Oxygen isotope composition of Mesoproterozoic (~1360 Ma) seawater constrained by clumped isotopes of North China limestones

The oxygen isotope composition (δ¹⁸O) of Phanerozoic seawater has been widely investigated, but the δ¹⁸O values of Precambrian seawater remain poorly constrained, with ongoing debate over whether they were substantially lower than those of Phanerozoic seawater. To address this question, we analyzed the clumped isotopes of limestones from the North China Craton to reconstruct Mesoproterozoic seawater temperature and δ¹⁸O values. Our results indicate that the Mesoproterozoic seawater had a temperature of 26.9° ± 0.4°C and a δ¹⁸O value of -6.3 ± 0.2 per mil (relative to standard mean ocean water). This δ¹⁸O estimate aligns with previous inferences from geochemical modeling, marine iron oxides, and oxygen isotope ensembles, supporting the hypothesis that Mesoproterozoic seawater was isotopically lighter than its Phanerozoic counterpart. These findings provide insights into the Earth's paleoclimate and the evolution of seawater composition during the Mesoproterozoic era.