Mineralogy and pore water chemistry of fine-grained carbonate sediments from Bermuda and south Florida indicate that aragonite and high-Mg calcite do not tend to recrystallize to low-Mg calcite and dolomite in sediments that always have been in contact with sea water; in these sediments ratios of Mg (super ++) and Sr (super ++) to Cl (super -) in pore water are similar to ratios in the overlying sea water. Pore waters from brackish mangrove swamp sediments contain more Mg (super ++) than would be expected from Mg (super ++) /Cl (super -) ratios of the overlying brackish water and of sea water; presumably transformation from high-Mg calcite to low-Mg calcite takes place by interaction with fresh water.