Reports

Evolution of drowned shelf edge reefs in the Great Barrier Reef

Authors: Webster, J.M., Davies, P.J., Beaman, R.J., Williams, S., Byrne, M.

Year: 2008

Publication: Voyage Summary SS07/2007, CSIRO Marine National Facility, Hobart, Australia, pp.18.

Abstract

Our first voyage objective was to map four study sites along the Queensland margin where the approximate location (Ribbon Reef, Noggin Pass, Viper Reef, and Hydrographers Passage) of submerged reefs were known (Fig. 1). Each planned study area was about 20 square nautical miles. Depths for surveys ranged from shoal areas marked on charts to 200 metres on the upper slope. For each survey box, we aimed for 100% multibeam swath coverage, and then required several lines of sub-bottom profiles (Topas PS-18 and Sparker) through each survey box. Detailed multibeam bathymetric and backscatter surveys using the Simrad EM300 determined the spatial distribution, depth and morphology of the reefs.

These data established if the submerged reefs were regionally significant geomorphic features with consistent depths, and their relationship with shelf width and slope angle and finer-scale geomorphologic details. The Topas PS-18 and Sparker sub-bottom profilers provided information about the acoustic reflection and geometric characteristics of the drowned reefs and associated sediments.

A second voyage objective was to conduct optical groundtruthing of the seabed using high-resolution underwater stereoscopic images, as well as high-resolution multibeam bathymetry, mounted on a state-of-the-art Autonomous Underwater Vehicle (AUV). The AUV surveyed transects across the drowned reefs and inter-reef areas to assess the substrate morphology and character of the modern epibenthic assemblages associated with the shelf edge reefs. The AUV's onboard Seabird CTD also took continuous measurements establishing the present day oceanographic conditions on the shelf edge.

A third voyage objective was to collect dredged rock samples from the tops of the shelf edge reefs. The detailed bathymetric and AUV surveys provided targeted site locations in each study area to obtain rock samples using a standard rock dredge and a Smith-McIntyre sediment grab to recover sediments between the reefs. Rock dredges were towed parallel to contours and along the features in order to collect samples of similar age and composition from the last phase of reef growth. To ensure the fidelity of post-voyage geochemical analyses, CTD and water sampling were collected at each site.

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