Elsevier

Geochimica et Cosmochimica Acta

Volume 264, 1 November 2019, Pages 67-91
Geochimica et Cosmochimica Acta

Partitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem

https://doi.org/10.1016/j.gca.2019.08.001Get rights and content

Abstract

The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’ subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with published cave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previous estimates at similar temperatures (∼8 °C). The DSr (0.100 ± 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (<1 μm a−1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 μm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation.

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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