HOLOCENE REEF BUILDING ALONG THE SHELF-EDGE OF SW PUERTO RICO

Five cores off SW Puerto Rico record the complex Holocene history of a shelf-edge reef system 15 km from the present shoreline near La Parguera. The reef complex was ca. 150m meters across and geomorphic evidence suggests that it extended along much of the southwestern part of the island. The cores record ~20m of Holocene accretion and reflect Caribbean zonation patterns at slightly deeper paleo-depths than what has been documented for modern reefs. The antecedent Pleistocene shelf margin flooded 12,000 years ago, but the oldest core intervals are dominated by coralgal sediment, suggesting that carbonate production by corals was totally offset by bioerosion. The preservation of coral-dominated framework began atop two shore-parallel Pleistocene ridges near the shelf break at ~10,000 CalBP. Water depth over both ridges was greater than 8m but the reef community was dominated by branching Acropora palmata and accretion rates averaged 3.1 m/ky (max = 7.9 m/ky). By 8,000 CalBP, the reef crest had built to within 4 m of rising sea level when A. palmata largely disappeared. Reef building by A. palmata did not return along the shelf edge despite its documented importance at sites closer to shore. Over the same interval, deeper sites between and in front of the active ridges were dominated by massive species and vertical accretion was slower (avg = 2 m/ky). However, the massive-coal community persisted throughout the 10,000-year history of the reef after A. palmata died off and accretion rates accelerated to 3.5 m/ky by 1,000 CalBP.

Accretionary patterns at this site are enigmatic. Possible triggers of A. palmata decline are not clear (i.e., the reef was catching up with rising sea level at the time of the die-off), but are probably different from those at Florida and other Caribbean sites where synchronous events have been described. Today, branching corals are rare at the site and widely scattered massive and soft corals dominate the reef community. The overall rate of Holocene accretion (3-3.5 m/ky) suggests that the reef was capable of keeping up with sea level rising at a rate similar to what is occurring today. However, the present reef community is probably incapable of keeping pace despite its isolation from obvious sources of terrestrial stresses.