Fibrous calcite cements from limestones of the Paleoproterozoic Pethei Group in northwest Canada have relatively constant δ 13C values, near 1‰, that indicate the C exogenic cycle of ca 1.9 Ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the Phanerozoic. 87Sr/ 86Sr ratios of these cements are near 0.7051, and support a progressively increasing influx of radiogenic Sr from continental weathering into the Paleoproterozoic oceans. δ 18O values of the cements vary from −14 to −8.5‰, although the original values of the cements were probably higher because the cements have been subseqently altered, as evidenced by their variable trace element contents. Proposed higher temperatures or substantially lower δ 18O values of ancient seawater based on such values may be in error. Fabric retentive dolomites, generally regarded as syndepositional, also have variable δ 18O values, the highest of which are ca 30 greater than the highest values of the fibrous calcite cements. The dolomites have more radiogenic Sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones. These fluids may have partially altered the δ 18O values and chemical compositions of the fibrous calcite cements. Late-stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar δ 18O values and radiogenic Sr isotopic ratios as the fabric retentive dolomites. If these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids, this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions.