Relict Reefs on the Great Barrier Reef - QLD
In late 2007 a scientific expedition, SS07/2007, set off to survey drowned reefs along the shelf edge of the Great Barrier Reef (GBR) in Queensland, Australia. The drowned reef features may reflect a complex history of growth and erosion during lower sea-levels, and are now capped by reef material from the last deglaciation. The key objectives for the cruise were to improve our understanding of the relationship between the structure, composition and spatial distribution of drowned and modern reefs, to investigate any variations within the succession of drowned shelf edge reefs and to characterise the biological communities associated with these areas.
To carry out these objectives the study used high resolution multibeam swath bathymetry, sub-bottom profiling, AUV-based stereo imaging and rock dredge sampling, providing an unparalleled view of the spatial distribution and morphologic details of the drowned reefs. The primary role of the AUV Sirius was to collect geo-referenced optical imagery to provide validation of interpretations made from sonar multibeam surveys as well as sub-bottom profiling. This will provide crucial baseline data about the modern substrates, habitats and biological communities that characterise these poorly studied shelf environments.
- (a) The ship track taken during the research cruise showing the four survey locations selected (b) early multibeam profiles showing the drowned reefs
The primary field objective of this cruise was to map four study sites along the Queensland margin, shown above, where the approximate location of submerged reefs was known. Detailed multibeam bathymetric and backscatter (sea floor reflectivity) surveys (Simrad EM300) were used to help determine their spatial distribution, depth and morphology. This data will establish if the submerged reefs are regionally significant features with consistent depths, as well as their relationship with shelf width and slope angle and finer-scale bottom features. Sub-bottom profiling using the shipboard Topas PS-18 and a sparker array provided information as to whether shelf edge reefs were built up or the surroundings were eroded away (erotional/constructional) features, and to provide estimates of the thickness and character of sediments between the succession of drowned reefs.
- The AUV Sirius being retrieved after a mission aboard the R/V Southern Surveyor
A second field objective was to validate the interpretations of the ship-based sonar data with targeted, high quality optical imagery of the seafloor. While a towed camera sled is traditionally used for optical characterization, an AUV-based approach offers improved image quality (through improved altitude control), precise positioning and the ability to operate over very rough bottoms. Typically the AUV will operate at slower speeds than a sled so that coverage is reduced, but in the case of optical ground truthing this tends to be compensated by the ability to target specific bottom features.
The AUV was programmed to maintain an altitude of 2 meters above the seabed while traveling at 0.5 m/s (1 knot approx.) during surveys. Missions lasted from two to four hours with the AUV surveying transects across drowned reef and inter-reef areas to collect data that would allow for the assessment of the substrate types and character of the modern epibenthic assemblages associated with shelf edge reefs. In addition to 2 Hz stereo imagery and multibeam data, the AUV's onboard CT and Ecopuck sampled the water column, establishing the present day oceanographic conditions on the shelf edge.
A third field objective was to collect dredged rock samples from the tops of the shelf edge reefs. The detailed bathymetric and optical surveys provide well characterised and geo-referenced target sites in each study area to obtain rock and sediment samples using rock dredges. These dredges were towed parallel to depth contours and along the features in order to collect samples of similar age and composition from the last phase of reef growth. This method has proved effective in sampling similar deposits in the southern GBR, Hawaii and Tahiti . Post-cruise sample analyses involve sedimentary facies, radiometric dating, and palaeoclimate investigations to establish the age and composition of the reefs, the timing and rate of drowning and ecosystem response to rapid sea level rise, environmental stress, and changes in sea surface temperatures and ocean chemistry. The data will be integrated with the geophysical data sets to better understand reef growth and demise.
Over the course of the three week voyage the AUV was deployed at nine locations undertaking both overlapping grid surveys of particular features as well as cross shelf transects to document the variability in benthic habitats at varying depths. The AUV imagery shows a variety of benthic communities and substrates that include red algae-encrusted fossil rock, thriving hard and soft coral, gorgonians (sea whip or fan), sponge communities and Halimeda (green algae).
- Image collected by AUV on the Great Barrier Reef.
