This study documents the pattern and rate of reef growth during the late Holocene as revealed by unique geological conditions at the subsiding NW Gulf of Aqaba. We discovered that the modern fringing reef near the city Elat grows on top of a fossil submerged mid-Holocene reef platform. Four coral cores from the fossil platform were dated using the radiocarbon and U-Th methods. The fossil corals range from 5.6 ± 0.1 to 2.4 ± 0.03 ka, constraining the initiation of the modern reef to 2,400 years ago at most. We documented the detailed morphology of the reef using aerial photographs and scuba diving. The survey shows that at its northern end, growth of the 2-km-long reef is inhibited by an active alluvial fan, and it is composed of isolated knolls that are just approaching the sea surface. Towards the south, the knolls are progressively larger and closer together, until they form a continuous reef platform. Along this north-to-south trend we follow the evolution of reef morphology, changes in coral distribution, and the development of a lagoon separated from the open sea. Based on these observations, we suggest a four-stage reef growth model: (a) the reef initiates as coral colonies, forms knolls, and begins to grow upward, limited by the sea surface. (b) Upon reaching the surface, the knolls spread laterally, preferentially parallel to the dominant wave direction assuming an elongated morphology. (c) Continued growth results in adjacent knolls eventually coalescing to form a continuous jagged reef. We interpret the spurs-and-grooves morphology that can be traced across the reef at Elat as remnants of the original trends of knolls. (d) While reef expansion continues, the original knoll trends may be obscured as a massive reef front takes shape. Considering reef growth rates and observations from the modern reef at Elat, this evolution scheme predicts an age range of 103 years for corals on the reef platform. The range and distribution of radiometric ages we obtained from the fossil reef platform underlying the living Elat reef confirm this hypothesis.
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Acknowledgements We wish to acknowledge the help and support provided by the Interuniversity Institute at Elat. We also thank D. Zakai, and the Israel Nature and Parks Authority for supporting the work at the Elat Nature Reserve beach. E. Boaretto of the Weizmann Institute of Science undertook radiocarbon dating. This research was funded by the Israel Ministry of National Infrastructures, and the Shapira Fund, Hebrew University. Careful reviews by C. Braithwaite and C. Perry greatly improved this manuscript.
- Fringing reefs
- Reef structure