INFLUENCE OF FRESH-WATER FLUX ON SR-87 SR-86 CHRONOSTRATIGRAPHY IN MARGINAL MARINE ENVIRONMENTS AND DATING OF VERTEBRATE AND INVERTEBRATE FAUNAS

Sr-87/Sr-86 chronostratigraphy is an important tool for dating and cor relating vertebrate and invertebrate faunas preserved in marginal mari ne sequences. Freshwater flux in marginal marine environments can infl uence the Sr-87/Sr-86 of mollusks and, consequently, Sr-chronostratigr aphic interpretations based upon them. To appraise the potential probl em we have used a two-component mixing equation to evaluate levels of ''measurable effects'' (defined as +/- 5 x 10(-5) departure from the m arine 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 analy ses of mollusks from estuaries in the Mississippi Sound and coastal Fl orida. Analyses document the largely regular variation in Sr-87/Sr-86 predicted, but also show that a simple two-component model cannot acco unt for all of the variation. Carbonates formed in restricted marine s ettings may not record a marine Sr-87/Sr-86 signal, emphasizing the ne ed to consider freshwater flux for Sr-87/Sr-86 chronostratigraphy.