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

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Science  19 Mar 1993:
Vol. 259, Issue 5102, pp. 1733-1736
DOI: 10.1126/science.259.5102.1733


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

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