Sr-87/Sr-86 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 Sr-87/Sr-86 of mollusks and, consequently, Sr-chronostratigraphic interpretations based upon them. To appraise the potential problem we have used a two-component mixing equation to evaluate levels of ''measurable effects'' (defined as +/- 5 x 10(-5) departure from the marine Sr-87/Sr-86 ratio) in marginal marine environments. A measurable effect occurs at 12 parts per thousand salinity for a weighted world average river, but can occur at salinity > 34 ppt 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 Sr-87/Sr-86 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 Sr-87/Sr-86 signal, emphasizing the need to consider freshwater flux for Sr-87/Sr-86 chronostratigraphy.
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