Aragonite-calcite speleothems; identifying original and diagenetic features

Despite the increasing use of speleothems as high-resolution archives of past climate proxies, few efforts have been made to improve understanding of diagenetic pathways affecting this material and to assess the degree of postdepositional transformation of initial features, including associated geochemical signatures. To provide criteria for distinguishing primary (spelean growth history) and secondary (spelean diagenetic history) features, this study documents a process-based approach combining mineralogical, textural, and geochemical data at different scales to characterize various diagenetic processes undergone by aragonite and calcite speleothems from the Pont-de-Ratz Cave (Herault, S. France). Results show that the initial precipitation of carbonate minerals forming the stalagmites (vadose precipitation) is represented alternatively by aragonite and calcite and consists of two types of primary aragonite and a primary columnar calcite. Four different types of growth interruption and inclusion horizons are evident, and attest to the discontinuous growth of stalagmites in this cave. Postdepositional diagenetic processes include void-filling calcite cementation, selective dissolution of either aragonite or calcite, and recrystallization. Recrystallization processes include 1) textural change of original aragonite precipitates (isomineralogical recrystallization), and 2) two distinct types of aragonite-to-calcite recrystallization. These results highlight the diversity of potential synformational and postformational processes controlling the formation and evolution of speleothems and their impact on the geochemical properties of the primary and secondary carbonate phases, and the potential importance in paleoclimatic interpretation.