Plate 21 - 00113.pdf

PPRT 11: OTHER FEATWS by F. R . Fosberg

S o i l s

The loose sediments p i l e d above high-tide l e v e l on a t o l l i s l e t s a r e composed alrslost e n t i r e l y of clcistic limestone debris, mostly of organic o r i g i n and varying i n s i z e from t h e f i n e s t s i l t - s i z e p a r t i c l e s t o enormous boulders many t o n s i n rieight ( p l . 2 3 ~ ) . This m a t e r i a l i s remarlrably uniform i n chemical ne.ture but 'diverse i n b i o l o g i c a l o r i g i n , comprising skeletons and fragments of skeletons of such lime-secreting organisms a s c o r a l s , Foraminifera, niolluslrs, echinoids, and calcareous red and green algae, a s riel1 a s a l l s i z e s of frapgents of limestone con- s i s t i n g of such skeletons cemented t o g e t h e r b y ' r e c r y s t a l l i z e d Calcareous and phosphatic materials ( p l s . 28-31, 33). These a r e mixed i n varied proportions rrithout a regulnr p a t t e r n of d i s t r i b u t i o n . The t e x t u r e range i s extremely wide and t h e range i n s o r t i n g i s f a r wider than t h a t described above f o r t h e beach m a t e r i a l s . They a r e i r r e g u l a r l y s t r a t i - f i e d ( p l . 25C), t h e s t r a t a mostly representing stages o r events i n t h e deposition 0.2 t h e m a t e r i a l . There seems t o be l i t t l e r e g u l a r i t y about t h e v e r t i c a l o r a r e a l arrangement of d i f f e r e n t grades and mixtures of t h e s e sediments, although i n general t h e lagoon s i d e s of t h e i s l e t s a r e more l i l r e l y t o be sandy i n t e x t u r e , and t h e seay.rnrd s i d e s a r e more o f t e n of coarse m a t e r i a l ( p l s . 21-23). The sampling, unfortunately,

s c a r c e l y represents t h e coarser r ~ m g e of t h e s e sediments. Previous des- c r i p t i o n s of s i m i l a r m a t e r i a l have been given by S o l l a s and o t h e r s

(1904), David and Sweet ( l 9 0 4 ) , Cloud (1952), Newel1 (1954a, 1954b, 1956), Posberg (1954), T e r c i n i e r (1955), Schlanser and Brookhart (1955), McKee (1956, 1958), and Fosberg and o t h e r s (1956).

The loose, unconsolidated m a t e r i a l s on t h e a t o l l s a r e t h e parent m a t e r i a l s of t h e majority of t h e s o i l s developed.

The s o i l s on t h e i s l e t s f a l l i n t o f i v e p r i n c i p a l types (Stone,

1951a.j

Fosberg, 1954) ^:

1. E s s e n t i a l l y unaltered sands and gravels.

2. Stony and veiy stony a r e a s .

3.

Shioya s e r i e s .

4.

Arno A t o l l s e r i e s . 5. Jemo s e r i e s .

These a r e described i n d e t a i l i n P a r t

IV.

Drainage

The m a t e r i a l of t h e i s l e t s i s g e n e r a l l y so porous t h a t drainage by p e r c o l a t i o n down through t h e ground i s p e r f e c t and almost instantaneous.

There i s no running surface .:rater, except; during typhoons o r tsunamis,

when greo,t sea waves may sweep across t h e i s l e t s . Norimally t h e r e i s no standing surface water except where depressions o r t a r o p i t s extend down t o below t h e maximum prater t a b l e . An exception t o t h i s occurs on i s l e t s

subjected t o i n t e n s i v e m i l i t a r y tra;ffic, such a s K ~ r a j a l e i n . Here t h e surface l a y e r s become so compacted and, apparently, so cemented a s t o become more o r l e s s impervious. Water puddles may stand i n such a r e a s a s long a s 24 hours a f t e r heavy r a i n s .

Vegetation

Most of t h e o r i g i n a l vegetation o f t h e northern Marshall I s l a n d s has been replaced by coconut p l a n t a t i o n s . This i s e s p e c i a l l y t r u e on t h e l a r g e r i s l e t s . Only on Taongi ( ~ o k a k ) ( p l . 2 4 ~ ,

B);

Bikar, and Wotho A t o l l s was it p o s s i b l e t o study considerable a r e a s of apparently undisturbed vegetation; on Lae, Ujae, Ujelang, and Krrajalein A t o l l s

smaller areas were s t u d i e d ( s e e Fosberg,

1953, 1955,

Fosberg and o t h e r s , 1956). As extensive r e p o r t s on t h e vegetation a r e t o be published e l s e - vhere only a b r i e f sunmary need be given here.

The coconut p l a n t a t i o n s (pl. 24C, D ) range i n d e n s i t y from almost complete cover i n more moist a r e a s t o q u i t e sparse i n t h e dry northern- most a t o l l s . The ground cover under t h e t r e e s ranges from g r a s s and

o t h e r herbs t o a t h i c k t a n g l e of bushes, vines, and t r e e s , depending; on t h e climate and on how d i l i g e n t l y t h e weeds a r e kept c l e a r e d .

On t h e seaward s i d e s of most i s l e t s a b e l t of thicls scrub and scrub f o r e s t ( p l . 2 ~ ) i s l e f t t o p r o t e c t t h e coconut t r e e s f r o m exces- s i v e wind and s a l t spray. On t h e vind3rard s i d e t h i s tends t o be very dense and t o slope gradually t o t h e beach. On more s h e l t e r e d s i d e s t h i s b e l t tends t o be narrower, t a l l e r , l e s s dense, and with a more abrupt slope t o t h e beach, both because 'trees grow c l o s e r t o t h e beach and because shrubs and t r e e s a r e l e s s stur-ted by wind and s a l t spray.

011 sinaller i s l e t s , and on a r e a s l e f t undisturbed on l a r g e r ones, t h e r e a r e s e v e r a l types of f o r e s t - - p u r e stands of g i a n t soft-wooded Pisonia t r e e s ( p l . 25), of f a n t a s t i c Pandanus, of umbrella-lilse

Ochrosia, o r mixed stands or^ and s e v e r a l hardvood species. These f o r e s t s comnonly have dense canopies and l i t t l e undergrowth. Around t h e edges a dense scrub f r i n g e gives an appearance of impenetrability.

On sand s p i t s , bars, and narrow places on i s l e t s a sparse scrub of pioneer species ( p l s . 26, 27) i s found, grading i n t o f o r e s t .

On very small o r very rocky i s l e t s t h e woody vegetation may be low and dense, o r of i r r e g u l a r l y s c a t t e r e d t r e e s m d shrubs with

patches of s p a r s e bunchgrass and t h i n low scrub, rock f l a t s may be com- p l e t e l y bare of vegetation. Such a s p e c t s become more and Inore pre- dominant a s one goes northward i n t h e Marshall I s l a n d s , u n t i l they c h a r a c t e r i z e almost t h e e n t i r e vegetation of a r i d Taonyi ( ~ o k a k ) A t o l l ( p l . 24A, B ) .

P r i n c i p a l reef -for~ning animals and p l a n t s

Corals, by J. W . Wells

The r e e f b u i l d i n g c o r a l s of t h e Marshall I s l a n d s include 52 genera of t h e S c l e r a c t i n i a , 2 genera of t h e Alcyonaria ( ~ e l i o p o r a and Tubipora), and 1 hydrozoan genus ( M i l l e p r a )

.

The slreletons o f y l l t h e s e a r e a r a g o n i t i c , analyses showing CaC03, 98.05-99.71 percent;

14gCO3, 0.09-1.11 percent; and minute amounts of Si02, ( ~ l , F e ) ~ c ~ , and t r a c e s 0.f CaSOq and C c ~ ~ ( P 0 4 ) ~ . The t e x t u r e of t h e skeleton ranges from porous o r spongy i n such r a p i d l y growing forms a s Acropora, P o r i t e s , and Montipora t o r e l a t i v e l y dense and s o l i d i n such r e l a t i v e l y slo-"7 growers a s

-'

Favia PocilLopora, and Heliopora.

The protean s c l e r a c t i n i a n genus Acropora, with a bevildering arre,y of species

(298 w,

b u t l e s s than one-third v a l i d ) , i s e a s i l y t h e dominant c o r a l everywhere on t h e Indo-Pacific r e e f s except i n regions

geographically p e r i p h e r a l t o t h e reef zone. Judging from i t s frequency i n reef rock and loose d e b r i s , it accounts inmany p l a c e s f o r tinree- q u a r t e r s o r even more of t h e mass. Locally, according t o e c o l o g i c a l c o n t r o l s , Acropora niay be q u m t V ~ a t i v e l y secondary t o a few o t h e r genera: on a l g a l r i d g e s t o vind\rard, Pocillopora corninonly _.- i s almost t h e only s c l e r a c t i n i a n , follo%red by t h e hydrozoan Millepora. Behind t h e ridge, Acropora i s dominant over those p a r t s of t h e reef f l a t t h a t do not "dry" a t - o r d i n a r y lorl t i d e . Onmany Indo-Pscific reef f l a t s , b u t only r a r e l y i n t h e Marshall 1slands;branching Montipora may be

l o c a l l y jbundant even t o v i r t u a l exclusion of o t h e r corals- Near shore, e s p e c i a l l y where t'ne substratum i s of s h i f t i n 2 loose deb-is, Acropora diminishes and dominance i s assumed by species of % r i t e s o r

--

t h e alcyonarizn "blue coral" Heliopora. Lagoon r e e f s a r e e a s i l y dominated t o considerable depths by A c r o p r a , except c l o s e t o t h e shore. On Marshall I s l a n d s r e e f s t h e o v e r a l l order of a u a n t i t a t i v e importance appears t o be: Acropora, P o r i t e s , Pocillopora, a d Helio-

porn,

with Montipora, Astreopore, t h e f a v i i d genera ( a s Favia

- J

F a v i t e s , Platygyr?, Goniastrea, Leptastrea, Cyphastrea, and P l e s i -

zstrea),

and a l l otiners much f n t h e minority.

Foraminifera, by Ruth Todd

. . . . ,

Four species of F'oraninifera a r e t h e chief r e p r e s e n t a t i v e s of t h i s group of animals found i n t h e sediments a d s o i l s of t h e northern

Marsha.11 I s l a n d s : _{. .}

Calcarina s p e n g l e r i

inel el in)

I n c e r t a i n of t h e sands t h e four t o g e t h e r cofi~prlse t h e b u l k of t h e m a t e r i a l and, i n some places, C a l c a r i m s p e n g l e r i (Gmelin) alone ac-

counC,s f o r most of i t .

T'ne e n t i r e skeletons of these four genera a r e composed of c a l c i t e . A l l a r e r a t h e r thick-walled forms with numerous i n t e r i o r chambers ~ m d

consequently they a r e more r e s i s t a n t t o abrasion on t h e beaches than a r e t h e smaller, thinner-walled and more f r a g i l e specimens of F o r m i n i - f e r a t h a t comnp,rise t h e remainder of t h e l a r g e foraminifera1 fauna of t h e Marshall Islands.

So f a r a s i s known a t present, Calcarina spengleri ( ~ m e l i n ) l i v e s only on t h e reef f l a t (~ushman, Todd, and Post, 1954, p . 364). Margino-

pora

v e r t e b r a l i s B l a i n v i l l e l i v e s both on t h e reef f l a t and i n water of shallow t o moderate depths i n s i d e and outside t h e a t o l l . The l a r g e r , t h i c k e r and more robust forms of t h i s species a r e presumed t o have l i v e d on t h e reef f l a t o r i n s h a l l o ~ , ~ ~rrater. Homotren~a rubrum (Lamarck) i s an encrustation t h a t occurs both on t h e reef f l a t and i n water of probably only shallow depth.

Amphistegina madagascariensis D'Orbigny probably does not a c t u a l l y l i v e o n t h e reef a t a l l , although it - i s found t h e r e i n small q u a n t i t i e s . Elsewhere, it i s very abundant. It probably l i v e s both i n s i d e and out-

s i d e t h e a t o l l s from shallow t o moderate depths, with l a r g e r and t h i c k e r - walled specimens (such 0.8 those i n t h e samples l i s t e d below) o r i g i n a t i n g i n t h e shallower p a r t s of t h e lagoons.

Table 1 records t h e d i s t r i b u t i o n of these major species i n t h r e e groups of samples. Samples i n ( a ) with mainly f r e s h and unworn specimens, i n d i c a t e l i t t l e t r a n s p o r t from t h e i r place of origin; i n ( b ) with worn and some polished specimens, i n d i c a t e long t r a n s p o r t and ( o r ) pro-

longed abrasion; and i n (c), with corroded specimens, may i n d i c a t e attacls by a c i d solutions.

Algae, by M. S. Doty

I n t r o p i c a l seas c e r t a i n algae a r e major accumulators of t h e material t h a t becomes deposited a s calcareous sediment o r rock t o form

t h e a t o l l s and i s l a n d s wound and on igneous rock bases (Ladd and others, 1353). These algae dominate t h e a t o l l r e e f s ^andmuch of t h e lagoon

bottom area. A r e a d i l y available i l l u s t r a t e d resume of these algae has been published by Johnson (1354). Fragments of algae broken from t h e reef patches and a t o l l reef edges by wave a c t i o n o r t h e browsing a c t i v i - t i e s of such animals a s t h e p a r r o t f i s h e s ( ~ c a r i d a e ) o r surgeon f i s h e s (Acanthuridae) may wash i n t o t h e 1a.goon where they contribute t o i t s f i l l i n g o r accumulate a s i s l a n d s on reef t o p s . Calcareous algae, when ingested by animals such a s these f i s h e s , a r e defecated l a r g e l y a s f i n e sediment.

Q u a n t i t a t i v e r e l a t i o n s h i p s of f o u r major constituents of present- day r e e f s a r e given i n t a b l e 2. Among t h e red algae t h r e e o r four genera a r e most conspicuous on t h e r e e f s of today, Porolithon, Gonio- l i t h o n ,

Jania,

and Amphiroa.

Table 1.--Foraminifera in various types of terrest.Nelsediments on a$olls in the northern Eplrshall Islands Analyst: Ruth Todd

(a) Samples containing fresh (unworn) Foraminifera

Foraminif era 14 18 53 57 60 75 76 92 105 112 126 129 139 156 253 275

Marginopora vertebralis X X . X X X

X

X X

....

^{X' X}

...

^X ^X ^X ^X

Amphisteginamadagascariensis X X X

...

^X ^X ^X ^X

...

I...

...

^X ^X

Calcarina spengleri

...

^X ^X ^X ^X ^X

^...

^X ^X ^X ^X ^X

^...

^X ^X

Houotrema

...

^X ^X

...

^X ^X

...

(b) Samples containing worn Foraminifera

22 25 29

34

36 44

70

96

104

u o

115 116 120 122 132 150

166

251 254 255 Marginopora vertebralis X X X

...

^X ^X ^X

...

Amphisteginarcadagascariensis X X

...

^...

^X ^X ^X ^X ^X ^X ^X ^X ^X ^X ^X

...

Calcarina spengleri X X X X X X X X

...

^X ^X ^X

...

^X ^X ^X ^X

...

^X ^X

Houotreme X

...

^X ^X

...

'/P P P P P P P

n

^{loo 102}

^{u 4}

, -

Marginopora vertebralis

...

^X ^X ^X

Amphistegina msdagascariensis X X

...

^X ^X

Calcarina spengleri X X X X X X X

polished specimens.

The genus Porolithon i s t h e b u i l d e r of sea edges of r e e f s , it i s perhaps t h e p r i n c i p a l organism m.alcing up t h e pink stony c r u s t s and heads t h a t may coalesce t o farm t h e b u t t r e s s e s and ridges of t h e reef margin. These b u t t r e s s e s and overhangs a r e , on occasion, broken o f f by storms and deposited inland o r on t h e reef f l a t , o f t e n a s huge and conspicuous reef boulders. Smaller fragments make up much of t h e i s l a n d gravels and sand g r a i n s . It i s t o be noted t h a t some of the huge chunks broken from t h e reef edge by storms become lodged i n t h e

grooves between t h e marginal reef b u t t r e s s e s and t h e r e , a~fter being overgrown, contribute t o tile roofing-over of t h e inward ends of t h e grooves. I n t h e c e n t r a l P a c i f i c t h e material seen i n such a r e s, o f t e n a s a pink pavement, i s perhaps 90 percent Porolithon onkodes.

9'

Goniolithon, another of 'the lime-producing genera, i s a den- d r i t i c a l l y branched a l g a of t h e more inward reef f l a t s and passes between a t o l l i s l a n d s . It does not form l a r g e d e t r i t a l pieces, being f r i a b l e , and i s most notably a sand producer.

Jania

and Rmphiroa a r e f l e x i b l e jointed c o r a l l i n e algae t h a t form patches of branches a few centpmeters high and a millimeter o r s o i n diameter. Upon breaking loose from t h e i r s i t e of growth on t h e reef f l a t s they d i e , and t h e s o f t p a r t s of t h e j o i n t s decay leaving calcareous sedin~ent o r sand.

This red a l g a l d e t r i t a l m a t e r i a l around Johnston Island (Emery 1956, p . 1511) may be the p r i n c i p a l l a g o o n - f i l l i n g sediment. I n many places a green alga, H~limeda, i s t h e p r i n c i p a l constituent of t h e sand t h a t b u i l d s t h e i s l a n d s , a s i n t h e Caribbean and Bermuda, o r one of t h e p r i n c i p a l constituents of lagoon sedlrilents, a s i n t h e northern Marshall Islands a t o l l s (Emery and o t h e r s 1954., p. 58). Halimeda i s composed of branches t h a t a r e made up of heavily c a l c i f i e d , f l a t , wal'erlike segments. These segments p e r s i s t a s sand grains of a r a t h e r coarse kind a f t e r t h e death of t h e p l a n t . While Halimeda does grow on reef f l a t s , e s p e c i a l l y t h e broader and what we f e e l t o be t h e older reef f l a t s , it grows most conspicuously i n t h e lagoons where it may densely cover t h e s i d e s of t h e lagoon r e e f s and reef patches and form meadows on the shallower bottoms.

The chemical composition of t'nese algae i s sho>m on t a b l e

3 .

Further s t u d i e s of chemical composition a r e reported by Clarlre and Wheeler (1917), Lemoine (1911, p. 38-43), and Johnson (1954). The high magnesium content of t h e red algae i s t o be noted. This may be one of t h e sources of t h e dolonlitizing magnesium s a l t s i n a t o l l a r e a s . Whereas t h e r e a r e t h e o r i e s t h a t c e r t a i n phosphorites have a r i s e n under marine conditions upon decay of marine organisms, from t h e very white color of young f o s s i l m a t e r i a l and t h e low P2O5 content, it i s unlike13 t h a t these marine algae play such a r o l e . L i t t l e i s yet known of t h e biomass of these rocklilce organisms. From t h e chemical analyses it i s

c l e a r t h a t they a r e mostly inorganic and thus could be regarded a s mostly nonliving:.

ir/

The o f t - r e f e r r e d - t o genus Lithothamion is, v i r t u a l l y , not t o be found on reef surfaces.

Table 2.

-

Quantitative r e l a t i o n s h i p s of t h e four major constituents

-

^of

r e e f s

-

Pearl and Southeastern

Constituents Hermes Reef F l o r i d a ~ a h a m a a Australia Algae, calcareous

Mollusk

Coral, madreporarian Foraminifera

T o t a l ( p e r c e n t ) Constituent r a t i o s Algae/coral

Algae/mollusk Algae/Foraminifera Mollusk/Foraminifera Mollusk/coral

Coral/Forminif e r a

g ~ a t a from T . W. Vaughan,

191.7.

Y ~ a t a from Thorp, 1936, p. 52. ^'

The r e s u l t s of Odum and

Oaum

(1955) indicated t h a t t h e r e i s uniform concer~tration of chlorophyll (equivalent t o between 0.05 and 0.10 of Codium edule,

dry

weight) over t h e reef surface regardless of substratum, whether

it

be c o r a l l i n e c r u s t , animal coral, o r d e t r i t a l . Sargent and Austin (1949) and Odum and Odum (1955) have shown t h a t t h e r e may grow and be deposited a s much a s

1.4

^{t o}

1.6

cm of m a t e r i a l per year over a reef surface. Under more i d e a l conditions growth may be much f a s t e r . For all. p r a c t i c a l purposes, reef surfaces a r e generally

i n equilibrium with a s much m a t e r i a l being removed by erosion as i s deposited by t h e l i v i n g organisms. !Tnus t h i s increase of about

1.5

cm per year times t h e a r e a involved can be used as a f i g u r e f o r ca;Lculating a

m a x i m

amount of m a t e r i a l depositable a s e l a s t i c s t h a t could become s o i l .

Table 3.--Partis1 chemical composition of some sediment-forming red algae

percent of dry :reight --

Sources: Cle.rke

and

Wheeler, 1917; Lemoine, 1911, Johnson, 1954

C 0 2

i n C02 i n Moisture

ashed sarcples i n

Algal s a n p l e s g not Organic a i r - d r i e d

species i g n i t e d matter samples Si02 R2O3 CaO

fCg0

SO3 P2O5 SrO Totcl Porolithon . . . ^37.5 g l 2 . 9 1.34

T r T r

33.3 7.49 .68

1.03 94.2

onlrodes ... ^35.3 g l 2 . 9 1.40

a .

do. 32.7 7.16 2.32

1.21 33.0

Porolithon 3.43 36.8 12.7 .67 do. do. 34.0 7.6k 1.03 .25

1.10

94.2 gardneri 2.96 37.4

11.8

.68 do.

20.

34.3 7.62 .28 .38 1.18 93.6

Poroli thon 3.27 28.2 21.5 1.72

60.

do. 31.9 8.62 2.05

.97 96.0

aequinoctiale 2.88 28.4 21.0 1.54 do. do. 31.9 8.46 1.84

1 1.10

94.2 -- --

W ⁰

1/ Detemined by

H C 1

t i t r a t i o n . Loss i n ~ r e i g h t t o 550'

C . I

Note: These percentages should not t o t a l

100

percent,

a l l t h e constituents $!ere not deterz!ined. The

figures f o r

C 0 2

i n ashed s m p l e s should not be included i n t o t a l s .

Legends of plates

Plate

23.

Miscellaneous deposits.

A. Dunes on sand s p i t , south end of Enajelar i s l e t , east side of Ailuk Atoll.

B. Large boulder i n i n t e r i o r of Kamwome i s l e t , Taongi ( ~ o k a k ) Atoll, apparently carried some hundred

meters

, inland by storm waves.

C. Mass of pumice pebbles on surface of ground, i n t e r i o r of south extension of Ebbetyu ( ~ b e j u ) i s l e t , Ujae Atoll.

D.

Top of broad boulder ridge, apparently piled up by ty- phoon, Utirik i s l e t , U t i r i k Atoll.

Photos by Fosberg.

Plate

24.

Types of vegetation on i s l e t s .

A. Lepturus grassland with shrubs of

-

Sida f a l l a x and scat- tered t r e e s of Messerschmidia argentea, boulder covered with guano, on Shioya s o i l series, Kamwome i s l e t , Taongi

( ~ o k a k ) Atoll.

B. sparse; half -dead Messerschmidia woodland on cobble f l a t s , Kammme i s l e t , Taongi (Pokak) Atoll.

C. Coconut grove with thick undergrowth on cleared, disturbed Jemo s o i l , Jemo island.

D.

Grassy opening i n coconut grove on Shioya s o i l , Jemo i s - land.

Photos by Fosberg.

Plate 25. Pisonia f o r e s t and f o r e s t s o i l .

A. Giant Pisonia grandis tree, Ebbetyu ( ~ b e j u ) i s l e t , Ujae Atoll.

B. Pisonia f o r e s t with grassy ground cover, Jemo island.

C. Soil t e s t p i t i n Pisonia f o r e s t on Bikar i s l e t , Bikar Atoll, showlng buried hardpan layer of Jemo s o i l series.

.

Pisonia forest, Bikar i s l e t , Bikar Atoll, showing dark humus layer on the surface of the ground, Jemo s o i l series.

Photos by Fosberg.

Plate

26.

Shore and sand-flat vegetation.

A. Mixed f o r e s t on stony s o i l , Bokerok i s l e t , Ujae Atoll. Wave- c a s t log, probably Douglas f i r from northwest America, i n foreground.

B.

Scrub f o r e s t of Pemphis acidula on rock platform surface, Jabwe i s l e t , Ailuk Atoll.

C. Well-developed fringe of Scevola scrub a t top of gravel beach, leeward side, W o i s l e t , Ujelang Atoll.

D.

krkening of bare coral sand by crust of blue-green algae, open areas on west end of Lae i s l e t , Lae Atoll. This crust may contribute t o the nitrogen supply of the s o i l .

Photos by Fosberg.

Plate

27.

Pioneer vegetation.

A. Low shrubs of Pemphis acidula on denuded rock platform surface, Jabanngit islet, Ailuk Atoll.

B.

Young Scaevola md Messerschmidia plants colonizing gravel bar on south reef of Lae Atoll.

C. Scaevola m d Messerschmidia colonizing gravel flat on Kabben islet, Wotho Atoll.

D.

Young Scaevola plants colonizing upper part of gravel beach, lagoon side of Bwdije (Breje) islet, ~aongi(~0ka.k) Atoll.

Photos by Fosberg.

PART 111: LABORATORY EXAMINATION OF UNCONSOLIDATED SEDIMElVS by Dorothy C a r r o l l

Unconsolidated calcareous i n a t e r i a l occurs on t h e beaches sur- rounding t h e i s l e t s on t h e a t o l l s , and i n t h e i n t e r i o r s of t h e i s l e t s a s s o i l s . Beach sands have been aescribed from Bikini by Emery, Tracey, and Ladd (1954), Onotoa by Cloud (1952), Raroia by Byrne (& Newell, 1954b, 1956), Kapingamarangi by McKee (1958, 1959), and from various o t h e r P a c i f i c I s l a n d s by urentworth and Ladd (1931). The s o i l s on Arno A t o l l have been described by Stone (1951.a,b), and on Bilrini by Stone ( i n Emery and o t h e r s , 1954). Fosberg (1954) gave a general description o T t h e s o i l s of t h e northern Marshall I s l a n d s and recognized a new s o i l type, t h e Jemo s o i l .

The m a t e r i a l s present a s beach sands together with l a r g e r f r a g - ments, s o l i d r e e f materials, and organic matter, provide t h e parent m a t e r i a l f o r t h e s o i l s . The sands described here a r e representative of t h e f i n e r r a t h e r than t h e coarser m a t e r i a l s of t h e beaches.

Tlle a t o l l s from which beach sands were c o l l e c t e d and described, and t i e number of samples from each a.toll a r e : A i l u l c 1, Bikar

15,

Kwajalein 8, Lae 2, Likiep 2, Taka 4, Taongi (Pokak)

31,

Ujae 2,

Ujelmg

8,

U t i r i k

5,

and Wotho

7,

& . t o t a l of

85

samples. The l o c a l i t i e s a r e l i s t e d i n Appendix I m ~ d indicated on f i g u r e s 5-15. A description of 28 of t h e s e samples i s given i n Appendix I.

The a t o l l s from which

-

s o i l s were c o l l e c t e d end described, &d t h e n w ~ b e r of samples from each a t o l l are: Ailuk 11; Bikar 26, Jemo 18, Kwajelein

5,

Lae 12, Likiep 6, Taka 29, Taongi ( ~ o k a k )

15,

Ujae

16,

Ujelang 3, U t i r i k 22, nnd Wotho 23, a t o t a l of 186. Vhen t h e samples were collected, a number of p i t s was dug through t h e s o i l s so t h a t m a t e r i a l from d i f f e r e n t depths could be c o l l e c t e d . Each vertLcal

sequence of samples i s r e f e r r e d t o a s a s o i l p r o f i l e . The positions on t h e i s l e t s from which s o i l p r o f i l e s were c o l l e c t e d (except t h e Kwajalein p r o f i l e s ) a r e shown on f i g u r e s 16,

17,

and 18. F i e l d descriptions of t h e analyzed s o i l s a r e given i n Appendix

11.

Unanalyzcd s o i l s a r e i n d i -

cated by appropriate sflt~bols on t h e maps i n order t o give a more adequate idea o f t h e d i s t r i b u t i o n of t h e s e v e r a l s o i l s s e r i e s within t h e i s l e t s .

Size distri'bution

---

To obtain t h e s i z e d i s t r i b u t i o n of t h e m a t e r i a l s i n t h e sands and s o i l s , a l l +,he.samples were sieved dry through a s e t of U.S. Standard Sieves t o give grades as o r i g i n a l l y described by Wentworth (1922). The s i z e s an6 sieves a r e :

, ' .

Size, mm Descriptive term Sieve No.* I "

above 16

16 - ⁸

8 - 4 4 - 2 2

-

^0.5

0.5

-

^0.25

0.25

-

^0.12

0.12

-

^0.06

l e s s than 0.06

Pebble Pebble Pebble

Granule

5

Very coarse sand

9

Coarse sand 18

Medium sand

35

Fine sand

60

Very f i n e sand 120

S i l t 230

---*---

*Sieve through ghr11ich sample passed; it was r e t a i n e d on t h e next f i n e r sieve i n t h e s e r i e s .

.In some samples t h e coarser material, generally c o r a l fragments, was removed with a one-half inch (12.7 m ) o r a one-quwter inch (6.35

mm)

s i e v e before t h e f i n e r grades vere sieved.

The s m p l e s c o l l e c t e d by F. S t e a r n s MacNeil were sieved by him through sieves

3,

9, 14

and 35, corresponding t o openings of 6.35, 4.76, 2.0,

1.9,

and 0.42 m, r e s p e c t i v e l y . These sieves do not corres- pond t o those used f o r most of t h e samples, but a r e s u f f i c i e n t l y close n o t t o cause s i g n i f i c a n t d i f f e r e n c e s i n t h e s i z e d i s t r i b u t i o n of t h e sands. - MacNeil's niaterial passing t h e

35

mesh sieve was not o r i g i n a l l y sieved i n t o f i n e r f r a c t i o n s , but a s a number of these samples contained more than 50 percent by weight of smaller than

35

mesh grains, these were resieved t o conform t o t h e rernalnder of t h e samples.

B i o t i c composition

The beach sands and s o i l s c o n s i s t of reef organisms and those t h a t ^! l i v e d i n t h e lagoons. The o r i g i n a l shape, s i z e , and buoyancy of t h e s e I

organisms determine t o some e x t e n t t h e s i z e d i s t r i b u t i o n , s o r t i n g , and

general appearance of t h e s e m a t e r i a l s . The admixture of organic matter

1 1

with calcareous sands forms t h e s o i l s of t h e a t o l l s .

1

A l l t h e reef organisms, a s described by Wells, Todd, and Doty (on ^I pages

25-30)

have c h a r a c t e r i s t i c s i z e s , shapes, and h a b i t s . Corals and

algae of t h e r e e f s a r e massive. Large fragments a r e broken off by waves

and c a r r i e d t o t h e shores of t h e i s l e t s . .Halimeda, i n t h e lagoons 'orealcs

1

i n t o f r a m e n t s about one-half t o one cm i n 1enp:tii. The commorlv occur- _..

-

r i c g Foranunifera, Calcarina, Marginopora, and Amphistegina, range from about one-half t o 2 m i n s i z e , e.lthough some a= smaller and a few r e . The detached spines of Calcarina spp. a r e generally unbroken i n t h e f i n e s t f r a c t i o n s of t h e sand. Sea urchin spines a r e broken i n t o a l l s i z e s from l a r g e t o small Traynents.

Each grade s i z e of each sample of beach sand and of s o i l was ^I examined under a binocular microscope t o i d e n t i e t h e kinds of organisms _I p r e s e n t . Estimates were made of t h e percentages of maJor c o n s t i t u e n t s

i n each sample. The r e s u l t s were p l o t t e d a s h i s t o g r m s combining t h e g r a i n s i z e and b i o t i c composition.

The composition of t h e various grade s i z e s was found t o be:

S i z e , mm Faunal coaposition

Larger than 16, and

16 - 8

0.25

-

^-12

0.12

- ^.O6

Smaller than 0.06

Large whole o r broken c o r a l s . l a r g e fragments of algae;

l a r g e s h e l l s

Coral, algae fragments, s h e l l s Coral, algae fragments, s h e l l s Foraminifera, broken c o r a l , algae Foraminifera, broken c o r a l , algae Few small Foraminifera, mostly

broken p i e c e s

Broken, u n i d e n t i f i a b l e fragments Broken, u n i d e n t i f i a b l e fragments Broken, u n i d e n t i f i a b l e fragments The appearance of t h e s e m a t e r i a l s i s shown i n P l a t e s 28-30.

Mineralogic composition

by John C . Hathawax

The various s i z e f r a c t i o n s of

19

samples from 6 s o i l p r o f i l e s were examined by X-ray powcer d i f f r a c t i o n methods t o determine t h e i r

mineralogic composition, and t o r e v e a l my v a r i a t i o n s occurring amongst t h e d i f f e r e n t s i z e grades a s well a s w i t h i n t h e s o i l p r o f i l e s .

A sample of each f r a c t i o n was ground t o pass a 230-mesh (0.06 mm) s i e v e and was packed i n t o an aluminum sample holder. Care was taken t o minimize t h e p r e f e r r e d o r i e n t a t i o n of t h e p a r t i c l e s . An X-ray d i f f r a c - togram was then made of each sample, and t h e amount of c a l c i t e and a r a g o n i t e p r e s e n t d e t e m i n e d by comparison of t h e i n t e n s i t y of t h e most s t r o n g l y d i f f r a c t e d l i n e s of t h e minerals. The i n t e r p i m a r spacing represented by t h e s t r o n g e s t c a l c i t e l i n e , 2. ( l o b ) , was a l s o measured f o r each sample. The r e s u l t s of t h e X-ray examinations a r e shown i n f i g u r e

19,

where t h e approximate m o u n t s of c a l c i t e and a r a g o n i t e i n each s i z e f r a c t i o n a r e shown on histograms of t h e p a r t i c l e s i z e d i s - t r i b u t i o n f o r t h e i n d i v i d u a l samples. There was found t o be a v a r i a t i o n i n

2

spacing of t h e s t r o n g e s t r e f l e c t i o n of c a l c i t e (104) i n t h e mater- i a l of t h e s i z e f r a c t i o n s . Inasmuch a s t h e organic m a t e r i a l s v a r i e d i n chemical composition, some samples gave two s t r o n g

-

d (104) r e f l e c t i o n s f o r c a l c i t e .

The s i g n i f i c a n c e of t h e v a r i a t i o n of t h e i n t e r p l a n a ? spacings i n c a l c i t e i s di.scussed by Chave (1952, 1954a, 1954b) and by Goldsmith, Graf, and Joensuu (1355). They have shown t h a t decreases i n t h e d (104) spacing: from those of pure c a l c i t e a r e t h e r e s u l t of t h e smaller &it

c e l l t h a t occurs when magnesium s u b s t i t u t e s f o r calcium i n t n e c a l c i t e s t r u c t u r e , and t h a t t h e amount of magnesium csll be correlated with t h e amount of s h i f t i n spacing p o s i t i o n . Goldsmith, Graf, and Joensuu (1955) show this s h i f t t o arflount t o about 0.0029 % per rnol percent MgC03. Nagnesiun s u b s t i t u t i o n has not been observed i n aragonite.

I n t h e s e Marshall Islands samples d (104) was measured t o t h e nearest 0.01 %, which represents incremznts of about

3.5

mol percent MgCO The t o t a l v a r i a t i o n i n spacing found was 0.06 ?A (2.98 t o

3.04%), representing about 20 mol percent MgC03. Ns.gnesium s u b s t i t u - t i o n increases up%rards i n t h e graphs ( f i g . 19). Conversion of rnol percent t o lt~eight percent i s iflade by using t h e l"ollo~i%ng formula:

Weight

$

MgC03 = B4 ^84.33

X

rnol

$

MgCO?

.33

X rnol '$ MgC03

+

100.09 (100-mol M&O$

Figure 20 shows t h e aveyage p a r t i c l e s i z e d i s t r i b u t i o n f o r a l l t h e samples with t h e average m~ount of c a l c i t e and aragonite i n each s i z e grade. The number of samples i n which a given type of m a t e r i a l was dominant was determined f o r each grade size, end t h e percentage f i g u r e f o r t h e occurrence of t h e type i s shown over each histogram block. A t t h e t o p of t h e graph t h e average c a l c i t e d (lo)+) spacing i s given.

-

Figure 21 shows t h e aragonite content of t h e d i r f e r e n t grades a s a percentage of t h e t o t a l amount of m a t e r i a l i n each s i z e f r a c t i o n .

A c o r r e l a t i o n between aragonite content and type of m a t e r i a l i s d i s t i n c t l y present i n f i g u r e 19. Aragonite content i s high i n t h e coarse f r a c t i o n where c o r a l i s t h e dominant constituemt, and decreases considerably where algae and Foraminifera make up a l a r g e p r o p o r t i o ~ l of .the smaller grade s i z e s . The r i s e i n aragonite content i n t h e

0.5-0.06 w i ~ range r e f l e c t s t h e presence of c o r a l fragments mixed with fragments of Foraminifera and algae. The r e l a t i v e dearth of aragonite i n t h e 2-0.5 mm grades a s sho7.m i n f i g u r e 19 a l s o suggests t h a t t h e c o r a l fragments a r e e i t h e r l e s s common o r r e l a t i v e l y unstable i n t h i s s i z e . The l o v content of aragonite i s possibly due t o t h e r e l a t i v e i n s t s b i l i t y of t h e mineral i n f i n e p a r t i c l e s . The wide range of com- p o s i t i o n shown by t h e extremes i n t h e coarser f r a c t i o n s i n figure 21 a r i s e s from t h e d i f f i c u l t y of sampling t h e l a r g e c o r a l fragments r e p r e s e n t a t i v e l y .

The c a l c i t e i n these smples i s high i n magnesium a s shown by t h e r e l a t i v e l y low d (104) spacings. C a l c i t e of more than one composition i s present i n 8-of t h e seinples a s revsaled by double maxima 5.11 t h e region of t h e d (104) spacing. A d i s t i n c t decrease i n t h e amount of .nagnesium o c c u ~ s i n t h e l e s s than 0.06 mm f r a c t i o n of

16

of t h e samples.

This i s r e f l e c t e d i n t h e upper curve of f i g u r e 20 showing t h e average d (104) spacings of t h e samples. A s i m i l a r , although smaller, decrease

.-

occurs i n t h e c o a r s e s t f r a c t i o n . The c a l c i t e present may be t h e f i n e - grained m a t e r i a l low i n magnesium t h a t occupies t h e many small c a v i t i e r

in t h e coarse c o r a l l i n e m a t e r i a l . Algae i n t h e coarse f r a c t i o n s tend t o show high magnesium contents f o r most o f t h e samples. Only i n sample 62

( f i g . 1 9 ) does a low-magnesium c a l c i t e occur where algae a r e important c o n s t i t u e n t s , and t h e r e only a s minor members of two types of c a l c i t e . An a l t e r n a t i v e p o s s i b i l i t y f o r t h e occurrence of low-magnesium c a l c i t e i n t h e coarser f r a c t i o n s i s t h e a l t e r a t i o n of ara.gonitic nia'terials t o c a l c i t e . I n t h i s s i t u a t i o n low-niegnesium c a l c i t e should not be confined t o t h e coarsest and f i n e s t f r a c t i o n s but should be found i n t h e medium grades a l s o , inasmuch a s aragonite i n t h e smaller frafpents of c o r a l might be expected t o a l t e r e s r e a d i l y a s it does i n t h e coarser f r a g - ments. However, "contanination" of fragments by fine-grained, magnesium-

f r e e c a l c i t e m a y be minor i n t h e medium grades because t h e g r e a t e r amount of rgorlcing and washing t h a t pro3uced t h e fragments of these grades might tend t o f r e e t h e a l t e r e d m a t e r i a l . The l i b e r a t e d low-magnesium c a l c i t e would then appear i n t h e f i n e s t f r a c t i o n . If such a l t e r a t i o n t a k e s place it probe.bly does so before deposition of t h e m a t e r i a l i n i t s present environment. A l t e r a t i o n i n a s o i l p r o f i l e would be expected t o remove aragonite a t t h e point of g r e a t e s t weatheri.ng, presumably near t h e surface. I n f i g u r e 22, a l l t h e p r o f i l e s except 30 show increases of

aragonite toward t h e surface, suggesting t h a t a l t e r a t i o n of aragonite t o c c l c i t e i s n o t important ir t h e soil-forming processes i n t h e s o i l s studied. This, together with t h c e r r a t i c v a r i a t i o n s of mechanical com- p o s i t i o n a t d i f f e r e n t depths i n t h e p r o f i l e s , suggests t h a t depositional f a c t o r s a r e more important i n determining t h e c a l c i t e : a r a g o n i t e r a t i o i n these s o i l s than i s a l t e r a t i o n of t h e i n a t e r i a l s i n place. The deposi- t i o n a l f a c t o r should be assessed i n t h e l i g h t of t h e lov r a i n f a l l t h a t p r e v a i l s i n many of t h e areas fro!:; which s o i l samples were collected.

Analytical treatment of grain s i z e d i s t r i b u t i o n data

The d a t a f o r g r a i n - s i z e d i s t r i b u t i o n and b i o t i c composition of a l l t h e beach sands were p l o t t e d a s histograms, and t h e histogram f o r each sample i s placed on t h e o u t l i n e map of t h e a t o l l from which it was col- l e c t e d i n f i g u r e s

5-15.

These histograms show t h e s i m i l a r i t i e s and d i s - s i m i l a r i t i e s between samples collected from difl'erent posi.tions on t h e ssme a t o l l and from t h e same geographic aspects on d i f f e r e n t a t o l l s . Cumulative frequency curves were d r a m f o r t h e data so t h a t t h e median g r a i n d i ~ m e t e r , th e s o r t i n g c o e f f i c i e n t , and skewness could be calcu- l a t e d . Traslc's (1932) s o r t i n g c o e f f i c i e n t , So=-> where Q3 i s t h e coarse q u a r t i l e and Q1 i s t h e f i n e q u a r t i l e of t h e d i s t r i b u t i o n , and skewness, Q Q ~ / M ~ ~ were used. The mean diameter. f o r each sample of sand

Z

rics calculn ed by a modification of a formula suggested by Folk and Ward.

(1957,

p. 1 2 ) > M = l6 +

$ 50

⁺

(d

.

The modification consisted of using t h e g r a i n diameter i n i t e a d of t h e phi n o t a t i o n .

The ske1:mess o r aspimetry of t h e d i s t r i b u t i o n curve of t h e grain- s i z e d i s t r i b u t i o n i s a l s o used t o describe t h e samples. Skewness i n sedimentary m a t e r i a l s i s not v e l l understood, but it has been suggested by K m b e i n and Tisdel (1940) t h a t g r a i n d i s t r i b u t i o n s with excess coarse g r a i n s ( p o s i t i v e slse~mess) 'indicate d i s i n t e g r a t i n g . igneous rocks, whereas g r a i n d i s t r i b u t i o n s with excess f i n e g r a i n s (negative skewness)

i n d i c a t e d i s i n t e g r a t i n g sedimentary roclrs. However, t h e s i z e d i s t r i b u - t i o n i n t h e Northern Narshall Islands beach sands i s Cependent on both t h e kinds of organisas present and on t h e s o r t i n g by vaves and wind t o

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