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Temperature variations of the “Kuroshio” and crustal movements in Eastern and Southeastern Asia 7,000 years B.P
Palaeogeography, Palaeoclimatology, Palaeoecology, 20(1976) 257--259 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Discussion T E M P E R A T U R E V A R I A T I O N S OF T H E " K U R O S H I O " AND CRUSTAL MOVEMENTS IN E A S T E R N AND S O U T H E A S T E R N ASIA 7,000 YEARS B.P. 1
F. C. TAN Bedford Institute of Oceanography, Dartmouth, N.S. (Canada) (Received January 29, 1976) COMMENT A recent paper by Taira (1975) on " T e m p e r a t u r e Variations of the 'Kuroshio' and Crustal Movements in Eastern and Southeastern Asia, 7,000 Years B.P." relates the rise in isotopic t e m p e r a t u r e of the Kuroshio around 7,000 B.P. to crustal m ove m ent s in eastern Asia on the basis of three isotopic temperatures c o m b i n e d with ot her related evidence. While it is quite conceivable that the t e m p e r a t u r e rise of the Kuroshio could have arisen as a result of a "change of its axis", a few remarks regarding the justification for this hypothesis would appear to be in order, in particular, I wish to c o m m e n t on the nature and suitability of the various samples used to obtain the isotopic temperatures. (1) In performing the isotopic analyses, Taira analyzed the outer layers of shell samples and stated t ha t " t h e calcium carbonate occurred mostly as crystalline aragonite". It should be poi nt ed out t hat although the preservation of the aragonitic structure is usually taken as a good indication of the preservation o f the original isotopic composition, it cannot be regarded as a positive p r o o f (Spaeth et al. 1971). Quantitative information on the calcite c o n t e n t of the shells would have been m or e revealing and satisfactory. In an earlier paper, Keith et al. (1964) have shown that the 5 lsO values of the interior and exterior layers of recent marine pelecypod shells can differ by as much as 0.87%0 corresponding to an isotopic t em perat ure difference of 4.0°C. Th ey also observed that the 51So values could not be correlated with the aragonite or calcite c o n t e n t of shell carbonate. In view of these findings, the question arises as to which layer would provide the true isotopic temperature record of ancient seawater. A f ur t he r complication arises as a result of the observation of Tarutani et al. (1969) who showed that at 25°C the aragonite--water fractionation is 0.6%° greater in 5180 than the calcite-water fractionation. l Contribution of the Bedford Institute of Oceanography. 257
258 (2) Taira has compared his paleotemperatures with the average temperatures of seawater observed in recent times. Such a comparison is open to discussion for the following two reasons: (a) Work by Epstein and Lowenstam (1954) indicates that pelecypods lay down their shell carbonates only during the warmer part of the year while coexisting gastropods appear to deposit their shell carbonate throughout the yearly temperature range. The average difference in growth temperatures is a b o u t 2°C. (b) Keith et al. (1964) observed a constant enrichment by 0.3 to 0.5 %,, in the 180 content of recent marine gastropods in comparison with coexisting pelecypods. It appears possible, therefore, that the higher temperatures (4--9°C) observed by Taira could in part be a reflection of growth pattern of the pelecypods. For example, Taira compared an isotopic temperature of 29°C for Kodakia with the present average yearly seawater temperature of 25°C for the Hengchun Peninsula in Taiwan. This observed 4°C difference might be a result of the deposition of shell carbonate during the high temperature period of the year and not a true indication of the temperature difference in Kuroshio between 7,000 B.P. and present. In view of the varying shell temperature characteristics exhibited by different species and classes of fossils, one must be very cautious in arriving at conclusions based on limited observations from the same class of fossils. An accurate assessment of the actual temperatures existing in the ancient sea would require an examination of many types of fossils in a fossil assemblage. • (3) Although the three pelecypod genera analyzed by Taira were taken from marine terraces and were associated with corals, there remains a possibility that the seawater in which these three genera lived could have been contaminated with low ~SO meteoric water. For example, Horibe and Ogura (1968), in their H/D study of the Kuroshio, have demonstrated the influence of meteoric water on ocean water down to a depth of 100 m. The effect of meteoric water contamination in this instance would result in erroneously high isotopic temperatures being recorded by the fossil shells. In conclusion, it is probably fair to state that the high isotopic temperatures observed by Taira may not be due to the rise in seawater temperature of the Kuroshio in the climatic optimum but may well be explained by the nature, shell characteristics and the environmental condition of the samples. Oxygen isotope measurements are a useful approach in deciphering the paleooceanography of the Kuroshio region if great care is exercised in examining the problem. ACKNOWLEDGMENT I wish to thank Drs. A. Walton and G. Vilks of the Bedford Institute of Oceanography for commenting on the manuscript.
259 REFERENCES Epstein, S. and Lowenstam, H., 1953. Temperature--shell growth relations of recent and interglacial Pleistocene shoal water biota from Bermuda. J. Geol., 61 : 424--438. Horibe, Y. and Ogura, N., 1968. Deuterium content as a parameter for water mass in the ocean. J. Geophys. Res., 73: 1239--1249. Keith, M. L., Anderson, G. M. and Eichler, R., 1964. Carbon and oxygen isotopic composition of mollusk shells from marine and freshwater environments. Geochim. Cosmochim. Acta, 28: 1757--1786. Spaeth, Chr., Hoefs, J. and Vetter, U., 1971. Some aspects of isotopic composition of belemnites and related paleotemperatures. Geol. Soc. Am. Bull., 82: 3139--3150. Taira, K., 1975. Temperature variation of the "Kuroshio" and crustal movements in eastern and southeastern Asia 7,000 years B.P. Palaeogeogr., Palaeoclimatol., Palaeoecol., 17: 333--338. Tarutani, T., Clayton, R. N. and Mayeda, T., 1969. The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta, 33: 987--996.
REPLY FROM K. TAIRA I a p p r e c i a t e t h e a b o v e c o m m e n t s o n m y p a p e r a n d as t h e y s e e m well f o u n d e d , I shall r e - e x a m i n e m y o x y g e n i s o t o p e d a t a .
Discussion T E M P E R A T U R E V A R I A T I O N S OF T H E " K U R O S H I O " AND CRUSTAL MOVEMENTS IN E A S T E R N AND S O U T H E A S T E R N ASIA 7,000 YEARS B.P. 1
F. C. TAN Bedford Institute of Oceanography, Dartmouth, N.S. (Canada) (Received January 29, 1976) COMMENT A recent paper by Taira (1975) on " T e m p e r a t u r e Variations of the 'Kuroshio' and Crustal Movements in Eastern and Southeastern Asia, 7,000 Years B.P." relates the rise in isotopic t e m p e r a t u r e of the Kuroshio around 7,000 B.P. to crustal m ove m ent s in eastern Asia on the basis of three isotopic temperatures c o m b i n e d with ot her related evidence. While it is quite conceivable that the t e m p e r a t u r e rise of the Kuroshio could have arisen as a result of a "change of its axis", a few remarks regarding the justification for this hypothesis would appear to be in order, in particular, I wish to c o m m e n t on the nature and suitability of the various samples used to obtain the isotopic temperatures. (1) In performing the isotopic analyses, Taira analyzed the outer layers of shell samples and stated t ha t " t h e calcium carbonate occurred mostly as crystalline aragonite". It should be poi nt ed out t hat although the preservation of the aragonitic structure is usually taken as a good indication of the preservation o f the original isotopic composition, it cannot be regarded as a positive p r o o f (Spaeth et al. 1971). Quantitative information on the calcite c o n t e n t of the shells would have been m or e revealing and satisfactory. In an earlier paper, Keith et al. (1964) have shown that the 5 lsO values of the interior and exterior layers of recent marine pelecypod shells can differ by as much as 0.87%0 corresponding to an isotopic t em perat ure difference of 4.0°C. Th ey also observed that the 51So values could not be correlated with the aragonite or calcite c o n t e n t of shell carbonate. In view of these findings, the question arises as to which layer would provide the true isotopic temperature record of ancient seawater. A f ur t he r complication arises as a result of the observation of Tarutani et al. (1969) who showed that at 25°C the aragonite--water fractionation is 0.6%° greater in 5180 than the calcite-water fractionation. l Contribution of the Bedford Institute of Oceanography. 257
258 (2) Taira has compared his paleotemperatures with the average temperatures of seawater observed in recent times. Such a comparison is open to discussion for the following two reasons: (a) Work by Epstein and Lowenstam (1954) indicates that pelecypods lay down their shell carbonates only during the warmer part of the year while coexisting gastropods appear to deposit their shell carbonate throughout the yearly temperature range. The average difference in growth temperatures is a b o u t 2°C. (b) Keith et al. (1964) observed a constant enrichment by 0.3 to 0.5 %,, in the 180 content of recent marine gastropods in comparison with coexisting pelecypods. It appears possible, therefore, that the higher temperatures (4--9°C) observed by Taira could in part be a reflection of growth pattern of the pelecypods. For example, Taira compared an isotopic temperature of 29°C for Kodakia with the present average yearly seawater temperature of 25°C for the Hengchun Peninsula in Taiwan. This observed 4°C difference might be a result of the deposition of shell carbonate during the high temperature period of the year and not a true indication of the temperature difference in Kuroshio between 7,000 B.P. and present. In view of the varying shell temperature characteristics exhibited by different species and classes of fossils, one must be very cautious in arriving at conclusions based on limited observations from the same class of fossils. An accurate assessment of the actual temperatures existing in the ancient sea would require an examination of many types of fossils in a fossil assemblage. • (3) Although the three pelecypod genera analyzed by Taira were taken from marine terraces and were associated with corals, there remains a possibility that the seawater in which these three genera lived could have been contaminated with low ~SO meteoric water. For example, Horibe and Ogura (1968), in their H/D study of the Kuroshio, have demonstrated the influence of meteoric water on ocean water down to a depth of 100 m. The effect of meteoric water contamination in this instance would result in erroneously high isotopic temperatures being recorded by the fossil shells. In conclusion, it is probably fair to state that the high isotopic temperatures observed by Taira may not be due to the rise in seawater temperature of the Kuroshio in the climatic optimum but may well be explained by the nature, shell characteristics and the environmental condition of the samples. Oxygen isotope measurements are a useful approach in deciphering the paleooceanography of the Kuroshio region if great care is exercised in examining the problem. ACKNOWLEDGMENT I wish to thank Drs. A. Walton and G. Vilks of the Bedford Institute of Oceanography for commenting on the manuscript.
259 REFERENCES Epstein, S. and Lowenstam, H., 1953. Temperature--shell growth relations of recent and interglacial Pleistocene shoal water biota from Bermuda. J. Geol., 61 : 424--438. Horibe, Y. and Ogura, N., 1968. Deuterium content as a parameter for water mass in the ocean. J. Geophys. Res., 73: 1239--1249. Keith, M. L., Anderson, G. M. and Eichler, R., 1964. Carbon and oxygen isotopic composition of mollusk shells from marine and freshwater environments. Geochim. Cosmochim. Acta, 28: 1757--1786. Spaeth, Chr., Hoefs, J. and Vetter, U., 1971. Some aspects of isotopic composition of belemnites and related paleotemperatures. Geol. Soc. Am. Bull., 82: 3139--3150. Taira, K., 1975. Temperature variation of the "Kuroshio" and crustal movements in eastern and southeastern Asia 7,000 years B.P. Palaeogeogr., Palaeoclimatol., Palaeoecol., 17: 333--338. Tarutani, T., Clayton, R. N. and Mayeda, T., 1969. The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta, 33: 987--996.
REPLY FROM K. TAIRA I a p p r e c i a t e t h e a b o v e c o m m e n t s o n m y p a p e r a n d as t h e y s e e m well f o u n d e d , I shall r e - e x a m i n e m y o x y g e n i s o t o p e d a t a .