Voyage Plans and Summaries

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Franklin Voyage Summary No. FR01/2002

Title

Cross-shelf sediment transport in the Torres Strait - Gulf of Papua Region.

Itinerary

Departed Brisbane 1000 hrs, Thursday 17 January 2002
Arrived Cairns 1100 hrs, Saturday, 9 February 2002

Principal Investigator(s)

Dr Peter Harris (Chief Scientist)
Principal Research Scientist
Geoscience Australia
University of Tasmania
PO Box 252-80
Hobart, TAS 7001
Tel: (03) 6226-2504; Fax: (03) 6226-2973 e-mail: P.Harris@utas.edu.au

Dr Michael Hughes
University of Sydney

Scientific Objectives

The objectives of this cruise were to test the hypothesis that a zone of high tidal current energy across the northern margin of the Great Barrier Reef was a cross-shelf corridor of sediment movement. The methods employed involved conducting high resolution seabed mapping at strategic locations in conjunction with seabed sampling and photography to identify mobile bed sediments and depositional end-members of the transport system.

The key goals were to: collect sufficient data to verify whether such cross-shelf transport is occurring; document benthic biological communities associated with different substrate types; and collect sediment cores to extract a time series of cross-shelf terrigenous flux.

The research is of relevance to environmental managers who require knowledge of the processes governing sediment dispersal and the distribution of biota in this environmentally sensitive and economically important geographic location.

Cruise Objectives

The methods employed were swath mapping, shallow high-resolution seismic surveying, suspended sediment sampling, current metering, coring and grab sampling. Three areas were to be mapped using a high-resolution swath mapper and Chirp shallow seismic system. It was also planned to take shallow gravity cores and surface sediment grab samples, deploy the Franklin’s CTD-turbidity meter, collect water samples, and deploy and underwater camera. One current meter instrument frame was deployed to measure sediment flux in Area "A".

Area "A" is located adjacent to the seaward margin of the Fly River Delta. It is a fairly flat seabed, but previous sonar surveys have indicated the presence of subtidal bedforms in the area. Hence it is an area of active bedload transport. Area "B" is located amongst the northernmost reefs of the Great Barrier, where a 100m deep submarine valley crosses the shelf. This valley is the low sea level river channel of the Fly River and it is of interest to determine whether it remains an active sediment transport conduit at the present time. Area "C" is located at the seaward terminus of the palaeo Fly River channel, and should be the locus of low sea level sediment deposition. Sonar data show this area is characterised by low sea level (drowned) reefs, Halimeda biostromes and possible gas escape features (pockmarks) on the seabed.

Cruise Track

Figure 1. Location of the three study areas in the northern Great Barrier Reef region.

Results

Our instrument mooring BRUCE was successfully recovered and, after inspection, was found to have logged all data as per the program. Further results will be available once the data have been more thoroughly analyzed.

Swath mapping and Chirp sub-bottom profile data were collected along evenly spaced track lines as shown in Figures 2, 4 and 6. A total of 75 stations were occupied spread more or less evenly between the three survey areas. Station locations are listed in Appendix 1 and shown in relation to bathymetric maps in Figures 3, 5 and 7. In summary, the following samples were recovered:

None of the sample data have been analysed to date but results will be published in a Geoscience Australia record.

Cruise Narrative

The RV Franklin sailed from Cairnscross Shipyard, Brisbane, @ 1000 hrs local time. Pilot dropped at Caloundra @ 1500 hrs and ship headed north with a brisk following (southerly) breeze.

Summary

The RV Franklin sailed from Cairnscross Shipyard, Brisbane on 17-1-02 and returned to Cairns on 9-2-02. All equipment functioned properly, apart from the main winch that was barely functional at times and which caused considerable anxiety for the crew, who were understandably concerned for its operational safety. It is clear that a major overhaul of the winch and its hydraulics system are urgently needed before it is next sent to sea. On all other fronts, however, the weather, working conditions, personnel morale etc. there were no difficulties whatsoever and it was overall a very productive and positive experience for all.

From a scientific perspective, the cruise was a tremendous success. All our equipment worked smoothly and the instrument mooring was recovered safely and found to have functioned properly. All sampling stations and survey targets were met (and exceeded). The cruise discovered that a zone of strong tidal currents at the northern end of the Great Barrier Reef prevents the southward advance of sediment derived from Papua New Guinea's rivers that would otherwise bury the coral reefs. The Fly River, located in close proximity to the northern end of the Great Barrier Reef, discharges about 120 million tonnes/yr of sediment, equal to more than that all of Australia's rivers combined. This sediment does not penetrate as far south into the reef area as might be expected, because southward prograding deposits are eroded by tidal currents.

Swath sonar mapping and underwater video equipment was used to map a series of channels up to 220 m deep that extend for more than 80km from eastern Torres Strait across the northern end of the Great Barrier Reef. It appears that there are two sorts of channels - those in the north are clearly relict fluvial channels, exhibiting lateral accretion surfaces and incised channels that intersect and truncate underlying strata. Over-deepened channels in the south, however, appear to have formed by tidal current scour. They exhibit closed bathymetric contours at both ends and are floored with well sorted carbonate gravely sand. Oceanographic observations indicate that the channels provide a conduit onto the shelf for cool and saline (and nutrient-rich?) upwelled Coral Sea water. The deepest channels form isolated depressions, and possibly were the sites of lakes during the last ice age, when Torres Strait formed a land-bridge between Australia and Papua New Guinea.

Scientific Personnel

Peter Harris¹ , Geoscience Australia, Cruise Leader
Michael Hughes⁴ , Sydney University, Co-Cruise Leader
Andrew Heap² , Geoscience Australia, Sedimentologist
Vicki Passlow³ , Geoscience Australia, Sedimentologist
Rick Smith¹ , Geoscience Australia, Computing & GIS
Jon Stratton³ , Geoscience Australia, Science Technician
Lyndon O'Grady³ , Geoscience Australia, Science Technician
Kevin Hooper⁵ , James Cook University, Geophysical Technician
Rob Beaman⁶ , University of Tasmania, PhD Student
Mark Hemer⁷ , University of Tasmania, PhD Student
Eric Madsen⁸ , CSIRO DMR, Science Technician
Bernie Heaney ³ , CSIRO DMR, Science Technician

1. Geoscience Australia and Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart TAS 7001

2. Geoscience Australia, c/o School of Geography and Environmental Studies, University of Tasmania, GPO Box 252-80, Hobart TAS 7001

3. Geoscience Australia, Petroleum and Marine Division, GPO Box 378, Canberra, ACT 2601

4. School of Geosciences, University of Sydney, Sydney NSW 2006, Australia

5. School of Earth Sciences, James Cook University of North Queensland, Townsville Qld.

6. School of Geography and Environmental Studies, University of Tasmania, GPO Box 252-80, Hobart TAS 7001

7. Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart TAS 7001

8. CSIRO Division of Marine Research, GPO Box 1538, Hobart TAS 7001

Ships’ Crew

Ian Taylor, Master
Arthur Staron, Chief Officer
John Boyes, Second Officer
Gordon Gore, Chief Engineer
Greg Pearce, First Engineer
Paul Dickson, Second Engineer
Malcolm McDougall, Bosun
Louis Jacomos , IR
Graham McDougall, IR
Tony Hearne, IR
Phillip French, Greaser
Shaun McQuaid, Chief Steward
Peter Williams , Chief Cook
Aung Zin, Second Cook

Acknowledgements

Thanks to the National Facility Steering Committee for making the ship time available and to Geoscience Australia for logistical and financial support. Thanks also to captain Ian Taylor, the ships crew and to the CSIRO support personnel for making the voyage a pleasant experience and a successful scientific operation. This cruise is also listed as Geoscience Australia cruise 234.

Peter T. Harris
Chief Scientist.

Figures

Figure 2. Survey track lines completed in Area "A", adjacent to the Fly River Delta.

Figure 3. Location of stations occupied in Area "A" in relation to regional bathymetry.

Figure 4. Survey track lines completed in Area "B", on the middle part of the Torres Strait shelf, north of Darnley Island.

Figure 5. Location of stations occupied in Area "B" in relation to regional bathymetry.

Figure 6. Survey track lines completed in Area "C", outer shelf of the Gulf of Papua, adjacent to East Cay.

Figure 7. Location of stations occupied in Area "C" in relation to regional bathymetry.

Updated: 31/01/03