MODEL OF THE OXYGEN ISOTOPE COMPOSITION OF SEAWATER THROUGH THE PHANEROZOIC

Spencer and Hardie (1990) proposed a model to explain the major element composition of seawater through mixing of river waters with mid-ocean ridge brines and the precipitation of calcium carbonate. The fluxes of each ion into and out of seawater are relatively large compared to each ions mass in seawater. Thus major ions have short residence times in seawater, on the order of less than one million years to several tens of millions of years. Concentrations of major ions in seawater are subject to change through geologic time because of potential changes in the relatively large fluxes in their input and removal. Therefore, changes in the major ion chemistry of seawater through the Phanerozoic can be estimated using steady state models. Oxygen isotopes in seawater are altered by the same sources and sinks that affect the major dissolved ions. However, the fluxes of oxygen isotopes in and out of seawater are small in comparison to the mass of oxygen in seawater. Therefore steady state models can not be used to estimate the oxygen isotope composition of seawater through time. Instead, a variable flux model, strongly dependent on the mass of oxygen (water) in the oceans, must be employed. This paper presents a model for the oxygen isotope composition of seawater through the Phanerozoic. The model modifies the first order mixing model proposed by Spencer and Hardie and adds additional inputs and outputs to the model. Additional inputs and outputs include isotopic fractionation into marine sediments and addition of continental-derived ‘oil field' type brines. The final model obtains a close match with published data on the oxygen isotopic composition of seawater through time and leads to a record of Phanerozoic temperature fluctuations.