the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A Greenland-wide empirical reconstruction of paleo ice-sheet retreat informed by ice extent markers: PaleoGrIS version 1.0
Tancrède P. M. Leger
Christopher D. Clark
Carla Huynh
Sharman Jones
Jeremy C. Ely
Sarah L. Bradley
Christiaan Diemont
Anna L. C. Hughes
Abstract. The Greenland Ice Sheet is a large contributor to global sea-level rise, and current mass losses are projected to accelerate. However, model projections of future ice-sheet evolution are limited by the fact that the ice sheet is not in equilibrium with present-day climate, but is still adjusting to past changes that occurred over thousands of years. Whilst the influence of such committed adjustments on future ice-sheet evolution remains unquantified, it could be addressed by calibrating numerical ice sheet models over larger timescales and, importantly, against empirical data on ice margin positions. To enable such paleo data-model interactions, we need Greenland-wide empirical reconstructions of past ice-sheet extent that combine geomorphological and geochronological evidence. Despite an increasing number of field studies producing new chronologies, such a reconstruction is currently lacking in Greenland. Furthermore, a time-slice reconstruction can help: i) answer open questions regarding the rate and pattern of ice margin evolution in Greenland since the glacial maximum, ii) develop a standardised record of empirical data, and iii) identify understudied sites for new field campaigns. Based on these motivations, we here present PaleoGrIS 1.0, the first Greenland-wide isochrone reconstruction of ice-sheet extent evolution through the Late-Glacial and early-to-mid Holocene informed by both geomorphological and geochronological markers. Our isochrones have a temporal resolution of 500 years and span ~7.5 kyr from approximately 14 to 6.5 kyr BP. We here describe the resulting reconstruction of the shrinking ice sheet and conduct a series of ice-sheet wide and regional analyses to quantify retreat rates, areal extent change, and their variability across space and time. During the Late-Glacial and early-to-mid Holocene, we find the Greenland Ice Sheet has lost about one third of its areal extent (0.89 million km2). Between ~14 and ~8.5 kyr BP, it experienced a near constant rate of areal extent loss of 170 ± 27 km2 yr-1. We find the ice-sheet-scale pattern of margin retreat is well correlated to atmospheric and oceanic temperature variations, which implies a high sensitivity of the ice sheet to deglacial warming. However, during the Holocene, we observe inertia in the ice-sheet system that likely caused a centennial to millennial-scale time lag in ice-extent response. At the regional scale, we observe highly heterogeneous deglacial responses in ice-extent evident in both magnitude and rate of retreat. We hypothesise that non-climatic factors, such as the asymmetrical nature of continental shelves and onshore bed topographies, play important roles in determining the regional-to-valley scale dynamics. PaleoGrIS 1.0 is an open-access database designed to be used by both the empirical and numerical modelling communities. It should prove a useful basis for improved future versions of the reconstruction when new geomorphological and geochronological data become available.
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Tancrède P. M. Leger et al.
Status: open (until 07 Oct 2023)
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RC1: 'Review of Leger et al', Evan Gowan, 15 Aug 2023
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Publisher’s note: the supplement to this comment was edited on 22 August 2023. The adjustments were minor without effect on the scientific meaning.
Please see the attachement for the review
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AC1: 'Reply on RC1', Tancrède Leger, 15 Aug 2023
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Dear referee,
The attachment displayed here seems to relate to comments made on a different paper (Marshalek et al). There seems to be a technical error of some kind.
Tancrede Leger
Citation: https://doi.org/10.5194/cp-2023-60-AC1 -
RC2: 'Reply on AC1', Evan Gowan, 22 Aug 2023
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My apologies, I had uploaded the wrong file by mistake. The editorial team has kindly replaced the file with the correct review document.
Best regards,
Evan J. Gowan
Citation: https://doi.org/10.5194/cp-2023-60-RC2
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RC2: 'Reply on AC1', Evan Gowan, 22 Aug 2023
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AC2: 'Reply on RC1', Tancrède Leger, 19 Sep 2023
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Main comment 1: ".....I think one thing that will
truly be valuable to add to this paper would be a sort of “instruction manual” for how other research
groups should structure their investigations so that the data can be easily incorporated into PaleoGrIS...."Authors' response: Many thanks for this comment, that is a very good point and something that was still being discussed when submmitting the manuscript. We have decided to add a text file with clear instructions regarding this in the supplementary data: i.e. the PaleoGrIS 1.0 database, in the home folder, so that it cannot be missed. Anyone who downloads the database will easily see it. However, please note that this work is different from databases such as Bedmachine in a sense that the mapping of isochrones involves a lot of interpretation by the person mapping and involves mannual data filtering. This means future iteration following a similar method cannot be achieved by re-running a standard code or by using an online tool/platform that people can upload their new data to. It is more complex and requires people to re-work on the product for many months.
However, we still think there are ways by which the community can collaborate and contribute to future versions: by making the process of keeping up with new data easier for investigators whishing to produce new versions.
We here paste the content of these instructions:"HOW TO CONTRIBUTE NEW DATA TO FUTURE VERSIONS OF PaleoGrIS:
We would like new future versions of PaleoGrIS to be a wider community efforts and thus believe there are numerous opportunities for collaboration and contributions from a wide range of research teams and scientists.
We believe the best way for new empirical investigations to actively contribute to new versions of PaleoGrIS when providing new constraints
on the retreat of the GrIS margin, as well as periphery outlet glaciers, is to download the PaleoGrIS database and to locally (offline) update
the products (under the same format as original) with their new data (only if published peer-reviewed). We then reccomend to attach the updated products as as supplementary data/info to their new publication. The next useful step could be to contact the main authors of PaleoGrIS (Dr Tancrede Leger, Prof. Chris Clark, Dr Jeremy Ely) with the link to the newly published article which includes access to the the supplement and updated products of PaleoGrIS.The data that could easily be modified and improved would for instance include the Excel tables for the Geochronological database: simply by adding lines with new data at the bottom of the spreadsheets. A very useful contribution would be to modify the PaleoGrIS isochrone polyline shapefiles (with 4 different confidence levels) by updating them in the specific area studied by the investigators, provided that the same mapping methodology as original product (detailed in PaleoGrIS paper) is employed.
Alternatively, if you believe that entirely different and new Greenland-wide products resulting from your research would suit being incorporated into future versions of PaleoGrIS, please get in touch with the main authors, we would be happy to discuss! "
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Main comment 2: "....The shapefiles are full of tiny triangular polygons, and they intersect in ways that will make certain kinds of GIS analyses fail. I have included a picture showing what I mean (Figure 1). If possible, I suggest finding some way to simplify the margin files so that there are fewer of these small polygons."
Authors' response: We thank the reviewer for spotting this issue. We have now modified the isochrone polygon shapefile products to remove these shapes. To do so, we re-rasterized the shapefiles to a lower resolution (75m) to simplify the margins, and re-converted to polygon shapefiles but keeping the "pixelized" aspect of margins to avoid the triangular shaped polygons to show up, which was a result of vectorization when converting from raster to polygon shapefile (Arc simplyfying things). We also removed small holes (<10 km2) in BedMachine mask product to simplify the resulting polygon shapefiles: this did not change the reconstructed ice-sheet area by more than 0.5 %. Thanks to this comment we also spotted numerous polygons that were disconnected from the main ice-sheet perimeter. These have now been removed, resulting in a cleaner product. We however do not reccommend using these polygons for model-data comparisons (see ReadME in relevant folder) and we instead encourage users to favour the isochrone polyline shapefiles, or the NETCDF isochrone or buffer products.
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Minor comment 1: "The light blue text and lines on this figure is very hard to read...."
Authors' response: Thank you for pointing this out, the colour of text and lines has now been changed to a bright yellow.Minor comment 2: "There is no Yang et al 2016 in the reference list"
Authors' response: This has now been added, thanks for spotting the error.Minor comment 3: "Google Earth has inaccurate georeferencing in Greenland...."
Authors' response: That is good to know, thanks for pointing that out. However Google Earth was not used for direct mapping of features.
We mapped from Arc using the Arctic DEM and georeferenced imagery products, while Google Earth was exclusively used to inform the mapper when we wanted to visualize the area of concern in 3 dimensions. We therefore do not expect the innacurate georeferencing in Google earth to have any impact on our mapping.Minor comment 4: "In some places, MIS6/5e (and older) aged sediments have been preserved...."
Authors' response: We agree that this sentence needs to be more conservative, as although it is rare at the scale of the
entire ice sheet periphery, it has been observed in some places, as pointed out here.
The sentence was thus modified to that is reads: ".... and less likely to be preserved (with exceptions: i.e. Mejdahl & Funder, 1994; Kelly et al., 1999), we consider our mapped ice-marginal landforms to have been deposited during the last deglaciation"Minor comment 5: "In some places around Greenland, there has been over 100 m of uplift since deglaciation...."
Authors' response: We agree that this will be important to re-assess once a publicly-available sea level indicator and proxy database exists.
However, we note that recent assessments of the impact of GIA-related uplift on cosmogenic exposure ages in Grenland (i.e. Jones et al., 2019) show that ages would be corrected corrected by a maximum of 7%: which, for Holocene ages (vast majority of deglacial ages in Greenland), would not represent a correction in the order of thousands of years, but rather in the order of a few hundred years. The key factor to consider is when did the uplift happen, and how quickly, along with how much. If most of the rebound had already occured prior to, or towards
the onset of the exposure period (say early Holocene), than the impact of GIA correction on the resulting exposure age would be minimal and well below analytical uncertainties.Minor comment 6: "I find the phrase “continuous perimeters” to be confusing"
Authors' response: The term "perimeters" has now been replaced by "margins", more widely used in this specific context.Minor comment 7: "The Dalton et al. (2020) dataset is essentially unchanged from Dyke (2004) for Greenland
(aside from the Nares Strait), so I suggest using a different reference here"
Authors' response: Thanks for pointing this out. The reference in this sentence was replaced by the more appropriate review by Funder et al. (2011)Minor comment 8: "I would propose that maybe the reason there has been no data found from prior to 14 kyr BP is because it is possible the ice sheet remained at the shelf edge until the Bølling–Allerød sea level rise event"
Authors' response: We agree with this hypothesis. Our team is currently running a numerical modelling experiment using PISM and
producing an ensemble of numerous simulations with varying parameter configurations from 24 kyr BP to present. Despite highly different parameter configurations and despite modifying climate and ice dynamics forcings by significant amounts, all simulations produce a GrIS that remains at its maximum extent untill 16-15 kyr BP, prior to rapid collapse during the BO warming: see preliminary findings here: https://doi.org/10.13140/RG.2.2.31786.80328Minor comment 9: "I am curious about the choice to have the isochrone intervals be centered intermediate of the 500 year time time steps...."
Authors' response: The isochrones were imagined to be given a time range rather than one time stamp, from the early days of us thinking about the reconstruction. It was important to us to try to better acknowledge uncertainties in dating and also better seperate the geochronological uncertainties from the ones arising from spatially-variable spread in empirical constraints, when attaching time to our isochrones. Knowing this we chose to have time ranges with simple rounded values (e.g. 10 kyr BP - 9.5 kyr BP) which makes it slightly easier for both:
1- Mapping isochrones with numerical dates and labels turned on in Arc and rounded to 1 decimal: the mapper can more easily identify dates belonging to a specific time range when reading them on a map with 1 decimal rather than 2.
The likelihood to make mistakes is lower.
2- Easier for writing in the paper when referring to a specific isochrone and time range, and easier to read. We believed isochrones with time ranges such as 8.75 - 8.25 kyr BP would be more difficult and confusing to read, and would rely on thresholds between two isochrones being expressed in decades, which from a geochronological point of view would feel a bit odd given that uncertainties in the dating are in the order of centuries. We however do acknowledge that users of the database wanting to use a single time stamp (+/- uncertainty), i.e. for comparing with model time for instance, will have to use rather odd centre values (e.g. 8.75 +/- 0.25 kyr BP) for a single isochrone. Although this might be a bit different than usual, it would not change the result or the analysis in any way, as is already pointed out in the reviewer's comment.Minor comment 10: "I suggest referencing England et al. (2006) for a detailed overview of the Innuitian Ice Sheet, including the glaciation of the Nares Strait...."
Authors' response: Highly relevant and incredible paper indeed ! Many thanks for the suggestion, the reference has now been added to this paragraph.Minor comment 11: "Based on the GIA modelling by Lecavalier et al. (2014) (and my own, yet to be published study), I expect that the more expansive scenario is more likely."
Authors' response: We agree, and believe that new arising empirical data constraining Last glacial extent will likely increasingly make that argument, as this has been the trend with studies from various regions of Greenland lately.Minor comment 12: "The margin lines in these figures are too thin to make out the differences in the levels of confidence...."
Authors' response: Indeed the margin lines are thin, but they are visible to make out different confidence levels if the viewer zooms in to expand a single subpanel to about the size of their screen. However we tried and realized that making them thicker to the point where they could be distinguished at the whole figure scale would result in hiding most of the information regarding where the actual margin is located: which is the primary goal of this figure. We would thus be more inclined to not change the thickness of those lines, if that sounds reasonable.
Regarding the caption: we were struggling to fit the full legend information on the figures with numerous sub-pannels filling the whole space. In order to not compromise their size, and in an attempt to address the comment, we have added a sentence in the figure captions referring the reader to the legend of the first figure to appear in this series (8-13) which all share identical styles and scales/colormaps: i.e. a reference to Figure 8, which features more white space and thus the complete legend with all labels.Minor comment 13: "I am not sure what is meant by “miscellaneous ice configuration”.
Authors' response: Indeed we agree the word miscellaneous is inappropriate. We have decided to use the term "characteristic" instead and modified the sentence accordingly.Citation: https://doi.org/10.5194/cp-2023-60-AC2
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AC1: 'Reply on RC1', Tancrède Leger, 15 Aug 2023
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Tancrède P. M. Leger et al.
Tancrède P. M. Leger et al.
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