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Uranium-series age of coral reef growth on Rottnest Island, Western Australia
Marine Geology, 29 (1979) M l l - - M 1 5 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Letter Section URANIUM-SERIES AGE OF C O R A L R E E F GROWTH ON R O T T N E S T ISLAND, WESTERN A U S T R A L I A
BARNEY J. SZABO
U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colo. 80225 (U.S.A.) (Received June 2, 1978; accepted August 28, 1978)
ABSTRACT Szabo, B.J., 1979. Uranium-series age of coral reef growth on Rottnest Island, Western Australia. Mar. Geol., 29: M l l - - M 1 5 . Dating samples of corals and shell from the elevated coral reef terrace on Rottnest Island, Western Australia, indicate that in this region away from active plate boundaries the sea stood at least 3 m above present sea level 132,000 -+ 5,000 years ago. There is no geologic evidence of other ancient reef-forming periods on this island. INTRODUCTION
Rottnest Island is situated west of Perth and Fremantle a b o u t 18 km off the coast of Western Australia. The geology of the island is described in detail by Teichert (1950). A fossil coral reef is exposed in Salmon Bay on the south side of the island. The elevation of this exposure is 2--3 m. The uranium-series age of one coral sample from this reef exposure was reported earlier by Veeh (1966) and Teichert (1967) to be 100,000 + 20,000 years. W.S. Moore with C. Teichert revisited the locality in 1967 and resampled thereof corals and mollusks for uranium-series dating; the age determination of these samples is the subject of this report. Accurate dating of this southerly coral reef (lat. 32 ° S) of the Pacific--Indian Ocean region is important because the area is believed to be relatively stable tectonically; therefore, it should provide undisturbed datum for comparison of past fluctuations of sea level. PREVIOUS DATING IN THE PACIFIC--INDIAN OCEAN REGION
The dating of fossil corals from terraces of tectonically rising islands of New Guinea and the R y u k y u is reported by Veeh and Chappel (1970), Bloom et al. (1974), Chappell (1974), Chappell et al. (1974), Sakanoue et al. (1967), Konishi et al. (1974), and Chappell and Veeh (1978). These studies reveal the complex Pleistocene variations of sea level in the Pacific Ocean region. Dating of coral samples from the subsiding atolls of Enewetak, Bikini and Mururoa reveals coral growing periods separated by solution
M12 unconformities (Thurber et al., 1965; Labeyrie et al., 1969; Tracey and Ladd, 1974; and Szabo and Tracey, 1977). The coral growing intervals represent interglacial high sea levels; the unconformities represent periods of emergence and weathering during glacial low sea levels. Fewer studies are reported from presumably tectonically stable areas of the Pacific--Indian Ocean region where the separation of eustatic and tectonic effects is less complicated. Veeh (1966) reported first on dating corals at a number of localities of the Pacific--Indian Ocean region that are now 2--9 m above sea level. The best dated elevated shoreline is the Waimanalo on Oahu, Hawaii that stands a b o u t 7.6 m above present sea level (Ku et al., 1974). The average of 23 age determinations of corals is 122,000 -+ 7,000 years. The elevation of the fossil reef is about 5 m on the apparently stable islands of Aldabra Atoll, Indian Ocean region (Thomson and Walton, 1972); the average age of 8 samples is 129,000 -+ 7,000 years. The average age of 3 samples of coralline deposits at an elevation of a b o u t 5 m on the east coast of Australia is 120,000 + 5,000 years (Marshall and Thom, 1976). All of these investigators conclude that the last interglaciation was an exceptional event in that sea level was above the present level and presumably also above earlier high sea levels during the middle Pleistocene. EXPERIMENTAL PROCEDURE The coral and shell samples chosen for uranium-series dating were cleaned by ultrasonic scrubbing and crushed to a fine powder which was ignited at 600°C for a period of about 2 hours. The concentrations of uranium and thorium were determined by mass spectrometry; the 234U/23sU and 23°Th/234U activity ratios were measured by alpha spectrometry utilizing the methods described by Szabo and Rosholt (1969). The activity ratios of 231pa/23sU were determined by neutron activation and alpha spectrometry (Rosholt and Szabo, 1969). The abundance of aragonite was determined by X-ray diffraction analysis. RESULTS AND DISCUSSION The results of the analyses and the calculated 23°Th and 231pa ages of samples of four corals and one shell are shown in Table I. The percent aragonite is 96 to nearly 100, indicating that no significant recrystallization occurred in these samples through their geologic history. The 23°Th/232Th activity ratios are 80 or larger, indicating no initial 23°Th contamination in these samples. The average 23°Th age of the five samples is 132,000 -+ 5,000 years. Only two samples (C-2 and C-3) yielded finite 231pa ages, but all 231pa ages are concordant with their respective 23°Th ages, within the limits of experimental error. The average 234U/23sU ratio of the five samples is 1.11. Using the average :3°Th age of 132,000 years, the initial 234U/23sU ratio is calculated
TABLE I Analytical data and calculated Ages of several fossil corals and a mollusk f r o m coastal l i m e s t o n e of R o t t n e s t Island, Western Australia Sample
Uranium
Thorium
Aragonite
23°Th'1
2s4U*~
23°Th*~
2S°Th Age .2
231Pa'1
2SlPa Age *s
(ppm)
(ppm)
(%)
2S2Th
2ssU
2s4U
(× l 0 g years)
2ssU
(× l 0 g years)
C-1.4
3.89 ±0.04
0.120 ±0.006
93
80
1.11 ±0.02
0.735 ±0.029
139±12
0.96 ±0.04
118
C-2 .4
2.83 ±0.04
0.10
97
660
1.10 ±0.02
0.705 ±0.028
129±10
0.92 ±0.04
118±
1.12 ±0.02
0.710 ±0.028
130± 9
0.89 ±0.04
103±
32
C-3 .4
3.79
0.020 ±0.004
>97
457
18 21 14
C-4 .4
2.86 ±0.03
0.040 ±0.001
96
171
1.10 ±0.02
0.717 ±0.029
133±10
0.97 ±0.04
>124
S-1 *s
0.642 ±0.006
0.016 ±0.002
93
96
1.12 ±0.02
0.704 ±0.028
128±10
0.95 ±0.05
>108
*1Isotopic activity ratios. *2Calculated using half-lives of 23°Th and 234U o f 75,200 and 244,000 years, respectively. *3Calculated using half-lives o f 231Pa o f 32,500 years. *4Unidentified corals. *SMainly M a r m a r o s t o m a sp., a mollusk.
M14
to be 1.16; the same as the 234U/23sU ratio in present day seawater or 1.15 + 0.02 (Szabo, 1969). Mollusk shells often yield unreliable ages, mainly because these materials do not remain ideal closed system with respect to the isotopes of uranium, thorium, and protactinium (Szabo and Rosholt, 1969; Szabo and Vedder, 1971; Kaufman et al., 1971). In this case, however, the shell sample yielded concordant 23°Th, 231Pa and 234U ages which agree with the coral dates. Results indicate that at a b o u t 132,000 years ago, during which time the coral reef was forming at the Salmon Bay area, the sea stood at least 3 m above the present level at Rottnest Island. There is no evidence of other ancient reef-forming events at Salmon Bay and the reported age agrees well with dates of coral reefs of similar elevation on tectonically stable islands. ACKNOWLEDGEMENTS
I wish to thank W.S. Moore, Dept. of Geology, Univ. of South Carolina, Columbia, S.C. for supplying the samples for this study.
REFERENCES Bloom, A.L., Broecker, W.S., Chappell, J.M.A., Mattbews, R.R. and Mesolella, K.J., 1974. Quaternary sea level fluctuations on a tectonic coast: New 23°Th/~34U dates from Huon Peninsula, New Guinea. Quat. Res., 4: 185--205. Chappell, J., 1974. Geology of coral terraces, Huon Peninsula, New Guinea: A study of Quaternary tectonic movements and sea level changes. Geol. Soc. Am. Bull., 85: 553--570. Chappell, J. and Veeh, H.H., 1978. Late Quaternary tectonic movements and sea-level changes at Timor and Atauro Island. Geol. Soc. Am. Bull., 89: 356--368. Chappell, J., Broecker, W.S., Polach, H.A. and Thorn, B.G., 1974. Problem of dating upper Pleistocene sea levels from coral reef areas. Proc. 2nd Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 563--570. Kaufman, A., Broecker, W.S., Ku, T.L. and Thurber, D.L., 1971. The status of U-series methods of mollusc dating. Geochim. Cosmochim. Acta, 35 : 1155--1183. Konishi, K., Omura, A. and Nakamichi, O., 1974. Radiometric coral ages and sea level records from the late Quaternary reef complexes of the Ryukyu Islands. Proc. 2nd. Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 595--613. Ku, T.L., Kimmel, M.A., Easton, W.H. and O'Neil, T.J., 1974. Eustatic sea level 120,000 years ago on Oahu, Hawaii. Science, 183: 959--961. Labeyrie, J., Lalou, C. and Delibrias, G., 1969. Etude des Transgressions marines sur l'atoll de Mururoa par la datation des diff~rents niveaux de corail. Cah., Pac., 13 : 59--68. Marshall, J.F. and Thorn, B.G., 1976. The sea level in the last interglacial. Nature, 263: 120--121. Rosholt, J.N. and Szabo, B.J., 1969. Determination of protactinium by neutron activation and alpha spectrometry. In: J.R. Devoe (Editor), Modern Trends in Activation Analysis. U.S. Natl. Bur. Standards, Spec. Publ., 312: 1: 327--333. Sakanoue, M., Konishi, K. and Komura, K., 1967. Stepwise determination of thorium-protactinium and uranium isotopes and their application for geochronological studies. In: Radioactive Dating and Methods of Low-Level Counting. Int. Atomic Energy Agency, Vienna, pp. 313--329.
M15 Szabo, B.J., 1969. Uranium-series dating of Quaternary successions. In: M. Ters (Editor), Etudes sur le Quaternaire dans le Monde. 8th Congress INQUA, Paris, pp. 941--949. Szabo, B.J. and Rosholt, J.N., 1969. Uranium-series dating of Pleistocene molluscan shells from southern California -- An open system model. J. Geophys. Res., 74: 3253--3260. Szabo, B.J. and Tracey, J.I., Jr., 1977. Uranium-series and radiocarbon dating of coral samples from Eniwetok Atoll cores reveal hiatus in coral growth between 8,000 and 132,000 years B.P. 10th INQUA Congress, Birmingham, Vol. of Abstracts, p. 456. Szabo, B.J. and Vedder, J.G., 1971. Uranium-series dating of some Pleistocene marine deposits in southern California. Earth Planet. Sci. Lett., 11: 283--290. Teichert, C., 1950. Late Quaternary changes of sea level at Rottnest Island, Western Australia. Proc. R. Soc. Victoria, 59 (part 2): 63--79. Teichert, C., 1967. Age of coastal limestone, Western Australia. Aust. J. Sci., 30: 71. Thomson, J. and Walton, A., 1972. Redetermination of chronology of Aldabra Atoll by 23°Th/234U dating. Nature, 240: 145--146. Thurber, D.L., Broecker, W.C., Blanchard, R.L. and Potratz, H.A., 1965. Uranium-series ages of Pacific atoll corals. Science, 149: 55--58. Tracey, J.I., Jr. and Ladd, H.S., 1974. Quaternary history of Eniwetok and Bikini Atolls, Marshall Islands. Proc. 2nd. Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 537--550. Veeh, H.H., 1966. Th23°/U23a and U234/U23s ages of Pleistocene high sea level stand. J. Geophys. Res., 71: 3379--3386. Veeh, H.H. and Chappell, J., 1970. Astronomical theory of climatic change: Support from New Guinea. Science, 167: 862--865.
Letter Section URANIUM-SERIES AGE OF C O R A L R E E F GROWTH ON R O T T N E S T ISLAND, WESTERN A U S T R A L I A
BARNEY J. SZABO
U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colo. 80225 (U.S.A.) (Received June 2, 1978; accepted August 28, 1978)
ABSTRACT Szabo, B.J., 1979. Uranium-series age of coral reef growth on Rottnest Island, Western Australia. Mar. Geol., 29: M l l - - M 1 5 . Dating samples of corals and shell from the elevated coral reef terrace on Rottnest Island, Western Australia, indicate that in this region away from active plate boundaries the sea stood at least 3 m above present sea level 132,000 -+ 5,000 years ago. There is no geologic evidence of other ancient reef-forming periods on this island. INTRODUCTION
Rottnest Island is situated west of Perth and Fremantle a b o u t 18 km off the coast of Western Australia. The geology of the island is described in detail by Teichert (1950). A fossil coral reef is exposed in Salmon Bay on the south side of the island. The elevation of this exposure is 2--3 m. The uranium-series age of one coral sample from this reef exposure was reported earlier by Veeh (1966) and Teichert (1967) to be 100,000 + 20,000 years. W.S. Moore with C. Teichert revisited the locality in 1967 and resampled thereof corals and mollusks for uranium-series dating; the age determination of these samples is the subject of this report. Accurate dating of this southerly coral reef (lat. 32 ° S) of the Pacific--Indian Ocean region is important because the area is believed to be relatively stable tectonically; therefore, it should provide undisturbed datum for comparison of past fluctuations of sea level. PREVIOUS DATING IN THE PACIFIC--INDIAN OCEAN REGION
The dating of fossil corals from terraces of tectonically rising islands of New Guinea and the R y u k y u is reported by Veeh and Chappel (1970), Bloom et al. (1974), Chappell (1974), Chappell et al. (1974), Sakanoue et al. (1967), Konishi et al. (1974), and Chappell and Veeh (1978). These studies reveal the complex Pleistocene variations of sea level in the Pacific Ocean region. Dating of coral samples from the subsiding atolls of Enewetak, Bikini and Mururoa reveals coral growing periods separated by solution
M12 unconformities (Thurber et al., 1965; Labeyrie et al., 1969; Tracey and Ladd, 1974; and Szabo and Tracey, 1977). The coral growing intervals represent interglacial high sea levels; the unconformities represent periods of emergence and weathering during glacial low sea levels. Fewer studies are reported from presumably tectonically stable areas of the Pacific--Indian Ocean region where the separation of eustatic and tectonic effects is less complicated. Veeh (1966) reported first on dating corals at a number of localities of the Pacific--Indian Ocean region that are now 2--9 m above sea level. The best dated elevated shoreline is the Waimanalo on Oahu, Hawaii that stands a b o u t 7.6 m above present sea level (Ku et al., 1974). The average of 23 age determinations of corals is 122,000 -+ 7,000 years. The elevation of the fossil reef is about 5 m on the apparently stable islands of Aldabra Atoll, Indian Ocean region (Thomson and Walton, 1972); the average age of 8 samples is 129,000 -+ 7,000 years. The average age of 3 samples of coralline deposits at an elevation of a b o u t 5 m on the east coast of Australia is 120,000 + 5,000 years (Marshall and Thom, 1976). All of these investigators conclude that the last interglaciation was an exceptional event in that sea level was above the present level and presumably also above earlier high sea levels during the middle Pleistocene. EXPERIMENTAL PROCEDURE The coral and shell samples chosen for uranium-series dating were cleaned by ultrasonic scrubbing and crushed to a fine powder which was ignited at 600°C for a period of about 2 hours. The concentrations of uranium and thorium were determined by mass spectrometry; the 234U/23sU and 23°Th/234U activity ratios were measured by alpha spectrometry utilizing the methods described by Szabo and Rosholt (1969). The activity ratios of 231pa/23sU were determined by neutron activation and alpha spectrometry (Rosholt and Szabo, 1969). The abundance of aragonite was determined by X-ray diffraction analysis. RESULTS AND DISCUSSION The results of the analyses and the calculated 23°Th and 231pa ages of samples of four corals and one shell are shown in Table I. The percent aragonite is 96 to nearly 100, indicating that no significant recrystallization occurred in these samples through their geologic history. The 23°Th/232Th activity ratios are 80 or larger, indicating no initial 23°Th contamination in these samples. The average 23°Th age of the five samples is 132,000 -+ 5,000 years. Only two samples (C-2 and C-3) yielded finite 231pa ages, but all 231pa ages are concordant with their respective 23°Th ages, within the limits of experimental error. The average 234U/23sU ratio of the five samples is 1.11. Using the average :3°Th age of 132,000 years, the initial 234U/23sU ratio is calculated
TABLE I Analytical data and calculated Ages of several fossil corals and a mollusk f r o m coastal l i m e s t o n e of R o t t n e s t Island, Western Australia Sample
Uranium
Thorium
Aragonite
23°Th'1
2s4U*~
23°Th*~
2S°Th Age .2
231Pa'1
2SlPa Age *s
(ppm)
(ppm)
(%)
2S2Th
2ssU
2s4U
(× l 0 g years)
2ssU
(× l 0 g years)
C-1.4
3.89 ±0.04
0.120 ±0.006
93
80
1.11 ±0.02
0.735 ±0.029
139±12
0.96 ±0.04
118
C-2 .4
2.83 ±0.04
0.10
97
660
1.10 ±0.02
0.705 ±0.028
129±10
0.92 ±0.04
118±
1.12 ±0.02
0.710 ±0.028
130± 9
0.89 ±0.04
103±
32
C-3 .4
3.79
0.020 ±0.004
>97
457
18 21 14
C-4 .4
2.86 ±0.03
0.040 ±0.001
96
171
1.10 ±0.02
0.717 ±0.029
133±10
0.97 ±0.04
>124
S-1 *s
0.642 ±0.006
0.016 ±0.002
93
96
1.12 ±0.02
0.704 ±0.028
128±10
0.95 ±0.05
>108
*1Isotopic activity ratios. *2Calculated using half-lives of 23°Th and 234U o f 75,200 and 244,000 years, respectively. *3Calculated using half-lives o f 231Pa o f 32,500 years. *4Unidentified corals. *SMainly M a r m a r o s t o m a sp., a mollusk.
M14
to be 1.16; the same as the 234U/23sU ratio in present day seawater or 1.15 + 0.02 (Szabo, 1969). Mollusk shells often yield unreliable ages, mainly because these materials do not remain ideal closed system with respect to the isotopes of uranium, thorium, and protactinium (Szabo and Rosholt, 1969; Szabo and Vedder, 1971; Kaufman et al., 1971). In this case, however, the shell sample yielded concordant 23°Th, 231Pa and 234U ages which agree with the coral dates. Results indicate that at a b o u t 132,000 years ago, during which time the coral reef was forming at the Salmon Bay area, the sea stood at least 3 m above the present level at Rottnest Island. There is no evidence of other ancient reef-forming events at Salmon Bay and the reported age agrees well with dates of coral reefs of similar elevation on tectonically stable islands. ACKNOWLEDGEMENTS
I wish to thank W.S. Moore, Dept. of Geology, Univ. of South Carolina, Columbia, S.C. for supplying the samples for this study.
REFERENCES Bloom, A.L., Broecker, W.S., Chappell, J.M.A., Mattbews, R.R. and Mesolella, K.J., 1974. Quaternary sea level fluctuations on a tectonic coast: New 23°Th/~34U dates from Huon Peninsula, New Guinea. Quat. Res., 4: 185--205. Chappell, J., 1974. Geology of coral terraces, Huon Peninsula, New Guinea: A study of Quaternary tectonic movements and sea level changes. Geol. Soc. Am. Bull., 85: 553--570. Chappell, J. and Veeh, H.H., 1978. Late Quaternary tectonic movements and sea-level changes at Timor and Atauro Island. Geol. Soc. Am. Bull., 89: 356--368. Chappell, J., Broecker, W.S., Polach, H.A. and Thorn, B.G., 1974. Problem of dating upper Pleistocene sea levels from coral reef areas. Proc. 2nd Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 563--570. Kaufman, A., Broecker, W.S., Ku, T.L. and Thurber, D.L., 1971. The status of U-series methods of mollusc dating. Geochim. Cosmochim. Acta, 35 : 1155--1183. Konishi, K., Omura, A. and Nakamichi, O., 1974. Radiometric coral ages and sea level records from the late Quaternary reef complexes of the Ryukyu Islands. Proc. 2nd. Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 595--613. Ku, T.L., Kimmel, M.A., Easton, W.H. and O'Neil, T.J., 1974. Eustatic sea level 120,000 years ago on Oahu, Hawaii. Science, 183: 959--961. Labeyrie, J., Lalou, C. and Delibrias, G., 1969. Etude des Transgressions marines sur l'atoll de Mururoa par la datation des diff~rents niveaux de corail. Cah., Pac., 13 : 59--68. Marshall, J.F. and Thorn, B.G., 1976. The sea level in the last interglacial. Nature, 263: 120--121. Rosholt, J.N. and Szabo, B.J., 1969. Determination of protactinium by neutron activation and alpha spectrometry. In: J.R. Devoe (Editor), Modern Trends in Activation Analysis. U.S. Natl. Bur. Standards, Spec. Publ., 312: 1: 327--333. Sakanoue, M., Konishi, K. and Komura, K., 1967. Stepwise determination of thorium-protactinium and uranium isotopes and their application for geochronological studies. In: Radioactive Dating and Methods of Low-Level Counting. Int. Atomic Energy Agency, Vienna, pp. 313--329.
M15 Szabo, B.J., 1969. Uranium-series dating of Quaternary successions. In: M. Ters (Editor), Etudes sur le Quaternaire dans le Monde. 8th Congress INQUA, Paris, pp. 941--949. Szabo, B.J. and Rosholt, J.N., 1969. Uranium-series dating of Pleistocene molluscan shells from southern California -- An open system model. J. Geophys. Res., 74: 3253--3260. Szabo, B.J. and Tracey, J.I., Jr., 1977. Uranium-series and radiocarbon dating of coral samples from Eniwetok Atoll cores reveal hiatus in coral growth between 8,000 and 132,000 years B.P. 10th INQUA Congress, Birmingham, Vol. of Abstracts, p. 456. Szabo, B.J. and Vedder, J.G., 1971. Uranium-series dating of some Pleistocene marine deposits in southern California. Earth Planet. Sci. Lett., 11: 283--290. Teichert, C., 1950. Late Quaternary changes of sea level at Rottnest Island, Western Australia. Proc. R. Soc. Victoria, 59 (part 2): 63--79. Teichert, C., 1967. Age of coastal limestone, Western Australia. Aust. J. Sci., 30: 71. Thomson, J. and Walton, A., 1972. Redetermination of chronology of Aldabra Atoll by 23°Th/234U dating. Nature, 240: 145--146. Thurber, D.L., Broecker, W.C., Blanchard, R.L. and Potratz, H.A., 1965. Uranium-series ages of Pacific atoll corals. Science, 149: 55--58. Tracey, J.I., Jr. and Ladd, H.S., 1974. Quaternary history of Eniwetok and Bikini Atolls, Marshall Islands. Proc. 2nd. Int. Symposium on Coral Reefs, Great Barrier Reef Committee, Brisbane, Australia, pp. 537--550. Veeh, H.H., 1966. Th23°/U23a and U234/U23s ages of Pleistocene high sea level stand. J. Geophys. Res., 71: 3379--3386. Veeh, H.H. and Chappell, J., 1970. Astronomical theory of climatic change: Support from New Guinea. Science, 167: 862--865.