286-10 Pliocene Marine Temperatures and Nutrient Sources on the Florida Platform: Evidence from Molluscan Stable Isotopes and Trace Elements Signatures

See more from this Division: Topical Sessions
See more from this Session: Sclerochronological Archives from Rivers to the Sea: Documentation, Interpretation, and Utility

Wednesday, 8 October 2008: 10:30 AM
George R. Brown Convention Center, 330A

Kai Tao, Dept. of Geology & Geophysics, Texas A&M University, College Station, TX and Ethan L. Grossman, Dept. of Geology & Geophysics, Texas A&M Univ, College Station, TX
Abstract:
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.

See more from this Division: Topical Sessions
See more from this Session: Sclerochronological Archives from Rivers to the Sea: Documentation, Interpretation, and Utility