STABLE ISOTOPIC INVESTIGATION OF COASTAL UPWELLING AS AN EXPLANATION FOR HIGH PLIOCENE PRODUCTIVITY ON FLORIDA'S WEST COAST

Studies have suggested that the high level of extinction and contemporaneous high level of origination at the Plio-Pleistocene boundary is the result of a decrease in nutrient supply following the Pliocene which may be the result of a termination in coastal upwelling. This system potentially supplied deep, nutrient-rich waters to these shallow ecosystems. Previous research has utilized δ¹⁸O and δ¹³C profiles to hypothesize regions of upwelling, suggesting that a seasonal depletion in δ¹³C concurrent with enrichment in δ¹⁸O indicates upwelling. Allmon and Jones (1995) have documented this signature in a single Turritella gastropod and Mercenaria bivalve along Florida's southwest coast and used it as the basis to support the upwelling hypothesis. The present study investigates the molluscan isotopic signature, utilizing 17 Mercenaria campechensis specimens from Pliocene, Pleistocene, and modern deposits from both the east and west coast of Florida, and from specimens in conditions both independent of, as well as influenced by, freshwater input. The isotopic profiles of these specimens were compared to see if they were consistent with high Pliocene upwelling and a subsequent decrease and shutdown leading towards the Recent. This study produced results inconsistent with that model, suggesting little to no Pliocene upwelling, with more prevalent upwelling signatures present in Pleistocene specimens. Of the Pliocene specimens that presented unambiguous seasonality, none exhibited a synchronous δ¹⁸O enrichment and δ¹³C depletion. In the Pleistocene, several specimens exhibited multiple potential upwelling signatures, although few specimens indicated upwelling for each annual cycle and in some cases seasonality was not clearly delineated by the isotopic proxies used. These results indicate either a more complex nature of isotopic variation reflecting a broad range of chemical and biologic influences, or that upwelling was not a ubiquitous element in the Pliocene of southern Florida.