Geochronology of drowned Hawaiian coral reefs

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary

Geochronology

Geology 99 (1995) 233-242

of drowned Hawaiian coral reefs Anthony

T. Jones

’

Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, 1000 Pope Road, Honoi’uk~, Hawaii 96822, USA

Received 3 June 1993; revised version accepted 5 June 1994

Abstract Two deeply submerged coral reefs were sampled with the HURL submersible Pisces V off the islands of Hawaii and Maui. An ancient fringing reef at 950 m depth on the Kohala slope of Hawaii was observed from deep-towed camera photographs and during submersible operations. A second reef at 1650 m depth on the Haleakala slope of Maui was observed on side-scan images, deep-towed camera photographs and during submersible observations, Ages of the two reefs were determined by electron spin resonance dating with a signal intensity for aragonitic coral at g = 2.0007 used to ascertain the accumulated dose by the additive dose method. Uranium concentration was measured by neutron activation analysis. The chronology of the reefs indicates that drowning coincided with periods of rapid global climate change at the termination of continental glaciation at approximately 250 ka and 340 ka. The

drowning events correlate with oxygen isotope stage boundaries drowning on subsiding volcanic edifices.

1. Introduction The evolution of coral reefs and volcanic islands has been closely linked to the tectonic process of subsidence (e.g. Darwin, 1837). Fossil reefs provide a useful palaeobathymetric marker with which to interpret the history of sea level and infer island subsidence. The discovery of deeply submerged coral reefs off the Hawaiian Islands in a series of submersible dives in the Alenuihaha Channel confirms rapid subsidence of hot spot volcanoes due to crustal loading. The

’ Present address: Oceanus Flotation Technologies Inc., 1401-I 166 Alberni Street, Vancouver, B.C. V6E 323, Canada.

7/8 and 9/10. These data support models of reef

reefs also provide a test of a simple model of reef drowning on subsiding edifices (Campbell, 1984). This paper reports the age of two deeply submerged reefs determined by electron spin resonance (ESR) dating. ESR dating was chosen because of the suitable range of the technique (g 1 Ma, Ikeya, 19841, the small amounts of material required for dating (- 1 g>, and the physical procedure of measuring the ESR spectra allows subsequent destructive chemical techniques to be performed on the same sample. This technique has been successfully applied to marine deposits on Oahu, Hawaii (Briickner and Radtke, 1989) and elsewhere (Ikeya and Ohmura, 1983; Koba et al., 1985; Skinner, 1985, 1988; Pirazzoli et al., 1991, 1993; Gray et al., 1992).

OO37-0738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0037-0738(95)00046-l

234

A. T. Jones / Sedirnmtmy Geology 99 (1995) 233-242

2. Study site

The Alenuihaha Channel separates the islands of Maui and Hawaii in the Hawaiian Island Chain (Fig. 3). The island of Hawaii is the youngest in the Hawaiian Archipelago and comprises five young volcanoes. Active volcanism at Kilauea on the southeastern end of the island contrasts with the extinct Kohala volcano on the northwestern end of the island. The Alenuihaha Channel reaches a water depth of approximately 2000 m. Bathymetric surveys identified a barrier reef system off northwest Hawaii at about 1000 m depth (Campbell, 1984). Reconnaissance surveys of the northwest Kohala submarine slope with deep-towed camera systems photographed coral reef rubble in water depths of 1000 m (Campbell, 1984) confirming earlier dredging of hermatypic corals from the Kohala Terrace by Campbell and Erlandson (1981). Predive deep-tow photographs showed in situ reef corals near the shelf slope break in approximately 950 m water depth on the Kohala slope with a sandy surface upslope of the reef and basalt talus downslope of the reef (Fig. 2). On the Haleakala slope, reef ledges were identified in bottom photographs in water depths of approximately 1100

Fig. I. Bathymetry

m. Pre-dive SeaMARC II side-scan imagery along the southern slope of Haleakala detected linear structures with significant relief that were interpreted as scarps with near vertical faces (Fornari and Campbell, 1987). Several submersible dives with the Hawaii Undersea Research Laboratory Pisces V were undertaken to characterize the benthic macrobiota (Chave and Jones, 1991) and to examine the major geological features of the channel. Samples of reef corals were collected during two dives (P5-59 and P5-64). Dive tracks are plotted in Fig. 1. The geology of the Alenuihaha Channel will be discussed in a companion paper.

3. Method-ESR

dating of corals

ESR dating is based on measurement of the accumulation of a radiation-sensitive signal in corals. The coral, thus, records natural environmental radiation principally from uranium bound within the skeletal lattice. The coral acts as a passive dosimeter. Because corals incorporate approximately 3 ppm of uranium from seawater into their skeleton during formation @wart and Hubbart, 19821, radioactive decay of this uranium

Jones.

1991).

235

A. T. Jones /Se~i~merrtury Geology 99 (1995) 233-242

where D, is the accumulated radiation dose the coral has absorbed. The radiation dose rate as a function of time, &t>, is estimated from analysis of radioactive elements (U, Th, and K) in the sample and its surroundings. The SI unit for radiation is the Gray (Gy). The age of the sample T can be expressed simply as:

causes electrons to become unpaired. Electron holes or “traps” in the crystal lattice facilitate retention of these unpaired electrons so that the ESR signal, which measures the paramagnetic signal from the unpaired electrons, can be used to date fossil corals (Ikeya, 1978; Griin, 1989). Three radiation-sensitive ESR signals are present in corals. The paramagnetic centre at g = 2.0007 is absent in modern corals, but increases in intensity as a function of the accumulated radiation absorbed by the coral over time (Fig. 3). This paramagnetic centre was used to determine the past radiation dose. Greater detail on ESR dating can be found in Ikeya (1978) and Griin (1989). An ESR age is derived from the relation:

D, = /‘ ( 0

Area

(Photography

photograph

of photograph

of in-situ coral head on Kohala is approximately

courtesy of F. Spiess.)

DE(@) B(Gy yr-‘)

A comparison of ESR dates with other dating techniques indicates that there is generally good agreement between the techniques (Fig. 4), especially considering that investigators have used different functions (linear, exponential) to extrapolate an accumulated dose.

t)dt

Fig. 2. Deep-tow camera.

T(u) =

terrace.

5 m X 7 m. For

Strobe light at left of photograph

further

details

on deep

is suspended

- 8 m below

tow see Spiess and Lonsdale

(1982).

236

A. 7: Jones /Sedimenia,y

Corals collected during submersible dives P5-59 and P5-64 were prepared for ESR dating by methods outlined in Griin (1989). Crushed aragonitic coral samples were handpicked under a binocular microscope to avoid obvious diagenetitally altered skeletal material. Sample X-ray diffraction revealed > 99% aragonite. The accumulated dose was determined by the additive dose method (Griin, 1989). Several aliquots of each sample were artificially irradiated with yrays from a “‘Co source at doses ranging from 10 to 1200 Gy. The intensity of the ESR signal was measured on a Briiker ESR spectrometer after annealing aliquots for 60°C for 3 days. The D, was extrapolated from an exponential curve fitting program and associated errors were determined by a jackknifing procedure which removes each point and recalculates the curve (Griin and Macdonald, 1989). Because the corals have been bathed in water in excess of 200 m for most of their history, the influence of cosmic radiation and any external radiation from sediments in determining the annual dose rate is assumed to be very small. Internal radiation was determined by measuring the concentration of radioactive elements, U and Th. Powdered subsamples were analyzed for U and Th by neutron activation analysis. Ages were calculated with a program that includes correction

Modulation 0.8 G,, Microwave Power 2 mW

A

g = 2.0007

Geology 99

(I 995) 233-242 ?? “C Age (ka)

0

23”Th/23sU Age (ka)

A “‘Pa/235~J +

Age (ka)

He/U Age (ka)

ESR Age (ka) Fig. 4. Comparison of ESR dates from corals with other independent methods. The 1:l line is plotted for reference. Data are from Ikeya and Ohmura (19831, Skinner (1985. 1988), Radtke et al. (1988), Radtke and Griin (1988), Peng et al. (1989) and Briickner and Radtke (1989).

for U-series disequilibria (Griin, 1989). The initial 234U/238U ratio in corals is assumed to be 1.14 based on mass spectrometric measurement of modern Hawaiian corals (Moore et al., 1990). The coral (Y efficiency (a/r effective ratio) is based on empirical studies of corals from Barbados and Papua New Guinea (0.06 f. 0.01, Radtke et al., 1988; Griin et al., 1992) and no Rn loss is experienced by corals (Skinner, 1986).

4. Results

346

347

340

349

350

Magnetic Field (mT)

Fig. 3. ESR spectra for samples P5-59 and P5-64

The ages for the deeply submerged reefs are presented in Table 1. The U concentration in the samples was 3.22 and 3.34 ppm for P5-59 and P5-64, respectively. These values are consistent with published U concentrations of 2-4 ppm for fossil corals Weeh and Burnett, 1982). The ESR ages generally concur with U-series ages (B. Szabo, pers. commun., 1991). Neutron activation analysis for selected other elements is presented

A. T. Jones /Sedinwlfury Table 1 ESR results Dive

P5-59 PS-64

from deeply

submerged

coral reefs 234u,238u

U

DE

ESR age

bpd

(GY)

(ka)

(ka)

3.22 3.34

225 334

248 + 18 338 + 36

259*6 380+7

” B. Szabo, pers. commun. 1 gray (Gy) = 100 rads.

age

n

(1991).

in Table 2. Note that Th and K are at or near detection limits. The submersible sample P5-59, identified as Porites compressa, was collected from the reef crest (Figs. 5, 61. Porites spp. is the dominant member of the reef at the shelf break in approximately 950 m water depth. The majority of the coral seen from the submersible was in growth position except for a pile of broken coral fragments downslope of the reef crest. Individual hermatypic coral heads clustered as patch reefs or coral mounds were observed south of the reef crest separated by calcareous sand flats. A surficial lava flow seen in the SeaMARC imagery was confirmed by submersible observations. Thus, sample P5-59 represents an in-situ coral from the reef crest and reflects the age of drowning. In the Alenuihaha Channel, there are no reef terraces deeper than the slope break at 950 m on the Kohala slope (Chave and Jones, 1991). Upslope, however, bathymetric terraces are indicated at - 150 and -400 m on the Kohala slope (Fig. 7). The age of the 950-m Kohala terrace (- 250 ka) is younger than those reported from a chronological study of a flight of terraces off northwest Hawaii by Ludwig et al. (1991) who dated a 925-m reef terrace at 287 + 10 and 314 f 10 ka. Their data, however, are consistently younger than their model age of 340 ka for this terrace. Sample P5-64 was collected from a coral framework exposure (Fig. 8). The age of the 1650-m Haleakala terrace at 340 f 36 ka is significantly younger than the reported age of 750 _+13 ka for dredged material (1555-1705 m> from the Haleakala Ridge off east Maui (Moore et al., 1990). Palaeomagnetism on sample P5-64 indicated normal polarity (D. McNeil& pers. commun., 1990) and is interpreted as within the

23-l

Geology 99 (1998) 233-242 Table 2 Selected elemental ples from Hawaii

analysis

of deeply

submerged

Concentration P5-59

P5-64

24Na 4zK 4”sc 47Ca s”Mn 58Ni 5yFe Yo ‘sZn 14”La IQE”

3960+ 130 n.d. 0.055 f 0.008 60.5 f 2.4% n.d. nd. n.d. 3.0550.20 n.d. 0.62+0.07 < 0.11 n.d. 7.2 + 0.6

4040* 130 260 f 40 0.068 * 0.005 64.9*2.5% 18+2 n.d. n.d. 1.77*0.10 n.d. 0.76 + 0.05 0.080 f 0.016 n.d. 7.3kO.4

a All concentrations

a

in ppm except

sam-

Detection limits

Element

233Th *“Np

coral

8 190 0.024 1% 6 5 310 0.4 21 0.15 0.04 0.2 1.4

47Ca, which is in percent.

Brunhes chron (O-750 ka) which supports the 340 ka age. The discrepancy of the younger age of the 1650 m terrace may be reconciled by two facts: the ESR date of the sample would appear younger, if recent marine cementation occurred, but under this condition, the correlation of the ESR with the U-series should not hold. Contamination of the dredged material cannot be ruled out, and the discrepancy indicates the need for more chronological studies of the deeply submerged Hawaiian terraces using carefully selected materials. The ESR ages agree with the most likely times for reef drowning (fig. 2 in Ludwig et al., 1991) at the end of oxygen isotope stages 8 and 10 based on interpretation of the deep-sea oxygen isotope

i,::~~

Fig. 5. Sketch of barrier reef on Kohala slope, Alenuihaha Channel. Note that sample P5-59 was collected from reef crest.

Fig. 6. Photograph

of Kohala

reef terrace

chronology (Imbrie et al., 1984), reef growth rates for Hawaii (Grigg and Epp, 19891, and models for reef drowning (Campbell, 1984). Campbell (1984) proposed a model to explain the occurrence of shallow-water marine fossils dredged and photographed from deep terraces off the island of Hawaii: As the island sinks from addition of new mass and sea level falls, the combined effect is a stable shoreline for forma-

N

Fig. 7. Bathymetric profile across the Alenuihaha axis. Drowned reefs are indicated by arrows.

s

Channel

at 950 m water depth (HURL

photo 5059-29).

tion of nearshore coral reefs (Fig. 9). At the conclusion of continental glaciation, melt water raises sea level causing reefs on subsiding edifices to experience rapid relative sea-level rise, thereby drowning the reefs. Once a coral reef sinks below about 30 m, the reef growth rate declines dramatically. Therefore, the ages of the fossil reef crest should correlate with the termination of glacial periods. To verify the model, the ages of the reef must coincide with periods of rapid rise in sea level. Oxygen isotope records from deep-sea cores provide a continuous record of the continental glaciation. Glacial and interglacial oxygen isotope “stages” were established by Emiliani (1955). Periods interpreted as low stands of the sea (high continental ice volume) have been assigned even numoers, whereas periods of high sea level (low continental ice volume) are identified by odd

A. T. Jones / Sediuzcrnlnry Geolofiy 99 (1995) 233-242

239

Fig. 8. Photograph of Haleakala reef terrace at 1640 m water depth with a branch of a dead gorgonian with attached barnacles, ophioroids and crabs in the foreground. Note vertical columns on left side of photographthat are frameworkcorals(HURL photo 5064-46).

A

I

I3

Fig. 9. Campbell’s model of reef volcano (after Campbell, 1984).

drowning

on

a sinking

numbers. The results from the ESR age of drowned Hawaiian coral reefs indicate that drowning coincided with periods of rapid global climate change at the end of continental glaciation approximately 340 and 250 ka (Fig. 10). The drowning events correlate with oxygen isotope stage boundaries (7/8 and 9/10), thus supporting Campbell’s model of reef drownings on subsiding foundations. A rapid sea-level rise of greater than 10 mm yr-’ will cause drowning of coral reefs. Deposits indicative of sea-level low stands are generally buried by subsequent transgressions and are generally unavailable to test sea-level chronologies. The low stands reported here provide new data for examination of glacial maxima. These results also direct attention to the potential for palaeoceanographic studies of reef corals which can provide insights into palaeoceano-

A. 71 Jot~es/Sedimentary

240

o Hawaiian Drowned Reefs E g -1000 $

1

I

I

II

I

I

- ,I

I

I

I

I

1

coo -cc

I

1

a

100

200

I

I

300

400

I

500

I’ 600

Time(1000 yr BP.)

Fig. 10. ESR age of deeply submerged Hawaiian coral reefs plotted with deep-sea oxygen isotope record of continental ice volume (from Imbrie et al., 1984).

graphic conditions during (e.g., Druffel, 1989).

low stands

of the sea

5. Rates of subsidence The construction of a volcanic island over a hot spot places a tremendous load on the underlying lithosphere. The result is an isostatic adjustment that depresses the lithosphere leading to Table 3 Subsidence

rates for the Hawaiian

Geology 99 (1995) 233-242

partial sinking of the growing island. Most of the Hawaiian Islands have subsided 2-4 km since breaking the ocean surface with the greater part of their subsidence occurring within the first 1 Ma of the island’s existence (Moore, 1987). Thus, at a horizontal plate movement rate of 10 cm yr-’ (Jackson et al., 1980) the maximum subsidence occurs within 100 km of the hot spot. Moore (1987) reviewed the subsidence along the Hawaiian Ridge using a wide variety of observations to infer subsidence rates, including tide gauge measurements, geophysical measurements, studies of deeply submerged reef terraces, drill holes, dredge hauls, and drowned cultural artifacts. Estimates for the rate of subsidence of the Island of Hawaii range from 1.8 mm yr-’ to greater than 3 mm yr-’ (Table 31, with the latest estimate being 2.6 mm yr-’ (Ludwig et al., 1991). Submerged carbonate platforms Several authors have used deeply submerged marine terraces around Hawaii to estimate subsidence of the island. Moore and Fornari (1984), assuming a uniform subsidence rate for Hawaii, inferred rates of subsidence for deep terraces along the western portion of Hawaii, basing their study on 14C dating of reef material from the

islands Subsidence rate (mm yr-‘)

Reference

Submerged archaeological features Honoaunau, Hawaii

3

Apple

Tide records Hilo, Hawaii Kahului, Maui

2.4 0.3

Moore, Moore,

Submerged carbonate platforms N.W. Hawaii W. Hawaii Kealakekua, Hawaii Kohala, Hawaii

2.6 2.5 1.8-3 + 2

Ludwig et al., 1991 Moore and Campbell, 1984 Moore and Fornari, 1984 Campbell, 1984

Chemical signatures in lauas Sulfur content

2.4

Moore and Thomas,

Height history of Haleakala Laser Station Haleakala, Maui

O+2

Smith et al., 1988

Feature

and location

and Macdonald,

1966

1987 1987

1988

A.T. Jones /Sedimmtur)~

shallowest terrace at - 150 m. Campbell (1984) calculated a subsidence rate of 2 mm yr-’ based on the depth of the 1000 m reef terrace and the youngest age of the subaerial portion of the Kohala volcano. Next, by matching terrace depths with the time of glacial maxima from oxygen isotope records of Shackleton and Opdyke (1976), Campbell estimated the different rates of subsidence for west Hawaii (2.43 mm yr-‘) and west Lanai (1.91 mm yr-‘) (Campbell, 1986). Szabo and Moore (1986) determined the age of reef material from the -360 m terrace off northwest Hawaii and calculated subsidence rates for northern Hawaii (2.7 mm yr-‘). More recently, Ludwig et al. (1991) dated a series of submerged reef terraces off northwest Hawaii. The six reef terraces, ranging in age from 17 to 475 ka, suggest that Hawaii has subsided approximately 2.6 mm yr-’ for the past 475 ka. Nevertheless, because of differential subsidence along the Hawaiian Ridge, Moore (1987) cautioned against age correlation of submerged terraces based only on depth. In fact, some terraces tilt toward the region of greatest volcanic loading, especially the terraces off Maui designated the “H” and “K” terrace (Moore and Campbell, 1987; Moore, 1987). Although the actual ages of the tilted terraces have not been determined, Moore (1987) set limits for the terrace ages based on subsidence rates and the age of the volcanic pedestal. Reefal carbonates, 1280 m below sea level, were recovered from a scientific observation hole drilled in the geothermal field along the Puna Ridge of Kilauea (D. Thomas, pers. commun., 1990). An in-situ limestone section, about 180 m thick, was composed of coralline algae, corals, shallow-water foraminifera, mollusk shells, and sponge spicules (J. Resig, written commun., 1991). The thickness indicates that more than one transgression-regression of the sea may have been required to built up this amount of shallow-water carbonates. This subsurface geological evidence implies that Kilauea subsided at least 1200 m probably within the last 0.5 Ma (assuming a rate of 2.4 mm yr-I). Alternatively, the reef developed on an antecedent foundation of the volcanic predecessor to Mauna Loa and the flow from

Geology 99 (1995) 233-242

241

Kilauea buried the carbonate section. Down faulting of the carbonate section has been proposed and is being evaluated (D. Thomas, pers. commun., 1992). In summarizing the available information on subsidence along the Hawaiian Ridge, Moore (1987) concluded that subsidence was fairly rapid for the initial 1 Ma of volcanic shield building. As the volcano was carried off the hot spot by the northwestward moving Pacific plate, the rate of subsidence declined rapidly until the volcano reached the present position of Oahu where it became stabilized. The evidence for subsidence shows that the island of Hawaii sinks at a rate of a few millimeters per year (2.6 mm yr-‘). The island of Maui is intermediate to Hawaii and Oahu. Oahu was believed to have been stable for the last 120 ka and possibly longer (Veeh, 1966; Ku et al., 1974; Stearns, 1978; Moore, 1987). Re-evaluation of geochronological data from marine terraces on Oahu has suggested that Oahu has undergone uplift during the last 500 kyr and may not be a suitable “stable” platform (Jones, 1993). The Moore model for differential but constant subsidence can be tested by dating fossil reef terraces along the Hawaiian Ridge.

6. Conclusions Submersible observations on the sunken Kohala barrier reef system support Campbell’s (1984) interpretation of bathymetric profiles and bottom photographs offshore of the northwest section of the island of Hawaii. The chronology of the drowned reefs in the Alenuihaha Channel provide a measure of subsidence for the islands of Hawaii and Maui during the Quaternary. The likely cause of drowning was a rapid rise in sea level following the termination of continental glaciation combined with rapid volcanic subsidence due to loading of the lithosphere. Accordingly, the islands of Hawaii and Maui have subsided on the order of 3-4 mm yr-’ within the last 250 ka. The reef terraces chronology confirms that the drowning occurred at the termination of glacial periods and correlates to well-documented deep-sea oxygen isotope stratigraphy.

242

Acknowledgements operations staff The interest of the HURL The during this project is greatly appreciated. assistance of B. Blackwell, N. Porat and H. Schwartz at McMaster University in ESR dating is gratefully acknowledged. B. Szaho, U.S. Geological Survey, provided the mass spectrometric U-series dating. This paper was improved by reviewer’s suggestions. This work was supported by NOAA’s National Undersea Research Program. Submersible dives were supported in part by the State of Hawaii Deep Water Cable Program.

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