Kuliah Deep Sea 1

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2 The physical environmentof the deep sea

THE SHAPEOFTHE SEAFLOORThe topography of the floor of the deep ocean (Fig. 2.1) is a balancebetween the parameters of seafloor spreading and sedimentation ofinorganic and organic particles. Around the periphery of the ocean basinslies a continental shelf ofvarying width ending at the shelfbreak, usually ata depth of c.200m, below which plant lifeis supposed to be absent. ln theAntarctic, owing to the weight of the ice cap, the shelf edge is at c.500m.Ifwe accept topographic criteria, the deep sea may be said to begin at theshelf break. This is a safer distinction than one based on photosyntheticdepth since attached seaweeds have been found as deep as 268m off theBahamas (Littler et al. 198 5 .Seaward of the shelf edge there is a marked increase in the downward

gradient of the seabed indicating the continental slope (Fig.2.2).The slopemay be a simple structure where the isobaths are parallel to each otherand evenly spaced or it may contain a series of irregularities togive a veryuneven gradient. The slope marks the underlying boundary betweenoceanic and continental crust. The theory of plate tectonics tells us thatthese areas of crust consist of a dynamic system of plates where crust isboth being formed at mid-ocean ridge spreading centres and consumedby subduction at 'active', or seismic margins typical in the Pacifie (Fig.2.3). 'Passive', or aseismic margins are typical of the Atlantic (Leeder,1985). The gradient of the slope may be interrupted by terra ces andsubmarine canyons. The latter appear as irregular, fissure-like channels cutdown the continental slope which may act as conduits for transport to thedeep ocean basin beyond, but probably were most active as such duringglacialperiods when sea levelswere lower, and downslope processes farmore intense than today. However, bottom currents, intense enough toresuspend the sediment, may occur from internal tides focussed alongthe canyon axis(Gardner, 1989).Their V-shaped profiles are probably theresult oferosion by turbidity currents (seep. 24).Many can be traced on tothe adjacent continental shelf, often at the mouths of major rivers.


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10 1.HISTORY, ENVIRONMENT AND METHODS

Fig. 2.1. Topography of asection of the northern N.E.Atlantic lying to the west ofthe British Isles showing

sorne of the chiefphysiographic features ofthe deep ocean. The viewlooks northeast and showsthe rugged topography ofthe Reykjanes Ridge (RR)

extending southwards frornIceland (I), and the E.-W.trending Charlie-GibbsFracture Zone (GFZ) thatsepara tes it frorn the

northern section of the Mid-Atlantic Ridge (MAR).The

westward flank of theReykjanes Ridge extends

into the part of theLabrador Basin lying east ofsouthern Greenland; whilethe eastward flank of theRidge rnerges into the

gentler topography of theIceland Basin (IB).Thisbasin isbounded to the E.

by a chunk of alrnostcornpletely subrnergedcontinental crust, the

RockallPlateau (RP)and itsassociated northern Banks.

continued

FSCWTRNBRP

At the base of the slope of passive margins there is typically a thickwedge of slope-derived sediment termed the contin ental ri se (Fig. 2.2)or,if the base of the slope is offshore from a large river, there may be amuch larger formation of alluvial sediment called a su bm arin e fa n. Thetopography of the rise is usually much smoother than the slope, but maybe cut by channels extending from canyoned slopes. Byadepth of c.4 kmthe seabed has levelled off to give awide expanse of relatively flat abyssalplain which extends gently from 4 km to 6 km depth. These are oftenundulating or quite featureless, or they may be interrupted by numerousflat-topped guyots or seamounts (Fig. 2.1),which are inactive ocean-floorvolcanoes that do not rise above sea level, and sometimes occur in chains(Epp Smoot, 1989),Seamounts can rise several kilometres above theocean floor and their profiles show declivities as great as 25, muchsteeper than any major seafloor feature elsewhere in the ocean. Theabyssal plains do not extend across the oceans but are separated by themid-ocean rid ge (Figs2.1,2.3,15.2). The ridge is the site of formation ofnewocean crust and is amore or less continuous system occupying about 33of the area of ocean floor. The generally symmetrical process ofextrusionof new crust along both sides of the mid-ocean ridge results in theseparation of the flanking lithospheric plates. The solid geometry of theEarth necessitates the formation of a series of cracks, called transformfaults, which appear as great slashes at right angles to the main axis (Fig.


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PHYSICAL ENVIRONMENTThis 'microcontinent' is

separated from thecontinental crust ofnorthern Europe by theRockall Trough (RT).Shallow sills, including the

lceland-Faeroe Rise (IFR)and the Wyville ThomsonRidge (WTR)that separatethe Norwegian Basin (NB)from the more southerly

Atlantic basins allowoverflow of cold Arctic-cooled water from theNorwegian Sea into the

Iceland Basin and RockallTrough to contribute to thedeep-sea water mass via the

Faroe Shetland Channel(FSC)(see p. 15).Two

seamounts, the flat-toppedAnton Dohrn Seamount(ADS)and Hebridean

Seamount (HS) lie on thecontinental rise of the

eastern Rockall Trough.The latter basin opens into

the deeper PorcupineAbyssal Plain (PAP).Thesteep continental marginlying southwest of Irelandis broken by a large bight-like terrace, the PorcupineSeabight (PSB)and furtherby numerous canyons onthe continental slope of the

Bay ofBiscay (BB).(Computer-generated chartcourtesy Dr G. Robinson,NERCUnit for Thematic

Information Systems,Reading University.)

Fig. 2.2. Profile of typicalpassive (aseismic)continental margin. (FromAnikouchine Sternberg,

1973.)

112.3).Mid-ocean ridges are usually about 2.5 kmbelow sea level, but, withincreasing distance from the ridge, depth increases to about 5 to 6 kmwith the depression of the thin ocean crust by an ever-increasing accumu-lation of pelagie sediment blanketing the uneven topography of theoceanic crust. Here lie the featureless expanses of the abyssal plains withgradients typically in the region of 1 : 1000.Trenches occur if the abyssalplain is bordered by an active margin, when the oceanic crust (thelithosphere) buckles and deepens as it is eventually destroyed by subduc-tion beneath an adjacent continent (Fig.2.3).Trenches are best developedin the PacificOcean where 'active' continental margins are dominated bysubduction zones. As a result, the Pacifie, in contrast to the Atlantic, isshrinking in size despite the relatively high seafloor spreading ratesfound there. Trenches are seismically active areas that are best developedwhen associated with island arc systems where one plate is subductedunder an adjacent oceanic plate (Fig.2.3).Trenches are greater than 6 kmdeep and can extend to depths greater than 11km in the case of theChallenger Deep in the Mariana Trench. The relative proportions of theEarth' s surface at any given level may be shown by a hypsographic curve(Fig. 2.4b which should not be confused with its superficial resem-blance to the profile of a passive continental margin (Fig.2.2).The terms applied so far are physiographical whilst the ecological

depth zones associated with them are labelled:(i) sublittoral or subtidal- Low water mark to 0.2 km(ii) bathyal or archibenthal- 0.2 to 2 km(iii) abyssal- 2 to 6 km(iv) hadal or ultra-abyssal- >6 km.

This terminology will be used throughout this book for the generaldescription of depth zones. However, as we shall see, this depth zone

1 1 1 b o 100 2001

1 11 .. -- - - 11 11

11

01 1E Shelf /. : . : 1.. < : : break. . 2< I . Je l 3 ]Continental 1 : : :4 < I . JShelf . : :5 0U6 0 100 200 300

Vertical exaggeration x 417 Iii400 500 600 700111 Continental1 margin111

f f1000 1100

Verticalexaggeration x 50

Abyssal Plain800 900 1000 1100


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12 I. HISTORY, ENVIRONMENT AND METHODSFig. 2.3. Tectonic structureof ocean basin showingmovement (arrows) of

oceanic crust (ocr) overlyingthe mantle (ma), that is intum overlain by the rafted

continental crust (ccr) of thecontinent (co) along thepassive margin shown on

the right. The linear patternof the spreading centre atthe mid-ocean ridge (mor)may be offset by transformfaults (tf). The zone of crust

subduction found at thetrench (tr) along the activemargin on the left is shownbordered by the volcanoesand volcanic islands (vi)typical of the western

Pacifie,

Fg. 2.4. Distribution ofEarth' s surface lying at

different levels: a frequency distribution, b

cumulative-frequency curvebased on a termed theHypsographic Curve. Thisshould not be confusedwith the superficiallysimilar profile of the

continental margin showninFig.2.2.

ccr

terminology cannot be rigidly applied, and vertical zonation of fauna,which in the deep sea seems determined much more by a complex ofsometimes interacting ecologicalfactors thanby simple physical variablesassociated with the depth gradient, needs to be described in the deep seaby multivariate quantitative methods (see Chapter 9).

DEEP WATER MASSES AND THEIR FORMATIONln terms of topography, the deep sea starts at the edge of the continentalshelfbut in terms ofhydrography it isusually considered tobe that regionbelow the permanent thermocline (Fig. 2.5). This latter is the transitionlayer in the water column in which temperature falls rapidly withincreasing depth until values below 4C are reached and the downwardtemperature gradients become small. ln most of the world ocean thismore stable temperature regime is entered at 0.8 to 1.3km depth, exceptin the N. Atlantic where the injection of Mediterranean outflow atintermediate depths depresses the 4C isotherm to about 4 km. As

a 108km 420

bMaximum height (8.85 km)High mountainrangesMean heightof land (0.8km)

Continental shelf Sea level../

108642

km 0 ~ :::: . J24 ~-- ---=:::;6 Ir:--=-----81 L L ~__L L__

11 Mean depth1 of oceans (3.2km)

10 20 30 40Percentage of earthssurface

o 20 40 60 80Cumulative percentage


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PHYSICAL ENVIRONMENT 13

5 a 50 5

E 1 0.. r : :: 1 5~Q Je l 2 0

Fig. 2.5.Vertical 2 5 Hightemperature profiles for latitudesdifferent latitudinal zones. 3 0

Temperature, Ca 5 l a 15 20 25 a 5 l a 15 20

Seasanalthermacline --

Law latitudes

the upper waters are heated by solar radiation (in mid-latitudes witha seasonal cycle to form a more superficial seasonal thermocline),freshened by coastal runoff, and mixed by wind, the permanentthermocline isolates the deep ocean from the direct effect of surfaceparameters. Because of the much greater depth of the mixed layer as aresult of wind stress (particularly evident in the N.E. Atlantic), the deepwater mass below the permanent thermocline deepens at mid-latitudes,with a concomitant depression in isotherms away from the Equator.They shoal again at higher latitudes towards the areas of deep waterformation nearer the poles (Fig. 2.6). This has had a marked bearing oninterpreting the depth-related distributions of deep-sea animals in termsof temperature (see Chapter 9).However, the water below the permanent thermocline is not of con-

stant temperature and salinity. Most of the floor of the world's majoroceans is bathed in water masses which originally formed in either theAntarctic or in the Greenland/Norwegian Seas of the Arctic Ocean (Fig.2.7).For surface water tobecome dense enough for it to sink to the bottomof the ocean, it must become either more saline by evaporation and iceformation or colder by heat loss. The water then sinks to its new density

a ,,Fig. 2.6.North-South / ,1 /section of western Atlantic 1showing distribution of -, , ::---4---- - temperature 0C . This E 2 , - /illustra tes the sharp ..;L . - gradient in temperature .. r: : :~ 3near the Equator with a Q J

trend to submergence of e lisotherms towards the poles 4where the water massbecomes increasingly 5isothermal. (From Svedrup 70 60 50 40 30 20 l a a 10 20 30 40 50 60 ONet al. 1942. S


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14 1.HISTORY, ENVIRONMENT AND METHODS

Fig. 2.7.Main movementsof water masses in a North-

South section of theAtlantic Ocean, showingthe origin of dense, coldAntarctic BottomWater

(AABW)from the WeddellSea (left), which can be

traced as far as 40 N, andthe overlying cold, lessdense AntarcticIntermediate Water

(AAIW).North AtlanticDeep Water (NADW)

originates in the NorwegianSea (right) near Greenlandfrom sinking of a mixture ofdense surface water fromthe Gulf Stream with coldArctic water. Warm, denseMediterranean Water (M)intrudes from the E. fromthe Straits of Gibraltar. Asimilar northward-flowingpattern exists in the Pacifie,but deep flow from the N. is

only poorly developed.(FromTurekian, 1976.)

o

levelwhere it spreads out. To form the very deepest waters in the ocean,the surface layers must become very cold. It eventually returns to thesurface by gradual upwelling until, in time, it all returns to the surface asreturn flows from low to high latitudes.The deepest waters found in the ocean are formed in the cold surface

layers close to the coast ofAntarctica especially in the Weddell Seawherewinter-cooled surface water is exceptionally cold (-1.9 C). This water(Fig. 2.8) mixes with the upper part of the saline warm deep water(Circumpolar Water) to give a modified deep water that flows along thewestern shelf edge of theWeddell Seaalong the Palmer Peninsula, mixingwith Western Shelf Water. This proceeds to sink downslope as WeddellSeaBottomWater. Thiswater then mixeswith deeper saline waters of theCircumpolar Water to form Antarctic BottomWater (AABW)(Mantyla Reid, 1983).Antarctic Bottom Water is generally accepted as a generic term for

water masses of very similar characteristics -0.4 C: 34.660/00 which areformed in the Ross Sea and off the Adelie Coast and contribute to thisbottom water. AABW sinks to form a circumpolar bottom water withbranches penetrating all the main oceans (Fig. 2.7). ln the Atlantic thiswater flows up the west side at depths >5 km but is prevented fromdoing so on the east side by the Walvis Ridge. Sorne of this water willpenetrate the N.E.Atlantic by passing through the major fracture zones inthe Mid-Atlantic Ridge. Branches of AABWextend into the lndian andPacifieOceans basins unless its spread is impeded by ridges such as theEast PacifieRise.

Equator

E 2 - - - -.Y . -.d - - - - - -0..j) 30 45


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16 I. HISTORY, ENVIRONMENT AND METHODS

Fig. 2.9. Formation andtrack of dense, cold deepwater (NADW)in theNorwegian Sea andsubpolar N. Atlantic fromcooling and sinking

(denoted by curled ends) ofwarm (>4 C) surface

currents. Other, lighter,cold overflows are alsoshown. (FromMcCartney Talley, 1984.) ArnericanMeteorological Society,1984.

~ Surface currents~ : : . 0 : NADW overflowQther light cold overflow

Water from the first two sources entrains overlying Atlantic water to formNortheast Atlantic Deep Water. The core gradually descends the con-tours of the continental slope south of lceland and then those of theeastern flank of the Reykjanes Ridge, eventually escaping westwards andnorthwestwards into the N.W.Atlanticbasin at 1.5to 3 km depth throughthe Charlie Gibbs Fracture Zone at 53N (Fig. 2.1). There it joins theNorthwest Atlantic Bottom Water formed from the Denmark Straitoverflow, to produce the North Atlantic Deep Water (NADW), whichflows south along the eastern continental slope ofN. America at 1to 5 kmdepth (Rowe Menzies, 1968)in the Western Boundary Undercurrent.Although the NADW is more saline than AABWowing to the entrainedupper waters, it remains less dense because it is warmer. Consequentlywherever both water masses penetrate, NADW overlies the AABW.NADWspreads throughout the S.Atlantic and eastwards round S.Africa,and its high-salinity core may be traced into the northern Indian andPacifieOceans (Reid Lynn, 1971).Although these water masses (AABWand NADW) coyer a significant

part of the floor of the deep ocean there are other deep waters that batherestricted parts ofthe ocean floor. For example, the European Basin of theN.E. Atlantic receives only a little NADW through the Gibbs Fracture


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PHYSICAL ENVIRONMENT 17Zone, where the deep flowis predominantly westwards. Direct evidenceof AABW presence in the deepest parts of the basin is restricted toanomalously high silicate content (Mann, Coote Garner, 1973).North-east Atlantic Deep Water, derived from the easternmost Scotland-Greenland Ridge overflows, occurs where bottom depths are 2 3.5 km(Ellett Martin, 1973;Ellett Roberts, 1973;Ellett et al. 1986)and in theshallower northern areas Labrador Sea Water (LSW) of lesser densitycovers much of the seabed in the depth range 1.5 to 2 km (Lonsdale Hollister, 1979).LSWis formed by deep winter mixing south and west ofGreenland, and spreads eastwards at intermediate depths, where itsdensity is somewhat greater than the much more saline but warmerGibraltar, or less precisely 'Mediterranean' water (Cooper, 1952). Gibral-tar Outflow Water is formed by the subsurface Mediterranean outflowplunging into the Atlantie from the Gibraltar Sill.It spreads northwardsand westwards and blankets much of the continental slope west ofEurope. It reaches to depths of 2.45 km in the south (Meincke et al. 1975but to the west of the British Isles has amore restricted depth range of 0.8to 1.2 km.There areno major sources ofdeep water in the northern Pacifie.This is

because the surface waters in the N. Pacifie are of such low salinity thateven intense wintertime cooling does not increase their density enoughfor them to sink (Warren, 1981).ln the Mediterranean, the opposite situation creates conditions for

deep water formation. The surface water flowing into the Mediterraneanthrough the Straits of Gibraltar is relatively saline, it flows eastwardsalong the North Afriean coast with branches travelling northwards.Within the enclosed basin, evaporation is high so salinities increase. lnwinter, in the northern Aegean, Adriatie and Ligurian Seas, coldoffshorewinds cool the high salinity surface water, causing massive overturningand the formation ofvery dense deep and bottom water. However, this isnot the water mentioned above as providing a mid-water high salinitycore in the Atlantic; this is the Levantine Intermediate Water (Wst, 1961)formed with lesser density off the Turkish south coast in winter, andwhich flows west to the Gibraltar Sill at 0.2 to 0.6 km depth.The main ecological significance of these deep water masses liesnot in

their salinity/temperature characteristies but in that they areweIloxygen-ated. AlI these water masses form at the surface so that their oxygencontent is in equilibrium with the atmosphere. After they sink, althoughthe oxygen is slowly used up by metabolic processes, with the exceptionoflimited areas such as the oxygenminimum zone and anoxiebasins, e.g.the Black Sea, the supply of oxygen in the deep waters is sufficient tomaintain the surficial sediments of the world' s oceans in an oxidizedstate. ln the open ocean, oxygen concentration near the seabed decreasesnorthward in the Indian and Pacifie Oceans as these areas are the mostremote from the supply of oxygenated water at the main site of deepwater formation in the North Atlantic (Mantyla Reid, 1983).Beneathregions ofhigh production (e.g. eastern tropical Pacificand in the ArabianSea) oxygen minima formwhieh can lead to anoxie conditions in bathyal


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18 1.HISTORY, ENVIRONMENT AND METHODSsediments. ln sorne regions, such as the Mediterranean, black layers inthe sediments called sapropels are considered to have been derivedduring similar anoxie conditions during previous eras.

PHYSICAL PROPERTIES OF THE WATERS COVERINGTHE DEEP OCEAN FLOORThe main feature of the physical properties in the deep sea is that, withthe exception of hydrostatic pressure and current energy, these para-meters show avery narrow range at any specific site below the permanentthermocline. Unlike coastal waters, solar radiation has no direct ecologi-cal significance, as alilight (except bioluminescence), has disappeared by1 km depth. It does, however, have an indirect effectby being the energysource for surface phytoplankton production, sorne of which enters thedeep sea ecosystem via the food chain (see Chapter 11).TEMPERATURE AND SALINITYThe temperature of the waters of the deep sea varies from 4C to-1C (Svedrup et al. 1942).Exceptions are the Mediterranean which is c.13C between 0.6 and 4 km, the Red Sea where the bottom tem-perature can be 21.5C at 2 km depth, and the very high temperaturesin the immediate vicinity of hydrothermal plumes (see Chapter 15).The lowest temperatures found are -109C in the deep waters of theAntarctic.The sa linity is also relatively constant and below 2 km is close to34.80/00 0.30,declining to 34.650at the very deepest levels (Svedrup et

al. 1942;Menzies, 1965).OXGYEN CONCENTRATION

The oxygen values are near saturation except where the oxygen mini-mum layer, found at 0.5-0.6 km depth in the open ocean (Fig. 2.7),impinges on the upper continental slope, and in enclosed basins such asthe Black Sea which, below 250m, is anoxie and azoic. However, as thedeep water masses progress further from their site of origin, oxygen willbe consumed by metabolic processes, and water in the deep N. Pacifiehasa relatively low oxygen concentration of 3.6 ml 1- (Mantyla Reid,1983).There is sorne evidence (Bruun, 1957)that, immediately above thedeep-sea bed, there is a slight reduction (0.15ml 1- in oxygen concen-tration.HYDROSTATIC PRESSURE

The most predictable physical variable is hydrostatic pressure. Thisincreases by 1 atmosphere (1 bar or 105 pascals) per 10m increase indepth. This increase in pressure, particularly in relation to the lowtemperature, affects the rates of enzymatic catalysis in deep-sea organ-isms (Somero et al. 1983 .


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DEEP SEA BIOLOGY:A natural historyof organisms atthe deep sea floor

John D GageScottish Marine BiologicalAssociationDunstaffnage Marine LaboratoryOban, UK

Paul A TylerDepartment of OceanographyUniversity of SouthamptonSouthampton, UK

C M RIDGEUNIVERSITY PRESS

Kuliah Deep Sea 1

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