Partitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem
Section snippets
Introduction and background
The trace-element geochemistry of calcium carbonate minerals is widely used to help reconstruct past environments across a wide range of time scales (Fairchild and Treble, 2009). The mechanisms by which these elements are incorporated vary depending on the physical properties of the element (e.g. especially the charge and ionic radius – Bourdin et al., 2011), the carbonate mineral in question (usually either calcite or aragonite, e.g. Balboni et al., 2015, Chen et al., 2016, Wassenburg et al.,
Study site and sampling
The water and calcite samples used in this study were collected from Antro del Corchia, a large cave system developed in Mesozoic dolomites, marbles and dolomitic marbles of the Alpi Apuane of western-central Italy (Piccini et al., 2008, Baneschi et al., 2011) (Fig. 1). The samples come from a cave pool (‘Laghetto Basso’) (Fig. 2, Fig. 3) that has developed on the floor of a large, well-decorated chamber called the Galleria delle Stalattiti (GdS). The chamber is situated ∼400 m vertically below
Water sampling and analysis
Water temperature, electrical conductivity (EC) and pH were measured directly in the Laghetto Basso water using a portable multi-parameter meter (Delta OHM Instruments). Accuracy is 0.5% for conductivity, 0.25% for temperature and 0.1 for pH. The pH and EC sensors were calibrated prior to each sampling trip with certified standard buffer solutions (pH 4 and 7 solutions, and 147, 717.5 and 1413 µS cm−1 solutions).
Total alkalinity was measured in the field within 1–2 hours of sampling by HCl
Pool-water chemistry
The pool-water chemistry of Laghetto Basso is summarised in Table 2; additional data, including charge balances, saturation indices and PCO2 values, are contained in Table A3. The Laghetto Basso water temperature is stable at 7.9 ± 0.4 °C (2σ), which is to be expected from a deep-interior chamber. The pH of the waters is slightly alkaline, and for most of the sampling period only fluctuated within a narrow envelope (±0.2 units). The ion chemistry indicates that the waters are of low ionic
Low ionic strength waters of Laghetto Basso
The Laghetto Basso pool waters are of low ionic strength, with concentrations of major ions (Ca2+, Mg2+ and HCO3−) being at the lower end of the spectrum of cave percolation waters. Part of the reason for this is the likelihood of low PCO2 of the infiltrating waters as they enter the karst, which can be attributed to the low soil cover above the cave (Drysdale et al., 2004). Much of the surface above Corchia Cave is steep and devoid of substantive soil cover. Here, the ground is criss-crossed
Conclusions
In this study, we have sought to advance understanding of how trace elements are incorporated into natural calcites by investigating the partitioning of Mg, Sr, Ba and U into subaqueous cave calcite, a rarely studied form of speleothem. We have shown that the pool-water chemistry of Laghetto Basso displays decadal-scale stability, in contrast to percolation-waters reported in a number of other cave studies (e.g. Fairchild et al., 2000, McDonald et al., 2004, Tremaine and Froelich, 2013). This
Acknowledgements
This research was supported by funding from the Australian Research Council (Discovery Project number DP160102969, awarded to RD, GZ, ER and JW; Laureate Fellowship FL160100028 awarded to JW; and Future Fellowship FT130100801 awarded to JH. We are grateful to the Gruppo Speleologico Lucchese and the Federazione Speleologica Toscana for outstanding support. This paper benefitted from discussions with Professor Richard Reeder. This paper has benefitted significantly from three very meticulous and
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