Chemical Erosion of 

Beach Rock and 

Exposed Reef Rock

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-T
Chemical Erosion of 

Beach Rock and 

Exposed Reef Rock

By ROGER REVELLE and K. O. EMERY

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-T

A study of the solution of calcium 

carbonate in the intertidal %one

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 65 (paper cover)
CONTENTS

Page 

Abstract_ ________________________________________ 699

Introduction and acknowledgments ___________________ 699

Measurements of diurnal variations in solution-basin and

reef water..______________________________________ 700

Results_.________________________________________ 703

Page 

Conclusions.___________________--__-_____---__-___- 705

References cited-___________________________________ 706

Appendix—Method of calculation of the percentage 

saturation of calcium carbonate___-_________________ 709

ILLUSTRATIONS

[Plates follow page 710]

PLATE 225. Etching and solution basins in the intertidal belt, Bikini and nearby atolls.

226. Solution basins in the intertidal belt, Guam and Australia.

227. Undercut sea cliffs (nips), Tonga, Guam, and Australia.

228. "Mushroom" rocks, reef flats, and undercut sea cliffs (nips), Fiji.

229. Undercut sea cliffs (nips), reef flat, "mushroom" rocks, and sea cave, Australia, East Indies, Red Sea, and Tonga.

Page 

FIGURE 233. Changes in water characteristics of two solution basins at Bikini island.________.__._____-___-_______---_ 701

234. Changes in water characteristics of normal sea-water basin 4 at Guam.__________________________________ 702

TABLES

Page 

TABLE 1. Summary of data for water from solution basins and reef flats.___________--__----_._-_-------_--------- 703

2. Computed amounts of calcium carbonate dissolved and precipitated in solution basins and on reef flats______ 704

419498—57

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

CHEMICAL EROSION OF BEACH ROCK AND EXPOSED REEF ROCK

By EOGER REVELLE i and K. O. EMERY 2

ABSTRACT

Above the low-tide line the beach rock and reef rock of Bikini 

and nearby atolls have a highly irregular surface with numerous 

small depressions, many of which are separated only by knifeedge ridges. This and other geological evidence suggests that 

some process of solution is of great importance in removing 

limestone in the intertidal zone. Marked diurnal changes in 

the alkalinity-chlorinity ratio of the water in the depressions 

strongly indicate that intertidal solution of calcium carbonate 

does in fact occur, probably at a comparatively rapid rate. 

But computations based on the experimentally determined solubility product of calcium carbonate in sea water show an apparent supersaturation in the basin and reef water that ranges 

from 175 to 800 percent. Possible rationalizations of these 

opposing lines of evidence are that (1) solution may occur 

within the limestone of the basin floors or in the interstitial 

water of the algal mats which cover some of the floors; (2) solution may occur in the films of water left on the basin walls 

by the receding tide; or (3) much of the calcium may be con> 

plexed or hydrated, and the time involved in the formation and 

dissociation of complexes containing calcium is long relative 

to the time required to precipitate free calcium ions or to ot>- 

tain them from solution of calcium carbonate. If this is true, 

solution can occur when the carbonate concentration or temperature, or both, are rapidly decreased even though the product of 

the concentrations of total calcium and carbonate remains above 

the equilibrium saturation value. Erosion of calcium carbonate 

in warm ocean water is very widespread but is confined to the 

intertidal zone, where marked diurnal changes in conditions occur. These facts, coupled with the slow attainment of calcium 

carbonate saturation equilibrium in the laboratory 5 support 

the hypotheses of slow complexing or slow hydration and 

dehydration.

In the intertidal basins, cooling at night and respiration by 

plants and animals cause a drop in temperature and pH and 

an increase in carbon dioxide content of the water, all of which 

favor solution. Precipitation of calcium carbonate during the 

daytime is facilitated by warming of the water and by the decrease in carbon dioxide and increase in pH corresponding to 

the excess of plant photosynthesis over respiration. In some 

of the basins the water shows a stable density stratification 

at night with more extreme conditions for solution near the 

bottom.

1 Scripps Institution of Oceanography, University of California.

2 University of Southern California.

NOTE.—Contribution from the Scripps Institution of Oceanography, 

New Series, no. 930; Contribution from the Allan Hancock Foundation, no. 183.

INTRODUCTION AND ACKNOWLEDGMENTS

Above the low-tide line the beach rock and reef rock 

of Bikini and nearby atolls have highly irregular surfaces with numerous small basin-shaped depressions 

(pi. 225), many of which are separated only by knifeedge ridges. The walls of some basins are overhanging 

so that their maximum diameters are below their rims. 

Very irregular basins are obviously the result of lateral 

coalescence of two or more smaller ones. Where two 

basin floors are at different levels, their junction may 

form a steplike structure and cause complete drainage 

of the higher basin. The floors of the basins are fairly 

flat, but the floors and especially the walls exhibit small 

sharp irregularities suggestive of corrosion rather than 

abrasion. Similar irregularities are present on the surfaces of melting ice, dissolving saltpans, and some 

meteorites. The basins seldom contain cobbles that 

might have served as tools to abrade potholes; moreover, some are floored by a thin mat of fragile foraminiferal sand held together by filamentous algae.

Erosion of limestone in the intertidal zone has been 

reported from the warm and temperate belts in many 

parts of the world (see, for example, MacFadyen, 

1930; Kuenen, 1933; Fairbridge, 1952; and Guilcher, 

1953). Solution basins in the upper parts of reef flats 

(pi. 226), nips (deep notches or undercuts) and caves 

at the base of sea cliffs in all kinds of calcareous rock 

from Pleistocene reef material to dense Paleozoic limestone, and nips around negroheads atop reef flats are 

common and widespread (pi. 227-229). The very existence of the broad and dead reef flat just below lowtide level indicates the efficacy of erosion of limestone 

in the intertidal zone. This erosion is as effective in 

sheltered lagoons as on exposed shores.

In some respects the irregular microtopography of 

the intertidal zone is similar to that of the karrenfelds 

on the elevated limestones of Guam and other high 

islands of the western Pacific. These karrenfelds have 

extremely jagged surfaces formed by vertical and

699
700 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

lateral corrosion of the limestone. Parts of the surfaces are exposed only to the air, and here solution 

must be brought about by films of rain water rather 

than by ground water. Unlike the intertidal basins, 

many of the steep-walled hollows in the karrenfelds are 

not closed basins capable of retaining water.

The geologic evidence for solution in the intertidal 

zone of warm tropical waters is in conflict with the 

results of laboratory studies of the solubility of calcium 

carbonate in sea water. The problem has been well 

stated by Kuenen (1950, p. 436) :

Geological observations * * * appear to establish beyond doubt 

the solvent action of tropical surface waters on reef limestone

* * *. Investigation of sea water from the surface in tropical 

seas has led chemists to the conclusion that it is always saturated 

or even supersaturated with lime. Hence they have concluded 

that solution of limestone is not possible. But it is exceedingly 

difficult to establish whether solution under natural conditions 

is really excluded.

He left the question essentially as it was in 1933 (p. 75) 

when he wrote that—

* * * whether it is the greater range in temperature, the contact with the atmosphere, some influence of surface tension, the 

alternate wetting and drying, or some other unsuspected influence which was overlooked in the experimental work, I am 

unable to say.

Solution in the upper part of the intertidal zone has 

been attributed to rainwater or to fresh ground water 

by some workers (Wentworth, 1938; Hills, 1949), but 

this is not likely to be important where rainfall is slight 

and where ground water cannot reach basins and nips, 

as on negroheads. Other investigators have supposed 

that animals burrowing into or grazing upon the rocks 

and algae boring below the rock surface are chiefly responsible for intertidal erosion of limestone (Duerden, 

1902, p. 326-332; Gardiner, 1931, p. 68; and Ginsburg, 

1953, p. 61-68).

Emery (1946) demonstrated that in the relatively 

cool waters of La Jolla, Calif., basins in the sandstone 

along the shore form in part through solution by biochemically modified sea water. The calcium carbonate 

cement of the sandstone is dissolved during the nighttime, when the carbon dioxide content of the water in 

the pools is raised by animal and plant respiration. In 

the daytime carbon dioxide is removed by photosynthesis, and calcium carbonate is precipitated, probably 

as small loose crystals that wash out of the pool at the 

next high tide. Thus, by alternate solution of calcareous cement and flushing out of the precipitate, the 

calcium carbonate is removed. Growth of the pools is 

aided by sand-ingesting organisms, particularly snails 

of the genus Littorina (North, 1954, p. 190-194). At 

La Jolla the characteristics of intertidal rock surfaces

indicate that some chemical and organic erosion also 

occurs outside of pools.

The basins at Bikini differ in several ways from those 

at La Jolla. Obvious points of difference are their 

presence in limestone instead of in sandstone, the absence of Littorina, the lesser abundance of other conspicuous animals and of algae, the greater rainfall, and 

especially the higher temperature.

A brief investigation of the chemical and physical 

processes taking place in the solution basins at Bikini 

was carried out in 1946 during Operation Crossroads. 

Measurements of temperature, oxygen content, and 

hydrogen-ion concentration of the water in basins and 

on the reef flat were made during the scientific resurvey 

in 1947 and the Mid-Pacific Expedition in 1950. In an 

attempt to resolve uncertainties resulting from these 

researches, supplementary studies of 13 basins at Guam 

were carried out by Emery in 1952. These included 

field determinations of alkalinity, temperature, oxygen 

content, and hydrogen-ion concentration of the basin 

and reef waters.

The field work at Bikini in 1947 was done by T. S. 

Austin and T. Goreau; the investigations in 1950 were 

under the direction of R. F. Dill and R. Y. Morita. 

Their careful work under difficult conditions is gratefully acknowledged. E. D. Goldberg, Wilson Orr, and 

N. W. Rakestraw made valuable suggestions concerning the interpretation of the results, particularly with 

regard to the problems of complexing or hydration of 

calcium. We are indebted to R. W. Fairbridge for 

helpful discussions, for assembling many of the photographs in plates 226 to 229, and for invaluable assistance in reviewing the literature. The studies at Guam 

were carried out during geologic mapping in the islands 

of the western Pacific accomplished as a part of a joint 

program of the Corps of Engineers, U. S. Army, and the 

U. S. Geological Survey.

MEASUREMENTS OF DIURNAL VARIATIONS IN SOLUTION-BASIN AND REEF WATER

Measurements of temperature, oxygen content, and 

hydrogen-ion concentration in basins and on the reef 

flat at Bikini were made at intervals of 2 to 4 hours over 

several days, and water samples were collected for later 

chlorinity determinations.

Dissolved oxygen was determined by the Winkler 

method. Hydrogen-ion concentration was measured 

with a pH meter, using a glass electrode. The meter 

was carried to each basin in turn, and the pH determined on a freshly collected water sample.

Five basins in the beach rock near the southeastern 

end of Bikini island were studied in 1950. Three of
CHEMICAL EROSION OF BEACH ROCK AND EXPOSED REEF ROCK 701

these were below mean-tide level and consequently were 

flooded much of the time. Basin 3 was near the hightide line and was only occasionally flooded. Basin 4 

was above high tide during the survey and was not 

reached by sea water. These two basins show a more 

extreme range of conditions than the others and serve

more easily to elucidate the processes taking place on 

the reef. Measurements from the two basins are shown 

in figure 233; selected data are given in table 1, as are 

data from the reef flat. Kesults very similar to these 

were, obtained during the investigations of 1946 and 

1947.

r 38

1400 1800 2200 0200 0600 1000 1400 1800 2200

SEPT. 25 SEPT. 26

0200 0600 1000 1400 1800 

SEPT. 27

FIGURE 233.—Changes in water characteristics of two solution basins at Bikini island in 1950. Basin 4 contains fresh rainwater; basin 3 contains

normal sea water.
702 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

The basic difference between the two basins was in 

the chlorinity of the water. Basin 4 contained nearly 

fresh water having a chlorinity of less than 0.2 %0 

(parts per thousand). Until the last morning, when 

a heavy rain diluted the water, the chlorinity in basin 

3 was close to that of the sea water around the atoll, 

ranging between 18.4 and 21.0%0 . In both basins the 

water temperature increased during the day and decreased during the night, with a total range of 13° C 

in the brackish basin and 9° C in the saltier one.

Oxygen content and pH showed marked diurnal 

cycles. Computations from the data given in figure 233 

indicate that oxygen in basin 3 varied from about 30 

percent of saturation at night to 180 percent of saturation in the afternoon. Koughly the same range of 

percentage saturation, 50 to 190 percent, occurred in 

basin 4. On the reef flat the diurnal variation was 

much smaller, from 85 to 140 percent. The oxygen increase in the daytime was due to the excess of photosyiithetic-oxygen production by marine algae living on 

the basin floors and within the water, over oxygen consumption by animal and plant respiration. A corresponding decrease in carbon dioxide content must have 

occurred. At night, when photosynthesis ceased, oxygen was removed by respiration, and carbon dioxide was 

liberated. These inferred changes in carbon dioxide

content are reflected in the observed cycle of pH, which 

was high during the day, when the carbon dioxide content was small, and low during the night, when the 

amount of carbon dioxide was relatively large.

Unfortunately, alkalinity was not determined in the 

field because it was believed that changes in the oxygen 

content would suffice to compute changes in carbon dioxide. Subsequent computations showed, however, that 

oxygen was being exchanged with the atmosphere so 

rapidly that the change in oxygen content was not a 

reliable measure of the change in carbon dioxide. 

Therefore, alkalinity of a few samples was measured 

nearly 2 years after the samples had been collected. Uncertainty in these results led to more complete field 

measurements of water in 13 basins and a reef flat at 

Guam in 1952 (Emery, in manuscript). In addition to 

measurements of temperature, oxygen content, and pH, 

water samples were filtered within a few hours after 

collection, and alkalinity was determined by the titration method of Wattenberg (1930). Representative 

data from the reef flat and from three basins of widely 

varying chlorinity (designated as brackish water, normal sea water, and hypersaline water) are given in 

table 1. A 24-hour series of observations of the normal sea-water basin (Guam 4) is illustrated in figure 

234. The diurnal variations of alkalinity in the basins

3.0 - 6

1.5

CJ

o

- 2

- 0

r 25

- 20

X 

CL

r 34

- 32 -

- 10

0600 0800 1000 I20O 1400 1600 1800 2000 2200 2400 0200 0400 0600

JULY 12 I JULY 13

FIGUEE 234.—Changes in water characteristics of normal sea-water basin 4 at Guam in 1952. Basin is 75 cm long, 50 cm wide, 10 cm deep.
CHEMICAL EROSION OF BEACH ROCK AND EXPOSED REEF ROCK 703

were large and in any one basin were roughly inverse 

to the variation in pH.

RESULTS

In the eight solution basins at La Jolla investigated 

by Emery (1946), the carbonate concentration increased 

during the daytime as carbon dioxide and bicarbonate 

decreased; consequently, calcium carbonate was precipitated. This precipitation resulted in a proportionate decrease of the alkalinity. During the night the 

carbon dioxide content of the water increased; the alkalinity increased as calcium carbonate was dissolved. 

The water was apparently supersaturated with calcium 

carbonate in late afternoon but undersaturated at night. 

Computations based on Smith's (1941, p. 240) apparent solubility product for calcium carbonate (aragonite) show as much as 425 percent saturation in the 

daytime and only 45 percent saturation at night. A 

minor error in the previously published computations

of percentage saturation, kindly pointed out to us by 

R. N". Ginsburg, is corrected in table 1.

Similar computations, using Smith's solubility product, indicate that at a temperature of 25° C, a chlorinity 

of 19°/00 , and an alkalinity of 2.34 mequiv per 1 (milliequivalents per liter), typical values for the tropics, 

solution can take place only if the pH is below 7.74. 

None of the pH values for tropical-basin and reef water 

given in table 1 are less than 8.00. The water is apparently supersaturated with calcium carbonate at all 

times. The greatest excesses occur in the afternoon 

and the least just before dawn, but the degree of saturation, using Smith's apparent solubility product for 

aragonite, is never less than 175 percent and rises to 

more than 800 percent (table 1). Thus, tropical sea 

water is apparently incapable of dissolving calcium carbonate at any time. This conclusion is opposed both 

by the geological evidence and by the observed changes 

in alkalinity.

TABLE 1.—Summary of data for water from solution basins and reef flats 

[Symbols: B, bottom; T, top; X, 400 ft from shore; Y, 1,400 ft from shore]

Type of feature Date

Local 

time

Sampled locality

Temp (°C) 01

(°/oo) PH

Oxygen

Ml per 1 Percent 

saturation

Total 

CO2 

(mg per 1)

Alkalinity 

(mequiv 

peri)

Alkalinity: 

chlorinity

Calcium 

carbonate 

(percent 

saturation)'

La Jolla

1944 

Apr. 28

Apr. 28

0500

1700

0645

T

T

T

13.3

24.8

15.9

18.60

18.98

8.00

9.00

7.62

93.39

44.07

107. 28

2.298

1.905

2.418

0. 1234

.0863

.1274

125

425

45

Bikini

Reef flat-....-----.-----,---.-.----.-.-.

I960

Sept. 26

Sept. 26

Sept. 26

Sept. 27

Sept. 27

Sept. 25

Sept. 26

Sept. 27

Sept. 27

1415

0200

1420

0430

1220

1500

1435

0615

1200

0610

1545

T

T

T

T

T

T

T

T

26.4

34.8

26.0

25.1

37.0

36.0

32.0

26.4

31.1

27.2

30.0

18.95

19.34

.11

.02

18.99

18.95

18.58

18.71

8.50

8.20

8.60

9.95

8.55

8.30

8.00

8.35

8.20

8.25

4 30

7 30

2.95

7.33

3.38

9.35

3.33

4.91

4.58

4.30

6.21

4.08

4.80

89

178

61

146

57

187

57

96

105

89

140

85

106

22.302

2 2. 474

22.302

2 2. 276

2 2. 421

0. 1215

.1279

.1216

.1224

.1294

175

265

175

245

325

Guam

Reef flat'.— ————— — —— — .——

/952 

July 12

July 12

July 12

July 12

July 13

July 13

July 12

July 24

July 25

July 24

July 25

July 5

July 6

July 5

July 6

0600

0600

1800

1800

0600

0600

1400

1600

0400

1600

0400

1840

0610

1815

T

B

T

B

T

B

B

B

B

B

B

X

X

Y

Y

26.7

27.3

28.3

29.9

26.4

27.2

33.9

37.2

97 R

27.8

26.4

31.7

22.8

30.0

28.8

22.75

22.89

23.80

23.61

22.41

22.89

10 AC

20.10

Ifi fifiL*>

.32

.36

19.10

18.90

19.12

19.50

8.41

8.27

8.52

8.50

8.29

8.20

8.84

9.39

8 4.7

9.10

8.20

8.28

8.24

8.17

8.24

2.29

H2S

4.11

3.15

2.10

H2S

6.41

11.56

2

EA

5.23

3.10

6.00

3.44

5.27

3.36

50

0

94

74

46

0

43

143

294

91

52

137

67

117

73

3.73

5.60?

3.86

3.82

3.61

4.09

3.05

2.64

2.03

2.91

.80

3.21

2.676

2.676

2.750

2.793

0.164

.244

.162

.162

.161

.179

.169

.131

.108

.154

2.50

8.90

.140

.143

.144

.143

625

590

785

810

495

515

330

805

770

490

180

250

345

335

355

1 Computed from Smith's (1941) apparent solubility product for aragonite.

2 Analyses made nearly 2 years after collection of water samples, 

s Reef flat 2,200 ft wide.

419498—57———2
704 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Reference to table 1 and figure 234 shows that in the 

brackish-water and normal sea-water (Guam 4) basins 

at Guam, and in the bottom of the hypersaline-water 

basin, the alkalinity increased at night and decreased 

in the daytime. These changes could have been caused 

by evaporation at night and dilution during the day, by 

solution of calcium carbonate at night and precipitation 

during the day, or possibly by other processes affecting 

the alkalinity. We are convinced that any other such 

processes were insignificant in the Guam and Bikini 

basins.

Under some circumstances the alkalinity may change 

because of solution or deposition of magnesium carbonate (Eiviere, 1940, p. 41; Fairbridge, 1950, p. 385). 

This is probably not a significant factor at Bikini and 

Guam because magnesium carbonate is only a minor 

constituent of the rock. In localities where organic 

matter is rapidly decomposing, the alkalinity may be 

increased appreciably through production of salts of 

organic acids and, under extreme conditions, through 

reduction of sulfates to sulfides. In the clear and 

nearly colorless waters of the basins at Bikini and 

Guam, the concentration of organic material is undoubtedly too small to affect the alkalinity measurably. 

Hydrogen sulfide was noted in only one basin, and, as 

shown below, the amount was insufficient to account for 

more than a small fraction of the observed change in 

alkalinity.

Evaporation and dilution would have affected the 

chlorinity and the alkalinity in the same proportion. It 

is probable that the slight changes in alkalinity observed on the reef flats were due to these causes, because 

the alkalinity-chlorinity ratio was constant within the 

limits of experimental error. But table 1 shows that 

in the basins the alkalinity-chlorinity ratio was significantly lower in the daytime than at night, and therefore precipitation and solution, respectively, are 

indicated.

For the normal sea-water basin (Guam 4) the diurnal 

change of alkalinity on July 12 was 0.41 mequiv per 1. 

This is not extreme either for this basin or for the other 

basins, even though it is veiy much larger than the 

changes observed in the reef water. The change in 

calcium computed from the change in alkalinity is illustrated in table 2. When this computed change in 

calcium concentration is corrected to a constant chlorinity, that of the reef water, it is found that 9 mg per 

1 (milligrams per liter) of calcium was dissolved during the night and nearly 5 mg per 1 was precipitated 

during the day. If the calcium carbonate dissolved at 

night is derived entirely from solution of limestone, the 

rate of solution is such that about 33 years would be 

required to dissolve a layer of limestone 1 cm thick.

Cl

Ca balanced against weak bases

Do,

%0

, computed from

.. _ ...me ner 1—

Ca balanced against weak bases, computed from 

alkalinity-chlorinity ratio in reef water— ..do.--. 

Ca dissolved.. - - - - do

Thickness of CaCOa (assumed

—— _ — -do— .

——— —— .do——

_ ———— __ _do---

density 2.5) disNormal sea-water 

basin (Guam 4)

0600

18.08

1.53 

61.1

52.1 

9.0 

0 

22.5 

0 

36,000 

3,900

.83

1400

20.10

1.32 

63.0

57.8 

0 

4.8 

0 

12.0 

36,000 

3,900

Reef-flat 

water 

(Guam)

1815

19.12

1.38 

55.1

55.1

This should be considered a minimum time because 

much of the calcium carbonate added to the water at 

night must have originated from solution of calcareous 

organic remains or re-solution of the calcium carbonate 

precipitated during the day. The net rate of removal 

of limestone depends in part on the tidal renewal of 

water at night and in part on the rate at which calcium 

carbonate, precipitated as small particles during the 

day, can be washed out of the basin by the tidal flow.

TABLE 2.—Computed amounts of calcium carbonate dissolved 

and precipitated in a typical solution basin

In the bottom of the hypersaline-water basin at 

Guam, some of the nighttime increase in alkalinity evidently resulted from the reduction of sulfate to sulfide, 

a weak Bronsted base, during the latter part of the 

night after the free oxygen was depleted. Because of 

the relative inefficiency of anaerobic respiration, the 

rate of hydrogen sulfide production must have been 

lower than the rate of decrease of oxygen during the 

earlier part of the night. The total hydrogen sulfide 

produced was probably less than 1 ml per 1 (milliliters 

per liter). This corresponds to a change in alkalinity 

of only 0.05 mequiv per 1 whereas the observed changes 

in alkalinity were 0.3 and 1.8 mequiv per 1. Thus, the 

larger part of the diurnal change in the alkalinity of 

the bottom water must have resulted from solution and 

precipitation of calcium carbonate.

The average alkalinity-chlorinity ratio of the reefflat water at Guam is 10 percent higher than the value 

of 0.123 which is characteristic of surface water in the 

open sea. This difference probably means that the reef 

water contains a considerable admixture of island 

ground water which has dissolved calcium carbonate in 

its passage through the limestone. A mixture of 90 

percent sea water with 10 percent brackish water having 

an alkalinity of 3 mequiv per 1 (similar to that in the 

brackish-water basin) would give the alkalinity-chlorinity ratio of about 0.142 found on the reef. Subsequent evaporation would raise the chlorinity to the
CHEMICAL EROSION OF BEACH ROCK AND EXPOSED REEF ROCK 705

prevailing value in the open sea around Guam. Water 

of the normal sea-water basin (Guam 4) is frequently 

exchanged with water from the reef, and consequently 

their average alkalinity-chlorinity ratios are nearly the 

same. In the hypersaline-water basin at Guam the 

higher average alkalinity-chlorinity ratio indicates 

that under the extreme conditions in this isolated basin 

more calcium carbonate remains dissolved throughout 

the day than in the basin with normal sea water. In 

the brackish-water basin the very high alkalinitychlorinity ratio accords with the well-known fact that 

calcium carbonate is easily dissolved by fresh water.

Some of the calcium which the high alkalinities indicate as being present in solution actually may be in 

colloidal form. The water was filtered before the alkalinity titrations were made, but finely divided colloidal particles might have been able to pass through 

the filter paper. However, the existence of such a suspension could not explain the changes in alkalinity between day and night.

CONCLUSIONS

Geologic evidence suggests that some process of solution is of great importance in removing limestone in 

the intertidal zone. This process is faster than solution 

by rainwater, as indicated by the existence of intertidal nips in water sheltered from wave action (pi. 

227-229). That it is much faster than solution below 

the intertidal zone is shown by the fairly level surface 

of the reef flat at a depth less than 30 cm below normal 

low tide.

The diurnal changes in alkalinity-chlorinity ratios 

given in table 1 strongly indicate that intertidal solution of calcium carbonate does in fact take place, 

probably at such a rapid rate that it is an important 

factor in reducing islands and raised reefs to the monotonously uniform level of the reef flat. The problem 

remains as to how solution can occur even though, in 

comparison with published solubility products, the 

water is apparently always supersaturated with calcium 

carbonate.

Smith's value for the apparent solubility product, 

•K'cacosj is based on laboratory measurements of the 

ion product in samples of sea water, collected over the 

Great Bahama Bank, which had been equilibrated with 

calcareous bottom mud from the bank for periods of 11 

to 38 days. The mud contained large numbers of 

minute aragonite needles. Other work (Hindman, 

1943; Wattenberg and Timmerman in Sverdrup, 

Johnson, and Fleming, 1942, p. 206), in which equilibrium was approached by dissolving or precipitating 

calcite, yields values of the apparent solubility product for calcite about half as great as Smith's solubility 

product for aragonite. Revelle and Fleming (1934), 

in experiments in which aragonite was precipitated 

by passing air free of carbon dioxide through sea 

water for periods of several weeks, obtained an apparent solubility product for aragonite twice as large as 

Smith's value, but it is likely that the sea water was 

still supersaturated at the termination of their experiments. If solution and precipitation are determined 

by the apparent solubility product, the value of this 

product in the tropical basins must be at least twice 

that found by Eevelle and Fleming, and 4 to 5 times 

Smith's value. Smith's apparent solubility product, 

when corrected for temperature and salinity, appears 

to be applicable to the La Jolla basins, and there is 

no evident reason why the solubility should be different in the tropics. Other explanations must be sought.

Clearly the diurnal cycles of oxygen, pH, and carbon 

dioxide reflect respiration by plants and animals, counterbalanced during the daytime by photosynthesis by 

plants. The greater range of the diurnal variations in 

the basin water, compared to the water of the reef flat 

or the open sea, is a result of the greater concentration 

of plant and animal tissue, relative to the amount of 

water, in the basins. Vertical gradients of chemical 

properties in the hypersaline-water basin indicate that 

more extreme conditions for carbonate solution or precipitation may exist at the bottom than within the 

main mass of water; this is in evident response, to the 

bottom position of most organisms. In the hypersalinewater basin, bacterial respiration actually brings about 

anaerobic conditions near the bottom at night, with 

complete depletion of oxygen and production of hydrogen sulfide. Both sets of morning observations from 

this basin show a stable density gradient because the 

higher chlorinity near the bottom more than compensates for the slightly higher temperature. Evaporation 

during the daytime raises the density of the surface 

water so that overturning probably occurs in the late 

afternoon.

The presence of vertical gradients suggests that solution may occur under special local conditions. It may 

take place within the rock surface of the basin floors 

by organic acids liberated by boring algae, or by carbon 

dioxide produced in respiration beneath the algal mats 

that cover some of the floors. Measurements in 1953 

by H. Eotschi while on the Capricorn Expedition of 

the Scripps Institution of Oceanography showed that 

the water beneath living algal masses on the reefs of 

Tahiti had a pH range of 6.0 to 8.7, much wider than 

in the overlying water. The diurnal changes of alkalinity given in table 1 demonstrate, however, that so-
706 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

lution and precipitation wherever they occur are reflected in changes of chemical composition of the main 

mass of basin water.

Solution may take place on the walls of the basins 

above the pool of standing water, as indicated by the 

highly irregular surfaces and the fact that the basins 

coalesce by lateral enlargement. Although it has not 

yet been tested, the suggestion is made that solution of 

calcium carbonate on the walls occurs in the film of 

water remaining on the rock during ebb tide. The ratio 

of plant and animal bulk to water bulk must be very 

large in such films; consequently the films should exhibit extreme ranges of pH which may permit the solution of limestone. Examination of these films should 

be most fruitful for understanding the development of 

solution basins and nips in the intertidal zone.

But the universal solution of intertidal limestone 

suggests that something other than localized processes 

is involved. Two facts are characteristic of the problem: in warm ocean water, solution is confined to the 

intertidal zone, where rapid and profound changes in 

the water conditions occur; and in the laboratory, several weeks are required for apparent equilibrium to 

be attained when calcium carbonate is being dissolved 

or precipitated in sea water (Hmdman, 1943). These 

two facts suggest that the problem is one of reaction 

rates. If the calcium is involved in two simultaneous 

reactions—solution or precipitation of calcium carbonate, and formation of complexes with chloride or sulfate or of hydrates—and if these reactions have 

markedly different reaction rates, the observations 

could be reconciled with the laboratory equilibrium 

values for the apparent solubility of calcium carbonate. 

This would require that much of the calcium is normally in the complex or the hydrated form and that 

the time constant for the complexing reaction or for hydration is much longer than the time required for solution or precipitation.

Let it be supposed that at a particular time the water 

is supersaturated; that is, calcium is slowly proceeding 

from the complex or the hydrated form to simple ionic 

form and is being precipitated as it is freed from the 

complex or the hydrated state. Let a marked change 

in conditions suddenly occur, for example a decrease 

in temperature and a decrease in pH, with a corresponding decrease in carbonate-ion concentration. Calcium carbonate will be dissolved, even though the overall concentrations of carbonate and of calcium in both 

complex or hydrated and simple ionic form are still 

sufficiently high that the apparent ion product is above 

the equilibrium saturation value. Eventually, if the 

new conditions continue, solution will cease and precipitation will again occur. But, if while solution is

still proceeding, the temperature, pH, and carbonateion concentration are increased to the previously existing value, it is possible to restore the solution nearlv 

to its original condition, provided the equilibrium constant for the complexing reaction or for hydration and 

the rates of complexing or hydration, solution, and precipitation vary sufficiently with temperature. A pseudoequilibrium may thus tend to persist, when there is 

a diurnal oscillation of pH and temperature, around a 

point where the average product of total calcium and 

carbonate considerably exceeds the laboratory equilibrium value.

Complexing of a sufficiently high proportion of calcium with organic substances could conceivably reduce 

the calcium activity to a level so low that the product 

of calcium and carbonate concentrations, multiplied by 

the activity coefficients, would be less than the solubility 

product, particularly at night. However, this would 

require a far higher content of organic matter in the 

water (of the order of 1 g per 1) than has been observed. Fox, Isaacs, and Corcoran (1952, p. 42), found 

a 0.25 to 35 ing per 1 range of concentration of colloidal 

organic matter in inshore surface-water samples. It 

is concluded that either slow complexing with chloride 

or sulfate or slow hydration and dehydration are more 

probable explanations of the nightly solution of calcium carbonate by the apparently supersaturated basin 

water. An excellent review of the problem of complexion formation is given by Young and Jones (1952).

REFERENCES CITED

Duerden, J. E., 1902, Boring algae as agents in the disintegration

of corals: Am. Mns. Nat. History Bull., v. 16, p. 326-332. 

Emery, K. O., 1946, Marine solution basins: Jour. Geology, v.

54, p. 209-228. 

Fairbridge, R. W., 1950, Recent and Pleistocene coral reefs of

Australia: Jour. Geology, v. 58, p. 330-401. 

———— 1952, Marine Erosion: Seventh Pacific Sci. Congr. Proc.,

New Zealand, 1949, v. 3, p. 347-359. 

Fox, D. L., Isaacs, J. D., and Corcoran, E. F., 1952, Marine

Leptopel: Jour. Marine Research, v. 11, no. 1, p. 29^-46. 

Gardiuer, J. S., 1931, Photosynthesis and solution in formation

of coral reefs: Proc. Linnean Soc. London for 1930-31,

p. 65-71. 

Ginsburg, R. N., 1953, Intertidal erosion on the Florida Keys:

Marine Science of the Gulf and Caribbean Bull., v. 3, p.

59-69. 

Guilcher, Andre, 1953, Essai sur la zonation et la distribution des

formes littorales de dissolution du calcaire: Ann. de Geogr.

(Paris)), v. 62, p. 161-179. 

Harvey, H. W., 1945, Recent advances in the chemistry and

biology of sea water: London, Cambridge Press, 164 p. 

Hills, E. S., 1949, Shore platforms: Geol. Mag., v. 86, p. 137-152. 

Hindman, J. C., 1943, Properties of the system CaCO3 —CO3—H2O

in sea water and sodium-chloride solution: Scripps Institution of Oceanography, doctoral thesis.
CHEMICAL EROSION OF BEACH ROCK AND EXPOSED REEF ROCK 707

Kuenen, Ph. H., 1933, The Snellius Expedition—Geology of 

coral reefs: Leyden, Brill, v. 5, p. 2, 126 p.

——— 1950, Marine geology: New York, John Wiley and Sons, 

Inc., 568 p.

MacFadyen, W. A., 1930, The undercutting of coral-reef limestone on the coasts of some islands in the Red Sea: Geog. 

Jour., v. 75, p. 27-34.

North, W. J., 1954, Size distribution, erosive activities, and gross 

metabolic efficiency of the marine intertidal snails, Littorina planaxis and L. scutulata: Biol. Bull., v. 106, p. 

185-198.

Revelle, Roger, and Fleming, R. H., 1934, The solubility-product 

constant of calcium carbonate in sea water: Fifth Pacific 

Sci. Cong., Proc., Canada, 1933, v. 3, p. 2089-2092.

Riviere, Andre, 1940, L'eau de nier et les sediments calcaires: 

Comptes Rendus Sommaires, Soc. Geol. de France, no. 5, 

p. 40-42.

Smith, C. L., 1941, The solubility of calcium carbonate in tropical sea water: Jour. Marine Biol. Assoc., v. 25, p. 235-242.

Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., 1942, The 

Oceans: New York, Prentice-Hall, Inc., p. 186-212.

Wattenberg Hermann, 1930, f ber die Bestimmung der Alkalinitat des Meerwassers: Annalen der Hydrographie und 

Maritimen Meteorologie, v. 58, p. 277-282.

Wentworth, C. K., 1938, Marine bench-forming processes—Part 

I, Waterlevel weathering: Jour, of Geomorph., v. 1, p. 5-32.

Young, T. F., and Jones, A. C., 1952, Solution of electrolytes: 

Ann. Rev. of Phys. Chemistry, v. 3.

APPENDIX— METHOD OF CALCULATION OF THE PERCENTAGE SATURATION OF CALCIUM CARBONATE

The apparent percentage saturation of calcium carbonate 

is given by the ratio of the product of the calcium and the 

carbonate concentrations to the apparent solubility product. 

Thus

* 4.- [Ca++] X [CO3— ] X 100 percentage saturation =

CaCO3

For the usual range of temperature, chlorinity, and pH, the 

maximum changes1 of calcium, owing to solution or deposition 

produced by gains or losses of carbon dioxide, can amount to 

only about 0.5X10~3g ions per 1 (gram ions per liter), or about 

5 percent of the total calcium concentration. Under field conditions it is exceedingly difficult to measure the calcium directly 

with an accuracy of more than a few percent. Consequently 

the concentration of calcium was not determined experimentally but was estimated from the calcium-chlorinity ratio 

of sea water with the modification described below. In the 

open oceans the ratio of calcium to chlorinity is 0.02106 (Sverdrup, Johnson, and Fleming, 1942, p. 173), hence the concentration of calcium, expressed as gram ions per liter, is 

0.02106 Cl X 1.025

40

if it is assumed that the concentration of calcium is governed 

by evaporation or dilution of sea water. This equation is 

accurate only when solution or precipitation of calcium carbonate has not occurred and when the sea water has not been 

diluted with fresh water in which the calcium-chlorinity ratio 

differs from that in the open sea.

Calcium totals about 400 mg per 1 (milligrams per .liter) in 

ordinary sea water; of this .amount, about 50 mg can be 

balanced against weak Bronsted bases, principally carbonate, 

bicarbonate, and borate. The concentration of weak bases, 

expressed in mllliequivalents per liter (mequiv per 1), is called 

the alkalinity. In water near the surface of the open sea the 

ratio between the alkalinity and the chloride concentration in 

grams per liter is 0.120; hence, the alkalinity-chlorinity ratio 

is about 0.123.

If it is assumed that measured changes in the alkalinitychlorinity ratio correspond to changes in the calcium-chlorinity 

ratio, the concentration of the calcium is

C1X 1.025 , ([A]-Q.123 C1)X1Q-3

~~ 40 2

The carbonate concentration can be determined by measiurement of chlorinity, temperature, hydrogen-ion concentration, 

and either alkalinity or total carbon dioxide. Measurement

of total carbon dioxide content is impractical in the field, hence 

alkalinity was chosen for the study at Guam. The carbonate 

concentration in gram ions per liter was computed by means 

of the following equations adapted from Sverdrup, Johnson, 

and Fleming (1942, p. 198-200).

Jwhere

and

[SH3BO3] = 0.0221 C1X 1.025 X 10-3 .

Values of K'z, Kw, and K' B were taken from tables given by 

Harvey (1945).

For the basins within the normal range of chlorinity of sea 

water the apparent solubility constant of calcium carbonate was 

based upon Smith's (1941) value corrected, by Wattenberg's 

coefficients, for the observed chlorinity and temperature :

P#'caco3 =5.933 + 0.019(20.0— Cl) +0.012(30.0- T).

For the brackish-water basin the apparently solubility was 

obtained by interpolation between Wattenberg's value of 

18X10"8 at a chlorinity of 5 %><> and the solubility for distilled 

water, 0.5 XlO"8 (Sverdrup, Johnson, and Fleming, 1942, p. 205, 

207). A "virtual chlorinity" corresponding to the supposed 

ionic strength of the solution was estimated from the measured 

chlorinity and alkalinity.

The symbols used above are as follows :

[Ca++]—gTB,m. ions per liter of calcium

— ] = gram ions per liter of carbonate 

[H+] = gram ions per liter of hydrogen = 10- pH 

[A] = total alkalinity = ([HCO3-] + 2[CO3— ] + [H2BO3-] +

[ACo2 ] = carbonate alkalinity = ([HCO8-] + 2[CO8~]) X 103 

K'z — apparent second-dissociation constant for carbonic

acid= 10-»*'»

.KV=ion product for water =lQ~pKw 

K' B— apparent first-dissociation constant for boric

acid = 10-"*' B 

o =apparent solubility product of calcium carbonate —

10-"*'C aco3

Cl= grams of chlorine and chlorine equivalent of 

bromine and iodine per kilogram of sea water, 

in parts per thousand 

T= temperature, in degrees centigrade.

709

PLATES 225-229
PLATE 225

A. Stereopair of karren (lapies) etched in beach rock on upper part of shore ramp, about 4-5 feet above low-tide level. Bikini island. (Photograph by J. I. Tracey, Jr.)

B. Solution basins, now partly filled with sand, on highest part of shore ramp, about 5-6 feet above low-tide level. Bikini island. (Photograph 

by K. O. Emery.)

C. Solution basins, showing coalescence; note relatively smooth floors, and jagged surfaces between basins. Latoback island, Rongerik Atoll. 

(Photograph by J. I. Tracey, Jr.)
GEOLOGICAL StTRVEY PROFESSIONAL PAPER 260 PLATE 225

ETCHING AND SOLUTION BASINS IN THE INTERTIDAL BELT, BIKINI AND NEARBY ATOLLS
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 226

Ti.

SOLUTION BASICS IN THE INTERTIDAL BELT, GUAM AND AUSTRALFA
PLATE 226

A. Solution basin 5, Toguan Bay, Guam. (Photograph by K. 0. Emery.) 

B. Solution basin 6, Gogna Cove, Guam. (Photograph by K. 0. Emery.)

C. Marine solution basins, with rills and undercut runnels, etched in beach rock 5 feet 

above low-tide level. Western Australia, between Dongara and Geraldton. 

(Photograph by R. W. Fairbridge.)

D. Solution basin showing scalloped margins and undercuts and outlet channel following 

a joint plane. Western Australia, north of Murchison River. (Photograph by 

R. W. Fairbridge.)
PLATE 227

A. Undercut cliffs of an isolated island of emergent Pleistocene coral limestone in the 

protected bay of Vavau; undercuts reach from low-tide level to about 5 feet above 

it. Tonga Group, Vavau. (Photograph by J. B. MacFall.)

B. Undercut cliff of limestone in intertidal zone, 6 feet high. Guam, near Inarajan. 

(Photograph by K. 0. Emery.)

C. Undercut cliff of Pleistocene coral limestone, 6 feet high; note protection from 

mechanical erosion by offshore reef; maximum depth of notch corresponds approximately to mean sea level. Western Australia, Port Denison, near Dongara. 

(Photograph by R. W. Fairbridge.)
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 227

UNDERCUT SEA CLIFFS (NIPS), TONGA, GUAM, AND AUSTRALIA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 228

'ML'SHJKOOAI" KOCKS, REEF FLATS, AI\D UNDERCUT SEA CLIFFS (MPS), FIJI
PLATE 228

A. Undercut cliff, showing remnants of horizontal reef platform approxi mately 10 feet above present reef flat. Passage between Ongea Levu and 

Yanuia. (Photograph by Edwin H. Bryan, Jr.)

B. Undercut islands nearly separated by progressive intertidal corrosion, leaving small natural bridge. Bay of Islands, Vanua Mbalavu. (Photograph by J. Edward Hoffmeister.)

C. Undercut cliff, with very deep notch that in places exceeds 20 feet in depth but is only 5-8 feet high; note extensive landslide due to collapse 

of undercut. North shore of Ongea Ndriki. (Photograph by Max Agassiz.)

D. Well developed "mushroom" rock (undercut stack) at low tide; top surface of undercut is 5-8 feet above reef flat. Mango. (Photograph 

by E. C. Andrews.)

E. Narrow protected bay with steep limestone cliffs undercut just as in exposed places. East shore of Ngillangillah. (Photograph by W. 

McM. Woodworth.)
PLATE 229

A. Undercut cliff, 10 feet high, in Pleistocene lagoon limestone; seen at low tide, tidal 

range being 3 feet; note horizontally eroded reef flat in protected lagoon. Western Australia, Middle Island, in Pelsart Lagoon, Houtman's Abrolhos islands. 

(Photograph by R. W. Fairbridge.)

B. Very deep notch, at least 20 feet deep and 5 feet high, in emergent Pleistocene coral 

limestone cliff. Shore is protected here by off-lying reefs and land. East Indies, 

west point of Waaf. (Photograph by Ph. H. Kuenen.)

C. Undercut cliffs and "mushroom" rocks, showing prominent overhang and horizontal 

top, which is the remnant of a former reef platform 10 feet above the present one. 

Red Sea, Farsan islands, east coast of Kamaran island. (Photograph by W. A. 

MacFadyen.)

D. Undercut cliffs leading into large sea cave (believed due to Pleistocene karst formation). Note that undercut extends into dark, protected waters of the cavern. 

Tonga islands, Vavau. (Photograph by Alan Jones.)
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 229

UM3KRCUT SEA CLIFFS (NTPS), REEF FLAT, "ML'SHROOM" ROCKS, AM) SKA CAVE, AUSTRALIA, EAST UNDIES, RED SEA, AND TONGA