(Table T1) Strontium isotope ratio and ages of ODP Hole 192-1183A and Hole 192-1186A sediments, Ontong Java Plateau

Strontium isotope stratigraphy of the Cretaceous sections recovered in Holes 1183A and 1186A generally matches trends expected from the seawater Sr isotope curve. Combined with biostratigraphic data, the 87Sr/86Sr measurements provide a good chronostratigraphic framework for these holes. In the mid-Cretaceous where the seawater Sr isotopic curve changes direction several times, biostratigraphy removes ambiguity in interpreting 87Sr/86Sr values. Furthermore, 87Sr/86Sr values refine the interpretation of several mid-Cretaceous unconformities suggested by nannofossil biostratigraphy. For the Campanian-Maastrichtian sediments in both holes, ages predicted from Sr isotope stratigraphy are as much as 2.5 m.y. younger than those from nannofossil stratigraphy. This offset likely results from differences in the way that ages are calibrated to nannofossil datums and the seawater Sr isotopic curve. Numerous Sr isotope values are high for their stratigraphic position, suggesting incorporation of excess 87Sr presumably derived from terrigenous clays. Other Sr isotope values suggest minor errors in the estimated ages of nannofossil datums.



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