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

Content Partner: Lincoln University. 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/(mSODSO₄/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.