| 08 Dec 2025
Temperature Dependence of Ice Crystal Size in Tibetan Ice Cores
Abstract. The ice crystal size in ice core can not only reflect the glacial strain process but also be connected to climate change. However, the process of ice crystal size variation along ice core in mountain glacier remains largely unexplored. Here, we continuously measured the ice grain areas along two deep ice cores drilled from the Tibetan Plateau, and found that the two ice cores exhibit vertical grain area differentiation at the hundred-meter scale, analogous to polar ice-core profiles, relatively higher temperatures significantly accelerate grain growth and result in larger grain areas. Refreezing under warm conditions gives rise to abrupt increases in grain area within melt-refrozen layers, whereas impurities result in abrupt decreases in grain area within cloudy bands. Together, these two factors drive centimeter-scale fluctuations in grain area. Even so, we also found that grain area exhibits a significant correlation with δ18O in ice layers where Rotation Recrystallization (RRX)-induced refinement is negligible, indicating that the ice crystal size of mountain glaciers ice core can retain temperature signals.
This manuscript is an important contribution to the study of ice microstructure in mountain glaciers. Microstructural investigations in these environments remain relatively scarce compared with polar regions (Greenland and Antarctic ice sheets), despite their considerable potential to improve our understanding of glacier dynamics and regional environmental changes. These types of studies are particularly valuable given the rapid retreat and ongoing degradation affecting many mountain glaciers worldwide due to global warming. In addition, due to their remoteness and difficult accessibility, obtaining ice cores from mountain glaciers often requires significant logistical effort, which further highlights the value of datasets derived from these archives.
The geographic setting of this study also adds considerable relevance. High Mountain Asia represents one of the most important cryospheric regions on Earth, often referred to as the “Third Pole”, and plays a critical role in regional hydrology and water resources. Investigations focusing on glacier records from this region therefore have clear scientific importance. The authors present microstructural observations that have the potential to provide meaningful insights into the evolution of glacier ice.
In order to further strengthen the manuscript and improve its clarity and scientific framing, I have several comments and suggestions that I hope the authors will find constructive. These mainly concern aspects of contextualization, methodological description, terminology, and the explanation of some interpretations presented in the text. I summarize these points briefly below before listing the specific comments in detail.
From a methodological perspective, several analytical procedures and aspects of the sampling strategy are not described in sufficient detail. In addition, some of the terminology and layer classifications used throughout the manuscript do not fully align with the more commonly adopted terminology in ice microstructure studies, which may create unnecessary ambiguity. Adopting more standard expressions could help enhance clarity. Finally, certain interpretations, particularly those linking microstructural patterns to temperature or climatic conditions, would benefit from clearer explanation and supporting information.
Introduction
lines 18-22
The statement presented in this section is conceptually sound; however, the introduction feels somewhat abrupt and would benefit from clearer contextual framing supported by relevant references. A brief overview of ice-core research would help situate the study within the broader field. Ice cores have provided long environmental and climatic reconstructions in polar regions such as Antarctica and Greenland, where this type of research is well established. Introducing this broader context before transitioning to mountain glacier ice cores would provide a smoother narrative. Although mountain records are generally shorter and more discontinuous, they offer regionally specific and highly sensitive environmental archives. Including representative references to previous studies on mountain glaciers would help highlight the significance and distinctive value of such records.
Lines 20-22
The link between ice-crystal characteristics, glacier dynamics, and potential implications for climate change is reasonable and would be clear to readers specialized in microstructural studies. However, in the introduction this connection remains somewhat implicit. It would strengthen the manuscript to briefly provide a concrete example illustrating how changes in crystal size or fabric can be related to climate variability or change.
Line 23
Change: “After decades of research, the polar ice core ‘three-stage model’ is widely adopted to characterize ice…” for “After decades of research, the polar ice core “three-stage model” has been widely adopted to describe ice…”.
Lines 28 and 29
The statement that ice crystal evolution is more complex in mid-latitude mountain glaciers effectively highlights the relevance of such studies. However, the explanation provided requires clarification. In particular, the reference to faster flow velocities and higher accumulation rates as inherent characteristics of warmer environments appears overly generalized. Warmer conditions alone do not necessarily imply faster glacier flow or higher accumulation, as these relationships depend on several other factors (e.g., impurities, slope, anisotropy, etc.) and may vary regionally. A brief clarification or justification of these links would improve the accuracy of the introduction.
Line 31
The term “typical mid-latitude glaciers” is used without a clear definition. It would be helpful to specify what is meant by mid-latitude in this context, at least by providing an approximate latitudinal range. In addition, including information on the typical elevation of these glaciers would strengthen the regional characterization.
The Tibetan Plateau is introduced without sufficient regional context. Given that the broader geographical framework of this study is High Mountain Asia (HMA), it would be important to clearly situate the Tibetan Plateau within this setting. HMA (often referred to as the “Third Pole” due to the extent of its glacierized area) represents a climatically and hydrologically critical region. Therefore, briefly clarifying that the Tibetan Plateau forms part of the third pole, and outlining its relevance, would improve clarity and help emphasize the broader significance of the study.
Line 32
The statement referring to “regional climatic and topographic heterogeneities” remains too general. It would be helpful to briefly specify which climatic and topographic factors are being considered (e.g., precipitation gradients, temperature variability, monsoonal/westerlies influence, relief, slope, etc.)
Line 33
The term “divergent microstructural characteristics” is unclear in this context. It is not evident with respect to what these characteristics are considered divergent. If the authors intend to emphasize truly divergent behavior, this distinction should be clearly explained and supported. Otherwise, a more neutral term such as “different” may be more appropriate, as the current wording suggests a stronger contrast than what is clearly justified in the text.
Line 34
It is necessary a more complete review of previous work on mountain glacier ice cores. Several ice cores have been retrieved from glaciers on the Tibetan Plateau, and these studies should be acknowledged with appropriate references.
In addition, microstructural investigations have also been carried out in other mountain glacier regions, including the Alps, the Pyrenees, and other mountain ranges worldwide. Although such studies remain relatively limited, they do exist and should be considered when framing the present research. Including these references would provide a more balanced overview of the existing literature.
Line 36
The use of a Microstructure Mapping system should be accompanied by appropriate references to the original developers or key methodological studies establishing this approach.
Line 37
The manuscript states that longitudinal microstructure images span from the firn to the base of the ice cores. It would be useful to clarify how the firn section was accessed. Was the firn exposed at the glacier surface, or was it reached by excavating down to the firn–ice transition before drilling? If excavation was required, a brief description of the procedure would improve methodological clarity.
Samples and Methods
Line 50
The drilling sites should be specified more clearly. It would be important to indicate whether the cores were retrieved from the accumulation or ablation zone, as the term “summit” does not necessarily imply a persistent accumulation area under current climatic conditions. The season of drilling should also be indicated to provide climatic context. If available, referencing previous studies on the glacier’s mass balance state (e.g., retreating, thinning, or stable) would further support the interpretation of the cores.
Line 51
Report the approximate snowline or equilibrium line altitude (ELA) for each glacier.
Line 53
Basic methodological details should be provided, including the type of drilling system used for core retrieval and the instruments used for temperature measurements.
Line 56
The description of the sampling interval requires clarification. The statement that “560 slices were obtained from 22.30 m to the base of the BJGR core” is ambiguous. It should be specified whether sampling starts at 22.30 m and continues downward to the base, and if so, why the upper section of the core was not included in the microstructural analysis. In addition, the meaning of “sampling rate” expressed as a percentage is unclear. It would be helpful to explain how this value is calculated and what it represents.
Line 57
The manuscript states that the sample “was trimmed flat using a Leica…”. While the term trimmed is sometimes used, it is not entirely clear which preparation method is being referred to. Based on the overall methodological description, it seems possible that the surface was prepared using a microtome. If this is the case, it may be more precise to state that the sample was microtomed to obtain a flat surface. Please, clarify the preparation method to improve the methodological description.
Line 58
It would be helpful to indicate in which facilities this process was carried out, as this would clarify the environmental conditions under which the sample preparation was performed.
Results and discussion
Line 76
The expression “firn layer (<35 m)” is somewhat ambiguous. If the authors intend to refer to the firn interval extending from the surface down to approximately 35 m, it would be clearer to specify this as “0–35 m”.
Line 78
The manuscript states that both ice cores reached the critical density of 0.830 g cm⁻³. If density measurements were performed on the cores, the methodology used to obtain these measurements should be described in the Methods section.
Line 79
The description of the grain area as “small” appears somewhat redundant, since the manuscript already provides the quantitative range (ANMQ: 2–5 mm2; BJGR: 1–3 mm2). Given that the values are explicitly reported, the qualitative descriptor may be unnecessary and could be omitted.
Line 80
It should be clarified whether these grain-size proportions refer to both ice cores or to a single core. As currently written, it is not clear whether the reported distributions apply to ANMQ, BJGR, or both.
Line 81
The subdivision into “3.1.1 Firn layer (<35 m)” and “3.1.2 Growth layer (35–45 m)” may require reconsideration. Grain growth appears to occur within the firn layer as well, meaning that both sections are characterized by grain-growth processes. As a result, the term “growth layer” may be somewhat misleading, since growth is not exclusive to that interval. It may be worth exploring alternative terminology that more clearly reflects the processes or structural characteristics distinguishing these layers.
Line 87
The transition between the general description of polar studies and the discussion of the present results is not clearly marked. In this sentence, the text appears to shift to the specific case of the studied cores, but this change is not explicitly indicated. It would be helpful to clearly signal when the discussion moves from general observations (polar areas) to the results of the present study in order to avoid confusion for the reader.
Line 92
Please clarify whether any post-drilling borehole measurements were performed, such as temperature logging or other physical profiling along the boreholes. Such data could help contextualize the processes discussed here.
Line 95
The term “stable layer” is somewhat confusing in this context. The description of this interval indicates ongoing changes, including decreases in average grain area and large-grain proportions, as well as increases in small-grain proportions and total grain number. This suggests that microstructural evolution is still occurring rather than remaining stable. It may therefore be helpful to reconsider the terminology or clarify what is meant by “stable” in this context.
Line 103
Minor typographical and formatting inconsistencies are present throughout the manuscript (e.g., missing spaces between words and symbols). A careful review of the document is recommended, as attention to these stylistic details improves readability and overall presentation.
Line 105
The manuscript suggests that higher temperatures enhance grain growth and lead to a larger equilibrium grain area. It would be helpful to indicate whether there is supporting evidence in the literature for this interpretation. If such relationships have been previously documented, including appropriate references would provide better support for the argument.
Line 113
The manuscript states that SIBM typically occurs when ice temperatures exceed −10 °C and that the basal temperatures of the studied ice cores satisfy this condition. However, the basal temperatures of the cores are not reported in the manuscript. It would be helpful to specify the temperature values (measured or estimated).
Line 114
The term “oversized grains” is qualitative and would benefit from quantitative support. If possible, it would be helpful to indicate the approximate grain size or provide a representative value or range to better substantiate this description.
Line 114
The statement refers to a temperature transition at ~150 m depth, but the actual temperature values are not reported.
Line 116
The terms “mutation layer” and “stable layer” are not standard in ice microstructure studies and is inappropriate in this context, as it may lead to confusion. More conventional terminology should be adopted. It may be clearer to define the sections based on depth intervals and dominant processes (e.g., polygonization, RRX, SIBM).
Line 123
Further explanation is needed. The manuscript states that the BJGR core reflects characteristics of Cold-type Ice Genesis, but the connection between the reported observations (lower temperature, lower firn density, smaller grain size) and this interpretation is not fully developed. It would strengthen the argument to more clearly explain how these observations support the classification as cold-type ice formation.
Line 134
The comparison presented here appears somewhat limited. It would be helpful to verify whether additional mountain glacier ice cores (particularly from High Mountain Asia) are available for comparison. If other records exist in the region, including them would strengthen the contextual interpretation.
In addition, comparisons with well-documented mountain glacier cores from other ranges (e.g., the Alps or other high-altitude regions) could also provide useful context. Expanding the set of reference sites would help place the reported firn temperatures in a broader framework.
Line 139
The description of cloudy band thickness requires clarification. It is unclear whether the statement means that the combined thickness of all cloudy bands within one meter of core is <20 cm, or whether it refers to an average thickness of ~20 cm per band. As currently written, the phrasing is ambiguous and should be explained more clearly.
Line 140
The description of the cloudy bands in this interval also requires clarification. It is not clear whether the reported total thickness of up to 90 cm refers to the combined thickness of all cloudy bands within the 120–140 m interval, or to the thickness of an individual band.
Line 146
The discussion linking impurities to grain-size variability is well supported by the cited polar ice-core studies and by the mountain glacier example already included (e.g., Eichler et al.) It may also be worth noting that similar relationships between impurities and ice microstructure have been reported in other mountain glacier settings. For instance, the study by González-Santacruz et al. (Annals of Glaciology) on the Monte Perdido Glacier in the Pyrenees documents comparable interactions between impurities and ice microstructure. Including these type reference could further broaden the contextual framework for interpreting impurity-related grain growth processes in mountain glaciers.
In addition, the manuscript would benefit from a brief overview of previous microstructural studies conducted on mountain glaciers. There is a long-standing body of work examining ice fabrics and microstructural characteristics in non-polar glacier settings that could help contextualize the present study. Early investigations include classic work on glaciers such as Aletsch (Perutz and Seligman, 1939) and Emmons Glacier (Rigsby, 1951), followed by comparative studies across several mountain and high-latitude glaciers (e.g., Rigsby, 1960; Kamb, 1959). More recent contributions have examined ice fabrics and microstructural properties in glaciers such as Tsanfleuron, Colle Gnifetti, Storglaciären, Rhône Glacier, sites in the Mont Blanc massif, and Jarvis Glacier, among others. While these studies often focus primarily on ice fabric, they nevertheless provide important background for understanding microstructural evolution in mountain glacier environments. Including a brief acknowledgment of this body of work would help place the present results within the broader historical development of mountain glacier microstructural research.
Line 149
“Ice temperature” instead of only “temperature”
Line 162
The interpretation that low d18O values in the cloudy-band zone reflect colder conditions is reasonable and consistent with the isotopic signal. However, the expression “cold periods” may benefit from further clarification. As currently written, it is not entirely clear whether the authors refer to large-scale climatic phases (e.g., glacial conditions) or to shorter-term colder intervals.
It is understandable that resolving the exact temporal scale may be difficult given the limitations of the chronology. Nevertheless, it may be useful to acknowledge the possible range of temporal scales involved. Previous studies have linked higher dust concentrations in ice cores to glacial conditions through mechanisms such as increased continental aridity and stronger atmospheric circulation. However, it is less clear whether the same mechanisms apply to shorter-term or seasonal cold intervals. Briefly recognizing these different possible scales would help refine the interpretation and avoid ambiguity.
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