the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Moss kill-dates and modeled summer temperature track episodic snowline lowering and ice-cap expansion in Arctic Canada through the Common Era
Abstract. Most extant small ice caps mantling low-relief Arctic Canada landscapes remained cold-based throughout the late Holocene, preserving in situ bryophytes killed as ice expanded across vegetated landscapes. As Arctic summers warmed after 1900 CE, ice caps receded, exposing entombed vegetation. The calibrated radiocarbon ages of dead moss collected near ice-cap margins (kill-dates) define when ice advanced across the site, killing the moss, and remained over the site until the year of their collection. In an earlier study we reported 94 Last Millennium radiocarbon dates on in situ dead moss collected at the margins of two upland ice complexes on northern Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age ~1240 CE, and further expansion ~1480 CE, coincident with episodes of major explosive volcanism. Here we test the confidence in kill dates as reliable predictors of expanding ice caps by re-sampling those previously sampled ice complexes 14 years later, after ~250 m of ice recession. The probability density functions (PDF) of the more recent series of ages matches PDFs of the earlier series, but with a larger fraction of early CE ages; post 2005 CE ice recession has exposed relict ice caps that grew during earlier Common Era advances, and were preserved beneath later ice-cap growth. We compare 107 kill dates from the two ice complexes with 79 kill dates from 62 other ice caps within 250 km of the two densely sampled ice complexes. The PDF of kill dates from the 62 other ice caps cluster in the same time windows as those from the two ice complexes alone, with the PDF of all 186 kill dates documenting episodes of widespread ice expansion restricted almost exclusively to 250–450 CE, 850–1000 CE and a dense early Little Ice Age cluster with peaks at ~1240 and ~1480 CE. Ice continued to expand after 1480 CE, reaching maximum dimensions ~1880 CE, still visible as zones of limited vegetation cover in remotely sensed imagery. Intervals of widespread ice-cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a Community Earth System Modeling (CESM) fully coupled Common Era simulation, suggesting primary forcing of the observed snowline lowerings were both modest declines in summer insolation, and cooling resulting from explosive volcanism, most likely intensified by positive feedbacks from sea-ice expansion and reduced northward heat transport by the oceans. The clusters of ice cap expansion defined by moss kill-dates are mirrored in an annually resolved Common Era record of ice-cap dimensions in Iceland, suggesting this is a circum-North-Atlantic-Arctic climate signal for the Common Era. During the coldest century of the Common Era, 1780–1880 CE, ice caps mantled > 11,000 km2 of north-central Baffin Island, whereas < 100 km2 is glaciated at present. That state approached conditions expected during the inception phase of an ice age, and was only reversed after 1880 CE by anthropogenic alterations of the planetary energy balance.
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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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Preprint
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Supplement
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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.
- Preprint
(1753 KB) - Metadata XML
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Supplement
(875 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
- AC1: 'Additional Figure', Gifford H. Miller, 14 May 2023
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RC1: 'Comment on egusphere-2023-737', Anonymous Referee #1, 04 Jun 2023
The authors present a new dataset of Moss Kill dates on Baffin Island, and compare them with an earlier dataset (from 2005), as well as nearby paleoclimate datasets and model simulations. The differences in the two datasets allow the authors to explore how the recent retreat has exposed older mosses, and the implications for the Common Era glaciation history of the region. The authors argue that the observed pattern of glacial advances is representative of the North Atlantic Arctic due to the similarities with other data throughout the region. Finally, the authors compare their results to model simulations, allowing them to explore the whether or not the peak of Common Era glaciation in the region, 1780-1880 would have led to glacial inception in the absence of anthropogenic addition of greenhouse gases.
The manuscript presents important new data that provide key insights in to the recent glaciation history of the region. It is also well written and appropriate for publication in Climate of the Past. I recommend publication following a handful of mostly minor corrections.
Calibration bias testing approach
Investigating how the calibration curve may impose biases on the results is a good idea, however I'm somewhat concerned about the methodology. I worry that a single realization of a random 160 samples could get you spurious results and peaks that are either artificial, or artificially high or low.
I can imagine two alternate paths to improve this. The first would be to expand on what you did, but rather than randomly sampling ages once, repeat that sampling 1000 or so times. Looking across this ensemble of random results, and specifically extracting so high end quantiles (90, 95, 99) would robustly show how the calibration curve impacts the likelihood of finding calibrated ages in certain time periods, and also tell you how likely it is for a random collection of dates to give you the observed results. One note on this - I wasn't certain from the description, but rather than randomly sampling radiocarbon ages, it might make more sense to randomly, with a uniform distribution, sample calendar, and then use a reverse calibration to create radiocarbon ages. Then add realistic analytical uncertainty and calibrate again.
The alternative strategy would be similar, but where you'd be looking to quantify the cumulative probability bias of each decade in due to the calibration curve, and then adjust, or bias-correct your results, effectively lowering peaks around 1280 and 1450 CE, while raising peaks in the 1300s. I think the first strategy is a more direct test.
Minor points
I'm not sure I'm convinced by the argument on lines 347-349. Earlier you argue that the dead moss are removed within a few years except in rare circumstances. Having a multidecadal retreat where the moss is preserved, and then a new advance that entombs both old and new moss seems possible, but I would like to see other potential explanations for this discrepancy explored (and argued against, if appropriate).
446: typo - I think "in" should be "is" here
457: Be explicit about what the data are saying that the model is getting wrong here.
Figures 5, 6 and 8. I think it'd be very instructive to add a panel showing the difference between 2018 and 2005 data, highlighting what moss kill ages were exposed during recent retreat
The caption for figure 7 looks like it might be swapped with that of figure 8.
844: I couldn't readily find the Dewar lake data at that website. Could you provide a more specific link?
Please add where the Hvitavatn dataset can be found.
The article is generally well cited, and includes a good review of early work with the method and in the region, however I found several references cited in the text that weren't included in the references section, including Falconer 1966.
Citation: https://doi.org/10.5194/egusphere-2023-737-RC1 -
AC2: 'Reply on RC1', Gifford H. Miller, 18 Jun 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-737/egusphere-2023-737-AC2-supplement.pdf
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AC2: 'Reply on RC1', Gifford H. Miller, 18 Jun 2023
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RC2: 'Comment on egusphere-2023-737', Anonymous Referee #2, 20 Jun 2023
This is an excellent study and a well written and well illustrated manuscript. In fact one of the best I have seen in years and certainly suitable for Climate of the Past.The authors have dated a large material of Polytrichum mosses from ice caps on Baffin Island, and use the ages as a proxy for summer temperatures. Based on a comparison with data from Iceland, they develop a palaeoclimate record for the Late Holocene for the North Atlantic. The authors conclude that a new ice age may have been on its way during the cold period 1780 to 1880, had it not been for global warming caused by human-induced emission of greenhouse gasses.
I did not see any reference to figure 13 in the text
Also I could not find figure 16
Citation: https://doi.org/10.5194/egusphere-2023-737-RC2 -
AC3: 'Reply on RC2', Gifford H. Miller, 27 Jun 2023
GHM responds. We thank Anonymous Referee 2 for their comments. Figure 13 was incorrectly referenced as Figure 14 in the original text, which has now been corrected in a revision. We have also added a new figure (Fig. 9) to clarify why such different moss ages can be found in close proximity along an ice margin. A revised version of the ms will be uploaded with that Figure later this week.
Citation: https://doi.org/10.5194/egusphere-2023-737-AC3
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AC3: 'Reply on RC2', Gifford H. Miller, 27 Jun 2023
Interactive discussion
Status: closed
- AC1: 'Additional Figure', Gifford H. Miller, 14 May 2023
-
RC1: 'Comment on egusphere-2023-737', Anonymous Referee #1, 04 Jun 2023
The authors present a new dataset of Moss Kill dates on Baffin Island, and compare them with an earlier dataset (from 2005), as well as nearby paleoclimate datasets and model simulations. The differences in the two datasets allow the authors to explore how the recent retreat has exposed older mosses, and the implications for the Common Era glaciation history of the region. The authors argue that the observed pattern of glacial advances is representative of the North Atlantic Arctic due to the similarities with other data throughout the region. Finally, the authors compare their results to model simulations, allowing them to explore the whether or not the peak of Common Era glaciation in the region, 1780-1880 would have led to glacial inception in the absence of anthropogenic addition of greenhouse gases.
The manuscript presents important new data that provide key insights in to the recent glaciation history of the region. It is also well written and appropriate for publication in Climate of the Past. I recommend publication following a handful of mostly minor corrections.
Calibration bias testing approach
Investigating how the calibration curve may impose biases on the results is a good idea, however I'm somewhat concerned about the methodology. I worry that a single realization of a random 160 samples could get you spurious results and peaks that are either artificial, or artificially high or low.
I can imagine two alternate paths to improve this. The first would be to expand on what you did, but rather than randomly sampling ages once, repeat that sampling 1000 or so times. Looking across this ensemble of random results, and specifically extracting so high end quantiles (90, 95, 99) would robustly show how the calibration curve impacts the likelihood of finding calibrated ages in certain time periods, and also tell you how likely it is for a random collection of dates to give you the observed results. One note on this - I wasn't certain from the description, but rather than randomly sampling radiocarbon ages, it might make more sense to randomly, with a uniform distribution, sample calendar, and then use a reverse calibration to create radiocarbon ages. Then add realistic analytical uncertainty and calibrate again.
The alternative strategy would be similar, but where you'd be looking to quantify the cumulative probability bias of each decade in due to the calibration curve, and then adjust, or bias-correct your results, effectively lowering peaks around 1280 and 1450 CE, while raising peaks in the 1300s. I think the first strategy is a more direct test.
Minor points
I'm not sure I'm convinced by the argument on lines 347-349. Earlier you argue that the dead moss are removed within a few years except in rare circumstances. Having a multidecadal retreat where the moss is preserved, and then a new advance that entombs both old and new moss seems possible, but I would like to see other potential explanations for this discrepancy explored (and argued against, if appropriate).
446: typo - I think "in" should be "is" here
457: Be explicit about what the data are saying that the model is getting wrong here.
Figures 5, 6 and 8. I think it'd be very instructive to add a panel showing the difference between 2018 and 2005 data, highlighting what moss kill ages were exposed during recent retreat
The caption for figure 7 looks like it might be swapped with that of figure 8.
844: I couldn't readily find the Dewar lake data at that website. Could you provide a more specific link?
Please add where the Hvitavatn dataset can be found.
The article is generally well cited, and includes a good review of early work with the method and in the region, however I found several references cited in the text that weren't included in the references section, including Falconer 1966.
Citation: https://doi.org/10.5194/egusphere-2023-737-RC1 -
AC2: 'Reply on RC1', Gifford H. Miller, 18 Jun 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-737/egusphere-2023-737-AC2-supplement.pdf
-
AC2: 'Reply on RC1', Gifford H. Miller, 18 Jun 2023
-
RC2: 'Comment on egusphere-2023-737', Anonymous Referee #2, 20 Jun 2023
This is an excellent study and a well written and well illustrated manuscript. In fact one of the best I have seen in years and certainly suitable for Climate of the Past.The authors have dated a large material of Polytrichum mosses from ice caps on Baffin Island, and use the ages as a proxy for summer temperatures. Based on a comparison with data from Iceland, they develop a palaeoclimate record for the Late Holocene for the North Atlantic. The authors conclude that a new ice age may have been on its way during the cold period 1780 to 1880, had it not been for global warming caused by human-induced emission of greenhouse gasses.
I did not see any reference to figure 13 in the text
Also I could not find figure 16
Citation: https://doi.org/10.5194/egusphere-2023-737-RC2 -
AC3: 'Reply on RC2', Gifford H. Miller, 27 Jun 2023
GHM responds. We thank Anonymous Referee 2 for their comments. Figure 13 was incorrectly referenced as Figure 14 in the original text, which has now been corrected in a revision. We have also added a new figure (Fig. 9) to clarify why such different moss ages can be found in close proximity along an ice margin. A revised version of the ms will be uploaded with that Figure later this week.
Citation: https://doi.org/10.5194/egusphere-2023-737-AC3
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AC3: 'Reply on RC2', Gifford H. Miller, 27 Jun 2023
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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.
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(1753 KB) - Metadata XML
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Supplement
(875 KB) - BibTeX
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- Final revised paper