Triple Oxygen Isotope Composition of Marine Chert through Earth history: Implications for 18O/16O of Seawater

The ¹⁸O/¹⁶O of carbonates cherts and shales show a puzzling increase over geologic time. Long-term changes in temperature, seawater ¹⁸O/¹⁶O ratios, and late-post-depositional overprinting can all explain this trend, but few tracers can distinguish between these scenarios. Here, we constrain the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios of seawater through 3.5Ga of Earth's history using the oxygen triple-isotope composition in ancient chert. We observe a temporal trend of increasing chert δ¹⁸O and decreasing Δ'¹⁷O through Earth history. We find that Phanerozoic marine cherts are consistent with having formed in oxygen isotope equilibrium with a modern-like seawater in the early diagenetic environment between 35°C and 60°C. Similarly, cherts from the Proterozoic and Neo-Archean are consistent with having formed in equilibrium with a modern-like seawater but at elevated temperatures of 70°C - 90°C. A subset of paleo-Archean cherts are consistent with having formed in modern-like seawater at a further elevated temperature of 100°C, but the majority of samples possess lower δ¹⁸O compositions and require an alternate explanation which will be discussed. The results of this study indicate that oxygen isotope composition of seawater has not changed significantly as far back the Ordovician which is consistent with clumped isotope studies. A change in seawater δ¹⁸O as far back as the Neo-Archean is not needed to explain the results under the possibility that the temperatures of chert diagenesis were higher either in these specific outcrops or systematically due to a difference in precursor phase. The results of this study can be explained with a constant seawater δ¹⁸O over the last 3.4Ga of Earth history but the possibility of a secular evolution of seawater δ¹⁸O prior to the Phanerozoic and coincident with the trend of chert oxygen isotope compositions cannot be eliminated.