the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Nov 2023
Evidence of solid Earth influence on stability of the marine-terminating Puget Lobe of the Cordilleran Ice Sheet
Abstract. Understanding drivers of marine-terminating ice sheet behavior is important for constraining ice contributions to global sea-level rise. In part, the stability of marine-terminating ice is influenced by solid-Earth conditions at the grounded-ice margin. While the Cordilleran Ice Sheet (CIS) contributed significantly to global mean sea level during its final post-Last Glacial Maximum (LGM) collapse, the drivers and patterns of retreat are not well constrained. Coastal outcrops in the deglaciated Puget Lowland of Washington state – largely below sea level during glacial maxima, then uplifted above sea level via glacial isostatic adjustment (GIA) – record late Pleistocene history of the CIS. The preservation of LGM glacial and post-LGM deglacial sediments provides a unique opportunity to assess variability in marine ice-sheet behavior of the southernmost CIS. Based on paired stratigraphic and geochronological work with a newly developed marine-reservoir correction for this region, we identify that the late-stage CIS experienced stepwise retreat into a marine environment about 12,000 years before present, placing glacial ice in the region for about 3,000 years longer than previously thought. Stand-still of marine-terminating ice for a millenia, paired with rapid vertical landscape evolution, was followed by continued retreat of ice in a subaerial environment. These results suggest rapid rates of solid Earth uplift and topographic support (e.g., grounding-zone wedges) stabilized the ice-margin, supporting final subaerial ice retreat. This work leads to a better understanding of shallow marine and coastal ice sheet retreat; relevant to sectors of the contemporary Antarctic and Greenland ice sheets and marine-terminating outlet glaciers.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-2725', Anonymous Referee #1, 18 Dec 2023
In this manuscript, McKenzie et al. present stratigraphic and chronological constraints on the retreat of the Puget Lobe of the Cordilleran Ice Sheet (CIS) in Washington State. This area has been studied by glacial geologists for many years, but there is still some uncertainty about its history. McKenzie et al.’s work largely relies on facies analysis of sediment outcrops on Whidbey Island, supplemented by 14C dating of marine shells and OSL dating of quartz. From these data, the authors construct a general timeline of events across Whidbey Island, from prior to the LGM up to the present. A key conclusion of the manuscript is that the Puget Lobe lingered near Whidbey Island for several thousand years longer than previous work indicates; this interpretation is based on 14C-dated shells in a unit that the authors identify as glaciomarine. McKenzie et al. then suggest that the Puget Lobe was stabilized on Whidbey Island by solid-Earth uplift, topographic pinning points, and grounding zone wedges.
I think the sedimentological descriptions and facies interpretations mostly make sense (although that is not my primary research area, so I will yield to other reviewers on this point), but the major weaknesses of the manuscript in its present form are (1) chronology and (2) how these results fit in with existing age constraints in the Puget Lowland. In my view, the authors need to better situate their work in the context of previous research, and offer more thorough (and convincing) explanations for why their results differ so much from what has already been published.
My comments on this submission are below. As I have focused most of my attention on what I believe are major issues with the manuscript in its present form, I have only provided a few line-by-line comments. It is my hope that all of these comments are helpful for the authors as they revise the manuscript.
Specific major comments:
- I’d like to see a thorough treatment of the existing constraints on CIS and landscape evolution in the Puget Lobe region. Some review of the literature is present in the supplemental material, but I believe these points should appear in the main paper, and that they should appear early (rather than being brought up at the end of the discussion section). Points that need more discussion earlier in the manuscript include:
- Previous outcrop work in the Puget lowlands
- 14C constraints on the Puget Lobe – marine shells, lake sediments
- Grounding zone wedges on Whidbey Island
- Cosmogenic 36Cl ages on Whidbey Island – ideally these would be recalculated from Swanson and Caffee (2001) as the production rates have changed slightly, but they indicate deglaciation ~15 ka (much earlier than your results suggest)
- Related to point 1, I strongly recommend adding a figure that shows the regional context, as the inset map on figure 1 is not sufficient to set the stage. I recommend that at minimum, you include published stratigraphic column locations, age constraints from 14C and cosmogenic 36Cl, surficial geology (e.g., presence of GZWs), and previously mapped CIS margins (and perhaps the glacial lakes too).
- I would also like to see a more thorough discussion of your chronology and stratigraphy. There are a few things that would help the readers better evaluate your data, and areas that need clarification:
- Pictures of outcrops in the main text, or at least in the supplemental material. Without these, it is difficult to evaluate your interpretations of the outcrops, and therefore the geologic history.
- Double Bluff site: You find 14C-dead woody debris in the “deglacial” unit. How can you explain this? Remobilization of old material?
- Fort Casey site 1: How do you explain a 9.3 ka OSL age in your “LGM” unit? You say it’s a minimum age, but what processes could make that age so much younger than regional deglaciation?
- Fort Casey site 2 and Penn Cove: Unless I’m reading the figure incorrectly (which is possible, see my comments on figures below), you interpret a transition from glacial outwash to landscape emergence between units 4 and 3 at Fort Casey Site 2 and between units 5 and 4 at Penn Cove. Are you interpreting the lower units as submarine glacial outwash? If so, how are you distinguishing that from subaerial outwash? I don't see any discussion of that in the text.
- More discussion of disagreement between your shell ages and previously published constraints on deglaciation. You are making some pretty strong claims about the timing of Puget Lobe deglaciation (several kyr later than previous work suggests), which need to be backed up by strong evidence. How can you explain the large difference in inferred deglaciation ages, and why should readers trust your chronology over others? I don’t see much of this in the current version of the paper.
- The marine reservoir correction in the Puget lowlands is controversial, and I’m not sure that calculating the latest Pleistocene reservoir correction using ~1.8 ka shells resolves the issues brought up by other authors. The marine reservoir correction along the northeast Pacific coast has been shown to be variable during the late Pleistocene (e.g., Schmuck et al., 2021), and as you acknowledge, there has been a good bit of work on the MRC around the southwestern CIS already. I recommend including a more thorough discussion of the literature on this topic if you intend to bring it up in the introduction as a main goal of your paper (e.g., line 90).
- Is it possible that the quartz grains used for OSL dating in the glacial outwash deposits were only partially bleached, leading to too-old burial ages (common in glacial environments)? Could this be another explanation for the disagreement with 14C and OSL in the “pre-LGM” units?
- The main title of the paper relates the inferred CIS stability to solid-Earth processes, but the discussion section barely touches on this point – it’s just a few lines. If you want to keep the title as-is, then I strongly recommend expanding this section of the discussion.
Comments on existing figures:
- The colors on figure 1 are somewhat difficult to interpret because the unit numbers/colors aren’t correlated between sites (e.g., the dark purple unit at Double Bluff is not correlative with the dark purple unit at Penn Cove). Please make the key larger and label the stratigraphic units more clearly on the stratigraphic columns.
- Figure 1 also appears to show samples that have no ages (e.g., uppermost shell icon on double bluff site, when the text says that no samples were collected in the upper shell-bearing units).
- In the caption for Figure 1 (line 142), I am not sure what “white dots indicate changes to site collection of samples” means, or what the white dots actually show on figure 1.
- I also found figure 2 difficult to interpret. The blue/gray colors (at least on my screen) were hard to distinguish from one another. And I’m also curious as to why there isn’t a timeline on the side that more clearly shows the ages? It was inconvenient to go back and forth between the table, key for the units, and the main panel of the figure. And it’s not clear which ages these are in the results tables – consider adding sample IDs somewhere on this figure to make comparison easier.
A few technical comments:
- Line 433: not sure what “respectively” means here
- Line 501: Should this be MIS 3, not MIS 5?
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC1 -
AC1: 'Reply on RC1', Marion McKenzie, 08 Feb 2024
Firstly, thank you Anonymous Referee #1 for your constructive feedback. We hope to address all of your comments through the following revisions.
To major comment 1, this is an important improvement that can be made to frame our work in the context of previous work conducted in the Puget Lowland. I intend to include pieces currently in the supplemental material and discussion within a new section of the introduction to provide this background. This added section will include outcrop work and 14C constraints from the 1960s and 70s conducted by Don Easterbrook and others, as well as more recent outcrop work conducted by Brian Demet and others in 2019. Including more information from Brian Demet and other’s 2019 work will provide more context on the study of grounding zone wedges on Whidbey Island as well. Using Ice-D cosmogenic nuclide dates (with possibility for date recalculation, if raw data is available) from around the Puget Lowland will be included in the refined geochronology. This analysis, including dates from Balbas et al.(2017) and Shaun Marcott’s dissertation will provide greater context and a brief overview of Puget Lobe behavior over the last 50,000 years.
Including the aforementioned context, specifically, known 14C and exposure dates as well as historically marked margins in figure 1 will greatly improve our ability to appropriately orient readers to the region. An updated figure 1 will be included in the revised manuscript.
In response to major comment 3: an updated figure 2 with stratigraphic and grain size results will include some pictures of outcrop units. The radiocarbon dead wood found at Double Bluff is thought to be associated with remobilization of old material. This will be stated more clearly in the discussion. In the revised discussion, I will also address your point of the young deglacial OSL age from Fort Casey Site 1 – the sample with the 9.3ka age has a very larger overdispersion (78%). It has a large fraction of grains with low doses which indicate that young or light exposed grains were mixed with the actual sample. Possible reasons could be from bioturbation or the mixing with the light exposed surface during collection. We therefore cannot attribute too much relevance to this sample because of the disturbance. Due to the unreliable age, I will remove this sample result from the figure. Variation between subaerial and submarine outwash is identified by the presence or lack of shells in the deposits, primarily. This is highlighted in the discussion when the facies are defined, but I will make this more clear in the revised manuscript.
To your last point in major comment 3: The disagreement between our dates and those published in the literature could be due to the lack of stratigraphic context paired with previous radiocarbon dates. In the updated manuscript, I will highlight gaps in previous work and how this works’ ages contribute to this context.
Related to major comment 4, discussion of the marine reservoir correction as a main finding of this paper will be removed from the introduction. When this correction is discussed in the methods, the limitations due to the Pleistocene variability will be added. I will also include (Schmuck et al., 2021) as a reference in discussing variability and limitations of the latest Pleistocene reservoir correction we have developed.
In response to your major comment 5, some of the samples had a larger overdispersion (>20%) which indicates some mixing of grains or only partial bleaching. This includes Fort Casey Site 1 OSL1 (age is going to be removed, anyway) and samples from West Beach and Penn Cove Site 2, OSL1. Limitations and potential sources of error for the OSL ages, while provided in the supplemental material, will be added to the OSL methods section to better address this comment.
I believe your major comment 6 is well-justified. In light of this comment, and due to the relation between landscape evolution and glacial movement but not showcasing a causal relationship, I aim to change the title to "Spatial variability in marine-terminating ice sheet retreat in the Puget Lowland" in the revised manuscript.
Related to your comments on the figures, I intend to create a new figure 1 with regional context. A new figure 2 will include the stratigraphic columns and grain size data. In the newly developed figure 2, I will include images of the facies for examples, make the key more legible, and create a new classification system so as to not confuse coloring between units not correlated at various sites. The purpose of including the shell icon in regions where shells were not collected was to indicate that although the unit was shell bearing (such as the uppermost unit at Double Bluff), we were physically unable to reach a sample for collection. I would like to keep the indication that the unit is shell bearing, despite lacking an age, but I will make this distinction more clear in the new figure 2 text. In the newly developed figure 2, I will also clarify what is meant with the white dots. The stratigraphic columns developed for this figure are a conglomerate of visible units across several locations at each site. The white dots are meant to indicate the end of one visible region and start of a new location where visible units are mapped.
In the revised manuscript, the original figure 2 will become figure 3. I will adjust the colors so they are more distinct from one another. Additionally, I will integrate the results table of ages with the main panel to make interpretation more intuitive. I will also include sample numbers in the new timeline to make comparison easier. Thank you for this suggestion.
Line 433: I will remove the word “respectively” to make this sentence more clear.
Line 501: You are correct, this should read “MIS 3” not “MIS 5”. I will make this change in the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC1
- I’d like to see a thorough treatment of the existing constraints on CIS and landscape evolution in the Puget Lobe region. Some review of the literature is present in the supplemental material, but I believe these points should appear in the main paper, and that they should appear early (rather than being brought up at the end of the discussion section). Points that need more discussion earlier in the manuscript include:
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RC2: 'Comment on egusphere-2023-2725', Anonymous Referee #2, 21 Dec 2023
In this manuscript, McKenzie et al. present stratigraphic sedimentary analyses, new radiocarbon and OSL ages from coastal outcrops on Whidbey Island, Puget Sound, to reconstruct the behavior of the marine-terminating Puget Lobe during deglaciation. Their scenario differs from that of earlier studies, mainly in that final deglaciation occurred 3000 years later, i.e. ~12 ka.
The manuscript is globally well written, and the methods are well presented. However, figures need to be improved, as detailed below. Also, the new results and interpretation should be put better into the context of earlier findings. I read reviewer 1’s comments, which I agree with and which are very similar to the concerns I had while reading the manuscript. As reviewer 1, I am not a sedimentologists, and cannot assess the quality of the newly proposed marine reservoir correction, either.
Here are my major comments and suggestions:
Given that the subject of this study has been investigated earlier in numerous studies over several decades, I find it hard to understand what has been done and found before. In section 1.1 only a few lines present the state-of-the-art of the subject. The more detailed information given in the supplement should be moved to the main text. It would really be helpful if these earlier findings and controversies could somehow be illustrated in a more comprehensive manner. Would it be possible to show them on a map of the region?
Also, please better specify what is new in your study. E.g. lines 89-90 say that there is “lack of detailed stratigraphic context for age constraints”. Does this mean your study is the first to present detailed sedimentological analyses? Which other studies have used the same approach (at the same sites or different?), and how do your new analyses compare to theirs? Regarding dating, have your sites ever been dated before?
In the discussion your findings and interpretations should be compared to the earlier ones and you should give arguments why your interpretation is more reliable. E.g. specify if the observations and interpretations of all of your seven profiles are consistent with each other and with the proposed scenario? How do you explain the high discrepancy between the earlier and the new 14C ages? Are there any earlier data or observations that support your model?
Figures:
Figure 1: the text is way too small. A legend is missing for the signature of the sediment profiles and the dating symbol (shell etc).
Line 141: what does “correlations” mean here? E.g. yellow stands for the same grain size range in each profile?
Line142: I do not understand what the white dots indicate.
I suggest to make A, B and C individual figures with width that allow the text to be read. Put A in landscape format? Make the grain size plots bigger.
I think it would give great value to the manuscript if a sketch of the different stages of the glacier behavior could be included, either similar to the drawings in Fig. S2 or as schematic maps.
Other minor comments:
Line 182: The citation of Goehring et al. (2019) is surprising here. These authors use a similar method for a specific procedure for cosmogenic in situ 14C analyses, which is however not described in detail. It would be better to add a citation that relates to NOSAMS, if exists.
Table 1: what does “actual age” mean?
Lines 262: add the reference to the corresponding figure.
Title of section 4: better would be “Interpretation and discussion”
Supporting Information:
- Header: the Journal name is wrong
S1.2: Note that Swanson and Caffee (2001) did not produce cosmogenic nuclide measurements to date the ice retreat, but they used the radiocarbon age of the deglaciation to calibrate cosmogenic nuclide production rates. So, the statement “numerous cosmogenic exposure ages consistently indicate …” is not correct.
It needs to be checked with the editor if the supporting information should include a separate reference list. Right now, the citations in the supporting information are included in the reference list of the main text, i.e. there are references that are not cited in the main text.
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC2 -
AC2: 'Reply on RC2', Marion McKenzie, 08 Feb 2024
Thank you, Anonymous Reviewer 2 for your helpful comments. The following response aims to incorporate your constructive feedback to improve the readability and impact of this manuscript.
Similar to Reviewer 1’s comments, the regional context will be greatly expanded upon in the revised manuscript. This will include a new section in the introduction that will incorporate text from the supplemental material and a newly adapted figure 1 that will include previously collected information and visual interpretations. Also incorporating your later figure comments, revised figures 1 and 2 will have enlarged text that is easier to read as well as a legend that incorporates all of the symbols used in the stratigraphic columns.
As part of the updated introduction section with a greater number of regional findings, I will more clearly specify the importance of this work within the context of what has already been studied in the region. I will include site-specific information here that ties in known sedimentology and dating histories of the specific sites for which I present new data. Related to a comment from Reviewer 3, this section will also include an overview of what is known from geomorphic data in the region.
The revised manuscript will better incorporate the regional context with our findings. I will discuss possible reasons for discrepancies between my data and known information.
Line 141: correct – the color of the stratigraphic unit (e.g., yellow) is the same color of the grain size range profile.
The white dots, meant to convey the end of visible units at one location and start of newly described sections within a site, will be better described in the figure description. Reviewer 1 also raised this point, and due to the lack of a continuous stratigraphic column at every site, they are important in clarifying the number of locations within a site that are used to develop the overall stratigraphic interpretation. This will be made more clear in the figure caption.
Section B will be developed into standalone figure 1, section A will be developed into the new figure 2 which will include images from the field, section C will be moved to supplemental information, and the current figure 2 will become figure 3 in the revised manuscript. The grain size plots for figure 2 in the revised manuscript will be enlarged so text and data are more visible.
Figure S2 from the supplemental material will be adapted using comments from Reviewer 3 and possibly added to figure 3 to bolster the interpretations of timing.
Related to your minor comments:
I will change the citation of Goehring et al., 2019 in line 182 to NOSAMS, 2023 (https://www2.whoi.edu/site/nosams/wp-content/uploads/sites/124/2023/02/General-Statement-of-14C-Procedures_2023.pdf), thank you for clarifying this reference.
“Actual age” in Table 1 will be changed to “calendar year before present (BP) age”.
Line 262 – I will add the reference to figure 2 in the revised manuscript to this line.
The title of section 4 will be changed to “Interpretation and Discussion” in the revised manuscript.
Unfortunately, the published preprint was uneditable prior to submission to this journal. Changing the journal name to Climate of the Past in the supplemental material will be a priority in submitting the revised manuscript!
Thank you for the clarification to the data provided from Swanson and Caffee (2001). I will make sure the findings from their work are represented properly throughout the revised manuscript by changing the statement from “numerous cosmogenic exposure ages [...]” to “cosmogenic nuclide production rates, calculated from wide-ranging radiocarbon ages marking deglaciation, indicate [...]”.
I will clarify the journal’s guidelines on supporting information citations and will create a separate citation list if recommended. Thank you for this suggestion.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC2
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RC3: 'Comment on egusphere-2023-2725', Anonymous Referee #3, 05 Jan 2024
The manuscript, Evidence of solid Earth influence on stability of the marine-terminating Puget Lobe of the Cordilleran Ice Sheet (CIS) describes new glacial, deglacial, and coastal sedimentological and geochronological data from Whidbey Island in the Puget Lowland of Washington state, USA. The dates span the past approx. 50000 years or longer and the sediments record pre-LGM, LGM, and post-LGM environmental and relative sea level change, including glaciation and deglaciation. A lot of new, detailed sedimentological data are presented here, covering an interesting area near the southernmost margin of the former CIS and I agree with the authors that this area provides a unique opportunity to assess shallow marine and coastal ice-sheet retreat. The sedimentological data, although not presented as clearly as it could be, looks solid, and the dating, although there are some issues as described by the authors, is still useful.
I share many of the same concerns about the manuscript as the other two anonymous reviewers, including that the title is somewhat problematic. I feel that the significance of this work is also not well-anchored in the cited literature and needs more explanation of its relevance (specifically how this story is relevant to Greenland and Antarctica). Furthermore, one of the key findings of the article, that there was a pause in overall glacial retreat between approx. 12.1-12.9 ka BP (due to GIA-caused emergence providing stability to the ice), may be problematic. The five radiocarbon-dated shells from the Penn Cove section may not actually represent an approximately 1000-year timespan, given that they all more or less overlap within their 2 (or 1) sigma calibrated ranges (whether it is 1 or 2 sigma is not stated). I expand on all of these concerns in the line-by-line comments below.
Comments are organized according to the following themes and described below: figures; referencing and literature review; writing and organization; sedimentology, geomorphology, dating
Figures
As stated by Reviewers 1 and 2, Figure 1 is difficult to read and understand. A few points:
1A. Why are the strat logs, grain size, and MS data presented in the Introduction? These are results and as such, should be included in the Results section. I think a bigger, better map should be included as Figure 1 in the Introduction section, however, with the strat logs, etc. presented in the Results as its own figure.
The lower 14C date from Double Bluff says 48.0 +(?) kya (also on Figure 2), but in the text you have written 46.7+ kya.
A few things are missing from Figure 1, including a legend for the strat column fill patterns, symbols, and colours. Also, what are the white dots?
Generally speaking, larger font size and larger strat column drawings with a legend are needed. Especially if there are no photos of the sections.
For the Double Bluff grain size distribution curves, I cannot see the red curve - can it be placed over the purple curves?
What are the extra, discontinuous columns with different fill patterns on the Penn Cove, West Beach Site 2, and Cliffside strat columns?
The grain size distribution curves would be more reader-friendly if you added the names of the grain size categories below the micron scale on the x axis. I don't have the size range of pebbles in microns memorized, for example. Not 100% necessary, just a suggestion to make this figure easier to understand quickly.
1B. Map text is much too small and inset is also too small. If this were its own figure in the Introduction, you could make it bigger and more clear. Would be helpful to see previous reconstructions of LGM and deglacial ice sheet margins on the same map. Photos of the strat sections would also be helpful, but I appreciate that you put a couple in the Supplementary Info and also that there may be strict limits with the journal.
1C. Why do some outcrops have low-resolution sampling and others are higher?
Figure 2
A legend is needed for the symbols representing the dates. The shells and suns are fairly intuitive, but the wood/plant material is not.
With respect to the right-hand column labels, is deglaciation not also post-LGM? Also, you have dates of 6.2 and 4.1 ka BP from West Beach Site 1 units classified as ‘Deglaciation’. Was deglaciation still going on this late here? I don’t interpret that from reading the text.
Referencing and literature review:
A more thorough presentation of previous literature describing the last glaciation and deglaciation of the Puget lowland is needed. As Reviewer 1 mentioned, a pretty good review of the literature is included in the Supporting Information, and it was very helpful; however, it should be in the main paper.
Some important interpretations are made that are supported by only one or a few, sometimes quite old, references even though there is a great deal of highly relevant, peer-reviewed literature that could be used to better support your interpretations. For example:
Page 3, lines 71-74 and Page 18 line 572-573: I cannot see clearly where the Whitehouse et al. 2019 and Nield et al. 2014 references indicate that the rheology of the study area is similar to that of the Antarctic Peninsula, although I did not closely read these papers. I am also not sure what you mean by topographic similarities to Greenland – both mountainous? But there are many other factors that control the timing and style of deglaciation, so I am not convinced of the relevance being attributed to this.
Page 5, lines 147-152: references and more explanation of what you mean by multiple approaches, differing classifications, etc., are needed in this paragraph.
Page 12, lines 397-398. I am not a glacial sedimentologist, but I do not think the Boulton and Deynoux, 1981 reference is appropriate on its own, to classify a deposit or unit as glacial outwash. There are more recent textbooks and articles that should be referred to and utilized.
Page 13, line 410: winnowing is a process, not a sedimentary characteristic. Do you mean an absence of fines?
Page 13, line 413: seaward typo. Also, should it be sampled rather than samples? I had some trouble understanding this section.
Page 13, line 428-430: There is no mention of cross-bedded sand with parallel-to-bed oriented clasts in Unit 3 West Beach Site 1 (page 11, lines 332-333).
Page 13, lines 434-435: RSL fall at this time would be a function of GIA or tectonics, so I don’t think you need to state both ‘RSL fall outpacing eustatic SLR’ and ‘GIA response’ as separate factors in this sentence. Coarsening grain sizes may also be related to ice sheet/glacier proximity and have nothing to do with RSL fall/landscape emergence.
Page 13, lines 442-445: A study documenting coarsening-upward grain size trends in coastal outcrops interpreted as a transition from a marine to coastal environment on northern Svalbard is used to support your assertion that facies transitions where grain sizes coarsen up are associated with landscape emergence. Only two references are listed here, although the McCabe, 1986 reference is a study of glaciomarine facies on Northern Ireland, not Svalbard. Earlier in this paragraph two other references are listed, but both are sedimentology textbooks.
On page 14, lines 453-456, you write that facies transitions where grain sizes fine upwards and are accompanied by the appearance of marine shells are associated with submergence. Fining-upward grain-size trends and the appearance of marine sub-fossil shells represent solid evidence for deepening water/RSL rise - this is no problem. Your presentation of your interpretation here, however, is weakened by the references you use to support your assertion - two sedimentology textbooks and one article from 1986 that links similar facies transitioning from a subaerial to submarine environment in the offshore SW Pacific/onshore New Zealand. It feels a bit random and not quite strong enough to justify the use of the phrasing in the final sentence of this paragraph ('Therefore, the fining of material between Unit 4... are all interpreted as a transition to a submarine setting.'). This interpretation and the study as a whole needs to be better situated in existing, relevant literature.
Writing and organization
General comment: Just refer to diamict deposited subglacially as till, not glacial till. The glacial part is redundant. David Evans’ 2007 entry on Tills in the Encyclopedia of Quaternary Science provides a good overview of the correct terminologies for different glacial diamictons.
On page 14, the sentence beginning on line 439 (which goes to line 442) is somewhat confusing as written. I think you're saying that tectonic activity as a cause for coarsening-upward trends in the stratigraphy can be rejected because the till and sedimentary structures like cross-bedding have been preserved. If so, I think a more clear explanation of why the till and sedimentary structures wouldn't be preserved if tectonics caused land-level changes forcing a fall in relative sea level and coarsening-upwards trends in the stratigraphy is needed. Not sure if that's what you're driving at here, but the fact that I'm not sure means it could probably use a better explanation.
Suggest that you go through the manuscript and check your use of dashes. Lowermost, southernmost, northernmost do not need dashes, for example.
Text size and font seem to change in some places.
Figure 1B is referred to before Figure 1A or Figure 1 in the text.
Page 3, lines 66-67: you have already spelled out the acronym GIA on page 2, line 57 and do not need to do this again.
Page 3, line 100: It should be rate, not magnitude of landscape emergence (cm/a).
Page 5, figure caption, line 141: grain spelled incorrectly.
Page 6, line 156: Suggest: Sediment samples were collected from …
Page 6, line160: ‘…referred to as lenses…’.
Page 7, lines 196-197: ‘…range in beach-front collection date from 1911 to 1931 … and include species …’. This is awkwardly written/grammatically incorrect, and I am not sure why the font/size change for the bivalve spp. The genus and species should be italicized only (with capitalized first letter for genus name). It is also important to state that the modern bivalves were live-collected, i.e. not just shells that were collected, between 1911 and 1931.
Page 7, Table 1 plus lines 194-203. I am not sure why this is included in Methods, since this is new data and should therefore be in the Results section. It might also be important to clarify that the reservoir correction you have determined isn’t necessarily relevant for earlier periods, for example, during early deglaciation of this area. I realize that it is difficult to develop reservoir corrections through the Holocene and beyond, and great that you have determined one from live-collected bivalves in this area.
Page 7, line 211. You have an extra ‘and’ in this sentence between (Th), and potassium (K).
Page 7, line 221: Why write Materials instead of sediments?
Page 8, line 256: considerably spelled incorrectly.
Page 9, lines 262-264: Not sure what you mean here. What are ‘all possible interpretations that have been conducted across Whidbey Island’?
Page 10, line 297. A bit more clear explanation of what your sites 1 and 2 mean for Fort Casey and West Beach sections mean is needed earlier in the manuscript. I finally realized here that you mean that you logged one part of the section (Site 1) separately from a lower part of the section (Site 2), likely because a continuous section from top to bottom of the cliff face wasn’t accessible or visible or …
Page 12, line 388: The use of the term structure here is a bit confusing when you correctly pointed out at the beginning of this section that structureless diamicton is classified as till.
Page 16, line 508: choose glacimarine or glaciomarine and be consistent throughout the manuscript.
Second paragraph, second sentence in Supporting Information: Figure S2 does not depict a schematic of glacial retreat within a marine environment versus glacial retreat within a subaerial environment (at least according to the arrows drawn on Puget Lobe) – rather it shows Puget lobe advancing across a subaerial environment and retreating in a submarine environment.
Sedimentology, geomorphology, dating
As I read through the descriptions of the strat sections, I was curious about the glacial geomorphology on Whidbey Island. Has it been mapped? How does it relate to your strat sections? It seems to me that it could be quite helpful in interpreting some of this data.
Page 17, lines 515-518: This needs a bit more explanation. When I look at the shell dates on Figure 1 and in Table 1, it appears that all of the ages overlap to some extent. But what are the dates as presented? The midpoints of the 2 sigma ranges? This needs to be indicated in Table 1 and stated in the text. The overlap in ages opens the possibility that there was a potentially much shorter period of time with very high sedimentation rates at this particular location, rather than a 1000-year pause in glacier retreat around this time. Or could it be possible that this is a slide or slump deposit that didn’t manage to dis-articulate the shells? Either way, unless you have very clear peak probabilities within the calibrated age ranges indicating that one specimen is very close to 12000 years old and another very close to 13000 years old, it is difficult to say with confidence that they (together with sedimentological evidence) represent a 1000 year pause in deglaciation.
Page 17, lines 528-532: This is the first mention of grounding zone wedges. Where have they been mapped? Are they dated? I also can’t tell with the way the references are inserted into the text here if these are the references for the GZWs.
Final comment: I really appreciated the acknowledgements and wish I saw this on every paper. Well done.
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC3 -
AC3: 'Reply on RC3', Marion McKenzie, 08 Feb 2024
Thank you, Reviewer 3, for your comments and in-depth suggestions. We aim to sufficiently address your comments in the revised manuscript by making the following adjustments:
In the revised manuscript, a new figure 1 with greater regional context will be developed. All results, including stratigraphic logs, grain size, and MS data will be presented in a newly developed figure 2. Greater context for the results figure including an updated legend with all symbols and better explanation for the white dots in the figure description will be developed. Thank you for catching my error in the Double Bluff 14C date. I will update this and any other age discrepancies in the revised manuscript. Photos will be added to the new results figure 2 and legibility will be improved by increasing the font size. I will also adjust the order of grain size curves so underlying data are more visible, such as in the case of Double Bluff. The discontinuous fill patterns on some of the stratigraphic columns are where sedimentary structures are seen in the outcrops. I will add these fill patterns to the legend as well as the figure text in the revised manuscript. Adding the grain size categories to the grain size graph x-axis would be a great addition to these plots. I will add these in the revised figure, thank you for this suggestion.
Figure 1B will be converted to its own figure (figure 1 in the revised manuscript), including all of the suggestions you recommended to provide greater regional context in the introduction.
I will clarify sampling resolution for 1C in the updated figure description, but variable sampling resolution is a product of accessibility to outcrops from the beach front. Some areas were more readily accessible than others.
When updating figure 2 (which will become figure 3 in the revised manuscript), I will include a legend for the symbols representing the dates. You are correct in your comment that deglaciation is also post-LGM, so in this figure I will also reconsider the use of the terms “post-LGM” and “deglacial”. Dates from the deglacial deposits include sediments sourced from outwash, so I will change the word “deglacial” to better contextualize the ages collected from the outwash.
I will add the background text on Puget Lowland glaciation from the supporting information to a newly developed section within the Introduction.
In reference to page 3, lines 71-74 and page 18 line 572-573, Nield et al.(2014) and Whitehouse et al.(2019) do not explicitly state the rheology similarities between the regions; I am drawing the connection between these two publications independently. I will make this more apparent in the revised manuscript. Additionally, the topographic setting of a sedimentary basin surrounded by mountains is the topographic similarity being drawn between the Puget Lowland and outlet glaciers in Greenland, previously established from Eyles et al.(2018). I will also make this point more clear in the revised manuscript.
I will change the references and increase explanations for the highlighted areas of page 5, lines 147-152 and page 12, lines 397-398.
Page 13, line 410: I will clarify “winnowing” is meant to be an “absence of fines”. The process I’m referencing is the removal of fines via tidal currents in a submarine or coastal setting.
Page 13, line 413: in the revised manuscript, these typos will be corrected.
I will clarify continuity between page 13 lines 428-430 and page 11 line 332-333.
Page 13, lines 434-435. I will clarify the fall of sea level rise as a function of glacial isostatic adjustment in this context. Due to the lack of any additional evidence for glacial proximity to these locations from the transgressive and regressive facies, I do, however, maintain the interpretation that the pre-LGM coarsening of material is more likely to indicate sea level and glacial isostatic adjustment changes than glacial proximity in this location.
Page 13, lines 442-445: I will clarify the difference between the two references in this line and will find new references for the aforementioned lines for interpretations of coarsening-upward sediments. Similar to your comment on lines 453-456 on page 14, I will find better references to situate my interpretations.
Thank you for directing me to David Evans’ 2007 entry on tills – I will change all occurrences of “glacial till” to simply “till” in the revised manuscript.
You are correct in the interpretation of lines 439-442 on page 14. I will rewrite this sentence with your recommended edits.
The revision of the figures, into a new figure 1 with regional context and a new figure 2 with stratigraphic and grain size data, will resolve your comment related to figure 1B being referenced before figure 1A.
In the revised manuscript, the use of dashes and text size and font will be refined per your suggestion.
Thank you for catching the grammatical and spelling errors on pages 3 through 8. I will make corrections for all of the line errors you mentioned.
Your corrections for the bivalves collected for the marine reservoir age will be included in the revised manuscript. Additionally, Table 1 will be presented in Results as you suggest.
Page 9, lines 262-264: the statement ‘all possible interpretations that have been conducted across Whidbey Island’ will be reworded or removed for better clarity in these lines.
In reference to your comment given to line 297 on page 10, I will make this distinction more clear earlier on, specifically in the Methods so the distinction between locations with multiple sites is explicit.
Page 12, line 388: I will change the use of the word “structure” to “facies” in this sentence.
Page 16, line 508: In the revised manuscript, glaciomarine will be the consistently used term. All instances of “glacimarine” will be changed to “glaciomarine”.
The Figure S2 schematic will be changed to reflect the text and then included in the new figure 3. Rather than showing ice advance into a subaerial environment, the new schematic will show glacial retreat within a subaerial environment.
Page 17, lines 515-518. Thank you for this comment, however, it seems unlikely that these shells are part of a slide or slump deposit due to their growth-position (images will be included in the newly developed figures) within a region of continuous stratigraphy where there is little slope that could have supported a slump. I will clarify the overlap in ages and present an updated span of ages based on the 2 sigma range in the radiocarbon dates from Penn Cove.
I agree that adding information about glacial geomorphology across the Puget Lowland will be helpful in interpreting the stratigraphic data. In the discussion of the revised manuscript, I intend to include a paragraph depicting what is known of glacial dynamics in the region from glacial geomorphology. Related to your comment on the grounding zone wedges (GZWs) on page 17 lines 528-532, I will add a more thorough review of this data in the newly developed geomorphic section of the Discussion. I will also introduce the GZWs earlier in the paper, likely in the Introduction to create some background knowledge of past ice behavior in the region.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC3
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AC3: 'Reply on RC3', Marion McKenzie, 08 Feb 2024
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-2725', Anonymous Referee #1, 18 Dec 2023
In this manuscript, McKenzie et al. present stratigraphic and chronological constraints on the retreat of the Puget Lobe of the Cordilleran Ice Sheet (CIS) in Washington State. This area has been studied by glacial geologists for many years, but there is still some uncertainty about its history. McKenzie et al.’s work largely relies on facies analysis of sediment outcrops on Whidbey Island, supplemented by 14C dating of marine shells and OSL dating of quartz. From these data, the authors construct a general timeline of events across Whidbey Island, from prior to the LGM up to the present. A key conclusion of the manuscript is that the Puget Lobe lingered near Whidbey Island for several thousand years longer than previous work indicates; this interpretation is based on 14C-dated shells in a unit that the authors identify as glaciomarine. McKenzie et al. then suggest that the Puget Lobe was stabilized on Whidbey Island by solid-Earth uplift, topographic pinning points, and grounding zone wedges.
I think the sedimentological descriptions and facies interpretations mostly make sense (although that is not my primary research area, so I will yield to other reviewers on this point), but the major weaknesses of the manuscript in its present form are (1) chronology and (2) how these results fit in with existing age constraints in the Puget Lowland. In my view, the authors need to better situate their work in the context of previous research, and offer more thorough (and convincing) explanations for why their results differ so much from what has already been published.
My comments on this submission are below. As I have focused most of my attention on what I believe are major issues with the manuscript in its present form, I have only provided a few line-by-line comments. It is my hope that all of these comments are helpful for the authors as they revise the manuscript.
Specific major comments:
- I’d like to see a thorough treatment of the existing constraints on CIS and landscape evolution in the Puget Lobe region. Some review of the literature is present in the supplemental material, but I believe these points should appear in the main paper, and that they should appear early (rather than being brought up at the end of the discussion section). Points that need more discussion earlier in the manuscript include:
- Previous outcrop work in the Puget lowlands
- 14C constraints on the Puget Lobe – marine shells, lake sediments
- Grounding zone wedges on Whidbey Island
- Cosmogenic 36Cl ages on Whidbey Island – ideally these would be recalculated from Swanson and Caffee (2001) as the production rates have changed slightly, but they indicate deglaciation ~15 ka (much earlier than your results suggest)
- Related to point 1, I strongly recommend adding a figure that shows the regional context, as the inset map on figure 1 is not sufficient to set the stage. I recommend that at minimum, you include published stratigraphic column locations, age constraints from 14C and cosmogenic 36Cl, surficial geology (e.g., presence of GZWs), and previously mapped CIS margins (and perhaps the glacial lakes too).
- I would also like to see a more thorough discussion of your chronology and stratigraphy. There are a few things that would help the readers better evaluate your data, and areas that need clarification:
- Pictures of outcrops in the main text, or at least in the supplemental material. Without these, it is difficult to evaluate your interpretations of the outcrops, and therefore the geologic history.
- Double Bluff site: You find 14C-dead woody debris in the “deglacial” unit. How can you explain this? Remobilization of old material?
- Fort Casey site 1: How do you explain a 9.3 ka OSL age in your “LGM” unit? You say it’s a minimum age, but what processes could make that age so much younger than regional deglaciation?
- Fort Casey site 2 and Penn Cove: Unless I’m reading the figure incorrectly (which is possible, see my comments on figures below), you interpret a transition from glacial outwash to landscape emergence between units 4 and 3 at Fort Casey Site 2 and between units 5 and 4 at Penn Cove. Are you interpreting the lower units as submarine glacial outwash? If so, how are you distinguishing that from subaerial outwash? I don't see any discussion of that in the text.
- More discussion of disagreement between your shell ages and previously published constraints on deglaciation. You are making some pretty strong claims about the timing of Puget Lobe deglaciation (several kyr later than previous work suggests), which need to be backed up by strong evidence. How can you explain the large difference in inferred deglaciation ages, and why should readers trust your chronology over others? I don’t see much of this in the current version of the paper.
- The marine reservoir correction in the Puget lowlands is controversial, and I’m not sure that calculating the latest Pleistocene reservoir correction using ~1.8 ka shells resolves the issues brought up by other authors. The marine reservoir correction along the northeast Pacific coast has been shown to be variable during the late Pleistocene (e.g., Schmuck et al., 2021), and as you acknowledge, there has been a good bit of work on the MRC around the southwestern CIS already. I recommend including a more thorough discussion of the literature on this topic if you intend to bring it up in the introduction as a main goal of your paper (e.g., line 90).
- Is it possible that the quartz grains used for OSL dating in the glacial outwash deposits were only partially bleached, leading to too-old burial ages (common in glacial environments)? Could this be another explanation for the disagreement with 14C and OSL in the “pre-LGM” units?
- The main title of the paper relates the inferred CIS stability to solid-Earth processes, but the discussion section barely touches on this point – it’s just a few lines. If you want to keep the title as-is, then I strongly recommend expanding this section of the discussion.
Comments on existing figures:
- The colors on figure 1 are somewhat difficult to interpret because the unit numbers/colors aren’t correlated between sites (e.g., the dark purple unit at Double Bluff is not correlative with the dark purple unit at Penn Cove). Please make the key larger and label the stratigraphic units more clearly on the stratigraphic columns.
- Figure 1 also appears to show samples that have no ages (e.g., uppermost shell icon on double bluff site, when the text says that no samples were collected in the upper shell-bearing units).
- In the caption for Figure 1 (line 142), I am not sure what “white dots indicate changes to site collection of samples” means, or what the white dots actually show on figure 1.
- I also found figure 2 difficult to interpret. The blue/gray colors (at least on my screen) were hard to distinguish from one another. And I’m also curious as to why there isn’t a timeline on the side that more clearly shows the ages? It was inconvenient to go back and forth between the table, key for the units, and the main panel of the figure. And it’s not clear which ages these are in the results tables – consider adding sample IDs somewhere on this figure to make comparison easier.
A few technical comments:
- Line 433: not sure what “respectively” means here
- Line 501: Should this be MIS 3, not MIS 5?
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC1 -
AC1: 'Reply on RC1', Marion McKenzie, 08 Feb 2024
Firstly, thank you Anonymous Referee #1 for your constructive feedback. We hope to address all of your comments through the following revisions.
To major comment 1, this is an important improvement that can be made to frame our work in the context of previous work conducted in the Puget Lowland. I intend to include pieces currently in the supplemental material and discussion within a new section of the introduction to provide this background. This added section will include outcrop work and 14C constraints from the 1960s and 70s conducted by Don Easterbrook and others, as well as more recent outcrop work conducted by Brian Demet and others in 2019. Including more information from Brian Demet and other’s 2019 work will provide more context on the study of grounding zone wedges on Whidbey Island as well. Using Ice-D cosmogenic nuclide dates (with possibility for date recalculation, if raw data is available) from around the Puget Lowland will be included in the refined geochronology. This analysis, including dates from Balbas et al.(2017) and Shaun Marcott’s dissertation will provide greater context and a brief overview of Puget Lobe behavior over the last 50,000 years.
Including the aforementioned context, specifically, known 14C and exposure dates as well as historically marked margins in figure 1 will greatly improve our ability to appropriately orient readers to the region. An updated figure 1 will be included in the revised manuscript.
In response to major comment 3: an updated figure 2 with stratigraphic and grain size results will include some pictures of outcrop units. The radiocarbon dead wood found at Double Bluff is thought to be associated with remobilization of old material. This will be stated more clearly in the discussion. In the revised discussion, I will also address your point of the young deglacial OSL age from Fort Casey Site 1 – the sample with the 9.3ka age has a very larger overdispersion (78%). It has a large fraction of grains with low doses which indicate that young or light exposed grains were mixed with the actual sample. Possible reasons could be from bioturbation or the mixing with the light exposed surface during collection. We therefore cannot attribute too much relevance to this sample because of the disturbance. Due to the unreliable age, I will remove this sample result from the figure. Variation between subaerial and submarine outwash is identified by the presence or lack of shells in the deposits, primarily. This is highlighted in the discussion when the facies are defined, but I will make this more clear in the revised manuscript.
To your last point in major comment 3: The disagreement between our dates and those published in the literature could be due to the lack of stratigraphic context paired with previous radiocarbon dates. In the updated manuscript, I will highlight gaps in previous work and how this works’ ages contribute to this context.
Related to major comment 4, discussion of the marine reservoir correction as a main finding of this paper will be removed from the introduction. When this correction is discussed in the methods, the limitations due to the Pleistocene variability will be added. I will also include (Schmuck et al., 2021) as a reference in discussing variability and limitations of the latest Pleistocene reservoir correction we have developed.
In response to your major comment 5, some of the samples had a larger overdispersion (>20%) which indicates some mixing of grains or only partial bleaching. This includes Fort Casey Site 1 OSL1 (age is going to be removed, anyway) and samples from West Beach and Penn Cove Site 2, OSL1. Limitations and potential sources of error for the OSL ages, while provided in the supplemental material, will be added to the OSL methods section to better address this comment.
I believe your major comment 6 is well-justified. In light of this comment, and due to the relation between landscape evolution and glacial movement but not showcasing a causal relationship, I aim to change the title to "Spatial variability in marine-terminating ice sheet retreat in the Puget Lowland" in the revised manuscript.
Related to your comments on the figures, I intend to create a new figure 1 with regional context. A new figure 2 will include the stratigraphic columns and grain size data. In the newly developed figure 2, I will include images of the facies for examples, make the key more legible, and create a new classification system so as to not confuse coloring between units not correlated at various sites. The purpose of including the shell icon in regions where shells were not collected was to indicate that although the unit was shell bearing (such as the uppermost unit at Double Bluff), we were physically unable to reach a sample for collection. I would like to keep the indication that the unit is shell bearing, despite lacking an age, but I will make this distinction more clear in the new figure 2 text. In the newly developed figure 2, I will also clarify what is meant with the white dots. The stratigraphic columns developed for this figure are a conglomerate of visible units across several locations at each site. The white dots are meant to indicate the end of one visible region and start of a new location where visible units are mapped.
In the revised manuscript, the original figure 2 will become figure 3. I will adjust the colors so they are more distinct from one another. Additionally, I will integrate the results table of ages with the main panel to make interpretation more intuitive. I will also include sample numbers in the new timeline to make comparison easier. Thank you for this suggestion.
Line 433: I will remove the word “respectively” to make this sentence more clear.
Line 501: You are correct, this should read “MIS 3” not “MIS 5”. I will make this change in the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC1
- I’d like to see a thorough treatment of the existing constraints on CIS and landscape evolution in the Puget Lobe region. Some review of the literature is present in the supplemental material, but I believe these points should appear in the main paper, and that they should appear early (rather than being brought up at the end of the discussion section). Points that need more discussion earlier in the manuscript include:
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RC2: 'Comment on egusphere-2023-2725', Anonymous Referee #2, 21 Dec 2023
In this manuscript, McKenzie et al. present stratigraphic sedimentary analyses, new radiocarbon and OSL ages from coastal outcrops on Whidbey Island, Puget Sound, to reconstruct the behavior of the marine-terminating Puget Lobe during deglaciation. Their scenario differs from that of earlier studies, mainly in that final deglaciation occurred 3000 years later, i.e. ~12 ka.
The manuscript is globally well written, and the methods are well presented. However, figures need to be improved, as detailed below. Also, the new results and interpretation should be put better into the context of earlier findings. I read reviewer 1’s comments, which I agree with and which are very similar to the concerns I had while reading the manuscript. As reviewer 1, I am not a sedimentologists, and cannot assess the quality of the newly proposed marine reservoir correction, either.
Here are my major comments and suggestions:
Given that the subject of this study has been investigated earlier in numerous studies over several decades, I find it hard to understand what has been done and found before. In section 1.1 only a few lines present the state-of-the-art of the subject. The more detailed information given in the supplement should be moved to the main text. It would really be helpful if these earlier findings and controversies could somehow be illustrated in a more comprehensive manner. Would it be possible to show them on a map of the region?
Also, please better specify what is new in your study. E.g. lines 89-90 say that there is “lack of detailed stratigraphic context for age constraints”. Does this mean your study is the first to present detailed sedimentological analyses? Which other studies have used the same approach (at the same sites or different?), and how do your new analyses compare to theirs? Regarding dating, have your sites ever been dated before?
In the discussion your findings and interpretations should be compared to the earlier ones and you should give arguments why your interpretation is more reliable. E.g. specify if the observations and interpretations of all of your seven profiles are consistent with each other and with the proposed scenario? How do you explain the high discrepancy between the earlier and the new 14C ages? Are there any earlier data or observations that support your model?
Figures:
Figure 1: the text is way too small. A legend is missing for the signature of the sediment profiles and the dating symbol (shell etc).
Line 141: what does “correlations” mean here? E.g. yellow stands for the same grain size range in each profile?
Line142: I do not understand what the white dots indicate.
I suggest to make A, B and C individual figures with width that allow the text to be read. Put A in landscape format? Make the grain size plots bigger.
I think it would give great value to the manuscript if a sketch of the different stages of the glacier behavior could be included, either similar to the drawings in Fig. S2 or as schematic maps.
Other minor comments:
Line 182: The citation of Goehring et al. (2019) is surprising here. These authors use a similar method for a specific procedure for cosmogenic in situ 14C analyses, which is however not described in detail. It would be better to add a citation that relates to NOSAMS, if exists.
Table 1: what does “actual age” mean?
Lines 262: add the reference to the corresponding figure.
Title of section 4: better would be “Interpretation and discussion”
Supporting Information:
- Header: the Journal name is wrong
S1.2: Note that Swanson and Caffee (2001) did not produce cosmogenic nuclide measurements to date the ice retreat, but they used the radiocarbon age of the deglaciation to calibrate cosmogenic nuclide production rates. So, the statement “numerous cosmogenic exposure ages consistently indicate …” is not correct.
It needs to be checked with the editor if the supporting information should include a separate reference list. Right now, the citations in the supporting information are included in the reference list of the main text, i.e. there are references that are not cited in the main text.
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC2 -
AC2: 'Reply on RC2', Marion McKenzie, 08 Feb 2024
Thank you, Anonymous Reviewer 2 for your helpful comments. The following response aims to incorporate your constructive feedback to improve the readability and impact of this manuscript.
Similar to Reviewer 1’s comments, the regional context will be greatly expanded upon in the revised manuscript. This will include a new section in the introduction that will incorporate text from the supplemental material and a newly adapted figure 1 that will include previously collected information and visual interpretations. Also incorporating your later figure comments, revised figures 1 and 2 will have enlarged text that is easier to read as well as a legend that incorporates all of the symbols used in the stratigraphic columns.
As part of the updated introduction section with a greater number of regional findings, I will more clearly specify the importance of this work within the context of what has already been studied in the region. I will include site-specific information here that ties in known sedimentology and dating histories of the specific sites for which I present new data. Related to a comment from Reviewer 3, this section will also include an overview of what is known from geomorphic data in the region.
The revised manuscript will better incorporate the regional context with our findings. I will discuss possible reasons for discrepancies between my data and known information.
Line 141: correct – the color of the stratigraphic unit (e.g., yellow) is the same color of the grain size range profile.
The white dots, meant to convey the end of visible units at one location and start of newly described sections within a site, will be better described in the figure description. Reviewer 1 also raised this point, and due to the lack of a continuous stratigraphic column at every site, they are important in clarifying the number of locations within a site that are used to develop the overall stratigraphic interpretation. This will be made more clear in the figure caption.
Section B will be developed into standalone figure 1, section A will be developed into the new figure 2 which will include images from the field, section C will be moved to supplemental information, and the current figure 2 will become figure 3 in the revised manuscript. The grain size plots for figure 2 in the revised manuscript will be enlarged so text and data are more visible.
Figure S2 from the supplemental material will be adapted using comments from Reviewer 3 and possibly added to figure 3 to bolster the interpretations of timing.
Related to your minor comments:
I will change the citation of Goehring et al., 2019 in line 182 to NOSAMS, 2023 (https://www2.whoi.edu/site/nosams/wp-content/uploads/sites/124/2023/02/General-Statement-of-14C-Procedures_2023.pdf), thank you for clarifying this reference.
“Actual age” in Table 1 will be changed to “calendar year before present (BP) age”.
Line 262 – I will add the reference to figure 2 in the revised manuscript to this line.
The title of section 4 will be changed to “Interpretation and Discussion” in the revised manuscript.
Unfortunately, the published preprint was uneditable prior to submission to this journal. Changing the journal name to Climate of the Past in the supplemental material will be a priority in submitting the revised manuscript!
Thank you for the clarification to the data provided from Swanson and Caffee (2001). I will make sure the findings from their work are represented properly throughout the revised manuscript by changing the statement from “numerous cosmogenic exposure ages [...]” to “cosmogenic nuclide production rates, calculated from wide-ranging radiocarbon ages marking deglaciation, indicate [...]”.
I will clarify the journal’s guidelines on supporting information citations and will create a separate citation list if recommended. Thank you for this suggestion.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC2
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RC3: 'Comment on egusphere-2023-2725', Anonymous Referee #3, 05 Jan 2024
The manuscript, Evidence of solid Earth influence on stability of the marine-terminating Puget Lobe of the Cordilleran Ice Sheet (CIS) describes new glacial, deglacial, and coastal sedimentological and geochronological data from Whidbey Island in the Puget Lowland of Washington state, USA. The dates span the past approx. 50000 years or longer and the sediments record pre-LGM, LGM, and post-LGM environmental and relative sea level change, including glaciation and deglaciation. A lot of new, detailed sedimentological data are presented here, covering an interesting area near the southernmost margin of the former CIS and I agree with the authors that this area provides a unique opportunity to assess shallow marine and coastal ice-sheet retreat. The sedimentological data, although not presented as clearly as it could be, looks solid, and the dating, although there are some issues as described by the authors, is still useful.
I share many of the same concerns about the manuscript as the other two anonymous reviewers, including that the title is somewhat problematic. I feel that the significance of this work is also not well-anchored in the cited literature and needs more explanation of its relevance (specifically how this story is relevant to Greenland and Antarctica). Furthermore, one of the key findings of the article, that there was a pause in overall glacial retreat between approx. 12.1-12.9 ka BP (due to GIA-caused emergence providing stability to the ice), may be problematic. The five radiocarbon-dated shells from the Penn Cove section may not actually represent an approximately 1000-year timespan, given that they all more or less overlap within their 2 (or 1) sigma calibrated ranges (whether it is 1 or 2 sigma is not stated). I expand on all of these concerns in the line-by-line comments below.
Comments are organized according to the following themes and described below: figures; referencing and literature review; writing and organization; sedimentology, geomorphology, dating
Figures
As stated by Reviewers 1 and 2, Figure 1 is difficult to read and understand. A few points:
1A. Why are the strat logs, grain size, and MS data presented in the Introduction? These are results and as such, should be included in the Results section. I think a bigger, better map should be included as Figure 1 in the Introduction section, however, with the strat logs, etc. presented in the Results as its own figure.
The lower 14C date from Double Bluff says 48.0 +(?) kya (also on Figure 2), but in the text you have written 46.7+ kya.
A few things are missing from Figure 1, including a legend for the strat column fill patterns, symbols, and colours. Also, what are the white dots?
Generally speaking, larger font size and larger strat column drawings with a legend are needed. Especially if there are no photos of the sections.
For the Double Bluff grain size distribution curves, I cannot see the red curve - can it be placed over the purple curves?
What are the extra, discontinuous columns with different fill patterns on the Penn Cove, West Beach Site 2, and Cliffside strat columns?
The grain size distribution curves would be more reader-friendly if you added the names of the grain size categories below the micron scale on the x axis. I don't have the size range of pebbles in microns memorized, for example. Not 100% necessary, just a suggestion to make this figure easier to understand quickly.
1B. Map text is much too small and inset is also too small. If this were its own figure in the Introduction, you could make it bigger and more clear. Would be helpful to see previous reconstructions of LGM and deglacial ice sheet margins on the same map. Photos of the strat sections would also be helpful, but I appreciate that you put a couple in the Supplementary Info and also that there may be strict limits with the journal.
1C. Why do some outcrops have low-resolution sampling and others are higher?
Figure 2
A legend is needed for the symbols representing the dates. The shells and suns are fairly intuitive, but the wood/plant material is not.
With respect to the right-hand column labels, is deglaciation not also post-LGM? Also, you have dates of 6.2 and 4.1 ka BP from West Beach Site 1 units classified as ‘Deglaciation’. Was deglaciation still going on this late here? I don’t interpret that from reading the text.
Referencing and literature review:
A more thorough presentation of previous literature describing the last glaciation and deglaciation of the Puget lowland is needed. As Reviewer 1 mentioned, a pretty good review of the literature is included in the Supporting Information, and it was very helpful; however, it should be in the main paper.
Some important interpretations are made that are supported by only one or a few, sometimes quite old, references even though there is a great deal of highly relevant, peer-reviewed literature that could be used to better support your interpretations. For example:
Page 3, lines 71-74 and Page 18 line 572-573: I cannot see clearly where the Whitehouse et al. 2019 and Nield et al. 2014 references indicate that the rheology of the study area is similar to that of the Antarctic Peninsula, although I did not closely read these papers. I am also not sure what you mean by topographic similarities to Greenland – both mountainous? But there are many other factors that control the timing and style of deglaciation, so I am not convinced of the relevance being attributed to this.
Page 5, lines 147-152: references and more explanation of what you mean by multiple approaches, differing classifications, etc., are needed in this paragraph.
Page 12, lines 397-398. I am not a glacial sedimentologist, but I do not think the Boulton and Deynoux, 1981 reference is appropriate on its own, to classify a deposit or unit as glacial outwash. There are more recent textbooks and articles that should be referred to and utilized.
Page 13, line 410: winnowing is a process, not a sedimentary characteristic. Do you mean an absence of fines?
Page 13, line 413: seaward typo. Also, should it be sampled rather than samples? I had some trouble understanding this section.
Page 13, line 428-430: There is no mention of cross-bedded sand with parallel-to-bed oriented clasts in Unit 3 West Beach Site 1 (page 11, lines 332-333).
Page 13, lines 434-435: RSL fall at this time would be a function of GIA or tectonics, so I don’t think you need to state both ‘RSL fall outpacing eustatic SLR’ and ‘GIA response’ as separate factors in this sentence. Coarsening grain sizes may also be related to ice sheet/glacier proximity and have nothing to do with RSL fall/landscape emergence.
Page 13, lines 442-445: A study documenting coarsening-upward grain size trends in coastal outcrops interpreted as a transition from a marine to coastal environment on northern Svalbard is used to support your assertion that facies transitions where grain sizes coarsen up are associated with landscape emergence. Only two references are listed here, although the McCabe, 1986 reference is a study of glaciomarine facies on Northern Ireland, not Svalbard. Earlier in this paragraph two other references are listed, but both are sedimentology textbooks.
On page 14, lines 453-456, you write that facies transitions where grain sizes fine upwards and are accompanied by the appearance of marine shells are associated with submergence. Fining-upward grain-size trends and the appearance of marine sub-fossil shells represent solid evidence for deepening water/RSL rise - this is no problem. Your presentation of your interpretation here, however, is weakened by the references you use to support your assertion - two sedimentology textbooks and one article from 1986 that links similar facies transitioning from a subaerial to submarine environment in the offshore SW Pacific/onshore New Zealand. It feels a bit random and not quite strong enough to justify the use of the phrasing in the final sentence of this paragraph ('Therefore, the fining of material between Unit 4... are all interpreted as a transition to a submarine setting.'). This interpretation and the study as a whole needs to be better situated in existing, relevant literature.
Writing and organization
General comment: Just refer to diamict deposited subglacially as till, not glacial till. The glacial part is redundant. David Evans’ 2007 entry on Tills in the Encyclopedia of Quaternary Science provides a good overview of the correct terminologies for different glacial diamictons.
On page 14, the sentence beginning on line 439 (which goes to line 442) is somewhat confusing as written. I think you're saying that tectonic activity as a cause for coarsening-upward trends in the stratigraphy can be rejected because the till and sedimentary structures like cross-bedding have been preserved. If so, I think a more clear explanation of why the till and sedimentary structures wouldn't be preserved if tectonics caused land-level changes forcing a fall in relative sea level and coarsening-upwards trends in the stratigraphy is needed. Not sure if that's what you're driving at here, but the fact that I'm not sure means it could probably use a better explanation.
Suggest that you go through the manuscript and check your use of dashes. Lowermost, southernmost, northernmost do not need dashes, for example.
Text size and font seem to change in some places.
Figure 1B is referred to before Figure 1A or Figure 1 in the text.
Page 3, lines 66-67: you have already spelled out the acronym GIA on page 2, line 57 and do not need to do this again.
Page 3, line 100: It should be rate, not magnitude of landscape emergence (cm/a).
Page 5, figure caption, line 141: grain spelled incorrectly.
Page 6, line 156: Suggest: Sediment samples were collected from …
Page 6, line160: ‘…referred to as lenses…’.
Page 7, lines 196-197: ‘…range in beach-front collection date from 1911 to 1931 … and include species …’. This is awkwardly written/grammatically incorrect, and I am not sure why the font/size change for the bivalve spp. The genus and species should be italicized only (with capitalized first letter for genus name). It is also important to state that the modern bivalves were live-collected, i.e. not just shells that were collected, between 1911 and 1931.
Page 7, Table 1 plus lines 194-203. I am not sure why this is included in Methods, since this is new data and should therefore be in the Results section. It might also be important to clarify that the reservoir correction you have determined isn’t necessarily relevant for earlier periods, for example, during early deglaciation of this area. I realize that it is difficult to develop reservoir corrections through the Holocene and beyond, and great that you have determined one from live-collected bivalves in this area.
Page 7, line 211. You have an extra ‘and’ in this sentence between (Th), and potassium (K).
Page 7, line 221: Why write Materials instead of sediments?
Page 8, line 256: considerably spelled incorrectly.
Page 9, lines 262-264: Not sure what you mean here. What are ‘all possible interpretations that have been conducted across Whidbey Island’?
Page 10, line 297. A bit more clear explanation of what your sites 1 and 2 mean for Fort Casey and West Beach sections mean is needed earlier in the manuscript. I finally realized here that you mean that you logged one part of the section (Site 1) separately from a lower part of the section (Site 2), likely because a continuous section from top to bottom of the cliff face wasn’t accessible or visible or …
Page 12, line 388: The use of the term structure here is a bit confusing when you correctly pointed out at the beginning of this section that structureless diamicton is classified as till.
Page 16, line 508: choose glacimarine or glaciomarine and be consistent throughout the manuscript.
Second paragraph, second sentence in Supporting Information: Figure S2 does not depict a schematic of glacial retreat within a marine environment versus glacial retreat within a subaerial environment (at least according to the arrows drawn on Puget Lobe) – rather it shows Puget lobe advancing across a subaerial environment and retreating in a submarine environment.
Sedimentology, geomorphology, dating
As I read through the descriptions of the strat sections, I was curious about the glacial geomorphology on Whidbey Island. Has it been mapped? How does it relate to your strat sections? It seems to me that it could be quite helpful in interpreting some of this data.
Page 17, lines 515-518: This needs a bit more explanation. When I look at the shell dates on Figure 1 and in Table 1, it appears that all of the ages overlap to some extent. But what are the dates as presented? The midpoints of the 2 sigma ranges? This needs to be indicated in Table 1 and stated in the text. The overlap in ages opens the possibility that there was a potentially much shorter period of time with very high sedimentation rates at this particular location, rather than a 1000-year pause in glacier retreat around this time. Or could it be possible that this is a slide or slump deposit that didn’t manage to dis-articulate the shells? Either way, unless you have very clear peak probabilities within the calibrated age ranges indicating that one specimen is very close to 12000 years old and another very close to 13000 years old, it is difficult to say with confidence that they (together with sedimentological evidence) represent a 1000 year pause in deglaciation.
Page 17, lines 528-532: This is the first mention of grounding zone wedges. Where have they been mapped? Are they dated? I also can’t tell with the way the references are inserted into the text here if these are the references for the GZWs.
Final comment: I really appreciated the acknowledgements and wish I saw this on every paper. Well done.
Citation: https://doi.org/10.5194/egusphere-2023-2725-RC3 -
AC3: 'Reply on RC3', Marion McKenzie, 08 Feb 2024
Thank you, Reviewer 3, for your comments and in-depth suggestions. We aim to sufficiently address your comments in the revised manuscript by making the following adjustments:
In the revised manuscript, a new figure 1 with greater regional context will be developed. All results, including stratigraphic logs, grain size, and MS data will be presented in a newly developed figure 2. Greater context for the results figure including an updated legend with all symbols and better explanation for the white dots in the figure description will be developed. Thank you for catching my error in the Double Bluff 14C date. I will update this and any other age discrepancies in the revised manuscript. Photos will be added to the new results figure 2 and legibility will be improved by increasing the font size. I will also adjust the order of grain size curves so underlying data are more visible, such as in the case of Double Bluff. The discontinuous fill patterns on some of the stratigraphic columns are where sedimentary structures are seen in the outcrops. I will add these fill patterns to the legend as well as the figure text in the revised manuscript. Adding the grain size categories to the grain size graph x-axis would be a great addition to these plots. I will add these in the revised figure, thank you for this suggestion.
Figure 1B will be converted to its own figure (figure 1 in the revised manuscript), including all of the suggestions you recommended to provide greater regional context in the introduction.
I will clarify sampling resolution for 1C in the updated figure description, but variable sampling resolution is a product of accessibility to outcrops from the beach front. Some areas were more readily accessible than others.
When updating figure 2 (which will become figure 3 in the revised manuscript), I will include a legend for the symbols representing the dates. You are correct in your comment that deglaciation is also post-LGM, so in this figure I will also reconsider the use of the terms “post-LGM” and “deglacial”. Dates from the deglacial deposits include sediments sourced from outwash, so I will change the word “deglacial” to better contextualize the ages collected from the outwash.
I will add the background text on Puget Lowland glaciation from the supporting information to a newly developed section within the Introduction.
In reference to page 3, lines 71-74 and page 18 line 572-573, Nield et al.(2014) and Whitehouse et al.(2019) do not explicitly state the rheology similarities between the regions; I am drawing the connection between these two publications independently. I will make this more apparent in the revised manuscript. Additionally, the topographic setting of a sedimentary basin surrounded by mountains is the topographic similarity being drawn between the Puget Lowland and outlet glaciers in Greenland, previously established from Eyles et al.(2018). I will also make this point more clear in the revised manuscript.
I will change the references and increase explanations for the highlighted areas of page 5, lines 147-152 and page 12, lines 397-398.
Page 13, line 410: I will clarify “winnowing” is meant to be an “absence of fines”. The process I’m referencing is the removal of fines via tidal currents in a submarine or coastal setting.
Page 13, line 413: in the revised manuscript, these typos will be corrected.
I will clarify continuity between page 13 lines 428-430 and page 11 line 332-333.
Page 13, lines 434-435. I will clarify the fall of sea level rise as a function of glacial isostatic adjustment in this context. Due to the lack of any additional evidence for glacial proximity to these locations from the transgressive and regressive facies, I do, however, maintain the interpretation that the pre-LGM coarsening of material is more likely to indicate sea level and glacial isostatic adjustment changes than glacial proximity in this location.
Page 13, lines 442-445: I will clarify the difference between the two references in this line and will find new references for the aforementioned lines for interpretations of coarsening-upward sediments. Similar to your comment on lines 453-456 on page 14, I will find better references to situate my interpretations.
Thank you for directing me to David Evans’ 2007 entry on tills – I will change all occurrences of “glacial till” to simply “till” in the revised manuscript.
You are correct in the interpretation of lines 439-442 on page 14. I will rewrite this sentence with your recommended edits.
The revision of the figures, into a new figure 1 with regional context and a new figure 2 with stratigraphic and grain size data, will resolve your comment related to figure 1B being referenced before figure 1A.
In the revised manuscript, the use of dashes and text size and font will be refined per your suggestion.
Thank you for catching the grammatical and spelling errors on pages 3 through 8. I will make corrections for all of the line errors you mentioned.
Your corrections for the bivalves collected for the marine reservoir age will be included in the revised manuscript. Additionally, Table 1 will be presented in Results as you suggest.
Page 9, lines 262-264: the statement ‘all possible interpretations that have been conducted across Whidbey Island’ will be reworded or removed for better clarity in these lines.
In reference to your comment given to line 297 on page 10, I will make this distinction more clear earlier on, specifically in the Methods so the distinction between locations with multiple sites is explicit.
Page 12, line 388: I will change the use of the word “structure” to “facies” in this sentence.
Page 16, line 508: In the revised manuscript, glaciomarine will be the consistently used term. All instances of “glacimarine” will be changed to “glaciomarine”.
The Figure S2 schematic will be changed to reflect the text and then included in the new figure 3. Rather than showing ice advance into a subaerial environment, the new schematic will show glacial retreat within a subaerial environment.
Page 17, lines 515-518. Thank you for this comment, however, it seems unlikely that these shells are part of a slide or slump deposit due to their growth-position (images will be included in the newly developed figures) within a region of continuous stratigraphy where there is little slope that could have supported a slump. I will clarify the overlap in ages and present an updated span of ages based on the 2 sigma range in the radiocarbon dates from Penn Cove.
I agree that adding information about glacial geomorphology across the Puget Lowland will be helpful in interpreting the stratigraphic data. In the discussion of the revised manuscript, I intend to include a paragraph depicting what is known of glacial dynamics in the region from glacial geomorphology. Related to your comment on the grounding zone wedges (GZWs) on page 17 lines 528-532, I will add a more thorough review of this data in the newly developed geomorphic section of the Discussion. I will also introduce the GZWs earlier in the paper, likely in the Introduction to create some background knowledge of past ice behavior in the region.
Citation: https://doi.org/10.5194/egusphere-2023-2725-AC3
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AC3: 'Reply on RC3', Marion McKenzie, 08 Feb 2024
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Marion McKenzie
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