Leeuwin Sill

LETTERS

TO

THE

EDITORS

L e e n w i n Sill AImtmct--The evidence for the Leeuwin Sill, a name proposed by C~atmGY and FxmBnn~E (1954, p. 83) and FAmBRIVGE(1955, p. 162) is reviewed. Soundings from this area reported to the International Hydrographic Bureau give no evidence for a sill with a depth of under 2,000 fins extending southwards from Cape Leeuwin for some 200 miles. IN an article " S o m e bathymetric and geoteetonic features of the eastern part of the Indian O c e a n " by FAIRBRrDGE 0955) under the heading " N e w Features Designated" is included Leeuwin Sill. The latter, is described as '" a low ridge of under 2,000 fathoms depth, connecting the south-western point of Australia, Cape Leeuwin, with the South-West Australian Ridge (of VENING MEFNESZ, 1948) " and it is stated that " i t trends roughly North and South and is about 200 miles long." An editorial note inserted at the end of this article states that the British National Committee on the Nomenclature of Ocean Bottom Features which met on 26th November, 1954 " c a m e to the conclusion that the names Leeuwin Sill . . . . . would require further consideration." At a meeting of the International Committee on the Nomenclature of Ocean Bottom Features which was held at the headquarters of the International Hydrographic Bureau (hereafter referred to as the I.H.B.) in Monaco on 9-10 September, 1954, during which meeting the delegates had carefully examined the plotting sheets of oceanic soundings used by the I.H.B. for the preparation of the Third Edition of the General Bathymetric Chart of the Oceans, the Chairman stated that he was of the opinion that the sill (Leeuwin Sill/Rise) was a misnomer as there was considerable doubt of its existance. It was agreed that the I.H.B..should look into the matter. This question has since been given very careful consideration by the I.H.B. and the following remarks are offered : - (a) It should be noted that the I.H.B. has since 1929 been entrusted with the collation of information concerning all soundings taken outside the continental shelves, and with the keeping up to date of the General Bathymetric Chart of the Oceans of which the "Cabinet Sei6ntifique du Prince Albert de M o n a c o " had drawn up the first and second editions. (b) A new edition of Sheet A'li I of the General Bathymetric Chart, which covers the area in question, was issued in 1942 and contains a selection from all soundings received in the I.H.B. up to that year*. If however VENIN¢ MEINESZ received additional information between that date and 1948 for the preparation of the chart accompanying his work "' G r a v i t y E x p e d i t i o n s at S e a . 1923-38," referred to in the above mentioned article, such information has not been received in the I.H.B. Leeuwin Sill is not shown on Sheet A'IH of the General Bathymetric Chart of the Oceans. (c) It is noted that on SCHOTr'S 0935) chart, this sill is also not shown, the single unbroken South Australian Basin (over 5,000 m) extending from Longitude I l l ° East, South-westward from Cape Leeuwin, to 141 ° East. The attached figure shows a selection from all the available soundings (in fathoms) used for Sheet A'IH mentioned in (b). The I.H.B. has received no soundings in the area between that of 1,776 fathoms shown seventy five miles to the southward of Cape Leeuwin and the 2,487 fathoms, one hundred and fifty miles southward of the Cape, and we therefore have no knowledge as to how much further to the southward the 2,000 fathom line may extend than is shown. The available soundings do however tend to disprove the statement that " Leeuwin Sill is about 200 miles long." From the above it will be seen that no evidence supporting the existence of l_aguwin Sill has been r~mived by the I.H.B., but that on the contrary the soundings plotted indicate an unbroken area of over 2,000 fathoms extending in an east-west direction 130 miles, and may be at a lesser distance, to the southward of Cape Leeuwin, as is already shown on SCHOTT'S (1935) chart. * See I.H.B. Special Publication No. 30 - Part A'III "Information concerning the preparation of the Third Edition of Sheet A ' m of the G.B.C."

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152

Letters to the Editors

For the information of readers of Deep-Sea Research it is pointed out that, in preparing the new sheets of the General Bathymetric Chart of the Oceans, all available soundings are first plotted on a series of sheets 10 times the scale of the finished chart* and the contours are drawn accordingly; they are then adjusted to agree so far as possible with those shown on maps accompanying various reports of scientific expeditions, etc., received in the bureau. A selection from the soundings is then made on tracing paper and these are reduced by photography to the required scale and sent to the printers for the preparation of the published chart. It is the bureau's present practice to send a copy of the first proof, whenever possible, to an outside specialist for his criticism and any suggested amendments. It is therefore hoped that the final sheets published will meet the needs not only of hydrographers but of all scientists interested in deep-sea research. VICE ADMmAL J. D. NARES, D.S.O"

International Hydrographic Bureau, Monaco. REFERENCES CAR~GY, M. A. and F A m e ~ E , R. W. (1954), Recent sedimentation, physiography and structure of the continental shelves of Western Australia - J. Roy. Soc. West. Austral., 38, 65-95. FAmBRIDGE, R. W. (1955), Some bathymetric and geotectonic features of the eastern part of the Indian Ocean. Deep-Sea Res., 2, 161-171. SCHOTT, G. (1935), Geographie des lndischen und Stillen Ozeaas. Hamburg (Boysen), 413. VENING MEINESZ,F. A. (1948), Gravity Expeditions at sea 1923 - 1938, 4, Publ. Netherlands Geodetic Comm., 1-233. ~* Except in the vicinity of the Poles where the plotting sheets are 5 times that of the finished chart.

An oceanographical curiosity : the perpetual salt fountain In many tropical and subtropical regions of the ocean the warm surface layers have a salinity exceeding that found in the colder waters of polar origin below. If a long tube were lowered from the surface to depth of low salinity water, and the deep water were slowly pumped to the surface through the tube, and the pump then disconnected, the water would iq [t continue to flow, by itself, forever. This remarkable phenomenon occurs because slow motion through the tube allows the water inside to attain the same temperature as the surrounding water. Its salinity, and hence density, is therefore less than that of its surroundings outside the tube, and hence the entire column of water inside the tube is buoyant with respect to the fluid outside at the same level. If the direction of pumping is reversed, so that the fluid initially goes downward, it will of course continue to flow downward forever on account of its excess in density over that of the water outside the tube. Although the attempt has not been made it is likely that in the Central North Atlantic with a tube 2,000 meters long, one might develop a pressure head of as much as two meters at the surface. A simple experiment on a laboratory scale will demonstrate the Fig. 1. Laboratory experiment to principle, however. A vertical glass tube, about three demonstrate the salt fountain, showing inches in diameter (A in Fig. 1) is filled halfway with outside glass tube ,4, with hole in the hot fresh water. Then the same quantity of slightly bottom for filling B, and inner glass coloured cold fresh water is introduced through a hole tube C to produce the fountain. The shading indicates the coloured water in the bottom (13) care being taken not to produce too after the fountain has been in operation much mixing at the interface between the two fluids. An for some time.