Optimising a method for aragonite precipitation in simulated biogenic calcification media

Abstract

Resolving how factors such as temperature, pH, biomolecules and mineral growth rate influence the geochemistry and structure of biogenic CaCO3, is essential to the effective development of palaeoproxies. Here we optimise a method to precipitate the CaCO3 polymorph aragonite from seawater, under tightly controlled conditions that simulate the saturation state (Ω) of coral calcification fluids. We then use the method to explore the influence of aspartic acid (one of the most abundant amino acids in coral skeletons) on aragonite structure and morphology. Using ≥200 mg of aragonite seed (surface area 0.84 m2), to provide a surface for mineral growth, in a 330 mL seawater volume, generates reproducible estimates of precipitation rate over Ωaragonite = 6.9–19.2. However, unseeded precipitations are highly variable in duration and do not provide consistent estimates of precipitation rate. Low concentrations of aspartic acid (1–10 μM) promote aragonite formation, but high concentrations (≥ 1 mM) inhibit precipitation. The Raman spectra of aragonite precipitated in vitro can be separated from the signature of the starting seed by ensuring that at least 60% of the analysed aragonite is precipitated in vitro (equivalent to using a seed of 200 mg and precipitating 300 mg aragonite in vitro). Aspartic acid concentrations ≥ 1mM caused a significant increase in the full width half maxima of the Raman aragonite v1 peak, reflective of increased rotational disorder in the aragonite structure. Changes in the organic content of coral skeletons can drive variations in the FWHM of the Raman aragonite ν1 peak, and if not accounted for, may confuse the interpretation of calcification fluid saturation state from this parameter.