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

Measurements on 170 carbonate rocks show decreasing deltaO18 of similar magnitude for both limestones and dolomites over a time span of approximately 2800 m.y. The Proterozoic dolomites are on average heavier by approximately 5 parts per thousand in deltaO18 than their coeval limestones. The data may indicate displacement in deltaC13 of about 3 parts per thousand at approximately 570 m.y. ago, with Precambrian carbonates being heavier in deltaC13 at a given deltaO18 level than their Phanerozoic counterparts. The compilation of the previously published data is consistent with the above described features. (author)