Severe storms drive holocene island evolution and coral reef biodiversity of the Houtman Abrolhos Archipelago, WA

The unique setting of Houtman Abrolhos island allows for a precise storm chronology and influence on coral communities of Western Australia. All islands display coral rubble ridges caused by extreme disturbances which are one of the most important mechanisms affecting diversity of species. A clarification of this complexity can only be solved by a combination of geomorphology, biology and chronology. The archipelago consists of three main island groups, Wallabi (North), Easter (Centre) and Pelsaert (South) spanning 172 km at 290S. They are situated 2-6 km from the shelf edge (50 m water depth) and consist of a Pleistocene coral rubble pavement. Modern reefs laterally enlarged the Pleistocene pavement. Two destructives wave regimes influence reef and island development: Swell and storm waves from tropical cyclones and strong swell from the southern/ south-eastern “roaring forties” with recurrent southern storm events. Four islands were investigated (Long (North), Serventy (Centre), Post Office, and Pelsaert (South) island), representing a Southward tropical and Northward Antarctic storm gradient. Transects were chosen with oblique aerial photography and satellite images. More than 100 ridges were encountered and mapped. In total 152 coral samples were collected for age dating (14C, U/Th, ESR) and 8 ridges from Long Island (tropical cyclone influence) were sampled (n=8000) for coral species identification to reconstruct coral communities at time of destruction. The orientation of ridges within complexes was parallel due to identical swell direction over longer time. Geomorphology of ridge complexes allowed relative age identification between complexes. Inter ridge-complex relative ages could be identified by the sequence of ridges, state of erosion and weathering. Since sea-level dropped the last 6800 yrs, storm ridge crest height above sea-level in general gradually decreased seawards. Ridges within a complex were variable in absolute height, width, volume, and species composition. Storm ridge dimensions were determined by storm wave (swell) energy, availability of offshore reef material (time in between storms/ regeneration time of coral communities), and availability of onshore reef material (existing ridges mixed with subsequent storm material). Preliminary conclusions are that storm waves were largely constructive to these islands. The produced coral rubble ridges added land to existing structures or formed new islands; however storm waves also partially eroded some existing Pleistocene pavement. The variability in ridge dimension depicts different storm frequencies and intensities over the Holocene. Coral species analyses showed different community composition between ridges either representing different stages in reef succession or a change in dominating species due to change in storm frequency and intensity.