the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Apr 2026
Fidelity and stratigraphy of the Antarctic Allan Hills old ice archive from Continuous Flow Analysis
Abstract. The Allan Hills blue ice area, East Antarctica, offers a unique opportunity to extend the ice core record beyond 800 thousand years (kyr), with ice as old as 6 million years recently recovered. The ice in this area demonstrates several peculiarities—such as strong layer thinning and folding—that warrant an in-depth investigation of its stratigraphy and the fidelity of the climate record it contains. Here, we present a high-resolution Continuous Flow Analysis (CFA) of two shallow ice cores from the Allan Hills (ALHIC 2201 and ALHIC 2302), spanning the upper 69 m and 46 m, respectively.
Our CFA analysis includes methane, water stable isotopes, and particle concentrations, allowing us to characterize their variability and assess how well geochemical measurements are recorded and preserved in Allan Hills ice. Dating of the ice (using the 40Ar chronometer) revealed ages ranging from ~150–1200 kyr with substantial age discontinuities and folding, highlighting the complex stratigraphy of ice in this region. To assess glacial-interglacial variability, we conduct descriptive statistical analyses of each measurement relative to the EPICA Dome C (EDC) deep ice core record, which served as a reference benchmark over the last 800 kyr. Relative to EDC, we find data representing warm climate states are overrepresented at Allan Hills, and the Allan Hills records typically exhibit a narrower distribution compared to the EDC record. These observations confirm previous suggestions of interglacial bias and glacial cycle averaging in some sections of Allan Hills records. Differences between ALHIC 2201 and ALHIC 2302 further suggest that coring location within the Allan Hills region affects signal preservation in the record. Our high-resolution investigation of this ice is a critical step toward improved interpretation of the discrete, multi-million-year records from the Allan Hills.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.
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.- Preprint
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Status: open (until 16 Jun 2026)
- RC1: 'Comment on egusphere-2026-2045', Anonymous Referee #1, 27 May 2026 reply
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- 1
In their manuscript Hudak et al. present CFA analysis and Ar40 dates of two Allen Hills cores alongside a description of the CFA setup at OSU. The CFA data is than used to investigate differences in variance of the Allen Hills data in comparison to existing EDC data. The application of continuous flow analysis to the discontinuous AL ice is certainly a challenge in terms of data analysis opening interesting avenues of investigation. Overall, the manuscript is well written and argued and fits the scope of Climate of the Past.
I do, however, have a request and some suggestions that should be taken into consideration prior to publication:
All the data analysis that the authors present is based on re-sampled versions of the existing EDC data. Because most of the analysis is dependent on the choices made during the resampling these choices need to be very well justified and compared to the ALHIC data, i.e. its age density. This justification and its implications for the analysis is probably best placed in the EDC data section or in an extra section that describes the statistical analysis. I understand that the age density of the ALHIC data might be ill defined but as so much of the discussion hinges on the differences in the data distributions to the EDC data it is well spent time/space to argue this carefully to take the reader along.
The authors put a lot of focus on purely descriptive statistics (mean, variances, simple correlations). The quantitative analysis is hidden in section 3.3.2 and mostly on the appendix. This is unfortunate as I think the analysis of the variance reduction and the representation of the climate states is clever and provides a lot of substance to the observations. For my taste, most of the descriptive statistics tables/figures could be moved to the appendix instead of the variance reduction analysis.
Specific Remarks
Section 2.2.2: The information about the precision of the water isotope and CH4 data would be a valuable addition to this section to gauge the differences between the discrete and the continuous data.
L192ff: I think all is fine with the solubility correction, but with the large error on the average offset it is not possible to make any quantitative statements. What is also missing is the magnitude of the solubility correction that was applied. If that is on the same order or smaller than the uncertainty of the offset than it is even less advisable to make any quantitative statements. I suggest rewriting this paragraph.
Section 2.2.3+2.2.4: The information about how the delay times for each analyser were determined (and how large/variable they are) seems to be missing. What is also missing is how the missing data at sample breaks was handled. Depending on the measurement resolution a significant amount of data will represent a mixture of non-continuous ice. In the application to the discontinuous ice here, that might be okay, but a few sentences should be added for explanation.
L274: The final sentence here is probably better placed as the second sentence of the paragraph.
Section 2.4: This section is missing the age-scale information for the EDC data. Later AICC is mentioned, but I think it would be better to state that here. What is also missing here is how the original EDC datasets where (sub/re)-sampled for the statistical analysis. It is important to be very explicit with this because especially the variance of the data and its range is very much dependent on the choices made here. The choices made here also need to be justified carefully considering the age density of the ALHIC data presented. Ideally the age densities should match as much as possible.
L295: should be “are due to depths not analysed”
L321ff: I think the comparison Figure A4 should find a place in the main part of the manuscript. Especially if sections are discussed in detail here, as in the final sentence.
L389: Should probably be “The average ALHIC water isotope values…”
Section 3: From my point of view there is an over emphasis, both in text and figure on the simple descriptive statistics of the data. I feel that it does not bring a lot to the arguments the authors are trying to make and could mostly be moved to the appendix, to shorten the main part of the manuscript.
Section 3.3.2: I think the approach used here is quite clever and deserves to not be hidden in the supplement: This is the data analysis that goes beyond simple descriptive statistics and adds quite a bit of value to the paper, from my point of view. I would suggest presenting it a more prominently and extend the section.
Section 3.3.1: I think the decile plots are probably best presented with the deciles as the x-axis labels not the data ranges. That would make the plots cleaner and would also make it easier to look at a specific decile without having to count the labels. Alternatively, the deciles could be added as an additional axis.
Appendix A: Both figures A1 and A2 include linearly interpolated values over data gaps, these should be removed.
Fig A2: It is quite surprising to see so large depth offsets (>10cm) between the replicate measurements. I suggest the authors add some detail on the precision of the depth assignment to the discussion.