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Abstract:
Dissolution processes in karst regions commonly involve (meteoric) water whose stable isotopic (O, H, C) composition is distinctly different from that of the paleowaters from which the host rock (limestone, dolostone) formed. This, in theory, should lead to isotopic alteration of the host rock beyond the active solution surface as the modern karst water is out of isotopic equilibrium with the carbonate rock. No such alteration has been reported, however, in epigenetic karst systems. In contrast, isotopic alteration, commonly referred to as isotopic halos or fronts, are known from various hypogene systems (ore deposits, active hydro­thermal systems, etc.). These empirical observations suggest that stable isotope data may be a diagnostic tool to identify hypogene water-rock interactions particularly in cave systems whose origin is ambiguous.

We have been testing the applicability of this assumption to karst settings by studying the isotopic composition of carbonate host rocks in a variety of caves showing clear-cut hypogene morphologies. Cores drilled into the walls of cave chambers and galleries were stud­ied petrographically and the C and O isotope composition was analyzed along these cores, which typically reached a depth of 0.5 to 1.2 m. We identified three scenarios: (a) no isotopic alteration, (b) a sigmoidal isotope front within a few centimeters of the cave wall, and (c) pervasive isotope alteration throughout the entire core length. Type (a) was found in caves where the rate of cave wall retreat apparently outpaced the rate of isotopic alteration of the wall rock (which is typical, for example, for sulfuric acid speleogenesis). Type (c) was observed in geologically young, porous limestone showing evidence of alteration zones up to 5 m wide. The intermediate type (b) was identified in hypogene karst cavities developed in tight limestone, dolostone and marble.

Our data in conjunction with evidence from speleothems and their geochemical and fluid-inclusion composition suggest that the spa­tial extent of the isotopic alteration front depends on the porosity and permeability, as well as on the saturation state of the water. Wider alteration zones primarily reflect a higher permeability. Shifts are most distinct for oxygen isotopes and less so for carbon, whereby the amplitude depends on a number of variables, including the isotopic composition of unaltered host rock, the isotopic composition of the paleofluid, the temperature, the water/rock ratio, the surface of water-rock contact, the permeability of the rock, and the time available for isotope exchange. If the other parameters can be reasonably constrained, then semi-quantitative temperature estimates of the paleowater can be obtained assuming isotopic equilibrium conditions.

If preserved (scenarios b and c), alteration fronts are a strong evidence of hypogene speleogenesis, and, in conjunction with hypogene precipitates, allow to fingerprint the isotopic and physical parameters of the altering paleofluid. The reverse conclusion is not valid, however; i.e. the lack of evidence of isotopic alteration of the cave wall rock cannot be used to rule out hypogene paleo-water-rock interaction.