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Ice Dynamics in Caves
CHAPTER
ICE DYNAMICS IN CAVES
4.3
Aurel Pers‚oiu
Emil Racoviţă Institute of Speleology, Cluj-Napoca, Romania
C HAPTER OUTLINE 4.3.1 Introduction.................................................................................................................................... 97 4.3.2 Subannual Dynamics.................................................................................................................... 100 4.3.2.1 Hoar Frost........................................................................................................... 100 4.3.2.2 Ice Speleothems (stalagmites, stalactites, and columns).......................................... 101 4.3.3 Multiannual to Centennial Ice Dynamics........................................................................................ 103 References............................................................................................................................................ 107
4.3.1 INTRODUCTION Ice accumulations are highly dynamic occurrences in caves, changing their morphology in response to the cave morphology and climatic conditions on scales ranging from days to millennia. Ice in caves occurs mainly as a result of water freezing and to a lesser extent of snow densification and diagenesis. The latter processes occur in cave entrances, where snowpacks up to 100 m in thickness occur, being subsequently compacted to form crystalline ice. Transformation of snow into ice does not occur as a “pure” compaction; freezing of water that percolates through the snow mass also contributes to the genesis of perennial ice masses. The results of these processes are large bodies of perennial ice that will subsequently deform and flow gravitationally towards the lower parts of the caves that host them. Contrary to these, ice formed by the freezing of water takes a large variety of shapes and sizes, from millimeter-scale, ephemeral ice crusts on the walls of caves, to large ice bodies, tens of thousands of cubic meters in size. Climatic factors strongly determine their dynamics on time scales ranging from hours to millennia, while the caves' morphology acts on scales on the order of centuries to millennia. A third, less-obvious process that leads to ice formation and subsequent dynamics, is that of ice sublimation, that is, the formation of ice crystals on the undercooled walls of caves, as warm and moist air is pushed out from the inner sections of these caverns. As a result of the above summarized processes, two classes of ice forms develop in caves (Fig. 4.3.1): perennial ice bodies and seasonal to multiannual ice speleothems (stalagmites, stalactites, ice crusts, and crystals). While most authors agree that the later are true speleothems (secondary mineral deposits formed in caves, sensu Hill and Forti, 1997) as they are minerals (crystalline H2O), they are of secondary genesis (formed by the freezing of water) and they have formed in caves; no consensus exists on whether the large, perennial, ice bodies accumulated in caves are speleothems, glaciers, both, or neither. Ice Caves. https://doi.org/10.1016/B978-0-12-811739-2.00034-6 © 2018 Elsevier Inc. All rights reserved.
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FIG. 4.3.1 Various ice bodies in caves—large ice block, semiperennial stalagmites, ice domes and seasonal stalactites (Scărișoara Ice Cave, Romania).
Here I argue that the large, perennial ice bodies found in caves and that have formed through water freezing are both speleothems and glaciers, while the similar ones that have formed by snow diagenesis are glaciers only, not speleothems, regardless of the fact that they are in caves. Snow diagenesis, regardless where it occurs, is a pure glacial process: snow is being slowly compacted and as air is squeezed out, ice develops. Infiltration and freezing of water within the ice body speeds up the compaction of the snow pack, and the end result is a layered mass of ice, with variable density (higher for the congelation ice), that also incorporates organic and inorganic sediments. While the end result is a large ice body inside a cave, the processes that led to its genesis are restricted to the caves' entrances, or, at most, to those areas that are directly fed by snow (e.g., the bottom of long, dipping slopes below cave entrances), thus being external to the cave environment. Freezing of liquid water to form perennial, large ice bodies can only occur (with the notable exception of permafrost) in single entrance, descending caves that act as cold air traps, maintaining negative temperatures throughout the year. In these caves, ice can form either as thin sheets of water freeze on sloping surfaces, or as shallow lakes freeze in autumn and winter. Most commonly, both these processes occur in caves (Fig. 4.3.2). In summer, lakes up to 30 cm deep can accumulate on top of existing ice bodies and will start to freeze, top to bottom, during a cooling period in late autumn through early winter to form a layer of “lake ice,” 1–30 cm thick. Warmer intervals during winter could lead to snow melting and water infiltration in caves to form thin sheets of “floor ice” (Pers‚oiu and Pazdur, 2011) on top of the existing lake ice. The end result, on a multiannual scale, is a layered body of secondary genesis crystalline H2O, formed by a cave-specific processes—a speleothem. Once formed, the large perennial ice bodies to be found in caves display a dual dynamic: they both accumulate and lose ice on top, as a result of both short and long term climatic variability and also, like any large surface glacier, flow under the influence of gravity. This later behavior occurs for both types (in terms of genesis) of large cave ice masses, resulting in flow-specific structures—ice thinning along the flow path, folds, push-moraines in front of the advancing (and melting) ice tongues, etc. (Fig. 4.3.3).
FIG. 4.3.2 Genesis of ice in caves through the freezing of water: (A) Formation of seasonal forms (stalagmites, stalactites, columns) occur intermittently throughout the winter, intensifying in early spring, when temperatures inside the caves are still negative and large amounts of water (released by melting snow) are available. With increasing outside temperature, warm water will start melting the ice. (B) Perennial ice bodies form by the freezing of lakes (accumulated in summer) in the early periods of cave cooling (lake ice-L) and, additionally, freezing of infiltrating water (floor ice-F) on top of existing lake ice, during winter.
FIG. 4.3.3 The underground glacier of Scărișoara Ice Cave. Layers deformed by flow are visible on both sides of the vertical wall, and an ice tongue extends on the lower-left side of the image. A push-moraine is visible in front of the advancing glacier.
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Thus, while in terms of their genesis, only large, perennial ice blocks formed by the freezing of water can be termed speleothems, contrary to the ice accumulations formed by snow diagenesis, both these two types of cave ice bodies are glaciers, in terms of postgenesis behavior. The dynamics of all ice formations in caves are the result of external climatic influences, cave morphology, or the combination of the two. Whereas the former acts on time scales ranging from days to millennia, the latter is only active on centennial scales—and the dynamics of cave ice will be analyzed further along these temporal differentiations (Fig. 4.3.4).
FIG. 4.3.4 Conceptual model of an ice cave showing the temporal range of dynamics for various ice formations: (A) subannual dynamics (accumulation and degradation); (B) multiannual dynamics (accumulation and ablation); (C) decadal to millennial dynamics (accumulation and ablation); (B′) multiannual dynamics— ablation only; (C′) decadal to millennial dynamics—flow and ablation only.
For clarity in the subsequent text, I will use the term “speleothem” for stalagmites, stalactites, and columns only, and “glacier” for perennial ice blocks.
4.3.2 SUBANNUAL DYNAMICS Accumulation and ablation of ice in caves on daily to monthly time scales affects all ice speleothems— stalagmites, stalactites, and columns, as well as the seasonally occurring hoar frost and the large, perennial glaciers.
4.3.2.1 HOAR FROST Hoar frost is the most dynamic form of ice in caves. Two processes are acting in conjunction to allow for the development of hoar frost: substratum (rock wall) undercooling and circulation carrying moisture towards the cold walls. The process starts in early winter, when external cold air, triggered by its higher density, sinks into the caves, leading to the slow undercooling of the walls. As this cold air reaches the
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caves, it pushes out the warmer and moister cave air, which will condensate on the wall as liquid water. Continuous inflow of cold air, combined with water evaporation from the rock will cool the walls to temperatures below 0°C, thus setting the scene for the initiation of hoar frost development (Pflitsch et al., 2007). Continuous inflow of cold air leads to the development of a bidirectional air circulation, with colder air swiping on the surface of ice and pushing out the warm and moist air along the ceilings, resulting in the deposition and growth of large ice crystals (Fig. 4.3.5). The moisture feeding the growth of crystals has a dual origin: the in situ moisture existing in the warmer sector of the caves, and moisture resulting from the sublimation of cave ice under the influence of cold and dry inflow of cave air, flowing on top of the ice. The hoar frost deposits develop and survive until early to mid-summer, when inflow of cold air ceases and both air and rock start to warm, leading to rapid degradation of the crystals. Warming of the walls results in crystals falling to the ground, where they either melt away, as in most cases, or, where temperatures are still below 0°C, they can accumulate on top of the existing ice to contribute to the growing of the ice block (Marshall and Brown, 1974). As a result of these processes, the dynamics of hoar frost in ice caves follows a simple, annual cycle, being present between mid- and late winter (when they reach a maximum) and absent throughout the rest of the year. The size of the crystals is dependent on the intensity of wall undercooling, the strength of air circulation, and amount of moisture available, reaching a maximum of 50–60 cm, and usual values of 5–15 cm (Fig. 4.3.5).
FIG. 4.3.5 Mechanism of hoar frost deposition in ice caves (A) and close-up view of the resulting ice crystals (B) in Scărișoara Ice Cave. The blue star marks the location of hoar frost deposits, and the two red dots indicate the sources of moisture. Length of scale bar is 3 cm.
4.3.2.2 ICE SPELEOTHEMS (STALAGMITES, STALACTITES, AND COLUMNS) Ice stalagmites and stalactites are common occurrences in cave entrances during the cold season in mid-to-high-latitude and/or altitude caves. They form as drip water freezes and usually melt as the temperature of the water feeding them becomes positive. In caves with perennial ice, these stalagmites can be perennial, the negative temperatures preserved throughout the year in the vicinity of underground glaciers helping their survival. However, differences exist between stalagmites, on one hand, and stalactites and columns, on the other, as the latter melt earlier and usually completely, the warm water leading to their detachment from the ceiling and collapse. The dynamics of these ice formations have been studied in great detail in Scăris‚oara Ice Cave, Romania (SIC), by Viehmann and Racoviţă (1968), Racoviţă et al. (1987), and Racoviţă (1994), and it is summarized below, as a “template” for other caves, as well.
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CHAPTER 4.3 ICE DYNAMICS IN CAVES
In Scăris‚oara Ice Cave, the dynamics of ice speleothems were studied separately for stalagmites, ice massifs, and ice crusts on the cave floor (Fig. 4.3.6), on both annual and subannual time scales. ∆h (cm) 4
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FIG. 4.3.6 Seasonal variations (10 years average) of the upper face of the ice block in SIC (A), ice massifs (B), ice stalagmites (C), and floor ice crust (D). The ice massifs are similar to the one visible on the left in Fig. 4.3.1. Modified from Racoviţă, G., 1994. Eléments fondamentaux dans la dynamique des spéléothèmes de glace de la grotte de Scărișoara, en relation avec la météorologie externe. Theor. Appl. Karstol. 7, 133–148.
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For all these speleothems, a maximum was reached between March and June, with the larger ice bodies having a delayed onset of melting. Ice crusts of the floor (Fig. 4.3.6D) grow outwards from the edges of the ice block, thinning with increased distance, their dynamics being controlled by the inflow of cold air which sweeps the upper surface of the ice block and leads to water freezing. However, this genetic mechanism renders them extremely vulnerable to melting, a process which initiates once the inflow of cold air ceases in early spring (Figs. 4.3.2 and 4.3.6D). The melting of the larger ice forms, both stalagmites and ice massifs (Fig. 4.3.6B and C), is delayed by the larger thermal inertia of the ice (Pers‚oiu et al., 2011). However, regardless of their size and shape, all ice forms in caves (including the large ice blocks) reach a minimum at the end of the melting season, just before the onset of freezing, usually in November (Figs. 4.3.2 and 4.3.6). Both growing and melting of ice is a complex process, controlled by the variable interplay between air temperature and precipitation amount and distribution, thus there is no clear correlation with either of these two (Pers‚oiu et al., 2011). The effect of precipitation amounts reaching ice caves on ice dynamics is strongly dependent of air temperature during winter, with water input superimposed on below 0°C conditions leading to rapid ice build-up, while the same input occurring during periods with positive temperature anomalies resulting in ice loss. However, in summer, temperature doesn't play too important a role, as the latent heat of the ice prevents temperature inside the caves reaching above 0°C. Thus, water input is the main factor leading to ice ablation, the heat delivered to the ice by inflowing warm water being the main factor in the ablation of ice. A peculiar type of subannual ice dynamics of ice is that of the “thermoindicator” speleothems (Viehmann and Racoviţă, 1968, Fig. 4.3.7). These form in winter, during periods of alternating cold and mild weather, with the translucent, bulky sections developing when temperatures are between 0°C and −2°C, and drip water will freeze slowly, expelling gas and calcite impurities. When temperatures drop below −3°C, dripping water will tend to freeze quickly, incorporating both air bubbles and in situ precipitated cryogenic cave calcite (Žák et al., 2008), hence the semiopaque appearance of the ice.
4.3.3 MULTIANNUAL TO CENTENNIAL ICE DYNAMICS Long-term dynamics of ice has been observed in a few caves, and monitoring programs exist in even fewer ones (Kern and Pers‚oiu, 2013). In Scăris‚oara Ice Cave, a monitoring program was in place for ice speleothems as well (summarized in Racoviţă, 1994) between 1965 and 1992, the results indicating an inverse relationship between the lowering of the upper face of the ice black (mass loss) and increase in the height of stalagmites. Racoviţă et al (1987) suggested that this contrasting behavior is the result of long-term periodicity in the dynamics of ice that could be possibly linked to climatic conditions outside the cave. However, owing to the short time series available (53 years between 1963 and 2017) and long-term suggested periodicity (50+ years) no such link has been found yet (Pers‚oiu and Pazdur, 2011), although photographic evidence from 1947 (Șerban et al., 1948) suggest that ice speleothems at that time were taller than in the 1960s. The dynamics of the glaciers in caves has been investigated sporadically in the past century, by combining various sources of data: photographic evidence, markers in the ice and/or cave walls, absolute dating, regular monitoring programs (Kern and Pers‚oiu, 2013 and references therein). However, with a few exceptions, monitoring programs longer than a few years are very rare (Ohata et al., 1994; Rachlewicz and Szczucinski, 2004; Fuhrmann, 2007; Pers‚oiu and Pazdur, 2011; Kern and Thomas, 2014). With little exception, most monitoring programs indicate a continuous and accelerated melting of cave ice, especially in the past 20–30 years (Fig. 4.3.8).
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CHAPTER 4.3 ICE DYNAMICS IN CAVES
FIG. 4.3.7 Thermoindicator speleothems in Scărișoara Ice Cave.
The causes of the decadal to centennial mass balance changes in caves can be grouped in three classes: (1) morphologic, (2) climatic and (3) endogenic. The morphologic ones are the consequences of changes in the dynamics of airflows and associated heat transfer in caves related to changes in the morphology of the open spaces as ice grows and shrinks (while the morphology of the rock walls in ice caves could changes as a result of collapses, no such occurrences have been reported so far in the literature). These changes in turn affect the climate of the caves and the behavior of ice, resulting in long-term modifications of the ice morphology and dynamics. In Scăris‚oara Ice Cave (Romania) such a case was described by Șerban et al. (1948), with the ice block switching between several phases of growth and decay (Fig. 4.3.9).
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FIG. 4.3.8 Cumulative ice loss in ice caves in the Northern Hemisphere during the last 100 years. From Kern, Z., Perșoiu, A., 2013. Cave ice—the imminent loss of untapped mid-latitude cryospheric paleoenvironmental archives. Quat. Sci. Rev. 67, 1–7.
FIG. 4.3.9 Growth and retreat of the ice block in Scărișoara Ice Cave (Romania). Original sketches by Mihai Șerban.
In phases I–III, ice grew through water freezing in layers of “lake ice,” however the growth of ice progressively isolated the lowermost sections of the incipient ice block, and the constant geothermal heat flux (70 mW/m2, Demetrescu and Andreescu, 1994) led to the melting of the ice and progressive growth towards the surface of the resulting opening (phases IV–VIII in Fig. 4.3.8). Once this opening reached the surface, inflowing water started to freeze on the vertical, exposed ice wall, progressively
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closing the opening (phases IX–XII) reinitiating a new cycle. Presently, the ice block is a stage similar to phase IX (Fig. 4.3.3), with newly formed ice growing on top of the exposed, older, ice. A secondary dynamic process related to melting near the base is that of ice flow, leading to progressive growth of folds in the ice (Fig. 4.3.10).
FIG. 4.3.10 Ice flow and folding in Svarthammarhola, Norway.
The climatic conditions affecting the dynamics of cave glaciers have are acting in a more predictable fashion, with simple, annual cycles (Fig. 4.3.6A) superimposed on long-term trends, generally pointing towards continuous melt (Fig. 4.3.8). While increasing air temperature (and possibly diminishing precipitation during the growing season) are likely the main causes leading to general ice loss, observations of a peculiar ice loss case in the Romanian Carpathians suggest that cave glaciers might respond nonlinearly to climatic conditions: in “Adevăratul Gheţar de la Vârtop” Ice Cave, a 6 m thick, ~2000 m3 ice block vanished completely between 2008 and 2012, its demise being possibly triggered by summer inflow of warm water that lead to catastrophic disintegration once the glacier was dissected in smaller ice blocks. Only a few ice caves have ages going back more than a thousand years, so that the dynamics over centennial to millennial scales is difficult to assess. Studies in Swiss (Luetscher, 2005), Austrian (Spötl et al., 2014) and Romanian (Pers‚oiu et al., 2017) caves have shown that ice in these caves responded sensitively to the main climatic swings during the past 2000 years, with ice accumulating in caves during cold and/or wet periods (the Dark Ages Cold Period of the 4–7th centuries AD and the Little Ice Age, between the 14th and 19th centuries AD), and melting during the medieval Warm period (between the 9th and 13th centuries AD). Stoffel et al. (2009) and Pers‚oiu et al (2017) have further shown that long-term changes in ice mass balance and dynamics could be controlled by large-scale circulation
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p atterns affecting Europe and these could also be recorded by the isotopic composition of cave, thus being a valuable tool in reconstructing past climatic conditions. Geothermal heat delivered to the base of cave glaciers (Fig. 4.3.4C) has been shown (Tulis and Novotný, 2003; Luetscher, 2005; Pers‚oiu, 2005) to lead to variable rates of melt: 8 cm/year in Monlesi Ice Cave (Switzerland), 1.53 cm/year in Scăris‚oara ice Cave (Romania) and 1 cm/year in Dobsinska Ice Cave (Slovakia). However, these basal melting rates were calculated based on limited observation, and the resulting mass turnover rates are not in agreement with the age of the cave glaciers (Kern, this volume), thus suggesting that either the estimates are wrong (by a factor of 10 in Scăris‚oara Ice Cave), or that basal melting was not happening at a constant rate during the existence of the ice blocks.
REFERENCES Demetrescu, C., Andreescu, M., 1994. On the thermal regime of some tectonic units in a continental collision environment in Romania. Tectonophysics 230 (3–4), 265–276. Fuhrmann, K., 2007. Monitoring the disappearance of a perennial ice deposit in Merrill Cave. J. Caves Karst Stud. 69, 256–265. Hill, C.A., Forti, P., 1997. Cave Minerals of the World, second ed., National Speleological Society Inc, Huntsville, Alabama, pp. 217–225. Kern, Z., Pers‚oiu, A., 2013. Cave ice—the imminent loss of untapped mid-latitude cryospheric paleoenvironmental archives. Quat. Sci. Rev. 67, 1–7. Kern, Z., Thomas, S., 2014. Ice level changes from seasonal to decadal time-scales observed in lava tubes, lava beds national monument, NE California, USA. Geogr. Fis. Din. Quat. 37, 151–162. Luetscher, M., 2005. Processes in Ice Caves and their Significance for Paleoenvironmental Reconstructions. Swiss Institute for Speleology and Karst Studies, La Chaux-de-Fonds, Switzerland, p. 154. Marshall, P., Brown, M.C., 1974. Ice in Coultard Cave, Alberta. Can. J. Earth Sci. 11, 510–518. Ohata, T., Furukawa, T., Osada, K., 1994. Glacioclimatological study of perennial ice in the Fuji ice cave, Japan. Part 2: interannual variation and relation to climate. Arct. Alp. Res. 26, 238–244. Pers‚oiu, A., 2005. Evidence of basal melting of the ice block from Scăris‚oara Ice Cave. In: Mavlyudov, B.R. (Ed.), Glacier Caves and Glacial Karst in High Mountains and Polar Regions, Institute of Geography of the Russian Academy of Sciences, Moscow, pp. 109–112. Pers‚oiu, A., Pazdur, A., 2011. Ice genesis and its long-term mass balance and dynamics in Scăris‚oara Ice Cave, Romania. Cryosphere 5 (1), 45–53. Pers‚oiu, A., Onac, B.P., Pers‚oiu, I., 2011. The interplay between air temperature and ice dynamics in Scăris‚oara Ice Cave, Romania. Acta Cardiol. 40 (3), 445–456. Pers‚oiu, A., Onac, B.P., Wynn, J.G., Blaauw, M., Ioniță, M., Hansson, M., 2017. Holocene winter climate variability in Central and Eastern Europe. Sci. Rep. 7, 1196. Pflitsch, A., Piasecki, J., Sawiński, T., Strug, K., Zelinka, J., 2007. Development and degradation of ice crystals sediment in Dobšinská Ice Cave (Slovakia). In: Zelinka, J. (Ed.), Proceedings of the 2nd International Workshop on Ice Caves, Demänovská dolina, Slovak Republic, pp. 38–49. Rachlewicz, G., Szczucinski, W., 2004. Sesonal, annual and decadal ice mass balance changes in the ice cave Jaskinia Lodowa w Ciemniaku, the Tatra Mountains. Poland. Theor. Appl. Karstol. 17, 11–18. Racoviță, G., 1994. Eléments fondamentaux dans la dynamique des spéléothèmes de glace de la grotte de Scărișoara, en relation avec la météorologie externe. Theor. Appl. Karstol. 7, 133–148. Racoviță, G., Șerban, M., Viehmann, I., 1987. Tendances de long terme dans la dynamique des formations de glace de la Grotte de Scărișoara (Monts du Bihor). Theor. Appl. Karstol. 3, 143–164. Șerban, M., Coman, D., Givulescu, R., 1948. Découvertes récentes et observations sur la glacière naturelle dite, Gheţarul de la Scărișoara. Bull. Soc. Sci. Cluj X, 174–210.
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Spötl, C., Reimer, P.J., Luetscher, M., 2014. Long-term mass balance of perennial firn and ice in an Alpine cave (Austria): constraints from radiocarbon-dated wood fragments. The Holocene 24, 165–175. Stoffel, M., Luetscher, M., Bollschweiler, M., et al., 2009. Evidence of NAO control on subsurface ice accumulation in a 1200 yr. old cave-ice sequence, St. Livres ice cave, Switzerland. Quat. Res. 72, 16–26. Tulis, J., Novotný, L., 2003. Changes of glaciation in the Dobšinská Ice Cave. Aragonit 8, 7–9. Viehmann, I., Racoviță, G., 1968. Stalagmitele de gheață termoindicatoare. Dări de Seamă Comit. Geol. LIV, 353–363. Žák, K., Onac, B.P., Pers‚oiu, A., 2008. Cryogenic carbonates in cave environments: a review. Quat. Int. 187 (1), 84–96.
ICE DYNAMICS IN CAVES
4.3
Aurel Pers‚oiu
Emil Racoviţă Institute of Speleology, Cluj-Napoca, Romania
C HAPTER OUTLINE 4.3.1 Introduction.................................................................................................................................... 97 4.3.2 Subannual Dynamics.................................................................................................................... 100 4.3.2.1 Hoar Frost........................................................................................................... 100 4.3.2.2 Ice Speleothems (stalagmites, stalactites, and columns).......................................... 101 4.3.3 Multiannual to Centennial Ice Dynamics........................................................................................ 103 References............................................................................................................................................ 107
4.3.1 INTRODUCTION Ice accumulations are highly dynamic occurrences in caves, changing their morphology in response to the cave morphology and climatic conditions on scales ranging from days to millennia. Ice in caves occurs mainly as a result of water freezing and to a lesser extent of snow densification and diagenesis. The latter processes occur in cave entrances, where snowpacks up to 100 m in thickness occur, being subsequently compacted to form crystalline ice. Transformation of snow into ice does not occur as a “pure” compaction; freezing of water that percolates through the snow mass also contributes to the genesis of perennial ice masses. The results of these processes are large bodies of perennial ice that will subsequently deform and flow gravitationally towards the lower parts of the caves that host them. Contrary to these, ice formed by the freezing of water takes a large variety of shapes and sizes, from millimeter-scale, ephemeral ice crusts on the walls of caves, to large ice bodies, tens of thousands of cubic meters in size. Climatic factors strongly determine their dynamics on time scales ranging from hours to millennia, while the caves' morphology acts on scales on the order of centuries to millennia. A third, less-obvious process that leads to ice formation and subsequent dynamics, is that of ice sublimation, that is, the formation of ice crystals on the undercooled walls of caves, as warm and moist air is pushed out from the inner sections of these caverns. As a result of the above summarized processes, two classes of ice forms develop in caves (Fig. 4.3.1): perennial ice bodies and seasonal to multiannual ice speleothems (stalagmites, stalactites, ice crusts, and crystals). While most authors agree that the later are true speleothems (secondary mineral deposits formed in caves, sensu Hill and Forti, 1997) as they are minerals (crystalline H2O), they are of secondary genesis (formed by the freezing of water) and they have formed in caves; no consensus exists on whether the large, perennial, ice bodies accumulated in caves are speleothems, glaciers, both, or neither. Ice Caves. https://doi.org/10.1016/B978-0-12-811739-2.00034-6 © 2018 Elsevier Inc. All rights reserved.
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FIG. 4.3.1 Various ice bodies in caves—large ice block, semiperennial stalagmites, ice domes and seasonal stalactites (Scărișoara Ice Cave, Romania).
Here I argue that the large, perennial ice bodies found in caves and that have formed through water freezing are both speleothems and glaciers, while the similar ones that have formed by snow diagenesis are glaciers only, not speleothems, regardless of the fact that they are in caves. Snow diagenesis, regardless where it occurs, is a pure glacial process: snow is being slowly compacted and as air is squeezed out, ice develops. Infiltration and freezing of water within the ice body speeds up the compaction of the snow pack, and the end result is a layered mass of ice, with variable density (higher for the congelation ice), that also incorporates organic and inorganic sediments. While the end result is a large ice body inside a cave, the processes that led to its genesis are restricted to the caves' entrances, or, at most, to those areas that are directly fed by snow (e.g., the bottom of long, dipping slopes below cave entrances), thus being external to the cave environment. Freezing of liquid water to form perennial, large ice bodies can only occur (with the notable exception of permafrost) in single entrance, descending caves that act as cold air traps, maintaining negative temperatures throughout the year. In these caves, ice can form either as thin sheets of water freeze on sloping surfaces, or as shallow lakes freeze in autumn and winter. Most commonly, both these processes occur in caves (Fig. 4.3.2). In summer, lakes up to 30 cm deep can accumulate on top of existing ice bodies and will start to freeze, top to bottom, during a cooling period in late autumn through early winter to form a layer of “lake ice,” 1–30 cm thick. Warmer intervals during winter could lead to snow melting and water infiltration in caves to form thin sheets of “floor ice” (Pers‚oiu and Pazdur, 2011) on top of the existing lake ice. The end result, on a multiannual scale, is a layered body of secondary genesis crystalline H2O, formed by a cave-specific processes—a speleothem. Once formed, the large perennial ice bodies to be found in caves display a dual dynamic: they both accumulate and lose ice on top, as a result of both short and long term climatic variability and also, like any large surface glacier, flow under the influence of gravity. This later behavior occurs for both types (in terms of genesis) of large cave ice masses, resulting in flow-specific structures—ice thinning along the flow path, folds, push-moraines in front of the advancing (and melting) ice tongues, etc. (Fig. 4.3.3).
FIG. 4.3.2 Genesis of ice in caves through the freezing of water: (A) Formation of seasonal forms (stalagmites, stalactites, columns) occur intermittently throughout the winter, intensifying in early spring, when temperatures inside the caves are still negative and large amounts of water (released by melting snow) are available. With increasing outside temperature, warm water will start melting the ice. (B) Perennial ice bodies form by the freezing of lakes (accumulated in summer) in the early periods of cave cooling (lake ice-L) and, additionally, freezing of infiltrating water (floor ice-F) on top of existing lake ice, during winter.
FIG. 4.3.3 The underground glacier of Scărișoara Ice Cave. Layers deformed by flow are visible on both sides of the vertical wall, and an ice tongue extends on the lower-left side of the image. A push-moraine is visible in front of the advancing glacier.
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Thus, while in terms of their genesis, only large, perennial ice blocks formed by the freezing of water can be termed speleothems, contrary to the ice accumulations formed by snow diagenesis, both these two types of cave ice bodies are glaciers, in terms of postgenesis behavior. The dynamics of all ice formations in caves are the result of external climatic influences, cave morphology, or the combination of the two. Whereas the former acts on time scales ranging from days to millennia, the latter is only active on centennial scales—and the dynamics of cave ice will be analyzed further along these temporal differentiations (Fig. 4.3.4).
FIG. 4.3.4 Conceptual model of an ice cave showing the temporal range of dynamics for various ice formations: (A) subannual dynamics (accumulation and degradation); (B) multiannual dynamics (accumulation and ablation); (C) decadal to millennial dynamics (accumulation and ablation); (B′) multiannual dynamics— ablation only; (C′) decadal to millennial dynamics—flow and ablation only.
For clarity in the subsequent text, I will use the term “speleothem” for stalagmites, stalactites, and columns only, and “glacier” for perennial ice blocks.
4.3.2 SUBANNUAL DYNAMICS Accumulation and ablation of ice in caves on daily to monthly time scales affects all ice speleothems— stalagmites, stalactites, and columns, as well as the seasonally occurring hoar frost and the large, perennial glaciers.
4.3.2.1 HOAR FROST Hoar frost is the most dynamic form of ice in caves. Two processes are acting in conjunction to allow for the development of hoar frost: substratum (rock wall) undercooling and circulation carrying moisture towards the cold walls. The process starts in early winter, when external cold air, triggered by its higher density, sinks into the caves, leading to the slow undercooling of the walls. As this cold air reaches the
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caves, it pushes out the warmer and moister cave air, which will condensate on the wall as liquid water. Continuous inflow of cold air, combined with water evaporation from the rock will cool the walls to temperatures below 0°C, thus setting the scene for the initiation of hoar frost development (Pflitsch et al., 2007). Continuous inflow of cold air leads to the development of a bidirectional air circulation, with colder air swiping on the surface of ice and pushing out the warm and moist air along the ceilings, resulting in the deposition and growth of large ice crystals (Fig. 4.3.5). The moisture feeding the growth of crystals has a dual origin: the in situ moisture existing in the warmer sector of the caves, and moisture resulting from the sublimation of cave ice under the influence of cold and dry inflow of cave air, flowing on top of the ice. The hoar frost deposits develop and survive until early to mid-summer, when inflow of cold air ceases and both air and rock start to warm, leading to rapid degradation of the crystals. Warming of the walls results in crystals falling to the ground, where they either melt away, as in most cases, or, where temperatures are still below 0°C, they can accumulate on top of the existing ice to contribute to the growing of the ice block (Marshall and Brown, 1974). As a result of these processes, the dynamics of hoar frost in ice caves follows a simple, annual cycle, being present between mid- and late winter (when they reach a maximum) and absent throughout the rest of the year. The size of the crystals is dependent on the intensity of wall undercooling, the strength of air circulation, and amount of moisture available, reaching a maximum of 50–60 cm, and usual values of 5–15 cm (Fig. 4.3.5).
FIG. 4.3.5 Mechanism of hoar frost deposition in ice caves (A) and close-up view of the resulting ice crystals (B) in Scărișoara Ice Cave. The blue star marks the location of hoar frost deposits, and the two red dots indicate the sources of moisture. Length of scale bar is 3 cm.
4.3.2.2 ICE SPELEOTHEMS (STALAGMITES, STALACTITES, AND COLUMNS) Ice stalagmites and stalactites are common occurrences in cave entrances during the cold season in mid-to-high-latitude and/or altitude caves. They form as drip water freezes and usually melt as the temperature of the water feeding them becomes positive. In caves with perennial ice, these stalagmites can be perennial, the negative temperatures preserved throughout the year in the vicinity of underground glaciers helping their survival. However, differences exist between stalagmites, on one hand, and stalactites and columns, on the other, as the latter melt earlier and usually completely, the warm water leading to their detachment from the ceiling and collapse. The dynamics of these ice formations have been studied in great detail in Scăris‚oara Ice Cave, Romania (SIC), by Viehmann and Racoviţă (1968), Racoviţă et al. (1987), and Racoviţă (1994), and it is summarized below, as a “template” for other caves, as well.
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In Scăris‚oara Ice Cave, the dynamics of ice speleothems were studied separately for stalagmites, ice massifs, and ice crusts on the cave floor (Fig. 4.3.6), on both annual and subannual time scales. ∆h (cm) 4
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FIG. 4.3.6 Seasonal variations (10 years average) of the upper face of the ice block in SIC (A), ice massifs (B), ice stalagmites (C), and floor ice crust (D). The ice massifs are similar to the one visible on the left in Fig. 4.3.1. Modified from Racoviţă, G., 1994. Eléments fondamentaux dans la dynamique des spéléothèmes de glace de la grotte de Scărișoara, en relation avec la météorologie externe. Theor. Appl. Karstol. 7, 133–148.
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For all these speleothems, a maximum was reached between March and June, with the larger ice bodies having a delayed onset of melting. Ice crusts of the floor (Fig. 4.3.6D) grow outwards from the edges of the ice block, thinning with increased distance, their dynamics being controlled by the inflow of cold air which sweeps the upper surface of the ice block and leads to water freezing. However, this genetic mechanism renders them extremely vulnerable to melting, a process which initiates once the inflow of cold air ceases in early spring (Figs. 4.3.2 and 4.3.6D). The melting of the larger ice forms, both stalagmites and ice massifs (Fig. 4.3.6B and C), is delayed by the larger thermal inertia of the ice (Pers‚oiu et al., 2011). However, regardless of their size and shape, all ice forms in caves (including the large ice blocks) reach a minimum at the end of the melting season, just before the onset of freezing, usually in November (Figs. 4.3.2 and 4.3.6). Both growing and melting of ice is a complex process, controlled by the variable interplay between air temperature and precipitation amount and distribution, thus there is no clear correlation with either of these two (Pers‚oiu et al., 2011). The effect of precipitation amounts reaching ice caves on ice dynamics is strongly dependent of air temperature during winter, with water input superimposed on below 0°C conditions leading to rapid ice build-up, while the same input occurring during periods with positive temperature anomalies resulting in ice loss. However, in summer, temperature doesn't play too important a role, as the latent heat of the ice prevents temperature inside the caves reaching above 0°C. Thus, water input is the main factor leading to ice ablation, the heat delivered to the ice by inflowing warm water being the main factor in the ablation of ice. A peculiar type of subannual ice dynamics of ice is that of the “thermoindicator” speleothems (Viehmann and Racoviţă, 1968, Fig. 4.3.7). These form in winter, during periods of alternating cold and mild weather, with the translucent, bulky sections developing when temperatures are between 0°C and −2°C, and drip water will freeze slowly, expelling gas and calcite impurities. When temperatures drop below −3°C, dripping water will tend to freeze quickly, incorporating both air bubbles and in situ precipitated cryogenic cave calcite (Žák et al., 2008), hence the semiopaque appearance of the ice.
4.3.3 MULTIANNUAL TO CENTENNIAL ICE DYNAMICS Long-term dynamics of ice has been observed in a few caves, and monitoring programs exist in even fewer ones (Kern and Pers‚oiu, 2013). In Scăris‚oara Ice Cave, a monitoring program was in place for ice speleothems as well (summarized in Racoviţă, 1994) between 1965 and 1992, the results indicating an inverse relationship between the lowering of the upper face of the ice black (mass loss) and increase in the height of stalagmites. Racoviţă et al (1987) suggested that this contrasting behavior is the result of long-term periodicity in the dynamics of ice that could be possibly linked to climatic conditions outside the cave. However, owing to the short time series available (53 years between 1963 and 2017) and long-term suggested periodicity (50+ years) no such link has been found yet (Pers‚oiu and Pazdur, 2011), although photographic evidence from 1947 (Șerban et al., 1948) suggest that ice speleothems at that time were taller than in the 1960s. The dynamics of the glaciers in caves has been investigated sporadically in the past century, by combining various sources of data: photographic evidence, markers in the ice and/or cave walls, absolute dating, regular monitoring programs (Kern and Pers‚oiu, 2013 and references therein). However, with a few exceptions, monitoring programs longer than a few years are very rare (Ohata et al., 1994; Rachlewicz and Szczucinski, 2004; Fuhrmann, 2007; Pers‚oiu and Pazdur, 2011; Kern and Thomas, 2014). With little exception, most monitoring programs indicate a continuous and accelerated melting of cave ice, especially in the past 20–30 years (Fig. 4.3.8).
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FIG. 4.3.7 Thermoindicator speleothems in Scărișoara Ice Cave.
The causes of the decadal to centennial mass balance changes in caves can be grouped in three classes: (1) morphologic, (2) climatic and (3) endogenic. The morphologic ones are the consequences of changes in the dynamics of airflows and associated heat transfer in caves related to changes in the morphology of the open spaces as ice grows and shrinks (while the morphology of the rock walls in ice caves could changes as a result of collapses, no such occurrences have been reported so far in the literature). These changes in turn affect the climate of the caves and the behavior of ice, resulting in long-term modifications of the ice morphology and dynamics. In Scăris‚oara Ice Cave (Romania) such a case was described by Șerban et al. (1948), with the ice block switching between several phases of growth and decay (Fig. 4.3.9).
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FIG. 4.3.8 Cumulative ice loss in ice caves in the Northern Hemisphere during the last 100 years. From Kern, Z., Perșoiu, A., 2013. Cave ice—the imminent loss of untapped mid-latitude cryospheric paleoenvironmental archives. Quat. Sci. Rev. 67, 1–7.
FIG. 4.3.9 Growth and retreat of the ice block in Scărișoara Ice Cave (Romania). Original sketches by Mihai Șerban.
In phases I–III, ice grew through water freezing in layers of “lake ice,” however the growth of ice progressively isolated the lowermost sections of the incipient ice block, and the constant geothermal heat flux (70 mW/m2, Demetrescu and Andreescu, 1994) led to the melting of the ice and progressive growth towards the surface of the resulting opening (phases IV–VIII in Fig. 4.3.8). Once this opening reached the surface, inflowing water started to freeze on the vertical, exposed ice wall, progressively
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closing the opening (phases IX–XII) reinitiating a new cycle. Presently, the ice block is a stage similar to phase IX (Fig. 4.3.3), with newly formed ice growing on top of the exposed, older, ice. A secondary dynamic process related to melting near the base is that of ice flow, leading to progressive growth of folds in the ice (Fig. 4.3.10).
FIG. 4.3.10 Ice flow and folding in Svarthammarhola, Norway.
The climatic conditions affecting the dynamics of cave glaciers have are acting in a more predictable fashion, with simple, annual cycles (Fig. 4.3.6A) superimposed on long-term trends, generally pointing towards continuous melt (Fig. 4.3.8). While increasing air temperature (and possibly diminishing precipitation during the growing season) are likely the main causes leading to general ice loss, observations of a peculiar ice loss case in the Romanian Carpathians suggest that cave glaciers might respond nonlinearly to climatic conditions: in “Adevăratul Gheţar de la Vârtop” Ice Cave, a 6 m thick, ~2000 m3 ice block vanished completely between 2008 and 2012, its demise being possibly triggered by summer inflow of warm water that lead to catastrophic disintegration once the glacier was dissected in smaller ice blocks. Only a few ice caves have ages going back more than a thousand years, so that the dynamics over centennial to millennial scales is difficult to assess. Studies in Swiss (Luetscher, 2005), Austrian (Spötl et al., 2014) and Romanian (Pers‚oiu et al., 2017) caves have shown that ice in these caves responded sensitively to the main climatic swings during the past 2000 years, with ice accumulating in caves during cold and/or wet periods (the Dark Ages Cold Period of the 4–7th centuries AD and the Little Ice Age, between the 14th and 19th centuries AD), and melting during the medieval Warm period (between the 9th and 13th centuries AD). Stoffel et al. (2009) and Pers‚oiu et al (2017) have further shown that long-term changes in ice mass balance and dynamics could be controlled by large-scale circulation
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p atterns affecting Europe and these could also be recorded by the isotopic composition of cave, thus being a valuable tool in reconstructing past climatic conditions. Geothermal heat delivered to the base of cave glaciers (Fig. 4.3.4C) has been shown (Tulis and Novotný, 2003; Luetscher, 2005; Pers‚oiu, 2005) to lead to variable rates of melt: 8 cm/year in Monlesi Ice Cave (Switzerland), 1.53 cm/year in Scăris‚oara ice Cave (Romania) and 1 cm/year in Dobsinska Ice Cave (Slovakia). However, these basal melting rates were calculated based on limited observation, and the resulting mass turnover rates are not in agreement with the age of the cave glaciers (Kern, this volume), thus suggesting that either the estimates are wrong (by a factor of 10 in Scăris‚oara Ice Cave), or that basal melting was not happening at a constant rate during the existence of the ice blocks.
REFERENCES Demetrescu, C., Andreescu, M., 1994. On the thermal regime of some tectonic units in a continental collision environment in Romania. Tectonophysics 230 (3–4), 265–276. Fuhrmann, K., 2007. Monitoring the disappearance of a perennial ice deposit in Merrill Cave. J. Caves Karst Stud. 69, 256–265. Hill, C.A., Forti, P., 1997. Cave Minerals of the World, second ed., National Speleological Society Inc, Huntsville, Alabama, pp. 217–225. Kern, Z., Pers‚oiu, A., 2013. Cave ice—the imminent loss of untapped mid-latitude cryospheric paleoenvironmental archives. Quat. Sci. Rev. 67, 1–7. Kern, Z., Thomas, S., 2014. Ice level changes from seasonal to decadal time-scales observed in lava tubes, lava beds national monument, NE California, USA. Geogr. Fis. Din. Quat. 37, 151–162. Luetscher, M., 2005. Processes in Ice Caves and their Significance for Paleoenvironmental Reconstructions. Swiss Institute for Speleology and Karst Studies, La Chaux-de-Fonds, Switzerland, p. 154. Marshall, P., Brown, M.C., 1974. Ice in Coultard Cave, Alberta. Can. J. Earth Sci. 11, 510–518. Ohata, T., Furukawa, T., Osada, K., 1994. Glacioclimatological study of perennial ice in the Fuji ice cave, Japan. Part 2: interannual variation and relation to climate. Arct. Alp. Res. 26, 238–244. Pers‚oiu, A., 2005. Evidence of basal melting of the ice block from Scăris‚oara Ice Cave. In: Mavlyudov, B.R. (Ed.), Glacier Caves and Glacial Karst in High Mountains and Polar Regions, Institute of Geography of the Russian Academy of Sciences, Moscow, pp. 109–112. Pers‚oiu, A., Pazdur, A., 2011. Ice genesis and its long-term mass balance and dynamics in Scăris‚oara Ice Cave, Romania. Cryosphere 5 (1), 45–53. Pers‚oiu, A., Onac, B.P., Pers‚oiu, I., 2011. The interplay between air temperature and ice dynamics in Scăris‚oara Ice Cave, Romania. Acta Cardiol. 40 (3), 445–456. Pers‚oiu, A., Onac, B.P., Wynn, J.G., Blaauw, M., Ioniță, M., Hansson, M., 2017. Holocene winter climate variability in Central and Eastern Europe. Sci. Rep. 7, 1196. Pflitsch, A., Piasecki, J., Sawiński, T., Strug, K., Zelinka, J., 2007. Development and degradation of ice crystals sediment in Dobšinská Ice Cave (Slovakia). In: Zelinka, J. (Ed.), Proceedings of the 2nd International Workshop on Ice Caves, Demänovská dolina, Slovak Republic, pp. 38–49. Rachlewicz, G., Szczucinski, W., 2004. Sesonal, annual and decadal ice mass balance changes in the ice cave Jaskinia Lodowa w Ciemniaku, the Tatra Mountains. Poland. Theor. Appl. Karstol. 17, 11–18. Racoviță, G., 1994. Eléments fondamentaux dans la dynamique des spéléothèmes de glace de la grotte de Scărișoara, en relation avec la météorologie externe. Theor. Appl. Karstol. 7, 133–148. Racoviță, G., Șerban, M., Viehmann, I., 1987. Tendances de long terme dans la dynamique des formations de glace de la Grotte de Scărișoara (Monts du Bihor). Theor. Appl. Karstol. 3, 143–164. Șerban, M., Coman, D., Givulescu, R., 1948. Découvertes récentes et observations sur la glacière naturelle dite, Gheţarul de la Scărișoara. Bull. Soc. Sci. Cluj X, 174–210.
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Spötl, C., Reimer, P.J., Luetscher, M., 2014. Long-term mass balance of perennial firn and ice in an Alpine cave (Austria): constraints from radiocarbon-dated wood fragments. The Holocene 24, 165–175. Stoffel, M., Luetscher, M., Bollschweiler, M., et al., 2009. Evidence of NAO control on subsurface ice accumulation in a 1200 yr. old cave-ice sequence, St. Livres ice cave, Switzerland. Quat. Res. 72, 16–26. Tulis, J., Novotný, L., 2003. Changes of glaciation in the Dobšinská Ice Cave. Aragonit 8, 7–9. Viehmann, I., Racoviță, G., 1968. Stalagmitele de gheață termoindicatoare. Dări de Seamă Comit. Geol. LIV, 353–363. Žák, K., Onac, B.P., Pers‚oiu, A., 2008. Cryogenic carbonates in cave environments: a review. Quat. Int. 187 (1), 84–96.