Ocean Acidification

Experiments were conducted on CaCO₃ nucleation in seawater of estimated Cretaceous Period “calcite sea” (i.e., low-magnesium calcite being the primary inorganic marine calcium carbonate precipitate) composition to determine the influences of alkalinity and pCO₂ on calcite versus aragonite formation. The pH was continually monitored throughout the experiments and it was used, in combination with the initial alkalinity, to calculate the pCO₂ and saturation state of aragonite and calcite at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy and X-ray diffraction analyses. It was observed that the initial alkalinity affected greatly the nucleation pCO₂ and the CaCO₃ polymorph that is precipitated. In seawater with Mg²⁺/Ca²⁺ = 1.2, ∼10 mM alkalinity, and a pCO₂ <2500 μatm, calcite is the initial polymorph that nucleates. Aragonite nucleates when the pCO₂ is >2500 μatm. Seawater with Mg²⁺/Ca²⁺ = 1.2 and a wide range of initial alkalinities (5–50 mM) produces variable results. Seawater with Mg²⁺/Ca²⁺ = 1.7 produces only aragonite at lower alkalinities (<∼11 mM), but calcite is formed when the alkalinity is >∼18 mM. These results demonstrate the need to also consider the effects of alkalinity and pCO₂ in the critical Mg²⁺/Ca²⁺ ratio region of ∼1–2 for calcite seas versus “aragonite seas” (in which the primary inorganic carbonate precipitates are aragonite and high-magnesium calcite) at various times throughout the Phanerozoic Eon and to use reasonable estimated values of alkalinity and pCO₂ in experimental studies. The results of this study indicate that it may have been possible for CaCO₃ to commonly nucleate directly from Cretaceous seawater due to elevated calcium and alkalinity concentrations, even though atmospheric pCO₂ was higher.



Lee, J., & Morse, J. W., 2010. Influences of alkalinity and pCO₂ on CaCO₃ nucleation from estimated Cretaceous composition seawater representative of “calcite seas”. Geology 38(2): 115-118. Article (subscription required).