom part of the core. Maybe it would be worth mentioning it.
Line 415: Please see my comment for part 4.1.2: the actual knowledge of dislocation slip systems activity and activation and the too weak information make this hypothesis of secondary slip system activity in EastGRIP very unlikely, and one of the less likely hypotheses over the ones mentioned in part 4.1.2. By bringing it back in the discussion part that way you put a relatively high weight on it and this is misleading.
At minimum, an estimation of the level of stress required to activate a secondary slip system with a high enough level of activity to explain the crossed-girdle texture must be provided. I doubt that this level of stress can be achieve at EastGRIP.
Lines 424-425: Why would shear occur DUE TO a strong recrystallization?? I would expect shear near the bedrock to induce high level of strain that, together with the high temperature, favor discontinuous dynamic recrystallization and therefore large grain size. Shear-induced fabrics are similar in deformation and recrystallization (at least for high level of strain) and should lead to a strong cluster. So either this cluster is hidden by the large grain size (too few measured orientations) or the main stress component at the bottom is not shear??
Citation: https://doi.org/10.5194/egusphere-2024-2653-RC1
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