Sulphate partitioning into calcite: Experimental verification of pH control and application to seasonality in speleothems

Peter M Wynn,Ian J Fairchild,Andrea Borsato,Christoph Spötl,Adam Hartland,Andy Baker,Silvia Frisia,James U.L Baldini

doi:10.1016/j.gca.2018.01.020

Abstract

Carbonate-associated sulphate (CAS) is a useful carrier of palaeoenvironmental information throughout the geologic record, particularly through its stable isotope composition. However, a paucity of experimental data restricts quantitative understanding of sulphate incorporation into carbonates, and consequently CAS concentrations and their diagenetic modifications are rarely interpreted. However, in the case of calcite speleothems, the remarkably high-resolution CAS records which are obtainable via modern microanalytical techniques represent a potentially invaluable source of palaeoenvironmental information. Here, we describe the results of controlled experiments of sulphate co-precipitation with calcite in freshwater solutions where pH, saturation state, and sulphate concentration were varied independently of each other. Solution pH is confirmed as the principal control on sulphate incorporation into calcite. The relative efficiency of incorporation was calculated as a partition coefficient DSO4 = (mSO4/mCO3)solid/(mSO4/mCO3)solution. High crystal growth rates (driven by either pH or saturation state) encouraged higher values of DSO4 because of an increasing concentration of defect sites on crystal surfaces. At low growth rates, DSO4 was reduced due to an inferred competition between sulphate and bicarbonate at the calcite surface. These experimental results are applied to understand the incorporation of sulphate into speleothem calcite. The experimentally determined pH-dependence suggests that strong seasonal variations in cave air PCO2 could account for annual cycles in sulphate concentration observed in stalagmites. Our new experimentally determined values of DSO4 were compared with DSO4 values calculated from speleothem-drip water monitoring from two caves within the Austrian and Italian Alps. At Obir cave, Austria, DSO4 (×105) varies between 11.1 (winter) and 9.0 (summer) and the corresponding figures for Ernesto cave, Italy, are 15.4 (winter) and 14.9 (summer). These values approximate predicted DSO4 values based on our chamber experiments containing both low (2 ppm) and high (20 ppm) sulphate concentrations. Our experimental values of DSO4 obtained at crystal growth rates typical of stalagmites, closely match those observed in other cave sites from around the world. This validates the universality of the controls behind DSO4 and will enhance the use of speleothem CAS as a palaeoenvironmental proxy.

Highlights

Carbonate-associated sulphate (CAS) has long been recognized as a tool for understanding past environmental processes (Burdett et al, 1989)
Calcite grown under controlled laboratory conditions provides the first quantitative description of the controls on annual cycles in speleothem sulphate. pH is confirmed as the dominant variable controlling the overall abundance of sulphate incorporated into calcite
Increased DSO4, despite the lower sulphate ion abundance, probably reflects an increase in defect sites at calcite crystal surfaces associated with high levels of supersaturation, as well as bicarbonate competition at the calcite surface

Summary

Introduction

Carbonate-associated sulphate (CAS) has long been recognized as a tool for understanding past environmental processes (Burdett et al, 1989). Most literature focuses on its isotope composition in marine archives as being indicative of global processes affecting sulphur cycling In contrast to isotopic studies, interpretations based purely on CAS concentrations are more limited due to uncertainty surrounding the nature of sulphate incorporation into the carbonate lattice. SeO42- has been demonstrated by Reeder et al (1994) and substitution of carbonate by sulphate was directly demonstrated by the X-ray absorption studies of Pingitore et al (1995), the lack of a quantitative understanding of sulphate incorporation into carbonates currently limits interpretation. The only experimental study on CAS incorporation was carried out by Busenburg & Plummer (1985)

Results

Discussion

Conclusion