Carbonate-rich periplatform sediments represent an active carbon reservoir containing metastable aragonite and magnesian calcite (Mg-calcite, > 4 mol % MgCO(3)). Since Mg-calcite is highly soluble, the preservation state of this mineral provides information on past carbonate systems at water depths shallower than the lysocline. The mineralogy and geochemistry of the carbonate-rich fine fraction (< 63 mu m) was determined for the last 940 ka in a sediment core collected from the Walton Basin (Northern Caribbean Sea, 846 m water depth). Mg/Ca ratios for Mg-calcite showed clear glacial-interglacial cycles with high values (12 mol % MgCO(3)) during interglacials. Glacial Mg/Ca ratios were approximately 8 mol % MgCO(3) for the period from 940 ka to 400 ka, and approximately 10 mol % MgCO(3) for the last 400 ka. The Mg/Ca shift is concomitant with a preferential loss of Mg-calcite relative to aragonite. The preservation state of Mg-calcite revealed that the bottom water mass of the studied site was slightly more corrosive for the earlier period, possibly relating to a composition change in intermediate water and/or to the ventilation rate in the Atlantic Ocean.