A global assessment of the geochemistry of carbonate mineral formation and the dynamics of calcite precipitation in alkaline lakes

Complex interactions are known to govern calcium carbonate precipitation in alkaline lake water conditions. Whilst comprising a small fraction of Earth's water volume, they exhibit unique sedimentological and geochemical characteristics that may be crucial for understanding palaeoclimate and planetary surface evolution. Here, I investigate the geochemical dynamics of CaCO 3 supersaturation in alkaline lakes and the possible roles of Mg/Ca ratios and total dissolved phosphate (ΣPO 4 ) in modulating CaCO 3 precipitation kinetics. Through a comprehensive analysis of global alkaline lake water data and thermodynamic modelling, it is suggested that 80–93 % of these lakes are supersaturated with respect to calcite, despite generally low calcium concentrations (defined in terms of saturation state or saturation index, expressed as Ω calcite ≥ 1 and SI calcite ≥ 0, respectively). To account for the presence of ΣPO 4 in generally high ionic strength and alkaline waters, this assessment is underpinned by original contributions, comprising the incorporation of amorphous and metastable Ca‑carbonate and Ca-phosphate phases in a geochemical thermodynamic database and an implementation of a Pitzer ion interaction activity coefficient model that explicitly considers the species H 3 PO 4 , H 2 PO 4 − , HPO 4 2− and PO 4 3− . These implementations enable a more robust characterisation of phosphate speciation and distribution at high ionic strengths and over the range of relevant pH values (mostly bracketed between 8 and 11 in these systems) in continental waters. This fundamental approach provides further insight into our current understanding of individual phosphate species' involvement in the inhibition of CaCO 3 precipitation in natural alkaline waters, with H 2 PO 4 − and PO 4 3− being the predominant species in the lower and higher pH values, respectively. However, individual phosphate species concentrations and distribution are also strongly dependent on the concentration of Ca 2+ ion, as well as dependent on the solid phosphate phase setting the solubility threshold (likely either octacalcium phosphate, OCP, or amorphous calcium phosphate, ACP), as shown by simple model calculations. In line with observations from recent studies, total phosphate concentrations tend to increase with increasing alkalinity (ALK T ) and dissolved inorganic carbon (DIC). Consequently, it may promote further CaCO 3 supersaturation through its known inhibitory effect. High CaCO 3 supersaturation lowers the necessary critical energy of nucleation, promoting the nucleation and growth of metastable polymorphs of CaCO 3 , such as monohydrocalcite (MHC) and amorphous calcium carbonate (ACC) and/or calcium-Mg carbonate (ACMC) phases. High supersaturation implies most actual calcite forms far from equilibrium in these lakes. Based on actual lake water chemistry data and theoretical considerations, likely MHC is the predominant precursor of calcite in alkaline lake waters, whilst ACC/ACMC phases are limited to very high calcite supersaturation conditions which may not be very commonly achieved. Calcite minerals forming via these pathways may not only deviate compositionally and in overall geochemical properties from those conditions expected at equilibrium, but are also likely to develop growth textures incorporating more defects and consisting of polycrystalline phases with non-coherent optical properties. The latter may be the case of familiar examples of unusual calcite crystal morphologies such as those now widely documented and studied in the Cretaceous continental carbonates of the South Atlantic sedimentary basins.