Geo-Marine Letters (1988) 8:159-165 Geo-Marine

Letters ~3 1988 Springer-Verlag New York Inc.

Observations of a Fast Burst of the Deep Western Boundary Undercurrent and Sediment Transport in South Wilmington Canyon from DSRV Alvin

Stephen J. Culver , 1 Charlotte A. Brunner , 2 and Charles A. Nit trouer 3

~Department of Geological Sciences, Old Dominion University, Norfolk, VA 23529-0496, 2Department of Geological Sciences, California State University, Hayward, CA 94542, and 3Marine Science Research Center, State University of New York, Stony Brook, NY 11794

Abstract

On October 3 and 4, 1986, DSRV Alvin dives encountered a strong current at 2,300 m in South Wilmington Canyon. The current, estimated at 1 knot, transported surficial sediment and constructed and modified bedforms. It appears to have been constant in its direction of flow from 30 to 40 ° . The observed current was prob- ably a burst of fast flow in a region of slow average currents in the Deep Western Boundary Undercurrent. Such episodic events may have a greater influence on the stratigraphic record than the temporally longer more tranquil flow conditions.

Introduction

Firsthand observations of deep sea oceanographic events are rarely possible. In October 1986, two dives by Deep Submergence Research Vehicle (DSRV) Al- vin took place at upper rise depths in South Wil- mington Canyon (Fig. 1) specifically to observe and Sample slump block ridges and the steep northern wall of South Wilmington Canyon. The main objective was to help understand the timing and processes of sub- marine canyon formation. During the first dive a strong CUrrent was unexpectedly encountered that caused problems with navigation, inhibited sampling efforts, and prevented completion of the dive's objectives. The current was still flowing on the next day's dive. It was Vigorous enough to transport surficial sediment and Construct and modify bedforms during the dives. This paper presents observations made on the two dives

relevant to the development and effects of the current, including modification of sediment bedforms, re- sponse of benthonic organisms, and indications of current direction, velocity, and duration.

The Dive Sites

McGregor and others (1982) surveyed South Wil- mington Canyon using a midrange side-scan sonar system which revealed pronounced lineations trans- verse to the canyon axis at depths of 2,200 to 2,300 m (Fig. 2). Observations from DSRV Alvin revealed clay ridges 10 to 20 m high with beds dipping steeply to the northwest (Stubblefield and others 1982). The steep, northern wall of South Wilmington Canyon to the north of the ridge field stood some 160 m above the canyon floor (Fig. 2).

DSRV Alvin dive 1750 on October 3, 1986 was designed to observe in detail the morphology of the sediment ridges and take pushcores and boxcores of the ridge troughs and crests to determine lithology, environment of deposition, and stratigraphy. The dive was to continue with similar objectives at the north wall but these were not achieved due to current-caused delays. Hence, dive 1751 was redesigned to sample the north wall of South Wilmington Canyon and a mesa-like structure to the southeast, and to observe effects of the current experienced during dive 1750. The track lines are reconstructed from navigational data in Figure 2.

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38"20'N

38 "10'

73" / . 0 'W 73*20 '

Figure 1. General location map of South Wilmington Canyon (modified from McGregor and others 1982).

73*00'

Methods of Observation

Observations were transcribed from audio field notes recorded by observers during the two dives. Time, heading from magnetic compass, and water depth were noted regularly and can be compared with shipboard estimates of the location of Alvin by sonic range and direction. A bathymetric map of the dive site, com- piled the preceding evening by SeaBEAM, made nav- igation simple for the submersible crew using Alvin's sonar. Audio field notes were supported by photo- graphs taken by handheld cameras aimed through ob-

38°10 .

09'

08'

07' . 0

Figure 2. Reconstructed tracklines of the two DSRV Alvin dives plotted on SeaBEAM bathyrnetry. Station numbers are indicated on tracklines (contours in meters).

servation ports, an externally mounted video camera, and an external 35 m m camera which took from 4 to 20 frames per minute. The observations were grouped into two categories: 1) those which demonstrate the speed, direction, and duration of the current; and 2) those which characterized the current's interaction with surficial sediments.

A kasten core 255 cm in length was taken at 38°03'N and 73°00'W at a depth of 2,480 m. Several 10 cm pushcores were taken from the canyon floor and the intercanyon area and mesa. X-radiographs of cores were made on shipboard.

The Contour Current

Direction of current

The observed current was constant in its direction of flow throughout the two successive dives. The gen- eral direction of flow was to the southwest, but a more accurate estimate can be made from observations dur- ing the first dive. A heading of 000 ° carded Alvin to the west and a similar problem occurred at a heading of 020 °. At a heading of 060 ° Alvin was carried to the east. Alvin was finally steered at a heading of 040 ° in an attempt to make headway. Hence, the current flowed from a direction of 30 to 40 ° east of north during the first dive. During the next dive the same current di- rection was indicated by the current-striated substrate.

The current direction generally paralleled regional

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Figure 3. (A) Outcropping block on seabed characterized by circular hummocks and hollows. Epizoan (a) on block is leaning down- current. Dive 1750, between stations 4 and 5 (Fig. 2). Current: left to right. (B) Relatively smooth sea bed with circular hummocks (a) and hollows (b). Dive 1750, station 3 (Fig. 2). Current: left to right as indicated by epizoan (c) on outcropping block. (C) Circular hummocks and hollows, some beginning to be remodeled by the current into "barchan" shapes (a). Dive 1750, between stations 4 and 5 (Fig. 2). Current: right to left. (D) Scour on upcurrent side of ca. 10 cm diameter cobble and regular urchins (a). Tail of sediment on lee side on urchin (b). Barchan-shaped mound on extreme left of field of view (c). Dive 1750, between stations 4 and 5 (Fig. 2). Current: upper right to lower left.

bathymetric trends which strike 35 ° but the current did not paraUel local geomorphic features. It flowed across the north side of the mesa, across the north wall of the canyon and transversely acrgss the canyon floor (Fig. 2). The current definitely did not flow up or downcurrent like a tide or density-driven current.

Duration and velocity of the current

A strong, generally southwest-setting current was noted from the beginning of the first dive at 1359 GMT on October 3 to the end of the second dive at 1828 GMT on October 4, a period of over 28 hours. Neither the onset nor the cessation of the current was observed. No current was noted on the preceding dive, on a site about 45 km to the southwest in 1,500 m of water. A slow, north-setting, contour-following current was noticed during the subsequent dive, also about 45 km to the southwest in 1,300 m of water. These sites, however, were probably too shallow to be affected by the southerly contour current.

The current accelerated from the start to the end of the first dive when the crew cut short its mission be- cause the submersible could no longer make head- way. Based on the behavior of the submersible, the pilot estimated that the current flowed at approxi- mately 1 knot (50 cm/sec) at its maximum velocity at the end of the first dive. The current apparently flowed more slowly during the second dive than dur- ing the previous one because the submersible main- tained steerageway.

Effect o f the Current on Surficial and Subsurface Sediments

The sea floor in South Wilmington Canyon consisted o f outcropping ridges 2 to 20 m high composed of clay strata dipping to the northwest. Sediment cov- ered the troughs and flanks of the ridges, whereas strata outcropped intermittently from the crests. Detached blocks of variable size littered the flanks. Some out- crop material appeared bedded, and blocks were dis-

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Figure 4. (A) Flattened hummock (a), sediment tails in lee of regular urchins (b) that are tilted upeurrent. Dive 1750, between stations 4 and 5 (Fig. 2). Current: upper right to lower left. (B) Sea bed flattened and streaked (a). Sediment tails (b) and scour (c) with tilted regular urchins. Dive 1750, six minutes after leaving station 6 (Fig. 2). Current: upper right to lower left. (C) Sediment stripped off pavement gravel leaving streaked seabed (a). Finer sediment tails behind regular urchins (b). Dive 1750, 22 minutes after leaving station 6 (Fig. 2). Current: upper right to lower left. (D) Asymmetric linguoid tipples and outcropping clay near base of north wall of South Wilmington Canyon. Dive 1751, between stations 1 and 2 (Fig. 2). Current: left to tight.

tinctly undercut. Animals attached to the outcrops and large blocks included bryozoans, sea pens, anemones, sponges, and terebratulid brachiopods. All observed blocks were heavily bored particularly near their bases, thus aiding or causing undercutting (Fig. 3A). It is also possible that erosion by saltating bedload played a role in sculpting the blocks.

The surface of the sediment cover changed in ap- pearance as the current increased during the first dive. Early in the dive, the sea bed surface was character- ized by circular mounds and depressions 10 to 20 cm across and 5 to 10 cm in relief (Fig. 3B). The inter- mound sea floor was quite smooth due to the relative paucity of faunal trails. The rare trails (probably hol- othurian) had levees 1 to 2 cm high and wide. Urchins set in circular depressions and small worm tubes and worm holes were abundant. An hour and a half into the dive, the water became noticeably turbid as the current increased in strength. Sediment ejected from a worm tube was immediately propelled downstream by the current.

Two and a half hours into the dive, barchan-shaped

features (Fig. 3C) were observed forming as modi- fications to small circular mounds. The sea bed be- came subtly imprinted with striations. At the same time, scour marks were noted on the upcurrent side of peb- bles and urchins while sediment tails accumulated in their lee (Fig. 3D). Within an hour, scour had pro- gressed to the point where urchins became tilted up- current into the scour marks (Fig. 4A). The urchins were no longer seated in depressions, but rather stood on pedestals exhumed as surrounding sediment was swept away. Forty-five minutes later urchins were ob- served cartwheeling downcurrent (Fig. 5). The sea bed became flattened and streaked in appearance (Fig. 4B) as turbidity increased. Patches of gravel pavement ap- peared next, swept partially clean of sand-sized sed- iment by the current. Exposed pavement alternated with streaks of sand oriented parallel to the current (Fig. 4C). At this point in the dive, in addition to cart- wheeling urchins, the current carried fish of various sizes downstream as they attempted to swim upcur- rent.

During the second dive the current was still active.

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G

E

K

C

F

"r

I

Figure 5. Sequence of 12 frames taken at 3 to 6 second intervals at station 6 (Fig. 2) showing regular sea urchin rotated out of its SCoured depression and rolled downcurrent. (A) Zero seconds, stationary urchin (arrowed) with upcurrent scour and downcurrent sediment tail; (B) 0 + 3 sec, urchin rotating out ¢;f its scoured,resting place; (C) 0 + 8 sec, urchin has rolled downcurrent and is again stationary. Its original scoured resting place is visible; (D) 0 + 14 sec, urchin stationary; (E) 0 + 17 sec, urchin stationary; (F) 0 + 23 see, urchin stationary; (G) 0 + 27 sec, urchin recommencing its cartwheeling downcurrent. Oral side down; (H) 0 + 30 sec, urchin is oral side up; (I) 0 + 33 see, urchin is oral side down; (3) 0 + 36 sec, urchin is oral side up; (K) 0 + 39 sec, urchin is oral side down; (L) 0 + 42 see, urchin is again stationary with its oral side up. Photos taken by external 35 mm camera mounted on bow of DSRV Alvin.

The canyon f loor was characterized by a pavement o f COnsolidated sediment streaked with lines o f sand and pebbles. Scour undercut the upstream side o f strewn boulders, and sediment tails were preserved in their lee. Asymmet r ic l inguoid ripple fields were observed Occasionally (Fig. 4D). These features were also pres- ent north and s o u t h o f the mesa (Fig. 2). The facies Was similar to that • observed at the end o f the first dive.

The gently inclined slope o f the intercanyon area Was covered with a drape o f sandy sediment and w a s distinctly streaked and marked with small crag-and-

tail features. Areas o f barchan-shaped ripples alter- nated with areas of small mounds. Soft corals and other sessile animals bent to the south in the downcurrent direction. The water co lumn was turbid with trans- ported material. Sand grains saltated downhil l with the current on station 6 and uphill on the north side o f the mesa.

X-radiographs o f the cores exhibit sedimentary structures characteristic o f contourites. The 10 c m pushcores contained ungraded sandy mud at the top and mud at the base. Contacts were highly irregular and convoluted, mudballs lay within the base o f the

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sandy units , and the muds were thoroughly biotu- bated. The kas ten core was P le i s tocene in age at its base based on p lanktonic foramini fera l b ios t ra t igra- phy and conta ined typical contour i te deposi ts throughout its length (S. Kuehl , personal communi -

cat ion, 1988).

Discussion

A complex image o f spat ial (Barret t 1965, Rowe and Menzies 1968) and temporal (Stanley and others 1981, Lai 1984) var iabi l i ty in f low has emerged f rom study o f the sha l low marg in o f the Deep Wes te rn Boundary Undercurrent (Hogg 1983). Evidence points to epi- sodic current events supe r imposed on a gent le average current. For example , Betzer and others (1974) and Richardson (1977) repor ted current bursts of 47 c m / sec at 2 ,575 m super imposed on an average o f 10.9 c m / s e c of f Cape Hatteras . The pulses were charac- ter ized by a quick increase o f sou thwes te r ly speed last ing severa l days fo l lowed by a two-week decrease in speed. Lai (1984) suspected that current pulses are re la ted to ant icyc lonic eddies advected southward in the s low average f low o f the deep current .

Geo log i c ev idence supports the ep isodic nature o f current bursts on the lower slope. Bedforms at local - i t ies on the s lope are t empora l ly intermit tent based on bot tom photographs (Betzer and others 1974, Mac- l lva ine and Ross 1979, Tucho lke and others 1985). A l ignmen t of magne t ic grains and mean silt s ize in surficial sed iment o f s lope transects show spat ial vari- abi l i ty in di rect ion and intensi ty o f the Deep Wes te rn Boundary Undercurrent (Bulf inch and others 1982). Concent ra t ions o f cur ren t - suspended turbidi ty above the sea f loor are t empora l ly var iable based on nephe- lomet ry (Swif t and others 1985).

The bedforms observed during these two Alvin dives are s imi lar to those descr ibed from beneath benthic s torms on the deep cont inenta l r ise (Hol l i s ter and M c C a v e 1984, Tucho lke and others 1985, Swif t and others 1985, Nowel l and Hol l i s te r 1985). The features descr ibed in this paper are typical of slow to modera te currents and take hours or days to construct accord ing to the preceding authors. W e did not see longi tudinal r ipples which take months to accumula te (DeMas te r and others 1985) and can persis t through mul t ip le ep- isodes o f current const ruct ion and b io turba t iona l de- s truction (McCave and others 1980).

The nature o f the observed bedforms leads us to conc lude that the current that we exper ienced was an episodic event strong enough to build bedforms, smooth b iogenic s tructures, and dis turb some benthic organ- isms, but not long enough to construct features that take months to form. The bedform reg ime is consis-

tent with reports o f occas iona l fast currents o f severa l days dura t ion in regions o f s low average currents in the Deep Wes te rn Boundary Undercurrent .

Sed imenta ry structures ev ident on x- rad iographs o f cores indicate that contour currents have inf luenced sed imenta t ion in this region s ince the Ple is tocene. Current bursts may be short in dura t ion c o m p a r e d to per iods o f s lower f low, but such ep i sod ic events m a y be more impor tan t in const ruct ing the s t ra t igraphic record.

Acknowledgments

We thank the captain and crew of Atlantis H and Alvh~ pilots, John Salzig and Dudley Foster for their help. R. Gannon provided Fig- ures 3 and 4 and Dr. L. Atkinson provided information on Gulf Stream rings. Drs. W. Stubblefield and B. McGregor invited us to join the dive team. Research supported by NSF grants OCE 8610365 to Culver, OCE 8609160 to Brunner, and OCE 8614159 to Nittrouer and Kuehl. NOAA's Undersea Research Office sup- ported the dive program.

References

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Betzer, PR, Richardson PL, Zimmerman HB (1974) Bottom cur- rents, nepheloid layers and sedimentary features under the Gulf Stream near Cape Hatteras. Marine Geology 16:21-29

Bulfinch, DL, Ledbetter MT, Ellwood BB, Balsam WL (1982) The high-velocity core of the Western Boundary Undercurrent at the base of the U.S. continental rise. Science 215:970-973

DeMaster DJ, McKee BA, Nittrouer CA, Brewster DC, Biscaye PE (1985) Rates of sediment reworking at the HEBBLE site based on profiles of naturally occurring Th-234 and Pb-210. Marine Geology 66:133-148

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tinental margin of Cape Hatteras, identified by petrologic method. Marine Geology 40:215-235

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Manuscript received 3 March 1988; revision received 20 June 1988,