ATOLL RESEARCH BULLETIN
No. 157CARBONATE LAGOON AND BEACH SEDIMENTS OF TARAWA ATOLL, GILBERT ISLANDS
by Jon N. Weber and P e t e r
M.J.
WoodheadI s s u e d by
THE SMITHSONIAII INSTITUTION Washington, D. C . , U . S. A .
L'ecember
31, 1972
CARBONA'fE LAGOON AND BEACH SEDIMENTS O F TARAWA ATOLL, GlLBERT ISLANDS
by Jon
N.
Webur' and P e t e r M,.I. ~ o x l h s a d 'SUMMARY
A large, si~allow lagoon, a l m o s t isolated from the open ocean, d i s t i n e ~ i s l ~ e s Tarawa f r o m other atolls w!~era carbonate sedimentation has been investigated. Pass-?:; through the atoll reef a r e lacking except along the western periphery. Size distribution s t a t i s t i c s (miidian grain s i z e , coefficients of sorting and skewness) wereobtainedfor 57 sediment s a m p l e s from Tarawa Lagoon and 12 s a m p l e s f r o m the fringing reefflats. Samples wdre subdivided into ten separate g r a i n s i z e fractions and the mineralogy of each fraction was determined. Compclnent analysis of the l a r g e r g r a i n s was undertaken to identify the different s o u r c e s of carbonate particles.
Lime muds a r e accumillating in shallow water (6-10 m ) o v e r a l a r g e a r e a of the southeastern lagoon. Evidence provided by m i n e r a l o m , carbon isotopic cornlosition, and electron micro- scopy indicates that the fines consist of a mixture of a l g a e - s e c r e t e d n e e d l e s a n d p a r t i c l e s derived f r o m the physical and biological breakdown of skeletal detritus. The abundance of carbonate muds in shallow water is attributed to the absence of c u r r e n t winnowing.
INTRODUCTION
Tarawa p o s s e s s e s s e v e r a l f e a t u r e s which distinguish it f r o m ,other atolls where carbonate sedimentation h a s been studied. These f e a t u r e s especially concern the depth of the lagoon and the relatively poor connection between it and the open ocean. The m t e r in much of T a r a w a Lagoon is murky, a fact which i s readily evident f r o m the a i r . At the extren~b? southeastern c o r n e r of the lagoon, underwater visibility i s l e s s than 2 m , and associated with the high degree of turbidity a r e fine-grained carbonate muds accumulating in shallow water.
The lagoon a t Tarawa d i f f e r s significantly f r o m those of Bikini, Eniwztok, and other atolls where detailed sediment studies were conducted by E m e r y e t a l . (1954). Tarawa L a g o m i s shallow, averaging about 12 to 15 m , with a maximum depth of 25 m. Rongelap, E n i w t o k , and Bikini Atolls, however, have l a g o m s averaging about 50 m in depth (maximum 70 m).
F u r t h e r m n r e , Taraw? Lagoon is encircled by c o r a l r e e f s a t s e a level except along the western margin where the atoll reef top i s submerged to about 9 m.. There i s only one deep (20-25 m ) p a s s into Tarawa, whereas Rongelap Lagoou is connected to the s e a by nine p a s s e s with maximum depth of 66 m , and Bikini h a s eight p a s s e s o r channels with a nlaximum depth of 60 m.' These variations in lagoon physiography might e x e r t considerable influence on the c h a r a c t e r i s t i c s of sedim'3ntary m a t e r i a l accumulating on the atoll.
The dominant sediment contributors in m o s t c o r a l reef environments a p p e a r to be Halinzeda, corals, m ~ l l u s c s , foraminifera, and t h e coralline algae. In unusual circumstances, other types of sediment may f o r m , f o r example, non-skeletal carbonates in the lagoon of Hogsty Reef (Millimnn, 1967) o r the pelagic foraminifera, benthonic foraminifera, and mollusc sands on the Seychelles Bank (Lewis and Taylor, 19661. Considerable variations in the abundance of carbonate grains Erom different s o u r c e s a r e observed f r o m one reef to another and often within a single reef complex. Halimeda and f o r a m h i f e r a , f o r examide, a r e imllortant in the Great B a r r i e r Reef (Maxwell, 1968) and the Florida reef t r a c t (Ginshurg, 1956) provinces.
Over large a r e a s of the f l o o r s of Eniwetok and Bikini Lagoons, Halimeda is a m l j o r sediment - ~ . ~
' ~ e n n s y l v a n i a State University, University P a r k , Pennsylvania 16802.
Z ~ 2 m o r i a l University, St. John's, Newfoundland, Canada.
(Manuscript r e c e i v e d June 1971--Eds.)
compment, often comprising between 75 and 100% of the total (Emory e t al., 1954). In Fanning Lagoon, however, foraminifera and Haliwzeda a r e not substantial c o n t r ~ b u t o r s (Roy, 1970).
ffalimeda fragments a r e a l s o relatively s c a r c e in the sediments of Kure and Midway Atolls (Gross e t a l . 1969). At P e a r l and H e r m e s Reef, carbonate of foraminifera1 derivation i s considerably l e s s abundant than g r a i n s f r o m ,algae, corals, and molluscs (Thorp, 1936). Thus the striking differences in the proportions of v a r i o u s carbonate contributors reported f o r Pacific islands and atolls is a n additional incentive f o r investigating the sedimentary deposits of Tarawa.
PHYSICAL SETTING
Tarawa Atoll, p a r t of the Gilberts Islands group, is located in the c e n t r a l Pacific Ocean just north of the equator (1°30'N, 173OE). The atoll is triangular in shape, with approximate dim+?nsions of 25 km E-W and 30 km N-S. Although the dominant surface ocean c u r r e n t is f r o m the e a s t , the wind regime changes f r o m NE T r a d e s in Jan.-Feb. to SE T r a d e s in July- Aug. The c o r a l reef is continuous and r i s e s to the surface on the windwwd s i d e of the atoll,
supporting numerous islands and i s l e t s no m o r e than a few feet above s e a level. These islands a r e surrounded by reef flats a s m x h a s 300 to 800 m wide on both lagoonal and sea- ward s h o r e s , and they a r e separated f r o m each o t h ? r by n:%rrow ~ h a n x l s wlhich a r e geaerally em,irgen'. a t low tide. Along the weatern x i r g i n 3: th? atoll, the ree: is s:l:xnorged to d?,>ths of about 8 to 10 m. Th? msin entrance to ths lagoon is a breach in the leeward, submerged reef, located just north of Betio n e a r sampling station TA-28 (Fig. 1). A s t r o n g (1-3 km/hr) curren: flows through this channel, its direction r e v e r s i n g with each change in th? tide. D a : ~ i t e i t s s i z e (400 sq. km! tht? lagoon i s relatively shallow, with ?n :%vzrage de;lth betwee2 12 atid L5 m, a n i a maximum depth of 25 m. I r r e g u l a r topography characterizes the lagoon floor, and c o r a l knolls o r pinnacle r e e f s a r e fairly common.
METHODS
A total of 57 sedim'int sa.mYles ("TA-series") w;rs collected in Tarawa L a g m n by divinz o r by m*?ans of a mn,lifie:d Van Veen bottom xarn,,ler. Adtl.tioml sedimi-?t w w sam,?led .gn the f l a i s ,>f the r e e f s fringing Blzanibau Island on both thc? lagoon ("B:L..s::ries") and oceati ('%I-s::ries") facing sides. The locations of sampling stations a r e shown in Fig. 1. Sedimt?iit s a m p l e s w e r e p r o c e s s e d by mechanical techniques described in detail by Neumann (1965), atid s i z e d i s i r i b ; ~ t i o n s t a t i s t i c s were ca1c;aIated from the resultin2 s i z + f r q l e : i c y distributioiis.
Ce:iiral tendency i s i q r e s s e d by the m?:lian ,srain 3ize (Mdi in mm. Tras'i 93~:ing (SO) and s k e w i e s s (Skj coo.ffic;enfs a r e used to indicate dispersion and symm?try: So =-and
Slz
=
Q1Q3/(Md)f
where Q1 and Qg a r e the f i r s t and t h i r d quartilesof the cumulative frequency distribution. E a c h sample was divided into ten s i z e Fractions designated by the l e t t e r s A to J in o r d e r of decreasing grain size. The limits of t h e s e s i z e c l a s s e s , in m m , a r e :The abundances of aragonite, high-magnesium calcite (HMC),andlow-magnesium calcite (LMC) in each s i z e fraction were determined by X-ray diffraction analysis described in detail by Weber (1968). Carbon and oxygen isotopic composition data were obtained by mass spectro- m e t r i c methods described by Deines (1970). Size frequency distribution statistics for each sediment sample a r e given in Table 1. Mineralogical data a r e listed in Table 2.
SEDIMENTS OF THE LAGOON
There i s a considerable difference between the material deposited in the eastern, relatively sheltered corner of the lagoon, and that coveringthe floor of the more exposed western portion.
The sediments in the Bonrikl-Bikenibeuareaareessentially carbonate muds and oozes, whereas lime sands predominate elsewhere in the lagoon. In samples TA-12, 8, and 7 (Fig. 1) for ex- ample, the percentage of grainslessthan0.07mmin diameter amounts to 75, 78, and 71 respec- tively. Coarse-grained sediments a r e typical of the western lagoon margin. Sample TA-28 yielded nothing finer than 0.125mm, and grains larger than 0.18mm accounted f o r 98% of the total. Similarly, 95% of TA-29 and 93% of TA-53 consisted of particles with diameters greater than 0.18mm. The striking change in median grain size from east to west i s illustrated in Fig. 2. The fine-grained sediments in the eastern neck of the lagoon tend to be very poorly sorted (So : 8.49). Progressing westwards, sorting becomes increasingly better, with the T r a s k index decreasing to 1.58 a t station TA-28.
Aragonite i s the most abundant mineral component in a l l samples f r o m the lagoon.
Maximlm and minimum amounts were found in TA-26 (92%) and TA-28 (65%), but approximate- ly three-quarters of the sediments sampled contained between 75 and 90%. The average con- centration of aragonite increases from 66% a t the eastern corner of Tarawa Lagoon to about 80238% in the center and then decreases abruptly to 65% a t the western atoll reef edge (TA-28,
~ i g . 1). Aragonite is distributed a m m g all ten of the size fractions studied, but in general, the intermediate grain size categories tend to have slightly higher percentages of this mineral than do the very fine o r very coarse grades (Fig. 3). Changes in the distribution of aragonite a s a function of grain size a r e shown f o r the east-west transect in Fig. 4, and for the TA-53 to TA-57 transect of the northern corner of the lagoon in Fig. 5.
Low-magnesium calcite is a minor constituent of the sediment throughout the lagoon.
Average concentrations a r e generally between 2 and 8%, and with few exceptions, the mineral appears to be more o r l e s s equally distributed among the various size fractions of a given sample. Only a t the western edge of the atoll reef where lagoonal and open ocean waters mix does LMC become abundant. The increase in concentration of low-magnesium calcite a s the reef margin isapproachedis rather abrupt, a s illustrated by the sample s e r i e s TA-40 (9% LCM), 39 (14%) and 28 (17%). Almost a l l of the additional LMC in the reef edge sediments is found in a single size fraction. F o r e x a m ~ l e , 82, 73, and 89% of the total LMC i n samples TA-40, 39, and 28 respectively occurs in the 0.42 to 0.48mm grain s i z e range. With the exception of the western reof margin where the LMC co.iient i s rariable, the distriblttions of high-magnesium calcite an.1 aragonite in the lagoon sedimi'nts a r e complem~?ntary.
COMPONEVT ANALYSIS OF LAG9ON SEDIMENTS
Grains l a r g e r than 0.5mm :rom a numher of samples on the east-wrst lagoon transect were identified and counted under a binocular microscope. The results f o r major components a r e summarized in Fig. 6. Constituent carbonate grains f r o m the extreme western p a r t of the lagoon (e.g. s a m l l e s TA-28, 29, 39) a r e "fresh1' in appearance, a r e not greatly abraded, and hence a r e relatively easily identified. Progressing eastward, however, the grains become increasingly abraded and a r e characterized by a "weathered" appearance. iIz:irnedz frag- ments constitute a major component of the sediment throughout the lagoon but their abundance is greatest in the eastern part, for example, 63% of the
>
0.5mm grains in sample TA-7.The percentage of coral debris is highest in the west near the atoll reef margin. Coral fragments decline markedly in abundance towards the east, amounting to essentially zero per- cent a t station TA-7. There is a slight increase in the quantity of coral produced grains in the lee of the seaward (eastern) atoll reef, f o r example a t TA-9. Skeletal detritus attributed to m3llusca i s also a significant sediment comA30nent, and a s shown in Fig. 6, this m l t e r i a l is mast abundant in the center of the lagoon.
Compared with other carbonate sediments of c o r a l reef environments, echinoderm debris appears to be unusually common. The percentage of this com1)onent i s low in the western lagoon but i t steadily increases eastwards, reaching up to 20% of the ) 0.5mm p a i n s in the region where the fine carbonate muds a r e accumulating (e.g. TA-6, 7, 8, 9). Skeletal ossicles of the echinoid Echinonzetvu, ophiuroids, and crinoids constitute the greater portion of recog-
~ i z a b l e echinodern-derived grains from samples n e a r the western atoll reef a r e a , whereas elsewhere in the lagoon, heart urchin debris predominates. It is noteworthy that foraminifera contribute very little sand size carbonate throughout the lagoon except in the beach a r e a s and along the western atoll reef zone. The 0.4 to 0.8mm size fraction of beach sands is almost exclusivey com2osed of the forani Calcavina. In sample TA-28, the benthonic foraminifera constitute 25% of the 0.84 to 2.00mm and 45% of the 0.42 to 0.84mm size fractions. Most of the smaller t e s t s a r e Am$ikiste,rrinz, whereas Xetevostegina is the major contributor of the larger shells. Pelagic foraminifera were very seldom observed, even in samples f r o m the western atoll reef margin.
Because of their f r e s h appearance, alcyonarian spicules a r e conspicuous in sediment sam:~les from the western lagoon. They tendto concentrate in the 0.42 to 0.84mm size fraction but did not exceed 5% of any sample. Although coralline algae amounted to 9% in one sample, this component along with bryozoa, crustacean, andannefiddebris, etc. a r e minor constituents of the) 0.5mm grains in the lagoonal sediments of Tarawa Atoll.
Major s o u r c e s of aragonite a r e the green algae, especially iIali?n,?3a, the scleractinian corals, pelecypods, and gastropods. Smaller quantities of this mineral might be derived from hydrozoans, the octocoral lf@liol,ova, some bryozoans, scaphopods, ceplialopods, and some annelids. Producers of liigh-magnesium calcite include echinoderms, the coralline and red algae, benthonic foraminifera, alcyonarians, some bryozoans, calcareous sponges, decapods, and some annelids. Among the low-magnesium calcite contributors a r e a few benthonic fora- minifera, and some pelecypods and gastropods. The foram A!nl>histe,rrinit s e c r e t e s low- magnesium calcite (Fig. 7) which accounts for the unusually high percentage of LMC in sample TA-28.
COMPARISON O F SEDIMENTS ON REEF FLATS FACING OCEAN AND LAGOON
Reef flatsaroundthe islands extendbothseawardand into the lagoon, usually for appreciable distances (Fig. 1). A s a substantial proportion of the total sediment associated with the atoll is derived from these reefs, samples were collected from reef flat enviro~iments on each side of Bikenibeu Island for comparison with sediments depositedwithin the lagoon proper. Physical conditions a s well a s the relative abundance of carbonate contributors a r e significantly different on the two types of reef flat. That facing the ocean, f o r example, is subjected to constant, high-energy wave action. Coralline algae, regular echinoids, and corals such a s i2drillopwa and Ehvites a r e conspicuous on the exposed seaward reef flats, whereas irregular echinoids, Poviles, and calcareous green algae a r e more abundant on the f l a t s adjacent to the lagoon.
The species composition of the m3lluscan fauna of the two environments is also different.
As expected, the seaward reef surfaces retain little sediment with the exception of the beach and the small localized deposits in isolateddepressions. These sediments a r e relatively coarse grained throughout (av. median diameter 1.13; range of median diameter 0.53 to 1.80mm).
In contrast, the lagoonal reef flats a r e almost completely covered with carbonate sediment having a n average median grain size of 0.48mm (range of median 0.32 to 0.67). In this en- vironment, median grain size decreases slightly f r o m the middle of the flat to the edge of the lagoon. Samples f r o m 110% rezl flats a r e generally w 4 I s:,r*recl a s indicated by the low value3 of thz T r a s k sorting cocffic;e;l: (av. 1.8 for ths lagoon flat and 2.1 f o r the ocean flat). A:
Bikenibeu, the degree of sorting in.crzases from mid flat to lagoon edse but decreases f r o m mid flat to ocean edge, probably reflecting thefact that the only significant sedimt?nt accumula- tions in the exposed, high-energy wAv2 zon? on the seawxrd flat a r e fou?,l in comllaratively protecteJ. holes a n 5 depressions. Perhxp3 th,? :nost conspicuous difference be:w:en sediments of the lagoon and ocean sides of the island is the effect of abrasion caused by wave action.
Carbonate grains on the seaward f l a t s have a much higher degree of roundness and polish.
The vitreous l u s t e r enhances the Zolor of the component g r a i l l : m l imparts a distinctive a p p a r a n c e to the sedimtlnt.
Aragonite i s the most abundant mineral in a l l reef f l a t samples (Fig. 8) and in general, the lagoonal reef flat sedimznts contain a higher percentage than those on tho seaw?.rd flat.
From Bikenibeu beach to th,? edge of th? lagoon there is 1 regular increase in the aragonite content f r o m 50 to 84%, wlisreas on the ocean size of the island, the percentage of aragonite tends to decrease f r o m 65% a t the shore to 45% a t the reef edge. Low-magnesium calcite constitutes from to 2?h of the sediments on '.he lagoon reef flat but was seld,xn 'etected in s ~ m p l e s collected s n the seaward flat. Thus the aragonite and high-magnesium .calcite distri- bution patterns can be considered complem?ntary.
ORIG!:N O F THE LIME MUDS
The quantitative importance of limr: m u 1 in ?he stratigraphic record has be:?n emphasized by Matthews (19!35) who founditparadoxical th?tsiu.lies of Recent carbonate s e d i m m t s general- ly have concentrated on the genesis of the sand size particles rather than on the origin of the fine-grained constituents, In soms major coral reef provinces, carbonate sands a r e clearly
m o r e abundaot thnn carbonate silts o r milds. Sediinr-its of the Capricorn reef c o ~ n ~ ~ l e x of Australia, f o r example, seldom ,contain more than 2% mud (Maiklem, 1970). According to Maxwell (1968), very little fine carbonate is produced in the Grent B a r r i e r Reef system of Australia. a n 5 the mud present in sedimi.nis sf that region is predominanlly of terrigenous origin. Elsewhere the deficiency of fine particles and the high proportions of sand aud gravel may be explained by removal of the fines into deeper water, a s a t P e a r l aud Hermi,s Reef
(Thorp, 1936). Gross et al. (1969) found that particles l e s s than 125 microns in diameter a r e relatively r a r e on Kure and Midway Atolls except in the deeper p a r t s of the lagoons. Much of the fines apparently remxins in suspension and is transported out of the lagoon, leaving behind the coarse-grained sedim.?nt in shallow ,iv;lter. Calcareous oozes a r e reported in the deepest p a r t s of Kapingamarangi Lagoon, between 50 and 80 m , but McKee (1958) concludes that t h e m u d i s notlikely derivedprimarily from the residue of sedim.?nts occurring in shallower wlter.
The situation a t Fanning Island appears quite different in that fine-grained carbonates a r e unusually abundant. Muddy sediments in the lagoon a t Fanning a r e reported by Roy (1970) who attributes most of the material to physical and biological abrasion o r con~minution of corals, molluscs, and calcareous r e d algae. Water in Fanning Lagoon is characterized by
a
high degree of turbidity, with underwater visibility often limitedto 2 m o r less. According to Bakus (1968) the suspended particles a r e generally between 5 and 15 microns in diameter. Hence the presence o r absence of carbonate fines may be controlledmore by the degree of current winnowing than by actual production of thegrains. This is demonstrated in the case of Palmyra Atoll, a t least, by the drastic changes in the lagoon following the construction of inter-island causeways in the early 1940's. Dawson (1959) reported that reef growth in the lagoon ceased a f t e r beingdeprivedof circulatingwater. Thelagoonbecame murky throughout due to the suspen- sion of fine calcareous sediment, and inshore reef a r e a s which once supported luxuriantcoral growth are now deeply covered with carbonate mud.
Several possible mschanism.5 may be important in generating carbonate fines: inorganic precipitation; algal secretion of aragonite needles; and attrition of larger calcareous skeletons by mechanical or biological processes. The first of these, inorganic precipitation, i s contro- versial. Fine-grained sediments covering large portions of the Bahamz Banks, for exam?le, are believed to be of inorganic origin by some investigators (Cloud, 1962; Illing, 1954; Newel1 et al. 1960; Broecker and Takahashi, 1966) while others consider green algae to be the domi-
nant source (Lowenstam and Epstein, 1957). Milliman (1967) reports that most o f the lagoonal sediment at Hogsty Reef i s "non-skeletal" and probably precipitated inorganically. After studying sedimentation on Serrana Bank, Milliman (1969) later postulated that non-skeletal carbonates might form in open lagoon conditions only where biogenic carbonate secretion i s lacking.
Lowenstam's (1955) discovery o f alga-secreted aragonite needles established an im- portant endemic biogenic source of carbonate fines. Acicular crystals of aragonite, about 2 to 9 microns in length, areproducedinlargenumbers by species in the Codiaceae, Dasycladaceae, Nemalionaceae, and Chaetangiaceae. Fine-grainedsedimentsfromBermuda, Jamaica, Kayangle Atoll, Johnston Island, Guam? Ifalik, Eniwetok, and Bikini were found to contain such needles (Lowenstam ;and Epstein, 1957) and Lowznstam (1955) predicted that quiet water sediments in equatorial regions should carry them wherever the algae are present. Stockman et al., (1967) attribute most o f the lim? m ~ d in the south Florida area to the skeletal disintegration o f calcareous algae.
Although experimental destruction of large invertebrate skeletons by boring organisms and abrasion tend to demonstrate the difficulty in comminuting shells to very fine particle sizes (Driscoll, 19701, it appears that physical and mechanical breakdown i s an important mechanism o f mud formation in some areas. Matthews (1966) described lime muds of British Honduras which contain both calcite and aragonite butare virtually devoid o f aragonite needles.
His analyses indicate that mechanical comminution in shoal waters and breakage o f fragile mollusc and hyaline foraminifera skeletons in the lagoon are important processes in the production of these muds. Other significant mechanisms include mlsticatioll by echinoids and holothurians, and trituration of carbonates by scarid, chaetodontid, acanthurid, and pomacentrid fish (Cloud, 1952). Land (1970) reports that appreciable quantities of carbonate mud may be produced by epibiotic growth of coralline red algae and serpulid worm:? on the turtle grass Tlzalassia.
At Tarawa, lime muds are presently accumulating in the extreme southeastern corner of the lagoon in relatively shallow water (6-10 m ) . These sediments are cream to very light green in color, are extremely plastic, and poorly consolidated. The unusually soft bottom created minor sampling problem:3 in that the sediment grab, which is triggered on contact, would not always " f i r e J ' . Despite replicated sampling o f this area, heart urchins (Bvissopsis and Mavstia
3
and sand dollars (Laganurn sp.) were the only living macrobenthos recovered.The two spatangoids were considerably more abundant than Laganunz, and the significant con- centrations o f echinoderm skeletal debris in these sediments indicate that heart urchins are an important in situ source o f carbonate (Fig. 6).
Up to 80% o f the material in the eastern lagoon is found in the (0.074mm size fraction, and examination by both electron microscopy and scanning electron microscopy indicates that the bulk of the fines actually lies in the particle size range o f 1 to 10 microns. Although needles, presumably aragonite (Fig. 91 are not uncommon, electron photomicrographs show a variety of grain shapes including fragments of laths and needles, platy and tabular forms, and nondescript particles. Between 60 and 75% o f the carbonate and mud consists of aragonite.
Low-magnesium calcite amounts to 4 - 8%, and high-magnesium calcite accounts for the remainder.
7 In much of Tarawa Lagoon, the w t e r is turbid, and this i s especially true in the region Of
lime mud accumulation where underwater visibility i s limited to a few feet. The abundance of carbonate fines, the shallowness of the water, and the high degree of turbidity suggest the possibility of inorganic precipitation, although like the "whitings" in the Bahamas (Broecker and Takahashi, 19661, the turbidity i s probably due to the suspension and resuspension of previously formed carbonate rather than in situ precipitation by inorganic mechanismn.
Nevertheless, carbon and oxygen stable isotope ratio data were obtained to examine this possibility.
The isotopic composition Of the "J" size fraction (<0.078mm) was determined for four samples in the a r e a of lime mud deposition. 6 13C and 6 l a 0 values, with respect to the PDB C02 standard,* are:
6 1 Z c o
/oo 6 1 8 ~ o
/00
TA-8 t 1.95
-
2.53TA-9 t 1.70
-
2.77TA-10 t 1.86
-
2.55TA-12 + 1.53
-
2.79Admittedly the J size fraction is a mixture of aragonite, H M C , and LMC, each of which may have a different isotopic com:~osition. A plot of 6 1312 and 6 180 us. mineralogy, however, indicated no correlation. The average 6180 f o r the four lim,? muds in -2.66 O/oo, which corresponds to a crystallization temperature of 28.g0 C usingthe "pdleotemperature" equation of Epstein et al. (1953) and assuming precipitation of tWe carbonate in oxygen isotopic equili- brium. The 180 content of the ambient seavnter also enters into this equation, and since this was not determined, specimens of calcareous coelenterates which precipitate skeletal CaC03 in apparent oxygen isotopic equilibrium with surrounding seawater (Weber and Woodhead, 1970) were analyzed: Millepova
(N =
161, Tubipova (51, and HeUopova (13). The average6
180 of these 34 specimens is -2.29 (standard deviation 0.17 O/oo) which corresponds to a calculated crystallization temperature of 27.loC. The mean annual temperature of surface seawater a t Tarawa, a s obtained from monthly temperature distribution maps of the central Pacific, is 28.1°C.While the oxygen isotope ratios a r e similar to those predicted for inorganically precipitated carbonate, 6 1 3 ~ data for the limn mnds a r e several permil lower than the theoretical values, and a r e in the range of carbon isotopic compositions typical of biogenic carbonates. The presence of HMC and LMC in addition to aragonite further suggests that the lime m d s in Taraw?. Lagoon a r e derived from ihe breakdownof skeletal detritus. It is noteworthy that while the occurrence of aragonite needles i s indicated by electron microscopy, num?rous other grain shapes a r e also evident. It appears likely that the fines a r e a mixture of algal needles and particles produced by attrition of larger skeletal debris. On atolls, fine-grained carbonate i s generally found to be accumulating only in the deepest portions of the lagoon, e.g. between 50 and 80 m at Kapingamarangi (McKeeet al. 1959). According to McKee eI al. a "shallow- water environment favorable for such fine-grained deposits has not been reported from .any of the Pacific atolls". Tarawa, however, has extensive deposits of carbonate mud which a r e accumllating in shallow water. This can be attributed to protection of the sediment from current winnowing, which i n turn results from the near absence of deep channels connecting lagoon and ocean, and the windbreak provided by the reef islands.
*Using the well known 6 notation, where:
in per mil, relative to the PDB standard C02.
8
ACKNOWLEDGEMENTS
We a r e g r a t e f u l to D. Cudmore, G. J o n e s , a n d R. G. R o b e r t s of t h e Gilbert a n d E l l i c e Islands government f o r t h e i r g e n e r o u s s u p p o r t of t h e field operations, to N. Suhr f o r t h e elec- t r o n p h o t o n ~ i c r o g r a p h s , t o Dr. E. W. White f o r providing scanning e l e c t r o n m i c r o s c o p e facili- ties, to Dr. D. M. Raup f o r identification of the echinoids, t o Dr. A. L. Gnber f o r independent
examination of many of the s e d i m e n t s a m p l e s , a n d t o the U S . National Science Foulldatioll (J. N, W.) a n d the Australiaii R e s e a r c h G r a n t s Comnlission (P. M. J. W.) f o r r e s e a r c h support.
REFERENCES
Bakus, G. J., 1968. Sedlmentatloli aud benthlc I n v e r t e b r a t e s of Fanmnglsland, c e n t r a l Paclflc.
Marzn@ Geol. 6. 45-51.
B r o e c k e r , W.S. a n d Takahashl, T., 1966. Calclum carbonate preclpltatloil on the Bahama Banks. J . Geophqs. R e s . 71 1575-1602.
Cloud, P . E . Jr., 1952. P r e l l m n a r y r e p o r t on geology and m a r m e envlrotiments of Ouotoa Atoll, G l l b e r t Islands. Atoll R e s . Hull. 1 2 . 1-73.
- - -
1962. Environment of calcium carbonate deposition w e s t of A n d r o s Island, Baham;ls. U.S. Ceol. Surv. I'vof. I'a/)es 350: 1-138.Dawson, E. Y., 1959. Changes in P a l n i y r a Atoll a n d i t s vegetation through t h e a c t i v i t i e s of man, 1913-1958. I'ac. Naluv. 1 : 1-51.
Deines, P., 1970. M a s s s p e c t r o m e t e r c o r r e c t i o n f a c t o r s f o r the determination of s m a l l iso- topic composition variations of c a r b o n a n d oxygeu. InL. J. ilfass Sl)ectl*oin. Ion I'llys.
4: 283-295.
D r i s c o l l , E. G., 1970. D e s t r u c t ~ o t i of potential f o s s i l s by boring o r g a n i s m s a n d a b r a s i o n . Geol. Soc. Anlev. Abstv. f o r 1970 7 : 740-741.
E m ? r y , K.O., T r a c e y , J.I. Jr., and Ladd, H.S., 1954. Bikini a n d nearby a t o l l s , M a r s h a l l Islands. U.S. Ceol. Susu. 1'vo.f I'alx?~ 260A: 1- 265.
Epstein, S., Buchsbaum, R., Lowenstam, H.A., a n d Urey, H.C., 1953. Hevlsed carbonate- w a t e r isotopic t e m p e r a t u r e s c a l e . Dull. Gal. Sot. Ante?'. 64: 1315-1326.
Ginsburg, R.N., 1956. Environmental r e l a t i o n s h i p s of g r a i n s i z e a n d constituent p a r t i c l e s in s o m e south F l o r i d a carbonate sedim-ents. 13~11. A m w . A s s o c . l ' e t ~ o l . Geol. 40: 2384- 2427.
G r o s s , M. G., Mllllman, J.D., T r a c e y , J. I. Jr., a n d Ladd, H. S., 1969. Marnie geology of K u r e a n d M ~ d w a y Atolls, Hawall: a p r e l ~ m m a r y r e p o r t . i'uc. Scz. 23. 17-25.
illlng, L. V., 1954. Bahaman c a l c a r e o u s s a n d s . I3ull. A n t e l . Assoc. Petvol. Ceol. 38: 1-95.
L. S., 1970. Carbonate mud: procluctlon by eplhlont growth on Tltalassza lesludznunz.
J. Sed. I'et?fol. 40: 1361-1363.
Lewis, M. S. a n d T a y l o r , J. D., 1966. M a r i n e s e d i m e n t s a n d bottom communities of t h e Seychelles. I'vans. Ifo). Sac. Loizdon A259: 279-290.
Lowenstam, H.A., 1955. Aragonite needles s e c r e t e d by a l g a e and s o m e sedimentary impli- cations.
J.
Sed. Petzfol. 25: 270-272.Lowenstam, H.A.and Epstein, S., 1957. On the o r i g i n of s e d i m e n t a r y aragonite n e e d l e s of the G r e a t B a h a l n i Bank. .I.(kol. 65: 364-375.
Maiklem, W.R., 1970. Carbonate sediments i n the C a p r i c o r n Reef complex, G r e a t B a r r i e r Reef, Australia. J. Sed. k t v o l . 40: 55-80.
Matthews, R.K., 1966. Genesis of Recent l i m e mud in southern B r i t i s h Honduras. J. Sed.
Pelvol, 36: 428-454.
Maxwell, W. G. H., 1968. Atlas of the G r e a t B a r r i e r Reef. E l s e v i e r , Amsterdam.. 258 pp.
McKee, E.D., 1958. Geology of Kapingam:lrangi Atoll, Caroline Islands. B1d1 Ceol. SOC.
.4;lze7-, 69: 241-278.
McKee, E.D., Chronic, J., and Leopold, E.B., 1959. S e d i m ? n t a r y b e l t s i n lagoon of Kapinga- m a r a n g i Atoll. I%dl. Amev. Assoc. I'eWol. Geol. 43: 501-562.
Millimnn J.D., 1967. Carbonate sedimentation on Hogsty Reef, a Bahamian atoll. J . Sed.
p@lrol. 37: 658-676.
- - - - - - -
1969. Carbonate sedimentation on f o u r southwestern Caribbean a t o l l s a n d relatioil t o "oalite problem7'. Bull. Amez,. A s s o c . I'etvol. Geol. 53: 2040-2041.Neummn, A.C., 1965. P r o c e s s e s of Recent c a r b o n a t e sedimentation i n Harrington Sound, Bermuda. IjuZ1. I b l u ~ . Sci. 15: 987-1035.
Newell, N.D., P u r d y , E. G. andImSrie, J., 1960. Bahamianoolitic sand.
J.
GeoZ. 68: 481-497.ROY, K. J., 1970. Sedim?ntation and r e e f developmsnt inturbid-water areas of Fanning Lagoon.
IMl. A m m . Assoc. Petvol. Geol. 54: 867.
Stockmill, I<. W., Ginsburg, K. N., a n d Shinn, E.A., 1967. The production of l i m e m u d by a l g a e i n south Florida. J. Sed. PeWol. 37: 633-648.
Thorp, E.M., 1936. The sedilnents o f t h e P e a r l a n d H e r m e s Reef. J. Sed. I'etvol. 6: 109-118.
Weber, J . N . , 1968. Quantitative mineralogical a n a l y s i s of carbonate s e d i m e n t s : c o m p a r i s o n of X - r a y diffraction a n d e l e c t r o n probe m i c r o a n a l y z e r methods. J. &d. f'elvol. 38:
232-234.
Weher, J. N. a n d Woodhead, P.M. J., 1970. C a r b o n a n d oxygen isotope fractionation in the skeletal c a r b o l x t e of r e e f - h i l d i n g corals. Chem. Geol. 6: 93-117.
TABLE
1: SIZE DISTRIBUTlON STATISTICS
table 1 (continued)
TA- 26 27
28
29 30 31 32 33 34 35 36 37 38 39 40 12
43 4k
45 46 47 48 49
50
5 1
52
5?
1.2
table 1 (continued)
TA- 54 55 56 57 B E 1
2
3 4 5
6
B e 1 2
3 4 5 6
All TA samples a r e from Tarawa Lagoon e m e p t TA-22 which i s beach sediment from
B a i r i k i Island. BL = lagoonal. r e e f f l a t a t B i k t d b e u Is.; EO
=ocean r e e f
f l a t aBikenibeu Is.; Fd = median diameter i n m; 2 = Trask s o r t i n g c o e f f i c i e n t ; =
Trask c o e f f i c i e n t of skewness.
A B C D E F C Z -
- - = J - Av .
- 1 A 87 89 86 83 86 83 80 78 77 68 85
a0 9 2 . 2 15 13 u, 17 20 19 28 13
3 2 2 2 1 3 3 2 k b 2
T A - ~ A 88 89 90 91 88 86 88 81 78 79 W
m 9 7 7 7 9 u 9 u 1 7 1 7 13
3 3 2 3 3 3 5 5 b 3
3 A 89 89 89 88 91 86 86 84 79 78 87 m 7 7
89 7 l 2 U . 1 2 1 6 1 8 10
& I g : 4 4 3 3 2 2 3 b 5 4 3
TA-I, A $4 87 90 90 89 8 82 82 76 76 83
IP.rC13 9 7 8 8 8 lb 1 5 18 18 13
~ ~ ~ : 3 b 3 2 3 4 4 3 b 6 6
TA-5 A 83 89 86 94 91 83 87 82 83 75 85
7 1 0 k 6 1 3 9 Z 1 3 1 9 11
~ ~ ~ 3 b b 2 3 4 4 4 4 4 6
TA-6 A 76 76 76 88 95 96 9b 86 76 73 77 m 2 0
$5 9 4 3 5 1 0 1 6 1 9 1 5
L I E 4 1 6 1 9 3 1 1 1 4 8 8 8
TA-7 A 73 69 87 77 83 84 8 77 75 70 72
taK: 25 25 9 17 bl I 1 P1 17 19 23 22
W 2 6 4 6 3 5 5 6 6 7 6
TA-9
A73 7 1 72 78 7 b 85 - - 64 66
HMC
29 2L 2317
2511 - - -
2927
L W 7 5 5 5 1
L -- - 7 7
14
table
2(continued)
t a b l e 2 (continued)
?A-17
A86
ll
x x 3 TA-18
A84
Rwc 14
LMC 2 TA-19
A83
m u
LMC 3 TA-20 A 86
HHC 9
rn 5
TA-21
A76
DE 23
rn 1
TA-22 A 19 FiMC 8 1 LHC 0 TA-23
A80
HMC 17
rn 3
TA-24
A7 l
HMC
27
LMC 2
16
table 2 (continued)
t a b l e 2 (continued)
TA-33 A 88 88 H H C 9 9 W c 3 3
A -
A 82 76
RMC 16 19 L F I C 2 5
TA-35 A 91 90 H K 7 8
LHC 2 2 TA-36 A 88 85
Hm: 10 12 LMC 2 3
TA-37 A 87 84 H H C l l l l
m 2 5
TA-38 A 87 89
HMC 12 10
m 1 1
TA-39 A 59 55 HMC 34 16 I.243 7 29 TA-40 A 79 57 H X 18 25
LMC 3 18
18
table 2 (continued)
TA-12 A 89 79 10 16 L M : 1 5
?A-43 A 88 81 HWC 11 16 m c 1 3 TA-44
A89 79
HMC 7 17
mc 4 4 TA-45 A 89 83
HMC 9 13
m 2 4
TA-46 A 84 84 12 13 m 4 3 TA-47 A 80 78
HMC 17 23
L H C 3 2
TA-48 A 85 90 H M C 8 7
= 7 3
TA-49 A 87 84
HtE 10 13
LW 3 3
table 2 (continued)
table 2 (continued)
C
Wa 52 74 5b 39
2829 - - - 53
LMC 0 0 3 3 0 0 0 - - - 0
t a b l e 2 (continued)
3
mm range of s i z e f r a c t i o n s s p e c i f i e d i n text, A = aragonite; HXC
-^Mgh-magnesium c a l c i t e ; W = low-magnesium c a l c i t e .
SAMPLING
STATION
F i y v e 2: Median grain size of lagoonal sediments along the sampling traverse from Bonriki (eastern lagoon) to the western reef margin.
Location of sampling stations given in Fig. 1.
i s y e 3: Mineralogical compoaition a s a. function of grain size f o r typical samples ( n u m k r below curve corresponds to sampling station showninFig. 1). Curves represent the percentage of each grain size category that is aragonite. Grain size ranges corresponding to classes A and J a r e specified in text.
E'i,rn*~m 4: Distribution of total sam;>le aragonite among grain s i z e c l a s s e s f o r sediment s a m p l e s along the TA-I0 (easterillagoon) to TA-28
(western lagoonal reef margin) transect. The height of a column i n a histogram indicates the percentage of the total sample aragonite that is in the grain s i z e range designated by the corresponding alphabetic c h a r a c t e r (millimeter equivalents specified in text).
F i e 5: Systematic changes in the distribution of aragoilite amollg grain size classes for sediments along the TA-53 to TA-57 transect in the northern corner of the lagoon (locations in Fig. 1). Curves B and
J indicate the percentage of total sample aragonite that is in graill size classes B (coarse) and J (fine) respectively.
SAMPLING STATION
F i ~ v e 6: Percentage of four m a j o r components (>0.5mm grains) of the sediments in T a r a m Lagoon. Geographic locations of the sampling stations a r e shown in Fig. 1.
E'i-wve 8: Comparison of aragonite distribution in lagoon and seaward r e e f flat sediments.
Location of s a m p l e s indicated in Fig. 1. The height of a column in a histogram indicates the percentage of the total sample aragonite that is in the grain s i z e range desigmted by the c o r - responding alphabetic c h a r a c t e r (millimeter e q u i w l e n t s specified in text).
I..ip.we 9: Electron photolnicrograph of the fines ((0.074mm) f r o m sample TA-8, showitlg whole and fragmented aragotlite needles and
l a t h s . The i r o m i n e n t da,rk colored acicular c r y s t a l a t lower right i s between 1 and 2 m i c r o n s in length.
