South PacificStudyofVol.
20, No.
1, under1999 Oxic and Anoxic Conditions in Babeldaob Island, Palau
Geochemistry
Paleosols
Formed39

Geochemistry of Paleosols Formed under Oxic and Anoxic
Conditions in Babeldaob Island, Palau
Munetomo NEDACHI1), Kammen M. TAKTAI2), Yoko NEDACHI3)

Abstract
Two different types of paleosols (laterite vs. kaolinite types), developed on Eocene volcanic rocks in the Babeldaob
Island, Palau, were examined in order to understand the behaviors of elements during soil formation under different
redox conditions.
Laterite type paleosol on the Aimiliki volcanic rock was formed under ordinal atmospheric (oxic) condition. Si, Al,
Mg, V, Fe, Mn, Cu, Co, Ni, Zn and Ge were leached out from the upper zone. It is noticeable that Fe is dissolved evenunder oxic condition. REEs increase slightly with advancing weathering. Th and U typically increase. Other elements
show rather complicate pattern. On the other hand, kaolinite type paleosol on agglomerate and tuff of the Babeldaob
volcanics might develop under anoxic condition, because the overlying lignite formation has absorbed the atmospheric
oxygen from penetrating meteoric water. Many elements generally decrease upward, with the enrichment in the middle
section. It is suggested that these elements have moved downward during the weathering. The most mobile elements
such as K, Mg and Rb decrease uniformly upward in the section.
From the behaviors of relatively immobile elements, low pH as well as high PO2 conditions were predicted for thelaterite type paleosol developed on the Aimiriki volcanics, and slightly higher pH as well as lower PO2 conditions forthe kaolinite type paleosol on the Babeldaob volcanics. The Palauan data are compared with the Paleozoic andPrecambrian paleosols reported previously. The comparison supports the suggestion made from the Pronto
paleosol(NEDACHI et al., 1998) that the atmosphere at the Archean-Proterozoic boundary era was already oxic.
keywords:paleosol, geochemistry, Palau, laterite type, kaolinitetype.

Introduction
Soils develop in direct contact with the atmosphere, and weathering is a chemical reaction
between meteoric water equilibrated with atmospheric gases, and rocks. Therefore, the soil
chemistry is strongly affected by the atmospheric compounds such as carbon dioxide andoxygen gases. Paleosols have attracted attentions to many researchers to understand the ancient Earth’s environment, because the mineralogy and chemistry of paleosol were shaped inlarger part by the contemporaneous atmosphere (HOLLAND, 1984). Especially, the loss or retention of Fe from paleosols has been used as a guide for the paleo atmospheric oxygen level(e.g., HOLLAND, 1984). Because ancient paleosols have been modified to various degree by
various processes (diagenesis, metamorphism, and hydrothermal alteration) during their long
history, however, it has been disputed as to whether the loss of Fe has occurred by anoxic
weathering, by dissolution by organic acid, or by hydrothermal leaching (OHMOTO, 1996).
Hence, the recent soils and young paleosols provide valuable information on the behaviors ofvarious elements during soil formation. In this study, two contrasting types (laterite and ka1) Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan.
2) Division of Conservation and Entomology, Bureau of National Resources and Development, Ministry of Resources
and Development, Republic of Belau.
3) Kagoshima Immaculate Heart College, Kagoshima 890-8525, Japan.

40South Pacific Study Vol. 20, No. 1, 1999

olinite types) of young paleosols developed on Paleogene volcanic rocks were studied.
Palau (Republic of Belau) is situated in the tropical zone and is composed of many islands,
which construct a volcanic island arc between the Pacific and Philippine plates. Each island is
enclosed with lagoons and reefs. Soils and paleosols are observed everywhere. Among them,
relatively older weathering profiles were selected for this study. Weathering profiles of thekaolinite-montmorillonite type have often developed beneath the sediments in damp groundareas of Miocene age. As the overlying sediment contains many thin lignite beds, weatheringmight have proceeded under a reduced condition. A laterite type paleosol is also studied tocompare the elemental behaviors during weathering under different redox conditions. The
comparisons are expected to provide useful information in interpreting much older paleosol
data, especially in attempt to put constraints on the atmospheric oxygen level during Precambrian.

Geological Outline of Babeldaob Island
Babeldaob Island, Palau, is comprised mainly of the volcanic rocks, lagoon sediments andcoral reef. The volcanic rocks of Babeldaob Island erupted during Eocene and Oligocene, anddebilitated at late Oligocene (TAYAMA, 1952). The volcanic rocks occur as lava, agglomerate,volcanic breccia, tuff breccia and tuff, with the composition of olivine basalt, hornblendebearing two-pyroxene andesite and clinopyroxene hornblende dacite. These volcanic rocks
are grouped into three formations; the Babeldaob volcanics (early Eocene), Aimiriki volcanics(late Eocene) and Almonagui volcanics (Oligocene) (TAYAMA and SHIMAKURA, 1937; TAYAMA,
1952). The SiO2 contents of the volcanic rocks vary widely from 47% to 67%.
There are two types of volcanism; olivine tholeiite series and calc-alkaline series. NEDACHIet al., (1996) described the trace element geochemistry of the volcanic rocks, and suggested
that the volcanism was similar to those of the other island arc systems in the West Pacificregion, especially that of the Mariana island arc system (TATSUMI et al., 1983). The Palauan
volcanism is characterized by the coexisting of two rock series, back-arc and front-arc series,
in spite of the limited space and time. The patterns of the Babeldaob volcanics are similar to
those in the back arc side of other island arc systems (TATSUMI et al., 1983; YOSHIDA et al.,1995). On the other hand, the Aimiriki agglomerate is similar to those in the volcanic frontzone. Associated with these volcanisms, a small scale Zn-Au mineralization is observed in thesoutheastern end of Babeldaob Island. MILLER et al. (1987) pointed out that hydrothermal
alteration in and near the veins was of neutral pH and potassic type. NEDACHI et al. (1996),
from a fluid inclusion study, predicted the temperatures of about 320°C and the depth of about
1 km for the conditions of hydrothermal mineralization, and suggested that the main uplift
movement was finished in Neogene.
The Galdog bed is found between the Babeldaob and Aimiriki volcanics in the northern areaof Babeldaob Island. The Airai formation (Miocene) is scattered along the coastal line. The
formation contains many thin lignite beds, which suggest that the formation has developed indamp grounds changed from lagoonal conditions in Miocene.

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau

41

Weathering profiles of volcanic rocks are observed everywhere in Babeldaob Island. There
are two types of paleosol. Laterite type weathering is well recognized everywhere, and kaolinite-montmorillonite type weathering is only beneath the Airai formation. The kaolinite typepaleosol beneath the lignite formation changes gradually to the laterite type paleosol in theareas not overlain by lignite formation. Among many paleosols in Babeldaob Island, two places
shown in Fig. 1 were selected for this work.

Fig 1. Geological map of the Babeldaob island, Palau, and sampling sites.

42South Pacific Study Vol. 20, No. 1, 1999

Sampling Sites
The samples, A1 to A18, were collected from the southern end of Babeldaob Island. Alongthe road, a steep road-cut exposes a clear profile of weathering. As shown in Fig. 2, the paleosols
have developed on the Aimiriki volcanic rocks, and are overlain by a recent terrace deposit.
The zonal arrangement, from the onion structure of fresh andesitic basalt, through yellowish
colored layer, to reddish brown colored layer and to black colored paleosol is easily recognized on the section. All the layers without the crust of onion structure are almost unconsolidated. The upper part of the section is slightly eroded out and changed to modern soil. The
boundary between the modern soil and paleosol is obscure. The fresh rock is an agglomeratewith basaltic to andesitic composition, and contains olivine, clinopyroxene, orthopyroxene
and plagioclase, with miner amounts of magnetite, apatite and others. According to X-ray
diffraction analyses, Mg-chlorite, illinite, feldspar, epidote and other minerals are recognizedin the paleosol near the fresh rock side, and kaolinite, carbonate, rutile, goethite, hematite andothers are detected from the middle to upper parts of the paleosol. The mineral assemblage
suggests that the weathering has proceeded under oxic environment. The cliff is almost vertical, and the distance from the terrace almost corresponds to the depth from the paleo-surface.
The samples were collected along two traversed lines on the wall.

Fig 2. Schematic cross section of the paleosols on the Aimiriki volcanic rocks of Eocene Age, Babeldaob
Island, Palau. Open and solid circles represent the location of the samples of which major, andmajor and trace elements were analyzed, respectively.

Another sampling site is located 10 km north of the first sampling site, where the samples,
B1 to B10, were collected. A sketch of the sampling site is shown in Fig. 3. An agglomerate
and tuff of the Babeldaob volcanics were weathered to form a paleosol. The Airai formation ofGeochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau43

Fig 3. Schematic cross section of the paleosols on the Babeldaob volcanic rocks of Eocene Age, overlain
by the lignite bearing Airai formation of Miocene Age, Babeldaob Island, Palau. Open and solid
circles represent the location of the samples of which major, and major and trace elements wereanalyzed, respectively.

lignite-clay alternation overlies the volcanics and paleosol.
Kaolinite has dominantly formed in the paleosol. Illinite, quartz, rutile, hematite and others
are also detected in the paleosol. Smectite is found in some of the sample. In spite of theintense mineralogical change, the original texture of phenocryst and other rock texture haveclearly remained. The evidences suggest that the paleosol did not rework, and that both the tuff
and agglomerate were weathered in situ. It might be better to call as “saprolith”. The paleosol
is strongly removed the color as a whole, but shows spotted hematite red at the lower part. The
hematite reddish color decreases toward the paleo-surface, and in the middle part of the outcrop, another hematite shows an irregular banded texture. The upper 200cm of the section iscomprised mostly of homogeneous white colored kaolinite. The texture gradually disappears
to be completely homogeneous at the top (about 20cm) of the section.
The age of the Airai formation is Miocene. There are three possible stages of weathering: 1)
before deposition of the Airai formation under atmospheric (oxic) condition, 2) during and/or
after the deposition of the Airai formation under anoxic condition, and 3) modern weathering.
The land utilization by modern human suggests that the modern weathering might be only
hematitization. The geological evidence suggests that the weathering proceeded at the stage ofdeposition of the Airai formation or much later. The existence of lignite in the Airai formationshows that the weathering has proceeded under anoxic environment (STUMM and MORGAN,
1981). The bedding of the lignite formation is slightly dipping toward south, and surface declines toward west. The parent fresh rock can not be observed in this section. The closest
outcrop of fresh rock is 1.3 km north from the sampling site (Fig. 1). The samples, B11 to B13,
collected from this outcrop, are regarded as parent rocks of the kaolinite type paleosol. The

44South Pacific Study Vol. 20, No. 1, 1999

fresh rock is basaltic andesite of calc-alkaline rock series, and the main rock-forming minerals
are plagioclase, clinopyroxene and orthopyroxene. Sometimes olivine or hornblende is observed. Magnetite, ilmenite and apatite are included as minor minerals.

Residual Elements in the Paleosols
The compositions of major and trace elements were obtained, using X-ray fluorescencemethod, ICP mass spectrometry and other methods. The results are shown in Tables 1 and 2.
The changes of concentratios of major elements during weathering are shown in Fig. 4. The
chemical behaviors roughly coincide with the microscopic observation and X-ray diffraction
data. For example, the chemical profile of the laterite type paleosol differs from that of thekaolinite type paleosol. The chemical profile of major elements such as the decreases of MgO,
Na2O and CaO coincides to the decomposition of major minerals such as plagioclase andpyroxene. In the section of kaolinite type paleosol, the chemical discontinuity corresponds tothe boundary of tuff and agglomerate.

Fig 4. Variation of the contents of major element of the paleosols from Palau. Open and solid circles
represent the paleosols on the Aimiriki volcanic rocks and on the Babeldaob volcanic rocks, respectively. The bar represents the deviation among three fresh samples.

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau

Table 1. Compositions of major elements of the paleosols from Palau (wt%)

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Table 2. Compositions of trace elements of the paleosols from Palau (ppm)46South Pacific Study Vol. 20, No. 1, 1999

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During weathering, some elements have been removed from and others have remained in
the original rock. Then from the concentration expressed in wt%, we can not judge whether theelement has remained, has been added from other place, or has been removed. Usually thenormalization by the concentration of the immobile element is adapted to evaluate true grain
and loss. Al, Ti, and Cr have been commonly treated as immobile elements during weathering.
Zr, Nb and Hf are proposed as more immobile elements, and the ratios among those elements
were also used to identify the original rocks of paleosol (BRIMHALL and DIETRICH, 1987). Zr
was used as an element for normalization in this study. The similar tendencies were obtained if
Hf and Nb are used. Zr-normalized contents of the paleosols are plotted in Figs. 5 to 7 for the
Aimiliki volcanics, and in Figs. 8 to 10 for the Babeldaob volcanics.

Chemical Profile of the Laterite Type Paleosols
Figs. 5 to 7 show the variation of Zr-normalized values of each element in the laterite typepaleosols developed on the agglomerate of the Aimiriki volcanics.
Zr-normalized Si, Al, Mg, V, Fe, Mn, Cu, Co, Ni, Zn and Ge values decrease uniformly
toward the unconformity. It is noticeable that Fe and Al are dissolved even under an oxic
condition. Fe may have dissolved as Fe3+ irons.
Some elements do not show smooth trends through the section. Phosphorus decreases andincreases toward the unconformity. The main P-bearing mineral in the parent rock is apatite.
The decrease of phosphorus in the lower part of the section is related with the dissolution of
apatite, but the P enrichment in the upper part might be due to biological activity. As there isno crystalline phase, phosphorus is probably adsorbed into clay minerals. K, Rb and Cs showrather complicate pattern. They are high in the middle part of the section. These elements
decrease at early stage of weathering (the lower part of the section, around 300 cm from paleosol
surface), increase at the middle stage (around 200 cm from the surface), decrease again, andincrease near the surface of the paleosol. The decrease at early stage of weathering suggest thatthe enrichments of alkali elements at the middle stage is not by in situ chemical reaction, butthat the alkali elements were added from the upper zones or from the overlying terrace (HOLLAND, 1984). These additions may not have been limited only to middle part, and the weakerenrichments are also recognized at near the top of the section. The enrichments near the top of
the paleosol are recognized more easily on the light alkali elements (Na and K) than heavy
alkali elements (Rb and Cs). The similar enrichments are recognized in the alkali earth elements; Ca and Sr. On the other hand, the elements of III and IV groups in the periodic tablesuch as Nb and Hf, do not change through the section.
Fig. 7 shows the variation of REEs, Th and U, which increase toward the paleo-surface. It issaid that uranium is dissolved and leached out from the parent rock under oxic environment,
because U6+ is mobile. The enriched uranium might be interpreted by the addition from otherplace or overlying terrace at the stage later than that of weathering. The environment mightchange to anoxic, when the soil was migrated and was not supplied the atmospheric oxygengas, which was completely consumed in the overlying formation. Other interpretation is the

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Fig 5. Variation of Zr-normalized contents of Si, Al, P, alkali and alkali earth elements of the paleosols on
the Aimiriki volcanic rocks.

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau49

Fig 6. Variation of Zr-normalized contents of elements of the III and IV groups and transitional elementsof the paleosols on the Aimiriki volcanic rocks.

50South Pacific Study Vol. 20, No. 1, 1999

Fig 7. Variation of Zr-normalized contents of REEs, Th and U of the paleosols on the Aimiriki volcanic
rocks.

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau51

Fig 8. Variation of Zr-normalized contents of Si, Al, P, alkali and alkali earth elements of the paleosols on
the Babeldaob volcanic rocks.

52South Pacific Study Vol. 20, No. 1, 1999

Fig 9. Variation of Zr-normalized contents of elements of the III and IV groups and transitional elementsof the paleosols on the Aimiriki volcanic rocks.

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau53

Fig 10. Variation of Zr-normalized contents of REEs, Th and U of the paleosols on the Aimiriki volcanic
rocks.

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formation of insoluble carbonate, such as UO2(CO3). UO2(CO3) is produced through thereaction of U6+ with HCO3- from overlying coral limestone, and is rather stable under oxic
environment.

Kaolinite Type Paleosols Overlain by Lignite Deposits
Geochemistry of the paleosols on the Babeldaob volcanic rocks overlain by the Airai formation is rather complex. As shown in Fig. 9, Zr-normalized Nb and Hf are almost constantthrough the paleosol section, including the parent rock. But Zr-normalized Ti and Ga in theupper part of the section are distinctly lower than those of the lower part. The upper part
originates in tuff, and the lower part in agglomerate. The profiles suggest that Ti and Zr haveremained unchanged through weathering, and that the Ti/Zr ratios are fixed to the originalcompositions of both the rock which differ with each other.
Zr-normalized values of many other elements decrease toward the top of the paleosol section through a discontinuous jump. The distance of the discontinuous zone from theunconformity depends on the kind of element.
Fe decreases with advancing weathering, and jump off at the boundary from agglomerate to
tuff with advancing weathering. Mn and Ni also decrease with advancing weathering, butjump up at the boundary from agglomerate to tuff.
The values of Si, Al, Co, Zn, Cu, Ge, REEs, Ca, Sr and Ba also show similar tendency, but
the positions of maximum values are beneath the boundary between tuff and agglomerate.
These elements may have been dissolved from the upper part, and re-precipitated in the slightly
lower part.
Phosphorus decreases from the parent rock to the lower part of paleosol, and increases
toward the unconformity. As mentioned in the another section, the P-enrichment may be causedby biological activity.
Fig. 10 shows the variations of REEs, which decrease toward the top with the maximum
values in the middle section. The LREEs profiles have the maximum values at 145 to 215 cm.
Th and U are almost constant during weathering. As U4+ and Th are immobile under reduced
environment, the profile well coincides with the existence of overlying lignite. In this case, thepattern would be similar to those of Hf, Nb and Ta.

Discussion
Some elements are easily dissolved and transported from rocks by low temperature water,
and are precipitated again at the places of different chemical environment. The present study
shows that these phenomena are frequently observed in young paleosol. OHMOTO (1996) compared the solubility of minerals comprised of rather immobile elements, showing that the solubility of SiO2, Al(OH)3, Fe2O3, Fe(OH)3, and TiO2 depends on pH as shown in Fig. 11. Although silica is less soluble at the pH below 9, SiO2 is more mobile compared to Ti, Al and Fe

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau55

Fig 11. Thermodynamic solubilities of Fe, Al and Si compounds.

compounds under neutral condition. Al(OH)3 is almost insoluble under neutral conditions, andFe(OH)3 and TiO2 are less soluble than Al(OH)3 under neutral or alkaline condition. However
Al(OH)3 is soluble more than SiO2 under the pH less than 3.5, and Fe(OH)3 might be alsosoluble more strongly under the pH less than 2.0. The dependency of solubility on pH affects
the behavior of each element through weathering.
The geochemical profiles of each paleosol are compared together, using the content normalized by Zr and also normalized by the parent rock; (Metal/Zr)S /(Metal/Zr)P. Where, the subscripts, s and p, represent sample and parent rock. Before discussion, the features shown in
Fig. 11 are qualitatively expressed using (Metal/Zr)S/(Metal/Zr)P, assuming that Zr is completely insoluble. Fig. 12 is a model of behavior of Ti, Al, Fe and Si during weathering. The
solubility of chemical compounds of Ti, Al and Fe3+ increase with decreasing pH(-6). Undersame acid solution, TiO2 is most insoluble and Al2O3 is most soluble among TiO2, Fe(OH)3and Al2O3. In addition, as there are ferrous and ferric, the solubility of iron compounds isaffected by PO2. A set of chemical trends of Si, Ti, Fe and Al should suggest the environment

Fig 12. A model of behavior of Ti, Al, Fe and Si during weathering.

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South Pacific Study Vol. 20, No. 1, 1999

of weathering. For example, the environment of neutral and low PO2 can be expected, if boththe (Al/Zr)s/(Al/Zr)P and (Ti/Zr)s/(Ti/Zr)P decrease gently with decreasing the (Si/Zr)s/(Si/Zr)P, but the (Fe/Zr)s/(Fe/Zr)P decrease more steeply than (Al/Zr)S/(Al/Zr)P with decreasing(Si/Zr)s/(Si/Zr)P.

Fig 13. Comparisons of the variation of Zr-normalized values of Al, Fe and Ti versus Si, which arenormalized by Zr and by parent rock.

The results of various paleosols are shown in Fig. 13. In the figure, lines represent the
regression line estimated statistically. Although the deviations from the regression lines arerather large, some features and differences can be recognized. The(Al/Zr)s/(Al/Zr)P, (Fe/Zr)s/(Fe/Zr)P and (Ti/Zr)s/(Ti/Zr)P ratios generally decrease with decreasing (Si/Zr)s/(Si/Zr)P ratio(with advancing the weathering), but the decreasing manners slightly differ with each other.
The field evidence in Palau shows that the laterite type weathering on the Aimiriki volcanic
rocks has proceeded under ordinal atmospheric condition (oxic condition). On the other hand,
The kaolinite type weathering on the Babeldaob volcanics has advanced under anoxic conditions. The overlying lignite bed may have played a role of filter to reduce the surface water
when O2-saturated meteoric water penetrated through the ground. The difference of the environment affect the decreasing rate of (Fe/Zr)s/(Fe/Zr)P. The (Fe/Zr)s/(Fe/Zr)P for the laterite
type is slightly lower than that for the kaolinite type. The (Al/Zr)s/(Al/Zr)P ratios of the laterite

Geochemistry of Paleosols Formed under Oxic and Anoxic Conditions in Babeldaob Island, Palau

57

type are higher than that of the kaolinite type when the (Si/Zr)S/(Si/Zr)P ratios are higher than~0.5, but become much lower when the (Si/Zr)S/(Si/Zr)P ratio decreases below ~0.5. On theother hand, the (Ti/Zr)S/(Ti/Zr)P ratios of the laterite type decrease slightly, but those of thekaolinite type are almost constant or slightly increase with decreasing the (Si/Zr)s/(Si/Zr)P
ratios. The profiles suggest the low pH condition for laterite type and near neutral condition
for the kaolinite type. Hence, the conditions for the laterite type weathering on the Aimiriki
volcanic rocks might be of lower pH as well as higher PO2, and that for the kaolinite typeweathering on the Babeldaob volcanics overlain by lignite might be of near neutral as well as
lower PO2. The further mineralogical study should be obtained to confirm the weathering feature under different type.
The data of other paleosols of different ages are also plotted in Fig. 13 for comparison. The
data are from Permian paleosol on granodiorite (CONDIE et al., 1995) and Precambrian paleosolson the Archean granite at Pronto and on the Archean greenstone at Elliot Lake, Ontario, Canada(NEDACHI et al., 1998). These parent rocks are chemically and physically different from thosein present study, and the absolute contents of constitutions affect their own behaviors as discussed by HOLLAND (1984). The chemical profiles of the paleosols on the Archean granitefrom Pronto, Ontario, Canada, are very similar to those of the laterite type weathering. CONDIE
et al.(1995) analyzed in detail the behaviors of REEs in the paleosol formed under high PO2
conditions in the Front Range, USA. The Al, Fe, Ti, Si, and Zr behaviors in this paleosol (Fig.
12) suggest a neutral pH condition. However, the very large variation of (Fe/Zr)S/(Fe/Zr)P
ratios suggest some Fe addition during weathering.
In contrast, on the paleosols on greenstone at the Blind River area, the (Al/Zr)S/(Al/Zr)P and(Ti/Zr)S/(Ti/Zr)P are almost constant during weathering, but (Fe/Zr)S/(Fe/Zr)P decreases withadvancing weathering. Considering that ferrous mineral is more soluble than ferric mineral,
the weathering condition on the Archean greenstone could be estimated to be almost same orslightly higher pH condition and same PO2 condition with that of the kaolinite type paleosol in
Palau. The reason of low PO2 might be same with that of the kaolinite type paleosol in Palau.
As P-enrichment is observed in the paleosol on the Archean greenstone, and as is observedsome bio-activity might produce anoxic conditions there. However, attention should be paid
to the wide deviations of data.

Conclusions
Behavior of various elements in two different types of paleosols on Eocene volcanic rocks
at Babeldaob Island, Palau, was examined and compared with those of the old age.
In the laterite type paleosols on the Aimiriki volcanics, many elements were leached out
from the upper zone. It is noticeable that Fe is dissolved even under oxic condition. REEs, Th
and U typically increase with advancing weathering. Some alkali elements are leached out
from the upper paleosols, and re-precipitated in the middle part.
In the kaolinite type paleosol on agglomerate and tuff of the Babeldaob volcanics, the ratios
among Ti, Zr, Nb, Hf and Ta are almost constant, but differ between tuff and aggromerate. Zr-

58South Pacific Study Vol. 20, No. 1, 1999

normalized values of many elements decrease toward the unconformity while jumping to maximum values at the middle part. The maximum Zr-normalized values of Si, Al, Co, Zn, Cu, Ge,
Ba, Ca, Sr and REEs are recognized at slightly below the boundary between tuff and agglomerate. It is suggested that these elements have moved downward during soil formation. The
contents of more mobile elements, such as K, Mg and Rb decrease uniformly toward top of thesection. The chemical profile of paleosol strongly depends on the pH conditions as well as on
the oxygen partial pressure.
From the diagrams of (Al/Zr)S/(Al/Zr)P, (Fe/Zr)S/(Fe/Zr)P and (Ti/Zr)S/(Ti/Zr)P ratios against(Si/Zr)S/(Si/Zr)P, low pH as well as high PO2 conditions were estimated for the laterite typepaleosol developed on the Aimiriki volcanics, and slightly high (but less than neutral) as well
as low PO2 conditions for the kaolinite type paleosol on the Babeldaob volcanics. The datafrom the Precambrian paleosols were compared, and it suggests that the atmosphere of theArchean-Proterozoic boundary era contained rather high oxygen.

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(Accepted 28 February, 1999)