Did major changes in the stable-isotope composition of Proterozoic seawater occur?

Early Proterozoic marine carbonates of the Rocknest Formation (1.93-1.89 Ga) have very depleted δ ¹⁸ O values (about 8‰) relative to younger, Late Proterozoic marine carbonates that formed in similar depositional environments. Two isotopic trends are superimposed on the data for open-marine components. The first involves stabilization of tidal-flat sediments during early, possibly reflux-type dolomitization by evaporative pore fluids enriched in δ ¹⁸ O The second trend toward isotopically light δ ¹⁸ O values was established during dolomitization of open-marine facies in contact either with meteoric waters (mixing zone) or under conditions of higher temperatures during burial. This resulted in precipitation of blocky, pore-occluding cements. The isotopically most enriched ooids are the best preserved normal marine components and may suggest that the δ ¹⁸ O of seawater was about -9.75‰ ±1.0‰ (SMOW) at 1.9 Ga. This composition would require a major change in the balance of high-temperature oxygen isotopic exchange between seawater and basalt and low-temperature weathering in order to explain the 8‰ positive shift in inferred seawater δ ¹⁸ O between 1.9 and 1.0 Ga. Alternatively, the depleted δ ¹⁸ O values represent an approximately 30-35 °C higher temperature of surface waters at 1.9 Ga. The heaviest carbonate δ ¹³ C values are +1.75permil;, more enriched than previously reported for the Early Proterozoic on the basis of bulk-rock data.