Abstract
The 87Sr/ 86Sr ratio of seawater strontium (0.7091) is less than the 87Sr/ 86Sr ratio of dissolved strontium delivered to the oceans by continental run-off (∼0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current 87Sr/ 86Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10 12 g/yr) against a hydrothermal recirculation flux of 3.6 × 10 12 g/yr, during which the 87Sr/ 86Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the 87Sr/ 86Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required 87Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus. The post-Upper Cretaceous increase in the strontium isotopic composition of seawater (∼0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the 87Sr/ 86Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.
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