Submerged Late Pleistocene reefs on the tectonically-stable S.E. Florida margin: high-precision geochronology, stratigraphy, resolution of Substage 5a sea-level elevation, and orbital forcing.
A well-preserved, submerged, in situ fossil reef tract lies offshore of the southeast Florida carbonate margin in the Florida Keys. Cores in two transects enabled reconstruction of reef stratigraphy and paleoenvironments, paleo water depths, and paleo sea-levels. Because Florida is tectonically stable, the elevation data required correction only for the known, minor subsidence rate. Twenty eight pristine aragonitic corals were dated via high-precision TIMS U-series methods to produce a comprehensive late Pleistocene and early Holocene geochronology. The Pleistocene section of the fossil reef includes a thick section of Substage 5a reef growth recorded from ≈-15 to -11 m MSL, spanning an age range of 86.2-80.9 ka. Paleoenvironmental reconstruction, local water quality constraints, and elevations of concurrent Bahamian speleothem growth combine to limit sea-level to ≈-9.0 m MSL at ≈83 ka. This coincides, unlagged, with the 82 ka 65°N July insolation peak: the absence of any lag suggests that the earlier peak in obliquity at 92 ka also controls paleoclimate and sea-level. A below-present sea-level estimate for Substage 5a from the Florida fossil reef data coincides more closely with uplift-corrected reef data estimates of -15 to -13 m MSL from tectonic margins such as Barbados and Haiti, than with higher than present sea levels derived from detrital corals from Bermuda and the U.S. Atlantic Coastal Plain. The best TIMS dates on corals and speleothem constraining sea level combine to produce a new regional sea-level curve for the Atlantic-Caribbean region for late isotope stage 5 and early stage 4.