Does Gribb Bank exist?

320

NOTES

DOES GRIBB BANK EXIST? PATRICK G. QUILTY, R. J. THWAITES, R. M. BURBURY Antarctic Division, Department of Science and Technology Channel Highway, Kingston, Tasmania 7150 Australia H. M. STAGG Bureau of Mineral Resources, Geology and Geophysics GPO Box 378, Canberra, ACT 2601 Australia AND R. W. KRUMMEL Department of Geology, University of Melbourne, Parkville Victoria 3052 Australia Received February 1984

Several hydrographic charts of the southern Indian Ocean show a group of shallow soundings, suggesting a bank or group of seamounts, between 61° and 63° S, 87° and 91° E (Figure 1). It sppears for example on the recently-published soundings sheets for the General Bathymetric Chart of the Oceans (GEBCO) Chart 541 (Figure 2) and British 70°

80° Kerguelen Is.

90 /

KERGUELEN PLATEAU

ANTARCTICA

I

/ FIG. 1. Location map.

Downloaded from http:/www.cambridge.org/core. University of Tasmania Library, on 04 Jan 2017 at 05:20:17, subject to the Cambridge Core terms of use, available at http:/www.cambridge.org/core/terms. http://dx.doi.org/10.1017/S0032247400005465

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Admiralty Chart 3171. The area is commonly called Gribb Bank; the name appears also on Chart 5879 (Carte Polaire Sud) used by French and German hydrographic services, and on the American Geographical Society 1:5000 000 World Map, Sheet 13 (1970 edition). Some modern compilations do not show it (for example World Map: Antarctica, 1975 of the USSR), and the question arises as to whether or not the bank exists. Gribb Bank's earliest appearance as shallows 200-800 m deep was based on six echo-soundings, recorded in 310-510 m by the 1937 Lars Christensen Expedition; the feature was named after the expedition whaleboat, the Gribb. The name first appeared on Norwegian Chart 8 published in 1937, and subsequently on British Admiralty Chart 3171 by Large Correction in 1940, based on the Norwegian chart. Enquiries on our behalf by the Hydrographer, Royal Australian Navy, through the Royal Navy Hydrographer to the Norsk Polarinstitutt indicate that the original data are probably no longer in existence. Three further three readings in 450-810 m, which appear in Figure 2, are from US Navy Field Chart HO 6643 and British Admiralty Chart 3171. Their sources have not been identified. Soviet hydrographic investigations have suggested that Gribb Bank does not exist at the indicated position; a note to this effect was incorporated in 1960 on British charts 3171 and 3172. However, those investigations recorded two additional shallow soundings at about 62° 30' S, 90° 20' E, and it was suggested that this may be the true location of Gribb Bank. Houtz and others (1977) reproduced a bathymetric chart of this area based mainly on data obtained on cruises of the US research ship Eltanin; the relevant part of their chart appears in Figure 3, which features the track of Cruise 47. Although Houtz and others did not refer specifically to Gribb Bank, the form of this track would suggest an attempt to investigate it. Track data have been included in the production of Figures 4 and 5; interestingly, Cruise 47 passed over the top of the only significant seafloor elevation (unnamed) in the area. In 1982 MV Nella Dan, operating for the Australian National Antarctic Research Expeditions (ANARE) surveyed the area of Gribb Bank. The objective was to dredge over the supposed seamounts in an attempt to relate their structure (should they be

61 °S

87°

89°

91° E

FIG. 2. The area as shown on GEBCO sounding sheet No 541; contours in metres.


322

6I°S

89C

9I°E

FIG. 3. Bathymetry following results of Eltanin cruise; from Houtz and others 1977. Eltanin Cruise 47 track is shown as a continuous line. Note that the highest point shown is a little to the east of its position on Figure 5, which is drawn from a larger data base; contours in fathoms.

volcanic in origin) to lies Kerguelen, Heard and McDonald Islands, and Gaussberg (Tingey and others 1983; Sheraton and Cundari 1980). The operation, using equipment and personnel from the Melbourne University Program in Antarctic Science (including Thwaites and Krummel), the Bureau of Mineral Resources, Geology and Geophysics, Canberra (including Stagg) and ANARE (Quilty and Burbury), formed part of a more extensive marine geoscience cruise devoted primarily to a seismic and magnetic survey in the Prydz Bay area (Stagg and others 1983). Nella Dan was equipped with a Raytheon echo-sounder reading to abyssal depths greater than 5 000 metres, which transmits a 'chirp' signal using an array of up to eight transducers mounted inside the ship's hull. The returning echo is correlated and compressed into a normal signal displayed on a graphic recorder; digital depth is recorded on magnetic tape every 10 seconds. Position was determined by the US Navy Transit Satellite system, with satellite fixes received by a Tracor MK II Satellite Navigator. The ship's pressure log and gyro-compass provide automatic dead-reckoning between satellite fixes. The raw data (used to plot the accompanying ship's track) are accurate to within 2 km, the magnitude of a large satellite fix update. Results of this survey were negative. MV Nella Dan's track is seen to pass over six of the supposed seamounts (Figure 4: see also Figure 6), including the two later Soviet shallow readings. The shallowest depth encountered in the area was 3 900 m. Figure 5 has been contoured from soundings on GEBCO Sheet No 541, in conjunction with the


323

NOTES

86

90° ISOLATEO DEPTH READINGS

Dl99

«

91°

92°.

SHIPS TRACKS , With high density dato

SHALLOW READINGS , Used to identify GritJb Sunk (reputed depth). READINGS IGNORED, becouse of a few hundred metres inconslstentcy.

-NELLA

D A N , 1982

- ELTANIN 4 7 DATA APPROXIMATE AREA OF GRIB8 BANK

FIG. 4. Tracks of ships which have given bathymetric data through the area and which form a basis for Figure 5. 61'

86°

87°

88°

89°

90°

91°

92°

FIG. 5. Contoured data from tracks shown in Figure 4; contours in metres.


NOTES

324

62° 10-

62°20'

62°30'

87°30'

88°00'

10'

FIG. 6I Enlargement of the area around 62° 20' S, 88° 00' E, based on 1982 Nella Dan investigations; depths in metres.

Nella Dan data. Certain radically inconsistent GEBCO soundings in the west and in the south of the area and those 'seamounts' which were not traversed by the Nella Dan, have not been included in the contouring. In addition, a line of soundings in the southwest of the area has been deleted as it shows consistent uncertainties of about 300 metres at line intersections. It is not clear what caused the Lars Christensen and later Soviet expedition echo sounders to record the features plotted as seamounts. The depths (260-810 m) are too great to be phytoplankton concentrations of a kind reported elsewhere in these waters (Quilty and others, in press); more likely possibilities include myctophids (lantern fish), and the depths coincide well with other reports of the deep scattering layer (Hall 1981; Thurmann 1978), though diurnal migrations the layer have not as yet been documented


NOTES

325

in this area. Future ANARE marine biology/oceanography cruises will where possible pass through the region regularly, and if similar features are recorded, trawl through them to identify the causes. We thank our fellow workers for discussion on board ship, Dr M. Hall (RAN Research Laboratories) and Mr. R. Williams (Antarctic Division) for advice on the deep scattering layer and Dr K Kerry (Antarctic Division) for reviewing the typescript. H. M. J. Stagg's contribution is published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics. The text was typed by J. Hinden and L. Mansen and the figures are by J. Cox. References HALL, M. 1981. Measurements of acoustic volume backscattering in the Indian and Southern Oceans. Australian Journal of Marine and Freshwater Research 32: 855—976. HOUTZ, R. E. and others. 1977. Kerguelen Plateau bathymetry, sediment distribution and crustal structure. Marine Geology 25: 95-130. QUILTY, P. G. AND OTHERS (in press). A seasonally recurrent patch of Antarctic planktonic diatoms. Science. SHERATON, J. W. and CUNDARI, A. 1980. Leucitites from Gaussberg, Antarctica. Contributions to Mineralogy and Petrology 71: 417-427. STAGG, H. M. J. and others. 1983. Preliminary report of a marine geophysical survey between Davis and Mawson Stations, 1982. Antarctic Earth Science, Australian Academy of Science, 527-532. THURMANN, H. V. 1978. Introductory Oceanography. Columbus, Merrill. TINGEY, R. J. and others. 1983. The age and mode of formation of Gaussberg, Antarctica. Journal of the Geological Society of Australia 30: 241-246.

BRAZILIAN ANTARCTIC RESEARCH PROGRAMME JEFFERSON SIMOES Scott Polar Research Institute, Lensfield Road Cambridge CB2 1ER (Received June 1984)

The second Brazilian Antarctic research expedition took place from early January to early March 1984 in the South Shetland Islands. Two ships were used, departing from the recently-established Antarctic support station in Rio Grande, Brazil's southernmost harbour. A research station, Comandante Ferraz was established on Keller Peninsula, at the head of Admiralty Bay, King George Island, opening on 6 February. Research programmes concentrated on marine biology (especially krill), geology and geophysics. The Brazilian naval vessel Barao de Tefe (formerly Thala Dan), commanded by Captain Paulo Cesar Adriao, established the summer support station of prefabricated fibreglass modules with total area 240 m2, costing over US$240,000. A scientific group headed by Dr Monica Montu of the Marine Biology Centre, Federal University of Parana (FUP), concentrated on studies of krill, primary production of phytoplankton, pelagic polychaets, larval fish and fish parasites. In addition, fourteen marine projects were carried out during two twelve-day cruises of the Professor Wladimir Besnard (Captain Waldir da Costa Freitas), research vessel of the University of Sao Paulo (USP).
