The nature of O18/O16 and C13/C12 secular trends in sedimentary carbonate rocks

Measurements on 170 carbonate rocks show decreasing δO¹⁸ of similar magnitude for both limestones and dolomites over a time span of ~ 2800 m.y. The Proterozoic dolomites are on average heavier by ~5%. in δO¹⁸ than their coeval limestones. The data may indicate displacement in δC¹³ of about 3%. at approximately 570 m.y. ago, with Precambrian carbonates being heavier in δC¹³ at a given δO¹⁸ level than their Phanerozoic counterparts. The compilation of the previously published data is consistent with the above described features.