Journal of Conference Abstracts

Elemental Geochemistry of Karst Cave Waters and Speleothems: Keys to Palaeohydrology

Ian J. Fairchild (i.j.fairchild@keele.ac.uk)¹, Yi Ming Huang¹, Anna F. Tooth¹, Andrea Borsato², Frank McDermott³, Silvia Frisia², Andy Baker⁴ & Baruch Spiro⁵

¹ Department of Earth Sciences, Keele University, Staffs ST5 5BG, UK

² Museo Tridentino, Via Calepina, Trento 38100, Italy

³ Geology, University College Dublin, Ireland

⁴ Department of Geography, University of Newcastle-on-Tyne, UK

⁵ NERC Isotope Geosciences Laboratory, Keyworth, Notts, UK

Study of cave water and speleothem geochemistry at a number of European sites has revealed some common geochemical patterns which can be related to the bedrock chemistry modified by hydrological conditions above and within the cave. Theoretical calculations from dissolution kinetics and experiments on soil and bedrock from study sites demonstrate also that preferential dissolution of calcite over dolomite occurs. Longer water residence time will lead to enhancement in Mg/Ca due to a greater proportion of dolomite dissolution: this has been observed as a factor responsible for part of the variation in water chemistry between different sites, but it is not clear whether this is important at single dripsites on a short timescale. Other factors must also be important since the common observation in cave waters is that there is enrichment in Sr and Mg in crossplots of Sr/Ca versus Mg/Ca compared with mixing lines of limestone and dolomite bedrocks at the studied localities. Locally, there is evidence for selective leaching effects, which are enhanced under slowflow conditions. More generally water chemistry is modified by prior calcite precipitation which raises the Mg/Ca and Sr./Ca ratios. This precipitation occurs within the studied caves, but can also occur above the cave too. Such prior precipitation tends to be enhanced in drier conditions when degassing of karst waters occurs more readily. The partitioning of Sr and Mg between aqueous solution and speleothems has been made in new experiments using Ca contents of 75 ppm and ionic strengths close to those in the cave environments. Supersaturations were in the range 10-17 and temperatures fixed at 25 or 15°C. Experiments were of many weeks duration and products were analyzed using an ion probe. The Sr partition coefficient varies between 0.06 and 0.08 at precipitation rates of 0.3-0.8 nmol per min-cm², with a positive relationship between partition coefficient and growth rate. These values are lower than in previous studies but much closer to values typical of cave environments. In contrast, the literature values would imply that speleothem growth is highly spasmodic. The results are encouraging in that variations in cave water chemistry should be translated into the speleothems. The pronounced intra-annual variations in Sr and Mg composition seen in speleothems can thus be interpreted in terms of water chemistry variations which in turn reflect paleohydrology and hence the balance of atmospheric precipitation and evaporation in the past.

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