Deep-Sea Research, 1969, Vol 16, pp 99 to 101 Pergamon Press Printed m Great Britain

S H O R T E R C O N T R I B U T I O N S

Contour c u r r e n t s in the Wedde l i S e a *

CHARLES D HOLLISTER t and ROBERT B ELDER~

(Recewed 20 August 1968)

IN THE austral summer of 1968 the U S C G C Glacter penetrated the pack ice of the Weddell Sea to over 75°S During tins expedation, 157 compass-oriented bottom photographs§ were taken at 13 locations m water depths between 400 and 4000 m, on the continental shelf, slope and rise of the western Weddell Sea Data from these and from the over I000 unorlented bottom photographs obtained by the U S N S Eltanm (1963-1964) at 67 locations m the Scotia Sea, South AtlanUc and northwestern Weddell Sea form the basra for tlns report on the visual indications of bot tom currents (Fig 1) Those among the latter from the viczmty of the South Sandwich Arc and the northwestern edge of the Weddell Sea gtve ample evidence of strong bottom currents (I-IEEZ~N and JorrNsoN, 1965, HOLUSTER and HESZeN, 1967) However, the absence of a compass m the field of view of these pictures made it impossible to infer current dtrection The oriented pictures taken by the Glacier now make it possible to interpret the unoriented photographs in this region and to infer a vmble pattern of bot tom water circulatmn m the Weddell Sea

BOTTOM WATER CIRCULATION

The &stributlon of temperature, oxygen, sahmty and density of the deep and bottom water of the Southern Ocean mdacates that the far-reacinng Antarctic Bottom Current originates in the Weddell Sea (WuST, 1936, S T O O L , 1957, DEACON, 1937) Here the flow is northerly along the continental rise of the western South Atlantic (WuST, 1936, 1957, HEEZEN and HOLLISTER, 1964) In addttlon a branch of tins flow breaks through the ridge west of the South Sandwich Are (40°W) and then turns to the west to form a wesfward-flowmg current along the continental rise of the Antarctic Pemnsula (GORDON, 1966, HOLUST~ and I-IEEzer~, 1967) Patterns of bot tom circulation in the Weddell Sea south of about 65°S are essenttally unknown, however, and this prehrmnary report based on the visible effects of bot tom currents provides a first approx~matmn of the nature of bot tom circulation in this remote area

SEA-FLOOR PHOTOGRAPHS

The most obvmus features seen in photographs taken on the floor of the Weddell Sea are animals and their traces (Fig 2) Echlnolds and then" chstinctive furrows, ophlUrolds and their star-shaped impressions and holothurlan fecal coils dominate the seascape in depths greater than 2500 m How- ever, despite very evident s~gns, the number of recogmzable orgamsms rarely exceeds one Indwidnal per picture Between 2500 and 1500 m, where ophlurolds, sea-spiders, crustaceans and worm tubes form the benthic commumty, their number increases gradually to about five, and m water less than I000 m, where worm tubes, ophlufolds, crinoids, long-spined echinoids, sea-pens and actmarians dominate, there may be more than ten

The second most obvious feature is scattered, presumably ice-rafted pebbles, rocks and boulders (Fig 2) Deeper than 3500 m these have a spotty d~stnbutmn However, they are generally more

*Woods Hole Oceanographic Institution Contribution No 2148 tWoods Hole Oceanograplue Institution, Woods Hole, Mass , 02543, U S A SU S Coast Guard, Oeeanograpluc Umt, Wasinngton, D C , U S A §Pictures reported on here were taken with a THORSDIKe (1959) bottom-contact camera The

tugger wire and lens were adjusted for close-up oblique photography and each frame revealed the form of the bottom over an area of about 3 meters square

99

100 Shorter Contrxbutlons

5 0 ° 4 0 ° 3 0 °

N TIC

2 0 ° /

55 °

6 0 °

NCE

DM WATER -_- ' -

~PS AND TALUS

RECTION FROM HOTOGRAPHS

r TRANSPORT iON

:)CKS a BOULDERS

C A N I M A L S

~CGC GLACIER 196B AND IN, 4965 a t964

ZOO0 METERS

7 , 0 ° 6 0 ° 5 0 ° 4 0 ° 3 0 ° 2 0 ° (CONTOURS IN METERS)

F1g 1 The Antarctic Bottom Current ~s thought to originate m the Wcddell Sea as a northerly- flowing contour current and transports fine materml into the Scotm Sea and South Atlantic D1stnbutmn pattern of wslble current evidence was determined by examining over 1200 sea- floor photographs at 80 locatmns Current evidence was considered strong ff d~stmct hneatlons or exceptmnal smoothing was apparent (for &scussmn of current velocities see I-~EZEN and HOLLISTER, 1964, pp 169 FF) In such cases, bottom current velocmes of about 10 cm/sec (moderate) are sufficient to deflect attached orgamsms Weak (less than 5 cm/sec) bottom currents

are probably capable of only shght smoothing of a muddy sea-floor

abundant in shallower water Most In depths of less than 1500 m, are profusely decorated w~th coelenterates and echinoderms, but regardless of depth, they are generally free of sechment cover

The most rewarding aspect of bottom photographs in the Weddell Sea is the consistent orientation of llneatlons and deflection direction of sessile orgamsms such as worm tubes and sea-pens (Figs 2 and 3) In each of the 8-12 frames at each staUon, there is less than a 15 ° deviation m dlrect~on, and m each of the 157 oriented pictures obtained, the bottom current direction appeared to be parallel to bathymetrlc contours (Fig 1)

There was unusually murky bottom water m vartually all water depths over 2000 m along the continental rise on either s~de of the Antarctic Peninsula, along the South Sandwich Island Arc of

0 5 IO I I' , I

crn

F~g 2 Meander ing echmo~d furrow and scattered rocks on the floor o f the Weddell Sea Deflected o r g a m s m s and their shadows (shghtly re touched here for pubhcat~on) md~cate a

nor thwes tward flowing bo t t om current at th~s s ta t ion 67014 ' S, 49058 ' W, 3664 m

[factng p 100]

OJrren~

0 5 0 L [

cm

Fig 3 Deflected worm tubes (center) mchcate nor thwester ly current 74~51 ' S, 39 05' W, 384 m.

Shorter Contributions 101

the Southern Antilles and along the northern portion of the Seotm Sea In some photographs the bot tom was even partially obscured The distributional pattern of turbid, bot tom water suggests that the Antarctic Bottom Current flowing from the Weddell Sea carries a vast quantity of mud in suspension Indeed, mineralogical analysis of deep sea clays in the Atlantic (BISCAYE, 1965) indicates that a tongue of quartz-rich mud extends from the Weddell Sea into the South Atlantic

C O N C L U S I O N

The Antarctic Bottom Current flows parallel to bathymetrlc contours of the Antarctic continental margin in the Weddell Sea This contour current* is presently transporting sediment from the Weddell Sea, around the South Sandwich Arc and into the South Atlantic As the southerly-flowing Western Boundary Undercurrent smooths and shapes the large sediment accumulations in the North Atlantic (HEEZEN, HOLLISTER and RUDDIMAN, 1966), so must the Antarctic Bottom Current smooth and shape the sea floor of the Weddell Sea

Acknowledgements--Our thanks are due to the National Selence Foundation for making the camera system available and to Dr JAN A STROMME of the Universltetet i Bergen, Lt JAMES M SEABROOKE of the Coast Guard Oceanographic Unit and PH1 JOHN CLAYTON of the C G C Glacier for providing valuable assistance throughout the cruise Drs R MEADE and D FOLGER kindly reviewed the manuscript The semor author gratefully acknowledges the support of the U S Geological Survey (Project No 10875) Prof BRUCE C HEEZEN generously provided advice and encouragement through- out this investigation

R E F E R E N C E S

BISCAYE P E (1965) Mineralogy and sedimentation of Recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans Bull gem Soc Am, 76, 803-832

DEACON G E R (1937) The hydrology of the Southern Ocean ' Discovery' Rep, 15, 1-124 GORDON A L (1966) Potential temperature, oxygen and circulation of bot tom water in the Southern

Ocean Deep-Sea Res, 13, 1125-1138 HEEZEN B C and G L JOHNSON (1965) The South Sandwich Trench Deep-Sea Res, 12, 185-197 HEEZEN B C and C D HOLLISTER (1964) Deep-sea current evidence from abyssal sediments Mar

GeM, 1, 141-174 HEEZEN B C , C D HOLLISTER and W F RUDDIMAN (1966) Shaping of the continental rise by deep

geostrophic contour currents Scwnce, 152, 502-508 HOLLISTER C D and B C HEEZEN (1967) The floor of the Belllngshausen Sea In Deep-Sea

Photograph), J B HERSEY, editor, Johns Hopkins Press, Baltimore, 177-189 STOMMEL H (1957) A survey of ocean current theory Deep-Sea Res, 4, 149-184 THORNOIKE E M (1959) Deep-sea cameras of the Lamont Observatory Deep-Sea Res, 5, 234-237 WusT G (1936) Sehlchtung und Zlrkulatmn des Atlantlschen Ozeans Das Bodenwasser Die

Stratosphare Wlss Ergebn dt atlant Exped 'Meteor, '6 (1), 1-288 WUST G (1957) Stromgeschwlndagelten und. Strommengen in der Tmfen des Atlantischen Ozeans

unter besonderer Berucksichtigung des Tiefen- und Bodenwassers Wtss Ergebn dt atlant Erped ' Meteor,' 6 (2), 261-420

*Bottom currents, resulting from thermohallne circulation on a rotating earth, flow parallel to bathymetrlc contours and are called contour currents (HEEZEN, HOLLISTER and RUDmMAN, 1966) Dense, sediment-laden turbidity currents on the other hand flow downslope