The 18 O/ 16 O Ratio of 2-Billion-Year-Old Seawater Inferred from Ancient Oceanic Crust

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Abstract
An oxygen isotope profile of the 2-billion-year-old Purtuniq ophiolite overlaps with similar profiles of younger ophiolites and the modern oceanic crust. This overlap implies (i) that there was a similar style of seawater—ocean crust interaction during the past 2 billion years; (ii) that the oxygen isotope composition of early Proterozoic seawater was similar to the modern value; (iii) that early Proterozoic sea-floor spreading rates were similar to, or greater than, average modern rates; and (iv) that early Proterozoic carbonate rocks and cherts with low 18 O/ 16 O ratios do not reflect global-scale 18 O depletion of early Proterozoic oceans.