Behavior of Sm and Nd in a lateritic soil profile

logy,

Pergamon

Geochimica et Cosmochimica Acta, Vol. 68, No. 9, pp. 2043–2054, 2004Copyright © 2004 Elsevier Ltd

Printed in the USA. All rights reserved0016-7037/04 $30.00� .00

doi:10.1016/j.gca.2003.10.034

Behavior of Sm and Nd in a lateritic soil profile

JEROME VIERS1,2,* and G. J. WASSERBURG

1

1The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of TechnoPasadena, CA 91125, USA

2Laboratoire d’e´tude des Me´canismes de Transfert en Ge´ologie, UMR 5563 (CNRS/IRD/UPS), Equipe “Eaux-Sol-Erosion”,38 rue des 36 Ponts, 31400 Toulouse, France

(Received March 14, 2003;accepted in revised form October 14, 2004)

Abstract—A study of lateritic soils and samples of ground and river waters was carried out in theNsimi-Zoetele, a tropical watershed in the southern Cameroon. The Nd isotopic compositions and concen-trations of Nd and Sm were determined. It was found that the Nd isotopic composition of the river waters wasmuch more radiogenic than the parent rocks, and that the Nd in the waters is not homogeneous but is carriedby different dissolved and complexed components that are not isotopically homogenized. The soil profileshows a regular increase in�Nd going from the parent rock (�Nd � �36) to �Nd � �18 near the top of theprofile. The Nd transported in the river is thus not representative of the parent rock but reflects the results ofdifferential weathering of constituent minerals and the redeposition of REE in phosphates and a significantcontribution of radiogenic Nd from dust. The concentration of Nd in the river water is far above that foundin temperate climate rivers and thus this type of tropical river may play a dominant role in the marine Nd andREE budget. It is suggested that the correlation of REE with DOC is related to DOC fixing some dissolvedREE but that the REE in solution is governed by other mechanisms. No major shifts were found in Sm/Nd;however, a regular progression from the parent rock through the lateritic profile was found. The upper lateriteprofile shows large, almost uniform depletions in all REE below Tb and enrichment above. Complementarybehavior was found in the lower part of the section. The concentration of Nd relative to the immobile elementsZr and Ti in the laterite is depleted by a factor of�10. Th, Nd and Sm are enriched in the lowest zone sampledand must reflect redeposition of REE from the upper part of the weathering section and is associated withphosphate formation. It is concluded that the soil evolution involves both differential dissolution of primaryphases from the parent rock, significant to major input of REE from atmospheric dust from other regions, and
the formation of diagenetic phases, particularly phosphates.Copyright © 2004 Elsevier Ltd
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1. INTRODUCTION

The purpose of this study was to investigate the behaviNd and Sm under tropical weathering conditions in latesoils and in the associated waters. Precise isotopic mements of Sm-Nd were developed and used to study the gand evolution of the continental crust (De Paolo and Wasseburg, 1976; McCulloch and Wasserburg, 1978; Alle`gre andBen Othman, 1980). The Sm-Nd isotopic system was laapplied to the suspended matter from major rivers andments to study the evolution of the continental crust (O’Nionset al., 1983; Goldstein et al., 1984; Goldstein and Jaco1988). The study of Nd isotopes in rivers and the oceansused as a tracer to understand the geochemistry of theEarth Elements (REE), the different sources of REE inoceans, and mixing processes of water masses in oceabetween oceans (Piepgras et al., 1979; Piepgras and Wasburg, 1980, 1982, 1987; Stordal and Wasserburg, 1986; Gstein and Jacobsen, 1987; Tachikawa et al., 1999). Howeverthe Nd and Sm transport by rivers into the oceans is not sia reflection of the bulk rock chemical and isotopic composof the source but depends on the differential weatheringacteristics of the constituent minerals. In addition, the appweathering product of a rock may also involve input froutside sources (e.g., dust).

* Author to whom correspondence should be addre
([email protected]).
2043

Use of the Sm-Nd system to study rock weathering proceand REE mobilization in soils is more recent (McDaniel et al.1994; McFarlane et al., 1994; Bock et al., 1994; BorgBanner, 1996; Tricca et al., 1999; O¨ hlander et al., 2000; Aubeet al., 2001; Andersson et al., 2001). The REE were firsconsidered to be immobile or unfractionated during weatheprocesses. Since the pioneering work ofNesbitt (1979)severastudies have demonstrated that REE could be both strmobilized and fractionated during low temperature weathe(Duddy, 1980; Smedley, 1991; Nesbitt and Markovics, 1Braun et al., 1998). However, the mechanisms that conthese processes in soils are still not understood. This is mdue to a lack of studies that consider present day interacbetween soil/rock and surface waters (with very low Rconcentrations).

This study reports on the concentration and isotopic comsition of Sm and Nd in both soil/rock and surface wacollected from a small granitic watershed located in tropenvironment. Firstly this work aims to improve our knowleof the REE behavior in laterite formations and secondly tothis knowledge to constrain the formation of thick latercovers, a topic that is still a subject of strong debate. Latecover more than one-third of the exposed continental suand occur extensively in humid tropical regions. Understanof laterites is important in evaluating the fluxes of elemfrom the continents into the oceans considering that the tr
supply�50% of the annual water input to the oceans.

2044 J. Viers and G. J. Wasserburg

2. THE NSIMI-ZOETELE EXPERIMENTAL WATERSHED

2.1. General Description

The Mengong watershed (Fig. 1), is located �120 km southof the city of Yaounde, southern Cameroon (3°10�N–11°58�E),200 km east of the Atlantic coast, and is covered by a tropicalrainforest. The geomorphology is characterized by 700 m highconvex rounded hills surrounding a flat swamp zone, whichrepresents �20% of the whole basin area (0.6 km2). Such ageomorphology is characteristic of the erosion surface of the“South Cameroon Plateau” (Tardy and Roquin, 1998). Theclimate is of equatorial type and marked by four differentseasons, including two three-month wet seasons (from Septem-ber to November and from April to June), a four-month dryseason (from December to March) and a short dry season fromJuly to August. The mean annual rainfall is 1751 � 143 mmand the mean annual air temperature is 24°C � 1 (Olivry,1986).

2.2. The Parent-Rock and the Soil System

The watershed is underlain by lateritic formations, the max-

Fig. 1. A) Location of the Nsimi-Zoetele watershed, Bwith the localities of the soil/rock and water sampling poinare MEN1 (spring), 5L6 pit (hill zone soil profile), and S

imum thickness of 40 m is attained at hill tops. This laterite

cover was determined to have started to evolve in the Miocene(Tardy and Roquin, 1998). This pedological cover developedon a bedrock which is a mixture of two granitoid parent rocksoriginating from late Archean crustal evolution in the CongoCraton (calco-alkaline differentiation) (Tchameni, 1997).These rocks are a gray blue-colored granodiorite, fine to me-dium grained, and a leucocratic monzogranite, that is mediumto coarse grained. The granodiorite was sampled in the hill zone(close to the pit 5L6) while the monzogranite was sampled inthe swamp zone (close to the SZ pit) (see Fig. 1). Theirmineralogical compositions (major and accessory minerals) aregiven in Table 1. The proportions of these two rocks in thesubsurface are difficult to estimate because of the thick soilcover. The parent rock appearance at depths of less than 3 mfrom the surface is only 0.45% of the whole catchment area(Robain, pers. comm.).

There are two types of soils present in the watershed (Olivaet al., 1999): a) typical hill slope soil (5L6-soil profile), devel-oped in situ; and b) typical swamp zone soil (SZ-soil profile)composed of both allochtonous and autochtonous materials(Fig. 2A and 2B). The major minerals of these soils are pri-mary resistant minerals (i.e., quartz, and to a lesser extent

e-dimensional representation of the watershed (0.6 km2)circles are springs. Fulled circles are pits. Sampling sites

wamp zone soil profile).

) A threts. OpenZ pit (s

K-feldspar), and iron- and aluminum-rich weathering products

2045Behavior of Sm and Nd in a lateritic soil profile

(i.e., kaolinite, goethite, and hematite). Minor minerals aremainly Fe-oxides, Ti-oxides and zircon. The respective abun-dances of these minerals in the two typical soil profiles (hillslope and swamp zone) are shown in Figure 3.

2.3. The Sampling Points and the Hydrographic System(see Fig. 1)

At the basin head, several springs give rise to a small brook�1 km long flowing through the swamp zone. They are theheadwaters of the Mengong River. As one proceeds down-stream of the study area there are more inputs and the Mengonggreatly increases in size and flow. The Mengong River is asmall tributary of a larger hydrographic system dominated bythe Nyong River. One of the main springs (MEN1) and thebrook were collected monthly during a period of two years(December 1994 to December 1998). Spring waters are organicmatter-poor whereas brook waters contain large amounts oforganic matter (Viers et al., 1997). Ground waters were alsosampled using a network of pits with adjacent piezometers inthe hill and swamp zones. One type of ground water is organicmatter-rich and corresponds to the ground waters flooding theswamp zone, and is close to the brook. The other is organicmatter-poor and corresponds to the ground waters flooding thehill soils and reached in some pits of the toposequence 6 (e.g.,pit 5L6). Chemistry of these waters is extensively discussedelsewhere (Viers et al., 1997; Oliva et al., 1999; Dupre et al.,1999).

Annual average discharge of the Mengong River is �10 L/s(Ndam Ngoupayou, 1997). All through the year, the Mengongbase flow is supported by springs, with a mean annual dis-charge that represents �20% of the total outlet discharge(Ndam Ngoupayou, 1997). The mean discharge of the springsis less variable than that of the Mengong. During several daysof the driest months (February, March) the spring dischargemay be higher than that of Mengong. The daily Mengong waterdischarge is characterized by strong variability related to therain events. When there is a rain event there is a direct increasein the Mengong water discharge due to the permanent satura-tion of the swamp zone. The hydromorphic soils of the swampzone constitute a pool of water, which can be quickly mobilized

Table 1. Minera

Granodiorite

Major minerals Oligoclase (An25)QuartzK-feldsparHornblendeTi-rich biotite

Accessories MagnetiteTitanomagnetiteRutileAugiteApatiteZirconMonazite

(“fl ushed” ) during rain seasons (or rain events).

3. MATERIAL AND METHODS

3.1. Rock and Soil Sampling

Samples of parent rocks (1 to 4 kg) were collected from drill coreslocated in the hill zone (granodiorite) and from outcrops located in thebed brook (monzogranite). Besides bulk soil samples of �2 kg werecollected from drill cores or pedological pits. These samples werecollected in soft plastic boxes to preserve the soil structure. Theporosity of the soil, close to 0.50 in all the soil horizons, has beencalculated using the bulk density (obtained by the paraffin method) andthe grain density (obtained by the air picnometer method) (Braun, pers.comm.).

3.2. Water Sampling

Water samples (collected in 1994, 1995 and 1996) are representativesamples of a complete database whose samples have been collectedmonthly from October 1994 to December 1998. The different samplinglocations are shown in Figure 1. The water was collected in 10Lacid-washed polypropylene containers, and immediately filtered at thesite through a 0.22 �m membrane (Millipore, ester cellulose) using aSartorius Teflon frontal filtration unit (142 mm diameter). Groundwa-ters were collected at the bottom of the pit after performing twosuccessive draining/recharge stages with a motor pump. Some sampleswere ultrafiltered with a Sartorius tangential filtration unit (Sartoconmini). All procedures (sampling, filtration, and storage) are described inreferences (Viers et al., 1997, 2000) and were carried out under thecleanest possible conditions. “Blank” tests were performed to controlthe level of contamination introduced by our experimental protocols. Itwas found that Sm and Nd concentrations measured in the total blankwere below the ICP-MS detection limit (�1 ng L�1).

3.3. Analytical Methods

Nd and Sm concentrations and isotopic compositions were measuredon the Lunatic I thermal ionization mass spectrometer. The measure-ments for the water samples were performed following the iron hy-droxide precipitation and ion exchange separation techniques. (SeePapanastassiou et al., 1977; Piepgras and Wasserburg, 1980, 1982).Samples were spiked with 150Nd and 147Sm before the REE precipita-tion on iron hydroxides. After crushing, pulverization, and homogeni-zation, rock and soil samples were dissolved in HF-HNO3-HClO4 inTeflon beakers and the total solution spiked. Mass fractionation wascorrected using the 146Nd/142Nd and 148Sm/154Sm ratios with a dis-crimination factor calculated with an exponential law (see Wasserburget al., 1981). Nd and Sm concentrations are measured with an accuracyof �0.1% and the reported errors in the isotopic composition are 2� for�200 ratios. Uncertainties in the 147Sm/144Nd ratio are around 1‰.The measured 143Nd/144Nd ratios are presented as fractional deviationin parts in 104 (�-units) from 143Nd/144Nd in a Chondritic Uniform

ances in rocks.

wt% Monzogranite wt%

50 Albite (Ab95) 3419 K-feldspar 332 Quartz 29

24 Ti-rich biotite 22 —ND Magnetite 0.53 Epidote NDND Zircon 0.01ND Ilmenite ND0.7 Titanite ND0.02 —ND —

l abund

Reservoir (CHUR) as measured today:

corres


�Nd(0) � [(143Nd/144Nd)S/ICHUR(0) � 1] � 104

where (143Nd/144Nd)S is the ratio measured in the sample today andICHUR(0) is the 143Nd/144Nd in the CHUR reference reservoir today

Fig. 2. Distribution of the different soil horizons in a tyswamp zone soil profile (SZ-soil profile) (B). The numberswhere samples were taken. The depths of the horizons bounabundances of aluminum (Al2O3), iron (Fe2O3) and silicathat SiO2 is given as (SiO2)/2. The vertical lines give the

(ICHUR(0)�0.511847; Wasserburg et al., 1981). Using the following

relation (�Nd(0) � Q fSm/Nd T) (De Paolo and Wasserburg, 1976), it ispossible to calculate a model age, TCHUR in aeons. Q is a constant(�25.13 Æ�1, Jacobsen and Wasserburg, 1984) and fSm/Nd is theenrichment factor, which gives the Sm-Nd fractionation in the sample

ill zone soil profile (5L6-soil profile) (A) and in a typicalin the boxes (ex: 250) represent the depth from the surfaceare indicated. For each soil profile is reported the chemical

as a function of depth (data in Oliva et al., 1999). Noteponding values for the bedrocks.

pical hshowndaries(SiO2)

related to the bulk earth.


fSm/Nd � 147Sm/144Nd)S/(147Sm/144Nd)CHUR � 1]

where (147Sm/144Nd)S is the ratio in the sample and (147Sm/144Nd)CHUR

the ratio of the chondritic Uniform Reservoir (147Sm/144NdCHUR �0.1967; Jacobsen and Wasserburg, 1984).

4. RESULTS

4.1. Water Samples

Waters were collected from the sampling site of the incipientMengong River (see Fig. 1) during dry and wet seasons. Two

Fig. 3. Respective abundances (in %) of major and accessory min-erals in the typical hill slope (5L6) and swamp zone (SZ) soil profiles.

Table 2. Neodym

Date of sampling Sample typeSamplename

Filtrationsize

9/1/1996 dry season River water Mengong 0.20 �m8/10/1996 wet season River water Mengong 0.20 �m8/10/1996 wet season River water Mengong 5,000 Da5/18/1995 wet season River water Mengong 300,000 Da12/13/1994 dry season Ground water 5L6 pit 0.20 �m(5L6 soil profile) Soils — —

Parent rock — —

river water samples (10/08/96, 5/18/95) were collected in a wetseason. One of these samples (10/08/96) was successivelyfiltered through a 0.20 �m and a 5000 Da pore size membranes.The second one (5/18/95) was filtered through a 300000 Dapore size membrane. One river water sample (1/9/96) was takenin a dry season and filtered through a 0.20 �m pore sizemembrane. Ground water was sampled in the pit 5L6 at 6mdepth which is 15m above the granodiorite contact. The Ndconcentration (CNd) and �Nd(0) results for these samples areshown in Table 2. In addition we give values for the bedrockand a range of values for the soils of the 5L6 profile.

Samples (riverwater, groundwater) from similar locationshave already been analyzed in other studies (Viers et al., 1997;Oliva et al., 1999; Viers et al., 2000). Particularly, Viers et al.(2000) reported a complete time series for the Mengong River.The concentration range of the samples considered in thisstudy is in good agreement with the seasonal variations re-ported in other studies, and thus can be used to constrain thehydrological and geochemical processes occurring in thisdrainage basin.

4.1.1. The “dissolved” phase (i.e., �0.20 �m)

The value for the �0.20 �m river sample for the dry seasonis 184 ng/L while the wet season gives a higher value (273ng/L). In the literature there are reports on a large span in Ndconcentration within and between river systems (3–3150 ng/L,Goldstein and Jacobsen, 1987). Also, large temporal variationsare reported for some rivers such as boreal rivers (Andersson etal., 2001) with both high and low concentrations of Nd (30–300 ng/L). However, the Nd concentrations of the rivers con-sidered in this study belong to the higher part of the concen-tration range reported for rivers filtrated through the same poresize membrane (Goldstein and Jacobsen, 1987; Sholkovitz,1995; Andersson et al., 2001). The values of �Nd(0) for the�0.20 �m river waters (�18.73; �20.97) are close to thevalues in the upper part of the laterite (from �18.30 to �20.75)but are far greater than that of the granodiorite parent rock(�36.18) (see Table 2). The ground water in the dry season hashigh CNd (338 ng/L) and �Nd(0) that is more negative (�23.56)than that of the river samples. As will be shown later this is thesame �Nd(0) as soil sample from the same depth. It is evidentthat the Nd being carried away by the river is not at allrepresentative of the parent rock but is close to the net weath-ering material present in the laterite. The Sm/Nd in the water ishowever rather close to that of the parent rock.

water samples.

DOCppm

CNd

ppt 143Nd/144Nd �Nd(0)Sm/Nd

(ICP-MS)

11.44 184 0.510774 � 23 �20.97 � 0.45 0.11017.92 273 0.510888 � 22 �18.73 � 0.44 0.123

4.59 113 0.511166 � 23 �13.30 � 0.46 0.1107.71 125 0.511117 � 19 �14.26 � 0.38 0.125�dl 338 0.510641 � 28 �23.56 � 0.54 0.117— — — �26.50/�20.26 0.093/0.104— — — �36.18 0.100

ium in

pH

5.955.605.604.624.38——

20 � 3


4.1.2. The “ultrafiltrated” phase (i.e., �300000 Da and�5000 Da)

We now consider the waters filtrated through smaller cut offsize membranes (i.e., 5000 Da and 300000 Da). Inspection ofthe data in Table 2 shows that both the concentration of Nd andthe isotopic composition (around �14) are the same for the twosmallest filtration sizes. The �5000 Da filtrate is what isreasonably considered to be in solution (“ truly dissolved”phase). The terms “ truly dissolved” or “ in solution” are used toindicate that the elements present in the �5000 Da filtrate arepresent as free ions or associated with ligands (inorganic ororganic) of very low molecular weight. Considering the 10/8/96river sample, it appears that the “bulk” water (�0.20 �m) hasa higher concentration and a substantially lower �Nd(0). Itfollows that the total load of Nd in the water is comprised of atleast two different sources. Taking one of the components to bethe “ truly dissolved” load and the second to be “ground water”we calculate that 2/3 of the load is associated with largecolloids (higher 5000 Da). There is thus a considerable isotopicheterogeneity in the Nd within the bulk river load.

4.1.3. Nd sources in bulk river load

In the previous paragraph, we saw that a variety of distinctsources are responsible for the bulk Nd inventory in the riverwater. This must be due to differential weathering of theoriginal bedrock minerals with different �Nd values and topossible input from dust. It is well known in several tropicalweathering areas that dust input plays a major role for variouselements and for example Sr (Brimhall et al., 1988; Chadwicket al., 1999). We will show that this is also the case for Nd inthis area. The average concentration of Nd in rainwater in thiswatershed has been determined to be 39 ng/L using a timeseries of twenty samples (Freydier, 1997). The total annualprecipitation is 180 cm3/cm2 yr. The area of the watershed isapproximately 1 km2 so the net input of water is 60 L/s for thewatershed and with a net Nd input from the rain of 2400 ng ofNd/s. The net water outflow of the Mengong River consideringevapo-transpiration losses (of �120 cm3/cm2 yr) is found to be10 L/s. Thus, the dust, if dissolved, could provide 240 ng/L ofNd in the outflow at steady state. It can be seen that thisconcentration level is essentially that measured in the filteredriver water (�0.20 �m). Studies by Goldstein et al. (1984) andGrousset et al. (1988) of aerosol dust from N. Africa show �Nd

� �10 to �13. The typical Nd concentrations in the dust are

Table 3. Neodymium and S

Soil profile Sample nameDepth(cm)

CNd

ppm 143Nd

5L6 (hill zone) 50 50 14.78 0.5108150 150 16.02 0.5109250 250 16.61 0.5107400 Clay matrix 400 19.11 0.5105400 Iron nodules 400 12.40 0.5105650 650 75.77 0.5105675 675 137.36 0.5104granodiorite 1500 49.72 0.5099

SZ (swamp zone) 275 275 23.68 0.5098monzogranite 450 4.00 0.5096

� 30 ppm with fSm/Nd � �0.4. It follows that some of the very

low �Nd values in the river water, particularly in the fine andultra-fine fractions, can plausibly be attributed to the dissolu-tion of infalling Sahara dust. Of the net Nd removed from thewatershed, �50% may be due to the dissolution of dust con-sidering the isotopic variability in the waters and the Nd in thebedrock. It follows that the Nd budget (as well as all the REE)found for the river is complex and can not simply be attributedto differential weathering of the bedrock (see Section 4.4).

4.2. Soil and Rock Samples

The isotopic composition and the concentration of Nd andSm are given in Table 3. A detailed study was made on the 5L6profile on samples ranging from the underlying granodiorite inthe pit upward through the overlying zones which exhibitincreasing degrees of weathering, and alteration into a maturesoil profile. Only two samples were measured at the swampzone site (SZ profile). A complete database (major and traceelements) for the 5L6 profile has been presented by Oliva et al.(1999). The bulk samples of local parent rock and soils exhibitregular behavior. The granodiorite has fSm/Nd � �0.5 which isclose to the average crustal ratio. The granodiorite has �Nd(0) ��36.18 and a model age TCHUR� 2.81 Æ. This age of “crustalextraction” is in good agreement with U-Pb ages determined onseparated zircons from the same formation (but from a differentlocality) by Toteu et al. (1994). As the sampling progressesupward in the section and comparing the granodiorite and thefirst soil sample (e.g., 675) we see there is a large increase inCNd and CSm (factor 2.8) but with Nd/Sm unchanged. Thevalue of �Nd(0) shifts drastically from �36.18 in the parentrock to �26.5. Progressing further up in the section the con-centration decreases markedly to a roughly constant value(around 15 ppm) and the �Nd(0) increases to a roughly constantvalue of � �20. The values of fSm/Nd do not change in anymarked fashion throughout the section. The model ages de-crease from TCHUR� 2.81 to 1.5 Æ. For the swamp zoneprofile, there were only two samples: the monzogranite fromthe bottom of the pit and one soil sample. The monzogranitehas TCHUR� 2.83 Æ and a somewhat low value of fSm/Nd. Theconcentrations of Sm and Nd are much lower than the grano-diorite. There is a sharp increase in CNd in going from themonzogranite to the soil and an increase in �Nd(0) (from�43.50 to �38.59). The value of the fSm/Nd does not change.There are general similarities in the two profiles. We will focuson the 5L6 profile. As indicated in Figure 2A the ferruginous

in soil and rock samples.

�Nd(0)CSm

ppm 147Sm/144Nd fSm/Nd

TCHURNd

AE

0 �20.26 � 0.39 2.44 0.096 �0.512 1.574 �18.30 � 0.50 2.61 0.099 �0.499 1.468 �20.75 � 0.55 2.46 0.090 �0.545 1.522 �25.60 � 0.42 2.22 0.070 �0.643 1.582 �25.93 � 0.42 1.92 0.093 �0.525 1.969 �26.08 � 0.28 11.19 0.089 �0.546 1.904 �26.50 � 0.50 22.78 0.100 �0.490 2.150 �36.18 � 0.58 7.90 0.096 �0.512 2.814 �38.59 � 0.46 3.08 0.079 �0.601 2.561 �43.50 � 0.60 0.50 0.076 �0.612 2.83

amerium

/144Nd

10 � 210 � 285 � 236 � 220 � 212 � 191 � 295 � 372 � 2

horizon is heterogeneous in terms of lithology. It contains clay


matrix surrounding iron hydroxide nodules. To ascertain therole of iron hydroxides relative to clays in fixing REE a sampleof clay and the attached iron nodules were separated andmeasured. The concentrations are not greatly different with theclay having a significantly higher value. The values of �Nd(0)are indistinguishable. There is an enhancement of Sm/Nd in theiron nodules relative to the clays. In summary, the generalbehavior of �Nd(0), fSm/Nd, and CNd are shown in Figure 4. Itcan be seen that �Nd goes from the bedrock value to anintermediate higher value (dashed arrow) and then increasesfurther to much more radiogenic values in the top 300 cm. Theconcentration of Nd (and Sm) concurrently increases to valuesfar above the parent rock and then drops rapidly to values farbelow the parent rock (full arrows). The Sm/Nd ratio does notundergo any major change. No data are available between 675cm and the bedrock.

4.3. Nd in the Waters: Relation between REE andOrganic Matter

In considering the matter of aqueous transport of Nd, it isnecessary to consider the role of organics. In Figure 5 we showboth the concentrations of Nd and dissolved organic carbon(DOC) in river waters for the Mengong River (Viers et al.,1997), the Awout River (Dupre et al., 1999) and the presentstudy. The different points correspond to a cut off size filtrationbetween 0.20 �m and 5000 Da. As shown by previous workers,there is a strong correlation of CNd with dissolved organiccarbon suggesting a main role of colloids in the transport ofboth Nd (as well as all REE) and organic matter. This suggeststhat Nd attached to surface complex is liberated for transport bythe presence of DOC. While most of our results are in strongsupport of this rule, we note that the ground water extractedfrom pit 5L6 has no detectable DOC but has a rather elevatedCNd comparably to the river waters. This is in conflict with theassumption that DOC is the species governing CNd. The liber-ation and aqueous transport mechanism for Nd and the otherREE are thus not clear. It is possible that the DOC fixes REEthat are already in solution and are not the solubilizing agent.

4.4. General Considerations about Soils: REE Depletionand Enrichment

The behavior of Nd relative to other elements in the soil that

Fig. 4. Evolution of Nd concentration, isotopic composition (�Nd(0)),and fSm/Nd ratio in the 5L6 soil profile.

are often considered “ immobile” (Brimhall and Dietrich, 1987;

Brimhall et al., 1991, 1992, 1994; Colin et al., 1992) is shownin Figure 6 where Nd/Ti, Nd/Zr and Nd/Th are plotted. Fol-lowing these workers, the dilation factor (�) can be determinedfor a soil sample if some element I is conservative. CI(X) atsome position divided by the initial concentration in the parentrock determines the dilation:

CIX�/CI0 � �0/� x��

C is the concentration of the element I (in atoms per gm) and� is the density. For all immobile species this should then be thesame at a given depths for all conserved species. For an elementE that is not conserved, then the fraction of E remaining is

fE �CEX�/CIX�

CE0/CI

0�

The measurement of Ti, Zr and Th concentrations in the solutionspercolating through the soil column confirms the immobility (atleast at the present time) of these elements (Oliva et al., 1999). Aremarkably regularly pattern is shown for Nd with high values (farabove the parent rock) at the lowest level sampled and a regulardecrease following an apparently hyperbolic curve with depth.Examination of Figure 6 reveals that the Nd relative to the “ im-mobile” element is depleted, relative to parent rock (granodiorite)ratio, in the upper horizons (both ferruginous and soft clayeyhorizons) but enriched in the lower horizon (saprolitic horizon).For illustration, a curve as a function of depth in the form of

f � A B

C � X

was fitted (A � �4.2�10�2; B � �755; C � �725) and isshown for reference in the Figure 6. Careful examination of

Fig. 5. Nd versus DOC relationship during ultrafiltration experimentsperformed on the Mengong River water samples. 1: �0.20 �m 10/08/96 Mengong River, 2: �0.20 �m 1/9/96 Mengong River, 3: �300kDa 5/18/95 Mengong River, 4: �5 kDa 10/8/96 Mengong River. Thepoint relative to the ground water collected in the pit 5L6 is reported.Previous ultrafiltration experiments done by Viers et al. (1997) on theMengong River and By Dupre et al. (1999) on a similar organic-rich
river are also reported.


Figure 6 reveals that Zr and Ti behave similarly and differentlyfrom Th which seems to track Nd, being enriched at the lowestlevels (see Fig. 7). The ratios of Zr, Ti and Th in the soilsamples divided by the respective concentrations in the parentrock are shown in Figure 7. The ratios for Zr and Ti decreaseto value close to one with depth while the ratio for Th remainshigh even at 675cm depth. It seems that Th like Nd is enrichedin the 675cm zone. A graph similar to Figure 6 is shown inFigure 8 for the phosphate (P2O5). We can see that all along theupper part of the soil profile there is a major depletion of P2O5

relative to Ti. Note the wide scatter of data for P2O5 in thezones corresponding to the iron crust and mottled induratedhorizon but again with substantial enrichment (but below fP2

O5

� 1) at the 675 cm depth. Thus there appears to be a general butrough correlation of fNd with fP2

O5.A basic question is the nature of the carrier phases of Nd.

Analyses of strontium (Sr), barium (Ba), lanthanum (La) andphosphate (P2O5) were carried out at three sampling depths.The results are shown in Figure 9. It is seen that Sr, La and Baappear to decrease regularly with decreasing P2O5 starting withhigh values at the deeper soil sampling depth. A line is shownas a guide in the figure. It appears that the high REE concen-trations are associated with phosphates in the weathered zoneabove the parent rock and that there is a decrease in REE alongwith phosphate loss. Note that the samples from the top profileappear to indicate that Sr, La, and Ba begin to approach zeroconcentration at P2O5 � 0.05% so that at the very low P2O5

content near the surface another carrier phase appears to con-tain some of the REE. In general, we infer that phosphates arethe principal phases governing the concentration of REE andNd in particular.

Concentrations of all the REE relative to the abundances in

Fig. 6. Nd over Ti, Th and Zr ratio in the 5L6-soil profile normalizedto the same ratio in the parent rock (granodiorite). The vertical straightline represents the hypothetical immobility of Nd relative to these threeelements.

the parent rock are shown in Figure 10 for soil samples ranging

from 675 cm to the surface. No sampling was done between675 cm and the bedrock at 1500 cm depth. It can be seen thatthe lower sample (675) has a very high and approximatelyconstant enrichment factor of all the REE below Ho. Above Hothe enrichment factor decreases regularly and becomes a de-pletion for Yb and Lu. The value of �Nd(0) is �26 for 675cmdepth. The same behavior is exhibited at 650cm depth but witha substantially lower enrichment factor. The heaviest REE (Tm,Yb, Lu) show increasing degrees of depletion. For all samplesfrom shallower depths (250-0 cm) the pattern is reversed. All ofthe light REE (from La to Gd) are approximately uniformlydepleted. Above Gd all of the heavier REE are regularly de-pleted to a lesser degree and Tm, Yb and Lu are somewhatenriched. The �Nd(0) of these upper soils range from �18 to

Fig. 7. Zr, Ti and Th concentrations in the 5L6-soil profile normal-ized to the corresponding concentrations in the parent rock (granodio-rite).

Fig. 8. P2O5 over Ti ratio in the 5L6-soil profile normalized to the
same ratio in the parent rock (granodiorite).


�20. All of the soil samples have �Nd(0) far above that of theparent rock. In all of the samples, Sm/Nd ratio is close to thevalue of the parent rock.

5. DISCUSSION

5.1. REE Mobilization during Soil Formation

The results obtained appear to indicate the following char-acteristics for the soil evolution. The transition from parentrock (�Nd(0) � �38) to the first sampled stages of weathering(675 cm, �Nd(0) � �26) shows that some phases with the leastradiogenic Nd have been removed. There is a strong mecha-nism of concentration of REE in this lower zone far above thebedrock. This enhanced concentration is only for the lighterREE (below Ho). The heavier ones are less enriched or aredepleted. REE enrichment in this zone is clearly associatedwith the P2O5 content. Further up the section (650 cm sample),the same pattern appears but at a lower concentration. There arelittle relative changes in the proportions or isotopic composi-tion. Above this zone then there appears to be a rather sharptransition above which �Nd(0) shifts to an even higher valueand the concentrations become low. The upper section (0 to 250cm depth) shows a pattern that has an apparently uniform REEreservoir with �Nd(0) around �20 and a constant and relativelylow concentration. The REE pattern for this zone is the samefor all shallower samples. The REE pattern relative to theparent rock is essentially the complement of what was foundfor the enriched layer. We note that there is no Ce anomaly inany part of the section, which indicates that the REE mobili-zation took place under conditions where there was relativelylow fO2.

The inventory of REE in the constituent phases of the parentrock was not determined in this study. Extensive work byGromet and Silver (1983), Banfield and Eggleton (1989),Braun (1991), Braun et al. (1993, 1998) have shown that ingranodioritic rocks trace or minor phases (e.g., allanite, sphene,apatite, epidote, zircon) are the principle carriers of REE. The

Fig. 9. Relationship between Lanthanum (La), Strontium (Sr), andBarium (Ba) and phosphate (P2O5) concentrations in the 5L6-soilprofile.

major minerals biotite, amphibole and plagioclase are signifi-

cant carriers. The isotopic composition of Nd will be highlyvariable between these different primary minerals and is de-pendent on the Sm/Nd ratio. This is particularly true whenconsidering the 2.8 Æ age of the parent rock.

As for the principle phases in the parent rock we expect thefeldspar and amphibole (hornblende) to have low Sm/Nd andthe biotite to have high Sm/Nd with corresponding low andhigh �Nd(0), respectively. Feldspar and hornblende comprise 50and 24% of the total parent rock, respectively, so that they aresubstantial contributors of the REE in the total rock in spite ofthe low concentration levels. With regard to the accessoryminerals zircon would have high Sm/Nd. We can not estimatea priori what the other phosphates would have. Typically apa-tites are close to the whole rock in their REE pattern.

We must consider the bulk isotopic composition of thesamples to represent a mixture of residual primary phases andof secondary phases produced during the transformation fromrock to a soil layer. Feldspar and hornblende are most subjectto alteration to clays as is evidenced in the presence of claypseudomorphs of feldspar seen in the lower zone (saprolitelayer) of the weathering profile (Oliva et al., 1999). As thefeldspar has typically lower Sm/Nd than the total rock it is wellknown that this phase gives low �Nd(0) compared to the othermajor phases. Estimating fSm/Nd � �0.7 would give �Nd(0) ��49 for the feldspar. Removal of this abundant phase wouldthus grossly increase the value of �Nd(0) of the bulk system atthe early stages of weathering. Accessory minerals such asapatite may play a role in the same direction. Also, through thedepth of 650 cm and below there must be concomitant removalof unidentified primary phases greatly enriched in the heavyREE. This could be zircon as it has a very strong heavy REEenrichment as indicated above. This would require zircon dis-solution at the early stage in the weathering process withprecipitation of the Zr and removal of the REE in that phase.This is probably the first step in the soil evolution. It is true thatzircon is often the least soluble mineral in laterites; however,recent studies (Oliva et al., 1999; Hodson, 2002) have demon-strated that in certain conditions these minerals could bestrongly weathered. A detailed study of the trace phases mustbe done in this section to understand the actual role of thevarious mineral phases and their diagenesis as related to theREE patterns.

In the second step of evolution, we propose to try to explain

Fig. 10. Whole REE concentrations in the 5L6-soil profile samplesnormalized to the parent rock (granodiorite). The range of �Nd(0) val-ues for the soils and the granodiorite is given (see Table 2). Thenumbers in the legend indicate the sample depth in cm as in Figure 2A.
REE concentrations are reported in Oliva et al. (1999).


the REE (LREE and MREE) enrichment at the bottom profile(650, 675 samples) as observed today. When the weatheringfront progressively descends with time (i.e., the soil becomesdeeper), the enrichment of REE will increase in the bottom dueto depletion of REE in surface layers and with some REEdeposition at the very lower zone. We have seen the closeassociation between phosphate and REE and the depletion ofboth of these components in the upper part of the soil column(see Fig. 6 and 8). We propose that REE bearing phosphateminerals play a key role in the control of REE abundancesthrough the soil profile. In this model, secondary phases areformed with a uniform isotopic composition and with a REEchemical pattern that is generally the complement of what wasremoved at the earlier stages. We also note the enrichment ofHREE in the near surface layers. The regularly enriched heavyREE pattern (Tm and above) would be consistent with enrich-ment in zircon (using the pattern given by Gromet and Silver,1983) or some other HREE rich phase. The flat pattern mustreflect residual phases substantially more radiogenic than thetotal rock or else the deposition of secondary phase formed bydissolution of the primary minerals.

These observations are in support of the approach of Braunet al. (1998) who used a combined approach on soil/rock andwaters. They proposed a model to explain the mobilization ofREE in a lateritic cover developed on a gneissic basement(Goyoum site, Cameroon). At the weathering front, they pro-posed that there is a replacement of the REE-bearing primaryminerals (monazite, apatite, allanite) by REE-bearing phos-phates secondary minerals (crandallite group). With Al andP2O5, REE, Ba and Sr are the main constituents of the cran-dallite group minerals. This mineral family is known for itsability to control REE during low-temperature processes (Ban-field and Eggleton, 1989). While complete chemical-mineral-ogical modal abundances of the accessory minerals of theparent rock and the soils has not been done, we nonethelessconsider that some general rules have been obtained and spe-cific problems identified.

In the above discussion, we have assumed that the REE areonly the result of dissolution loss and transport downward inthe section due only to effects on the original parent rock.However, the input from dust was shown to be a very importantcontributor to the REE budget relating to the water transportfrom this watershed. The input of Nd from dust in the rain is2.4 � 10�7 ng/cm2sec or 7.4 ng/cm2 yr. A two meter section ofthe soil profile is �400 g/cm2 which at �15 ppm Nd containsa net amount of 6 � 106 ng/cm2 of Nd. It follows that the Ndinput from dust over 106 years is sufficient to dominate the Ndand REE budget in the soil. We note that Aubert et al. (2002)found atmospheric sources of both Sr and Nd in the chemistryof waters in the upper soil horizon of eastern France. It followsthat a real understanding of the isotopic and chemical system-atics for a system with strong external input from dust requiresa much more complete knowledge than is currently available. Itis possible that the very regular systematics reported here canbe better explained by considering dust input with subsequentweathering and downward transport concurrent with alterationand weathering of the bedrock, precipitation at depth and netremoval. It is event from the isotopic data on the river wateritself that there are fundamentally different contributions to Nd
transport going out of the system so that these waters must also
reflect the dust input and differential weathering in differentzones of the laterite section. The constancy of the relative REEabundances of the lighter and medium elements may reflect thisbalance of infall and rock input and outflow loss.

5.2. Nd and Sm in the Waters Draining the Catchment

The Nd isotopic composition of the Mengong River watersranges from �18.73 to �20.97 in the �0.20 �m fraction.These values do not reflect that of the parent rock with �Nd

� �36, but those of the weathered soils with some dust input.This result shows that the river chemistry for Nd and the REEis acquired by the net effective chemical weathering processesoccurring in the soil profile and not just from the assumedparent rock with “bulk” Nd dissolution. Similar but smallereffects have been seen in prior studies, for example Goldsteinand Jacobsen (1987) noted that in several temperate rivers thedissolved load has more radiogenic Nd than the suspendedload, a difference that was attributed to differential weatheringof minerals within the drainage basin.

When considering the Nd isotopic composition of the groundwater flooding the hill (collected at the bottom of the 5L6 soilprofile), it appears that this solution has an isotopic signature(around �23), which is intermediate between the values of theupper (�18) and lower soil horizons (�26). This result indi-cates that the REE contained in these waters have probably twoorigins, the upper soil horizons and the lower soil horizonsflooded today by the ground waters. REE from the upper soilhorizons are brought to the bottom profile by meteoric waters,which are percolating downward.

Considering the relation between the ground water and theriver water, it was suggested by Viers et al. (2000) that duringthe rainy season swamp groundwater mainly feeds the Men-gong River and that during dry season there is an increase ofthe contribution of waters flooding the hill soils. Isotopic com-position of these waters seems to confirm this; indeed, we sawthat during dry season the isotopic composition of the MengongRiver tend to be more negative (�20.97) which may indicate anincreasing contribution of waters flooding the bottom profileand which have a signature of �23. The different isotopiccomposition found in the Mengong River water as a function ofthe pore size filtration (see Table 2) shows that the Nd in thewaters is not isotopically homogenized but is a mixture ofisotopically and chemically distinct components.

We have seen before (see Fig. 5) that the relationship ob-tained between Nd (and which is also valid for the other REE)and DOC defines a straight line going through zero, withinanalytical errors. Based on this relationship, it was concludedby several authors (Viers et al., 1997; Dupre et al., 1999) thatNd was entirely associated with organic matter through theformation of complexes with the functional acidic groups.

Undoubtedly, we have two different sources of Nd. The first,more radiogenic, is contained in the small molecular size frac-tion and the second, more negative, is contained in the fractionof higher molecular size. Finally, there is not an homogeneousisotopic composition of Nd in the Mengong River water and Ndcan originate from different locations in the watershed anddifferent sources (e.g., dust and weathered bedrock). Takinginto account this result, two hypotheses can be enunciated: (i)
if we consider Nd as being present only as a dissolved species


(free ions) associated with acidic groups of organic matter, weshould involve the presence of very high REE affinity com-plexing sites, which prevent any exchange with the other REEpool. However, thermodynamically, this hypothesis appearshazardous. (ii) The second hypotheses we propose is that Nd iscontrolled both by complexes formed with acidic functions oforganic matter and by colloidal particles. The isotopic signatureof the colloidal pool may indicate that the Nd, which constitutesthis fraction, is derived from the soil surfaces, which givesimilar values (around �20). By contrast, the Nd contained inthe smallest filter fraction is considered to be the truly dissolvedspecies. Finally, our results show that we can not strictly usethe strong correlation between DOC and REE (see Fig. 5) todefine the nature of the REE in the solution. Even, though Ndversus DOC follows a straight line going through the origin, theisotopic values tell us that Nd is not exclusively present underthe form of a homogeneous dissolved species. Further, theobservation that the pit water with no detectable DOC has highCNd demonstrates that the DOC (which may really be dispersedcolloidal material) is not the controlling mechanism for solu-tion. We suggest that the DOC complexes act to fix the avail-able REE that are already in solution from different sources butmay not be the basic mechanism governing solution.

Whatever the surface water type, ground water or Mengongriver water, REE concentrations are high. Nonetheless, thesehigh concentrations reveal a strong control or relationship ofthe REE content by colloidal material, organic and mineral aspreviously shown by different works (e.g., Stordal and Wass-erburg, 1986; Sholkovitz, 1995; Dupre et al., 1996; Viers et al.,1997; Ingri et al., 2000).

One question concerns the origin of Nd found in the smallestsize filtration. Indeed, the isotopic value of the smallest fraction(� �14) is far more radiogenic than all the other water samplesor soils (from �18 to �43). Evidently, this must involveanother source in the watershed or from outside the watershed.We will not consider the presence of soil horizon with moreradiogenic values. One possibility will be the contribution ofSaharan aerosol (i.e., dust), which may present similar radio-genic values (Grousset et al., 1988; 1992). During dry seasons,atmospheric particles accumulated on the canopy surface andare then mobilized by leaching during rain events. The signif-icant contribution of derived atmosphere components to thebudget of elements in soils has been already reported for oldHawaiian soil (Chadwick et al., 1999).

6. CONCLUSIONS

● The Nd isotopic composition of the bulk water that is leavingthe watershed is not representative of that of the parent rock,but reflects that of the laterite profile and dust input. This hasimportant consequences when Nd isotopic measurements onnatural solutions are used to trace the source of materials.

● This study confirms that there is a high mobility of REE inthe waters draining these environments (laterite developedunder humid tropical climate). The Nd concentrations aremuch greater than found for temperate rivers. Use of thevariation of Nd isotopic abundances shows the complex anddiverse nature of the sources (bedrock weathering of differ-ent minerals and dust input) and phases (dissolved, colloids)
carrying REE in solution and their sources.
● By coupling Nd isotopic composition of soils (different ho-rizons) and waters (different reservoirs), we can reach abetter understanding of the hydrological pathways within thewatershed as a function of seasonal change.

● By using both chemical and isotopic data, we can improveour understanding of the genesis of lateritic soils.

Acknowledgments—This work was done during a fellowship atCaltech. Support for this work was from DOE DE-FG03-88ER13851.Caltech Division Contribution 8898(1100). The authors dedicate thissmall report to George Brimhall who has pioneered the study ofweathering in all sorts of climates. The Nsimi-Zoetele watershed wasdesignated since 1994 as a pilot-site for intertropical geosphere envi-ronmental studies, by the French scientific programs PEGI (Programmed’Etude de la Geosphere Intertropicale, CNRS/IRD) and DYLAT (Dy-namique de la Couverture Lateritique, IRD). We thank the French“Ministere de l’Education Nationale,” the INSU/CNRS and IRD agen-cies for funding. The samples used in this study belong to theseagencies. We would like to greatly acknowledge Henry Ngo and JimChen for their support in the chemistry and in the TIMS measurements.Bernard Dupre, Jean-Jacques Braun, Frederic Coppin, Remi Freydierand Magnus Land are thanked for their helpful comments.

Associate editor: G. R. Helz

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Behavior of Sm and Nd in a lateritic soil profile