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The phototropic phytospeleothems of Moss Palace, Mole
In Moss Palace, the presence of unusual speleothems further justifies
action towards conservation of the Dogs Head Hill karst at Mole Creek, Tasmania. A "symbiotic" carbonate deposition and growth of the moss
Distichophyllum microcarpum results in rare phototropic phytospeleothems, in the form of fan-shaped erratics.
Moss Palace cave was discovered during a reconnaissance of Dogs Head Hill by
MCCC founding members Phill Gregg, Deb Hunter and myself, March 1988. This cave was
unaffected by human visitation, and does not correspond to any record in the inventory compiled by Kiernan,
(1984). The rift-guided cave has 45 m of passage in two levels. The entrance leads to a narrow, high passage sloping down to a 2 m wide rift. The passage at the bottom of the rift returns underneath the upper level and pinches out a little
farther. The overall cave length is about 25 m, total depth about 6 m.
Cave walls are largely coated by white flowstone and delicate calcite speleothems, demanding extreme measures for the caver to avoid impact. Techniques include
use of several changes of clean clothes and footwear, and strict placement of feet and
hands, and refraining from
unnecessary visitation. The upper level is lined with smooth white flowstone, and floored with delicate gours, some of which contain oolites. The main features of the rift are the
flowstone wall with fan-shaped
erratics (described later) on one side and white knobbly flowstone on the other. There is also some moonmilk on this wall. The lower passage has more conventional types of speleothems, though
sometimes in unusual combinations. There is evidence towards the end of the lower passage that indicates at least four stages of flowstone and dripstone deposition and vadose solution. A further expedition produced a photographic record of the cave and its speleothems. In the context of the small size of the cave and its high sensitivity to visitation, a further expedition for mapping was
unwarranted, however a sketch map was produced using preliminary survey
and estimations. During a subsequent brief visit in October 1990, it was noted that an unidentified bird, possibly a Tawny Frogmouth or Owlet Nightjar, had taken up a nesting site on a high shelf in the upper level, and was depositing guano and debris onto the clean flowstone wall below.
The phototropic phytospeleothems
Part of the twilight zone of Moss Palace features unique phototropic phytospeleothem decorations, ranging from 2 mm to 200 mm in length. They are found (in small form) near the entrance and (in more spectacular form) on a patch of wall 2-6 m below a 0.5 mē skylight at the far end of the cave (photo 1). The best developed phytospeleothems are 5 m below the skylight. These fan-shaped erratics, (see photos), grow outwards from wall surfaces, oriented with the face of the fan perpendicular to the source of the light. They mostly occur in overlapping clusters, (photo 4), with only the outermost edges being readily visible from the direction of the light. These edges consist of a fringe of living moss, Distichophyllum microcarpum, (species described in Scott, Stone & Rosser, 1976). The moss appears to be in the process of calcification, being increasingly advanced back from the edge of the growing fringe. The calcium carbonate deposit completely replaces the older, dead moss, and the speleothem becomes thickened towards the base by a coat of calcite material. During and following periods of wet weather, these speleothems are heavily dowsed by water; at other times they were observed to remain wet or moist. The decorations have not been inspected during serious drought conditions, though the presence of what appear to be dead (i.e. lacking a living green fringe) stubs among the active phytospeleothems suggests they are intolerant to deep soil drought.
Hill and Forti (1986), and Ford and Williams (1989) classify the forms of cave deposits, and put forward mechanisms to explain their formation. Growth of erratics include situations of crystal growth when evaporation rates exceed water seepage flows, thus preventing formation of water drops, and hence normal straw stalactites. The observed water flows would contraindicate this mechanism. Jennings (1985) used the term "phytokarst" to describe solution features on limestone caused by biological activity. This term should not be used to describe carbonate deposit features. Goede (1988) pointed to the possibility of biological mechanisms for the formation of moonmilk deposits. DeSaussure (1961) concluded that green algae was the cause of phototropic cave coral in Teopisca Cavern, Mexico. Cox et al (1989) concluded that cyanobacteria causes phototropism of "crayfish" stalagmites in Nettle Cave and Arch Cave at Jenolan, NSW. Dalby (1966) described carbonate accumulation on the mosses Eucladium verticillatum and Barbula tophacea, growing at inclined angles from the ceiling of disused coal mine level in the twilight zone at Dorset (UK) by the seaside. D. microcarpum has previously been observed on bare karst substrates; George Scott (pers. comm.) observed the species growing on limestone under 5 m of water at Ewens Ponds, SA, but no carbonate growth was noted in this situation.
Andy Spate (pers. comm.) observed a number of poorly developed examples of phototropic phytospeleothems at Honeycomb Cave, Mole Creek. Subsequently, I observed copious growth of D. microcarpum at the entrance and twilight zone of Gillam Cave, Mole Creek. This was found to be devoid of any associated phytospeleothem growth.
Morphology of the Moss Palace speleothems indicates that their growth is controlled by the existence of the growing moss, hence the use of the term phytospeleothem. The orientation of the growth is directed by light, i.e., phototropic, and erratic with respect to gravity. The more specific term for these erratic speleothem decorations would thus be "phototropic phytoerratics".
The mechanism of calcium carbonate deposition may be explained by a net photosynthetic removal of
CO2 from the water supplying the moss, thus shifting the hydrogen carbonate equilibrium, resulting in carbonate deposition. There are likely to be two factors involved in this process; firstly, the photosynthesis of the moss removes
CO2 from solution to sustain the plants; secondly, the moss merely provides a physical substrate of large surface area for degassing of
CO2 into the cave atmosphere. It would be speculation at this stage to suggest which is the dominant factor. The subsequent thickening at the stems
of the speleothems is regarded as being a conventional flowstone deposit.
The significance of the Dogs Head karst was noted
earlier by Kiernan (1984); the hill is a rare and classic Hum, and there were already a number of known caves with speleological or archaeological significance. The value of the area was again highlighted by Hunter, in Cadman et al
(1990), in the context of the discovery of Moss Palace and its phytospeleothems. There remain serious threats to the values of this National Estate-listed
area, specifically from the mining operations of the nearby limeworks.
Forestry, insensitive recreation and farming represent additional threats, Hunter
I thank Mr Paddy Dalton, Dr George Scott and Dr Heinar Streimann for their assistance with
identifying moss samples.
S.Fearn, L.Goldsworthy, A.Hingston, D.Hunter, A.Kelly, and C.Spencer, 1990
The Great Western Tiers: the case for conservation. A Proposal for a Great Western Tiers National
Park. Deloraine Environment Centre, Deloraine. 49pp.