Uranium-Thorium Ages for Late Holocene Corals from the Southeast Florida Nearshore Ridge Complex

Coral samples were collected from the northern part of the Nearshore Ridge Complex off the coast of Pompano Beach, in northern Broward County, FL (26.219764, -80.085489). Whole coral colonies or coral fragments were collected by hand by SCUBA (self-contained underwater breathing apparatus) divers either haphazardly along 50 x 5 meter (m) belt transects, from 50-centimeter (cm) pits excavated to depths of 30 to 60 cm, or, for more rare target species (Dendrogyra cylindrus), these samples were collected at any location and depth where they were observed in the study area (see Modys and others, 2022; 2023 for more details).

Researchers used a tile saw with a diamond blade to collect small pieces of internal coral aragonite free of visually observable diagenesis for dating. Those samples were soaked in an 8% sodium hypochlorite solution for ~24 hours and then sonicated in deionized water to remove any detrital material. Subsamples of each coral sample were powdered and analyzed for evidence of secondary alteration to calcite by X-ray diffraction (XRD) in the Department of Chemistry and Chemical Biology at Rutgers University (Philips XPert Powder Diffractometer) Only samples with <0.2‰ calcite were used for Uranium-Thorium (U-Th) dating.

U-Th isotopes were measured on the ThermoScientific NEPTUNEPLUS Multi-collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) at the Department of Earth and Planetary Sciences at Rutgers University following the methods described in Mortlock and others (2005), and revised in Abdul and others (2016). These data were used to estimate ages before present (relative to 1950) with 2 standard error (two-sigma [σ]) uncertainties.