THE ROLE OF TRACE-METAL PROXIES AS UPWELLING INDICATORS ON THE FLORIDA PLATFORM DURING THE PLIO-PLEISTOCENE

Plio-Pleistocene units of southern Florida are characterized by dense fossil assemblages representing productive and nutrient-rich nearshore marine environments. The highest nutrient levels are hypothesized to be indicative of the Pliocene. Previous studies, using δ¹⁸O and δ¹³C as oceanographic proxies, have indicated that the distinctive variation in the isotopic signal suggested coastal upwelling off the Florida Platform, thereby increasing the nutrient supply. However, little work has been completed using trace metals to identify the source of the nutrients by examining the seasonal nutrient flux associated with the apparent climate shift during the epochs. This study examines Mercenaria campechensis specimens from eastern and western Florida coastal deposits that represent marine and brackish coastal environments. The bivalves have been collected from Pliocene and Pleistocene deposits and are amplified by modern day control specimens. Sample resolution was 1 sample per ~2mm and the sample powder was analyzed for Fe/Ca and Ba/Ca ratios using ICP-mass spectrometry. Fe/Ca and Ba/Ca have been used as proxies for terrigenous input influence and for productivity, respectively. The co-occurrence of Ba/Ca and Fe/Ca positive excursions suggest that productivity may be enhanced by terrigenous input rather then primarily by upwelling. The Ba/Ca ratios range from 2.2 x 10^-5 to 1.7 x 10^-4 and from 2.1 x 10^-5 to 8.0 x 10^-5 for the Pliocene and Pleistocene specimens, respectively, whereas Fe/Ca ranges from 6.1 x 10^-5 to 1.8 x 10^-3. The modern shells have no consistent pattern in signature or value range most likely reflecting the brackish environment they inhabited. As a whole, the proxy trends of the individual valves from a given temporal unit coincide with one another implying a platform-wide control. Using the ratios as proxies for productivity and terrigenous input, in combination with annual banding patterns in the specimens, suggest that in the Pliocene nutrient input was highest in the winter, while nutrient input was highest during the summer in the Pleistocene. These patterns suggest that winter precipitation may have been higher in the Pliocene potentially reflecting the dominance of El-Nino like conditions during this period which is the opposite pattern of Pleistocene and modern conditions.