the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2023
Combining traditional and novel techniques to increase our understanding of the lock-in depth of atmospheric gases in polar ice cores - results from the EastGRIP region
Abstract. We investigate the lock-in zone (LIZ) of the EastGRIP region, Northeast Greenland, in detail. We present results from the firn air pumping campaign of the S6 borehole, forward modeling, and a novel technique for finding the lock-in depth (LID, the top of the LIZ) based on the visual stratigraphy of the EastGRIP ice core. The findings in this work help to deepen our knowledge of how atmospheric gases are trapped in ice cores. CO2, δ15N, and CH4 data suggest the LID lies around 58 to 61 m depth. With the grayscale and bright spot analysis based on visual stratigraphy, we can pinpoint a change in ice properties to exactly 58.3 m depth, which we define as the optical lock-in depth (OLID). This visual change in ice properties is caused by the formation of rounded and enclosed air bubbles, altering the measured refraction of the light pathways. The results for the LID and OLID agree accurately on the depth. We furthermore use the visual stratigraphy images to obtain information on the sharpness of the open to closed porosity transition. Combing traditional methods with the independent optical method presented here, we can now better constrain the bubble closure processes in polar firn.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-1904', Anonymous Referee #1, 26 Oct 2023
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RC2: 'Comment on egusphere-2023-1904', Anonymous Referee #2, 25 Mar 2024
General review:
The article addresses the intricacies surrounding bubbles and LIZ formation within the firn layer of the Greenland ice sheet, employing both traditional and innovative optical methodologies. This research holds significant importance in advancing our understanding of gas ages and their associated smoothing effects. However, the previous conventional methods fell short in capturing the the subtle intricacies of LIZ formation. The authors have introduced a novel term, the optical lock-in depth (OLID), and conducted a comparative analysis with conventional approaches. These novel findings significantly augment our understanding, and the interpretations offered are judicious. Nonetheless, the methods elucidated poses challenges in comprehension, necessitating structural and terminological refinements to enhance clarity. Editorial revisions are imperative prior to publication.
Specific comments:
Line 20: What does δ15N signify? Is it δ15N of N2? Please specify.
Page 2, Move ‘1.3 Motivation’ before ‘1.2 Site Locations’ for better flow.
Page 2, In section 1.1 or 1.3, describe the issues with previous conventional methods.
Line 32: Can you provide coordinates for the S2 location?
Figure 1c caption: why is the photo from Little Dome C shown? Did you use the same equipment? Please clarify this choice.
Lines 48 and 50: should not be broken into separate paragraphs.
Line 59: Specify what you mean by “issues”.
Line 81: absence of gravitational enrichment => absence of further gravitational enrichment?
Figure 2b caption: transition depth of ‘58 m’? It is not likely to be about 60 m. Refer to text at Line 96.
Figure 2f caption: Witrant et al. 2012) => Witrant et al. (2012)
Line 96: Explain why the top of the LID is differently defined using data of δ15N-N2 and CO2 concentration.
Line 104~107: Add more reference. Similar features were reported also in Mitchell et al. (2015) and Jang et al. (2019)
Line 108: Erase ‘(fig. 2c)’
Line 127: Even the green line in fig. 2d does not well matched with the CH4 concentrations in high density layers (minima of CH4 concentration). The authors may suggest plausible reasons.
Line 156-158: Mention other definitions of the close-off zone. Consider citing Martinerie et al. (1992).
Line 161: Between 55 and… => Between 50(?) and…
Line 165-174: Specify that details of observations are described in Appendix C.
Figure 4 caption & Line 181: please define ‘pixel value’
Figure 5a caption: please address Appendix B if it is related to the method.
Line 221: Define ‘percolation transition’
Line 227: Specify ‘clusters’
Line 231: add Jang et al. (2019) to the references
Line 241: Define ‘maximum pixel value’
Line 273-274: Address ‘Appendix D’ where the ‘pixel values above 20’ are described.
Line 281: 55 to 58 m depth => 55 to 58 m depth?
Line 298-299: Change ‘agree very well’ to ‘agree well’. We see a difference in the LIDs defined by CO2 and δ15N-N2
Line 325-336: Relocate and shorten ‘Appendix A’ in the main text’s site description section.
Figure D1b: Specify the meaning of ‘gray value’ on the y-axis label? Is it the pixel value?
Line 432: Erase ‘Appenix:’
Refercence:
Martinerie, P., Raynaud, D., Etheridge, D. M., Barnola, J. M., and Mazaudier, D.: Physical and Climatic Parameters which Influence the Air Content in Polar Ice, Earth Planet. Sc. Lett., 112, 1–13, https://doi.org/10.1016/0012-821X(92)90002-D, (1992)
Mitchell, L., Christo Buizert, Edward Brook, Daniel Breton, John Fegyveresi, Daniel Baggenstos, Anais Orsi, Jeffrey Severinghaus, Richard B. Alley, Mary Albert, Rachael H. Rhodes, Joseph R. McConnell, Michael Sigl, Olivia Maselli, Stephanie Gregory and Jinho Ahn. Observing and modelling the influence of layering on bubble trapping in polar firn, Journal of Geophysical Research, 120, doi:10.1002/2014JD022766 (2015)
Youngjoon Jang, Sang Bum Hong, Christo Buizert, Hun-Gyu Lee, Sang-young Han, Ji-Woong Yang, Yoshinori Iizuka, Akira Hori, Yeongcheol Han, Seong Joon Jun, Pieter Tans, Taejin Choi, Seong-Joong Kim, Soon Do Hur and Jinho Ahn, Very old firn air linked to strong density layering at Styx Glacier, coastal Victoria Land, East Antarctica, The Cryosphere, 13, 2407-2419 (2019)
Citation: https://doi.org/10.5194/egusphere-2023-1904-RC2 -
EC1: 'Comment on egusphere-2023-1904', Ed Brook, 31 Mar 2024
In addition to the posted two reviews I have a comment from another reviewer, Jeff Severinghaus, which I will paste below as there was a complication getting this in the system. This review is very positive and just makes some minor comments for the authors to consider.
Review of “Combining traditional and novel techniques to increase our understanding of the lock-in depth of atmospheric gases in polar ice cores – results from the EastGRIP region”
Authors: J. Westhoff et al. March 20, 2024
This manuscript presents a novel and very interesting new method of determining the depth in an ice core at which air bubbles are effectively closed off, taking advantage of the optical properties of bubbles and ice. In polar settings, slow densification of the firn (porous ice) typically leads to formation of ice with trapped air bubbles after several hundred years.
In detail, the authors show that “bright spots” appear in the optical images around 61 m depth, very close to the depth that is found via classical methods for finding the depth of the bubble close-off. The authors make a convincing case that these “bright spots” have a mechanistic linkage to the ice properties at around 61 m depth. With their new optical methods, the authors show persuasively that the “bright spots” record a critical change in the firn geometry and structure.
They proposed that the “bright spots” are due to the creation of bubbles with semi-spherical geometry, which is known to produce bright reflections As such, the “bright spots” are shown to have a mechanistic and meaningful origin that can be further probed in the future. The authors also use traditional methods such as density measurements to verify the closure of bubbles at this depth. Overall, this is an excellent contribution to our understanding of the firn-to-ice transition in polar ice sheets, and represents a new and valuable metric of where the “classical bubble close-off depth” happens. I recommend that this work be published with only minor edits, as spelled out below.
Jeff Severinghaus, reviewer
Minor and editorial comments:
Line 56 this would be made clearer for the reader by writing “inserted an inflatable rubber bladder”
Line 57 perhaps this should be ¼ inch, not 1.4 inch? Normally the purge line is about 3/8” to ½” in diameter, and the air sampling line is ¼”
Line 59 say “were monitored on-site during pumping to detect contamination issues…”
Line 64 “and diffusive mixing in the vertical direction essentially stops” [This is an important distinction because many studies have shown that horizontal diffusion and advection can remain prevalent due to horizontal high-permeability layers (typically summer layers) even when vertical air flow is completely shut off]
Line 72 “evolution of pore closure”
Line 78 for the reader, it might help to cite a ref here at 2), since thermal fractionation isn’t widely known in the community. You could cite Severinghaus et al., 1998 Nature
Line 79 your interpretation of an extremum in 15N at 12 m from the previous winter’s cold is mistaken - in fact the extremum at 12 m is due to the recent summer atmospheric warmth just months before the pumping. The reason is that the atmosphere cannot change its 15N, due to its virtually infinite reservoir, so the nitrogen gas in the top dozen meters of firn (which is colder than the atmosphere in local summer) must become fractionated with the heavy isotope 15N becoming enriched. The signal of the previous winter, on the other hand, shows at 18 m, with a slightly depleted 15N. See Severinghaus et al. 2001 (G Cubed) for a more complete explanation of the phenomenon, including a wintertime firn air sampling at South Pole along with the usual summertime sampling of firn air. As expected, the top 12 meters of firn shows very negative 15N in winter (because the atmosphere is so much colder than the air in the firn).
Thermal fractionation of air in polar firn by seasonal temperature gradients – G-Cubed J. P. Severinghaus, A. Grachev, M. Battle 2001
Line 84 The slight decrease of 15N with increasing depth within the lock-in zone is well known to be due to global warming and resulting firn thermal fractionation over the past 4 to 5 decades, not to contamination. This effect has been extensively documented by Orsi et al., 2017. You should cite her work: The recent warming trend in North Greenland , Geophysical Research Letters, AJ Orsi et al., 2017
Line 91 Check your calculation of “almost 400 years of snow accumulation” in 66 m. It’s probably more like 330 years. You have to take account of the fact that annual layers are quite thick in the upper part of the firn, with snow densities of only 0.35 to 0.55 kg per liter, in comparison to snow densities of 0.83 to 0.84 kg per liter in the lock-in zone.
Line 109 Fix the statement “around 66 m the bubbles are essentially closed off”. This is not consistent nor accurate, since the same sentence states that “layers with not fully closed pores can be found down to 71.5 m”.
Line 135 “11 cm annual layer thickness” This doesn’t seem right – please check
Line 213 It would be helpful to include a reference here on “percolation transitions” – which some readers might not be familiar with.
Line 235 please provide a reference to “strangulation” – it is the first time this word is used in the paper, and many readers will not know what this means.
Citation: https://doi.org/10.5194/egusphere-2023-1904-EC1
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