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.