6
Coral Reefs (1982)1:29 34 Coral Reefs Springer-Verlag 1982 Darwin Point: A Threshold for Atoll Formation* R.W. Grigg Hawaii Institute of Marine Biology, P.O. Box 1346, Kaneohe, Hawaii 96744 Received 5 January 1982; accepted 21 January t982 Summary. A threshold for atoll formation, herein termed the Darwin Point, exists at the northern end of the Hawai- ian Archipelago at 29~ latitude. Hawaiian atolls and coral islands transported northwest by tectonic movement of the Pacific Plate appear to have "drowned" near the Darwin Point during the last 20 million years. Measures of gross carbonate production by corals across the archi- pelago show that growth rates decrease with increasing latitude. At the Darwin Point, corals may contribute only 20 % of the calcium carbonate necessary to keep pace with recent changes in sea level and thus appear to be more im- portant as builders of framework than producers of lime- stone. Reduction in this function rather than total carbon- ate production may be the determining factor in the for- mation of atolls and coral islands. Elsewhere in the world other Darwin Points may exist but probably not at the same latitude due to differences in ecological conditions, coral species composition, island area, rates of erosion and tectonic histories. Introduction During the voyage of the Beagle in 1836, Charles Darwin conceived his well known theory on the development of coral islands, now known as atolls (Darwin 1842, 1962). In writing about the islands of Tahiti and Moorea on 12 April 1836, Darwin made the following entry in his diary: "Hence if we imagine such an island after long successive intervals, to subside a few feet in a manner similar but with a movement opposite to the continent of South America; the coral would be continued up- wards, rising from the foundation of the encircling reef. In time, the cen- tral land would sink beneath the level of the sea and disappear but the coral would have completed its circular wall. Should we not then have a coral island? Under this view we must look at a lagoon island as a monument raised by myriads of tiny architects to mark the spot where a former land lies buried in the depths of the ocean." Darwin's subsidence theory of atoll formation was confirmed in 1951 when two holes were drilled which reached the volcanic rock basement beneath Enewetak Atoll at depths of 1,267 m and 1,405 m, respectively (Ladd * Hawaii Institute of Marine Biology Contribution No. 627 et al. 1970). The limestones recovered were all of shallow water origin demonstrating both subsidence of the atoll and the upward growth of shallow water coral since Eocene time, approximately 49 million years B.P. (Schlanger 1963). While the mechanism of atoll formation is now under- stood, much less is known of the interaction between the process of reef building and rates of subsidence, erosion and past changes in sea level. In the Hawaiian Archi- pelago, an opportunity exists to examine this interaction across latitudes from zones of optimal reef development to the limit of atoll formation. The Hawaiian Archipelago stretches diagonally 2,450 km across the north Pacific from the island of Hawaii (19 ~ N) to Kure Atoll (29 ~ N). Kure Atoll is the Pacific's northernmost atoll (Dana 1971). Beyond Kure Atoll the Hawaiian chain extends northwestward as a series of drowned atolls (guyots) and seamounts which merge with the Emperor Seamounts (Davies et al. 1972). It is now generally accepted that is- lands and almost all of the seamounts in the Hawaiian and Emperor chains are geologically contiguous and orig- inated over a relatively fixed melting anomaly or "hot spot" in the Pacific lithospheric plate situated at 15~+ 4~ N latitude (Wilson 1963; Morgan 1972; Jackson et al. 1980). Drift of the Pacific Plate to the north and then northwest has resulted in the formation of an almost lin- ear series of progressively older islands, atolls and seamounts. As islands drifted northward by seafloor spreading they gradually subsided and eroded (Schlanger and Gillett 1976). Islands which reached sea level at lati- tudes where coral growth could keep pace with changes in sea level developed into atolls or coral islands. Continued drift to the north beyond latitudes where corals could keep pace with sea level led to further subsidence and drowning. Hence, the evolutionary succession of island formation, subsidence, erosion, atoll or coral island formation, north- ward drift and eventual drowning appears to be a con- tinuing and long-term process in the Hawaiian Archi- pelago. In this paper, I report the results of an experiment designed to measure coral growth and reef development 0722-4028/82/0001/0029/$01.20

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Page 1: Darwin Point: A threshold for atoll formation - · PDF fileCoral Reefs (1982)1:29 34 Coral Reefs 9 Springer-Verlag 1982 Darwin Point: A Threshold for Atoll Formation* R.W. Grigg Hawaii

Coral Reefs (1982)1:29 34 Cora l Reefs �9 Springer-Verlag 1982

Darwin Point: A Threshold for Atoll Formation*

R.W. Grigg

Hawaii Institute of Marine Biology, P.O. Box 1346, Kaneohe, Hawaii 96744

Received 5 January 1982; accepted 21 January t982

Summary. A threshold for atoll formation, herein termed the Darwin Point, exists at the northern end of the Hawai- ian Archipelago at 29~ latitude. Hawaiian atolls and coral islands transported northwest by tectonic movement of the Pacific Plate appear to have "drowned" near the Darwin Point during the last 20 million years. Measures of gross carbonate production by corals across the archi- pelago show that growth rates decrease with increasing latitude. At the Darwin Point, corals may contribute only 20 % of the calcium carbonate necessary to keep pace with recent changes in sea level and thus appear to be more im- portant as builders of framework than producers of lime- stone. Reduction in this function rather than total carbon- ate production may be the determining factor in the for- mation of atolls and coral islands. Elsewhere in the world other Darwin Points may exist but probably not at the same latitude due to differences in ecological conditions, coral species composition, island area, rates of erosion and tectonic histories.

Introduction During the voyage of the Beagle in 1836, Charles Darwin conceived his well known theory on the development of coral islands, now known as atolls (Darwin 1842, 1962). In writing about the islands of Tahiti and Moorea on 12 April 1836, Darwin made the following entry in his diary:

"Hence if we imagine such an island after long successive intervals, to subside a few feet in a manner similar but with a movement opposite to the continent of South America; the coral would be continued up- wards, rising from the foundat ion of the encircling reef. In time, the cen- tral land would sink beneath the level of the sea and disappear but the coral would have completed its circular wall. Should we not then have a coral island? Under this view we must look at a lagoon island as a monumen t raised by myriads of tiny architects to mark the spot where a former land lies buried in the depths of the ocean."

Darwin's subsidence theory of atoll formation was confirmed in 1951 when two holes were drilled which reached the volcanic rock basement beneath Enewetak Atoll at depths of 1,267 m and 1,405 m, respectively (Ladd

* Hawaii Institute of Marine Biology Contribution No. 627

et al. 1970). The limestones recovered were all of shallow water origin demonstrating both subsidence of the atoll and the upward growth of shallow water coral since Eocene time, approximately 49 million years B.P. (Schlanger 1963).

While the mechanism of atoll formation is now under- stood, much less is known of the interaction between the process of reef building and rates of subsidence, erosion and past changes in sea level. In the Hawaiian Archi- pelago, an opportunity exists to examine this interaction across latitudes from zones of optimal reef development to the limit of atoll formation. The Hawaiian Archipelago stretches diagonally 2,450 km across the north Pacific from the island of Hawaii (19 ~ N) to Kure Atoll (29 ~ N). Kure Atoll is the Pacific's northernmost atoll (Dana 1971). Beyond Kure Atoll the Hawaiian chain extends northwestward as a series of drowned atolls (guyots) and seamounts which merge with the Emperor Seamounts (Davies et al. 1972). It is now generally accepted that is- lands and almost all of the seamounts in the Hawaiian and Emperor chains are geologically contiguous and orig- inated over a relatively fixed melting anomaly or "hot spot" in the Pacific lithospheric plate situated at 15 ~ + 4 ~ N latitude (Wilson 1963; Morgan 1972; Jackson et al. 1980). Drift of the Pacific Plate to the north and then northwest has resulted in the formation of an almost lin- ear series of progressively older islands, atolls and seamounts. As islands drifted northward by seafloor spreading they gradually subsided and eroded (Schlanger and Gillett 1976). Islands which reached sea level at lati- tudes where coral growth could keep pace with changes in sea level developed into atolls or coral islands. Continued drift to the north beyond latitudes where corals could keep pace with sea level led to further subsidence and drowning. Hence, the evolutionary succession of island formation, subsidence, erosion, atoll or coral island formation, north- ward drift and eventual drowning appears to be a con- tinuing and long-term process in the Hawaiian Archi- pelago. In this paper, I report the results of an experiment designed to measure coral growth and reef development

0722-4028/82/0001/0029/$01.20

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ZONE 4 ZONE 3 ZONE 2 ZONE I SUBDUCTION IN I REEF CORAL GROWTH I FRINGING REEFS I SUBAERIAL EROSION, I KAMCHATKA TRENCH [CEASES, CONTINUED I BECOME ATOLLS, I SUBSIDENCE, AND I

I SUBSIDENCE, ATOLLS I CONTINUED I FRINGING REEFS I [DROWN TO FORM GUYOTS I SUBSIDENCE I 19~176

<'~ 4~ 165~ DARWIN POINT tl A/~A [ I

/

~ , 2 8 ~ 24~ Ci It /v l SEA LEVEL ~ - A r] 11 II I I

~ . ~ W Y ~ ' - - pAciFic a ~ ~ ,~,\ ~/,

- ~ , ~ ~ ASTHENOSPHERE AN%MRA LY

~ ~ "HOT SPOT"

Fig. 1. Schematic representation of evolu- tionary history of the Hawaiian Archipelago showing location of the Darwin Point sepa- rating zones 2 and 3. Age of islands in millions of years shown for six geological features (a-f). (Modified after Rotondo 1980)

across the en t i re a r c h i p e l a g o to the l imi t o f a to l l f o r m a -

t ion , he re in t e r m e d " T h e D a r w i n P o i n t " (Fig. 1). I a lso

d r a w u p o n geophys ica l ev idence f r o m the l i t e ra ture tha t

suggests the D a r w i n P o i n t has exis ted n e a r the same lati-

tude fo r a t least the pas t 20 mi l l ion years.

Materials and Methods

The experiment was conducted on 14 major islands which span the archipelago from Hawaii to Kure Atoll. Stations were selected off southwest exposures of each island at depths of 10 m in zones of maxi- mum reef development. At each station coral growth was measured by averaging the width of at least ten annual bands (Knutson et al. 1972) in skeletal cross sections of at least ten colonies of the most abundant spe- cies, Porites Iobata (Figs. 2 and 3A). The mean density of each colony was measured on a densitometer using aluminium step wedges as standards (Fig. 3B). Colony mass accretion in kg CaCO3 was calculated as the product of mean linear growth rate and mean colony density (Fig. 3C). Mean values of mass accretion for colonies of P. lobata were then com- puted for each island and multiplied by measures of mean coral cover for all species of coral to produce mean rates of accretion due to all corals in kg CaCO3/m2/year (Fig. 4B). P. lobata is considered representative of other Hawaiian corals because its growth approximates the growth in mean solid radius of corals of substantially different growth form, i.e. branching and encrusting (Maragos 1972), and because it is intermediate in growth rate (Buddemeier et al. 1974). Also it is the overwhelming dom- inant framework builder in the Hawaiian Archipelego (Grigg and Dollar 1980).

It could be argued that carbonate production should be measured in water shallower than 10 m or in the lagoons in order to obtain maxi- mum rates. However, colonies of P. lobata collected at 1-5 m depth at a number of locations in the archipelago contained comparable or slightly narrower rather than broader annum growth bands. Also at depths shal- lower than 10 m in areas exposed to waves, hiatus lines are common sug- gesting growth is frequently interrupted. Hence measures at 10 m can probabl7 be considered near maximal for offshore reefs. Inside the lagoons the proportion of carbonate contributed by corals may be greater than it is on offshore reefs, particularly at Midway and Kure Atolls where many shallow patch reefs support thickets of Porites com- pressa. Nevertheless, growth studies were conducted on seaward reefs be- cause many islands, banks and shoals in the archipelago lack lagoons. Also before lagoons can develop, seaward reefs must be capable of keep- ing pace with changes in sea level. Therefore, in terms of understanding the factors which control atoll formation, the development of seaward reefs is of first order importance.

In this study, since measurements of growth were made along axes of maximum growth in each colony and because estimates of coral bot-

tom cover were taken from optimal areas (southwest exposures), rates of accretion should be considered estimates of maximum gross production for corals. Chave et al. (1972) have distinguished between potential, gross and net carbonate production. Potential production refers to the calcifi- cation rate of an individual organism or colony, gross production is the calcification rate of the community (the product of potential production and the proportion of reef covered by calcifying organisms), and net pro- duction is the carbonate permanently retained by the reef. With no car- bonate dissolution or mechanical gains or losses, net production is equiv- alent to gross production. In optimal environments in Hawaii rates of erosion and dissolution are small and gross production is a reasonable approximation of net production. In fact, estimates of calcification based on the method used here agree remarkably well with measures of net cal- cification based on alkalinity depression for comparable areas in Hawaii (Kinsey 1979).

Reef accretion depends on net production of all calcifying or- ganisms, that is, the amount of CaCO 3 retained by the reef. In optimal environments in Hawaii where coral cover is high, gross production by corals is a good approximation of net production on the reef. Where cor- al cover is low, gross production by corals is an underestimate of net pro- duction on the reek

Results

T h e resul ts s h o w tha t ca lc i f i ca t ion o f i nd iv idua l cora l s and

co ra l reefs acc re t i on due to cora l s decl ine in a l inear fash-

i on as a f u n c t i o n o f l a t i tude f r o m H a w a i i in the sou theas t

to K u r e A t o l l in the n o r t h w e s t (Figs. 3 A - C , 4 A - B ) . A t the

s o u t h e a s t e r n end o f the c h a i n the we igh ted m e a n g r o w t h

ra te o f co lon ie s o f P. Iobata is 1 3 m m / y e a r o r 1 8 k g

C a C O 3 / m 2 / y e a r . E q u i v a l e n t ra tes for the n o r t h e a s t e r n ex-

t r eme are 3 m m / y e a r and 5 kg C a C O 3 / m 2 / y e a r . These

ra tes c o r r e c t e d d o w n to a c c o u n t fo r co ra l c o v e r for all spe- cies are 11 m m / y e a r and 1 5 k g C a C O 3 / m 2 / y e a r a n d

0 . 2 m m / y e a r a n d 0.3 kg C a C O 3 / m 2 / y e a r , r espec t ive ly (Fig. 4B).

L a t i t u d e - d e p e n d e n t ca lc i f i ca t ion by cora l s is consis-

t en t w i th the m o d e l sugges ted by M c K e n z i e et al. (1980) b u t impl ic i t ly con t r ad i c t s the conc lus ions o f K i n s e y

(1979), K i n s e y a n d D a v i e s (1979), a n d S m i t h a n d K i n s e y (1976). W h i l e a c k n o w l e d g i n g a large degree o f va r i ab i l i ty

in ca lc i f i ca t ion a s soc ia t ed w i t h hab i t a t , K i n s e y (1979) has sugges ted tha t cora l reefs as a who le exh ib i t a " s t a n d a r d "

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Fig. 2A-D. X-radiographs of cross sections from colonies collected from Hawaii (A), French Frigate Shoals (B), Pearl and Hermes Reef (C) and Kure Atoll (D). Note progressive decrease in thickness of annual growth bands and change in growth form from lobate to veneer moving northwest

or uniform metabolic performance independent of lati- tude. Kinsey (1979) claims that in general, Pacific coral reefs can be characterized by a bimodal scheme consisting of reef flats or perimeter zones (3-5 kg CaCO3/m2/year) and sand-rubble zones (0.5 kg CaCOa/m2/year). Chave et al. (1972) have summarized the literature up to 197t and report values of gross production for Pacific and Indian Ocean reefs of 5-8 kg CaCO3/m2/year. In 1978, Adey summarized the literature for net production during the Holocene for reefs in the tropical ( < 24 ~ latitude) Carib- bean (13-22 kg CaCO3/m2/year based on an accretion rate of 9-15 ram/year 1) and the Pacific (4-14 kg CaCO3/ m2/year, based on an accretion rate of 3-10 ram/year1). The coral reefs considered by Kinsey (1979) and others are all situated at latitudes between 23 ~ S and 25 ~ N. Within these latitudes, with the possible exception of the Carib- bean the notion of uniform calcification of coral reefs is reasonably well supported by existing data.

1 Assuming a skeletal density of 2.9 g/cm 3 and porosity of 0.50

Looking again at the data for the Hawaiian chain, values at the southeastern end are relatively high suggest- ing optimal conditions. In the middle of the chain, figures are more comparable to reefs elsewhere in the world, while at the northwestern extreme estimates are unusually low. In view of the data for more tropical reefs discussed above, the inverse relationship reported here between latitude and coral calcification may only apply near latitudinal limits of coral reef development.

Discussion

The rates for coral growth and reef accretion due to corals at Kure Island (0.2 mm/year and 0.3 kg CaCO3/m2/year, respectively [Fig. 4B]) are more than sufficient to offset subsidence at this latitude in the chain (0.04mm/year) (Greene et al. 1978), but not nearly large enough to have kept pace with sea level rise during the Holocene trans- gression which was about 14 ram/year between 15,000 and

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32

25

"C >-, 2C

E E

15 h i I-- ,< rr

I0 T I--

O 5 n," (_O

0

2.0

1.8 r,~

E E

"~ 1.6 E

1.4 03 z Ld C~ 1.2

I.O

0.0"

E 25

0 0 o 0 20

z 15 0 I - - I..d o~ l 0 c) <

>- 5 z o / o o o ~

9

A GROWTH RATE

AWAII Y= 3 5 . 5 - 1 . 0 8 ( X ) , p< 0.01, r = - O . 8 7 �9 -- ~ OAHU

- _ _ �9 NECKER

I ~., �9 ; o�9 -- LAYSAN M~Od-

~ ~ ~MAUI - - - - - - - T - _ . ~ ! " -- - - MIDWAY

�9 FFS ~ __.____ NIHOA GAR N t - - I ~

I I I I I, I LISIA, NSK~" ~I~ ~ I I

LISIANSKI

KAUAI GARDNER

I I " IAWAII OAHU NECKER �9

MARO

I I �9 l �9 MAUl NIHOA �9 FFS

I

MIDWAY

-1 I

P ; H �9149

e l

"1 KURE

LAYSAN

Y= 0 .67 + 0 , 0 3 6 ( X ) B D E N S I T Y

i i i I I I I I I I I

C COLONY ACCRETION OAHU

~ ~ ~ NECKER

..AWAII �9 �9 o ~ ~ �9 Y= 44 .9 -1 .39 (X ) , p<O.OI, r = - 0 . 8 6

�9 �9 , I L ~ �9 �9 ~ LAYSAN

--_ M20, " "MAROI "--_

"KAUAI -- ~ �9 I" KAUAI- FFS �9 ~ I'

.... : : - _ �9 ? ~ I NIHOA ----GARDNER I pl .~

~ LISIANSKI I ~

.TKURE I I I I l I f I ~ 1 I

20 21 22 23 24 25 26 27 28 29 50

LATITUDE (~

Fig. 3A-C. Growth rate (A), colony density (B), colony accretion (C) of P. lobata in the Hawaiian Archipelago. Data plotted in Fig. 3A are averages based on at least ten annual bands per colony and in this respect represents decade long means

6,000years B.P. (Bloom 1971; Chappell 1974a; Adey 1978). Surely Midway and Kure Atolls would have "drowned" during this period were they not emergent features.

Evidence is now accumulating that many coral atolls, coral islands and reefs were probably above sea level dur- ing the last glaciation (Kinsey and Davies 1979). During this period, sea level receded to about -130 m about 18,000 years ago (Chappell 1974b) and persisted below present sea level for as long as 120,000 years B.P. (Adey 1978). If Midway and Kure were exposed during this time, a solution unconformity (Schlanger 1963) should be pres- ent below the layer which represents Holocene growth. No

dates are available for cores taken at Midway (Ladd et al. 1970), but such unconformities have been found at other places in the Pacific. At Enewetak an unconformity exists at approximately 10 m depth which separates horizons 6,000 and 100,000 years in age (Thurber et al. 1965; Tracey and Ladd 1977). Other unconformities which match this time period have been found at 20 m at Heron Island on the Great Barrier Reef(Davies and Kinsey 1977) and 6 m at Muroroa (Ladd et al. 1970). If the core data for Ene- wetak and Mururoa can be applied to Midway and Kure Atolls, then present day reefs there may have formed from pre-existing karst foundations (Purdy 1974) about 6-10 m below present sea level during the last 6,000 years.

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I00

8o

n 6 0

rF taJ > 0 4 0 (.9

_1 < r r 2 0 O 0

�9 HAWAII

�9 MAUl

�9 OAHU �9 FFS

A CORAL COVER

� 9

I I I I I

�9 NIHOA �9 LAYSAN

�9 NECKER l LISIANSKI �9 P~H

MIDWAY l I KURE �9 GARDNER I I I I I 0

B REEF ACCRETION E 25

o ~ o 2o (.9

_~ I /Y= 4556-160(X), r=-0.98, N=IO ~ 1 5 ~ i~ /

fY=4263-,53(x,, ,~ N~

~ �9 MARO p & ~

,,~r_ .... @LISIANSKI O* I' l I I I I I �9 GARDNER i ~ " ~ O @ KURE

19 20 2, 22 23 2 4 25 26 27 28 29

LATITUDE (ON)

33

310 �9

Fig. 4A, B. Mean coral cover (A) and reef accretion (B) on seaward reefs off all major Hawaiian Islands. Mean values of coral cover, although based on data collected in one year, represent longer time periods since reef transects tend to integrate small scale patches (< 50 m) of different successional age. In (B) the upper regression line was calculated without data for Nihoa, Necker and Gardner Pinnacles. These Islands would be "drowned" banks were it not for emergent outcrops of basalt

Over the last 6,000 years sea level rise has moderated considerably. A sea level curve based on Florida and Micronesian data, for this period yields a rate of 0.89 mm/ year (Bloom 1971). Estimates for the Pacific alone (Chap- pell 1974a; Adey 1978) give an average value of 0.33 ram/ year.

I f a figure for sea level rise during the last 6,000 years of between 0.33 and 0.89 mm/year is applied to Kure Atoll, the mean growth rate obtained for corals there in optimal environments (0.2 ram/year) would represent be- tween 22% and 60% of the carbonate necessary to keep pace with sea level. Since the reef at Midway has kept pace with sea level (surface sediments are Holocene in age [Ladd et al. 1970]), and Kure is situated adjacent to Mid- way, the difference (40%-78%) in carbonate production may be an estimate of limestone produced by other cal- careous organisms, such as coralline algae, molluscs, bryozoa, echinoderms and foraminifera as well as lithifi- cation processes (Land and Goreau 1970).

I f a foundation at - 6 m is taken as the horizon from which Holocene growth started 6,000 years ago, then an upward rate of 1 ram/year would be required to keep pace with sea level. In this case the amount of limestone con- tributed by coral would be only 20%. The finding of Gross et al. (1969) that CaCO3 production by coralline algae at Midway is about double that produced by coral lends sup- port to this conclusion. Since corals provide the basic fra- mework and control the accumulation of sediments in, on and around coral reefs (Hoffmeister and Multer 1964), re- duction in this function rather than total carbonate pro- duction may be the determining factor in atoll formation.

Selective drowning of banks southeast of the Darwin Point has occurred in a number of instances in the Hawai- ian chain demonstrating that the Darwin Point is not an absolute threshold. Why some banks keep pace with sea level and others do not, remains a mystery but could be re- lated to local differences in subsidence or environmental conditions such as erosion by high waves (Adey 1978). Some support for the erosion theory is a negative correla- tion between area of bank tops and drowning. At or near the Darwin Point, environmental conditions would be ex- pected to be marginal, where even small differences in- cluding chance might determine the difference between drowning and keeping pace with sea level changes. Indeed, surrounding Midway and Kure Atolls are five shallow banks which are essentially drowned ( > 30 m depth).

The Darwin Point in the Hawaiian Archipelago ap- pears to have been relatively fixed in space since the Miocene. Based on Pacific Plate motion over the past 70 million years, Rotondo (1980) has derived the pale�9 tion of many drowned seamounts and guyots presently northwest of Kure Atoll. For at least during the past 20 million years, the most northerly latitude at which "drowning" of guyots occurred was between 27-31 ~ N. I t is reassuring that the most recent reconstruction of sea surface temperatures in the Pacific during the Pleistocene does not show significant differences between the 18,000 B.P. values at 30 ~ N latitude and 180 ~ W longitude and the present (Moore et al. 1980).

Elsewhere in the world other Darwin Points undoubt- edly exist, however, not necessarily at the same latitude as in the Hawaiian Archipelago. Also, gross and net values

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34

o f c a r b o n a t e p r o d u c t i o n at a D a r w i n P o i n t w o u l d n o t be

expec ted to be un ive r sa l because o f g loba l d i f fe rences in

o c e a n o g r a p h i c c l ima t e a n d tec ton ic histories . In H a w a i i ,

however , the D a r w i n P o i n t appea r s to have d e t e r m i n e d

the n o r t h e r n l imi t o f the a r ch ipe l ago and d r o w n i n g o f nu-

m e r o u s a tol ls ca r r i ed n o r t h w e s t by sea - f loor sp read ing fo r

at least 20 mi l l ion years. N o r t h o f the th resho ld , co ra l is-

l ands no l onge r keep pace wi th sea level and n o w "l ie

bur i ed in the dep ths o f the o c e a n " as o r ig ina l ly con-

ce ived by Char l e s Da rwin .

Acknowledgements. I thank Stephen V. Smith, Seymour D. Schlanger and Keith E. Chave for critically reviewing the manuscript and Steven J. Dollar and Randi Schneider for field and laboratory assistance.

References

Adey W (1978) Coral reef morphogenesis: A multidimensional model. Science 202:831-837

Bloom A (1971) Glacial-eustatic and isostatic controls of sea level since the last glaciation. In: Turekian K (ed) The late cenozoic glacial ages. Yale University Press, New Haven pp 355-379

Buddemeier R, Maragos J, Knutson D (1974) Radiographic studies of reef exoskeletons. 1. Rates and patterns of coral growth. J Exp Mar Biol Ecol 14:179-200

Chappell J (1974a) Relationship between sea levels, 180 variations and orbital perturbations during the last 250,000 years. Nature 252:199- 202

Chappell J (1974b) Late quaternary glacio- and hydro-isostacy on a layered earth. Quaternary Res 4:405 428

Chave K, Smith S, Roy K (1972) Carbonate production by coral reefs. Mar Geol 12:123-140

Dana T (1971) On the reef corals of the world's most northern atoll (Kure: Hawaiian Archipelago). Pac Sci 25:80-87

Darwin C (1942) Coral reefs. Smith Elder, London Darwin C (1962) Coral islands with introduction, map and remarks by

D. Stoddart. Atoll Res Bull 88:1-20 Davies P, Kinsey D (1977) Holocene reef growth One Tree Island,

Great Barrier Reef. Mar Geol 24:1-11 Davies T, Wilde P, Clague D (1972) Koko Seamount: a major guyot at

the southern end of the Emperor Seamounts. Mar Geol 13:311-321 Greene H, Dalrymple G, Clague D (1978) Evidence for northward move-

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