Zeigler 1964 Golfo de Venezuela MUY BUENA

8/13/2019 Zeigler 1964 Golfo de Venezuela MUY BUENA

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INIT3ODUCTIONThe Gulf of Venezuela is a large kid-ncy-shaped embayment along the north-ern coast of South America between Venc-zuela and Colombia. It is approximately200 km long on a line between Aruba andthe entrance to Lake Maracaibo and 84km wide between Espada Point and Para-guan& Waters from the open Caribbeanpenetrate the gulf over a broad shallowshelf ( Fig. 1). The Gulf of Venezuela isdivided into two parts, Calabozo Bay andthe Outer Gulf, by a submarine sill thatmarkedly influences the water movementsbetween the two basins. The basin of Cal-

abozo Bay is 9 m deeper than the deepestportion of the sill. The bottom of theOuter Gulf slopes toward the CaribbeanSea somewhat regularly from the coast ofFalc6n to the sudden deepening at theshelf edge. Four small islands off the LosMonges group in the northwestern part ofthe Outer Gulf are within the area of study.Field data presented in this paper (Ta-ble 1) were collected on two cruises of thel Contribution No. 1395 from the Woods HoleOceanographic Institution, Woods Hole, Massa-chusctts.

THE HYDROGRAPHY AND SEDIMENTS OF THE GULFOF VENEZUELA1John M. Zeigler

Woods Hole Oceanographic Institution, Woods Hole, MassachusettsABSTR4C1

It is the object of this paper to examine the sediments of the Gulf of Vcnezucla andto relate them to the hydrography. The hydrography is, therefore, described in somedetail. New data support the general conclusions of Rcdfield (1955) that the Gulf ofVcnezucla and Calabozo Bay opcratc as two estuarine cells driven by the trade winds andby outflow from Lake Maracaiho. New data also show that water enters the Outer Gulffrom the northwest and flows at mid-depths towards the southeast in response to upwell-ing taking place off the coast of Paraguanh.Sediment parameters that reflect circulation best arc shells of pelagic foraminifera, sanddistribution, and silt clay distribution. Of unknown importance is the distribution ofresistant fecal pellets and the distribution of carbonate (shell material), The distribut ionof foraminifera indicated that water does not enter Calabozo Bay over the central part ofthe sill but rather a long both coasts. The silt clay distribut ion reflects an outflow ofwater from Calabozo Bay into the Outer Gulf over the sill, a flow that had been first seenin the hydrographic stations.Rainfall data showed the great dependence of water character in the Gulf of Vcnczuelaon rainfall and outflow from Lake Maracaibo. Hydrographic stations made in 1958, fol-lowing a protracted dry period showed differcnccs in salinity of l-3%, between stationsoccupied in 1954 that wcrc taken following 2 months of heavy rainfall,

RV Atlantis (November 1958 and Febru-ary 1960). Eight days were spent collect-ing bottom samples, hydrographic data,plankton tows, fish tows, and water sam-ples for nutrient and geochemical studies.The position of the hydrographic and sedi-ment stations are shown in Fig. 1. Thehydrographic stations are numbered, the

FIG. 1. Hydrographic stations, Gulf of Ventzuela. Bathymetric contours in meters.397


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398 JOHN M. ZEIGLERTABLE 1. Elydrographic stations in the Gulf of Venezuela, November 1958

Stationnumber LocationLatitude LongituclrSampled;jP:m

Tcmpcr-nturecc>Oxygen(ml/liter)

5700 12 lO' N 70 20 W 55

5701 ll 60' N 70 20' W 35

5702 ll 50' N 70 20 W 42

5703 ll 50' N 70 25' W 55

5698 12 14' N 7O lO' W 33, 01020305699 12 20' N 70 09' W 44 1102030401102030405011020301102030401102#03040505704 ll 50' N 70 28' W 56 110203040505705 ll 58' N 70 26' W 64 11020304050605706 12 20' N 70 30' W 71 11020304050705707 12 20' N 70 40' W 78 1;2030405070

25.9325.9225.6324.5226.2226.2126.1925.9725.0127.1627.1525.0323.7023.1223.0327.1127.0825.4824.7627.2427.2127.1424.9123.8427.4327.3227.2126.4223.9823.8027.4327.2327.1924.8323.9523.4327.2227.2225.9324.3923.3923.2723.3127.7227.2327.2325.5023.6122.4821.9226.9226.9726.8324.8723.3422.9622.77

36.59036.59436.59836.64036.51536.51836.52636.54936.61936.54836.54836.59836.70636.73736.73936.58436.58236.58936.61336.58436.58336.57836.60036.69036.57836.57136.56836.56736.68536.69536.563536.56336.56636.64036.70036,.71436.57436.57536.54736.64436.71836.73436.70136.55236.54536.59836.69736.71736.74636.55636.56036.55136.61036.69936.71636.714

4.174.174.033.284.214.314.294.103.78,4.314.374.263.693.152.974.274.324.052.954.754.344.272.972.794.274.324.264.243.272.844.264.274.214.163.562.974.324.374.324.193.263.152.864.324.324.324.284.053.443.194.374.354.414.083.513.373.29


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3,99YDROGRAPHY AND SEDIMENTS OF TIIE GULF OF VENEZUELATABLE 1. Continued

Stationnumber LocationLatitude LongitudeWateryz$

SamplecB

Tcmpcr-ature(Cl5708 12OO N 7040 W

5709 1151 N 7040 W

5710 ll50 N 7050 W

5711 1235 N 7050 W

5712 1235 N 71OO W

5713 1220 N 71OO W

5714 12OO N 71OO W

57155716

57175718

5719

5719A ll30 N 7134 W

20

ll50 N 71005 W 12ll40 N 7120 W 25

1120 N 7150 W 16ll20 N 7140 W 24

ll30 N 7140 W 24

24

65

56

51

78

75

43

1102030405060110203040551102030405011530456075115

3045607011020

215121

101102011211020110221

1022

27.20 36.550 4.3227.20 36.601 4.2827.04 36.544 4.3124.74 36.655 4.2023.65 36.687 3.9123.05 36.786 3.1622.81 36.735 3.1927.32 36.588 4.2127.21 36.593 4.3027.19 36.578 4.2825.09 36.593 4.0324.15 36.652 3.1123.84 36.688 2.9527.86 36.352 4.4127.62 36.363 4.5027.08 36.547 4.4025.02 36.~87 4.1424.64 36.636 3.5824.54 36.618 3.2427.22 36.481 4.2427.20 36.494 4.2926.97 36.582 4.2724.37 36.652 3.6-i23.12 36.729 3.4323.12 36.728 3.4027.12 36.558 4.2727.13 36.554 4.3026.95 36.557 4.3026.43 36.620 4.5023.61 368.709 3.6223.40 36.711 3.2927.32 36.458 4.3227.32 36.460 4.5925.23 36.601 4.6224.19 36m.666 3.7324.11 36.670 3.6927.25 36.566 4.3227.26 36.574 4.2727.28 36.562 4.3028.29 32.912 4.4328.09 36.280 4.3228.60 32.932 4.5028.47 32.9 10 4.4828.34 33.227 3.8127.85 27.012 4.5128.34 31.990 3.7528.03 30.684 4.4728.19 30.843 4.5728.26 33.449 4.1828.19 32.410 4.2128.19 32.410 4.2128.33 33.639 3.7028.21 32.883 4.5428.22 32.897 4.5028.25 34.026, 3.97


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400 JOHN M. ZEIGLERTABLE 1. Continued

StationnumberLocation water Sample

Latitude Longitude w m d(%tpTempcr-ature(Cl

Oxygen(ml/liter)

5720 ll 30' N 71 30' w 2357215722

ll O6' N 71 20' Wll 20' N 71 20' W

917

5723 ll 20' N 71 OO' w 175724 ll 40 N 71 00' w 1757255726

ll 20' Nll 30' N

70 40' w70 40' w

2,l30

5727 ll 40' N 70 40' w 36

5728 ll 40' N 70 26' W 43

5729 ll 30' N 70 30' W 30

57305731

ll 30' N 70 20' W 30ll 40 N 70 20 W 42

1 28.03 28.6rll 4.62,,5 28.04 28.667 4.6410 28.06 28.662 4.641 28.34 34.743 4.487 28.14 34.876 4.641 28.18 33.857 4.466 28.20 33.842 4.4813 28.16 34.054 4.271 28.34 34.456 4.277 28.35 34.453 4.2814 28.32 34.440 4.2871 28.14 33.374 4.245 28.11 33.382 4.2'113 28.08 34.352 3.921 26.70 36.627 4.098 26.60 36.6286, 4.081 26.95 36.595 4.2414 26.97 36.599 4.2728 26.95 36.601 4.233 27.02 36.605 4.2313 27.01 36.603 4.2123 27.00 36.599, 4.1633 26.70 36.599 3.62

1 26.96 36.599 4.2710 27.00 36.599 4.2820 27.02 36.593 4.2730 24.21 36.663 3.8940 24.01 36.684 3.221 26.62 368.607 4.2110 26.63 36.609 4.2020 26.56 36.605 4.0928 26.29 36.603 3.783 24.87 36.648 3.5416 24.87 36m.640 3.5829 24.76 36.638 3.361 26.82 36.606 4.2314 26.83 36.610 4.2427 23.85 36.695 2.7940 23.73 36.705 3.22

sediment stations are not. All hydro-graphic stations, including some made in1954 by the Atlantis, were occupied dur-ing the windy season. Previous writings,summarized by Murphy ( 1936)) show thatthis part of South America has a wet anda dry season and that the strength of thetrade winds that are a primary drivingforce of surface water is also seasonal. Theoutflow from Lake Maracaibo also fluctu-ates from year to year ( Carter 1955).Therefore, until hydrographic surveys are

made in nonwindy seasons the existinglimited hydrographic descriptions will beincomplete.ACKNOWLEDGMENTS

The assistance of the officers and crewof the RV Atlantis and of the authors col-leagues is gratefully acknowledged. Thestudy would not have been possible with-out the encouragement and cooperationof the Republic of Venezuela throughits representative Dr. Ramon Perez Mena


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HYDROGRAPHY AND SEDIMENTS OF THE GULF OF VENEZUELA 401of the Ministry of Mines and Hydrocar-bons, I want to thank the Creole Pctro-leum Corporation for its hospitality andinterest, particularly to Dr. Parke Dickeyand Mr. L. A. Earlston Doe. Dr. Dickeywas also kind enough to read the manu-script.Dr. A. C. Redfield gave generously ofhis time to reading and interpreting thismanuscript. His assistance is gratefullyacknowledged. The writer wishes to thankMrs. Barbara Gill and Mr. Carlyle Hayesfor laboratory analyses and Professor K. 0.Emery for his valuable suggestions regard-ing the manuscript.

The cruises and subsequent work werefinanced by the Geophysics Branch of theOffice of Naval Research under ContractNonr 2196( 00)-NRO 83004.IIYDROGRAPIIY

A description of the hydrography of theGulf of Venezuela, based on a surveymade in December 1954, has been pub-lished by Redfield (1955). Our observa-tions made in November 1958 generallyconfirm his conclusions but are somewhatmore detailed.The circulation of the gulf appears to beinfluenced by the wind, the introductionof freshwater from Lake Maracaibo, andthe submarine sill. Both s~u~ys weremade during the period of strong north-east trade winds. The outflow of fresh-water from the lake was much greater in1954 than in 1958 and resulted in lowersalinities and some differences in the pat-tern of circulation,The circulation of the Gulf of Venezuelaconsists of two estuarine cells delimited bythe submarine sill that separates CalabozoBay from the Outer Gulf. Freshwater fromLake Maracaibo dilutes the water of Cala-bozo Bay, particularly in the upper layers.Over the sill and on its eastward slope,where the surface waters of Calabozo Baymix with those of the Outer Gulf, there isa marked change in salinity (Figs. 2 and3). The distribution of salinity at the sur-face ( Fig. 3) indicates the position of thistransition zone.

NE

FIG. 2. Cross section A-A, Gulf of Venezuela,showing vertical distributions of tcmpcrature, sa-linity, and oxygen.The Maracaibo outflow appears as atongue of low-salinity water that projectstoward the center of Calabozo Bay,flanked on either side by water of greatersalinity. The distribution of the isohalinessuggests a westward indraft of saline wa-ter along the coasts, with a compensatingescape of water across the central part ofthe sill. This distribution is somewhat dif-ferent from that observed by Redfield in1954. In 1954, the volume: of water de-rived from Lake Maracaibo was muchgreater, as evidenced by the lower salini-ties of the bay water. The outflow ap-pears to have spread northwestward alongthe coast and to have suppressed the in-flow along the Guajira coast, thus distort-ing the salinity pattern without changingits fundamental character.The hydrography of the Outer Gulf ap-pears to be dominated by upwelling that

occurs off the western coast of the penin-sula of Paraguana as the result of the


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402 JOHN M. ZEIGLER72 710 700

. . . .*.

. .

. . . . . .. .

. . 3.:.,.L

Frc. 3. Distribution of temperature and salin-ity 1 m below the sdrfacc, Gulf of Vcnezucla.

northeast trade winds (Fig. 4). The net-work of stations occupied in 1958 permitsthe description of this phenomenon insome detail. Fig. 5 shows the distributionof salinity, temperature, and oxygen alonga section that approaches the peninsulafrom the northwest, then turns to thesouth and west parallel to the coast. Theseisolines all slope upward in a similar pat-tern. The isolines for 36X$& salinity, 26C,and 4 ml/liter of oxygen all rise from adepth of about 50 m at the continentalslope to 20 m as the coast is approached.Thev remain at this depth along the Para-

guanj coast but rise to the surface in theoffing of the Gulf of Coro.The horizontal extent of the upwardwater movements illustrated in this sectionis shown in Fig. 6, in which the contoursshow the depths of specified isolines. Thecontours for the 24C isotherm and the36.7& isohalinc (Fig. 6A,B) suggest thatthe indraft of dee,p water moves in asoutheasterly direction from offshore.Such water does not upwell to the seasurface. The contours for the 26C isother-mal surface and the 36.6%0 salinity surfacebreak the surface off the Gulf of Coro( Fig. 61),E). Water of this salinity ex-tends westward into shallow water alongthe coast of Falcbn and northwesterlywhere it appears to overflow less salinewater lying at a greater depth (Fig. 6E).The temperature of the upwelled water

TEMPERATURE

FIG. 4. Cross section B-B showing upwellingnear the coast of Paragud.


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404 JOHN M. ZEIGLER

-TEMPERATURE, SALINITY AND OXYGEN ISOTHERMS-BOTTOM CONTOURS IN METERS

FIG 6. Charts showing the depths of specified isolines of temperature, salinity, and oxygen overthe Gulf of Venezuela.m3. Of this, about two-thirds of the fresh-water difference is found in Calabozo Bayand the remainder in the Outer Gulf.Data on the outflow of Lake Maracaibomake it clear that the differences in thesalinity of the gulf are related to variationsin rainfall in the Maracaibo Basin, Thesedata were supplied by the Creole Petro-leum Corporation and are based on esti-mates, made in accordance with the pro-cedure of Carter ( 1955)) of the monthlyrainfall corrected for evaporation. Theestimated outflows for the years 1952through 1958 are shown in Fig. 8. Theoutflows vary greatly from month tomonth and frequently arc negative whenevaporation exceeds precipitation. Thiscondition occurs especially during winter,the dry season. The survey of DecemberI954 was made after 2 months of excep-

tionally high rainfall following 2 yearsof average precipitation. In contrast, thesurvey of November 1958 followed a pro-tracted period when the outflow was smalland frequently negative, that is, when wa-ter flowed from the gulf into Lake Mara-caibo. When the data for the 12 monthsimmediately preceding the surveys areadded together, the annual exchange is asfollows:

Exchange 1954 1958Outflow positive 1.78 x 10 mx 0.18 x 10 m3Outflow negative( inflow ) 0.45 x lOlo rn 0.84 x 10 m3Net outflow 1.33 X 10 m -0.66 X 10 mVery little of the lake water returns to thelake during periods of inflow, when theinflow is composed of gulf water that isonly slightly diluted. Consequently, thepositive outflows give the best indication


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HYDROGRAPHY AND SEDIMENTS OF THE GULF OF VENEZUELA 405of the effect of lake water in diluting the 023 24 25 26 27C 5 16 37x.s 44 m///qergulf for the two periods. . IXIARUEACHANNELThe difference in the volume of outflow T10 ,.A 1 ii T I\ Iin 1954 and 1958 is 1.6 x UYO m3. This isabout the same as the estimated differencein the water content of the gulf. It isprobable that the residence time of waterin the gulf is not more than 1 or 2 years.Of this, about two-thirds of the time isspent in Calabozo Bay and one-third inthe Outer Gulf.

SEDIMENTSBottom sediment in the Gulf of Vene-zuela is composed of three principal corn--ponents: terrigenous silt clay, terrigenous

sand, and shells ( carbonate).Tedgenous sand

Terrigenous sand is defined as grains ofterrigenous minerals that did not pass a 8 40screen with hole size of 0.062 mm. The $ 5.sand is composed mostly of quartz butincludes minor amounts of heavy mineralsand feldspars. Five areas where sand ismore than a trace can be seen (Fig. 9).The large sand area in the southeast cornerof the gulf is probably a relict deposit thatwashed through the Gulf of Coro beforethe strait was closed on the east by thetombolo connecting Paraguana to the 20mainland. Bits of broken barnacles, cal-careous 1gae, bryozoans, and heavyshelled mollusks are mixed with sand.These animal remains were probablywashed from the shallow Gulf of Coro atthe same time.Sand in the central part of the gulf, offEspada Point, was probably eroded fromsandy outcrops on the sea floor, One ofthe samples contained many oolitcs andbits of massive limestone. Limestoneshave been examined in formations exposedon shore by the author and reported byRenz ( 1956)) and since neither oolites norsand were found between the offshoresandy area and the coast, it is assumedthat some of the coastal formationscropped out on the sea floor and wereeroded when sea level was lower and wa-ter turbulence greater. Present depth ofwater in this sandy area is 60-65 m.

Frc. 7. Comparison of tcmpcraturc, salinity,and oxygen at stations occupied in 1054 and in1958.

Sand near the Los Monges Islands prob-ably was derived from those islands orfrom outcrops now below the sea. Rocksof the islands, as reported by Carmonaand Bellizzia ( 1952, p, 6, S), are igneousor metamorphic hornblendites and aplitesthat contain quartz stringers. How fast the


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406 JOHN M. ZEIGLER

+ 10

+8

+6

Frr,. 8,

52 53 54 55 56 57 58YEAR

, Estimated outflow from Lake Maracaibo into Calabozo Bay 1952-59 and dates ographic surveys.present islands are being eroded or howmuch quartz is being supplied to the seais not known. It is surprising that nostriking increase could be seen in theamounts or kinds of heavy minerals pres-ent in the sediments near the islands. Pos-sibly the recent rate of erosion does notsupply large amounts of material to thesea.

Sand eroded from the Guajira and Fal-con coasts is being moved westward nearthe coast by waves driven before the tradewinds. The result is that sand is accumu-lating along the western end of CalabozoBay. Fine sand, which is present in smallamounts all over the Gulf of Venezuela, isprobably carried from the land by wind.Silt clay

The term silt clay includes all materialssmaller than 0.062 mm and not soluble inacid. A blanket of fine material dominates

f 1

the sediment in Calabozo Bay and extendsoutward into the Outer Gulf (Fig. 10).Most of the fine material is produced byerosion along the Guajira or Falcon coastsor is carried in suspension in the outflowfrom Lake Maracaibo. Nearshore circula-tion under the influence of trade windsdrives the muddy coastal water into Cala-bozo Bay, which is a natural topographiccollecting basin. Most of the fine materialsettles out before the water enters the cir-culation of the Outer Gulf. However, themuddy water that escapes from CalabozoBay does so across the center of the ridge,and a tongue of silt and clay, outlined bythe 60% contour, concides closely with thezone of escape. That the less saline waterescaping from Calabozo Bay is taking thisroute is also indicated by Fig. 6D, whichshows the 26C isotherm greatly depressedby the outflowing water over a tonguc-shaped area.


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HYDROGRAPIIY AND SEDIMENTS OF TIIE GULF OF VENEZUELA 407

WEIGHT PER CENTt ERRIGENOUS SANDCONTOUR INTERVAL .0,IO PER CENT

72O 71 70FIG. 9. Distribution of tcrrigcnous sand, Gulfof Venczueln.

Carbonate (shells)Broken and whole shells form a majorsedimentary constituent in the outer partof the Gulf of Venezuela. Inasmuch as allof the carbonate particles readily visibleunder low magnification were shell frag-

ments except for the one sample that con-tained oolites and broken limcstoncs, itwas concluded that a map of the carbon-ate content of the sediment would repre-sent a reasonable approximation to shellcontent. Samples were analyzed for ap-proximate carbonate content by acidifica-tion and back-titration ( Jackson 1958).The carbonate content is generally lowover Calabozo Bay and most of the south-ern part of the Gulf of Venezuela, exceptfor two areas off the Gulf of Coro; thecarbonate content increases abruptly tothe north, and one can describe the sedi-ments of the northern part of the Gulf ofVenezuela as muddy or sandy shell dc-posits.The shelly areas can be distinguishedsomewhat in terms of dominant character-istics; for example, the area around LosMonges contains many algal bank-type orreef-type carbonate remains such as bryo-zoans or small corals. Sediments along thedeeper outer edge of the Gulf of Venczu-ela, and particularly near Aruba, are dom-

72 710

FIG. 10. Distribution of terrigenous silt clay,I

Gulf of Vcnczuela.inantly pelagic, that is, broken and wholeshells of globigerina and swimming mol-lusks plus other foraminifera. The morecentral and eastern shelly areas are char-acterized by mixtures of heavy shelled,badly worn (often only hinge lines re-maining) mollusk shells, and foraminifera,both pelagic and agglutinated. The inneredge of the carbonate deposits is domi-nated by pelagic foraminifera and thinfragile shell fragments. Parker and Curray( 1956) and Curray (1960) found shell de-posits in deeper waters in the Gulf ofMexico that were composed, at least inpart, of the shells of nearshorc organisms.Among other things, these authors usedthe occurrence of the shallow-water formsas evidence that some of the shell depositswere, in reality, relict shorelines, The rel-atively narrow transition zone in which theshell content increases rapidly from 20 to40% in the Gulf of Venezuda suggeststhat this zone may also rcprescnt a relictshoreline similar to those in the Gulf ofMexico; however, R. II. Parker (personalcommunication ) failed to find distinctbeach or lagunal deposits (based on shells)in 17 samples selected from the zone ofincreasing shell content, although he didfind shallow-water shells in some of thesamples. Inasmuch as the entire Gulf of


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408 JOHN M

WEIGHT PER CENTFECAL PELLETS

. . . .12

72 71 70

J?K. 11. Distrihtion of resistant fecal pellets,


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HYDROGRAPHY AND SEDIMENTS OF THE GULF OF VENEZUELA 409hydrography indicates that the bottomcurrents are probably weak. Perhaps thepellets represent the range of some as yetunidentified animal, or they may indicatean environment that is favorable to glau-conitc formation. The close relationshipbetween glauconite formation and organicmatter has been pointed out by Takahashiand Yagi (1929) and by Takahashi (1955),who also believe that an oxygen-low cnvi-ronment is conducive to glauconite forma-tion. Perhaps the fecal pellet distributionthercforc represents the distribution of re-ducing conditions. Pyritized tests of foram-inifcra were collected from core samplesin Calabozo Bay. Pyrite was present fromnear the surface of the cores to 3 m belowthe surface, indicating the prescncc of areducing environment at least below thebottom-water interface. Our hydrographicstudies revealed low concentrations of ox-ygen in Calabozo Bay. Redfield reportedeven lower values at a time when the out-flow of lake water was greater. It is pos-sible that during the summer wet season,when the outflow of the lake is cvcngreater, organic production is higher, andwinds are lighter, the water over the bot-tom may become anoxic. A period of calmcoupled with an increase in organic detri-tus from lake discharge would probablyuse up the already small amounts of oxy-gen present. Production of organic matterin Lake Maracaibo is among the highestin the world, and the outflow from thelake through Tablazo Bay is directly intoCalabozo Bay, the main locus of the pel-lets. The physical setting, therefore, leadsone to believe that oxygen-poor waters arethe norm and anoxic bottom water is notitipossible. The present study cannot es-tablish validity for the idea that partiallyglauconitized fecal pellets represent oxy-gen-low environments, but it does presentsupporting evidence. Coincidentally, VanAndel and Postma (1954) reported thelargest concentrations of fecal pellets inthe Gulf of Paria near Trinidad to be inthe Guiria Trough, a basin within thegulf near the Venezuelan coast, No hydro-graphic data for the trough were given.

I

71 70PELAGIC FORAMINIFERA

72 71 700

FIG. 12. Numbers of pelagic foraminifera pergram of dry scdimcnt, Gulf of Vcnezucla.Foraminifera

Shells of dead benthonic and pelagicforaminifera constitute a visible and im-portant part of the sediment of the Gulfof Venezuela. The various species fromthese samples were identified by Bermu-dcz ( 1960). During routine examinationof sediment constituents, it became appar-ent that benthonic foraminifera occurredall over the Gulf of Venezuela, includingCalabozo Bay, but pelagic species wereentirely missing from the greater part ofCalabozo Bay ( Fig. 12).Pelagic foraminifera drift with the cur-rents, and their preferred habitat seems tobe the open ocean seaward of the conti-nental shelves, although they probablyalso live and reproduce to some extentover the more seaward parts of the broadshelves. In a sense, therefore, their shellsrepresent microscopic drift bottles andunder some conditions might be useful totrack oceanic currents that invade near-shore waters or open bays such as the Gulfof Venezuela. This hypothesis was testedby counting the number of shells of pe-lagic species in dried sediment samplesfrom the Gulf of Venezuela ( Fig. 12). Inspite of the numerous errors that can af-fect the count, such as errors in sampling,


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410 JOHN M. ZEIGLER

FIG. 13. Circulation in the Gulf of Venczucla.Heavy arrows represent movement of clccper wa-ter, light arrows upper water layers.splitting, or counting, the overall distribu-tion is thought to be significant. Not onlyare foraminifera scarce in Calabozo Bay(entirely absent from 23 stations in thecentral part of the bay), but those fewthat are present are confined to two bandsof water adjacent to the two coasts. Theyare present in small numbers over the sub-marine sill, but their numbers increase oneor two orders of magnitude in a short dis-tance seaward, while they disappear alto-gether in going in the opposite directionto the central basin of Calabozo Bay. Thewriter interprets this to mean that verylittle open gulf water enters Calabozo Baydirectly across the sill, but rather thatwater from the open gulf enters the bayalong both coasts and water from Cala-bozo Bay moves seaward over the sill.This circulation must be reasonably per-mancnt, because the shell counts representsamples of the bottom that required yearsto deposit. Counts from cores taken inCalabozo Bay show that the scarcity ofpelagic foraminifera persists some metersbelow the present sea bottom at the placeswhere cores were taken.

SUMMARYThe distribution of sediments has beenshown to be related to the general hydro-

graphic features of the Gulf of Venezuela.Thus, terrigenous sand predominates alongthe coasts of Calabozo Bay where waveaction provides its source and wind-drivencurrents its distribution, while terrigenoussilt clay underlies the central part of thebasin. In contrast, shell fragments makeup the major part of the bottom depositsin the deeper waters of the Outer Gulf.Pelagic species of foraminifera are limitedalmost wholly to the Outer Gulf and pene-tratc the warmer, less saline, and more tur-bid waters of Calabozo Bay only in smallnumbers, being carried by wind-drivencurrents along the coasts. In some areaswhere hydrographic data are lacking, thedistribution of sediments, and particularlyof the pelagic foraminifera, may bc usedto supplement details of the circulation.In Fig. 13, the general character of thecirculation of the Gulf of Venezuela, asdeduced from the hydrography and sedi-ment distribution, is shown diagrammat-ically.Water draining from Lake Maracaiboacross Calabozo Bay must remain more inthe basin than close to the coast, eventhough the tongue of fresher water comingout of the lake will be shifted more to thewest and north in times of strong tradewinds. Inflow of saline water into the bayis confined to the two coasts, while dis-charge from the bay is across the centralpart of the sill. The primary inflow ofwater into the Gulf of Venezuela is in thedeeper layers from northwest to southeast.This water, in turn, upwells along thecoast of ParaguanA. Wind drift in theshallow layers is an important part of themechanism, but this drift does not pene-trate far into Calabozo Bay, turning in-stead toward the coasts or toward the opennorthwest part of the gulf. Circulation inCalabozo Bay is weak, and water spendstwice as much time there as in the OuterGulf, this number being based on compari-son of hydrographic stations taken in 1954and 1958.

REFERENCESI~EILMUL)EL, I. J. 1900. Foraminifcras plancto-nicos clcl Golfo de Venczucla. Man. Sot. Ci-enc. Nat. La Salle, 20( 55).


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IIYDROGRAPIIY AND SEDIMENTS OF TIIE GULF OF VENEZUELA 411CAIWONA, C. L., AND A. BELLIZZIA. 1952. Re-conocimicnto Gcol6gico dcl Flanco nor-ori-cntnl de la Sicrre dc Pcrijk e Isln do Toas,Estado Zulia y dcl Archipi&go dc Los Mon-jes, depcndcncia Federal. 18 p.CAIITM, D. B. 1955. The water balance of theLake Maracaibo Basin during 1946-1953.Drcxel Inst. Tcchnol., Lab. Climatol., Publ.Climatol., 3 : 209-227.CURRAY, J. R. 1960. Scdimcnts and history ofHolocene transgression, continental shelf,northwest Gulf of Mexico, p. 221-266. InF. P. Shepard and others, [eds.], Rcccnt scd-imcn ts, northwest Gulf of Mexico. Am. As-soc. Petrol. Geologists, Tulsa, Oklahoma.-. 1961. Late Quatcrnary sea lcvcl: acliscussion. Bull. Gcol. Sot. Am., 72: 1707-1712.A~OORE, D. G. 1955. Rate of deposition shown

by relative abundance of foraminifera. Bull.Am. Assoc. Petrol. Geologists, 39: 1594-1600.MURPIIY, 13. C. 1936. Oceanic birds of SouthAmerica, v. 1. Am. Museum Nat. Hist.,New York. 640 p.PARKER, R. II., AND J. R. CUWAY. 1956. Faunaand bathymetry of banks on continental

shelf, northwest Gulf of Mexico. Bull. Am.Assoc. Petrol. Geologists, 410 2428-2439:REDFIELD, A. C. 1955. The hyclrography of theGulf of Venezuela. Papers Marinc Biol.Occnnog. Deep-Sea Rcs., 3: (Suppl. ) 115-133.RENA, 0. 1956. Crctaccous in wcstcrn Venczu-ela ancl the Guajira ( Colombia ). ( Unpub-lishcd paper prescntcd at the 20th Intern.Gcol. Congr. Mexico City, 1956. )

TAKAHASIII, JUN-ICIII. 1955. Synopsis of glau-conitization, p. 503-512. In P. Trask, [cd.],Rcccnt marine sediments. Am. Assoc. Pe-trol. Geologists, Tulsa, Oklahoma.AND T. YAGI. 1929. Peculiar mucl-g&ins and their relation to the origin ofglauconite. Econ. Gcol., 241: 838-852,.

VAN ANDEL, T., AND H. POSTMA. 1954. Rcccntscdimcnts of the Gulf of Paris. Vcrhandcl.Koninkl. Ned. Akad. Wetenschap. Afdcl. Na-tuurk. Sect. I, 20. 244 p.ZEIGLER, J. M. 1959. Scdimcntary cnviron-mcnts on the continental shelf of northernSouth America. Preprints, Intern. Occanog.Congr., New York. AAAS, Washington, D.C.p. 670.

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Zeigler 1964 Golfo de Venezuela MUY BUENA