Chemical Diagenesis of Some Modern Carbonate Sediments: ABSTRACT

Abstract

Mineralogical determinations and pore-water chemical analyses have been conducted on fine-grained carbonate sediments from Bermuda and southern Florida. For the pore waters of normal marine sediments, the results clearly demonstrate no change within experimental error of the ratios Mg++/Cl- and Sr++/Cl- from values found in the overlying sea water. Also, there is a lack of consistent change in carbonate mineralogy with depth. No evidence was found for the recrystallization of metastable aragonite or high-Mg calcite to low-Mg calcite and dolomite in sediments which have been in contact with sea water since deposition. Carbonate saturometer measurements indicate equilibrium of the pore waters with low-Mg calcite, even though it is a minor phase in many samples. This conclusion is corroborated by calcium, alkalinity, and pH determinations, and by laboratory studies of the solution behavior of the sediments. Values of dissolved Ca++ lower than those expected for simple CaCO3 solution by dissolved CO2, in sediments rich in H2S, can be explained partly by the addition of excess HCO3- from bacterial sulfate reduction and partly by the precipitation of calcium phosphate. Pore waters from the brackish mangrove-swamp sediments of the Everglades contain a slight excess of dissolved Mg++ over that expected from the Mg++/Cl- ratio of the overlying brackish waters and of sea water. This suggests that transformation of high-Mg calcite to low-Mg calcite takes place in these sediments by interaction with fresh water. Loss of Mg from calcite in fresh water is directly demonstrated by the chemistry of Bermuda cave waters. The lack of diagenetic transformation of carbonate minerals in sea water may be caused mainly by the interaction of dissolved Mg++ with the surfaces of the mineral grains. End_of_Article - Last_Page 606------------