Modelling of the oxygen isotope evolution of seawater: Implications for the climate interpretation of the delta O-18 of marine sediments

It is often argued that the delta(18)O value of oceanic water was maintaine d close to 0 parts per thousand for hundreds of millions of years, as a con sequence of oxygen isotope exchange between oceanic crust and seawater. How ever, for several decades, the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because, with increas ing age, a general trend of decreasing delta(18)O values (about 6 parts per thousand) is observed in most carbonates, cherts and phosphates, especiall y for the Paleozoic and early Mesozoic. We developed a dynamical model of s eawater-crust interaction that computes the delta(18)O value in these two r eservoirs as function of time. This model takes into account the continuous production of crust at oceanic ridges, its expansion rate, the permeabilit y profile with space and time, the mineralogical mode of the crust, and the kinetics of oxygen isotope exchange between rock-forming minerals and seaw ater. The model indicates that the delta(18)O value of seawater may vary by +/-2 parts per thousand with a time response ranging from 5 to 50 Ma for e xpansion rates of 1 to 10 cm.a(-1). The variation of +/-2 parts per thousan d is fixed by both integrated water-rock ratio and closure time of the seaw ater-crust system by sediments. Variations in the oxygen isotope ratio of s eawater through time have important implications for the interpretation of the systematically low delta(18)O values of pre-Jurassic marine sediments. According to our model, marine paleotemperatures could be up to 10 degrees C lower than those expected when applying the classical hypothesis of an ic e-fret: ocean with a delta(18)O value of -1 parts per thousand. Copyright ( C) 1999 Elsevier Science Ltd.