Pacific Science (1973), Vol. 27, No.3, p. 290-303Printed in Great Britain

Calcium Carbonate and Gross-Size Analysis of SurfaceSediments, Western Equatorial Pacific I

MARK]. VALENCIA 2

ABSTRACT: Surface-sediment samples taken from the tops of 47 free-fall, trigger­weight, or piston cores from an area in the western equatorial Pacific (long 1550 E­1750 E, lat 100 N-10° S) were separated into three size-fractions « 44 p" 44-246 p"

> 246 p,), and the calcium carbonate content of the total sample and of each size­fraction was determined. Subaerial volcanic dilution from the direction of theSolomon Islands prompted exclusion of some samples from carbonate and size­fraction profiles. An abrupt decrease in carbonate content in the western equatorialPacific occurs at 3,500 m, whereas the compensation depth is found at 5,250 m.Comparisons of previous works and examination of the present data prompt theassertion that, under specified conditions, the sedimentary lysocline may be ap­proximated by the slope-break in plots of carbonate content versus depth. A strongpositive correlation (0.92, P < 0.001) of the < 44-p, fraction with depth suggeststhat anomalous values for this weight-fraction may be useful in delineating dis­placed surface sediments in the area studied.

MATERIALS AND METHODS

This work establishes for the western equa­torial Pacific the approximate depth of theinitial abrupt decrease in carbonate content andthat of carbonate compensation for the totalsediment and for three size-fractions « 44 p"

44-246 p" > 246 p,). In addition, the relation­ships between size-fraction, carbonate content,foraminiferal solution indices (Berger 1968),and water depth are explored.

Surface-sediment samples were extractedfrom the tops of 47 free-fall, trigger-weight, orpiston cores obtained by Hawaii Institute ofGeophysics personnel on cruises of the R.V.Mahi over a 3-year period (Fig. 1). The coretops were all of Quaternary age (]. Resig andV. Buyannanonth, personal communication),although the top few centimeters or more maynot have been collected.

Samples were separated by U.S. standardsieves (no. 60, 246 p,; no. 325, 44 p,) into threesize-fractions and the weight of each fractionrecorded. Weight-percent calcium carbonatewas determined for the total sample and foreach of the size-fractions by Hiilsemann's (1966)

290

MURRAY AND RENARD (1891) first noted that anegative relationship exists between calciumcarbonate content in pelagic sediments andwater depth. This relationship has since beenmore specifically documented and discussed byRevelle (1944), Bramlette (1961), Turekian(1964, 1965), Lisitzin (1970), and others. Recentinvestigations involving profiles of calciumcarbonate solution in the water column (Peter­son 1966, Berger 1967) and detailed analyses ofsolution effects on the foraminiferal componentof pelagic sediments (Ruddiman and Heezen1967; Berger 1968, 1970; Kennett 1966), to­gether with a reevaluation of previous results(Heath and Culberson 1970, Broecker et al.1968, Broecker 1971), have led to the definitionof five important conceptual horizons in pro­files of calcium carbonate distribution withdepth in the ocean; i.e., calcium carbonatecompensation depth, foraminiferal compensa­tion depth, critical depth, lysocline, and depthof calcium carbonate saturation (Table 1).

I Hawaii Institute of Geophysics contribution no.529. Manuscript received 12 January 1973.

2 Hawaii Institute of Geophysics, University ofHawaii, Honolulu, Hawaii 96822, U.S.A. Presentaddress: School of Physics and Mathematics, UniversitiSains Malaysia, Pulau Pinallg, Malaysia.

Surface Sediments, Western Equatorial Pacific-VALENCIA

TABLE 1

VARIOUS CONCEPTS OF CALCIUM CARBONATE DEPTH-SOLUTION BOUNDARIESAND THE DEPTH OF OCCURRENCE IN THE EQUATORIAL PACIFIC

291

HORIZON

Compensation Depth

Planktonic ForaminiferalCompensation Depth

Critical Depth

Lysocline

Depth of Calcium CarbonateSaturation in Water Column

DEFINITION

Depth at which the rate of calcite supplyequals the rate of dissolution (Bram­lette 1961)

Depth at which whole tests of plank­tonic foraminifera are absent (Parkerand Berger 1971)

Depth at which the sediment contains10 percent calcium carbonate byweight (Lisitzin 1970)

Depth of maximum rate of change inBerger's (1968) solution index

Depth at which the ratio of ion productCa++ COa- and the concentrationproduct of solubility equals 1(Lyakhin 1968)

DEPTH:EQUATORIAL

PACIFIC(km)

4.7(Bramlette 1961)

5.0(Parker andBerger 1971­

fig.,14a)

5:5(Lisitzin 1970,

fig. 17)

3.8(Parker andBerger 1971,

fig. 140)

0.5(Lyakhin 1968,figs. 5, 6-4)

DEPTH:WESTERN

EQUATORIALPACIFIC

(km)

5.0-5.25(Lisitzin 1970,

fig. 20)

3.75-4.0(Berger 1968,

fig. 15)

0.5;1.0-2.5

(Lyakhin 1968,fig.6-4b)

rapid gas volumetric technique. In addition,foraminiferal species counts were made on anapproximated minimum of 300 specimens forthe > 246 # fraction of each sample (Table 2),and Berger's (1968) solution indices calculated.Computer correlation analysis using the Univer­sity of Hawaii's IBM 360 facilitated the de­lineation of significant relationships betweenwater depth, size-fraction, carbonate content,and solution indices.

RESULTS

Calcium Carbonate

A contour map of percent calcium carbonatein the sediments superimposed on a bathy­metric map of the area (Fig. 2) reveals that thepercent values decrease markedly offthe OntongJava Plateau into deeper water, presumably dueto solution of the calcium carbonate. Themarked decrease in the direction of the Solo­mon Islands is anomalous, however, as thevalues in those samples are much too low forthe depth of water. In the plot of percent car­bonate versus depth (Fig. 3), the values in the

hatched area are all from the region betweenthe Plateau and the Islands. Microscopic exami­nation of the anomalous samples revealed thepresence of significant amounts of volcanic ashand traces of pumice incorporated in roundedcalcareous pellets coarser than 246 #. Theseobservations suggested dilution by volcanicmaterial in this area, and prompted the ex­clusion from the carbonate and size-fractionprofiles of 17 samples situated southwest of the80-percent-carbonate contour.

In the plots ofpercent carbonate versus depthfor the total sediment (Fig. 4) and for the threesize-fractions (Fig. 5), the following observa­tions are noteworthy:

1. For all profiles, the initial decrease in car­bonate content occurs in the vicinity of3,500-m depth, whereas the compensationdepth is at about 5,250 m.

2. Above 4,000-m depth, the < 44-# fractioncontains less weight-percent carbonate thanis contained in the other size-fractions.

3. Between depths of 2,500 and 3,500 m, the< 44-# fraction exhibits a maximum weight­percent of carbonate.

292 PACIFIC SCIENCE, Volume 27, July 1973

150' E 160' 170' 180'10·r------------r.------,---...,...------,-,--c:-----,---------.....---,10·

N D- N

'30"59

'28

0'·

~23

ZS,,o 1

10' °2 10'S S

150' E 170' 180'

SYMBOLS

• = 67 PISTON CORE 0 = 68 FREE FAll CORE

Ii = 68 PISTON CORE • = 70 FREE FAll CORE

• =70 PISTON CORE 0: 70 TRIGGER CORE

FIG. 1. Sample locations and general bathymetry of the Ontong Java Plateau area. Bathymetric contour interval:1 km.

4. At depths below 4,250 m, all the carbonateprofiles except that for the > 246-,u fractiondisplay a linear relationship with depth.Since the 44-246-,u and the > 246-,u frac­tions comprise nearly equal proportions ofthe total sediment and have equal weight­percents of carbonate above 4,250 m depth,the attenuation of the > 246-,u profile isparticularly striking.

If the rate of dissolution of carbonate (1) iszero above the saturation level, (2) is slow in theinterval of slight undersaturation between thesaturation level and the lysocline, and (3) in­creases linearly with depth below the lysocline,then carbonate profiles may be mathematicallygenerated from estimates of the original cal­careous content of the settling sediment and the

lysocline and compensation depths (see Table 1for horizon definitions) (Heath and Culberson1970). A comparison of a theoretical profile tothat of the observed total percent carbonate(Fig. 4) (see Table 3 for calculations) will revealthat the latter appears to decrease more rapidlywith depth than Heath's and Culberson's modelpredicts, although additional deep data wouldstrengthen this conclusion.

A profile of the weight-percent carbonatecontributed by each size-fraction to the totalpercent carbonate (Fig. 6) shows considerablescatter above 2,500 m; below this depth the< 44-,u carbonate fraction clearly predomi­nates, reaching a maximum at about the 4,000-mdepth. The calcareous contribution of the othersize-fractions exhibits a relatively consistentdecrease below 2,500 m.

.......... ....., ........ .L..oI _

FORAMINIFERAL SPECIES COUNTS FOR THE> 246-1' FRACTION IN SURFACE SEDIMENT SAMPLES(fl(::t-t

1967 1970~

Year 1967 1967 1968 1967 1967 1968 1967 1967 (")(1)

Core FFC7 FFC6 PC20 FF27 FF5 PC15 FF9 FF8 FF14 PC62 (fl

Depth (m) 2,178 2,232 2,250 2,254 2,256 2,535 2,670 2,688 2,830 2,910 (1)p...

SPECIES NUMBER OF SPECIMENS S'(1)

Globorotalia crassaforn,is 0 0 5 0 2 4 0 0 1 2 ::J...Globigerina calida 6 7 7 6 0 0 3 2 0 0 S"

Globorotalia Cl/ltrata 14 11 21 5 18 30 8 9 7 32 ~Globorotalia tUl71ida 0 0 0 3 0 2 3 0 4 8 (1)

'"5phaeroidinella dehiscens 1 1 1 1 2 0 2 1 1 1 ...(1)t-t

Globigerinoides conglobatus 23 15 22 7 75 10 31 20 17 6 ::JGlobigerinoides sacculifer 109 74 114 79 167 64 122 76 91 81 I:J1GlolJigerinoidu ruber 169 104 148 112 238 36 123 81 97 78 ..gGlobigerina conglomerata 3 1 10 1 4 3 4 1 4 8 l'>...Globo]uadrina dutertrei 62 37 50 25 78 37 37 36 18 44 0

i:!,Globorotalia scitllia 0 1 0 1 0 0 1 0 0 1 e.Globigerineila siphonifera 67 39 70 57 86 42 61 40 64 70 '"dCandeina nitida 4 4 8 3 6 1 4 4 0 1 l'>

(")

Orbulina flniversa 5 2 5 0 2 2 4 2 1 1 EhPulleniatina obliquiloculata 45 57 52 79 88 123 59 32 66 120 1Globigerinoides quadrilobattls 64 58 78 54 108 53 94 46 66 58

>t-'

Year 1967 1968 1968 1968 1967 1967 1970 1970 1967 1967 ~Core FF2 PC12 PCll PC23 FF30 FF15 pcn FFC59 FF28 FFl nDepth (m) 3,360 3,430 3,900 3,940 4,216 4,232 4,255 4,280 4,312 4,312 >

SPECIES NUMBER OF SPECIMENSGloborotalia crassaformis 0 0 0 0 0 0 0 1 0 0Globigerina calida 0 0 0 0 0 0 0 0 0 0Globorotalia cultrata 8 2 13 15 15 4 24 13 5 14Globorotalia tUl71ida 5 2 10 9 4 0 2 3 4 55phaeroidinella dehiscens 3 2 6 0 1 1 0 2 2 4GlolJigerinoides conglobatus 14 7 5 18 5 6 4 1 2 3Globigerinoides sacculifer 99 136 32 62 29 60 52 13 22 15Globigerinoides ruber 31 38 10 19 2 23 7 6 9 5Globigerina conglomerata 14 7 6 9 3 2 10 0 2 3Globoquadrina dutertrei 37 16 22 16 15 41 43 5 27 11Globorotalia scitula 0 0 0 1 0 0 0 0 0 0Globigerinella siphonifera 12 80 27 50 9 17 21 6 16 13Candeina nitida 0 0 0 2 0 0 0 0 0 0Orbulina universa 1 0 0 1 0 0 1 0 0 0 NPulleniatina ob/iquiloclliata 114 122 246 120 213 164 238 170 154 182 \0

'JJGlobigerinoides quadrilobatus 70 87 19 49 21 50 36 8 18 10

294 PACIFIC SCIENCE, Volume 27, July 1973

10·N

160·

:,.:i.:.i.:.:..:l,.:.:i.••..:.·..:l.:.:l..:.: ~I:.:.I:.:I..:..:.i.:!:'.....•I:I..:..:.,!:.:I:1::..: :;:::::::::::::::::::::::::;:;::::::;::;::::::::::;:::

il!il!! !IIIIIII 1IIIIIIi II illlill1~· :::::::'i :.;.:.:.;.:::.:::.;:::::::,:,:,:::,:::,::::::::::::::.;.: .

150· E

O·

150· E10·N

170·

FIG. 2. Percent calcium carbonate in surface sediments superimposed on the bathymetry of the Ontong J~lateau area. Carbonate contour interval: 10 percent, contours dashed where approximate' bathymetric cont~:~mterval: 1 km. '

Size-Fractions

Weight-percent < 44 # correlates remarkablywell with water depth (r = 0.92, P < 0.001),especially between depths of 2,500 to 4,500 m(Fig. 7). This size-fraction approaches a con­stant 90 percent at about 4,750 m. Weight­percent > 246 # shows a high negative cor­relation .with depth (r = -0.93, P < 0.001).The weight-percent of the 44-246-# fractionand that of the > 246-# fraction approaches aconstant 10 to 15 percent at 3,900 m and 2 to 3percent at 4,750 m, respectively (Figs. 8 and 9).

Faunal Parameters

Parker and Berger (1971, fig. 14A) indicatea lysocline depth of 3,800 m for the entire

equatorial Pacific region. Solution index pro­files for my data (Fig. 10) and for data extractedfrom Parker and Berger (1971, Appendix A)for the region long 150° E-180° E, lat 10° N­10° S (Fig. 11) delineate a lysocline at about3,500 m. This depth does not agree with thedepth range of 3,750-4,250 m which Parker'sand Berger's fig. 15 depicts for the westernequatorial Paci~c. Data from Parker and Berger(1971, Appendix A) for the eastern equatorialPa~ific (F~g. 12) agree with their entire equa­tOrIal PacIfic depth of 3,800 m for the lysocline.Lysocline depth is greater in the eastern thanin the western equatorial Pacific; if data fromthese ~wo regions of disparate productivity arecombmed, this difference is masked.

•

295Surface Sed'Iments W, estern E =~-----------quatorial PaeifiC-VALENCIA

----

20% CoCO:! IN TOT40 AL SAMPLE

60 80

100

1000

...fooO ~ ,

'000 ~ 'LY"''''TNYj--

5000 ~///////////~,;;;__ CS>~P.5~ATlON

F

---~~~

IG.3 P ---. ercent cal . - - -clUm carbonate . - - -10 total sample versus water depth

%C C .20 a 03 IN TOTAL S40 AMPLE

60

1000

OBSERVED CoCOCONTENT 3 •

~.

1500

~ 2000UJ....~ 2500z::I: 3000....a..UJ03500 - "

____LYSOClIl\JE"

4000 THEORETICAL-o- - - -~RAr~ISSOLUTION

4500 THEORETICAL •CONTENTCoC03

5000r--.----~~~~~

296 PACIFIC SCIENCE, Volume 27, July 1973

1000

'"eo:: 2000w....w:EzJ: 3000....e>.w0

4000

5000

- - ---" LYSOCLlNE"-- ----

FIG. 5. Percent calcium carbonate in each size-fraction versus water depth. Open triangle = < 44 1"; opensquare = 44-246 1"; open circle = > 246 It.

DISCUSSION

Lysocline and Compensation Depth

Although compensation depth may bemapped on the basis of carbonate content, thelysocline content, according to Berger (1968,1970), should be defined in sediments by usingthe concentration of resistant species of for­aminifera. Depending on the initial compositionof the sediment, a sample consisting entirelyof calcium carbonate may have undergone moresolution than one with a very small calcareouscomponent. However, an examination of themathematics of lysocline generation leads tothe possibility of delineating a depth approxi­mating the lysocline in nature and origin onprofiles of carbonate content for a uniformsedimentary regime.

Consider the formula (from Berger 1971):

L = 100 (l-RoIR)

where L is the loss of sediment necessary toincrease the insoluble fraction Ro to R percent.If Ro equals 5 percent and R equals 10 percent,loss L equals 50 percent; i.e., 50 percent of the

TABLE 3

CALCULATION OF THEORETICAL CaC03 CONTENTWITH DEPTH

d d-L R C

3,500 0 0 90*3,750 250 14.3 88.64,000 500 28.6 86.54,250 750 42.9 83.74,500 1,000 57.1 79.54,750 1,250 71.4 72.35,000 1,500 85.7 55.85,250 1,750 100.0 0

NOTE: L = lysocline depth = 3,500 m; Do ~ com­pensation depth = 5,250 m; d = a depth at or betweenL and Do in meters; R ~ rate of solution expressed as apercentage of the rate of supply; R = d - L/D0 - L x100; Co = carbonate content of undissolved sediment;N = noncarbonate content of undissolved sediment;C = calcite content of sediment ~ [(1 - R/l00) x Co x100]/[(1 - R/100) x Co + N].

* Observed average carbonate content above 3,500meters.

sediment dissolves as the carbonate contentdecreases from 95 to 90 percent. In comparison,if Ro equals 50 percent and R equals 60 percent,

Surface Sediments, Western Equatorial Pacific-VALENCIA 297

LEGEND_._. L

....•.. <44 jL

---- 44-246jL

- >246jL

%(0(°3

40 6020

":::::~:·;;.~N~

, ,~ ." -0.I 'I ...

I ...t '.

r o ---,

(\,,-)\.IIi

/.I...... ::...::::.::: _.";-

....,::...:;::.:.::;.::~:>;.. .... _.1'-

1...:,;:;;;;:.;.:<.::-;':'_.

00

1000

V'll>:: 2000wI-w~

z 3000::cI-0..W0

FIG. 6. Weight-percent calcium carbonate contribution of each size-fraction to total (% CaCOa x wt. % size­fraction) calcium carbonate content.

WEIGHT 1 < 441'

00-;._ ___.---=:2;.0--r-...::.;40=--r---..::6TO~---.---=8;.0--r--'il00

500

1000

1500 • •on 2000

'"w...~ 2500

~

~ 3000wo

3500

•• •

•".

••0>

••

4000• •. :..

4500

•5000

•• •5500IL -.l.--.l.--.l--...I.--...I--...L--'----'----L---'

FIG. 7. Weight-percent < 44 Ii versus water depth.

298 PACIFIC SCIENCE, Volume 27, July 1973

WEIGHT" 44-2461'20 30

00

500

1000

1500

2000'"C<W

:;:; 2500:IE~J: 3000:;:

~ 3500

4000

4500

5000

•5500

10

• •

• •....:•,

•••.. ,•

••••

40

••

50

FIG. 8. Weight-percent 44-246 It versus water depth.

WEIGHT" > 2461'

0°i---.--.:,IO=----r---'2TO---,---=3i-0--,--~40=---_.,.__-=5iO

500

1000

1500 •••

.,. ..

'" 2000C<w

i 2500

~

t 3000

wCi

3500

4000

45001-

•

• •

e •••••

•-•••

•

5000 ,.55ooL--.L---'------'------L---L_--'--_-L_-L_-L_....J

FIG. 9. Weight-percent> 246 p, versus water depth.

--------=~~= 'VALENCIA' 1Paclfic­Equatona- WesternS diments,

Surface e LUTtON INDEX 30 1.50

BERGER SO 1.10 1._~.90,,--.--.70

299

-----~.

1.301.00 _--;..=-----,--o

4000

~ 20000­W

::z

;: 3000Q..

~

h's work).'fic (data from t !torial Pac!5000 r the western equa' d x profile lorI tion-In eO""gu 00 U X

F>G 1 • UTION INOE 20 2.50BE RGE R SOL 90 2... " :....-,-----,1.60 1.

1000

d Berger 1971,Parker an'fi (data fromtorial Pac! c5000 r the western equa' x profile lorlution-Inde11 Berger soFIG. ,

Appendix A),

300 PACIFIC SCIENCE, Volume 27, July 1973

1.00o

1000 f-

BERGER SOLUTION INDEX1.30 1.60 1.90 2.20

I I I I2.50

-

-

iQ 2000 I-wf­W

:EzJ: 3000 f-f­e..wCl

5000

.~

--I I I I

FIG. 12. Berger solution-index profile for the eastern equatorial Pacific (data from Parker and Berger 1971,Appendix A).

loss L equals 17 percent. As the percent ofinsoluble material approaches that of the cal­careous component, the same amount of solu­tion loss produces a greater decreas.e in thepercent soluble fraction. Thus, irrespective ofactual solution-rate variations, an abrupt changein slope in plots of carbonate content versusdepth would occur where the carbonate frac­tion becomes comparable to the noncalcareousfraction. Minimum dissolution loss in foramini­feral assemblages can be described by the sameequation, if resistant forms are considered tobe insoluble (Berger 1971). The solution indexis formed by multiplying the species percentby its solution rank, summing the products, anddividing by the average rank number. Becausethe solution indices are comparisons of resistantand nonresistant species, the lysocline can bethought of as a "compensation depth of easilydestroyed species" (Berger 1971 : 350).

The initial composition of the settling for­aminiferal assemblage in the South Pacific isno more than 5-percent resistant species (Berger

1971), and the noncalcareous fraction of theshallowest samples in the investigated area alsoapproaches 5 percent. The initial biogeniccalcite rain as well as the deep physicochemicalhydrographic structure is relatively uniformover the western equatorial Pacific. Samplescontaining anomalous amounts of noncal­careous material or exhibiting evidence of re­working have been excluded from the car­bonate profile. Thus, for this relatively uniformsedimentary environment, the carbonate profileand the solution index profile show a similarabrupt change of slope in the vicinity of 3,500 m(Figs. 4, 10, and 11). The lysocline depth in thesoutheastern Pacific is also consistent with thedepth at which carbonate content abruptlydecreases in the equatorial Pacific (Berger 1971).Of course, wherever the two initial insolublefractions are disparate, the profile horizonswould not be coincident.

It must be emphasized that the concept ofthe sedimentary lysocline as a measurable boun­dary produced by a hydrographic dissolution~

Surface Sediments, Western Equatorial Pacific-VALENCIA 301

rate change (hydrographic lysocline) is open toquestion. In fact, marked dissolution occurs atdepths as shallow as 3,000 m in the equatorialPacific (Berger 1971). Tentative associations byBerger (1968) of the sedimentary lysocline withthe actual levels at which solution rates increase-in the water column, as documented byPeterson (1966) and Berger (1967), remainunsupported.

The extrapolated compensation depth of5,250 m for the western equatorial Pacific issimilar to other previously reported depths forthe central equatorial Pacific, whereas the lyso­cline depth of 3,500 m is at the shallower endof the range of previously reported data for theequatorial Pacific. The depth and sharpness ofthe compensation level depend on the rate ofsupply of calcium carbonate, the hydrographiclysocline depth, and the slope of the dissolution­rate curve below the lysocline (Heath andCulberson 1970). The general pattern of thelysocline is primarily controlled by the massbalance of calcium carbonate (Broecker et al.1968), thermodynamics (Sillen 1967), and deepcirculation (Berger 1970); but this general pat­tern is supposedly altered by geographicallyvariable rates of surface production of organiccarbon (Heath and Culberson 1970, Berger1970). An increase in surface production wouldraise the lysocline by supplying more oxidizableorganic matter for carbon dioxide formationand, at the same time, would depress the com­pensation depth by supplying more calcareoustests (Heath and Culberson 1970). However,Kroopnick (1971) reports that oxygen con­sumption is not limited by a lack of carbon.Most of the dissolved oxygen is probably con­sumed in the oxidation of organic matter toform carbon dioxide which, in turn, increasesthe undersaturation of the water with respectto calcium carbonate. If oxygen consumptionin the formation of carbon dioxide is limitedby kinetics rather than by a lack of carbon, locallysocline depth may not be greatly affected bysurface productivity.

Surface production in the investigated areais less than 100 mgCfm2 per day, as comparedwith that of 100-250 mgCfm2 per day for theeastern equatorial Pacific (Koblentz-Mishkeet al. 1970, fig. 1). If the inverse relation ofproduction with these carbonate profile boun-

daries is also correct, and if surface productionis a locally dominant controlling factor, thenthe lysocline should be deeper and the com­pensation depth shallower in the western equa­torial Pacific than they are in the easternequatorial Pacific. The greater interval betweenthe lysocline and the compensation depthcoupled with lower production in the westernequatorial Pacific contradicts this hypothesis.The biological, physical, and chemical pro­cesses leading to local hydrographic and sedi­mentary lysocline-depth variations obviouslyrequire further elucidation.

A linear increase in the dissolution rate be­low the lysocline as postulated by Heath andCulberson (1970) is perhaps inconsistent withthe observed percent carbonate profile (Fig. 4).Berger's (1967) finding of successive doublingsofforaminiferal ooze weight-loss between 2,750and 5,250 m also supports the theory of non­linear increase of the dissolution rate. It shouldbe noted, however, that Heath's and Culber­son's model assumes that all dissolution occursat the depth of deposition. Actually, the biogenicparticles may undergo some dissolution duringsettling (Peterson 1966), possibly decreasingthe carbonate content in a nonlinear manner,and thus accounting for the observed dis­crepancy between the theoretical and observedprofiles.

Size-Fractions

If slumping and turbidity flows are dis­regarded, the grain size of the carbonate frac­tion of pelagic sediments is a function of itsoriginal biogenic constituents and dissolutionhistory. The < 44-,u fraction exhibits a highpositive correlation with depth, and Fig. 6confirms that a dominant solution effect above4,000 m is the fragmentation of particlescoarser than 44,u. Berger (1967) has also foundthat, in foraminiferal ooze, preferential frag­mentation of larger (> 125,u) specimens tendsto increase the finer fraction weight-percent.Calcareous particles of > 246 ,u show greaterresistance to solution than do the other size­fractions (Fig. 5), presumably due to the greaterresistance of some species of large foraminifera,e.g., Pulleniatina obliquiloculata.

Because the weight-percent of the > 246-,u

302

fraction is composed almost entirely of for­aminifera and because the decrease of this frac­tion with depth is linear (Fig. 9), its potentialusefulness as a solution index within a par­ticular core is obvious; although climaticallyinduced variations in the production ratios oflarge foraminiferal tests and coccoliths mayconfuse the interpretation. Similarly, anomalousvalues for this fraction or for the < 44-,u frac­tion in surface sediments for a given depth mayindicate recent tectonic or sediment movements.

CONCLUSIONS

1. Subaerial volcanics reduce the calcium car­bonate content of surface sediments on thewestern margin of the Ontong Java Plateau.

2. Two artificially generated, carbonate solu­tion boundaries-the sedimentary lysoclineand the slope-break in the percent carbonateprofile-are equivalent in the investigatedarea.

3. The lysocline depth in the western equa­torial Pacific is shallower (3,500 m) thanthat in the eastern equatorial Pacific (3,800m).

4. The western equatorial Pacific compensationdepth occurs at 5,250 m, and establishes agreater interval between this level and thelysocline in the western as compared withthe eastern equatorial Pacific.

5. Both the < 44-,u and the > 246-,u weight­percents exhibit highly significant correla­tions with depth and may be useful assolution indicators or delineators of dis­placed. surface sediments.

ACKNOWLEDGMENTS

The author is indebted to Drs. K. ]. Roy,S. V. Smith, and]. Resig for computer adviceand valuable discussion, and to Mr. R. D'Amicoand Ms. Lili Smith for efficient laboratoryassistance. This research was supported by theOffice of Naval Research contract N00014-70­A-0016-0001, G. P. Woollard and G. H. Sutton,principal investigators; and by the NationalScience Foundation grant GA-11412, J. Resig,principal investigator. It constitutes a portion

PACIFIC SCIENCE, Volume 27, July 1973

of a Ph.D. dissertation which was submitted tothe University of Hawaii, August 1972.

LITERATURE CITED

BERGER, W. H. 1967. Foraminiferal ooze: solu­tion at depths. Science 157: 383-385.

---. 1968. Planktonic foraminifera: selec­tive solution and paleoclimatic interpretation.Deep-Sea Res. 15: 31-43.

---. 1970. Planktonic foraminifera: selec­tive solution and the lysocline. Mar. Geol. 8:111-138.

---. 1971. Sedimentation of planktonicforaminifera. Mar. Geol. 11: 325-358.

BRAMLETTE, M. N. 1961. Pelagic sediments.Pages 345-366 in M. Sears, ed. Oceano­graphy. Publ. Amer. Ass. Advanc. Sci. 67.

BROECKER, W. S. 1971. Causes of temporalchanges in the carbonate compensation level.Page 332 in Program, 56th Annual Meeting,Amer. Ass. Petrol. Geol. (Abstr.)

BROECKER, W. S., R. HOROWITZ, T. TAKA­HASHI, and Y-H. LI. 1968. Factors influencingthe CaCOa compensation levels in the ocean.Page 38 in Program, Annual Meeting, 1968,Geol. Soc. Amer.

HEATH, G. R., and C. CULBERSON. 1970.Calcite: degree of saturation, rate of dis­solution, and the compensation depth in thedeep oceans. Bull. Geol. Soc. Amer. 81:3157-3160.

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