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CALCIUM CARBONATE SATURATION IN SEAWATER:EFFECTS OF DISSOLVED ORGANIC MATTERKeith E. Chase and Erwin Suess2Hawaii Institute of Geophysics and Department of Oceanography,University o f Hawaii, Honolulu 96822

ABSTBACISurface seawater is generally supersaturated with CaCOs; but carbonate will not prcdpi-tate from natural seawater in convenient experimental times. If the supersaturation isincreased by the addition of Ca2 or C03- ions, precipitation can occur in minutes or hours.Seawater, with 0.1 m Na&Os solution added to give a pH o f 9.5, will begin rapid CaCOs(aragonite) precipitation in as little as 15 min. The length of time between the additionof the N&CO3 and rapid precipitation increases with an increase in the dissolved organiccontent of the water. If CaCL, which produces no pI1 increase, is added to seawater,CaSOc.21140 (gypsum) is the only precipitate that will form within 12 hr.

INTRODUClTONThe supersaturation of CaC03 in surfaceseawater was first noted in the central At-lantic by Wattenberg and Timmerman( 1936). Others have since observed thisphenomenon in many widely scatteredareas (e.g., Cloud 1962; McIntyre andPlatford 1964; Pytkowicz and Fowler 1967;Lyakin 1968). It appears that most surface

waters of the worlds oceans are supersatu-rated. This disequilibrium state of theCaC03 system in surface seawater hasgenerally been attributed to some nonspe-cific, inorganic reaction kinetic control. Onthe basis of laboratory experiments, Pyt-kowicz (1965) showed the effect of mag-nesium in solution in seawater in theregulation of carbonate nucleation kinetics.Simkiss (1964) has shown that dissolvedphosphorus compounds inhibit CaC03 pre-cipitation.Chave ( 1965) and Chave and Suess( 1967) suggested that naturally occurringorganic compounds inhibited reactions be-tween carbonate minerals and seawater.This report presents the results of experi-ments to determine the effect of dissolvedorganic compounds on the nucleation of1 This work was supported by Office of NavalResearch Contract NR 083-194. Contribution 374from the Hawaii Institute of Geophysics.2 Present address: Geologisch-PalaontologischcsInstitut der Universitgt, 23 Kiel, Olshausenstrassc40/60, West Germany.

CaC03 from filtered natural and artificialseawatcrs.EXPERIMENTS

Calcium carbonate will not precipitatefrom naturally occurring supersaturatedsurface seawater in reasonable experimen-tal times, Thus, to study rates of nuclea-tion, it is necessary to increase the degreeof supersaturation artificially. This is eas-ily accomplished by the addition of Ca+or COs2- ions. If these are added in theform. of CaCl2 or Na&On solutions,, thegross chemistry of the seawater changeslittle since Na- and Cl- are the dominantions of seawater.

Natural seawaters from a variety of en-vironments around Hawaii were passedthrough 0.8-p Millipore filters and contin-uously stirred. Portions ( 160 ml) of thefiltrates were brought to pH 9.5 by addi-tion of 0.1 m Na2C03 solutions. Theamount of Na&03 solution required toreach this pH ranged, with one exception,from 5.7-6.7 ml; about 0.6 mmoles of C032-per sample was added.3 To approximate a consistent degree of super-saturation by addition of Na2COs to various sea-waters, the choice of adding a constant amount ofNaEOs, or of adding NaE03 to a constant pH,is arbitrary. Because of the disequilibrium in either

system, it is impossible to describe the absolutesupersaturation. Addition of NazCOa to a constantpH was chosen for these experiments.633

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634 KEIII-I E. CHAVE AND ERWlN SUES3

9.5 ---A:+,, . Y=c \;---*---.+.9.4

1 ?\ .

\ -.. 3.0 mG /L

9.3 \

9.0 .I , I I 1 I I0 IO 20 30 40 50 60 70TIME, MINPIG. 1. Change in VI-I with time after additionof NanCOs solutions. Rapid precipitation of CaCO:,

is indicated by a rapid decrease in pII. Dissolvedorganic carbon (mg C/liter ) of each sample isindicated.

Plots of pII change with time after theaddition of Na&08, Eor three typical ex-pcrimcnts, arc shown in Fig. i.- In theLxpcrimcnts, the $1 dccrca&s slowly atfirst and then rapidly as a thick whiteprecipitate of CaCOs (aragonite) is formed.Total dissolved o,rganic carbon was dc-termined on each of-the filtered seawatersamples bcforc the prccipita tion expcri-ments, using the method of Menzel andVaccaro ( 1964). The length of time bc-tween the addition of Na2CO:I and therapid precipitation of carbonate, as indi-cated by a rapid pH dccrcasc, appears tobe related to the dissolved organic contentof the water (Fig. 1, Table 1).Two samples of scawatcr from laboratoryaquaria, with unusually high dissollved or-ganic contents, did not precipitate CaCOsin 3 to 4 hr after additions of Na&03. Theexperiments were not continued longerbccausc WC: elt that after this period otherfactors such as microbiolo,gical activity,evaporation, or water-surface contamina-tion, might influence the results.To clucidatc the relationship betweendissolved organic compounds aid CaCORprccipitationr wc collcctcd samples of pre-cipitatcd material periodically during an-other set of experiments. The Na2COzIsolution was added to 8-lo-liter samplesof Filtered scawatcr until the pII was 9.5.

TAIHX 1. Effect of dissolved organic carbon. onCaCOs precipitation from seawater raised topH 9.5

Scawntersample Initialj?II

0.1 m Time toNa,C08 Dissolved rnpidadded/ org-C pII de-

ArtificiaIKancohe Bay( surface )Kancohc Bay( surface )Coconut Jsland(surface)Lab circulating( well)Lab circulating( well)Kaneohc Bay( surface )Aquarium( natural )Aquarium*( natural )Aquarium *(natural)

7.64 5.69 0.5 168.09 6.65 1.1 228.14 6.21 1.2 2618.12 5.71 1.2 3x27.92, 6.10 1.9 GO8.01 6.10 2.0 488.00 5.90 2.0 557.94 6.10 2.9 120

- - 3.3, >240240t--* Various aquaria around the laboratory containing or-ganisms in nntural seawater.t Iyo precipitation obscrvcd.

Immediately after addition of the Na&O:I,1 liter of the experimental solution waswithdrawn and filtered through a precom-busted Gclman type A glass-Fiber filter.The filter was then washed with 5 ml ofdistilled water. Withdrawal and filteringof 0.5-1.0-l&r aliquots continued at 5-l@min intervals until the initial seawater sam-plc was exhausted, or until the CaCO:lprccipitatc was very heavy.The glass-fiber filters were dried oversilica gel, and analyzed for CaC03 carbonand organic carbon. WC measured CaCO:$-carbon (in a Beckman IR-215 CO2 ana-lyzcr) in the form of CO, rclcascd bytreatment with 15% IIaPOL~. Organic car-bon was measured as CO2 rcleascd byoxidation with l&$208 after acid treatment.Both reactions took place in the same ves-scl. The method is described by Klim(1969).The results of two of these cxpcrimentsare shown in Fig. 2. Organic carbon isprecipitated rapidly from the NazCOR-cn-

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chCo3 SATURATION IN SEAWATER 6351.0

0.8 -

< 0.6 -435z-$%0 0.4 -

- NORMAL SEAWATER--- ORQANIC-RICH SEAWATERPI

IIII0 0

LII

ARAGONITE \::i

III

0 30 60 90TIME, MINFE. 2. Composition of precipitate from NaLIOs-enriched seawater as a function of time. Organiccarbon is removed from solution before rapidCaC,Os precipitation occurs.

richcd seawaters at the beginning of theexpcrimcnts, before the major part of theCaCOs precipitates. Organic compoundsmay be prccipitatcd because the increasedpH causes a decrease in their solubility,or they may be removed from solution byassociation with the few Ca.C03 nuclei thatfolrm early in the experiment (Fig. 2.). Ineither case, organic compounds are rc-moved from solution before rapid CaCOsprecipitation occurs.The maximum. amounts of dissolved or-ganic carbon precipitated during the cx-periments arc 0.10 mg C/liter for normalseawater and 0.25 mg C/liter for organic-rich ( aquarium) water. This representsabout 10% of the initial dissolved organiccarbon in these two types of samples. Suess(1970) reported that between 10 and 14%of the dissolved organic carbon in seawaterassociates with calcite surfaces when themineral is added to seawater as a finepowder. It is likely that in the expcri-ments reported here, dissolved organic

TABLE 2. Rates of CaCOs nuchtion from Udifi-da1 seuwater raised to pH 9.5 (clissolved mg-C,0.5 mg C/liter)

Mg concn(M)

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636 KEITH E. CHAVl3 AND ERWIN SUESSTABLE 3. Precipitates formed within 12 hr byaddition of CaCL to seawater

Mg CaCl, added (g/100 ml) Dissolvedconcn org-Cbd 1.0 1.25 1.50 1.75 (mg C/liter)

Artificial seawater

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C&03 SATURATION IN SEAWATER 637surfaces. This suggestion is in agreementwith the observation by Chave and Suess(1967) that the adsorption of dissolvedorganic matter from seawater onto carbon-ate surfaces is faster than the precipitationof CaC& onto the same surfaces.Furthermore, in our experiments withenriched seawater, rapid CaC03 precipi-tation took place only at high pH values,after organics had -been removed fromsolution (Fig. 2).Finally, inactivated nuclei may accountfor the CaC03 associated with organic ag-gregates observed in suspension in shallowand deep ocean water (Wangersky andGordon 1965 ) .On the basis of this model of carbonatesupersaturation in surface seawater, the seacan probably maintain a high degree ofsupersaturation if pH remains near 8.0,and equilibration will occur only if pH israised considerably,

REFERENCESCHAVE, K. E. 1965. Carbonates: Associationwith organic matter in surface seawater. Sci-ence 148 : 1723-1724.-, AND R. F. SCHMALZ. 1966. Carbonatc-seawater interaction. Gcochim. Cosmochim.Acta 30: 1037-1048..-, AND E. SUE!%. 1967. Susncnded min-erals in seawater. Trans. N.Y. *Acad. Sci. 229 : 991-1000.CLOUD, P. E. 1962. Environment of calciumcarbonate deposition west of Andros Island,Bahamas. U.S. Geol. Surv. Prof. Pap. 350.138 p.GARJTELS, R. M., AND M. E. THOMPSON. 1962.A chemical model for seawater at 25C and

one atmosphere total pressure. Amer. J. Sci.260 : 57-66.-- AND R. SIEVER. 1961. Controlof carbonaie solubility by carbonate com-plexes. Amer. J. Sci. 259: 24-45,.KLIM, D. G. 1969. Interactions between sea-water and coral reefs in Kaneohe Bay, Oahu,EIawaii. M.S. thesis, Univ. Hawaii, Honolulu.

LYAKIN, Y. I. 1968. Calcium carbonate satura-tion of Pacific water. Oceanology (USSR)8: 44-53.McINT~, W. G., AND R. F. PLATFORD. 1964.Dissolved CaCOs in the Labrador Sea. J.Fish. Res. Bd. Can. 21: 1475-1480.MENZEX, D. W., AND R. I?. VACCARO. 1964. Themeasurcmcnt of dissolved organic and partic-. ulate carbon in seawater. Limnol. Oceanogr.9: 138-142.PYTKOWICZ, R. M. 1965. Rates of inorganic cal-cium carbonate precipitation. J. Geol. 73:196-199.-, AND G. A. FOWLER. 1967. Solubility offoraminifera in seawater at high pressures.Geochim. J. 1: 169-182.SCE~LZ, R. F., AND K. E. CIIAVIL 1963. Cal-cium carbonate: Factors affecting saturationin ocean waters off Bermuda. Science 139 :1206-1207.%MKISS, K. 1964. The inhibitory effects of somemctabolitcs on the precipitation of CaC03from artificial and natural seawater. J. Cons.,

Cons. Perm. Int. Explor. Mer 29: 6-18.SUESS, E. 1970. Interaction of organic com-pounds with calcium carbonate, Geochim.Cosmochim. Acta 34 : 157-168.WANGERSKY, P. K., AND D. C. GORDON. 1965.Particulate carbonate, organic carbon, andMn++ in the open ocean. Limnol. Oceanogr,10: 544-550.WA~ENBERG, I-I., AND E. TIMMERMAN. 1936.Ober die SSittigung des Seewassers an CaC03,Ann. Hydrogr. Mar. Meteorol. 64: 23-31.WEYL, P. K. 1961. The carbonate saturometer.J. Gcol. 69: 3244.