the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
500-thousand-year-old basal ice at Skytrain Ice Rise, West Antarctica, estimated with the 36Cl/10Be ratio
Abstract. Dating the bottommost section of an ice core is often complicated by strong layer thinning and possible disturbances in the stratigraphy. The radioactive decay of atmospherically produced 36Cl and 10Be can provide age estimates, where traditional methods can no longer be used. In this study, we investigated ice from the bottom of the Skytrain ice core, which was drilled in West Antarctica next to the Ronne Ice Shelf and has previously been dated to 126 kyr BP about 24 m above bedrock.
Apart from decay, radionuclide concentrations in ice can be influenced by production rate variations, atmospheric transport and deposition variations, and, at low accumulations sites, by chlorine loss through hydrogen chloride outgassing. Using the 36Cl/10Be ratio largely removes production related variations and we were able to confirm that no 36Cl loss occurs at Skytrain Ice Rise, as the nuclear weapon test caused peak in 36Cl concentrations was found at the expected depth corresponding to the 1950s and 60s. An analysis of samples with known age showed that individual radionuclide concentrations and the 36Cl/10Be ratio are negatively correlated to the δ18O signal, which was used to apply a climate correction that enabled a higher precision for age estimates of previously undated samples. The deepest analysed section of the Skytrain ice core was found to be 552 ± 112 kyr old.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
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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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Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'minor revision', Kees C. Welten, 31 May 2025
The authors present the results of a study to estimate the age of old ice at the bottom of the ~650 m deep Skytrain ice core in West Antarctica, using the 36Cl/10Be method. I think the introduction could use a bit of historical perspective, since this method was first pioneered by Nishiizumi et al. (1983) for Antarctic ice from Allan Hills and then used by Elmore et al. (1987) for Greenland ice. Later measurements by Nishiizumi and Finkel (1998; Chinese Science Bull 43) showed that the 36Cl/10Be ratio varies systematically between ice cores (e.g. GISP2 vs. Siple Dome), so the method has not become a standard application to date old ice. The main challenge of the method is that it seems to require local calibration, as the authors have done in this study, so this seems a sound approach. Based on the measured 36Cl/10Be ratios in the top ~625 m of the core, which have been well dated, and a “climate correction” based on d18O, they used the climate-corrected 36Cl/10Be ratios in the bottom 24 m of the core to estimate their ages, yielding values increasing with depth from ~50 kyr to 550 +/- 110 kyr. This old age at the bottom is an important finding for the climate record of the Skytrain ice core; and the observation that the age increases monotonically with depth gives some confidence in the method. The authors have done a good job in explaining the experimental uncertainties in the measured 36Cl/10Be ratios as well as the systematic uncertainty in the 36Cl/10Be ratio of young ice,
However, I would like to see a bit more discussion of the 36Cl/10Be ages of the dated samples. For example, one sample in Fig. 2c (not sure what depth) has a 36Cl/10Be ratio of ~0.19, what age would that correspond to ? In fact, it would be illustrative to plot the 36Cl/10Be-derived ages of all 18 samples in the top 620 m of the core in Figure 3, just to give a sense of how much the ages scatter – given the uncertainty of ~100 kyr in the ages of the deeper samples I would expect them to plot within ~200 kyr of their true age. This may also give some insight into how reliable the climate correction is.
Secondly, I would like to see a bit of discussion on the implications of this old ice at the bottom of the core. What does it mean to see a ~400 kyr increase in age over 24 m of ice thickness? I am not a glaciology expert, but I seem to remember from the WAIS Divide core that the projected age at the bottom of the core depended on the geothermal flux, i.e., more heating from the bottom means younger ice. So does the old ice imply a low geothermal flux and is this consistent with what we know about West Antarctica or is this beyond the scope of this paper ?
In summary, this study provides a valuable contribution for the ice core and climate change community and is an interesting result that will probably be tested by other methods. I recommend publication of this manuscript in Climate of the Past after minor revision. Besides the two comments above, I have a few small edits and suggestions (listed below) that may help to further improve the clarity of this paper.
Minor edits/suggestions/comments.
L22 – Explain where the effective half-life of 384 kyr comes from. Audi et al. (2017) lists a half-life of 301 kyr for 36Cl and 1.51 Myr for 10Be, whereas the updated value of Chmeleff et al. (2010) is 1.387 Myr, so it is not clear which one was used. Later in the paper (L198) a value of 308 kyr is quoted for 36Cl or is that a typo ?
L33-34. I’m sure the accumulation rate has varied in time, so may not always have been 13 g/cm2/yr. So even though 36Cl has not been lost in the past 100 yr, is it possible that it may have been lost in the past when precipitation was lower ?
L78. Was there a particular reason to add more Cl carrier to the deeper samples ? Did the authors take the Cl component of the ice itself into account when converting 36Cl/Cl ratio to 36Cl concentration or is this contribution negligible compared to added carrier. If so, it would be useful to mention typical Cl concentration in Skytrain ice samples.
L198. Check 36Cl half-life – 301 kyr ?
- AC1: 'Reply on RC1', Niklas Kappelt, 08 Jul 2025
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RC2: 'Comment on egusphere-2025-1780', Anonymous Referee #2, 09 Jun 2025
In this paper by Kappelt and co-authors a chronology for the bottom ice of the Skytrain Ice Rise ice core is proposed. A previous chrononology was built via annual layers counting with the aid of selected accurate age markers, covering the last 126 kyr and reaching a depth of 627 m. The last 24 m of the ice core remained undated in the paper by Hoffman et al. (2022) due to the difficulty of dating this ice by traditional methods. This paper presents a method to retrieve a possible age (though with an high uncertainty) for the bottom ice by using radionuclides, namely the 36Cl/10Be ratio was used. The main drawback in using this approach for dating purposes is that 36Cl can easily undergo post depositional loss and this plays a significant role in what we can expect from the temporal pattern of the 36Cl/10Be ratio. While there is no doubt that it does not represent an issue at Skytrain ice rise during the Holocene or during interglacials, the authors should comment on what they expect to happen at this site during glacial periods, when the acc. rate is much lower. This consideration calls for a local calibration when using this dating approach, but the authors did a great work on this by comparing the 36Cl/10Be ratio with other chemical markers, including isotopes which are tightly linked to the accumulation rate.
This dating methodology is of great importance, representing a valid tool for deep and old ice where no other methods are useful. The 36Cl/10Be ratio could be particularly interesting in view of the BE-OI or MYIC projects. Thus, I recommend the publication of this paper with the following few comments.
It’s a good result that the 5 ages estimated in deep ice (figure 3) are getting older as depth increases but the discussion about the inconsistency of the first two points should be more detailed and the addition of the other experimental points (the younger ones) to this graph would greatly help in making clear how the estimated ages in the younger part relate to the official chronology.
A short discussion about the influence of possible artifacts in the bottom ice seems to me very useful. Since 36Cl and 10Be have a different behaviour as concerning their movements in the ice, while the diffusion of H36Cl is properly discussed in the text, the possibility of an accumulation at grain boundaries of 10Be and 36Cl in the deep section should be briefly taken into account. If some information about the physical properties of the ice are available (crystal dimansions, etc), this should be mentioned in the paper to corroborate the meaning of the 36Cl/10Be ratio.
Minor comments:
Line 77: are all the significant figures in 0.299 mg necessary?
Line 112: change to “… signal from Mulvaney et al. (2023)”
Line 149: the numbers in the equation are slightly different from those in fig. 2c.
Citation: https://doi.org/10.5194/egusphere-2025-1780-RC2 - AC2: 'Reply on RC2', Niklas Kappelt, 08 Jul 2025
Interactive discussion
Status: closed
-
RC1: 'minor revision', Kees C. Welten, 31 May 2025
The authors present the results of a study to estimate the age of old ice at the bottom of the ~650 m deep Skytrain ice core in West Antarctica, using the 36Cl/10Be method. I think the introduction could use a bit of historical perspective, since this method was first pioneered by Nishiizumi et al. (1983) for Antarctic ice from Allan Hills and then used by Elmore et al. (1987) for Greenland ice. Later measurements by Nishiizumi and Finkel (1998; Chinese Science Bull 43) showed that the 36Cl/10Be ratio varies systematically between ice cores (e.g. GISP2 vs. Siple Dome), so the method has not become a standard application to date old ice. The main challenge of the method is that it seems to require local calibration, as the authors have done in this study, so this seems a sound approach. Based on the measured 36Cl/10Be ratios in the top ~625 m of the core, which have been well dated, and a “climate correction” based on d18O, they used the climate-corrected 36Cl/10Be ratios in the bottom 24 m of the core to estimate their ages, yielding values increasing with depth from ~50 kyr to 550 +/- 110 kyr. This old age at the bottom is an important finding for the climate record of the Skytrain ice core; and the observation that the age increases monotonically with depth gives some confidence in the method. The authors have done a good job in explaining the experimental uncertainties in the measured 36Cl/10Be ratios as well as the systematic uncertainty in the 36Cl/10Be ratio of young ice,
However, I would like to see a bit more discussion of the 36Cl/10Be ages of the dated samples. For example, one sample in Fig. 2c (not sure what depth) has a 36Cl/10Be ratio of ~0.19, what age would that correspond to ? In fact, it would be illustrative to plot the 36Cl/10Be-derived ages of all 18 samples in the top 620 m of the core in Figure 3, just to give a sense of how much the ages scatter – given the uncertainty of ~100 kyr in the ages of the deeper samples I would expect them to plot within ~200 kyr of their true age. This may also give some insight into how reliable the climate correction is.
Secondly, I would like to see a bit of discussion on the implications of this old ice at the bottom of the core. What does it mean to see a ~400 kyr increase in age over 24 m of ice thickness? I am not a glaciology expert, but I seem to remember from the WAIS Divide core that the projected age at the bottom of the core depended on the geothermal flux, i.e., more heating from the bottom means younger ice. So does the old ice imply a low geothermal flux and is this consistent with what we know about West Antarctica or is this beyond the scope of this paper ?
In summary, this study provides a valuable contribution for the ice core and climate change community and is an interesting result that will probably be tested by other methods. I recommend publication of this manuscript in Climate of the Past after minor revision. Besides the two comments above, I have a few small edits and suggestions (listed below) that may help to further improve the clarity of this paper.
Minor edits/suggestions/comments.
L22 – Explain where the effective half-life of 384 kyr comes from. Audi et al. (2017) lists a half-life of 301 kyr for 36Cl and 1.51 Myr for 10Be, whereas the updated value of Chmeleff et al. (2010) is 1.387 Myr, so it is not clear which one was used. Later in the paper (L198) a value of 308 kyr is quoted for 36Cl or is that a typo ?
L33-34. I’m sure the accumulation rate has varied in time, so may not always have been 13 g/cm2/yr. So even though 36Cl has not been lost in the past 100 yr, is it possible that it may have been lost in the past when precipitation was lower ?
L78. Was there a particular reason to add more Cl carrier to the deeper samples ? Did the authors take the Cl component of the ice itself into account when converting 36Cl/Cl ratio to 36Cl concentration or is this contribution negligible compared to added carrier. If so, it would be useful to mention typical Cl concentration in Skytrain ice samples.
L198. Check 36Cl half-life – 301 kyr ?
- AC1: 'Reply on RC1', Niklas Kappelt, 08 Jul 2025
-
RC2: 'Comment on egusphere-2025-1780', Anonymous Referee #2, 09 Jun 2025
In this paper by Kappelt and co-authors a chronology for the bottom ice of the Skytrain Ice Rise ice core is proposed. A previous chrononology was built via annual layers counting with the aid of selected accurate age markers, covering the last 126 kyr and reaching a depth of 627 m. The last 24 m of the ice core remained undated in the paper by Hoffman et al. (2022) due to the difficulty of dating this ice by traditional methods. This paper presents a method to retrieve a possible age (though with an high uncertainty) for the bottom ice by using radionuclides, namely the 36Cl/10Be ratio was used. The main drawback in using this approach for dating purposes is that 36Cl can easily undergo post depositional loss and this plays a significant role in what we can expect from the temporal pattern of the 36Cl/10Be ratio. While there is no doubt that it does not represent an issue at Skytrain ice rise during the Holocene or during interglacials, the authors should comment on what they expect to happen at this site during glacial periods, when the acc. rate is much lower. This consideration calls for a local calibration when using this dating approach, but the authors did a great work on this by comparing the 36Cl/10Be ratio with other chemical markers, including isotopes which are tightly linked to the accumulation rate.
This dating methodology is of great importance, representing a valid tool for deep and old ice where no other methods are useful. The 36Cl/10Be ratio could be particularly interesting in view of the BE-OI or MYIC projects. Thus, I recommend the publication of this paper with the following few comments.
It’s a good result that the 5 ages estimated in deep ice (figure 3) are getting older as depth increases but the discussion about the inconsistency of the first two points should be more detailed and the addition of the other experimental points (the younger ones) to this graph would greatly help in making clear how the estimated ages in the younger part relate to the official chronology.
A short discussion about the influence of possible artifacts in the bottom ice seems to me very useful. Since 36Cl and 10Be have a different behaviour as concerning their movements in the ice, while the diffusion of H36Cl is properly discussed in the text, the possibility of an accumulation at grain boundaries of 10Be and 36Cl in the deep section should be briefly taken into account. If some information about the physical properties of the ice are available (crystal dimansions, etc), this should be mentioned in the paper to corroborate the meaning of the 36Cl/10Be ratio.
Minor comments:
Line 77: are all the significant figures in 0.299 mg necessary?
Line 112: change to “… signal from Mulvaney et al. (2023)”
Line 149: the numbers in the equation are slightly different from those in fig. 2c.
Citation: https://doi.org/10.5194/egusphere-2025-1780-RC2 - AC2: 'Reply on RC2', Niklas Kappelt, 08 Jul 2025
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Eric Wolff
Marcus Christl
Christof Vockenhuber
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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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