The strontium isotopic composition of seawater, and seawater-oceanic crust interaction

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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 × 1012 g/yr) against a hydrothermal recirculation flux of 3.6 × 1012 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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