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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RC1: 'Comment on egusphere-2024-2653', Maurine Montagnat, 20 Nov 2024
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 -
RC2: 'Comment on egusphere-2024-2653', David Prior, 19 Jan 2025
Review of Stoll et al: “EastGRIP ice core reveals the exceptional evolution of crystallographic preferred orientation throughout the Northeast Greenland Ice Stream”
Review by Dave Prior, University of Otago.
I’d like to congratulate the team on collecting an astounding data set. This is a unique piece of work that must be published. This is the first whole ice sheet thickness core that that has been collected to address issues of ice dynamics, rather than palaeoclimate. The density of CPO data together with the restoration of azimuthal orientations blows all previous core studies of ice CPO off the chart. The work gives us new insights into the processes, kinematics and conditions inside an ice stream. Since flow in ice streams dominates drainage of the Antarctic and Greenland ice sheets this work is extremely important. I felt a great sense of excitement in reading the paper.
Although unique and new - the paper can be improved significantly. It needs major revisions to have the impact that it deserves and to ensure this remains a definitive description of the internal structure of an ice stream. Most significantly it lacks any comprehensive microstructural data or descriptions. I would like the authors to consider spending significant time and effort in revising and modifying this paper to ensure it is as good as it possibly can be. Better to take several more months on it rather than to rush it out close to current form. I have tried to summarise significant issues in this review document and have made a list of some of the minor things at the end. In addition, I have an annotated version of the manuscript that will be better for seeing a thorough set of minor comments and also shows some of my un-filtered thoughts on reading. Apologies for the handwriting on this - most was done on two cramped and bumpy C130 flights to and from the ice! If there are comments that the authors cannot read and want interpreted then they can get in touch. My review here reached a cut-off time, rather than me completing everything I could write!
I don’t think the paper contains too much material - indeed some of my recommendations will involve putting more stuff in. The writing is verbose and the current text can be reduced by 20 - 30% simply by improving the tightness of the writing.
Major comments:
Evolution? The paper uses the word evolution a lot - it is there in the title. I think this is highly misleading. The paper presents the pattern/sequence of CPO (and other stuff) as a function of depth. There may be ice core scenarios where the sequence with depth is a proxy for time and can be inferred to be an evolution; the upper part of ice domes/ divides for example. If there is any scenario where the pattern/sequence of CPO with depth is not realistically a proxy for time or strain, it is here in an ice stream. Inferring evolution later on in the paper is important - but it does not fall out simply from the vertical sequence, it requires much more information including comparison with Greenland ice divide core data and discussion of the effects of ice transport through the shear margin. Please remove the words evolution (and other phrases that imply the same) in describing the sequence with depth. Remove “evolution” from the title. How about a title such as: “New insights into ice stream dynamics from crystallographic preferred orientations through the EastGRIP ice core.” ?????
Microstructures? The microstructures of the ice should go hand in hand with CPO data. Both are needed for the most robust interpretation of processes and history. Holding back basic microstructural information for another paper makes no sense as CPOs and microstructures are both needed here. The microstructural data presented is very limited - just two micrograph figures from the deeper part of the core and a graph of mean grain sizes.
- Figure 2 needs to have a fabric analyser microstructure picture to go with each CPO (or min a parallel extra fig). Please be careful in the colour scheme chosen for these, to maximise the comparability of microstructures, particularly within samples with similar CPOs (e.g. 910 to 2417m). The colour scheme used in figure 1 of (Stoll et al., 2021) is not good as the colours in each image change radically depending on the (arbitrary) azimuth of the girdle. A much better colour scheme would be to colour the pixels based on the inclination: the angle between the c-axis and the core axis. The only complication then is whether grain shapes depend on orientation of the section relative to the girdle. This can be dealt with by marking ticks on the outside of the c-axis stereonet to show where the section plane is.
I would also, for completeness, have a supp info document with the microstructural image of each sample, to complement the CPO file.
- In using grain size data to infer processes, grain size distributions are very useful. The means or medians alone are insufficient. I see that some distributions (down to 1340m) are shown in fig 3 of (Stoll et al., 2021). These show slightly skewed normal distributions (difficult to tell as grain area plotted on a log scale ~ same as plotting grain diameter on a linear scale. I presume the “counts” axis is number of grains not cumulative area?). I’d really like to see the pattern of grain size distributions over a greater depth and how they compare with GSDs from the lowermost section with multimaxima CPOs. I realise the latter may be complicated as the data sets will be small (in terms of grain number). A figure that parallels fig 2 and has a GSD for each sample from the FA data would be great. That would leave open the possibility of having a paper later on focused on the sublimation image data.
- Grain shape data - bubble shape data?
Inference of processes.
The inference of processes from the observations is very poor. It’s not necessarily that I disagree with the interpretations, more that there is not a clear separation of observations and interpretations and often the text jumps to the interpretation without a useful justification. In particular you should remove expressions such as “dynamic recrystallisation has been observed…” (line 312): it is not observed, it is an interpretation. Inferring that there has been “dynamic recrystallisation” in ice is not particularly useful. All terrestrial ice deforms at very high homologous temperature. If it has deformed it will have undergone dynamic recrystallisation, so that term by itself does not say anything useful about inferred processes. Using observations to infer much more specific processes such as recovery, subgrain rotation, subgrain rotation recrystallisation, grain boundary migration, nucleation, grain boundary sliding – see for example table 1 in (Trimby et al., 1998).
The inferences need to lean in more heavily into the results from experiments (physical rather than numerical), where often we have time or strain series microstructural data and mechanical data that allow the interpretation of process to be more robust. Comparison of microstructural observations in samples that represent a point in time/strain with those developed as a function of time/strain then provides a good basis for interpretation. In particular, a lot of our inference comes from experiments where microstructural change is observed (transparent polycrystal, metals in an electron microscope etc) see for example (Urai et al., 1986) Here’s my little checklist of common relationships between observations and reasonable interpretations. I’ve put some references where I think you may be unaware here but have not referenced completely.
- Strong CPO: dislocation glide and climb.
- Subgrain boundaries: dislocation glide and climb, recovery
- Cell like subgrains: recovery, subgrain rotation (particularly if evidence of a range of subgrain misorientations)- sometimes called polygonization. (Guillope and Poirier, 1979; Poirier and Guillope, 1979; Poirier and Nicolas, 1975)
- Cell like grains adjacent to subgrains of a similar size: subgrain rotation recrystallisation. (refs as above)
- Irregular-lobate grain boundaries: strain induced grain boundary migration (SIGBM). (Bestmann et al., 2005; Jessell, 1986)
- Highly irregular grains with strongly skewed grain size distributions: SIGBM with a nucleation process (subgrain rotation recrystallisation and/or bulging).
- Grains that are consistently smaller than subgrains: bulge nucleation associated with subgrain rotation. (Halfpenny et al., 2006)
- Polygonal grains with straight-slightly curved boundaries: absence of SIGBM. Normal grain growth(?), possibly associated with mechanisms involving grain boundary sliding. Grain size distributions likely to be normal or only slightly skewed.
- Polygonal grains and independent evidence of high strain: mechanisms involving grain boundary sliding.
- Jump in boundary misorientations between subgrain boundaries and grain boundaries (needs EBSD data): mechanisms involving grain boundary sliding (Bestmann and Prior, 2003; Craw et al., 2018; Fan et al., 2020), also change of miso axes from crystal control to random.
- Linear alignment of multiple grain boundaries: mechanisms involving grain boundary sliding. (Ree, 1994)
Eigenvector information, especially inclination.
The paper misses important information about the eigenvectors. This includes both the orientation of the eigenvectors relative to the CPOs and within the core reference frame. Things you should do:
- Mark the eigenvectors on the stereonets on figure 2. Red symbols would distinguish them nicely from the data points. I think eigenvectors should also be added to the stereonets in the supp info file. Given the code to generate the c-axis plots should be capable of generating these, it should not be a big deal to do. I would also consider including an error small circle or ellipse where this error is significant (error > than ~5 degrees?). This will do two really useful things. In the vertical girdle samples this will give an idea as to how well the horizontal maximum (within the girdle) is defined. In the multimaxima samples it will help illustrate the issue of the small number of grains in the data set. I understand that error circles may be a little complex and may be beyond scope- particularly for all samples in supp info, but if you can build it into the plotting code, I would add these.
- Provide a plot of eigenvector inclination to go with the eigenvector magnitudes in figure 4. I predict that this will have l3 vertical to ~200m, horizontal from ~600 to ~2400 (highlighting the horizontal maximum within the girdle- and departures from this showing where it gets weaker) and steep (with a fair bit of variation) at >~2400m. Plot inclinations of all three eigenvectors to reveal the complete pattern. The most crucial place to use these data would be in comparing with other ice cores in fig 10. I presume you have access to the NEEM and EDML data and can generate the eigenvector inclination pattern with depth. SPICE data are all available as text files and generating the inclinations should be trivial. The eigenvector inclination data for EastGRIP will be starkly different to all of these others. NEEM will have l3 ~ vertical at all depths. I think this will be true in EDML as well: it looks like the girdle CPOs in fig 2m of (Weikusat et al., 2017a) have maxima in the girdle that are steep to vertical? The girdle section for EDML in fig 10 also has a wavy pattern (like EastGRIP: although not recognized in this manuscript I am reviewing), and the orientation of the max (l3) within the girdle helps distinguish the EastGRIP and EDML patterns. SPICE is an interesting comparison - this is a vertical girdle dominated core- the pattern in fig 10 suggests to me that whilst l1 will be horizontal at all depths (as I’d predict for the vertical girdle section of EastGRIP) there will not be a consistent pattern in the inclinations of l3 and l2 (different to what I’d predict for EastGRIP).
Description of CPOs
The descriptions of the CPOs is not good. A few of specific things:
- Descriptions of the vertical girdle CPOs describes them as having two maxima. This is incorrect. Clustering at points on the primitive circle 180 degrees apart are not 2 maxima – this is a single maximum and the spatial separation of clusters simply a function of reference frame choice: in this case the maximum is around a horizontal axis and the choses stereonet reference frame has a horizontal primitive circle. A reference frame with the primitive in the vertical plane paerpendicular to flow with have one spot on increased point density on the net.
- The word asymmetric is used. I’m unsure what this relates to. Describing patterns on stereonets as symmetric or asymmetric is uninformative. Better is to describe the nature of the symmetry and the reference frames to which those statements refer. For CPOs associated with deformation it is commonly possible to describe CPOs as having orthorhombic (three orthogonal mirror planes) or monoclinic (A two-fold axis which is also the pole to a mirror plane) symmetry. If the internal symmetry elements (rotation axes, mirror planes) have the same orientations as symmetry elements of the external deformation kinematics, that gives a good reason to suggest that the CPOs relate to that deformation. Simple shear is a really great example: the two-fold axis of the monoclinic shear kinematics maps onto the 2 fold axis of monoclinic CPOs (Bouchez and Duval, 1982; Journaux et al., 2019; Qi et al., 2019). In your case see section “Kinematic significance of vertical girdles”
- You mention skeletal outlines, without ever using them or explaining. CPO skeletons are in common use in the structural geology community, particularly but not exclusively for quartz. They are very useful and I think you should incorporate some. Fig 6 of (Toy et al., 2008) has a lot of examples, including a variety of x-girdles. Skeletons to represent the key CPO types in fig 2 and/or as a column in table 1 gives you a useful shorthand for the CPO patterns that you can reuse in later analysis of interpretation. Skeletons of typical x-girdles will help significantly the description of these.
- Contoured data. I am glad that you present point data - many workers do not do this and the papers are poorer for it. I think you should also have contoured data - point and contour data are complementary. In particular, it is not so easy to see the horizontal maximum in some of the point data plots and there is no real sense of the intensity of this maximum. Contoured data would help the reader resolve these features.
- X-girdles. As you say these have never been described in natural ice. I don’t think that they have been described from experiments either. So they deserve a complete description. The one you provide is very superficial (almost nonexistent) - you jump straight to the quartz literature. The x-girdles need to be described in the observations section with skeletons to help clarity, before any link is made to qtz data. Pick several of the range of examples for description. 375_1, 375_3, 432_1, 436_1,438_6,486_1 are a suite of examples that could be used for this description, in an extra figure focused on x-girdles (with skeletons, point data and contoured figs). I am very intrigued as to what the x-girdles might mean. I think you can be a bit more discriminatory about the possible explanations. The quartz kinematic explanations (see summaries in (Law, 1990; Schmid and Casey, 1986) nearly all involve different slip systems (see fig 7 in (Toy et al., 2008). Quartz has extreme c axis patterns, for example the y-max in shear (e.g. (Cross et al., 2017) that cannot relate to basal dislocations. Although there is evidence in subgrain misorientations for non-basal dislocations in ice (Chauve et al., 2017; Fan et al., 2022; Weikusat et al., 2017b), I don’t think that there is evidence from experiments that we can get CPOs that have significant involvement of anything other than basal plane dislocations. Axial compression (Jacka and Maccagnan, 1984; Montagnat et al., 2015), simple shear (Bouchez and Duval, 1982; Journaux et al., 2019; Qi et al., 2019) are all explicable in terms of rotation on the basal plane plus preferred growth of grains with high resolved shear stress on the basal plane.
- You say that you have no clear evidence of overprinting; but what does an overprinted CPO pattern look like? For most crystalline materials at high temperature there is very little obvious evidence that old CPOs are preserved through significant strain. Some experiments in ice (Craw et al., 2018) show that a pre-existing girdle CPO can be annihilated with only 20% strain. This does not mean I don’t believe we can get CPOs with some overprinting I suspect it means that they have low chance of survival.
Kinematic significance of vertical girdles
I am really intrigued by girdle CPOs. These have not been reported from laboratory deformation experiments, although there are very few experiments in axial extension and even fewer that have CPO data. Warm axial extension experiments (-3C: (Jacka and Maccagnan, 1984), -5.5C (Craw, 2016)) give an open cone CPO indistinguishable from that generated in axial compression experiments at similar temperatures (e.g.(Jacka and Maccagnan, 1984; Montagnat et al., 2015)) with a difference in shape preferred orientation (parallel to the CPO cone axis for extension and perpendicular for compression). There are no lower temperature extension experiments I know of. Although you use the beautiful study from Fujita et al (シュウジ, 1987) as a justification of these being related to extension, I think you can and should do more to develop this idea from your own observations. Some of the kinematic context of the site is only outlined at the end (line 411-412). This provides a framework for understanding that should be earlier and more detailed. You can then relate the orthorhombic internal symmetry of your vertical girdle CPOs to the orthorhombic symmetry of the ice stream extensional reference frame, making the idea that these are related to axial extension really robust. These CPOs give me as an experimentalist a big challenge. I cannot explain them using the same ideas we use to explain axial compression and shear CPOs (Qi et al., 2017; Qi et al., 2019; Wang et al., 2024). At low stress natural conditions we expect the recrystallisation effect, giving orientations with high resolved shear stresses on the basal plane, to dominate. This would be an open cone rather than a girdle. Very puzzling.
Related to this point I would suggest that plotting Fig 2 could be plotted in a kinematic rather than geographic reference frame. Or at least make the kinematic reference frame clear within the geographic frame of this figure.
Strain terminology
The “strain” described in Line 411 is not precise. Stretch and strain are not the same and I get different implications if I take these (175%, 67%) both as stretch, both as natural strain or one as stretch and one as strain. Tighten up the language (ask Paul)
Kinematics, conditions and mechanisms.
The interpretive section is a bit mixed up. Key things to infer from CPOs and microstructure are kinematics, conditions and mechanisms. These are rather intermixed at the moment; for example title for 4.1.3. is kinematic, 4.1.5. is process (and not v useful). It would give much greater clarity to separate these three issues. Kinematics will come primarily from the CPOs and their relation to the ice core setting. Grain and bubble shapes could be important here as well and are given little mention. Mechanism interpretations come primarily from microstructures and a little from CPOs. The microstructural description needs beefing up as mentioned earlier. Some conditions (T for example) are available from borehole measurements others (stress magnitude, principal stress orientations- note this is not exactly the same as kinematics) need some discussion that involves data from the ice.
Changes at the ~glacial?
Discussion related to fig 5 seems oversimplified. The grain size changes at “glacial” are considered to be very similar- I disagree and think that this is important. The mean grain size as a function of depth at NEEM does have an abrupt step that corresponds to the depth marked as NEEM Glacial. The pattern of grain size in EastGRIP is a smooth transition that does not have an abrupt change at the depth marked as EastGRIP Glacial. Add to this that NEEM has an abrupt and substantial CPO change (from weak girdle to strong vertical max) at the depth marked as NEEM Glacial, whereas EastGRIP is a subtle change involving a gradual increase in strength of the horizontal maximum within the vertical girdle. If you scan through from 1000m to 2000m in the supplementary data there is no obvious change in CPO visible in the EastGRIP point data stereonets. You suggest that NEEM might represent a starting profile which has evolved in the ice stream to the EastGRIP profile. Maybe the Glacial is not so important in terms of processes and kinematics in EastGRIP. So the processes are trying to end up with the same kind of ice, but the pathway is different depending of the starting material: different above and below the Glacial depth.
Signature changes in fig 6.
A lot is made concerning the change in grain size at ~ 2618m. There are a large number of similar steps or perturbations in the EastGRIP DEP profile in fig 6a. Some of these have significant changes in CPO:
- 2417-2419 transition from vertical girdle to multimax
- Between 2419 and 2450 change from multimax to vertical girdle
- 2450-2499 complicated (and interesting) CPO transition.
I don’t understand the big deal with that grain size change when there are radical changes in CPO in the same sequence. What are the microstructural transitions (if any) corresponding to these CPO transitions?
Cartoon at the end.
The final discussion at the end of the paper is very hard to follow and really needs a cartoon to explain it. I would have in the cartoon:
- A simplified representation of key features of the EastGRIP core. Fig 7 has the CPO represented very nicely that could form the basis of this part of the figure. Fig 7 is too early in the paper. In terms of representing key aspects of the core it does not summarise the microstructure.
- A simplified representation of key features of an ice core that represents ice before it becomes involved in ice stream. This should highlight the same CPO and microstructural characteristics as 1. This could be an actual ice core (NEEM? – as is used in discussion), or could be a synthesis of features from several cores (NEEM, NGRIP, GRIP, GISP2).
- Stratigraphic tie lines- lines that link points in depth in the two core cartoons.
- For some specific depths, a series of cartoon sketches suggesting how CPO and microstructure has changed as a function of time / strain. You could also have schematic graphs of how eigenvector ratios and orientations change as a function of time/ strain.
- Some representation of what might happen closer to the shear margins and to ice passing through the shear margin and what influence this might have on features in the EastGRIP core.
I do hope that all these rambles are useful
Dave
Bestmann, M., Piazolo, S., Spiers, C. J., and Prior, D. J., 2005, Microstructural evolution during initial stages of static recovery and recrystallization: new insights from in-situ heating experiments combined with electron backscatter diffraction analysis: Journal Of Structural Geology, v. 27, no. 3, p. 447-457.
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