Influence of freshwater flux on 87Sr/86Sr chronostratigraphy in marginal marine environments and dating of vertebrate and invetebrate faunas


Bryant, J., Jones, D., and Mueller, P. (1995) Journal of Paleontology, v. 69, p. 1-6

Abstract


87Sr/86Sr chronostratigraphy is an important tool for dating and correlating vertebrate and invertebrate faunas preserved in marginal marine sequences. Freshwater flux in marginal marine environments can influence the 87Sr/86Sr of mollusks and, consequently, Sr-chronostratigraphic interpretations based on them. To appraise the potential problem we have used a two component mixing equation to evaluate levels of "measurable effects" (defined as +/- 5x10-5 departure from the marine 87Sr/86Sr ratio) in marginal marine environments. A measurable effect occurs at 12 per mil salinity for a weighted world average river, but can occur at salinity >34 per mil for rivers draining basins with ancient granitic rocks. Predictions were tested with analyses of mollusks from estuaries in the Mississippi Sound and coastal Florida. Analyses document the largely regular variation in 87Sr/86Sr predicted, but also show that a simple, two-component model cannot account for all of the variation. Carbonates formed in restricted marine settings may not record a marine 87Sr/86Sr signal, emphasizing the need to consider freshwater flux for 87Sr/86Sr chronostratigraphy.


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