Speleogenesis – oral 2013 ICS Proceedings

GLACIER ICE-CONTACT SPELEOGENESIS

Stein-Erik Lauritzen1, Rannveig Øvrevik Skoglund2 1Department of Earth Science, Bergen University, Allegaten 41, N-5007 Bergen, Norway, [email protected] 2Department of Geography, University of Bergen, Fosswinckelsgt. 6, N-5007, Bergen, Norway, [email protected]

The classic hypothesis of G. Horn’s (1935) subglacial speleogenesis as an explanation of the relatively small diameter cave conduits in the Scandinavian marble stripe karst is reviewed. Recent work, including accurate cave mapping and morphological analysis, radiometric dating of cave deposits, chemical kinetics experiments and computer simulations have challenged the old theory. Scandinavia has relatively small caves that often have surprisingly high ages, going beyond the limit of Th/U dating. The high ages are apparently compensated by correspondingly slow wall retreat rates in the ice- contact regime, and longer periods when the caves were inactive. Ice-contact speleogenesis varied in time and space, in pace with waxing and waning of wet-based ice. Maze or labyrinth morphology appears as a characteristic feature of caves ascribed to these processes.

1. Subglacial speleogenesis 3. Flow regimes and labyrinth morphology Between WW1 and WW2, the Norwegian geologist and Through accurate cave mapping and scallop morphometry, speleologist Gunnar Horn (1894–1946) studied caves in the we have been able to demonstrate that the corrosive flow marble karst of North Norway. Horn published his results that caused wall retreat was quite slow, corresponding to in German and Norwegian periodicals (Horn 1935, 1937, average groundwater velocities in non-glacial karst 1945, 1947). These ideas were new and radical for their (Lauritzen et al. 2011), and that “levels” or tiers in vertical time, resulting from a remarkable train of original maze caves did not operate independently, but were part of observations and thoughts. Horn’s main thesis was that a flow network that involved the whole (known) cave phreatic speleogenesis under subglacial conditions was a (Lauritzen 1982). Many relict labyrinths which slope into simple and feasible explanation for the formation of the hillside, like the Grønli system (Lauritzen et al. 2005) numerous relict and truncated phreatic caves that are and the Nonshaugen system (Skoglund Øvrevik and located in hanging position within the walls of glacially Lauritzen 2011) were effluent (discharge areas) when sculptured valleys. In modern terminology, the ice-contact active. Other caves display terrain-reversed phreatic flow regime beneath wet-based glaciers and ice-sheets (Lauritzen 1984). These features are best explained by ice- was a sufficient speleogenetic agent. Presently, more than contact under hydraulic gradients superimposed from a 2,000 karst caves have been recorded in the country, with surrounding glacier or ice-sheet. Modelling experiments maximum lengths and depths of 25 km and 580 m, suggest that when in contact with a wet-based glacier, karst respectively. However, most of them are relatively small aquifers tend to develop maze or labyrinth morphology (passage diameter) compared to caves in other countries, (Skoglund et al., 2010). and – from the viewpoint that size is a measure of (active growth) age – one may infer that the caves are relatively young. Horn suggested that the last glacial cycle would 4. Subglacial wall retreat rates have been sufficient to account for the development of these relatively small cave passages. Horn was also the first to Due to the chemical conditions in subglacial waters and the name the characteristic stripe-karst morphology (Horn general high content of alkaline rock flour with a large 1937; Lauritzen 2001) in the marble-mica schist setting. specific area, speleogenetic wall retreat rates are up to 50 times less effective under subglacial conditions than under non-glacial (interglacial) conditions. All these effects, 2. Chronology and field observations, provides a good explanation why we have relatively small caves of a comparably high age Our investigations, since the mid-1970’ies, have focussed (Lauritzen and Skoglund 2013). on morphological analysis, studies of cave sediments and radiometric dating of speleothems which made it possible to test Horn’s hypothesis. First, we have shown that many 5. The “car-wash” analogy caves, also relatively small ones, consist of labyrinths or mazes, either horizontally or vertically, depending on the Speleothem (stalagmite) dates represent periods when the attitude of the host rock in the marble stripes. Second, many cave was inactive (drained). Likewise, the periods of caves, even quite small ones, contain speleothems that date formation for calcareous concretions (Höhlenkrapfen, Kyrle beyond the last glacial cycle, some also exceed the present (1923), “doles” in French), reveal periods when the cave limit of the method (Th/U: 750,000 yr.). These caves may, was filled with sediments having pore-water that was in principle, even be older than the Quaternary ice-ages, super-saturated with respect to carbonate, which by although direct proof of pre-glacial deposits in them has yet inference mean that speleogenesis was stagnant. Many to be found. Höhlenkrapfen which we to-day find cemented onto clean-

368 Speleogenesis – oral 2013 ICS Proceedings washed passages date beyond the last glacial maximum, yet References they do not stay proud of the surrounding marble wall. Hence, no effective wall retreat has occurred since the Horn G, 1935. Űber die Bildung von Karsthöhlen unter einem Gletcher. Norsk Geografisk Tidsskrift, 5, 494–498. concretion was formed, even during the deglaciation, when water availability and presumably chemical aggressiveness Horn G, 1937. Űber einige Karsthöhlen in Norwegen. was optimal. If we plot all available data of speleothems Mitteilungen für Höhlen und Karstforschung, 1–15. and Höhlenkrapfen ages in the same diagram, together with Horn G, 1945. Om Dannelsen av de Nordlandske Karsthuler. the time-distance diagram of glacial cover during the last Norsk Geologisk Tidsskrift, 25, 180–182. glacial-interglacial cycle, an interesting picture appears Horn G, 1947. Karsthuler i Nordland. Norges Geologiske (Figure 1). If all these observations came from the same Undersøkelse, 165, 1–77. cave (they do not), there would hardly be any time-windows available for wall retreat (active speleogenesis). Since we Kyrle G, 1923. Grundriss der theoretischen speläologie. also have positive evidence that speleogenesis did occur Lauritzen SE, 1982. The paleocurrents and morphology of during this period, for instance through the Kvithola Pikhåggrottene, Svartisen, North Norway. Norsk Geografisk Mechanism, i.e. phreatic ice-contact speleogenesis under Tidsskrift, 36, 183–209. thin ice cover (Lauritzen 1986) and through interglacial Lauritzen SE, 1984. Evidence of subglacial karstification in speleothems that have become part of a scalloped wall, we Glomdal, Svartisen, Norway. Norsk Geografisk Tidsskrift, 38, must conclude that ice-contact speleogenesis was relatively 169–170. slow, and sporadic in time and space, in pace with waxing and waning of wet-based ice in the local topography. Lauritzen SE, 1986. Kvithola at Fauske; Northern Norway: an example of ice-contact speleogenesis. Norsk Geologisk Therefore, the so-called “car-wash analogy” (Lauritzen Tidsskrift, 66, 153–161. 2010) offers a feasible concept of this process, Figure 2. In this situation, we consider two caves, 1) and 2) situated in Lauritzen SE, 2001. Marble Stripe karst of the Scandinavian valley floor and wall, respectively. According to the position Caledonides: An end-member in the contact karst spectrum. of the ice-sheet/glacier front at any time, the two caves are Acta Carsologica, 30, 47–79. either activated or stagnant. Scenario a) represents a Lauritzen SE, 2010. Grotter. Norges ukjente underverden. (Caves. continental, mainly dry-based ice-sheet with the margin on Norway’s unknown underground), Tun Forlag, Oslo. the continental edge. At b) the ice front is closer, so that Lauritzen SE, Hestangen H, Skutlaberg S, Øvrevik R, 2005. The both caves are within the ablation area and can receive Grønli-Seter cave research project, Rana, North Norway. supraglacial and subglacial water. At c) the ice front is at Proceedings of the 14th International Congress of Speleology, the caves, which both experience maximum hydraulic Athen-Kalamos, August 2005, Paper P-22, 1–6. gradient, water flow and CO supply. At d) the glacier front 2 Lauritzen SE, Skoglund Øvrevik R, 2013. kap. 6.15: Glacier ice- is proximal to the caves and only the valley floor cave can contact speleogenesis in marble stripe karst. In Frumkin, receive water. A. (ed.) Treatise of Geomorphology, Vol. 6: Karst Geomorphology. Elsevier, London. Lauritzen SE, Skoglund Øvrevik R, Skutlaberg S, Lauritsen Å, Worthington SRH, 2011. What can scallops tell us about ice- contact speleogenesis in Scandinavia? In Heldal, T. (ed.) Winter Conference 2011, Norwegian Geological Society. Norwegian Geological Survey, Stavanger. Skoglund Øvrevik R, Lauritzen S-E, Gabrovsek F, 2010. The impact of glacier ice-contact and subglacial hydrochemistry on evolution of maze caves: A modelling approach. Journal of Figure 1. Time-distance diagram for glaciations on the Norwegian Hydrology, 388, 157–172. mainland during the last 80 kyr (purple areas). The black curve represents atmospheric CO2 content based on Antarctic ice. Skoglund Øvrevik R, Lauritzen SE, 2011. Subglacial maze origin Dots with error bars: radiometrically dated speleothems and in low-dip marble stripe karst: examples from Norway. Journal Höhlenkrapfen. Horisontal bars: periods with paleomagnetically of Cave and Karst Studies, 73, 31–43. dated cave sedimentation. From Lauritzen & Skoglund (2013).

Figure 2. The car-wash analogy of speleogenesis, where glacier contact may be likened to a car-wash moving through the landscape with stationary caves. From Lauritzen (2010).

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