THE OXYGEN ISOTOPIC COMPOSITION OF THE OCEANS, SEDIMENTS AND THE SEA-FLOOR

The oxygen isotope history of seawater remains controversial despite d ecades of study. Isotopic data from fossils and chemical sediments gen erally indicate significantly O-18-depleted seas during the Paleozoic and earlier, whereas the isotopic composition of contemporaneous ophio lites imply an ocean indistinguishable from today's. Uncertainty in de lta(18)O of seawater limits the usefulness of oxygen isotope geothermo metry as well as challenges current paradigms of plate tectonics. This O-18-balance within the hydrosphere. The conclusion is that paper rev iews the controversy and revises earlier estimates on the the delta(18 )O of seawater is mainly buffered by hydrothermal and weathering proce sses at mid-ocean ridges to a delta(18)O value of about 0 parts per th ousand (SMOW). At values other than 0 +/- 2 parts per thousand, a larg e countervailing flux of O-18 would return seawater to near 0 parts pe r thousand. The delta(18)O of Paleozoic, Proterozoic and Archean ophio lites support the above model. The discrepancy in delta(18)O between m ost (but not all) post-Mississippian fossils and chemical sediments an d ophiolites must be explained as a combination of: (1) loss of integr ity of delta(18)O in sediments and fossils; (2) warmer paleoenvironmen ts that lead to the precipitation of lower delta(18)O exogenic product s; (3) isotopically distinct water masses in inland seas that host the fossils but which are not well mixed with the open ocean that alters the seafloor. (C) 1998 Elsevier Science B.V. All rights reserved.