Carbon and Oxygen Isotope Ratios as Environmental Indicators: Anomalous Results from Carbonate Shells from Beach Sediments of Lake Managua, Nicaragua

Abstract

THE value of carbon and oxygen isotope ratios in determining the nature of the depositional environment for various types of carbonate rocks has been pointed out by a number of workers1–3, who showed that freshwater limestones are enriched in carbon-12 and oxygen-16 relative to the marine varieties. Moreover, detailed variations in 12C/13C ratios were found to be preserved in the Vanport limestone of Pennsylvanian age4; the ratios increase regularly toward the shoreline of the estuarine–deltaic basin complex and presumably indirectly reflect variations in salinity concurrent with the mixing of carbon-12 enriched continental bicarbonate of the distributary with carbon-13 enriched bicarbonate of the ocean.