HOLOCENE EXTINCTION PATTERNS ON THE SOUTHEAST FLORIDA OUTER REEF TRACT

The outer reef tract offshore southeast Florida is a relict shelf-edge reef complex composed primarily of Holocene Acropora palmata-dominated facies. Although Acropora-dominated reefs existed along the entire Florida Reef Tract throughout the late to mid Holocene (~11,000 – 6,000 ybp), deteriorating oceanic conditions led to latitudinal range contraction in the northern region offshore southeast Florida. Currently, active reef accretion terminates just north of the Florida Keys off Miami, FL (25°35’N). While a general southward contraction is evident, the exact timing and patterns associated with localized acroporid reef extinction are less clear. In order to better characterize this contraction, the outer reef tract near its northern extent off Boynton Beach, FL (26°31’N) was dated and compared to the previously dated outer reef tract 30 km south offshore Broward County, FL (26°15’N). Five calibrated AMS radiocarbon ages of well-preserved fossilized A. palmata samples collected at surface exposures of reef framework were determined. Sample preservation was evaluated using a combination of X-ray diffraction and light microscopy techniques, and only pristine coral aragonite with <1% detectable calcite was dated. The calibrated 2–sigma radiocarbon ages of surficial A. palmata reef framework at this site ranged from 9430 to 8522 cal yr BP at depths between 14 and 23m. These ages differ from the outer reef offshore Broward County FL, which was dated between 8770 and 8160 cal yr BP at depths between 23 and 16.5m. The difference in reef surface age at both locations suggests a distinct southward trending extinction pattern in the early Holocene fossil reef record off southeast Florida. Based on these dates, reef growth cessation appears to have initiated earlier (~9000 cal yr BP) near the northern extent of the reef tract. This may be due to a combination of factors characteristic of mid latitude coral reef environments, such as increased thermal stress associated with periods of cold water upwelling and rising sea level; however, more dating is necessary to identify the exact cause of localized acroporid reef extinction at the northern extent of the outer reef tract.