Contrasting genesis and environmental significance of aragonite inferred from minor and trace element variation in speleothems

This study compares minor and trace element variation in two speleothems from two caves of southwestern Sardinia (Italy), in order to verify the role of some elements in calcite growth inhibition and aragonite precipitation. The first sample is a drapery from a cave hosted in dolostones. The second is a flowstone from a natural cave intercepted by a gallery of a mixed sulphide mine. In the first sample, Mg concentration in the calcite layer is high just above the underlying aragonite, it shows a minimum in the middle of the calcite, and it increases to a maximum just below the overlying aragonite. The Mg variation in this depositional sequence suggests that aragonite forms when calcite growth is inhibited by Mg because of a greater Mg/Ca ratio in solution. We infer that climatic conditions evolved from drier to wetter for the transition aragonite-calcite, and then from wetter to drier for the transition calcite-aragonite. This hypothesis is supported by inverse correlation of P with Mg in the calcite layer. In the second sample from a mine cave, the inhibitory effect of Mg on calcite deposition cannot explain the appearance of aragonite, because Mg concentration is small in the calcite layer and even decreases in the upper part, nearest the overlying aragonite. Zinc concentration, which is greater than that of Mg, increases abruptly in calcite toward the aragonite layer, reaching its maximum just below the aragonite. Thus, in this case, calcite inhibition seems to be induced by Zn. An increase of the Zn/Ca ratio in calcite can be simply explained by an increase of dissolved Zn in the fresh (not residual) feeding water, due to greater oxidation of sulphides in wetter periods. This hypothesis is supported by positive correlation of Zn with P, Pb and Cd. If the presence of aragonite in the mine cave is truly controlled by the Zn/Ca ratio rather than the Mg/Ca ratio, aragonite could reflect wetter conditions rather than drier ones, counter to conventional understandings from caves where Zn is less abundant