WAMSI Node 3.4.1 - Reef Morphology and Growth History – Eastern Ningaloo Reef (Exmouth Gulf) Development and demise of a fringing coral reef during Holocene environmental change, eastern Ningaloo Reef, Western Australia

Brief description

The growth history component (WAMSI Node 3.4.1) was to specifically characterise the morphology and growth history of the Ningaloo Reef system, to provide an insight into and predictive capacity for potential and future climatic changes. Reefs lining the western Exmouth Gulf, located at the northern limit of the 300 km long Ningaloo Reef in Western Australia, represent modern incipient coral reefs and veneers of non reef-building coral/algal communities on exposed Pleistocene or ‘give-up’ Holocene reef surfaces. Acquisition of sixteen cores alongside U-series TIMS dates were used to confirm the nature of the Pleistocene foundation and characterise Holocene reef development. Three calcretised Pleistocene units were identified as: 1) the Last Interglacial (MIS 5e) reef directly underlying Holocene units, 2) a mid-Pleistocene (MIS 7?) bioclastic conglomerate unit, and 3) a Pleistocene alluvial fanglomerate. Eight Holocene reef facies (total thickness of 1.8-5.3 m) included coral framework facies (domal, arborescent, mixed, tabulate and encrusting) and detrital facies (carbonate sand, skeletal rubble and alluvial fan deposits). Holocene ages range from 7.93-5.8 ka BP with vertical accretion ranging from 1.46-9.88 mm/yr (avg. 4.11 mm/yr). Highest rates of accretion and thickest accumulation occurred in the most seaward and deepest cores composed of massive coral framestone and coralline algal crusts. A six stage Holocene chronology is proposed, including: 1) coastal inundation from 8-8.5 ka BP, 2) initiation ‘start-up’ from 8-7.5 ka BP, 3) rapid growth ‘catch-up’ and back-step from 7.5-7 ka BP, 4) rapid aggradational growth ‘catch-up’ from 7-6.5 ka BP, 5) reef decline ‘give-up’ and detrital buildup from 6.5-5.8 ka BP, and 6) detrital buildup and progradation from 5.8 ka BP to present. Changes in reef facies and the ultimate demise of the Holocene reef probably involved a combination of increased sea-level, coastal flooding and erosion during the mid-Holocene highstand, with associated increase in sedimentation, turbidity and decline in water quality; burial by sediment buildup during the mid-Holocene highstand and detrital progradation during the mid- to late-Holocene regression; and, the introduction of alluvial sediment during cyclones and other severe storms to an already stressed community. Modern communities have thus shifted from coral-dominated to bored macroalgal pavements. This study shows that integration of reef development processes with response to environmental change can be used to assess future pressures on coral reef ecosystems globally.

Lineage

Statement: For further information refer to final report Exmouth Marina core site: -21.957, 114.141 Other study areas: Bundegi Reef: -21.86, 114.165 Mowbowra Creek: -22.001, 114.125 Point Maxwell: -22, 458, 114.226 Vertical range from +1.5 metres to 10 metres Excel spreadsheet contains core data and U-series dating information.

Notes

Credit
The authors wish to thank the Western Australian Marine Science Institution (WAMSI) for funding as part of the Ningaloo Reef Program Project 3.4 and a WAMSI PhD top-up scholarship for ET. We are grateful to Cleve Flottmann (WA Department for Planning and Infrastructure) for allowing access to the cores for this study, and Marie Kospartov (coral), Viviane Testa (coralline algae) and Justin Parker (foraminifera) for help with identification. We would also like to thank Ben Fitzpatrick for shared fieldwork and volunteers for all their assistance. Collection of hyperspectral imagery was funded by BHP-Billiton through the Australian Institute of Marine Science (AIMS). Gratitude is expressed for the comments of Lucien Montaggioni, John T. Wells and an anonymous reviewer which have greatly improved the manuscript.