Augusto S. Auler and Peter L. Smart
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Auler A.S. and Smart P.L.

Abstract: Condensation corrosion is a little studied, but important dissolutional process that occurs within caves in many karst settings around the world (for a review see Dublyansky and Dublyansky, 2000). Condensation corrosion occurs when air equilibrates with the cave atmosphere, becomes acidic and dissolves the bedrock and speleothems. It is a later vadose process that apparently depends on air circulation patterns, number of entrances and general configuration (vertical range, presence of ponded water, passage shape, etc) of the cave. Both bedrock and speleothems can be affected by the process, resulting in weathered outer surfaces. Condensation corrosion in speleogenesis has been regarded as responsible for dissolutional modification during later stages of cave development of coastal (Tarhule-Lips and Ford, 1998) and hypogenic caves (Hill, 1987; Palmer and Palmer, 2000).

Keywords: Condensation corrosion, speleothems, Brazil.

Condensation corrosion is a little studied, but important dissolutional process that occurs within caves in many karst settings around the world (for a review see Dublyansky and Dublyansky, 2000). Condensation corrosion occurs when air equilibrates with the cave atmosphere, becomes acidic and dissolves the bedrock and speleothems. It is a later vadose process that apparently depends on air circulation patterns, number of entrances and general configuration (vertical range, presence of ponded water, passage shape, etc) of the cave. Both bedrock and speleothems can be affected by the process, resulting in weathered outer surfaces. Condensation corrosion in speleogenesis has been regarded as responsible for dissolutional modification during later stages of cave development of coastal (Tarhule-Lips and Ford, 1998) and hypogenic caves (Hill, 1987; Palmer and Palmer, 2000).

The Campo Formoso Karst area of northeastern Brazil holds very extensive cave systems, such as Southern Hemisphere?s longest cave, the 97 km long Toca da Boa Vista. These caves show remarkable features of condensation corrosion such as cupolas, weathered cave walls yielding dolomitic sand, ?air scallops? and corroded speleothems. Weathering rinds up to 5 cm thick occur in both dolomite bedrock and speleothem surfaces. Unlike the dolomite, speleothems usually do not disintegrate but change to a milky white opaque porous calcite that is in marked contrast with the fresh crystalline calcite. The area is presently under semi-arid climate and the cave atmosphere is characterised by high internal temperatures (27­29 °C) and low relative humidity (mean of 73% for sites away from entrances).

Despite being such a widespread process, rates of condensation corrosion have so far been reported only from caves in the coastal area of the Caribbean (Tarhule-Lips and Ford, 1998). In this study, rates of condensation corrosion in speleothems were derived by determining thickness of weathering rind and age of last unaltered calcite. These rates represent minimum rates because speleothem growth ceased later than age obtained, and also condensation corrosion may not be continuous in time. Due to variable thickness of weathering layer (usually thicker at the top and thinner at sides of stalagmites), maximum and minimum thickness were obtained for each sample. Dating was performed through the alpha spectrometric U-series method in the first unaltered calcite layer beyond the weathering rim. Dating of the weathered portion of the calcite is not possible due to radionuclide remobilisation. U-series analyses and condensation corrosion minimum rates are reported in Table.

Auler A.S. and Smart P.L. / Speleogenesis and Evolution of Karst Aquifers, 2 (2) December 2004, p.2

TABLE Uranium series analyses and condensation-corrosion minimum rates for selected samples from Toca da Boa Vista.

Sample

U (ppm)

234 U/ 238 U

230 Th/ 234 U

230 Th/ 232 Th

234 U/ 238 U initial

Age (ka)

TBV1-95

0.171 ± 0.002

1.46 ± 0.02

0.869 ± 0.016

57

1.76

182.9 ± 10.5

TBV-17T

0.420 ± 0.006

1.73 ± 0.02

0.744 ± 0.014

754

2.06

129.8 ± 5.5

TBV-32

0.050 ± 0.001

1.18 ± 0.02

0.962 ± 0.022

24

1.39

275.2 (+41 ?38)

TBV-41

0.103 ± 0.004

1.50 ± 0.07

0.801 ± 0.030

206

1.76

152.7 (+21 ?19)

TBV-46

0.043 ± 0.003

1.40 ± 0.10

0.707 ± 0.039

33

1.56

123.2 (+25 ?21)

 

Sample

Age (ka)

Weathering rind thickness (mm)

Rate (mm/ka)

   

Range

Range

TBV1-95

182.9 ± 10.5

5.0 - 71.0

0.026 - 0.41

TBV-17T

129.8 ± 5.5

0.1 - 1.0

0.00074 - 0.0080

TBV-32 TBV-41 TBV-46

275.2 (+41 ? 38) 152.7 (+21 ? 19) 123.2 (+25 ? 21)

3.0 - 16.0 1.5 - 7.0 0.1 - 1.0

0.0095 - 0.067 0.0086 - 0.052 0.00067 - 0.0098

The rates obtained vary over two orders of magnitude. They appear to be highly site specific, and are probably heavily dependent on the local atmospheric conditions, although more sampling is needed to confirm this relationship. The data shows that rates are dependent primarily on thickness measured, as range of ages is quite small. Tarhule-Lips and Ford (1998), in the very different littoral caves of the Caribbean, have estimated condensation corrosion rates based on experiments using gypsum tablets. Their reported mean value of 24 mm/ka, much higher than observed in the Campo Formoso caves, suggest that the process may be episodic in the area, not occurring during speleothem growth phases associated with wetter periods.

Although the rates reported by Tarhule-Lips and Ford (1998) indicate that condensation corrosion may actually enlarge cave passages in the normal (10 4 ? 10 6 ka) time range of speleogenesis, in the Campo Formoso caves the process appears to play a minor speleogenetic role, being responsible for later modification of cave walls and speleothems.

Acknowledgements

ASA wishes to thank the CNPq (grant 200711-95.4) for supporting this study. Field work was performed with the help of Grupo Bambuí de Pesquisas Espeleológicas.

References

  • Dublyansky V.N.; Dublyansky Y.V. 2000. The role of condensation in karst hydrogeology and speleogenesis. In: Klimchouk A.B., Ford D.C.;, Palmer A.N., Dreybrodt W. (eds): Speleogenesis: Evolution of Karst Aquifers. National Speleological Society, pp. 100-112.
  • Hill C.A. 1987. Geology of Carlsbad Caverns and Other Caves in the Guadalupe Mountains, New Mexico and Texas. New Mexico Bureau of Mines and Mineral Resources Bulletin 117: 1-150.
  • Palmer A.N., Palmer M.V. 2000. Speleogenesis of the Black Hills maze caves, South Dakota,
  • U.S.A. In: Klimchouk A.B., Ford D.C.;, Palmer A.N., Dreybrodt W. (eds): Speleogenesis: Evolution of Karst Aquifers. National Speleological Society, pp. 274-281.
  • Tarhule-Lips R.F.A., Ford D.C. 1998. Condensation corrosion in caves on Cayman Brac and Isla de Mona. Journal of Caves and Karst Studies 60: 84-95.