Variations in the O18/O16 and C13/C12 Ratios of Diagenetically Altered Limestones in the Bermuda Islands

Abstract

Marine and eolian Pleistocene sediments in the Bermuda Islands are composed largely of fragments of mollusks, Foraminifera, lithothamnoid algae, and Halimeda. The Pleistocene sediments range from unconsolidated carbonate sands to well-consolidated limestones. Recent carbonate sediments and major sediment-contributing organisms (except Halimeda) have similar and ratios which are distinctly different from the and ratios observed in the stalactites, soil bases, and secondary calcites from the limestones. Precipitation of secondary calcite in the limestones and alteration of the constituent grains cause the and ratios of the limestones to approach the values observed in stalactites and soil bases. Secondary calcites, precipitated from fresh water on the Bermuda Islands, have ratios which are controlled primarily by the average ratio of the rain water on the island. The ratios of the secondary calcites are controlled by the amount of carbon dioxide derived from the soil zone which is mixed with carbon derived from solution of the limestones through which the water has passed. Some of the limestones from the intertidal or surf-spray zone have and ratios which are distinctly different from the majority of the well-consolidated limestones. These limestones may have been lithified by cement precipitated from sea water or from fresh water which had been subjected to evaporation and to exchange of carbon with atmospheric carbon dioxide.