the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Mar 2026
Post-deposition processes affecting water stable isotope records at Little Dome C, Antarctica: new records from two firn cores and virtual firn core modelling
Abstract. The variability of water isotopes in Antarctic ice cores is of major interest for reconstructing past climate through changes in temperature and the hydrological cycle. Archived during the deposition of successive layers of snow, water molecules undergo various processes that can modify the isotopic signal initially imprinted in snowfall during the process of snow densification to ice (i.e. firn). Diffusion is a well-known effect that affects water isotope composition by smoothing the initial climate-related signal. Here, we focus on new water isotopes profiles of two firn cores from Little Dome C (LDC), dated thanks to sulphate concentration measurements, with the aim to identify the physical processes affecting the isotopic signal in the firn at the drilling site for the new Beyond Epica Oldest Ice (BELDC) deep ice core on the East Antarctic Plateau. We use a simple Virtual Firn Core model (VFC) to best fit our δ18O firn profiles. We conclude that the VFC should include a 8 cm surface mixing layer and that diffusion is overestimated below a depth of 3 meters.
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Status: open (until 27 Jun 2026)
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RC1: 'Comment on egusphere-2026-871', Anonymous Referee #1, 31 Mar 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-871/egusphere-2026-871-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-871-RC1 -
RC2: 'Comment on egusphere-2026-871', Anonymous Referee #2, 10 Jun 2026
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See attached review.
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RC3: 'Comment on egusphere-2026-871', Anonymous Referee #3, 12 Jun 2026
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The manuscript presents an important new dataset from Little Dome C and provides a useful attempt to reconcile observed firn isotope variability with a virtual firn core framework. The data are valuable and the comparison between observations and simulations is generally convincing. However, I have several concerns regarding the interpretation of the model results and the robustness of some conclusions.
My primary concern relates to the optimization strategy used to infer the preferred combinations of noise and mixing parameters. The model-observation agreement is quantified using an RMS score calculated from only two points in the power spectrum, namely the inflection point and the high-frequency limit. While this approach is computationally simple, it does not fully exploit the information contained in the observed spectra. Different parameter combinations may produce similar values at these two frequencies while exhibiting substantially different behaviour elsewhere in the spectrum. Because the inferred 8 cm mixing scale is one of the central conclusions of the manuscript, I encourage the authors to demonstrate that this result remains robust when alternative metrics based on the full spectrum, spectral slope, or integrated spectral differences are considered.
A second concern relates to the interpretation of the Subglacior-LDC record. The manuscript attributes much of the discrepancy between observations and simulations to storage diffusion occurring during the five years between drilling and analysis. While this explanation is plausible and supported by previous work, the additional storage-diffusion experiment still fails to reproduce the observed profile and spectral characteristics. The discussion subsequently suggests that even stronger storage diffusion may be required. At present, this argument remains somewhat speculative because storage diffusion is invoked to explain residual discrepancies that are not directly quantified. The authors should clarify which aspects of the mismatch can be explained by storage effects and which remain unexplained.
The interpretation of the inferred surface mixing process would also benefit from further clarification. The manuscript demonstrates that introducing a mixing parameter improves agreement between simulations and observations, but the physical meaning of this parameter remains somewhat ambiguous. Throughout the discussion, the mixing term is interpreted as representing barometric pumping, firn ventilation, wind-driven snow reworking, and surface redistribution. These processes operate on different spatial and temporal scales and may not necessarily be represented by a single effective parameter. The limitations of this interpretation should therefore be discussed more explicitly.
Finally, I encourage the authors to moderate several statements that imply a direct physical estimate of an 8 cm mixing layer. The analysis demonstrates that an effective mixing scale of approximately 8 cm provides the best agreement within the current modelling framework. However, given the uncertainties associated with model structure, forcing data, chronology, and optimization criteria, it would be more appropriate to describe this value as a model-derived effective parameter rather than a uniquely constrained physical property of the firn.
Citation: https://doi.org/10.5194/egusphere-2026-871-RC3
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