title: Trace element incorporation in stalagmite calcite as a paleo-volcanic record: a chemical perspective
Creators: name: Midgley, Scott D.
Publisher: name: University of Reading
Year: 2021

description: This thesis examines the incorporation of oxyanion impurities (sulphate, molybdate and bromate) in calcium carbonate minerals, using atomic-level computer simulations. The motivation is to understand the chemical nature and environment of these impurities in speleothems, as they constitute a record of past volcanic activity that can be used in past climate reconstruction models. Sulphur in its tetrahedral oxyanion form, sulphate (SO4 2- ), shows low thermodynamic tendency to incorporate in bulk crystalline regions of calcite, which is the calcium carbonate phase forming most speleothems. Incorporation thermodynamics in other calcium carbonate phases is highly dependent on the phase density. Incorporation stability in the anhydrous phases follows the order: vaterite &gt; calcite &gt; aragonite. Molybdenum, in its tetrahedral oxyanion form, molybdate (MoO4 2- ), shows similar incorporation behaviour to sulphate, though it requires around 40-50% more energy to incorporate across the naturally occurring calcium carbonate host phases. The incorporation in hydrated calcium carbonate phases is also discussed and compared with the behaviour in anhydrous phases. Given that bulk substitution is thermodynamically unstable, the incorporation of sulphate and molybdate at the calcite/water interface was investigated, using ab initio molecular dynamics. The incorporation at both the calcite (10.4) terraces and at step line defects with the same surface termination was examined. Oxyanion/carbonate exchange energies calculated at the calcite/water interface are far more favourable than those calculated for the bulk, indicating a clear tendency for sulphate and molybdate to accumulate at the surface, and even more strongly at the defect regions. The specific surface area or crystallinity can therefore be expected to play a large role in the uptake of sulphur and molybdenum in speleothem, which should be considered when interpreting experimentally trace element concentrations for paleovolcanic reconstruction models. Bromine in its trigonal pyramidal oxyanion form, bromate (BrO3 - ), was also studied as a possible anionic trace element in calcium carbonates (focusing on calcite and aragonite in this case). Bromate provides a geochemically stable redox state for bromine whilst inducing lower elastic strain effects on the host crystal compared with sulphate and molybdate, owing to one less apical oxygen atom. The charge imbalance introduced when substituting a singly charged bromate ion for a doubly charged carbonate ion was compensated by the co-incorporation of a singly charged cation. Lithium, sodium, and potassium ions were investigated for their effect on the thermodynamics of substitution in the crystalline bulk of calcite and aragonite. It was demonstrated that at ambient temperature the binding between the oppositely charged impurity dominated over the configurational entropy tendency to separate the defects. Therefore, the impurities are likely to be found in nearest neighbour configurations in both calcite and aragonite, which could have implications for their detection and measurement. Although bromate is also metastable in calcite in aragonite with respect to phase separation, the thermodynamic driving force for that is much lower than for sulphate and molybdate. Bulk crystalline incorporation of bromate would therefore be expected to be far higher than sulphate and molybdate. This higher stability could be related to the reported superior reliability of bromine in speleothems as a record of past volcanic activity. This theoretical work offers a first approximation to the chemistry of incorporation of molybdenum and bromine in calcium carbonate minerals, providing clear thermodynamic and structural data that will help geochemists to interpret their speleothem records, and hopefully motivate further research work.