application of the crystallinity ratio of free iron oxides

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 up-lift 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 valuesof (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

I. Introduction

In the previous paper (Nagatsuka and Nlae-

jima, 2001), the authors studied the absolute ages of soils developed on the raised coral reef terraces of Kikai Island in the Ryukyus, South-west Japan by using a combined method of the mean rate of tectonic uplift and the glacio-eustatic curve during late Quaternary. The soils on the raised coral reef terraces of Kikai Island showed a series of chronosequence in their development in the order of Coral Lime-

stone Lithosols (ca. 3.0 ka), Initial Rendzina-like soils (3.5-3.9 ka), Rendzina-like soils (35-40 ka), Brown Rendzina-like soils (50-55 ka), Terra f usca-like soils (70-80 ka), Terra rossa-like soils (95 -100 ka), and finally Intergrade between Terra rossa-like soils and Red-Yellow soils (120-125 ka). The years in the paren-theses indicate the absolute ages of the soils. Consequently, it was concluded that ca. 125 ka would be needed for Red-Yellow soils (Acrisols, Hapludults) to develop on coral limestone 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.

However, the mean rate of tectonic uplift is

not applicable for the area where the actual

uplift process in the past is not understood. So,

as an alternative, we tried to estimate the soil

age of other areas where the similar soils as

those existed in Kikai Island are formed by

using the index of soil age, which is highly

correlated with some physico-chemical prop-

erties of soil itself.

In the present paper, the authors examined

the applicability of the crystallinity ratio of

free iron oxides of soils for the estimation of

soil age, and tried to estimate the absolute age

of the soils derived from coral limestone in

Minami-Daito Island under the comparison

with other methodology.

II. Materials and methods

1. Soil samples The soil-sampling sites are shown in Figure

1. Soil samples were used from six soil profiles consisting of Initial Rendzina-like soil (Rendzic Leptosol according to WRB (FAQ, ISRIC and ISSS, 1998), Lithic Rendoll according to Soil Taxonomy (Soil Survey Staff, 1994)), Rendzina-like soil (Rendzic Leptosol, Lithic Rendoll) Brown Rendzina-like soil (Mollic Leptosol, Eu-tropeptic Rendoll), Terra fusca-like soil (Haplic Luvisol, Typic Hapludalf), Terra rossa-like soil

(Chromic Luvisol, Typic Hapludalf), Terra ros-

Fig. 1 Schematic location map of the soil-sampling

sites

sa-like soil and Red-Yellow soil (Haplic Lixisol, Typic Hapludalf) developed on the raised coral reef terraces in Kikai Island. Furthermore, soil samples from two soil profiles consisting of Lateritic Red soil (Typic Rhodudalf, Haplic Lixisol) and Lateritic Yellow soil (Typic Kandi-udult, Haplic Acrisol) developed on the raised coral reef terraces in Minami-Daito Island were also used for comparison. The profile descrip-tions, general physico-chemical and mineralog-ical properties of these soil samples have al-ready been reported in the previous paper

(Maejima et al., 1997 a, b; Nagatsuka and Mae-jima, 2001).

2. Iron and aluminum oxides and total iron Analyses were performed on the fine earth

fractions (≦2mm) and results are expressed on

an oven dry basis (% R2O3 per dry soil). The contents of total iron (Fet), iron and alu-

minum extractable by dithionite-citrate (Fed, Ald) and by acid ammonium oxalate (Feo, Alo) were determined for all horizons of the six

profiles. Fet was determined by the method described by Nagatsuka (1994). Fed, Feo, Aid and Alo were determined by the method of Blakemore et al. (1987).

III. Results and discussion

1. Distribution of iron and aluminum oxides in soil profile

The contents of total iron (Fet), dithionite-citrate-extractable iron (Fed) and aluminum

(Ald), and oxalate-extractable iron (Feo) and alu-minum (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 distribu-tion 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 the degree of soil development and soil age. 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 hori-

zons.

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 pedo-

genetic 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 iron oxides

Figure 3 shows the values of activity ratio of free iron oxides (Feo/Fed) (Blume and Schwert-

Fig. 2 Relationship between contents of clay and

dithionite-citrate-extractable iron or alumi-

num in Kikai Island soils ** Significant at 1% level .


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-Feo)/Fet] (Nagatsuka, 1972) of the studied soils. In Figure 3, the values of these ratios obtained from Minami-Daito Island soils derived from coral limestone are also shown for comparison.

In Kikai Island, activity ratio of free iron oxides ranged from 0.09 to 0.36, and the crys-tallinity ratio from 0.28 to 0.64. (Fed-Feo)/Fet

gradually increases from the youngest to the oldest soils, and Feo/Fed decreases in the same order (Fig. 3 and Table 1). Urushibara-Yoshino

(1988, 1992) also showed the increase of (Fed-Feo)/Fet in the B-horizon of the soils in Kikai Island associated with the decrease of Feo/Fed that was in accordance with the estimated age of the raised coral reef terraces. McFadden and Hendricks (1985) showed that the activity ratio

(Fea/Fed) in the chronosequence of southern California was moderately high to very high

(0.22-0.58) in middle Holocene to latest Pleis-tocene soils and progressively decreased to less than 0.10 in older soils. In Kikai Island, Feo/Fea of Initial Rendzina-like soil developed on the Holocene terrace was also moderately high

(0.32-0.36) and those of other soils developed on the Pleistocene terraces were less than 0.20 except A-horizon of Terra rossa-like soil. Arduino et al. (1984) showed that (Fed-Feo)/Fe, of the soils in the western Po Valley of north-ern Italy increased gradually with ages of the terraces, which range from early Middle Pleis-

tocene to Upper Pleistocene-Holocene. In addi-tion, Arduino et al. (1986) suggested that the ratios Fed/Fet and (Fed - Feo)/Fet were closely related to the ages of the terraces in northern Italy. Bech et al. (1997) also obtained similar results that the crystallinity ratio [(Fed-Feo)/ Fe] of Red Mediterranean soils was very close-ly related to the degree of weathering and/or age. Therefore, it is expected that the crystal-linity ratio of free iron oxides will be a good index of ages of Red-colored soils in Southwest

Japan. In general, the crystallinity ratios of free iron

oxides of Red-Yellow soils of Southwest Japan range from 0.5 to 0.7 (Nagatsuka and Urushi-bara-Yoshino, 1988). The (Fed-Feo)/Fet values of profile No. 6 in the present study range from 0.54 to 0.64. This fact agrees with our view that the profile No. 6 is an intergraded soil type that develops from Terra rossa-like soil to Red-Yellow soil.

According to Nagatsuka et al. (1983), the ac-tivity and crystallinity ratios of the soils derived from coral limestone in Ishigaki and Okinawa Islands ranged from 0.05 to 0.14, and from 0.53 to 0.75, respectively. The activity and crystallinity ratios of these soils were all distributed within the range of Red soil-region

[Feo/Fed≦0.4,(Fed-Feo)/Fet≧0.5](Nagatsuka,

1972). These soils have the morphological and

physico-chemical properties similar to those of


Rendzina, Terra fusca and Terra rossa, respec-tively, but the nature of free iron oxides is similar to that of Red soils. For this reason, these soils were designated tentatively as Rendzina-like, Terra fusca-like and Terra rossa-like soils, respectively. In Kikai Island, the activity and crystallinity ratios of soils were also distributed within the range of Red soil-region except Initial Rendzina-like and Rendzina-like soils. It is probable that the high content of organic carbon (84-129, 21-32g kg-1, respectively) in Initial Rendzina-like and Rendzina-like soils, of which activity ratios were relatively high and crystallinity ratios were low, prevented the crystallization of iron.

On the other hand, the crystallinity ratios of

the soils of Minami-Daito Island (Red soil and Yellow soil) also derived from coral limestone ranged from 0.77 to 0.81, and were similar to the Lateritic Red and Yellow soils which widely distributed in subtropical South China. The soil forming age as well as the develop-ment of these soils is considered to be in more advanced stage than the Red-Yellow soils

(Udults) in Southwest Japan (Maejima et al., 1997 a). However, the estimated age of these soils from Minami-Daito Island is not eluci-dated.

3. Estimation of soil age of Minami-Daito Island

The time-dependent trend of Feo, Fed, Fe, 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)


in Figure 4. There was no correlation between Feo (Fig. 4-a), Alo (Fig. 4-c) and the absolute soil age. However, it was clear that Fed and Fet

(Fig. 4-a), (Fed-Feo)/Fet (Fig. 4-b), and Ald (Fig. 4-c) were positively related to the absolute ages of Kikai Island soils. While, there was a negative correlation between Feo/Fed and soil ages (Fig. 4-b). Among these correlations, there was the highest and positive correlation between the soil age and (Fed-Feo)/Fet (y= 0.0687x0.1857, r=0.91). By using this relation-ship, the ages of Lateritic Red soil and Lateritic Yellow soil in Minami-Daito Island were es-timated as 500±60ka and 630±110ka, respec-

tively, by the extrapolation of this correlation.

The years indicate mean±standard deviation

(n=5 and 6, respectively). Makino (1983) re-ported 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 compositions and quartz content of red and/or

yellow colored soil materials in the southwest-ern part of Japan, using a multivariate statisti-cal method. Araki (1993) mentioned that al-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 correla-tion with each other, based on the factor analy-sis using the five variables consisting of activ-ity 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 designated the former as index of weathering degree (WIc) and deduced on the basis of multi-

ple regression analysis a formula for calculat-ing WIc from the original variables. The for-

mula is as follows:

WIc=1.89 [(Fed-Feo)/Fet]-0 .02 CEC of clay-4 .79 (Feo/Fed)-0 .14 sum of bases-0.33

When WIc was calculated from the data of

physico-chemical properties of the soils in Kikai Island (Nagatsuka and Maejima, 2001) and Minami-Daito Island (Maejima et al., 1997 a) by using this formula, we obtained the re-sults as shown in Figure 5. WIc of Minami-

Fig. 5 Relationship between weathering index

based on chemical properties (WIc) and

CEC of clay fraction

WIc was calculated from the equation

proposed by Araki (1993).

Fig. 6 Relationship between soil age and weather-

ing index based on chemical properties

(WIc) in Kikai Island soils

WIc was calculated from the equation

proposed by Araki (1993).


Daito Island soils are apparently higher than those of Kikai Island soils. It is suggested that Minami-Daito Island soils are in the state of much more advanced development stage than Kikai Island soils.

As shown in Figure 6, there is a high and

positive correlation between the absolute ages of Kikai Island soils and W Ic (y=-33.92+6.01 log x, r=0.84). The ages of Lateritic Red soil and Lateritic Yellow soil of Minami-Daito Island estimated by the extrapolation of thiscorrelation were estimated as 250±130ka and

450±140ka, respectively. The difference in the

estimated ages between by WIc and by crys-tallinity ratios alone, particularly in Lateritic Red soils, may be due to the difference in the nature of the soil parent materials, since non-calcareous soil samples were used for WIc. Judging from the correlation coefficients, 0.91 (crystallinity ratios in Fig. 4-b) and 0.84 (WIc by Araki (1993) in Fig. 6), the estimated ages of Lateritic Red soil and Lateritic Yellow soil of Minami-Daito Island would be more plausible,being 500±60ka and 630±110ka. respectivel

which are obtained from by the regression curve of the crystallinity ratios.

As mentioned above, the soil samples used in this study were calcareous soils, and the appli-cation of the crystallinity ratio of free iron oxides to other soils derived from other parent materials is not known. However, Arduino et al. (1986) also reported that the crystallinity ratio of free iron oxides was closely related to the ages of fluvial terraces in northern Italy. Moreover, according to Araki (1993), there was a high correlation between the crystallinity ratio of free iron oxides and the weathering index (WI) by Araki and Kyuma (1985), it may be also possible to evaluate the soil age from WI for the restricted studied area under similar weathering conditions.

IV. Conclusions

Following conclusions can be deduced about the soil age and the characteristics of status of free iron oxides of soils from Kikai and Minami-Daito Islands:

1. In Kikai Island, the crystallinity ratios of free iron oxides [(Fed-Feo)/Fet] gradually in-creased with the stage of soil development,

while the activity ratios (Feo/Fed) decreased. 2. There was a highly positive correlation

between the absolute ages of Kikai Island soils

(Nagatsuka and Maejima, 2001) and crystal-linity ratios of free iron oxides. Therefore, it is concluded that the crystallinity ratio of free iron oxides could be a good index of the ages of Red-colored soils.

3. The ages of Lateritic Red soils and Lateri-tic Yellow soils of Minami-Daito Island were estimated by the extrapolation of the correla-tion shown in Figure 4-b, and are 500±60ka

and 630±110ka, respectively.

Acknowledgements

We thank Dr. A. Tomikawa of Tsukuba Uni-

versity for his advise in the iron analysis and

for his helpful suggestions. This study was financially supported by the Grants-in-Aid for

Scientific Research, No. 04660067 from the Min-

istry of Education, Science and Culture of

Japan.

References

Araki, S. (1993) Comparative study of Kunigami Mahji and Feichisha soils in Okinawa Islands- Among Red-Yellow soils of the world. Pedologist,

37, 113-125. (J) Araki, S. and Kyuma, K. (1985) Characterization of

red and/or yellow colored soil materials in the southwestern part of Japan in terms of lithology

and degree of weathering. Soil Sci. Plant Nutr., 31, 403-410.

Arduino, E., Barberis, E., Carraro, F. and Forno, M.G. (1984) Estimating relative ages from iron-oxides/

total-iron ratios of soils in the western Po Valley, Italy. Geoderma, 33, 39-52.

Arduino, E., Barberis, E., Ajmone Marsan, F., Zanini, E. and Franchini, M. (1986) Iron-oxides and clay

minerals within profiles as indicators of soil age in northern Italy. Geoderma, 37, 45-55.

Bech, J., Rustullet, J., Garrigo, J., Tobias, F. J. and Martinez, R. (1997) The iron contents of some red Mediterranean soils from northeast Spain and its pedogenetic significance. Catena, 28, 211-229.

Blakemore, L. C., Searle, P. L. and Daly, BK. (1987) Extractable iron, aluminum and silicon. Blake-

more, L. C. et al. (eds.) NZ Soil Bureau Scientific Report 80 Methods for chemical analysis of soils: 71-

76, DISR, Lower Hutt. Blume, H. P. and Schwertmann, U. (1969) Genetic eval-

uation of profile distribution of aluminum, iron, and manganese oxides. Soil Sci. Soc. Am. Proc., 33,

438-444.


FAO, ISRIC and ISSS (1998) World reference base for soil resources, world soil resources reports 84. 88 p,

FAO Rome. 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 Intl. Coral Reef Symp., 2, Great Barrier Reef Comm., 595-613, Brisbane.

Maejima, Y., Nagatsuka, S. and Higashi, T. (1997 a) Classification of soils on the raised coral reef ter-

races of Minami-Daito Island. I. Occurrence of Haplic Lixisol or Typic Rhodudalf in Japan. Pedo-

logist, 41, 2-14. (J+E) Maejima, Y., Nagatsuka, S. and Higashi, T. (1997 b)

Classification of soils on the raised coral reef terraces of Minami-Daito Island. II. Clay Mineral Com-

position. Pedologist, 41, 15-22. (J+E) Makino, K. (1983) Marine terraces and age of the

formation in Kitadaito and Minamidaito Islands. Ann. Meeting of the Association of Japanese Geogra- phers, 24, 74-75. (J)

McFadden, L. D. and Hendricks, D. M. (1985) Changes in the content and composition of pedogenic iron

oxyhydroxides in a choronosequence of soils in southern California. Quat. Res., 23,189-204.

Nagatsuka, S. (1972) Studies on genesis and classification of soils in warm-temperate region of South-

west Japan, part 3. Some features in distribution and mode of existence of free iron and aluminum

oxides in the soil profile. Soil Sci. Plant Nutr., 18, 147-154.

Nagatsuka, S. (1994) Selective dissolution methods of iron. Japan Association for Quarternary Research

(ed.) A handbook of Quaternary Research, vol. 2: 22- 32, Univ. of Tokyo Press. (J)

Nagatsuka, S., Kaneko, S. and Ishihara, A. (1983) Soil genesis on the raised coral reef terraces of Ishigaki

and Okinawa-islands in the Ryukyu Islands, Japan part 1. Soil Sci. Plant Nutr., 29, 343-354.

Nagatsuka, S. and Urushibara-Yoshino, K. (1988) On the vertical zonality of soil distribution and soil

conditions in Xishuangbanna, South Yunnan. Cli- matological Notes, 38, 229-247.

Nagatsuka, S. and Maejima, Y. (2001) Dating of soils on the raised coral reef terraces of Kikai Island in

the Ryukyus, Southwest Japan: With special reference to the age of Red-Yellow soils. The Quat.

Res. (Daiyonki-Kenkyu), 40, 137-147. QOta, Y., Machida, H., Hori, N., Konishi, K. and Omura,

K. (1978) Holocene raised coral reefs of Kikai-jima- An approach to Holocene sea level study-. Geogr.

Rev. Japan (Chirigaku-Hyoron), 51, 109-130. (J+E) Schlichting, E. and Blume, H. P. (1962) Art and Au-

smab der Veranderungen des Bestandes mobiler Oxide in Boden aus jungpleistozanem Geschiebem-

ergel and ihren Horizonten, Z. Pflanzenernahr, Dung. Bodenk., 96, 144-156.

Soil Survey Staff (1994) Keys to soil taxonomy, sixth edition. 306 p, U. S. D. A. Soil Conservation Service.

Urushibara-Yoshino, K. (1988) The red soils on a limestone area in Nansei-island, Southwest Japan.

Gillieson, D. and Smith, D. I. (eds.) "Resource manage- ment in limestone landscapes" Special Publication,

No. 2: 183-189, The Australian Defence Force Academy, Australia.

Urushibara-Yoshino, K. (1992) The red soils on a limestone area in Kikai island, of Nansei-islands, South-

west Japan. Chardon, M., Sweeting, M. and Pfeffer, K. H. (eds.) Proceedings of the Karst-Symposium-Bl-

aubeuren: 71-81, Tubiringer Geographische Stud- ien H109.

Zainol, E. (1985) Pedogenetic forms of extractable iron in selected soils of Kedah, Malaysia. J. Rubb.

Res. Inst. Malaysia, 33, 115-123.

(J) in Japanese, (J+E) in Japanese with English abstract.


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

の離水サンゴ礁段丘上の土壌生成が石灰岩岩屑土→初生

レンジナ様土→ レンジナ様土→褐色レンジナ様土→ テラ

フスカ様土→ テラロッサ様土→ テラロッサ様土と赤黄色

土の中間型の順に段階的に進行することを明らかにし

た.そして,平均隆起速度直線と氷河性海面変動曲線を

用いて,各土壌型の絶対年代を測定し,さらに湿潤亜熱

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

めには,約12.5万年の年月を必要とすると推定した.

本報では,土壌の絶対年代と相関の高い指標を土壌自

体の理化学的性質から得ることを目的として,年代の明

らかな喜界島の各土壌中の全鉄(Fet),ジチオナイトーク

エン酸塩可溶鉄(Fed)およびアルミニウム(Ald),酸性

シュウ酸塩可溶鉄(Feo)およびアルミニウム(Alo)を定

量し,遊離酸化鉄およびアルミニウムの形態について検

討した.その結果,土壌の絶対年代(x)と遊離酸化鉄の

結晶化指数(y)との間に高い相関関係(y=0.0687

x0.1857,r=0.91)があることが明らかになった.さらに,

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

を外挿したところ,南大東島の離水サンゴ礁段丘上のラ

テライト性赤色土は50±6万年,ラテライト性黄色土は

63±11万年という推定生成年代を得ることに成功した.

これらの年代は,南大東島のサンゴ化石のESR年代値

と非常に近い値であることから,遊離酸化鉄の結晶化指

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

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 .

application of the crystallinity ratio of free iron oxides

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