IAEA-CN-80/P-20 SPELEOTHEMS AND CLIMATIC CHANGES: EQUILIBRIUM AND KINETIC FRACTIONATION EFFECTS IN CAVE DEPOSITS

L.F. MOLERIO LEON XA0100950 Grupo de Aguas Terrestres, Instituto de Geofisica y Astronomia, Ministerio de Ciencia, Tecnologia y Medio Ambiente, P.O. Box 6219, Habana 6, CP 10600, Ciudad de La Habana, CUBA

Speleothems are mainly calcitic deposits formed by vadose seepage waters supersaturated with calcium carbonate. (Fig. 1). These cave deposits could provide very useful information on past climatic changes, mainly on palaeotemperatures, when formed in isotopic equilibrium [1].

Fig. 1. Physical model for calcite speleothem deposition (from Holland, 1964 fide White, 1988)

CO,

Precipitation H2O

Infiltration uptake of COj

2 B ~"J~ Solution of CaC O3 at bedrock contact

Transport along joints

Deposition of CaCO-j in cave

by loss of CO2

Nevertheless, these estimations are significantly biased if microclimatic cave effects avoid isotopic equilibrium. As the basic principle for isotopic evaluation is that the heavier isotope is preferentially concentrated in the denser phase, the environmental temperature may determine the amount of such fractionation.

143 IAEA-CN-80/P-20 Two effects can therefore, alter the isotopic equilibrium, v.gr. the most ideal condition for palaeoenvironmental assessments. These effects are evaporation and outgassing of CO2 from vadose seepage. They produce the so-called kinetic fractionation.

Kinetic fractionation is often associated to the microclimatic cave environment, the pending time of solution on the roof of the cave, the speleothem surface and the flow of stalagmite and flowstone [1,2].

As these factors exert a remarkable influence on the genesis of speleothems, an association between kinetic fractionation and speleothem type is recognized. Advantage can be taken on this linkage for the proper selection of the speleothem to be sampled. Misleading of this well- know fact to speleologists has been one of the main causes of misinterpretation of speleothem environmental records or to incorrect interpretation of palaeotemperatures.

Drip yield and climate are the main factors control speleothem formation Montoriol [3].

Drip yield involves the concentration of calcite available for deposition and the flow rate of water feeding the speleothem. Climatic factor includes air temperature and relative humidity, wind velocity and CO2 partial pressure.

When drip factor dominates, rapid growth of formation is attained and speleothem usually grows in isotopic equilibrium. Partial pressure of CO2 of water and cave atmosphere approaches equilibrium very fast and candle, straw or spaghetti-like stalactites and stalagmites are formed. These speleothems were very little, often with negligible evaporation produced and isotopic fractionation is therefore, very low.

When climate factors dominate, kinetic fractionation is developed. Carrot or pagoda-like stalagmites are the common expressions of these kinds of speleothems that reflects singular variations of climate variations along its growth axis.

There are also mixed-type speleothems that are the best for palaeoenvironmental studies. These are usually concretions that have been grown continuously over long periods of time and therefore, often represent evaporation and no-evaporation cycles, v.gr. kinetic fractionation of equilibrium. Most flowstones, gours and stepped or some pagoda-like stalagmites are of this origin.

In most reports of stable isotopic studies of speleothems or dealing with absolute dating of cave formations, little or no information is given on the genetic type of the sampled speleothem. Usually, no information is given from the cave passage present and ancient hydrology and climatology or of the immediate environment surrounding the sampling site within the cave.

In Cuba, where 65% of the territory is karstified, a special attention have been paid to these factors when absolute sampling of cave deposits is performed. Temperature anomalies reported in the literature, derived from the 8 D- 8 18° fluid inclusion relationship is very often a consequence of a sampling procedure that did not account for the following factors:

The genetic type of the speleothem, where mixed drip-climatic and drip-controlled formations should be preferred over climate controlled concretions.

144 IAEA-CN-80/P-20 The past and present hydrology and climatology of the cave, avoiding comparison among dry and now submerged caves or were different thermodynamically passages were sampled, accounting that wind tube dynamic caves should be preferred for speleothem sampling. Locations of abundant reconstruction features are better for sampling because they resemble a more direct connection between cave passage and surface environment; concretions developed over or under breakdown slabs should be avoided for sampling.

Speleothems showing evidences of climatic corrosion or alternating reconstruction- decalcification-redisolution if sampled should be accounted that formation evolution represents different rates of fractionation, evaporation and flow rates of the feeding waters.

REFERENCES

[1] DULINSKY, M., GLAZEK, J. (1993): Speleothems as indicators of past climatic changes - Limitations in the interpretation of data derived from single samples-. An example from de Tatra Mountains, Poland. International Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere, IAEA Proc. Series, Vienna, 551-554. [2] SCHWARCZ, H., YONGE, C. (1983): Isotopic Composition of Paleowaters as inferred from Speleothem and its Fluid Inclusions. Palaeoclimate and Palaeowaters: A Collection of Environmental Isotope Studies. Panel Proc. Series, IAEA, Vienna: 115- 133. [3] MONTORIOL POUS, J. (1959): Relaciones entre la quimiolitogenesis y la termocirculacion. Speleon X :33-44.

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