Low-magnesian calcite limestones forming at the deep-sea floor, Tongue of the Ocean, Bahamas

Carbonate ooze in the deep troughs between the Bahama Banks is a mixture of pelagic and bank-derived material. It consists of aragonite, calcite and magnesium calcite in a ratio of about 3:2:1. Where exposed in erosional cuts at the sea floor, this ooze lithifies within 100,000 years and is transformed into calcite micrite of only 3.5-5 mol % MgCO₃. Where buried, the ooze maintains its original composition for at least 200,000-400,000 years and remains unlithified for tens of millions of years. Quite unexpectedly, the path of sea-floor diagenesis of peri-platform ooze was found to be the same as that of freshwater diagenesis. Most of the aragonite is leached, pteropod shells often leaving cement-lined moulds behind; magnesian calcite recrystallizes and loses magnesium; polyhedral calcite of 2-4 μm size appears as cement. The setting and the carbon-oxygen isotope ratios rule out any freshwater influence. Carbon isotope ratios remain heavy, oxygen ratios shift towards equilibrium with the cold bottom water. The calcite cement has 3.5-5 mol % MgCO₃ and can be interpreted as the least soluble form of calcite emerging from alteration at the sea floor or, alternatively, as a direct precipitate from cold sea water. The change in the composition of calcite cements with water depth supports the second interpretation. In the Bahamas and elsewhere in the world ocean, magnesium in calcite cements decreases from the warm surface waters down to 700-1200 m, i.e. the boundary between intermediate and cold deep-water masses. Below this level, calcite prevails and magnesian calcite and aragonite cements are restricted to semi-enclosed seas with exceptionally warm bottom waters.