Authors: Abbey, E., Webster, J.M., Beaman, R.J.
Publication: 2010 American Geophysical Union (AGU) Fall Meeting, 13-17 December 2010. AGU, San Francisco, U.S.A.
Australia has the largest extant barrier reef system in the world, the Great Barrier Reef. As sensitive indicators of their environment, tropical coral reefs are also valuable repositories of climate and sea-level histories. As sea-levels oscillate, reefs wax and wane along shallow shelf margins. During rapid transgressions, many reefs are unable to keep up and become drowned. Submerged, or drowned, reefs can provide a wealth of information as to the nature and timing of local oceanic conditions, and are well-recognised for their value as relative sea-level indicators. The Great Barrier Reef may hold the largest repository of Pleistocene and Holocene climate and sea-level records in the world, in the form of submerged reefs.
The current understanding of submerged reefs along the Great Barrier Reef shelf edge is based on widely-spaced singlebeam echosounder profiles and several small-scale (3-8 km2) multibeam surveys. In spite of these earlier studies that hinted at the wide distribution of submerged reefs, no regional-scale work has been undertaken using high-resolution multibeam swath bathymetry. Here we investigate four widely-spaced sites (200 km) from depths of 45-130 m ranging in size up to 400 km2 along Australia’s north-east margin using high-resolution multibeam swath bathymetry and surficial dredge samples. The aims of this study include characterising the morphology, distribution and variety of features, as well as identifying the processes associated with their origin, to gain a better understanding of the history of the Great Barrier Reef and its response to Quaternary sea-level changes.
The high-resolution (cell pixel size 5 m) dataset presented here has allowed an unprecedented view of the seabed topography. This highly detailed imagery reveals very subtle characteristics of features that can indicate their environmental setting: for example, submarine or subaerial, and constructional or erosional. Comprehensive mapping of each site has resulted in the identification and classification of a wide variety of submerged features. These range from nearshore discontinuous fringing reefs to high relief, laterally extensive double barrier reefs. Radiocarbon AMS confirms that these features are >10 ka and are likely multigenerational, representing multiple phases of reef building and subsequent erosion during past sea-level oscillations. These results give strong evidence that a vast and complex, once thriving reef system lies to seaward of the modern reefs, submerged at 45-130 m water depth. As many of these reef features are now confirmed to have developed during the most recent glacial-interglacial transition, a study of this nature will further our understanding of the effects of sea-level rise on modern reefs. Recent drilling by the IODP (Expedition 325) will reveal the internal structure and development of these reefs and their undoubtedly complex growth history.