Quantifying evaporation during fluid inclusion isotope analysis in speleothem samples

Speleothem fluid inclusion isotope analysis provides the oxygen and hydrogen isotope compositions of the parent water from which the carbonate was precipitated. In contrast to the carbonate isotopes, it is not affected by kinetic isotope effects or cave air temperature. However, in-cave evaporation has been identified as a potential control on drip water isotopes if drip rates are slow and or relative humidity in the cave is low.

Rainfall isotope compositions generally plot close to the global meteoric water line (GMWL) in a plot of d²Hversus d¹⁸O that can be described by the following regression equation: d²H = 8*d¹⁸O + 10. A lower deuterium excess (d-excess) value indicates post-condensation evaporation, and different fractions of evaporation typically result in so-called evaporation lines with shallower slopes. Recently it was shown that in-crusher evaporation results in water loss during analysis, which may significantly affect the speleothem fluid inclusion isotope composition. For fluid inclusion isotope compositions that have low d-excess values, it is thus key to find out where evaporation took place.

In this study, we examine the effect of analytical evaporation by quantifying the water loss during analysis. We target two layers with different calcite fabrics from a flowstone of Touhami Cave (GTOF2), Morocco, as well as a speleothem from Scladina Cave, Belgium. The Moroccan fluid inclusion isotope data agree well with the drip water isotope composition from a cave nearby. The white opaque layer from GTOF2 has high water contents of 3.4 µl/g, whereas the second transparent layer has only 0.12 µl/g. The speleothem from Scladina Cave yielded 2.0 µl/g. We observed that all replicates lose water up to 39% by evaporation, but only the Scladina speleothem shows a clear relationship between fractional water loss and d-excess. The replicates of the low water content layer in GTOF2 plot on an evaporation line, but the slope is steeper compared to the evaporation line from the Scladina speleothem. We suggest that the Touhami Cave flowstone may have been affected by in-cave evaporation.