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

Stable isotope data as constraints on models for the origin of coralloid and massive speleothems: The interplay of substrate, water supply, degassing, and evaporation

Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as “smoothing accretions”). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss fromacapillary filmof solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about δ13C and δ18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substratemorphology. In subaerial speleothems, data showenrichment in heavy isotopes both along the direction ofwater flow and toward the protrusions. The first effect is due toH2OevaporationandCO2 degassing during a gravity-driven flowof water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing duringwatermovement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol fromthe cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water toward different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the iso-depleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves

Inactive Hydrothermal Hypogenic Karst in SW Sardinia (Italy)

none4siIn Sardinia, no active hypogenic caves have yet been discovered or described. Although there are a few thermal springs, mostly correlated to Quaternary volcanic activity, none of these thermal waters have interacted with carbonate rocks. Nevertheless, in the SW of the Island many metal ore deposits hosted in Cambrian limestones have been exploited over the last two centuries, allowing the discovery of so-called mine caves, some of which are clearly of hypogenic origin. These caves formed by thermal waters in a phreatic setting and are now located far above the water table and are no longer active, apart from some recent dripstone formation. The mine tunnels in Mount San Giovanni, near Iglesias and Gonnesa towns, have cut most of these caves: among them the well-known Santa Barbara cave, covered with barite crystals, Santa Barbara 2 cave, with its unique oxidation vents, and Crovassa Ricchi in Argento. Other hypogenic caves have been discovered in the mines of Campo Pisano and Monteponi (Iglesias), Mount Onixeddu (Gonnesa), and especially Masua (Iglesias). A very special case of hypogenic cave is the Corona ’e Sa Craba quartzite system, known for its barite crystals and rich in many mineral species. This chapter summarizes these known inactive hydrothermal and sulfuric acid caves.openDe Waele, Jo; Gázquez, Fernando; Forti, Paolo; Naseddu, AngeloDe Waele, Jo; Gázquez, Fernando; Forti, Paolo; Naseddu, Angel