Recent Inorganic Nonmarine Carbonate Environments

Compared with modern carbonate environments associated with or derived from the marine milieu, the study of nonmarine carbonate depositional environments has been neglected, although these offer a much wider range of conditions under which carbonate formation and diagenesis can occur. During the past years I have studied the following environments: (1) lakes and ponds, (2) springs and rivers, (3) caves (speleothems), (4) soils (especially caliche, and (5) technical incrustations (scale). My investigations clearly reveal that the formation of primary carbonate minerals (calcite, high-magnesium calcite, aragonite, hydrous magnesium carbonates) and of secondary carbonates (dolomite, huntite, and magnesite) in these various environments having different hydrochemistry, salinity, climatic conditions, etc., is mainly dependent on the Mg/Ca ratio of the solution in which the formation or transformation occurs. By loss or extraction of carbon dioxide, evaporation concentration or mixing of different water bodies, calcite, high-magnesium calcite, aragonite, and hydrous magnesium carbonates precipitate in an order of increasing Mg/Ca ratios. Dolomite formation takes place only at elevated Mg/Ca ratios (> 7) if high-magnesium calcite is available. The reactions leading to the formation of huntite and magnesite at very high (> 30) Mg/Ca ratios are not fully understood; from observations in Turkish lakes, it seems evident that dolomite is the precursor of huntite and huntite is the precursor of magnesite. A comparison between inorganic carbonate minerals deposited in the marine and nonmarine environments shows that the only significant difference in carbonate mineralogy is that (low-magnesium) calcite does not form under marine (or marine-derived) conditions. The explanation is that the high Mg/Ca ratio of the seawater (about 5) does not allow the formation of (low-magnesium) calcite.