Application of the Crystallinity Ratio of Free Iron Oxides for Dating Soils

第四紀研究(The Quaternary Research)41(16)p.485-493 Dec. 2002

Application of the Crystallinity Ratio of Free Iron Oxides for Dating

Soils Developed on the Raised Coral Reef Terraces of Kikai

and Minami-Daito Islands, Southwest Japan

Yuji Maejima*1, Shizuo Nagatsuka*2 and Teruo Higashi*3

Absolute ages of soils developed on the raised coral reef terraces in Kikai Island in the Ryukyus, Southwest Japan were estimated by using the mean rate of tectonic uplift and glacio-eustatic curve during late Quaternary in the previous paper. In order to estimate the soil age of other areas where the similar soils as those existed in Kikai Island were formed by using the index of soil age, which was highly correlated with some physico-chemical properties of soil itself, total iron (Fet), iron and aluminum extractable by dithionite-citrate (Fed, Ald) and by acid ammonium oxalate (Feo, Alo) were determined for all horizons of the six profiles in Kikai Island. The results obtained are as follows: The crystallinity ratio of free iron oxides [(Fed-Feo)/Fet] gradually increased with the stage of soil development, while the activity ratio (Feo/Fed) decreased. There was a highly positive correlation between the soil age and (Fed-Feo)/Fet. By using this relationship, the ages of Minami-Daito Island soils, Lateritic Red soil and Lateritic Yellow soil whose age had not been determined, were estimated from the mean values of (Fed-Feo)/Fet, as 500±60 ka and 630±110ka, respectively. Therefore, it was concluded that the crystallinity ratio of free iron oxides could be a good index of the degree of soil development and age of Red-colored soils.

Key Words: Iron oxides, crystallinity ratio, Kikai Island, Soil age, Red-colored soil

stone Lithosols (ca. 3.0 ka), Initial Rendzina- I. Introduction like soils (3.5-3.9 ka), Rendzina-like soils (35- In the previous paper (Nagatsuka and Nlae- 40 ka), Brown Rendzina-like soils (50-55 ka), jima, 2001), the authors studied the absolute Terra f usca-like soils (70-80 ka), Terra rossa- ages of soils developed on the raised coral reef like soils (95 -100 ka), and finally Intergrade terraces of Kikai Island in the Ryukyus, South- between Terra rossa-like soils and Red-Yellow west Japan by using a combined method of the soils (120-125 ka). The years in the paren- mean rate of tectonic uplift and the glacio- theses indicate the absolute ages of the soils. eustatic curve during late Quaternary. The Consequently, it was concluded that ca. 125 ka soils on the raised coral reef terraces of Kikai would be needed for Red-Yellow soils (Acrisols, Island showed a series of chronosequence in Hapludults) to develop on coral limestone their development in the order of Coral Lime- under the humid subtropical rain forest cli-

Received February 13, 2002. Accepted July 27, 2002. *1 Research Fellow of the Japan Society for the Promotion of Science . Institute of Applied Biochemistry, Uni- versity of Tsukuba. 1-1-1 Tennoudai, Tsukuba, 305-0006, Japan. (Present address: Research Center of Nuclear Science and Technology, The University of Tokyo. 2-11-16 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan .) E-mail: *2 Japan Soil Research Institute Inc . Hibarigaoka, Takano Bd. 7F, 2-5-11 Yato-cho, Nishi-Tokyo, 188-0001, Japan. *3 Institute of Applied Biochemistry , University of Tsukuba. 1-1-1 Tennoudai, Tsukuba, 305-0006, Japan. 486 Y. Maejima, S. Nagatsuka and T. Higashi Dec. 2002 mate condition. sa-like soil and Red-Yellow soil (Haplic Lixisol, However, the mean rate of tectonic uplift is Typic Hapludalf) developed on the raised coral not applicable for the area where the actual reef terraces in Kikai Island. Furthermore, soil uplift process in the past is not understood. So, samples from two soil profiles consisting of as an alternative, we tried to estimate the soil Lateritic Red soil (Typic Rhodudalf, Haplic age of other areas where the similar soils as Lixisol) and Lateritic Yellow soil (Typic Kandi- those existed in Kikai Island are formed by udult, Haplic Acrisol) developed on the raised using the index of soil age, which is highly coral reef terraces in Minami-Daito Island were correlated with some physico-chemical prop- also used for comparison. The profile descrip- erties of soil itself. tions, general physico-chemical and mineralog- In the present paper, the authors examined ical properties of these soil samples have al- the applicability of the crystallinity ratio of ready been reported in the previous paper free iron oxides of soils for the estimation of (Maejima et al., 1997 a, b; Nagatsuka and Mae- soil age, and tried to estimate the absolute age jima, 2001). of the soils derived from coral limestone in 2. Iron and aluminum oxides and total iron Minami-Daito Island under the comparison Analyses were performed on the fine earth with other methodology. fractions (≦2mm) and results are expressed on an oven dry basis (% R2O3 per dry soil). II. Materials and methods The contents of total iron (Fet), iron and alu- 1. Soil samples minum extractable by dithionite-citrate (Fed, The soil-sampling sites are shown in Figure Ald) and by acid ammonium oxalate (Feo, Alo) 1. Soil samples were used from six soil profiles were determined for all horizons of the six consisting of Initial Rendzina-like soil (Rendzic profiles. Fet was determined by the method Leptosol according to WRB (FAQ, ISRIC and described by Nagatsuka (1994). Fed, Feo, Aid ISSS, 1998), Lithic Rendoll according to Soil and Alo were determined by the method of Taxonomy (Soil Survey Staff, 1994)), Rendzina- Blakemore et al. (1987). like soil (Rendzic Leptosol, Lithic Rendoll) III. Results and discussion Brown Rendzina-like soil (Mollic Leptosol, Eu- tropeptic Rendoll), Terra fusca-like soil (Haplic 1. Distribution of iron and aluminum Luvisol, Typic Hapludalf), Terra rossa-like soil oxides in soil profile (Chromic Luvisol, Typic Hapludalf), Terra ros- The contents of total iron (Fet), dithionite- citrate-extractable iron (Fed) and aluminum (Ald), and oxalate-extractable iron (Feo) and aluminum (Alo) are shown in Table 1. The contents of Fed and Fet increased with the degree of soil development and soil age. The distribution pattern of Fed in each profile, except the case of Initial Rendzina-like soil, showed the increase from the surface horizon to the lower horizon, and almost constant at the depths below second horizon. The distribution pattern of Fet was similar to that of Fed. On the other hand, Feo was very low (0.3- 1.3% Fe2O3) in each soil profile, and it was almost constant throughout the profile. There was no appreciable difference among soils and horizons studied. The contents of Ald gradually increased with

Fig. 1 Schematic location map of the soil-sampling the degree of soil development and soil age. sites The contents of Alo as well as Feo indicated no 2002年12月 Estimation of Soil Age by the Crystallinity Ratio 487

Table 1 Iron, aluminum and clay contents of the soils in Kikai Island*

*Oven dry basis . **cf Nagatsuka and Maejima (2001) ***The terrace No . of Kikai Island, accroding to Konishi et al. (1974) and Ota et al. (1978)

appreciable difference among all soils and horizons. Figure 2 shows the nearly constant Fed/clay and Ald/clay ratios (r=0.93** and r=0.94**, respectively, **indicates significant at 1% level). This indicates the presence of clay mi- gration (Schlichting and Blume, 1962) or co- illuviation of iron and aluminum oxides and clay minerals (Zainol, 1985). Since clay migra- tion and accumulation are very important proc- esses for soil development in longer pedogenetic time (>50 ka as Brown Rendzina-like soil in this study), it is very rational to incorpo- rate Fed and Ald for the estimation of soil age. 2. Activity and crystallinity ratios of free Fig. 2 Relationship between contents of clay and iron oxides dithionite-citrate-extractable iron or alumi- Figure 3 shows the values of activity ratio of num in Kikai Island soils free iron oxides (Feo/Fed) (Blume and Schwert- ** Significant at 1% level . 488 Y. Maejima, S. Nagatsuka and T. Higashi Dec. 2002

Fig. 3 Activity and crystallinity ratios of free iron oxides of the soils in Kikai Island R: Red soil, YB: Yellow-Brown Forest soil

mann, 1969) and the crystallinity ratio [(Fed- tocene to Upper Pleistocene-Holocene. In addi- Feo)/Fet] (Nagatsuka, 1972) of the studied soils. tion, Arduino et al. (1986) suggested that the In Figure 3, the values of these ratios obtained ratios Fed/Fet and (Fed - Feo)/Fet were closely from Minami-Daito Island soils derived from related to the ages of the terraces in northern coral limestone are also shown for comparison. Italy. Bech et al. (1997) also obtained similar In Kikai Island, activity ratio of free iron results that the crystallinity ratio [(Fed-Feo)/ oxides ranged from 0.09 to 0.36, and the crys- Fe] of Red Mediterranean soils was very close- tallinity ratio from 0.28 to 0.64. (Fed-Feo)/Fet ly related to the degree of weathering and/or gradually increases from the youngest to the age. Therefore, it is expected that the crystal- oldest soils, and Feo/Fed decreases in the same linity ratio of free iron oxides will be a good order (Fig. 3 and Table 1). Urushibara-Yoshino index of ages of Red-colored soils in Southwest (1988, 1992) also showed the increase of (Fed- Japan. Feo)/Fet in the B-horizon of the soils in Kikai In general, the crystallinity ratios of free iron Island associated with the decrease of Feo/Fed oxides of Red-Yellow soils of Southwest Japan that was in accordance with the estimated age range from 0.5 to 0.7 (Nagatsuka and Urushi- of the raised coral reef terraces. McFadden and bara-Yoshino, 1988). The (Fed-Feo)/Fet values Hendricks (1985) showed that the activity ratio of profile No. 6 in the present study range from (Fea/Fed) in the chronosequence of southern 0.54 to 0.64. This fact agrees with our view California was moderately high to very high that the profile No. 6 is an intergraded soil type (0.22-0.58) in middle Holocene to latest Pleis- that develops from Terra rossa-like soil to Red- tocene soils and progressively decreased to less Yellow soil. than 0.10 in older soils. In Kikai Island, Feo/Fea According to Nagatsuka et al. (1983), the ac- of Initial Rendzina-like soil developed on the tivity and crystallinity ratios of the soils Holocene terrace was also moderately high derived from coral limestone in Ishigaki and (0.32-0.36) and those of other soils developed Okinawa Islands ranged from 0.05 to 0.14, and on the Pleistocene terraces were less than 0.20 from 0.53 to 0.75, respectively. The activity except A-horizon of Terra rossa-like soil. and crystallinity ratios of these soils were all Arduino et al. (1984) showed that (Fed-Feo)/Fe, distributed within the range of Red soil-region of the soils in the western Po Valley of north- [Feo/Fed≦0.4,(Fed-Feo)/Fet≧0.5](Nagatsuka, ern Italy increased gradually with ages of the 1972). These soils have the morphological and terraces, which range from early Middle Pleis- physico-chemical properties similar to those of 2002年12月 Estimation of Soil Age by the Crystallinity Ratio 489

Rendzina, Terra fusca and Terra rossa, respec- the soils of Minami-Daito Island (Red soil and tively, but the nature of free iron oxides is Yellow soil) also derived from coral limestone similar to that of Red soils. For this reason, ranged from 0.77 to 0.81, and were similar to these soils were designated tentatively as the Lateritic Red and Yellow soils which Rendzina-like, Terra fusca-like and Terra widely distributed in subtropical South China. rossa-like soils, respectively. In Kikai Island, The soil forming age as well as the develop- the activity and crystallinity ratios of soils ment of these soils is considered to be in more were also distributed within the range of Red advanced stage than the Red-Yellow soils soil-region except Initial Rendzina-like and (Udults) in Southwest Japan (Maejima et al., Rendzina-like soils. It is probable that the high 1997 a). However, the estimated age of these content of organic carbon (84-129, 21-32g soils from Minami-Daito Island is not eluci- kg-1, respectively) in Initial Rendzina-like and dated. Rendzina-like soils, of which activity ratios 3. Estimation of soil age of Minami-Daito were relatively high and crystallinity ratios Island were low, prevented the crystallization of iron. The time-dependent trend of Feo, Fed, Fe, On the other hand, the crystallinity ratios of Feo/Fed, (Fed-Feo)/Fet, Alo, and Ald are shown

Fig. 4 Relationship between soil age and several soil properties in Kikai Island soils a) Oxalate-extractable (Feo), dithionite-citrate-extractable (Fed), and total iron (Fe) b) Activity ratio (Feo/Fed) and crystallinity ratios [(Fed-Feo)/Fet] c) Oxalate-extractable (Alo) and dithionite-citrate-extractable aluminum (Ald) 490 Y. Maejima, S. Nagatsuka and T. Higashi Dec. 2002

in Figure 4. There was no correlation between mula is as follows: Feo (Fig. 4-a), Alo (Fig. 4-c) and the absolute WIc=1.89 [(Fed-Feo)/Fet] soil age. However, it was clear that Fed and Fet -0 .02 CEC of clay (Fig. 4-a), (Fed-Feo)/Fet (Fig. 4-b), and Ald (Fig. -4 .79 (Feo/Fed) 4-c) were positively related to the absolute -0 ages of Kikai Island soils. While, there was a .14 sum of bases-0.33 negative correlation between Feo/Fed and soil When WIc was calculated from the data of ages (Fig. 4-b). Among these correlations, physico-chemical properties of the soils in there was the highest and positive correlation Kikai Island (Nagatsuka and Maejima, 2001) between the soil age and (Fed-Feo)/Fet (y= and Minami-Daito Island (Maejima et al., 1997 0.0687x0.1857,r=0.91). By using this relation- a) by using this formula, we obtained the re- ship, the ages of Lateritic Red soil and Lateritic sults as shown in Figure 5. WIc of Minami- Yellow soil in Minami-Daito Island were estimated as 500±60ka and 630±110ka, respectively, by the extrapolation of this correlation. The years indicate mean±standard deviation (n=5 and 6, respectively). Makino (1983) reported the age of surface corals in Minami-

0 Daito Island measured by the use of electron spin resonance method as 550ka to 1,200ka, and ca. 250ka for the oldest and the younger ridges, respectively. From this fact, it is also suggested that the crystallinity ratio of free iron oxides might be a good index of the degree of soil development and soil age on the raised coral reef terraces. On the other hand, Araki and Kyuma (1985) proposed lithology index (LI) and weathering index (WI) based on chemical and mechanical Fig. 5 Relationship between weathering index compositions and quartz content of red and/or based on chemical properties (WIc) and yellow colored soil materials in the southwest- CEC of clay fraction ern part of Japan, using a multivariate statisti- WIc was calculated from the equation cal method. Araki (1993) mentioned that al- proposed by Araki (1993). though WI and crystallinity ratio of free iron oxides are efficient to evaluate the degree of weathering for the restricted studied area under similar weathering conditions, they are not used universally. So, Araki (1993) extracted two independent factors that have no correlation with each other, based on the factor analysis using the five variables consisting of activity and crystallinity ratios of free iron oxides, CEC of clay fraction, sum of exchangeable bases and clay contents. The primary factor was related with the degree of weathering and the second one with clay contents. He Fig. 6 Relationship between soil age and weather- designated the former as index of weathering ing index based on chemical properties degree (WIc) and deduced on the basis of multi- (WIc) in Kikai Island soils ple regression analysis a formula for calculat- WIc was calculated from the equation ing WIc from the original variables. The for- proposed by Araki (1993). 2002年12月 Estimation of Soil Age by the Crystallinity Ratio 491

Daito Island soils are apparently higher than while the activity ratios (Feo/Fed) decreased. those of Kikai Island soils. It is suggested that 2. There was a highly positive correlation Minami-Daito Island soils are in the state of between the absolute ages of Kikai Island soils much more advanced development stage than (Nagatsuka and Maejima, 2001) and crystal- Kikai Island soils. linity ratios of free iron oxides. Therefore, it is As shown in Figure 6, there is a high and concluded that the crystallinity ratio of free positive correlation between the absolute ages iron oxides could be a good index of the ages of of Kikai Island soils and W Ic (y=-33.92+6.01 Red-colored soils. log x, r=0.84). The ages of Lateritic Red soil 3. The ages of Lateritic Red soils and Lateri- and Lateritic Yellow soil of Minami-Daito tic Yellow soils of Minami-Daito Island were Island estimated by the extrapolation of this estimated by the extrapolation of the correla- correlation were estimated as 250±130ka and tion shown in Figure 4-b, and are 500±60ka

450±140ka, respectively. The difference in the and 630±110ka, respectively. estimated ages between by WIc and by crys- Acknowledgements tallinity ratios alone, particularly in Lateritic Red soils, may be due to the difference in the We thank Dr. A. Tomikawa of Tsukuba Uni- nature of the soil parent materials, since non- versity for his advise in the iron analysis and calcareous soil samples were used for WIc. for his helpful suggestions. This study was Judging from the correlation coefficients, 0.91 financially supported by the Grants-in-Aid for (crystallinity ratios in Fig. 4-b) and 0.84 (WIc Scientific Research, No. 04660067 from the Min- by Araki (1993) in Fig. 6), the estimated ages of istry of Education, Science and Culture of Lateritic Red soil and Lateritic Yellow soil of Japan. Minami-Daito Island would be more plausible, References being 500±60ka and 630±110ka. respectivel which are obtained from by the regression Araki, S. (1993) Comparative study of Kunigami curve of the crystallinity ratios. Mahji and Feichisha soils in Okinawa Islands- Among Red-Yellow soils of the world. Pedologist, As mentioned above, the soil samples used in 37, 113-125. (J) this study were calcareous soils, and the appli- Araki, S. and Kyuma, K. (1985) Characterization of cation of the crystallinity ratio of free iron red and/or yellow colored soil materials in the oxides to other soils derived from other parent southwestern part of Japan in terms of lithology materials is not known. However, Arduino et and degree of weathering. Soil Sci. Plant Nutr., 31, 403-410. al. (1986) also reported that the crystallinity Arduino, E., Barberis, E., Carraro, F. and Forno, M.G. ratio of free iron oxides was closely related to (1984) Estimating relative ages from iron-oxides/ the ages of fluvial terraces in northern Italy. total-iron ratios of soils in the western Po Valley, Moreover, according to Araki (1993), there was Italy. Geoderma, 33, 39-52. a high correlation between the crystallinity Arduino, E., Barberis, E., Ajmone Marsan, F., Zanini, ratio of free iron oxides and the weathering E. and Franchini, M. (1986) Iron-oxides and clay minerals within profiles as indicators of soil age in index (WI) by Araki and Kyuma (1985), it may northern Italy. Geoderma, 37, 45-55. be also possible to evaluate the soil age from Bech, J., Rustullet, J., Garrigo, J., Tobias, F. J. and WI for the restricted studied area under similar Martinez, R. (1997) The iron contents of some red weathering conditions. Mediterranean soils from northeast Spain and its pedogenetic significance. Catena, 28, 211-229. IV. Conclusions Blakemore, L. C., Searle, P. L. and Daly, BK. (1987) Extractable iron, aluminum and silicon. Blake- Following conclusions can be deduced about more, L. C. et al. (eds.) NZ Soil Bureau Scientific the soil age and the characteristics of status of Report 80 Methods for chemical analysis of soils: 71- free iron oxides of soils from Kikai and 76, DISR, Lower Hutt. Minami-Daito Islands: Blume, H. P. and Schwertmann, U. (1969) Genetic eval- 1. In Kikai Island, the crystallinity ratios of uation of profile distribution of aluminum, iron, and manganese oxides. Soil Sci. Soc. Am. Proc., 33, free iron oxides [(Fed-Feo)/Fet] gradually in- 438-444. creased with the stage of soil development, 492 Y. Maejima, S. Nagatsuka and T. Higashi Dec. 2002

FAO, ISRIC and ISSS (1998) World reference base for Nagatsuka, S. and Urushibara-Yoshino, K. (1988) On soil resources, world soil resources reports 84. 88 p, the vertical zonality of soil distribution and soil FAO Rome. conditions in Xishuangbanna, South Yunnan. Cli- Konishi, K., Omura, A. and Nakamichi, O. (1974) matological Notes, 38, 229-247. Radiometric coral ages and sea level records from Nagatsuka, S. and Maejima, Y. (2001) Dating of soils the late Quaternary reef complexes of the Ryukyu on the raised coral reef terraces of Kikai Island in Islands. Proc. 2nd Intl. Coral Reef Symp., 2, Great the Ryukyus, Southwest Japan: With special ref- Barrier Reef Comm., 595-613, Brisbane. erence to the age of Red-Yellow soils. The Quat. Maejima, Y., Nagatsuka, S. and Higashi, T. (1997 a) Res. (Daiyonki-Kenkyu), 40, 137-147. Classification of soils on the raised coral reef ter- QOta,Y., Machida, H., Hori, N., Konishi, K. and Omura, races of Minami-Daito Island. I. Occurrence of K. (1978) Holocene raised coral reefs of Kikai-jima- Haplic Lixisol or Typic Rhodudalf in Japan. Pedo- An approach to Holocene sea level study-. Geogr. logist, 41, 2-14. (J+E) Rev. Japan (Chirigaku-Hyoron), 51, 109-130. (J+E) Maejima, Y., Nagatsuka, S. and Higashi, T. (1997 b) Schlichting, E. and Blume, H. P. (1962) Art and Au- Classification of soils on the raised coral reef ter- smab der Veranderungen des Bestandes mobiler races of Minami-Daito Island. II. Clay Mineral Com- Oxide in Boden aus jungpleistozanem Geschiebem- position. Pedologist, 41, 15-22. (J+E) ergel and ihren Horizonten, Z. Pflanzenernahr, Makino, K. (1983) Marine terraces and age of the Dung. Bodenk., 96, 144-156. formation in Kitadaito and Minamidaito Islands. Soil Survey Staff (1994) Keys to soil taxonomy, sixth Ann. Meeting of the Association of Japanese Geogra- edition. 306 p, U. S. D.A. Soil Conservation Service. phers, 24, 74-75. (J) Urushibara-Yoshino, K. (1988) The red soils on a lime- McFadden, L. D. and Hendricks, D. M. (1985) Changes in stone area in Nansei-island, Southwest Japan. the content and composition of pedogenic iron Gillieson, D. and Smith, D.I. (eds.) "Resource manage- oxyhydroxides in a choronosequence of soils in ment in limestone landscapes" Special Publication, southern California. Quat. Res., 23,189-204. No. 2: 183-189, The Australian Defence Force Nagatsuka, S. (1972) Studies on genesis and classifica- Academy, Australia. tion of soils in warm-temperate region of South- Urushibara-Yoshino, K. (1992) The red soils on a lime- west Japan, part 3. Some features in distribution stone area in Kikai island, of Nansei-islands, South- and mode of existence of free iron and aluminum west Japan. Chardon, M., Sweeting, M. and Pfeffer, oxides in the soil profile. Soil Sci. Plant Nutr., 18, K. H. (eds.) Proceedings of the Karst-Symposium-Bl- 147-154. aubeuren: 71-81, Tubiringer Geographische Stud- Nagatsuka, S. (1994) Selective dissolution methods of ien H109. iron. Japan Association for Quarternary Research Zainol, E. (1985) Pedogenetic forms of extractable (ed.) A handbook of Quaternary Research, vol. 2: 22- iron in selected soils of Kedah, Malaysia. J. Rubb. 32, Univ. of Tokyo Press. (J) Res. Inst. Malaysia, 33, 115-123. Nagatsuka, S., Kaneko, S. and Ishihara, A. (1983) Soil genesis on the raised coral reef terraces of Ishigaki (J) in Japanese, (J+E) in Japanese with English and Okinawa-islands in the Ryukyu Islands, Japan abstract. part 1. Soil Sci. Plant Nutr., 29, 343-354. 2002年12月 Estimation of Soil Age by the Crystallinity Ratio 493

喜界島および南大東島の離水サンゴ礁段丘上に発達した土壌の遊離酸化鉄の結晶化指数による年代推定

前島勇治*1・永塚鎮男*2・東照雄*3

〔要旨〕

前報(Nagatsuka and Maejima,2001)では,喜界島シュウ酸塩可溶鉄(Feo)およびアルミニウム(Alo)を定の離水サンゴ礁段丘上の土壌生成が石灰岩岩屑土 → 初生量し,遊離酸化鉄およびアルミニウムの形態について検レンジナ様土 → レンジナ様土 → 褐色レンジナ様土 → テラ討した.その結果,土壌の絶対年代(x)と遊離酸化鉄のフスカ様土 → テラロッサ様土 → テラロッサ様土と赤黄色結晶化指数(y)との間に高い相関関係(y=0.0687 土の中間型の順に段階的に進行することを明らかにし x0.1857,r=0.91)があることが明らかになった.さらに, た.そして,平均隆起速度直線と氷河性海面変動曲線を得られた回帰式に年代不詳の南大東島土壌の結晶化指数用いて,各土壌型の絶対年代を測定し,さらに湿潤亜熱を外挿したところ,南大東島の離水サンゴ礁段丘上のラ帯気候条件下でサンゴ石灰岩から赤黄色土が生成するたテライト性赤色土は50±6万年,ラテライト性黄色土はめには,約12.5万年の年月を必要とすると推定した. 63±11万年という推定生成年代を得ることに成功した. 本報では,土壌の絶対年代と相関の高い指標を土壌自これらの年代は,南大東島のサンゴ化石のESR年代値体の理化学的性質から得ることを目的として,年代の明と非常に近い値であることから,遊離酸化鉄の結晶化指らかな喜界島の各土壌中の全鉄(Fet),ジチオナイトーク数は赤色系土壌の絶対年代の指標として有効であると考エン酸塩可溶鉄(Fed)およびアルミニウム(Ald),酸性えられる.

*1 日本学術振興会特別研究員筑波大学応用生物化学系〒305-0006つくば市天王台1-1-1 .(現住所:東京大学原子力研究総合センタータンデム加速器研究部門〒113-0032文京区弥生2-11-16).E-mail: *2 (有)日本土壌研究所〒188 -0001西東京市谷戸町2-15-11ひばりヶ丘高野ビル7F . *3 筑波大学応用生物化学系〒305-0006つくば市天王台1-1-1 .