Bathymetry of Pacific seamounts Bowie and Union

Deep-Sea Research, 1971, Vol. 18, pp. 123 to 126. Pergamon l ~ ' e s s .

Printed in Great Britain.

Bathymetry of Pacific seamounts Bowie and Union J. A. SCRIMGER* and W. HALLIDAY*

(Received23 January 1970) W• Hhw recently had occasion to explore two seamounts off the West Coast of Canada--the seamounts Bowie and Union and wish to report the topography of their summits. Echo soundings for Bowie Seamount (Fig. 1) were obtained from C.S.S. Parizeau using a Simrad EH4 transducer (which operates at 11 kHz and produces an 11° wide beam) in conjunction with a Gift Type G.D.R.-1C-19-T recorder. Soundings assumed a constant sound speed of 1460 m/sec and were uncorrected for transducer beam width. A radar transponder mounted on a buoy moored near the seamount summit was used as a reference for the sounding lines. Contours are thus positioned

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z3( Fig. 1. The summit of Bowie Seamount above the 200-fm contour. *Defence Research Establishment Pacific, Defence Research Board of Canada, Victoria, B.C., Canada.

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SCRIMGER a,Rd W. HALLIDAY

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Fig. 2. The summit of Union Seamount above the 500-fro contour. The sounding line AA reveals a possible volcanic crater (cf. Fig. 4).

with an estimated error not exceeding :k 180m relative to an arbitrary datum on the seamount surface. Components of the error are: errors associated with the observed range and bearing of the transponder, and errors arising from movement of the transponder buoy allowed by the scope of the mooring. The latter component was estimated to be small ( ± 30 m) since the survey was carried out in a six hour period during which wind conditions remained stable; also the water depth at the buoy's position was shallow (70 m). Use of satellite navigation and observation of six satellite passes enabled an accurate geodetic position of Bowie Seamount to be obtained (Fig. 1). The shallow (30 m) peaks of the seamount are accessible to, and on two occasions have been inspected by, Scuba divers. The summit of Union Seamount is about eight times deeper than Bowie (Fig. 2) and is generally conical in form. The transponder buoy used for the sounding lines was anchored in about 380 m of water--a depth which we suspect permitted significant buoy movement during the 6-hr data gathering period. Contour accuracy is thus estimated to be -4- 360 m relative to an arbitrary datum on the seamount surface. The geodetic position of Union Seamount was obtained with a statistically insignificant number of Loran A fixes, therefore, no estimate of the accuracy of the seamount's

Bathymetry of Pacific seamounts Bowie and Union 500

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BOWIE SEAMOUNT

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Fig. 3. Profiles of the Seamounts Bowie and Cobb taken to the sea floor, obtained when leaving each of the seamount summits.

position (Fig. 2) was made. Echo soundings at Union Seamount were made from C.N.A.V. Endeavour using an Edo transducer Type 353 (which operates at 12 kHz and produces a 30 ° wide beam) and recorded on an Alden PrecisionGraphic Recorder. Soundings were not corrected for sound speed dependence on depth or beamwidth. The most notable bathymctric feature of the summit of Bowie Seamount is the 130-fro(238-m) terracewhich is prominent at the east and west ends of the ridge-like summit. This terraceis similarto that at the 100-fm (183-m) levelon Cobb Seamount (BuDINGER, 1967) and likely has a c o m m o n origin, namely wave erosion during a period of sea-levellowcring. The 30-fro(55-m) discrepancy between the terracedq~ths is presumably due to differentdegrees of subsidence undergone by these seamounts in recent geologicaltime. Evidence of subsidence or tiltingoccurs in our data. Profiles of the two seamounts taken from their summits out to the sea floor on an easterly bearing (Fig. 3) reveal depressions near their bases. Union Seamount, presumably because itnever rose far enough above the sea floor to become exposed to wave crosion,is not terraced. A featureof interest,however, is a cratcr-likedepression (Fig. 4) to the east of the peak (cf. Fig. 2, profileAA). Insufficientdata were obtained in the area of thisfeatureto permit a detaileddescription. The horizontal extent of Union Seamount (Fig. 2) at the 500-fro (915-m) depth differs from that shown on U.S. Coast and Geodetic Survey Chart No. 8500. At this depth the latter indicatcs about a 15 nautical mile (27.8km) east-west and a 5 nautical mile (9.27kin) north-south extent of the summit. However U.S. Hydrographic Chart No. B.C. 1410, dated January 1969, records an additional peak at

126

J.A. ,Scx~oEx and W. HALLIDAY

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Fig. 4. Profile from a sounding traverse (AA in Fig. 2) along a north-south bearing to the east of

the peak of Union Seamount.

290 fm (520 m) some 8 nautical miles (14.83 Ion) to the east of the peak we surveyed and connected by a ridge about 1000 fm (1830 m) deep which is in accord with our observation. Acknowledgements--We wish to thank Captains C. ANQUS and D . M A c F ~ of C.S.S. Parizeau and C.N.A.V. Endeavour for their support and cooperation in carrying out these surveys. REFERI~NCE

BUD~O~ T. F. (1967) Cobb Seamount. Deep-Sea Res., 14 (2), 191-201.