Results of the long term carbon cycle model (Berner, 1991) have been used to calculate the values of Sr-87/Sr-86 for seawater over Phanerozoic time. Rates of weathering of silicates and carbonates and the rate of sedimentary burial of carbonates over time are derived directly from the model. The Sr-87/Sr-86 ratio of carbonates undergoing weathering and marine burial are fixed by carbonate recycling also according to the model. Exchange of Sr isotopes via basalt-seawater reaction is assumed to vary directly over time with the rate of seafloor spreading, values of which are derived from the sealevel inversion model calculations of Gaffin (1987). Use of a constant Sr-87/Sr-86 for silicates undergoing weathering results in a calculated curve which is in bad agreement with measured Phanerozoic oceanic Sr-87/Sr-86 values for the period of 300 Ma to present, and this disagreement cannot be removed by varying critical parameters of the carbon model such as rates of global degassing or weathering as affected by orogenic uplift. By contrast, use of an expression relating values of the mean isotopic composition of silicates undergoing weathering to changes in eustatic sealevel results in crude first order agreement with the overall trend of the measurements (the silicate Sr isotopic ratio varies from .709 to .716). The expression reflects the reasonable assumption that the production of relatively nonradiogenic, mantle-derived silicates (volcanics, volcaniclastic sediments, et cetera) and their subaerial and submarine weathering is greater when sealevel is higher due to increases in seafloor spreading rate. Because sealevel is also affected by continental breakup and collision, the use of sealevel can also serve as a proxy for the addition of radiogenic Sr accompanying the uplift of old rocks during continental collision. It is likely that trends in the Sr isotopic composition of the oceans over time have been primarily due to changes in the isotopic composition of materials undergoing weathering and not to large changes in weathering fluxes and/or hydrothermal fluxes.
