Pliocene Marine Temperatures and Nutrient Sources on the Florida Platform: Evidence from Molluscan Stable Isotopes and Trace Elements Signatures

Stable isotopic signatures provide valuable information for understanding global climate change and nutrient sources. The mid-Pliocene Pinecrest beds of the Florida Platform show evidence of enhanced productivity, but the nutrient source, freshwater runoff versus upwelling, is still debated. We examine this discrepancy through high-resolution stable-isotopic and trace-elemental analyses on ten mollusk shells collected from Units 4 and 7 in the mid-Pliocene Pinecrest Beds in Sarasota and Highlands Counties. Unit 7 (3.5-2.5 Ma) contains great diversity of molluscan species, indicating tropical-subtropical offshore environment; Unit 4 (2.5-2.0 Ma) has great abundance of both marine and freshwater shells, indicating brackish water environment. Unit 4 samples average 2.3‰ higher in δ¹⁸O and 3.0‰ lower in δ13C than those of Unit 7. Using a latitude-corrected seawater δ¹⁸O of 0‰ (SMOW), the reconstructed Units 4 and 7 SSTs are 13°C and 3°C lower than modern SSTs, respectively. These low temperatures could imply cooler temperatures, or reflect higher-than-expected seawater δ¹⁸O. Sr/Ca measurements in these shells suggest no significant paleotemperature difference between Units 4 and 7 specimens, indicating that the δ¹⁸O differences reflect seawater δ¹⁸O differences. If Unit 4 was deposited in brackish water, the freshwater δ¹⁸O must have been higher than that of seawater. Such δ¹⁸O enrichment in freshwater has been reported for the northern margin of Florida Bay due to excess evaporation, accompanied with a δ¹³C depletion due to oxidation of terrestrial organic debris. The δ¹⁸O and δ¹³C for Units 4 and 7 mollusks fit exactly with the reported data for nearshore-lagoonal and open-sea environments in the Florida Bay (Halley and Roulier,1999, Estuaries, 22:358-368). Meanwhile, the positive covariance in δ¹⁸O and δ¹³C profiles of Unit 7 samples show little evidence for upwelling, but suggest salinity-induced changes. Therefore the high productivity in this region is more likely to have been caused by nutrient input from freshwater runoff.