Discovery of an early Holocene relict reef and shoreline off Grand Cayman

• Main Author: P. Blanchon
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Holocene relict reef; Grand Cayman; Breakwater; Ice Sheet
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.606.7684; http://www.coremap.or.id/downloads/icrs9th-blanch.pdf

Ten short cores drilled on the eastern shelf off Grand Cayman have revealed the presence of a relict, early Holocene, breakwater reef at a depth of 21 m below msl. Cores from the crest of the relict reef consist not of in-situ coral framework but of cobbles of Acropora palmata in a matrix of skeletal sand – a facies that is identical to the modern reef-crest deposit. The surface of the relict reef slopes seaward from –21 to –24 m and, in some areas, is overlain by up to 1m of mixed-coral framework containing severely bioeroded stumps of A palmata and other corals. In other areas, this veneer is absent and marine abrasion has produced furrows that cut down into the relict reef surface. Dating the reef surface gives U-Th TIMS ages that range from 9878 ± 97 to 8122 ± 101 Cal.years, indicating that reef growth has stopped by ~8.1 Cal.ka. The depth of the relict breakwater reef around Grand Cayman is close or identical to the depth of relict reefs reported from other Caribbean island. Radiocarbon dating of those reefs indicates that they ceased accreting in a narrow window between 7-814C ka and had re-initiated in new positions 5-10 m higher up slope by 6-714 C ka. When the dates of the reef demise on these islands are calibrated for secular variation in atmospheric 14C production, the demise of relict reefs across the Caribbean clusters around 8 Cal.ka – closely matching the age of the reef demise on Grand Cayman. The cause of this Caribbean-wide reef demise is uncertain., but has been previously attributed to a rapid sea-level jump caused by ice-sheet collapse (CRE-3)