Submerged Coral Reefs and Benthic Habitats of the Southern Gulf of Carpentaria: Post-Survey Report - GA Survey 276, RV Southern Surveyor

In 2003, Geoscience Australia discovered three large patch reefs in the southern Gulf of Carpentaria (GA Survey 238; SS-03/2004; Harris et al., 2004).  The submerged platform reefs (R1, R2 and R3) are located east of Mornington Island and appear to have been formed when sea level was ~30 m below its present position, however as the ship did not come prepared with a drill-core sampler, the sub-surface composition of the reefs was not determined. The submerged platforms support live hard corals in many locations and their discovery raised the question of the possibility of widespread reef occurrence in that region.  Survey 276 was designed to deliver some answers to these questions. 
The current survey used rotary drilling of reefs R1, R2 and R3 which recovered coral material from 8 sites and confirmed the coral reef composition of these features.  Multibeam sonar bathymetry and rotary drill cores were collected over two sections (R4 and R5) of a large (&gt;100 km long) submerged platform that extends westwards from Mornington Island.  The platform exhibits a Karst erosion surface, exhibiting drainage and depressions with raised rims, overprinting relict reef-growth geomorphic features.  Reef growth features include raised rims, spur and groove reef front and elevated back-reef mounds. Other platform reefs were mapped in the south-western Gulf (R6 and R7) and in the Arafura Sea (R8).  Rotary drilling has confirmed the coral reef composition of these features.
Preliminary assessments of the recovered drill cores indicate that reef growth has persisted in the region for several glacial cycles, extending over at least the past 120,000 years.  Dating of Holocene corals by the U/Th method demonstrates that a phase of rapid (1-2 m per kyr) reef growth occurred at most sites between 9 and 7 kyr before present, with zero or much reduced growth rates occurring after 7 kyr ago.  Although coral growth occurs in many areas, the production of carbonate has not been sufficient to build the reef-tops upwards to the present sea level.  The observations of live corals, but low carbonate production rates, are consistent with a 'catch-up' reef growth pattern, in which the upper surfaces of the reefs are submerged 20 to 30 m below present sea level, with isolated local reef-tops having reached to within 18 m of the sea surface.  An analysis of the hypsometry of the reef surfaces indicates that platform surfaces at all sites (R1 to R8) are confined to two narrow depth intervals, centred at 26.8 Â± 1 m and 30.7 Â± 0.3 m.  The good correspondence of hypsometric peaks indicates regionally significant phases of carbonate deposition during a prolonged, Pleistocene sea level still stand.
This voyage has proved that the southern Gulf of Carpentaria contains a previously unknown major coral reef province in Australia.  The reefs support locally diverse and luxuriant coral growth.  From a management perspective, the slow rates of coral growth point to the need for protection of these reef systems because of their limited capacity to recover from natural or human-induced disturbances.

Tags: AU-NT, AU-QLD, Earth Sciences, GA Publication, Published_External, Record, continental shelf, environmental, marine, seabed, sedimentology
Organization: Geoscience Australia Data