Journal of A/rican Earth Sciencea, VoL 10, No. 4, pp. 739-744, 1990 0899-5362/90 $3.00 + 0.00 Printed in Great Britain O 1990 Pergamon Press plc

Extractable Fe-A! in Quaternary paleosols on Mount Kenya, East Africa

W. C. MAHANEY* and K. SANMUGADAS**

*Geomorphology and Pedology Laboratory, Department of Geography Atldnson College, York University, 4700 Keele Street,

North York, Ontario, Canada M3J IP3 **Department of Geography, York University,

4700 Keele Street, North York, Ontario, Canada M3J IF3

Abstract - Ground soils and buried paleosols at ~3000 m on Mount Kenya were studied to determine if distributions of Fe and AI would provide information on age as well as present and past soil-forming environments. Extractable Fe and A1 distributions indicate that leaching was more vigorous in older buried paleosols. Analysis of the F%/Fe d (oxalate to dithionite extractable) distributions in the paleosols studied leads to the interpretation that greater age can be inferred firom conversion of Feo to Fe d. Age differences between loess (Brunhes Chron) and till (Matuyama Chron) in one section, as determined on paleomagnetic evidence, is clearly supported by Feo/Fe d distributions.

I N T R O D U C T I O N

Many s tud ies have a t t emp ted to u se ext rac table Fe and A1 to s epa ra t e sur face and bur i ed softs and p l a c e t h e m t a x o n o m i c a l l y ( B l u m e a n d Sch wer tmann , 1969; Lutwick and Dormaar , 1973; M a h a n e y and Fahey, 1988). Bur ied soils unde rgo phys ica l and chemica l t r ans fo rma t ions mak ing it difficult (and some t imes impossible) to u se color, organic ca rbon , pH, ca t ion exchange capaci ty, th ickness , t ex tu re and s t ruc tu r e to adequa te ly in terpre t the i r genesis . In th is s t u d y we a t t emp t first, to de te rmine if ex t rac table Fe and AI dis t r ibu- t ions provide age -dependen t evidence suppor t ing pa leomagnet ic de te rmina t ions on two sect ions. Second, we s e e k to es tab l i sh if ex t rac tab le Fe and A1 might prove use fu l in de termining the degree of downward m o v e m e n t b e t w e e n g r o u n d soils and bur i ed paleosols. Third, we cons ider it impor tan t to de te rmine if the bu r i ed A hor izons had been affected b y leaching effects.

F I E L D A R E A

The Hobley Valley site (HOB 19) (Fig. I) is located in the t imberl ine ecotone domina ted b y Hagenia abyssinica and Festuca pilgeri ( tussock grass). This is one of the we t t e s t s i tes on the m o u n t a i n wi th p rec ip i t a t i on e s t i m a t e d at ~ 2 0 0 0 m m (Coetzee, 1967). Precipi ta t ion is delivered b y the classical m o n s o o n with the heavies t rainfall from J u l y to October . Mean annua l t empe ra tu r e in the

t imberl ine a rea is e s t ima ted at 10 ° C with an es t imated t e m p e r a t u r e depress ion of 7 ° C for the las t (Liki, Wfirm) glaciat ion (Coetzee, 1967).

In lower Teleki Valley a pa leoso i s e q u e n c e at site TV61 is covered with t ropical suba lp ine forest e lements cons is t ing primari ly of P o d o c a r p u s (P. milan.]ianus) and b a m b o o (Arundinaria alpina~. Mean a n n u a l t e m p e r a t u r e at th is site is e s t ima ted at 10 ° C; us ing the s a m e t e m p e r a t u r e depress ion for the las t glaciat ion gives a m e a n annua l t e m p e r a t u r e of + 3 ° C dur ing the height of the Liki Glaciat ion (Mahaney, 1989). Precipi tat ion is de- t e rmined part ly b y the m o n s o o n and also by t h u n d e r s t o r m activity genera ted by hea t ing over the vas t p la ins b e t w e e n Mt. Kenya and the Aberdare Moun ta ins to the wes t (Mahaney, 1986).

M E T H O D S

Soft nomenc la tu re follows the Soil Survey Staff (1951, 1975} and Birke land (1984). Terminology for g round softs and pa leosols follows Ruhe (1965) and Pawluk (1978). Si tes were se lected on mora ine su r faces a r o u n d the marg in of glaciat ion on Mount Kenya at approximate ly 3 0 0 0 m. Particle size de te rmina t ions were m a d e by dry sieving the coarse part icles (2mm-63 ~n) and sed imen ta t ion for the fine f ract ions (< 63 fan) (Bouyoucos , 1962; Day, 1965}. Iron ext rac t ions follow Coffin (1963) and McKeague and Day (1976).

739

740 W.C. MArU~N~Y and K. SANMUGAI~AS

?-

<

.... ~ (,>.

- " " L . . ¢ . - h

,i

o

e ' v

', 9 '

%

!

~HOB191

Fig. I. Location of sites HOBI9 and TV61 on Mount Kenya, East Africa.

DISCUSSION

The Paleoso ls The Hobley Valley site (HOB 19) was selected in a

moraine ridge at - 3000m, approximately 0.5 km down valley from the Liki I (Mahaney, 1989) moraine limit described by Nilsson (1931). The paleomagnetism of these sediments, previously described by Barendregt and Mahaney (1988), is known to consist of younger Brunhes-age loess overlying reversed sediments emplaced in the Matuyama Chron (> 730 000 yr).

The texture (Table I) of these sediments ranges from loamy sand and sandy loam for the till to silt loam and loam for the loess. Pebble size clasts in the till consist of stones of basaltic and phonolitic composition suggesting that pre-Pleniglacial ice was eroding rock from the volcanic plug of the mountain. Because the loessic component of this section has normal magnet ism (relative to reversed magnetism in the underlying till) (Barendregt and Mahaney, 1988) we consider it to be considerably younger (middle to Upper Brunhes Chron). As a result of these age discrepancies we expect higher Feo/Fe d ratios in the loess.

In lower Teleki Valley at ~ 2950 m a sequence of paleosols in slope wash (mainly reworked loess), loess and transported regolith provides a setting in which an older paleosol formed in till (Unit IV) of the Naro Moru Glaciation (- 0.5 my) (see Mahaney,

Barendregt and Vortisch, 1989). Overlying loesses (unit III) probably predate the last glaciation (Liki = Wisconsinan). Above the loess unit, sandy colluvium (II) and slope wash (I) deposits belong to the late Wisconsinan (Liki II stage; Mahaney, 1989).

Particle Size As shown in Table i there are some age-

dependent t rends in the particle size data. In the HOB 19 profile silt plus clay is highest in the loessic component (I) partly as a result of aeolian deposition plus weathering since deposition. The composition of cemented materials in the C hori- zons include mainly MnO 2 and Fe20 a (verified by X-ray microanalysis) forming around sand, granule, and pebble size clasts (Derbyshire and Mahaney, 1989, for a discussion of these clay coatings).

Profile TV61, a complex section composed of several paleosols (Figs. 2 and 3), begins with an old till (unit IV) that contains considerable quantities of large boulders and a matrix of fine-grained material with a high percentage of clay (Table i), The overlying loess (unit Ill) contains less clay and silt with high percentages of fine and very fine sand (250-63 ~rn). Overlying the loess a unit of sandy colluvium (II) is nearly devoid of clay (:4C controls on buried A horizons in this sequence suggest it formed rapidly and was buried within 2 to 3

Extractable Fe-AI in Quaternary paleosols on Mount Kenya, East Africa

Table I. Par t ic le size cha rac t e r i s t i c s for two paleosols in the M o u n t Kenya Afroalplne a n d b a m b o o forest zones

Sim

FIOBI~

FV61

Depth Horizon (cm)

Sand Silt Clay (2000-63 lain) (63-2 larn) (< 2 lain)

AI 0-13 31.2 B2h 13-23 23.8 B22t 23-39 24.7

IIClm 39-50 36.6 l[C2m 50-64 52.5 HC3m 64-I 15 34.5

A1 0-9 33.6 B2t 9-34 19.1 Clox 34-71 56.8 C2ox 71-88 66.8 C3ox 88-160 84.9

l l A l l b 160-191 92.8 IIAl2b 191-226 91.8 IIClb 226-238 90.7 IIC2b 238-286 85.2 IIC3b 286-306 85.2 IHAb 306-330 88.8 IllB21bt 330-350 75.8

ilIIB22bt 350-370 83.3 [IIB23bt 370-390 84.1 IVCloxb 390-430 30.9 IVC2oxb I 430-480 36.7

47.7 50.2 49.6 55.4 42.0 59.7

47.9 54.9 32.7 28.2 14.8 7.2 8.2 9.3

14.5 14.6 I!.1 7.0 7.9

12.9 33.2 39.9

21.1 26.0 25.7

8.0 5.5 5.8

18.5 26.0 10.5 5.0 0.3 0.0 0.0 0.0 0.2 0.2 0.1

17.2 8.8 3.0

35.9 23.4

741

thousand years), contrasting sharply with unit I (ground soil formed in slope wash). The ground soil (Tropudalf) is typical of soils formed in the timber- line zone (Hagenia woodland) on Mount Kenya. Considerable clay movement has occurred in the solum of the ground soil indicating sufficient trans- location resulting in the development of an argillic horizon. From the field and texture data it is apparent we might expect some rather large differences between extractable Fe and AI in older and younger stratigraphic units depending on the time available for weathering prior to burial.

Extractable Fe ~md AI As shown in tables 2 and 3 there are a few

important age-dependent t rends between the dif- ferent paleoso ls. In the HOB 19 profile organically complexed Fe and Al sequentially decline down- ward in the profile producing a sharp break at the B22t/IIClm horizon contact. Oxalate extractable (amorphous) Fe plus A1 are both noticeably higher in the younger loess as expected. Within the crystalline Fe and Al (Table 2) a sequential decline occurs from the surface downwards in the profile. Higher extractable Feo and Ald in the B21t/B22t complex results from a higher weathering intensity in the soft solum.

Fig. 2. Uni t s I {to b o t t o m of m a t t o c k handle) a n d II in the "I~/61 profile.

Fig. 3. Uni ts III (loess) a n d IV (note large bou lde r of porphyri t ic phonol i te in till] depos i ted d u r i n g the

Naro Mon~ Glacia t ion (early B r u n h e s Chron).

742 W.C. ~ Y and K. SANMUGAOAS

Table 2. Extractable Fe and A1 in two Qua t e rna ry Paleosols on Mount Kenya, Eas t Africa.

S ite

HOB19

TV61

Dep th Pyrophosphate Extractable Oxalat¢ Extractable Dithionite Extractable Horizon (cm) Fe(%) Alp(% Fe,(%) Al,(%) Fed(%) Ald(%)

A1 B21t B22t

IIClm IlC2m IIC3m

A1 B2t Clox C20x C30x

lIAllb llA12b llClb IIC2b llC3b

IIIAb [llB21bt IIIB22bt ll~23bt IVCloxb IVC2oxb

0-13 13-23 23-39 39-50 50-64 64-115

0-9 9 -34 34-71 71-88 88-160

160-199 199-226 226-238 238-286 286-306 306-330 330-350 350-370 370-390 390-430 430-480

1.84 1.39 2.19 1.62 2.28 1.82 0.09 0.40 0.08 0.42 0.09 0.40

2.53 1.22 1.02 1.85 3.55 2.12 2.37 2.00 1.67 1.86 0.30 1.16 0.30 1.15 0.19 0.83 0.12 0.74 0.23 0.88 0.79 0.94 1.57 2.59 1.15 1.38 0.47 0.83 0.20 0.84 0.23 1.05

2.66 1.44 3.29 1.78 3.82 2.25 1.16 1.43 0.96 ] .45 0.95 1.40

3.25 1.48 4.15 1.98 4.96 3.80 5.43 5.36 5.54 5.75 1.32 5.59 3.25 5.59 3.47 4.58 2.65 4.20 4.27 4.82 4.99 3.65 4.11 1.57 4.76 1.78 4.82 2.62 2.47 0.84 1.96 0.76

3.07 1.38 4.06 1.55 3.87 2.00 2.50 0.70 3.16 0.86 2.06 0.73

3.27 1.40 4.26 1.80 4.94 3.77 5.66 4.33 5.85 4.60 5.33 3.87 5.93 4.03 6.55 3.35 7.10 3.18 6.40 3.z14 6.20 3.20 6.52 1.70 5.91 1.60 5.90 1.37 4.20 0.84 4.05 0.58

Profile TV61 shows some interest ing t rends for pyrophosphate-ext rac table Fe and Al where higher values occur in the g round soil (unit I) and in the bur ied loess (unit Ill) a n d till (IV). Overall the da ta indicate tha t movement is possible wi th in the g round soft and wi th in the bur ied soil (unit Ill). A l m o s t no movement has occurred wi thin uni t II (probably the resul t of its in ters tadia l origin rela- tive to un i t s I, Ill, and IV, which are postglacial (1) and interglacial (m-IV) respectively). The t endency for oxalate extractable Fe and AI to first increase, t hen decrease, and later increase upward in the profile, is possibly the resul t of the t ime required for a m o r p h o u s Fe and Al to change to crystall ine forms, a n d to posi t ion wi th in the profile relative to weather ing intensi ty.

Dithionite-extractable Fe and Al dis t r ibut ions also show some interes t ing var ia t ions tha t are related to t ime, posit ion within the profile, and possibly paleoclimate. If the tills (unit II) in the HOB19 profile were expossd for a while prior to deposit ion of the loess, the da ta suggest a t endency for h igher Fe and Al format ion in the IIC2m hori- zon. In the TV61 profile the general increase in Fe and Al with depth indicates increasing age; the higher values a r o u n d the IIC3b/IIIAb contac t probablyrefect the development of Fe and Al oxides in an envi ronment s imilar to or wetter t h a n today.

Analysis of the Feo/Fe d and Fep + A l / F e d + Ald ratios with depth (Table 3) in both profiles showed

some interest ing t rends t ha t are useful as relative age incidators. In the HOB 19 profile, a sha rp break at the B22t / I IClm contac t showed higher values for Feo/F % in the younge r loess. This t rend argues for h igher a m o r p h o u s Fe in younger sed iments which is wha t one would expect if there was insufficient t ime to form crystalllrle Fe. According to the da ta for Fe + Al /Fe~+ AL no movement of .P Fe has occurred in t h ~ profile"despite its well- dra ined topographic position, a b u n d a n t rainfall and old age.

Using the ratios in profile TV61 it is easy to differentiate younger un i t s (I and II) from older units . The Fep+ A I J F e d + A1 d rat ios are part icularly impor tan t because they show downward move- men t in the older paleosol (unit Ill) formed in loess, which con t ras t s with the g round soft a t the surface where no downward movement occurred. The da ta indicate t ha t the p re sen t -day env i ronment is inca- pable of moving organical ly-bound Fe and Al in surface soft sy s t ems (even though the particle size da ta shows movement of clay size material). Hence, the paleoclimate which existed dur ing the forma- t ion of uni t Ill m u s t have been wet ter (and war- meg?) t h a n today (last interglacial).

Informat ion on extractable Fe and A1 at o ther tropical t imberl ine localities in Africa, South America and New Guinea is lacking making it impossible to make comparisons . Compar ing the da ta d i scussed here in with o ther s tud ies in the

Site

HOB19

TV61

Extractable Fe-AI in Quaternary paleosols on Mount Kenya, East Africa

Table 3. Feo/Fe d and ratios for Two Mount Kenya paleosols

Depth Horizon

(cm)

A1 0-13 B21t 13-23 B22t 23-39

llClm 39-50 IIC2m 50-64 llC3m 64-115

AI 0-34 B2t 9-34 CIox 34-71 C2ox 71-88 C3ox 88-160

IIA11b 160-199 IIA12b 199-226 IIC1b 226-238 IIC2b 238-286 IIC3b 286-306 HIAb 306-330 IIIB21bt 330-350 IIm22bt 350-370 IIm23bt 370-390 IVCloxb 390-430 IVC20xb 430-480

Fe d + A1 d

0.87 0.73 0.81 0.68 0.99 0.70 0.46 0.28 0.30 0.12 0.46 0.18

0.99 0.80 0.97 0.47 1.00 0.65 0.96 0.44 0.95 0.31 0.25 0.16 0.55 0.15 0.53 0.10 0.37 0.08 0.67 0.11 0.80 0.18 0.63 0.51 0.81 0.34 0.82 0.18 O.59 0.21 0.48 0.28

middle la t i tude alpine belt of the Rocky Mounta ins in Colorado (Mahaney and Fahey, 1976, 1988) and Wyoming (Mahaney, 1978; Mahaney and Spence, 1984) it appears t ha t somewha t grea ter Fep, Alp, Fe d a n d A1 d have formed in the tropical t imberl ine area. This is probably in response to different vegetat ion, somewha t greater age (for the tropical paleoso ls), pa ren t mater ia l s with greater total Fe (about 9- 11%) and overall wa rmer c l imates with d iurna l t empera tu re regimes. As in the middle- la t i tude alpine localities Feo/Fe d rat ios give impor- t an t informat ion on relative ages of paleoso l s and on the soil-forming environment .

C O N C L U S I O N S

Dist r ibut ions of extractable Fe a n d AI in surface (ground) softs and in bur ied pa l eoso l s on Mount Kenya indicate t ha t leaching, and hence soft- forming envi ronments , were different in the past . Fur ther , ana lys i s of Feo/Fe d d is t r ibut ions in the paleosols s tud ied shows that , in general, greater age can be inferred from deplet ion of Feo. Age difrerences be tween loess (Brunhes} and till (Matuyama) in one sect ion (HOB 19) de te rmined on p a l e o m a g n e t i c evidence, is clearly suppor ted by Feo/Fe d dis t r ibut ions . In the TV61 profile dif- ferences be tween an older paleosol formed in loess and till a n d a younge r profile formed in slope wash

743

clearly show t h a t the bur ied un i t formed u n d e r a more in tense leaching env i ronment (presumably dur ing the Naro Moru-Teleki Interglacial of early B r u n h e s age).

A c k n o w l e d g ~ t s - We thank the Natural Sciences and Engineering Research Council of Canada (Grant A9021 to WCM) for financial support. The laboratory analyses were completed in the Geomorphology and Pedology Laboratory at York Univ. and at the Pacific Science Centre (Energy Mines and Resources ) , Vancouver, B. C. Rick Boyer and Drew Clarke assisted with the laboratory work. Field work was carried out under permit from the Office of the President, Republic of Kenya.

R E F E R E N C E S

Barendregt, R. W. and Mahaney, W. C. 1988. Paleo- magnetism of selected Quaternary sediments on Mount Kenya, E. Africa, J. of Afrt Earth ScL 7(1), 219-225.

Birkeland, P. W. 1984. Soils and Geomorphology, Oxford, N .Y. 372 p.

Blume, H. P. and Schwertmann, U. 1969. Genetic evaluation of profile distribution of aluminum, iron and manganese oxides, Sot/ScL Soc. Amer. Proc. 33 , 438-444.

Bouyoucos, G. J. 1962. Hydrometer methcxt improved for making particle size analyses of soils, Agronomy J. 5 4 , 464-465.

Coffin, E. E. 1963. A method for the determination of free iron in soils and clays. CanadlanJ. of Soft ScL 43, 7-17.

Day, P., 1965. Particle fractlonation and particle size analysis. In: Methods of Sot~Analysis, (Ed.hy Black, C. A.), Madison, Wise., Amer. Soc. Agron. p. 545-567.

Derbyshire, E. and Mahaney, W. C. 1989. A preliminary scanning electron microscope study of five dlamlctons from Mount Kenya. In: Quaternary and Environmental Research on East African Mountains, (Ed.by Mahaney, W.C.) Balkema, Rotterdam, p. 155-164.

Dormarr, J. F. and Lutwick, L. E. 1983. Extractable Fe and AI as an indicator for buried soil horizons, Catena, I0, 163- 173.

Lutwick. L. F. and Dormarr, J. F. 1973. Fe status of Brunisolic and related soil profiles, Can. J. of Soil Sct, 53, 185-197.

Mahaney, W. C. 1986. Fnvironmental impact in the Afroalpine and subalpine belts of Mount Kenya, East Africa, Mountain Research and Development, 8(3), 247- 260.

Mahaney, W. C. 1989. Quaternary glacial geology of Mount Kenya, In W. C. Mahaney, ed., Quaternary and Environmental Research on East African Mountains, Balkema, Rotterdam, 121- 140.

Mahaney, W. C. and Fahey, B. D. 1976. Quaternary Soil Stratigraphy of the Front Range Colo., In: Quaternary Stratigraphy ofNorthAmerica, (Ed. by W. C. Mahaney), Dowden. Hutchinson and Ross, Stroudsburg, Pa., 319-352.

Mahaney, W. C. and Spence, J. R. 1984. Glacial and periglacial sequence and floristics in Jaw Cirque, Central Teton Range, western Wyoming, Amer. Jour. ScL 284, 1056- 1081.

744 W. C. MAHA~V and K. SANMUGADA$

Mahaney, W. C. and Sanmugadas , K. 1986. Notes on the use of extractable iron and clay minerals for determination of soil age. GeograftskTidssk'rift 8B, 14- 20.

Mahaney, W. C. and Fahey, B. D. 1988. Extractable Fe and AI in late Pleistocene and Holocene paleosols on Niwot Ridge, Colorado Front Range, Catena, IB, 17- 26.

Mahaney, W. C. Barendregt, R. W. and Vortisch, W. B. 1989. Quaternary glaciations and paleoclimate of Mount Kenya, Eas t Africa, In: Glacier Fluctuations and Climatic Change (Ed. by Oedemans H.), Kluwer, Dordrecht, 13-35.

McKeague, A. and Day, J . 1966. Dithionite- and oxalate

extractable Fe and AI as aids in differentiating various classes of soils, Can. J. SoiISc£ 46, 13-22.

Nilsson, E. 193 I. Traces of ancient changes of climate in East Africa, Geografiska Annaler 12, 1-21.

Pawluk, S. 1978. The pedogenic profile in the stratigra- phic section, in W. C. Mahaney, ed., Quate rnary Soils, Geo-Abstracts, Norwich, U.K., 61-75.

Ruhe, R. V. 1965. Quaternary paleopedology. In: The Quaternary of the U.S., (Ed by H. E. Wright, Jr . and Frey, D. G.) Princeton Univ. Press, Princeton, 755-764.

Soft Survey Staff 1951. Soil Survey Manual, U.S.D.A., Washington,

Soft Survey Staff 1975. Soil Taxonomy, Ag. Handbook 436, U. S. D. A., Washington.