Recent littoral and sublittoral high-Mg calcite lithification in the Mediterranean

Lithification caused by inorganic precipitation of high-Mg calcite is reported from several localities in the Mediterranean. A micritic calcite cement with 13–15 mol. % MgCO3 in solid solution lithifies internal sediments and forms open-space fillings within a sublittoral algal framework down to 20 m, in which the biogenic structures are gradually obliterated through the activity of numerous boring organisms and the subsequent precipitation of micrite. The silt-mud fraction of unconsolidated bottom sediments is to a large extent stable low-Mg calcite, and is not the source for the mcrite; instead the cementing mineral is assumed to precipitate directly from interstitial sea water. The special physico-chemical régime is attributed to organism activities.

The same phase of high-Mg calcite also forms the cement in beachrock in the areas studied; also here the Mg-content is an original property and not the result of diagenetic changes. The cement occurs partly as micrite, identical to the micrite in the lithified algal framework, and partly as radiating microspar fringes. The littoral and sublittoral lithification described here indicates that within several niches in the present marine environment high-Mg calcite may substitute for aragonite as the mineral form of precipitated calcium carbonate.