Nepheloid Layers and Bottom Currents in the Arctic Ocean

J'ovaN^I. or G•ovi•vstc^t. Rv.sv.^acn VOL. 74, No. 28, D•C•MBV. a 20, 1969

KENNETHHUNKINS, EDWARD1•. THORNDIKE,2 AND GUY I•ATHIEU Lamont-Doherty Geological Observatory o/Columbia University Palisades, New York 10964

Between 1965 and 1969, fifty-one profiles of light scattering were made in the central Arctic Ocean from Fletcher's Ice Island (T-3). The profiles, taken with an in situ photographic nephelometerextend from just below the surface to the bottom. Two distinctly different types of profileswere observed.At all stationsthe strongestscattering occurs near the surface, decreasingwith depth in the upper layers.Over the Canada abyssalplain, light scattering is almost constantbelow 2000 meters,decreasing slightly with depth all the way to the bottom so that the bottom water is the clearest. Over the ridges and rises surrounding the Canada basin, however, scatteringincreases with depth below an intermediate scattering minimum. The zone of deep light scattering on the ridges and rises is called the bottom nepheloidlayer. The bottom nepheloidlayer is evidently causedby fine material that is maintained in suspensionby the turbulent flow. Four spot measurementsof bottom currents on the Mendeleyev ridge gave speedsof 4 to 6 cm/sec. One spot measurementover the Canada abyssalplain gave a speedof less than 1 cm/sec.This indication of swifter current speedsover the ridgesis supportedby bottom photographsin whichanimal tracksare much less evident on the ridges than on the Canada abyssalplain in spite of a greater abundance of life on the ridges. This is attributed to the higher current speeds,which obliterate the tracks. The observationssuggest a counterclockwisedeep circulation in the Canada basin with the currents confinedprimarily to the sloping margins of the basin. This pattern of deep circulationis in agreementwith ideas and experimentson deep circulationwith a con- centrated source and distributed surface sink. Deep water enters the basin over a sill and leavesby upward diffusionthrough the haloclineinto the surfacewater, which then flows out of the basin.

INTRODUCTION biologicalactivity and to sedimentaryprocesses. Jerlov [1968] reviews the techniquesand re- Small particles suspendedin ocean water, as sults of scatteringmeasurements in the ocean. well as the inhomogeneitiesin the water itself The marine nephelometerused in this in- causedby temperaturedifferences and molecular structure,scatter incidentlight in all directions. vestigation was developed as a simple and Measurementsof this scatteringby various in- rugged instrument for in situ light scattering intensityin a continuousprofile from just below vestigators have shown a large variability, which is attributed to the variations in the the surface to the ocean floor [Thorndike and concentrationand character of suspendedpar- Ewing, 1967a, b]. It was designedfor use in a reconnaissancesurvey of the three-dimensional ticles. Scattering from the water itself would showrelatively little changethrough the oceans. pattern of light scatteringin the world oceans. Thus, scatteringobservations in the oceangive The nephelometercontains a small tungsten primarily a measureof the quantity and dis- bulb as a light source,a baffle and attenuators, which obstruct the direct beam, and a shutter- tribution of suspendedmaterial. The particles less deep-seacamera as a detector.The sche- m•y be either organic or mineral in origin. Hence, light scattering is related both to matic arrangementof these parts is shownin Figure 1. Light rays that have been scattered in a forward direction may reach the camera x Lamont-Doherty Geological Observatory Con- after passingaround the baffle and attenuators. tribution 1416. Only the part of the direct beam that passes 2 Also Queens College, City University of New York, Flushing, New York 11367. through the attenuatorsreaches the film. This attenuated direct beam gives an indication of Copyright ¸ 1969 by the American Geophysical Union. absorptionby the water and constancyof the

6995 6996 HUNKINS, THORNDIKE, AND MATHIEU H BAFFLEAND •

LIGHT CAMERA

61cm. Fig. 1. Schematic diagram of the nephelometer. light source.The intensity of the attenuated SCATTERErIN UPPERL^¾ERS directbeam decreases at a regular rate through- Lightscattering measurements werebegun in out a nephelometerstation, owing to decreasethe ArcticOcean in 1965with the objectof in batteryvoltage. This effectis considereddetermining the pattern of lightscattering and smallenough to be negligiblein thisstudy. No of findingwhether nepheloid layers exist in irregular changesthat could be attributed to this ocean.From 1965 to 1969, fifty-one deep changesin absorptionwere noted. nephelometerstations were made from a drift- Early 'attenuatorswere madeof teflon,but ing ice researchstation, Fletcher's Ice Island morerecent ones are madeof opalglass, since (T-3). The instrumentis raisedand lowered the absorptionfor teflonwas foundto be in- with winchand cablethrough a holecut in the fiuencedby pressure.Intensity is recordedby ice. Depth is determinedfrom meter wheel the amountof darkeningof photographicfilm. readings,which are correlatedwith time marks During operation the 35-ram film is trans- on the nephelometerfilm. Wire angleis gener- portedcontinuously by a smallelectric motor. ally slighton the ice stationand so these depths Two attenuatorsare used to give two levels are consideredaccurate within --+10 meters. of direct illumination.The centerpart of the T-3 driftsin the polarpack ice with windsand film containslight and dark bandsfrom the currents,generally following the large clock- two attenuateddirect beams.On either side wisegyre that dominatesice circulationin this of thesecentral bands, the film receivesonly part of the ArcticOcean. During the period light that has been scattered.The scattered of over three years coveredby theseobserva- light is strongestnear the centralbands and tions, T-3 describeda. semicirclearound the weakensoutward toward the film edges.An westernhalf of the gyre (Figure 2). The lo- impressionof changesin scatteringwith depth cationsof the stationsare shownin Figure 3. may be had from direct observationof the Pertinent information about the stations is film strips. For more detailed analysis, the given in Table 1. filmsare scannedwith a recordingdensitometer. Precisionecho soundings collected along the The densitometerrecord providesa measure track showedthat T-3 crosseda number of of the forwardscattering of white,unpolarized physiographicprovinces during this period. light as a functionof oceandepth. Thus, scatteringprofiles were obtainedover The presenceof • layer of strongscattering severaldifferent types of bottom topography. near the oceanfloor in many areashas been one A number of stationswere obtainedover the of the principalresults from nephelometerob- Canadaabyssal plain (Figure3). This abyssal servationsin the Atlantic Ocean[Ewing and plain occupiesthe deepestpart of the Canada Thorndike,1965; Eittreim et al., 1969] and basin,covering an area of over250,000 km 2. It North Pacific Ocean [Ewing and Connary, is exceptionallyfiat, with a nearly constant 19703. These authors conclude that this depthof 3800meters over its entireextent. The nepheloidlayer is a permanentfeature of the boundariesof the abyssalplain are clearly oceansover many of the continentalrises and definedby a sharpchange in slope.Nephelom- deepbasins. They attributethe scatteringto eter stationswere also obtained over some of suspendedlutite in thewater. the more elevatedfeatures surrounding the NEPI-IELOID LAYERS IN TI-IE ARCTIC OCEAN 6997 basin.Stations were made over the Northwind Typicalrecords of densitometerdeflection are ridgeand the Northwindabyssal plain in 1966 reproducedin Figures4 and 5. Theserecords and over the Mendeleyevridge and Alpha showthe transmissivityof the nephe!ometer cordillerain 1967and 1968. filmin the regionwhere it hasreceived scattered

NORTH POLE

1/69

12 / 65

SCALE IN KILOMETERS

0 500 I000 Fig. 2. Drift trackof ice islandT-3 in the ArcticOcean from DecemberI, 1965,to January 1, 1969.BOrders of mapin Figure3 areshown in outline. 6998 HUNKINS, THORNDIKE, AND MATHIEU

ALPHA

CORDILLERA

/- /-

! • •o' / I

CANADA

ABYSSAL PLAIN CONTINENTAL/CONTINENTAL RISE/ SLOPE

8-14 •. C•0 I-7

150' W

ß ' STATIONS WITH BOTTOM NEPHELOID LAYERS

0 STATIONS WITHOUT BOTTOM NEPHELOID LAYERS

Fig. 3. Locations of nephelometer stations in the Arctic Ocean. Solid circles indicate stations showing bottom nepheloid layers. Open circles indicate lack of bottom nepheloid layers. Outlines of physiographicprovinces are shown. light as • function of ocean depth. Without light-struck region. Light scatteringthen de- further calibration and calculation, these rec- creasesrapidly with depth through the upper ords indicate the intensity of scattered light layers. The recordsgenerally show small fluc- only qualitatively, but sufficientlywell for the tuationsin scatteringsuperimposed on a general present discussion.For all stations, regardless trend. The significanceof the small-scalefea- of location,the strongestscattering occurs near tures of the record is not certain. They may be the surface.The film is generallylight-struck at natural fluctuations,or they may be introduced the start, so that the record for the upper- by the instrumentaltechnique. This discussion most layer is unreliable.However, the strongest is concernedonly with the generaltrend of reliable•Cattering values occur just belowthis scatteringwith depth.If smallfluctuations are NEPHELOID LAYERS IN THE ARCTIC OCEAN 6999

o o o o o

o o o o o o o o o o o o o o o o o o o o o o

oooo 7000 HUNKINS, THORNDIKE, AND MATHIEU NEPItELOID LAYERS IN THE ARCTIC OCEAN 7001 ignored, all records show a smooth decreasein o KM 0 KM scatteringwith depth throughthe upper part of thewate• column. At manystations a record is obtainedin both descentand ascent,pro- vidinga checkon the stabilityof the profiles. The generaltrends are alwayspresent on both descentand ascent, but the small variations are not always reproducedin both records. It is presumedthat much of the scattering material in the upper layers is of biological origin. Evidences of the marine life sometimes appeardirectly on the nephelometerfilm. Small STA. 17 linear markings often appear during lowering through the upper layers just below the light- struck zone. These markings are probably STA. 2 9 caused by plankton. The combination of the shutterlesscamera and moving film causessta- tionary, as well as moving, obiectsto appear LIGHT SCATTERING INCREASING as streakson the film. Evidencesof even larger objectsthroughout the water column were ob- Fig. 4. Light scattering profiles over the Can- ada abyssal plain showing absence of bottom tained when the nephelometerwas stopped at nepheloid layers. These are densitometer records various depths [Ewing et al., 1969]. When the made from the nephelometerfilm strips.Note that instrument was held stationary at one depth, depth scales differ between records and that the animalswere attracted by the light sourceand scale of scattering intensity is arbitrary. swam into the camera's field of view.. During loweringand raisingthese animals COuld not nepheloidlayer is assumedto be at the level swim rapidly enoughto reach the camerafrom of maximum clarity, bottom nepheloid-layer any distance.The animalcausing •[•e large thicknessranges between 114 and 1090 meters. streakswas identifiedas the amphipod'Parathe- There is a tendencyfor layer thicknessto in- misto abyssorum. • creasewith depth. Depth of the level of maximum clarity versus ocean depth is plotted in BOTTOM NEPHELOID LA•m• Figure 6 for all stationstaken over ridgesand Although the upper layers show the same rises.The differencein scatteringbetween these general scattering profile at all stations, the elevated regions and abyssal plains is not as- deeperlayers show two distinctlydifferent types sociatedwith differences•in water type, since of profiles.These two types of profilesare cor- the same Arctic Deep Water Mass covers the relatedwith location.On the Canadaabyssal entire basin at. depths below 900 meters. plain, light scatteringis almost constantbelow Some measure of the intensity of scattering 2000 meters (Figure 4). There is a very slight in the nepheloidlayer may be obtainedby com- decreasewith depth throughoutthe deepwater', •paringthe densitometervalue•.. at the level of so that the clearest water of aH is..f0und on maximum clarity to the bottom value. This the bottom. Fifteen of the 16 stations on the ratio, the R value, is 1.0 if scatteringis uniform Canada abyssal plain show this type of pro- throughout the water column or if the maxi- file. The remaining 35 stations which were mum clarity level is at the bottom. If the taken on sloping topography show a bottom scattering at the level of maximum clarity is nepheloid layer. Records from the Alpha the same at different stations, the R value cordillera in Figure 5 are typical. Bottom measuresthe scatteringstrength of the bottom nepheloidlayers occurredin oceandepths rang- nepheloidlayer. Although this assumptionof ing from 614 to 3295 meters. At mid-depth a .constantminimum value of scatteringis in these records,•.there is a zone of maximum probably only approximatelytrue for our data, clarity below which scatteringincreases all it is interestingto comparethe R valuesfor the way to the bottom. If the top of the the Arctic Ocean with those in other areas. R 7002 HUNKINS, THORNDIKE, AND MATHIEU

i i i i i , OKM

STA. 55 --

STA. 5 2

o KM , OKM

I I

STA. 41

LIGHT SCATTERING INCREASING Fig, 5. Light scattering::profiles over the Alpha cordillera showing::presence of bottom ne:phe]oid]ayers. Note that de:pthscales dJEer between records and that the scaleof scattering intensity is arbitrary. values for arctic nephelometerstations are by an abrupt scatteringincrease at 700 meters. listedin Table 1. An arbitraryscale of nepheloid- Scattering remains strong between 700 and layer intensity in terms of R values has been 1250 meters. There is another scatteringlayer given by Ewing and Connary [1970] in terms between 1700 and 1900 meters. This unusual of their experiencein the Pacific Ocean. The record may be due to the small size and isola- scaleis 1.0-1.5,weak; 1.5-2.5,moderate; greater tion of this basin. All other stations were made than 2.5, strong. On this scale the arctic in locationswith free accessto the generaldeep nepheloid layer is weak to moderate with one circulation of the Canads basin. The pro- measurementin the strong category. nouncedincrease at 700 meters may be an The recordtaken over the North Wind abyssal effect of the main sill of the North Wind basin. plain (record 21) is an exception that does The basin has not been exploredwell enough not fit into either of the two previous cate- for the sill level to be known exactly, but it gories.Scattering varies irregularly with depth is probably at about 700 meters. Above that at this station. There is the usual decrease level the water column communicatesfreely through the upper layers, which is terminated with the rest of the Arctic Ocean and has the NEPHELOID LAYERS IN THE ARCTIC OCEAN 7003 same characteristicsas the upper layers in oceans.The water remainingin the centrifuge other locations. Below 700 meters the water bowlafter extractionwas run througha MilIi- massesare cut off and acquirespecial scattering pore filter. The amount of material collected characteristics. on the Milliporefilter appeared to be negligible. RELATION BETWEEN SUSPENDEDMATERIAL AND After beingreturned to LamontObservatory, LIGHT SCATTERING the sampleswere washed,dried at 100øC,and weighed.The sampleweights given in Table 2 Water samples from nine levels near the includeorganic as well as mineral matter. positionsof nephelometerstations 23 and 24 There is a notable lack of correlation between were obtainedfor measurementof suspended the total concentrationof particulatematter particulatematerial [Jacobsand Ewing, 1969]. and light scattering.The maximum concentra- The particulate matter was extracted from the tion of particlesis 9.76 rag/200 1 at a depth of sampleand weighedto seewhat relationshipit 500 meters, although there is no intermediate had to the bottom nepheloidlayer, which was scatteringlayer at adjacentnephelometer sta- 680 to 690 meters thick in this area. Great tions. The four deepestsamples were obtained care was taken to ensure that no contamination within the bottom nephelometerlayer. The was introduced into the samples.A specially mean of these four weights,1.87 rag/200 l, is constructed200-liter water barrel with plastic greater than the value of 1.16 at 1000 meters. lining was used, and the sample was removed This fits somewhatwith expectations,but the from the bottom through a spigot so that no value at 500 metersis puzzling.Care must be hose and pump were present to introduce taken, however, in drawing conclusionsfrom foreign matter. The water was centrifugedat this result sinceonly a singlesample was taken the station to separate the solid material. The at each level. The possibilityof contamination efficiencyof the recovery of suspendedmaterial is alwayspresent when dealingwith suchminute for this technique was checkedby Jacobs and concentrations,even when the greatest pre- Ewing [1969] on selectedsamples from other cautionshave been taken. Further sampling

i i i

/ 3 KM / / / / / / / / /

/ / . ee e• / • / / / /

0 I 2 :• • KM

OCEAN DEPTH Fig. 6. Relationbetween ocean depth and the depthof maximumclarity for all nephelometer stations taken over ridges and risesin the Arctic Ocean. 7004 HUNKINS, THORNDIKE, AND MATHIEU

TABLE 2. ConcentrationData for SuspendedParticulate Material in Arctic OceanWater from Jacobsand Ewing [1969]

Initial Dry Weight, Water Depth, Position,* Sample mg/200 1 meters meters Latitude Longitude Water Mass

I 6.40 1537-1735 12 78 ø46'N 175 ø25'W Surface II 3.76 2500-2528 75 79ø25'N 171ø25'W Pac. (Max. Temp.) III 3.58 2286-2313 155 79ø24'N 173ø57'W Pac. (Min. Temp.) IV 9.76 2184-2284 500 79ø16•N 175ø05'W Atlantic V 1.16 2498-2522 1000 79ø37'N 171ø41'W Deep Water VI 1.92 2250-2407 2000 79ø43'N 173ø38'W Deep Water VII 1.68 2310 2210 79ø39'N 171ø52'W Near Bottom VIII 1.90 2310 2260 79ø39'N 171ø56'W Near Bottom IX 2.00 2304 2302 79ø38'N 172ø00'W Near Bottom

* Position of sample from the surface. seemsnecessary. Perhaps, also the nature and on the Mendeleyev ridge, and all showedbot- size of the suspendedmatter needs more in- tom currentsof 4 to 6 cm/secdirected generally vestigation. southward.These currentsthus flow generally parallel to the depth contours. These are BOTTOM CURRENTS AND I•EPIIELOID LAYERS essentiallyspot observationsand give no infor- Five current stations were taken from T-3 with mation about long-periodcurrents, such as the photographicbottom current meters developed currents associated with tides or inertial mo- by Thorndike[1963] and Bruce and Thorndike tions.Tides are smallin the Arctic Ocean,how- [1967]. The meter consistsof an underwater ever, and may not be of much influence on camera, mounted on a tripod, which photo- bottomcurrents there. Inertial andlonger-period graphs either the deflectionof vanes or the motionsof surfacewaters are knownand may movementsof a neutrally buoyant droplet in extend to deep levels. Thus caution is neces- the field of view. A magnetic compassis also sary, but the consistencyof speedand general mounted within the field of view for azimuth direction for the four stationsdo suggesta reference. Photographsare taken at approxi- steady componentof bottom flow to the south mately 2-minute intervals over a time span of alongthe Mendeleyevridge. 15 to 30 minutes. The one station (station 5) in the Canada Information about the stations is summarized abyssal plain indicated currents of less than 1 in Table 3. Stations I through 4 were all taken cm/sec. Other, lessquantitative, measurements

TABLE 3. Summary of Bottom Current Stations Taken from T-3

Corrected Sonic Oc•all Current Current North West Depth, Speed, Direction, Type of Physiographic Station Date Latitude Longitude meters cm/see øT Indicator Province*

1 April 28, 1967 79o33' 174o25' 2047 4-6 130 Vanes M.R. 2 May 1, 1967 79ø36' 173ø58' 2013 4-6 210 Vanes M.R. 3 May 11, 1967 79ø31' 171ø30' 2381 4-6 160 Vanes M.R. 4 May 15, 1967 79ø29' 171ø09' 2653 4-6 255 Vanes M.R. 5 Sept. 15, 1963 82ø32' 157ø10' 3790 <1 --. Suspended C.A.P. Drop

* M.R. designates Mendeleyev ridge; C.A.P., Canada abyssal plain. NEPItELOID LAYERS IN THE ARCTIC OCEAN 7OO5 of the behaviorof dye plumesnear the bottom timespresent on the rid•ee... and rises,but they with the sametripod cameraalso indicated very never completelycover the bottom in a mosaic lowcurr?nt si•eeds on the Canadaabyssal plain. as they do in the abyssal plain photographs. •-•Thes•'preliminary current data suggestthat It has been shown [Hunkins and Kutschale, c•irrentspeeds are higheralong the Mendeleyev 1967] that sedimentationrates have beensimilar ridgethan on the Canadaabyssal pla•? Since over the abyssalplain and over the rises for the nepheloidlayers are generally found on the past1400 years. In earlierperiods, turbidity the ridges, this implies an associationof currents must have made the sedimentation nepheloid layers and bottom currents. rate muchhigher over the abyssalplain, but, at Photographsof the oceanfloor showmarked least in the northwestern area, the Canada contrastsin fauna and microreliefbetween the abyssalplain has recentlybeen dormant.Thus, Canadaabyssal plain and adjacent•rises and the differences in microrelief and fauna are ridges [Hunkins, 1968]. Bottom photographs probably causedby bottom currents.In some on the abyssalplain show a network of small cases,scour marks are seen around rocks on animal tracks coveringthe entire field of view the risesindicating currents, although no ripple (Figure 7). No animals or rocks are seen. All marks have been noted. It is alsolikely that the abyssalplain .photographsare monotonously'higher speed of the currentson the risesquickly similar. Photographs from the surrounding removes small animal tracks by erosion even elevatedfeatures show more variety with fre- thoughanimals are more prevalent.The evi- quent appearances of ice-rafted rocks and dencefrom the bottomphotographs thus tends animals (Figure 8). Animal tracks are some- to support the direct measurementsshowing

Fig. 7. Bottom photographon the Canada abyssalplain. Note abundant animal tracks and lack of rocks, animals, or other large features. Station T-3/66-2; 3782 meters; 75ø17'N, 146ø01'W. 7006 HUNKINS, THORNDIKE, AND MATHIEU

Fig. 8. I]ottom photograph on the Alpha cordillera. Note mounds in upper left and sediment-draped rock in lower right with brittle star just beyond it. ,Station T-3/63-2; 3426 meters; 83ø02'N, 163ø31'W. weak bottom currents, currents on the rises, ble, however,to test whether the excessdensity and vanishingly small bottom currents on the of the suspendedmatter is sufficientto produce Canada Abyssalplain. bottom currents. The bottom currents flowing southward on the western side of the basin are •)YNAlV•ICAL ROLE OF •BOTTOlV•NEPI-IELOID LAYER in the proper sense to be produced by a If there is a sufficient load of particles in nepheloidlayer slopingin the samedirection as the nepheloid layer, it. is conceivablethat the the bottom. bottom currents are driven by a horizontal The concentrationsof suspendedmatter ob- density gradient in this layer. If the particle served in the Arctic Ocean permit a rough load is small, however,the bottom current must calculationof geostrophiccurrent velocity pro- be driven by other means.Is the excessdensity duced only by the excessdensity of particles. of the suspendedparticles sufficient to drive Assume a nepheloid layer of constant density the bottom current, or doesthe bottom current, bounded above by a sloping interface between driven by other means, maintain the particles it and the overlying clear water. If the currents in suspensionthrough turbulence ? Probably the are geostrophic,the equation of motion is currents are at least partly geostrophicwith gp•--p density gradientsarising from the variation in tan 0 temperatureand salinity. There are not enough f p oceanographicstations to determine whether where temperatureand salinitygradients are sufficient v, current speed parallel to the bottom to account for the bottom currents. It is possi- contours. NEPHELOIDLAYERS IN THE ARCTIC OCEAN 7007 densityof the nepheloidlayer. were measuredon the Mendeleyevridge and density of clear water. presumablyare also presenton the Northwind Coriolisparameter. ridgeand the Alpha cordillera.On the fiat floor tan O, slopeof the interfacebetween the nephe- of the Canadaabyssal plain neithernepheloid loid layer and overlyingclear water. layers nor bottom currents were detected. Cal- accelerationdue to gravity. culationsin the previoussection show that the From Table 2, the concentrationof suspended bottomcurrents must be drivenby pressure material in the nepheloidlayer is taken as gradientsother than the onesproduced by the suspendedmaterial. The flow of these currents 2.0 rag/2001 or 10 •g/1. If a densityof 2.0 g/cm8 over the bottom generates turbulence that is assumedfor the particles,the densityexcess, p' -- p, in the nepheloidlayer is 10-8 g/cm8. maintainsthe particlesof the nepheloidlayer Valuesused for the otherquantities are p ----1.0 in suspension.The tendencyof the particles g/cm8, f -- 10-4 sec-•, tan 0 -- 10-•, and g -- to sink is balancedby their upward diffusion in the turbulent flow. 108g/sec •. The current in the nepheloidlayer in this case would be It is reasonableto assumethat these deep currents around the margins of the Canada 103 basin are relatively steady and that their source v -- 1•-•.10-s.10 -• = 10-2 cm/sec is the inflow of deep water from the more eastern arctic basins and ultimately from the Since the observed currents are at least two NorwegianSea. A comparisonof potential tem- orders of magnitudegreater than this upper peratures between the Canada basin and the limit for currentsproduced solely by suspended Eurasia basin showsthat deep water flows into matter, the nepheloid layers cannot be con- the Canada basin over a sill in the Lomonosov sideredto have any dynamicalsignificance for ridge at about 2300 meters [Worthington, bottom currents. 1953]. The southward flow measured on the On the other hand, evidence from other western side of the Canada basin suggestsa sourcessuggests that the observed current ve- deep counter-clockwisecirculation. This is the locities are quite sufficient to maintain fine senseof motion to be expectedin the case of particles in suspension.Heezen and Hollister horizontal convergencein a deep layer. The [1964] presenteda summaryof experimental deep water in the Canada basin is well mixed evidenceon the minimum velocitiesrequired with a nearly adiabatic temperature gradient for transport of sediment.They state, 'Once below 2000 meters. The deep water must ulti- put into motion particlesof 0.02 mm would not mately mix upward through the halocline into come to rest until current velocities decreased the surfacewater, which then flows out of the to 0.03 cm/sec, and particles of 0.2 mm would Arctic Ocean in the East Greenland Current. not stop until velocitiesfell below 2 cm/sec.' All these observationssuggest that the deep Thus, it is clearthat the currentson the ridges circulation in the Canada basin resembles the and rises are capableof transportingmaterial flow pattern observedby Storereelet al. [1958] of the size capableof scatteringlight. in a rotating turntable experiment on density- It is likely then that the bottom nepheloid driven flow in a pie-shapedtank. A sourceof layer is an effect rather than a cause of the denser water marked with dye was located at bottom current. Bottom currents producedby the apex of the sector.The water level rose in other means produce sufficientturbulence near the tank during the experiment simulating a the bottom to keep fine material suspended. distributed surface sink as in the case of upward diffusion through a halocline. An intense D•scuss•o• southward-flowingboundary current developed The results presented here indicate a three- along the western margin of the tank. The in- way associationof bottomnepheloid layers, bot- terior of the basin was filled from the rim in tom currents, and the sloping topography of a sluggishnorthward flow. The experimentmay ridgesand rises.Nepheloid layers were observed actually be a closermodel of deep circulation on the Northwind ridge, Mendelyev ridge, and in the Canada basin than in the North At- Alpha cordillera. Significant bottom currents lantic, which it was intendedto model. It pro- 7008 HUNKINS, THORNDIKE, AND MATHIEU vides a working model for further exploration Hunkins, K., Geomorphic provinces of the Arctic of abyssal circulation in the Arctic Ocean. Ocean, in Arctic Dri]ting Stations, Arctic Insti- tute of North America, Washington, D.C., Acknowledgments. We are indebted to Allan 1968. Gill and Steve Teeter, who took many of the Hunkins, K., and H. K. Kutschale, Quaternary nephelometer stations at T-3. The assistance of sedimentatiqnin the Arctic Ocean,in Progress the Naval Arctic Research Laboratory at Barrow in Oceanography, vol. 4, edited by M. Sears, and at T-3 was indispensableto this research. Pergamon Press, London, 1967. Financial support was provided by the Office of Naval Research under contracts Nonr 266(82) and Jacobs, M., and M. Ewing, Suspendedparticulate N 00014-67-A-01084)016. matter' concentration in the major oceans, Science,163, 380-383, 1969. I•EFERENCES Jerlov, N. G., Optical Oceanography, 194 pp., Elsevier, New York, 1968. Bruce, J. O., Jr., and E. M. Thorndike, Photo- Storereel, H., A. B. Arons, and A. J. Faller, Some graphicmeasurements of bottom currents,chap. examples of stationary planetaw flow patterns 9, in Deep-SeaPhotography, edited by J. B. in bounded basins, Tellus, 10, 179-187, 1958. Hersoy,pp. 107-111,Johns HOpkinZ Press, Balti- more, 1967. Thorndike, E. M., A suspended-drop current Eittreim, S., M. Ewing, and E. M. Thorndike, meter, Deep-Sea Res., 10, 263-267, 1963. Suspendedmatter along the continentalmargin Thorndike, E., and M. Ewing, Light scattering in of the North Americanbasin, Deep Sea Res., the sea, presented at Society of Photo-Optical in press, 1969. Instrumentation Engineers, Underwater Photo- Ewing, M., and S. Connary, Nepheloid layer in Optics seminar, Oct. 1966, A-IV-1 ' to A-IV-7, the North Pacific, in Geological Investigations 1967a. o] the North Pacific, GSA Memoir, edited by Thorndike, E., and M. Ewing, Photographic J. Hays, GeologicalSociety of America, New nephelometers for the deep sea, chap. 10, in York, 1970. Deep-Sea Photography, edited by J. B. Hersoy, Ewing, M., K. Hunkins, and E. Thorndike, Some Johns Hopkins Press, pp. 113-116, Baltimore, unusual photographsin the Arctic Ocean, J. Md., 1967b. Mar. Tech. Soc., 3, 41-44, 1969. Worthington, L. V., Oceanographic results of Ewing, M., and E. Thorndike, Suspendedmatter project Skijump I and Skijump 2 in the Polar in deep ocean water, Science,147, 1291-1294, Sea, 1951-1952, Trans. Amer. Geophys. Union, 1965. 34, 543, 1953. Heezen, B., and C. Hollister, Deep-sea current evidencefrom abyssalsediments, Mar. Geol., 1, 141-174, 1964. (Received August 14, 1969.)