The oxygen isotope evolution of seawater: A critical review of a long-standing controversy and an improved geological water cycle model for the past 3.4 billion years

• Published in: Earth-science reviews 2007-07, Vol.83 (1), p.83-122
• Main Authors: Jaffrés, Jasmine B.D., Shields, Graham A., Wallmann, Klaus
• Format: Article
• Language: English
• Subjects: box modelling; diagenesis; Geological time; Geology; Isotopes; Marine; marine carbonates; Oxygen; paleotemperature; Seawater; seawater δ18O; δ18O record
• ISSN: 0012-8252; 1872-6828
• DOI: 10.1016/j.earscirev.2007.04.002

Controversy over the oxygen isotope composition of seawater began in the 1950's, since which time there has been no agreement over whether the oxygen isotope composition of the oceans has changed over time. Resolving this uncertainty would allow the δ 18O values of demonstrably well preserved marine authigenic precipitates to be used to reconstruct surface climate trends back to early Archean times and would help towards a more quantitative description of Earth's global water cycle on geological time scales. Isotopic studies of marine carbonate and silica reveal a trend of increasing δ 18O values with decreasing age since the Archean. This trend has been interpreted by some to reflect a progressive increase in seawater δ 18O through time; however, it is generally accepted on the basis of ophiolite studies and theoretical considerations that seawater δ 18O cannot change significantly because of the buffering effects of ocean crust alteration at mid-ocean ridges. As a result many alternative interpretations have been proposed, including meteoric alteration; warmer paleoclimates; higher seawater pH; salinity stratification and biased sampling. Here we review these interpretations in the light of an updated compilation of marine carbonate δ 18O from around the world, covering the Phanerozoic and Precambrian rock records. Recent models of the geological water cycle demonstrate how long-term trends in chemical weathering and hydrothermal circulation can indeed influence the O-isotope composition of the global ocean to the extent necessary to explain the carbonate δ 18O trend, with residual variation attributed to climatic fluctuations and post-depositional alteration. We present the further development of an existing model of the geological water cycle. In the model, seawater δ 18O increased from about − 13.3‰ to − 0.3‰ over a period of 3.4 Ga, with average surface temperatures fluctuating between 10 °C to 33 °C, which is consistent with known biological constraints. Similar temperature variations are also obtained, although with higher starting seawater δ 18O composition, when more conservative approaches are used that take into account the systematic effects of diagenetic alteration on mean calcite δ 18O values. In contrast to much published opinion, the average δ 18O value of ocean crust in the model remained relatively unchanged throughout all model runs. Invariable ophiolite δ 18O values can, therefore, not be used as a definitive argument against changing