THE O-18/O-16 RATIO OF 2-BILLION-YEAR-OLD SEAWATER INFERRED FROM ANCIENT OCEANIC-CRUST

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 O-18/O-16 ratios do not reflect global-scale O-18 depletion of early Proterozoic oceans.