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 southern Sardinia, Italy. Samples have been analysed by XRD and Laser Ablation-ICP-MS. The first sample (SPD) is a drapery from the Spaghetto Cave (Santadi), hosted in dolostones without sulphide mineralization. SPD consists of a layer of primary calcite between two layers of primary aragonite. The second sample (SDF) is a flowstone from a natural cave (Sesta Sorella) intercepted by a gallery of the mixed sulphide mine of Sa Duchessa (Domusnovas). SDF consists of a layer of primary calcite underlying a layer of primary aragonite. In the calcite layer of SPD, Mg concentration is high just above the underlying aragonite, decreases to a minimum in the middle of the calcite, and increases to a maximum of 5 mol% MgCO3 just below the overlying aragonite. Magnesium is commonly believed to inhibit calcite precipitation, and greater concentration of Mg in calcite is commonly attributed to upstream precipitation of CaCO3 and resultant increase of Mg/Ca ratio in the residual solution (because the partition coefficient for Mg in both calcite and aragonite is < 1). The Mg pattern in SPD suggests that the initial Mg/Ca ratio in concretioning water was sufficiently high to inhibit precipitation of calcite and favor deposition of the lower aragonite layer, but then smaller Mg/Ca ratio allowed calcite to form the middle layer. Finally, Mg/Ca ratio increased so that calcite precipitation ceased and aragonite was again deposited. This depositional sequence suggests that climatic conditions evolved from dry to wetter for the transition aragonite-calcite, and then drier for the transition calcite-aragonite. This hypothesis is supported by inverse correlation of P with Mg in the calcite, with maximum concentration of P in the middle of the calcite layer. In fact, concentration of P is known to decrease in drier periods, when Mg increases. In SDF, 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. However, the inferences regarding Mg in SPD apply very well to Zn in SDF. Zinc in the calcite layer increases abruptly toward the aragonite layer, reaching its maximum (2 mol% ZnCO3) just below the aragonite. Like Mg, Zn is known to inhibit calcite precipitation, although its role is less clear than Mg. However, the partition coefficient of Zn is > 1, so that an increase of 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 in the calcite, as well as with Pb, Cu and Cd. Thus, contrary to SPD from a natural cave, the presence of aragonite in SDF from a mine cave seems to be controlled by the Zn/Ca ratio rather than the Mg/Ca ratio, and to reflect wetter conditions rather than drier ones.