Wholesale stabilization of the unstable carbonate phases aragonite and magnesium calcite, and reprecipitation of calcite and dolomite are currently taking place where the phreatic zone (modern water table) invades 120,000-year-old Pleistocene biolithites (Falmouth Formation), north Jamaica.

Pleistocene rocks in the vadose zone are relatively unaltered, and consist of in situ, mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a "water-table cave," commonly is present. Below the water table, the rocks are invariably more highly altered than those above. Magnesium-calcites are very scarce, and considerable dissolution of aragonite commonly has occurred.

Dolomite occurs as 8-25 micron, subhedral crystals precipitated as void linings. The isotopic composition of the dolomite (O¹⁸ = -1.0 ^pmil, ^dgr C¹³ = 8.4 ^pmil) and its high-strongtium content (3,000 ppm) suggest precipitation as CO₂-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve magnesium-calcites and strontium-rich aragonites, and remove the gas. Because some dolomitized rocks are enriched in magnesium relative to primary biolithites, magnesium addition to the system is necessitated and probably is derived from seawater in the mixing zone.

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