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Nepheloid Layers and Bottom Currents in the Arctic Ocean

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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 Nepheloid Layers and Bottom Currents in the Arctic Ocean 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 sur- face, decreasingwith depth in the upper layers.Over the Canada abyssalplain, light scatter- ing 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 shown in 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
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