the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jun 2023
Insights into glacial processes from micromorphology of silt-sized sediment
Abstract. Meltwater plume deposits (MPDs) from marine sediment cores have elucidated clearly connected, yet difficult to constrain, relationships between ice-marginal landform construction, grounding-zone retreat patterns, and subglacial hydrology for several glacial systems in both hemispheres. Few attempts have been made, however, to infer coveted details of subglacial hydrology, such as flow regime, drainage style, and mode(s) of sediment transport through time from grain-scale characteristics of MPDs. Using MPD, till, and ice-proximal diamicton samples collected offshore of six modern and relict glacial systems in both hemispheres, we examine whether grain-shape distributions and microtexture assemblages (collectively, grain micromorphology) of the silt fraction are the result of subglacial meltwater action, or are indistinguishable from glacial proximal and subglacial sediments from the same region. We find that of all grains imaged (n=9,400), three-quarters can be described by one-quarter of the full range of measured shape morphometrics, indicating widespread and efficient abrasive processes in subglacial environments. Microtexture analysis reveals that while grains comprising MPDs show evidence of edge rounding more often than tills, fluvial microtextures occur in modest amounts on grain surfaces. Furthermore, MPDs retain many mechanical (i.e., glacial) textures in comparable abundances to tills. Significant alteration of MPDs from till sources is observed for systems (1) for which intensive, potentially catastrophic, meltwater drainage events in the Holocene are inferred from marine geologic records, and (2) with comparatively less mature till grains and a contribution of supraglacial melt to the bed, indicating that quantifiable grain-shape alteration of MPDs may reflect a combination of young till, high-energy flow of subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances. We encourage future works to integrate grain micromorphology into site-specific marine sediment analyses, which may distinguish periods of persistent, well-connected subglacial discharge from periods of sporadic or disorganized drainage and provide context needed to estimate sediment fluxes and characterize ice response to subglacial meltwater transmission. In addition, this work demonstrates that glacial and fluvial surface textures are retained on silts in adequate abundance for microtexture analysis.
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RC1: 'Comment on tc-2023-70', Bradley W. Goodfellow, 05 Jul 2023
Thank you for the opportunity to review Insights into glacial processes from micromorphology of silt-sized sediment by Lepp et al. The paper reports a study of micromorphology of silt-sized quartz in meltwater plume deposits (MPDs) offshore of six presently and formerly glaciated catchments. The micromorphologies of the silt-sized quartz from these deposits are compared with those in glacial tills, which are assumed to represent the parent material prior to glaciofluvial transport to form the MDPs. The authors investigate micromorpholgies to elucidate the dominant mechanism(s) of sediment transport by glaciers. The authors interpret their results as indicating that grain characteristics reflect mainly glacial abrasion, and that meltwater transport has made only a minor contribution to grain evolution. This information may provide a basis for inferences on glacial meltwater flow rates, distances, and pathways, which may also inform paleoglaciology and paleoclimatology.
I believe that the authors have completed an innovative study that could be of general interest to the glaciology community. I also thought that the results were well presented in the Figures. However, there is a large opportunity to improve the presentation of the study, including especially its key findings, through addressing important issues with the writing. At present, the key findings are not clearly and simply stated. In particular, the Abstract, Introduction, Methods and Conclusions are afflicted by a lack of clarity. The presentation of the Results and the Discussion is clearer. The main issues contributing to a lack of clarity are frequent grammatical errors, long and complex sentences, being indirect in the presentation of information, and frequent gaps in logic. It is this latter point that is probably the most critical to address in a future revision. The Introduction could better establish the key relevant micromorphological features, including a brief overview of the relevant mechanical work that appears (too briefly) in the Discussion. What might seem clear to a glacial sedimentologist is not necessarily clear to a more general reader and, as a non-sedimentologist, I felt that there are many important gaps in logic in the present manuscript. My general impression is that it is presently difficult for the reader to 'distinguish the forest from the trees', which is a pity because it detracts substantially from what I believe has been a good study.
I have appended a marked-up version of the manuscript with detailed comments and suggestions for improvement to the manuscript. Many of these are minor, but I am of the view that substantial rewriting would greatly assist the presentation of this study.
- AC2: 'Reply on RC1', Allison Lepp, 05 Oct 2023
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RC2: 'Comment on tc-2023-70', Anonymous Referee #2, 07 Sep 2023
Abstract
Is clear and well-written; possibly lacking in impact/significance at the end i.e. why is this research so important and how will it help future analyses of these types of deposits…and what will that mean for reconstruction purposes or for better understanding processes.
Introduction
Intro is well written and concise; again more significance/impact needed; other minor comments within the manuscript itself.
Materials and methods
Not sure why till and ice-proximal diamicton deposits are considered to be the same in terms of sediment transport processes and thus grain textures; sec 2.1 needs references; see other comments within the manuscript itself.
Results
The main issue with the description of the results is that it is unclear what deposits the grains have come from for each of the different site locations – MPD or till? Just please remind the reader throughout. Also, there is no real mention of the results of grains from ice-proximal diamictons – presumably because (according to an earlier sentence in the methods section) the till and ice-proximal diamicton results have been merged together?? But it remains unclear why this is, why not just keep them separate, since they are entirely different kinds of deposits? There seems to be some inclusion of interpretations (the bit about how the local geology may reflect some of the grain results) when this info is probably better off in the discussion?
Discussion
In general, there is a lack of reference to the actual sites/results from this study – would like to see the results and sites more systematically dealt with within each sub-section of the discussion. Many sentences in the discussion are not worded very well, the writing style could be tightened up significantly.
4.1– Geological differences at each site are not explained anywhere – what is the local geology for each site? this section is really difficult to follow since there is a lot of information that the reader is assumed to know (but doesn’t know); see comments within the Ms itself on where and how to make sentences clearer
4.2 – this reads much better than section 4.1; it tells the reader what has been seen (from these results) and what they mean (whereras 4.1 fails to do that)
4.3 – is fine, see comments within.
4.4 – needs clarification in place, see comments within.
Conclusions
Might be better off as bullet points? See comments within.
- AC1: 'Reply on RC2', Allison Lepp, 05 Oct 2023
Status: closed
-
RC1: 'Comment on tc-2023-70', Bradley W. Goodfellow, 05 Jul 2023
Thank you for the opportunity to review Insights into glacial processes from micromorphology of silt-sized sediment by Lepp et al. The paper reports a study of micromorphology of silt-sized quartz in meltwater plume deposits (MPDs) offshore of six presently and formerly glaciated catchments. The micromorphologies of the silt-sized quartz from these deposits are compared with those in glacial tills, which are assumed to represent the parent material prior to glaciofluvial transport to form the MDPs. The authors investigate micromorpholgies to elucidate the dominant mechanism(s) of sediment transport by glaciers. The authors interpret their results as indicating that grain characteristics reflect mainly glacial abrasion, and that meltwater transport has made only a minor contribution to grain evolution. This information may provide a basis for inferences on glacial meltwater flow rates, distances, and pathways, which may also inform paleoglaciology and paleoclimatology.
I believe that the authors have completed an innovative study that could be of general interest to the glaciology community. I also thought that the results were well presented in the Figures. However, there is a large opportunity to improve the presentation of the study, including especially its key findings, through addressing important issues with the writing. At present, the key findings are not clearly and simply stated. In particular, the Abstract, Introduction, Methods and Conclusions are afflicted by a lack of clarity. The presentation of the Results and the Discussion is clearer. The main issues contributing to a lack of clarity are frequent grammatical errors, long and complex sentences, being indirect in the presentation of information, and frequent gaps in logic. It is this latter point that is probably the most critical to address in a future revision. The Introduction could better establish the key relevant micromorphological features, including a brief overview of the relevant mechanical work that appears (too briefly) in the Discussion. What might seem clear to a glacial sedimentologist is not necessarily clear to a more general reader and, as a non-sedimentologist, I felt that there are many important gaps in logic in the present manuscript. My general impression is that it is presently difficult for the reader to 'distinguish the forest from the trees', which is a pity because it detracts substantially from what I believe has been a good study.
I have appended a marked-up version of the manuscript with detailed comments and suggestions for improvement to the manuscript. Many of these are minor, but I am of the view that substantial rewriting would greatly assist the presentation of this study.
- AC2: 'Reply on RC1', Allison Lepp, 05 Oct 2023
-
RC2: 'Comment on tc-2023-70', Anonymous Referee #2, 07 Sep 2023
Abstract
Is clear and well-written; possibly lacking in impact/significance at the end i.e. why is this research so important and how will it help future analyses of these types of deposits…and what will that mean for reconstruction purposes or for better understanding processes.
Introduction
Intro is well written and concise; again more significance/impact needed; other minor comments within the manuscript itself.
Materials and methods
Not sure why till and ice-proximal diamicton deposits are considered to be the same in terms of sediment transport processes and thus grain textures; sec 2.1 needs references; see other comments within the manuscript itself.
Results
The main issue with the description of the results is that it is unclear what deposits the grains have come from for each of the different site locations – MPD or till? Just please remind the reader throughout. Also, there is no real mention of the results of grains from ice-proximal diamictons – presumably because (according to an earlier sentence in the methods section) the till and ice-proximal diamicton results have been merged together?? But it remains unclear why this is, why not just keep them separate, since they are entirely different kinds of deposits? There seems to be some inclusion of interpretations (the bit about how the local geology may reflect some of the grain results) when this info is probably better off in the discussion?
Discussion
In general, there is a lack of reference to the actual sites/results from this study – would like to see the results and sites more systematically dealt with within each sub-section of the discussion. Many sentences in the discussion are not worded very well, the writing style could be tightened up significantly.
4.1– Geological differences at each site are not explained anywhere – what is the local geology for each site? this section is really difficult to follow since there is a lot of information that the reader is assumed to know (but doesn’t know); see comments within the Ms itself on where and how to make sentences clearer
4.2 – this reads much better than section 4.1; it tells the reader what has been seen (from these results) and what they mean (whereras 4.1 fails to do that)
4.3 – is fine, see comments within.
4.4 – needs clarification in place, see comments within.
Conclusions
Might be better off as bullet points? See comments within.
- AC1: 'Reply on RC2', Allison Lepp, 05 Oct 2023
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