Carbonate Layers (image) Arizona State University Caption Carbonate layers hold carbon isotope evidence of the late Precambrian greening of the Earth. These are located in the Old Dad Mountains in California. Credit L.P. Knauth, Arizona State University Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.