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.