Aragonite-calcite relationships in speleothems (Grotte de Clamouse, France): Environment, fabrics, and carbonate geochemistry

In Grotte de Clamouse (France), aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation. Prolonged degassing and evaporation yield needle aragonite, which is more enriched in O-18 and C-13 than aragonite ray crystals, which form near isotopic equilibrium. At present, aragonite ray crystals form at the tops of stalagmites at very low discharge (0.00035 ml/min), and when fluid Mg/Ca ratio is > 1.1. Temperature and evaporation do not seem to have a significant role in their formation. The presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology. Fossil aragonite was in part replaced by calcite in a time frame < 1.0 ka, possibly through the combined effects of dissolution of aragonite, and precipitation of calcite, which preferentially nucleated on calcite cements that had previously formed between aragonite rays. Commonly, the replacement phase inherited the textural and chemical characteristics of the precursor aragonite prisms and needles (and in particular the delta(13)C signal and U content), and preserved aragonite relicts (up to 16 weight %). The isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters. The real extent of aragonite, to-calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor.