the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2025
Air clathrate hydrates in the EDML ice core, Antarctica
Abstract. In the deeper part of polar ice sheets, air hydrates trap most of the ancient air molecules, which are essential for understanding past climate. We use digital image analysis to create a high-resolution record of air hydrate number, size and shape from ice thick section microphotographs of the EPICA Dronning Maud Land (EDML) ice core, Antarctica, over a depth range from 1255–2771 m. We confirm that the air hydrate number and size correlate with paleoclimate and that the correlation disappears in the deeper parts of the ice core, which was previously shown for the Vostok and Dome Fuji ice cores in Antarctica and the GRIP ice core in Greenland. We also observe that the air hydrates grow with depth. Furthermore, we identify two peculiarities: A distinctive change in air hydrate aspect-ratio and orientation at about 2030 m and a region of increased air hydrate clustering from 2392–2545 m depth. Remarkably, they coincide with regions of distinctive changes in ice microstructure as response to changes in local ice dynamics and, therefore, we discuss the influence of ice deformation on the air hydrate ensemble.
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RC1: 'Comment on egusphere-2025-633', Anonymous Referee #1, 17 Apr 2025
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General comments
The manuscript presents the first experimental data on the air-hydrate geometrical properties in the EDML ice core, below the bubbles-to-hydrates transition, which were obtained by automated image analysis of microphotographs of thick sections of ice taken in the field a few days after core retrieval. The resulting data set represents a significant addition to the existing experimental data on the geometric properties of air hydrates in polar ice sheets, which until recently were obtained only from GRIP, Vostok, and Dome Fuji ice cores. Specific conditions of the EDML site on the Antarctic Plateau (relatively warm temperature of ice and enhanced accumulation, location on the ice divide) allowed authors to reveal some new peculiarities of hydrate properties such as weakening of their correlation with the climate (isotope content of ice) on small time scales, and a distinctive change in air hydrate properties associated with changes in the deformation regime of ice reflected in its microstructure. The most important achievement of this work is a carefully developed technique of using automatic analysis of high-quality images of thick sections of ice in order to obtain various geometric characteristics of air-hydrate inclusions in polar ice. This technique was, for the first time, successfully applied to quantitatively describe air hydrates in an Antarctic ice core. Its further refinement will provide a reliable tool for the study of future ice cores, including those that will be obtained in Antarctica as part of the Oldest Ice project. To summarize, I believe that this work deserves to be published in TC after the small corrections and clarifications suggested below are made by the authors.
Concerning the presentation of the material, I would like to encourage the authors to make an additional effort to improve the structure of the text in order to make it easier to read and understand.To me, the most obvious idea in this regard would be to move sections 4.1 and 4.2 of the ms from Discussion to Results, and to merge them with sections 3.1 and 3.2, respectively. Some of my specific comments below also reflect the difficulties I encountered in reading the manuscript and therefore, hopefully, may help to improve its presentation.
Specific comments
L15-17: The BHTZ depth range depends on the temperature in the ice sheet (e.g., 500-1250 m at the cold Vostok site, but 1000-1500 m at the GRIP drilling site, which is warmer). What site does the 500-1500 m depth interval refer to? I did not find in the ms clear information about the BHTZ depths at the EDML site, which is important for interpreting the bubble and hydrate data. At L249 one reads: “Note that LGM-ice at EDML coincides with the top of the BHTZ”, which is a bit misleading, since the LGM ice at EDML is buried at a depth of about 1000 m. According to Bendel et al. (2013), the BHTZ at EDML is between 700 and 1200 m and therefore LGM ice coincides with the middle of the transition zone, which disrupts the correlation of bubbles’ properties with climate. Please state clearly the BHTZ depths at EDML in the introduction. It would also be useful to give in the introduction the maximal depth (from the ice core log) of the borehole when it reached the bedrock (at 2774.15 m?).
L28-30: To the best of my knowledge, the most recent review on this topic is given in Lipenkov V. Y. How air bubbles form in polar ice, Earth’s Cryosphere, 2018, 22, 16–28.
L186-188: For hydrate inclusions with non-isometric shape and preferred orientation in space, it is also important to choose the right plane for the thick section analysis (i.e. image plane) that would allow proper assessment of inclusions’ dimensions and aspect ratios. Have you tried to solve this problem, and if so, how?
L196 “Surprisingly, no air hydrates were found in the lowermost sample at about 2774 m depth”: Any comment on this: distance of this sample from the bedrock, measured air content in it, accreted ice?
L68-69 “For this study, mosaic images located about 1.5 mm below the sample surface were analyzed…”; L179 “We convert the measured air hydrate counts per sample to Nah using the measured ice-sample thickness…”; L180-182 “Air hydrate counts per sample and Nah have a good linear correlation (Fig. 4) and we conclude that the observed volume (i.e. hydrate mapping depth; Fig. 1) is consistent for all samples”; L253-254 “… the hydrate-mapping depth in our microscopic set-up might not correspond to the physical sample thickness (Fig. 1). In other words, the microphotographs from one focus plane might not display the entire sample volume”. The construction of the narrative is such that it is not until L253-255 that the reader begins to realize that you could not have obtained Nah (in cm-3) because you did not know the thickness of the ice layer (the hydrate mapping depth) within which the hydrate counts were made. Why not say this at the very beginning, and treat the hydrate count data as a reliable but relative metric of Nah (assuming the observed volume was the same in all samples), and then scale that metric with the measured air content of ice, and use the thus derived Nah in further consideration? (Such a relative metric of Nah can be the density of hydrates’ projections (cm-2) observed over an 80 x 30 mm area of the mosaic image of the sample).
Figure 5 caption: “Error-band for b) is explained in section 2.4”. I could not find an estimate of the resulting Nah error in Section 2.4. In any case, I now suspect that this resulting error shown in Fig. 5 does not account for the difference between the sample thickness that was used to calculate Nah and the hydrate mapping depth.
L226 “Naturally, the TACh and Vc signals show the same pattern (Fig. 6b)”. Not sure that this statement matters when both TACh and Vc are shown by a single graph. Delete the sentence?
L237-241: It would be appropriate here (or elsewhere) to provide information on the inclination of the borehole from which the studied ice core was obtained.
L247-248: In Fig. 3 from Bendel et al. (2013), the average Nab value in LGM ice appears to be closer to 400 cm-3.
L250-251: Number concentration of air bubbles and that of succeeding hydrates (if we assume one-to-one conversion) depends on the ice formation conditions (accumulation rate, firn temperature and surface snow density), so the properties of the Vostok ice cannot be simply projected onto the EDML ice core. Using a simple model, as described in Lipenkov (2018), one can estimate Nab ~400 cm-3 for present-day conditions at EDML (-44.5 °C; 6.4 g/cm2 yr; 0.38 g/cm3) and Nab ~475 cm-3 in the LGM ice (assuming: -54 °C; 3 g/cm2 yr; 0.38 g/cm3). The model estimate for Holocene ice is close to Bendel’s et al. data while that for the LGM ice is slightly higher than their experimental data (because LGM ice coincides with the transition zone), and lower than the Vostok-based value used in the ms (the extremely low LGM temperature at Vostok led to a very small grain size at close-off and hence a great number of bubbles forming).
L259: It appears that the CFA system used by Ruth et al. (2004) does not provide accurate absolute values of air content (it was designed primarily to document high-resolution relative TAC variations). For that reason, the obtained average value (0.0815 cm3/g) is lower than the TAC measured with absolute methods in the Holocene ice cores from the most elevated drilling sites in Antarctica (Dome F, Vostok, EDC). For the EDML elevation (2892 masl) and atmospheric pressure (~700 mb) we can expect the TAC to be slightly above 0.09 cm3/g (Martinerie et al., 1992) in Holocene ice and even higher in LGM ice. This prediction has been confirmed by the measurements made with an absolute barometric method in LGGE/IGE, Grenoble, which gave TAC=0.0906±0.0025 cm3/g in Holocene ice, 0.0970±0.0020 cm3/g in LGM ice, and a mean value equal to 0.0924±0.0031 cm3/g (unpublished data). I suggest that the authors use estimates based on Martinerie, P., Raynaud, D., Etheridge, D.V., Barnola, J.-M., Mazaudier, D. Physical and climatic parameters which influence the air content in polar ice. Earth Planet. Sci. Lett. 1992, 112: 1-113, rather than the experimental data from Ruth et al. (2004).
L260: It may be noted here that uncertainty in hydrate size determination can also be the cause of the observed mismatch between calculated and measured air content (Vc is very sensitive to even small errors in hydrate linear dimensions), but in this particular case the greatest contribution to this mismatch comes, of course, from the uncertainty in Nah.
L267: Replace “air hydrates dissociating” with “air hydrates dissolving”.
L269-273: This reasoning seems a bit odd to me, because: 1) below the BHTZ, almost all of the air trapped by ice (98% or so) is stored in air hydrates. Therefore, whatever evolution the hydrate system undergoes (growth of larger hydrates at the expense of smaller ones, coalescence, etc.), the geometric properties of the hydrate ensemble, if accurately measured, should allow us to correctly estimate the TACh; 2) if the cage occupancy in Eq. (9) is erroneously overestimated, the calculated TACh must also be overestimated, but it seems to show the opposite trend?
L286-289: The geometric properties of air hydrates (inherited from the properties of bubbles) depend on both the temperature and accumulation rate prevailing during ice formation. The correlation between accumulation and temperature (isotope content of ice) on small time scales is not as obvious as in the case of global glacial-interglacial climate changes, and may not exist at all. This may also be the reason for the weakened correlation between Nah and δ18O in the case of smaller-scale climatic fluctuations.
L299-300: I would rewrite as “Due to the greater surface curvature, smaller particles are more soluble than larger particles…”
L310-312: With the exception of two data points, I do not see particularly large mean hydrate diameters in region 2 in Fig. 5c. It would be good to say here how the inability of your image analysis to distinguish individual air-hydrate crystals within their clusters may affect the count of hydrate number? It seems like it should lead to an underestimation of the number of individual hydrates (although the hydrate number concentration shows anomalously high values in zone 2).
L312-313: I cannot agree with this explanation for the abnormal volume concentration and TACh. Whatever the mechanism of hydrate coalescence, this process cannot change the initial (pre-coalescence) volume concentration of hydrates (and TACh), since this property of ice is as constant as its air content. As for the quantitative assessment of the geometrical properties of all hydrate inclusions (without discrimination into clusters and individual hydrate crystals), one would expect an underestimation of the size of hydrate clusters rather than an incorrect count of the total number of hydrate inclusions (individual crystals and their clusters), which should lead to an underestimation of the calculated total volume concentration of hydrates, but not the other way around. The underestimation of cluster size in image analysis could be quite expected due to the partial overlap of the projections of two or more individual hydrate crystals included in the cluster (see Fig. 7c) and the difficulty in assessing the cluster size in the direction normal to the image plane.
L326-327: It is not quite clear what phenomenon the authors have in mind, but if it is the disappearance of the climate-related variations in the hydrate properties in the course of Ostwald ripening, then the fundamental reason for this is that the hydrate growth rates are inversely proportional to the square of the mean initial hydrate size, which leads to dumping of the climatically induced variations in the hydrates’ geometrical properties (Salamatin et al., 2003).
Technical corrections
The caption to Fig 5 needs some explanation of the zones 1 and 2 shown in the figure.
Is the subscript 0 at n0 really needed in Eq. (9)?
L207: Reference to Fig. 8 is given before references to Figs. 6 и 7. Consider changing the order of the figures.
In Fig. 6, the blue and green symbols are not well distinguishable on my screen. Consider changing the colors.
Citation: https://doi.org/10.5194/egusphere-2025-633-RC1
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