Plio-Pleistocene Stratigraphy, Paleoenvironments, and Isotope-Derived Water Temperatures at Beeline Quarry, Orange County, Florida, U.S.A.

Plio-Pleistocene marine sediments from Florida record glacial-interglacial sea level cycles and the evolution of shallow shelf ecosystems of the Florida Platform. These deposits also provide opportunities to reconstruct coastal ocean temperatures during past interglacial highstands and evaluate long term changes in West Atlantic oceanography. Here, we use carbonate clumped (Δ 47 ) and bulk oxygen (δ 18 O carb ) isotope paleothermometry to test whether shallow marine temperatures during two Pleistocene highstands on the Florida Platform differed from those on the modern Atlantic coast of Florida. We first characterize the lithostratigraphy and biostratigraphy of a newly exposed section at Beeline Quarry in Orange County, east-central Florida that contains fossiliferous marine deposits of the upper Nashua Formation (Early Pleistocene) and a younger unit that is temporally equivalent to the lower Fort Thompson Formation (late Middle Pleistocene). Amino acid racemization data from the Fort Thompson-equivalent unit indicate deposition prior to MIS 5e (~121 ka). Δ 47 -derived carbonate formation temperatures and water oxygen-isotope (δ 18 O water ) reconstructions are then presented from four fossil bivalve genera ( Mulinia, Mercenaria, Dinocardium , and Chione ). Reconstructed δ 18 O water values are consistent with marginal-marine deposition and seasonally-targeted Δ 47 measurements in Chione exhibit subannual δ 18 O water variations (>2‰) consistent with freshwater influence. Unexpectedly, Δ 47 temperatures from Mercenaria , Dinocardium , and Chione in the Fort Thompson are 4–5 °C cooler than modern coastal waters. Although Mulinia appears to exhibit a genus-specific cold bias, it indicates that temperature conditions were similar (cooler-than-modern) during both the Nashua and Fort Thompson highstand intervals. Rather than reflecting regionally-cooler marine climates, we propose that these cool temperature signals most likely record the influence of localized hydrologic processes, potentially including submarine groundwater discharge or seasonal upwelling along the ancient East Florida Shelf.