Major and trace elements of river-borne material: The Congo Basin

Geochimica

et Cosmochimica

Acta, Vol. 60, No. 8, pp. 1301-1321, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/96 $15.00 + .OO

PI1 SOO16-7037(96)00043-9

Major

and trace elements of river-borne

The Congo Basin

E GAILLARDET, 2,* DOMINIQUE ROUSSEAU, and CLAUDE J. ALLBGRE’ ‘CNES/GRGS, 18 Avenue E. Belin, 31055 Toulouse, France ‘Laboratoire de GCochimie et Cosmochimie, URA CNRS 1758, Institut de Physique du Globe, Universite de Paris 7. 75252 Paris, Cedex 05, France

BERNARD

DuP&,

material:

’ JBRGM

(Received

May

10,

1995; accepted

in revised

form

Jarmary

30,

1996)

Abstract-The Congo river Basin is the second largest drainage basin in the world, after the Amazon. The materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the African continent. This paper presents the chemical composition of the different phases carried in the Congo rivers and is followed by a companion paper, devoted to the modelling of major and trace elements. The Congo river between Bangui and Brazzaville as well as its main tributaries, including a few organic-rich rivers, also called Black rivers, were sampled during the 1989 high water stage. The three main phases (suspended load, dissolved load, and bedload) were analysed for twenty-five major and trace elements. Concentrations normalized to the upper continental crust show that in each river, suspended sediments and dissolved load are chemical complements for the most soluble elements (Ca, Na, Sr, K, Ba, Rb, and U). While these elements are enriched in the dissolved loads, they are considerably depleted in the corresponding suspended sediments. This is consistent with their high mobility during weathering. Another type of complementarity is observed for Zr and Hf between suspended sediments and bedload, related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads. Compared to other rivers, absolute dissolved concentrations of Ca, Na, Sr, K, Ba, Rb, and U are remarkably low. Surprisingly, high dissolved concentrations are found in the Congo waters for other trace elements (e.g., REEs), especially in the Black rivers. On a world scale, these concentrations are among the highest measured in rivers and are shown to be pH dependent for a number of dissolved trace elements. The dissolved loads are systematically normalized to the suspended loads for each river, in order to remove the variations of the element abundances owing to source rock variations. Normalized diagrams for REEs are presented and extended to the other elements. They strongly support the argument that the apparent higher solubility of trace elements in the Congo waters is due to the presence in the dissolved load of a colloidal phase (as a result of 0.2 pm filtration). An important result is that these colloids are strongly depleted in Fe and Al with respect to the other elements. Finally, the comparison of the dissolved, suspended, and sandy transport fluxes of each element in the Congo Basin rivers shows that, although the proportions of, for example, the REEs in the dissolved loads of the majority of the Congo Basin rivers is close to 10% of the total transport flux, up to 80% of the REEs are transported by the so-called “dissolved” load in the Black rivers. 1. INTRODUCTION

and Edmond, 1987; NCgrel et al., 1993). During erosion processes, elements are fractionated between a fluid phase (chemical weathering that supplies dissolved matter to the rivers) and a residual solid phase (removed from the soils by physical or mechanical erosion). Most studies have focused on the chemistry of dissolved loads in rivers, while the particulate phase (and its relation with the dissolved phase) has not attracted sufficient attention. Rare examples of studies focusing on the partitioning of elements in the different river phases can be found in Goldstein and Jacobsen ( 1988) or Zhang et al. ( 1994).

Major rivers are studied by geochemists to estimate fluxes of continental material supplied to the oceans (Potter, 1978; Martin and Whitfield, 1983; Milliman and Meade, 1983; Meybeck, 1988). However, rivers draining very large basins have proven to be useful in other, complementary fields: 1) Like glacial deposits (Goldschmidt, 1933 j or loess (Taylor et al., 1983), materials from large rivers represent precious samples, as they integrate the lithological and chemical diversity of the continental crust. Thus, they provide important information concerning the average chemical and isotopic characteristics of the continental crust (Goldstein et al., 1984; Taylor and McLennan, 1985). 2) Because of the differential mobility of elements during denudation processes, large rivers also provide some insights into erosion processes on a global scale (Stallard

The purpose of the present study is to initiate a trace element survey of the largest rivers of the world in order to 1 ) create a comprehensive data base for modern river erosion products, 2) ascertain the composition of large regions of the upper continental crust, and relate this to its mean age, and 3) assess erosion processes on a large scale in relation to parameters such as climate, lithology, relief, or vegetation.

* Author to whom correspondence should be addressed. 1301

B. Dupre

et al.

FIG. 1. Map of the Congo Basin showing sampling locations, locations are different. sand samples are marked by a star

The central importance of weathering on atmospheric CO2 levels and climate modelling is beyond the scope of our study. The first study of this survey is the Congo Basin in Central Africa. Preliminary results concerning the dissolved load have been published by NCgrel et al. ( 1993). This paper is devoted to the presentation and discussion of data for a number of major and trace elements in both particulate and dissolved loads. A discussion of this dataset in the context of a simple model of global erosion can be found in Gaillardet et al. (1995). 2. STUDY

AREA

AND

SAMPLING

Flowing in wet tropical Africa, the Congo river is the second largest river in the world with respect to its discharge and drainage area, after the Amazon. This area (3.82 lo6 km*) is an extensive platform mostly occupied by the Central African Shield (Fig, 1). The elevation at Brazzaville is 280 m and the basin is surrounded by highlands: plateaux above 800 m to the south and the north, mountains of the East African Rift Valley, with elevations greater than 4000 m to the east. Lake Tanganyika and its drainage basin is

River

sampling

points

are underlined

and when

the

also part of the Congo catchment (Fig. 1) Geologically, the central plain consists of Mesozoic sedimentary rocks (sand, sandstones, and red argillites) and is bordered by Precambrian basement, composed of crystalline and metamorphic rocks associated with a shale-limestone system with a stratigraphic age: 900 Ma (Ntgrel et al., 1993, and references therein). Like the Amazon Basin, the Congo drainage area is dominated by rain forests of high productivity and arborescent savannahs and is submitted to a wet tropical climate that constantly experiences precipitations higher than 1500 mm/y and temperatures above 25°C. The main tributaries of the Congo river are the Oubangui, the Zaire, the Kasai, and the Likouala-Sangha rivers. Their main features are described in Table I. It is worth noting that only the Likouala and Alima rivers flow exclusively under tropical rain forest. The Sangha river catchment has a greater surface area and is less dominated by the rain forest. Nevertheless, these three rivers have the features of the so-called Black rivers (or “Coca Cola rivers,” see Bemer and Bemer, 1987, for a review). Samples were collected during a cruise in November 1989 between Bangui (Central African Republic) and Brazzaville (People’s Republic of Congo) during the high water stage. The “Lobaye,” “Zaire.” “Sangha.” “Likouala,” and “Kasai” samples were collected just before they join the main stream of the Congo river. “Oubangui 1” and ‘Congo 64” were sampled at Bangui and Brazzaville respectively (Fig. 1). River pH was measured in the field

Transport

Table 1. Drainage rivers

studied

Oubangui 1 Lobaye Oubangui 34 Zaire Congo 42 Likouala Sangha Ahna Kasai Corm0 64

areas, water in this Paper.

of sediment discharges,

Drainage area S 103~2 475 31 600 1660 2265 60 250 50 900 3500

and dissolved

runoff

ions in rivers

and suspended

Water discharge W m3ls 3500 335

sediments

concentrations

Runoff W/S

cm2lyr 27 21

17OQO

32

160 1700 600 11000 43100

10 18 38 39 39

1303 of the

Suspended matter SM mgfl 30 16 33 31 25 6 19 14 17 21

The drainage areas are from Olivry et al. (1989). Lobaye, Zaire and Kasai water discharge are long term averages from UNESCO (1974 and 1979). Oubannui and Conrro 64 water discharge are from Olivxy et d. (1989) (mean annual discharge’for 198gand 1989)ySangha, Ahma and Likouala discharges are 1989/1990 mean values (Briquet, personnal communication). Suspended concentrations (SM) were determined daring the November 1989 cruise.

during sampling. Dissolved and suspended phases were isolated immediately after collection by filtration through 0.2 pm acetate celhlose filters ( 142 mm diameter) with a pressurized Sartorius’ Teflon filtration unit. For each sample, about 4 L were filtered and 1 L was collected in a polypropylene acid-washed container and acidified with ultrapure 16 N HNO? for trace elements analysis (for more details, see Ntgrel et al., 1993). At the same time, alkalinity (HCO;) was determined by acid titration and end point determination by the Gran plot method. For the determination of suspended sediment concentrations, a given volume (about 500 mL) of the river water was filtered through a pre-weighted Sartorius@ filter with a diameter of 47 mm and a nominal pore size of 0.2 pm. This filter was then dried in an oven at about 80°C for an hour and weighed. The weight of suspended matter was then converted to concentration (SM) in mg/L (Table 1). Sand samples were dredged on the river bottom at sixteen locations between Bangui and Brazzaville. These samples are referenced by the distance from the sampling point to Bangui (Fig. 1). For example, the sample pK87 was sampled in the main stream of the Oubangui river 87 km down stream from Bangui. Sand samples of three tributaries were also collected, they are named “Lobaye,” “Motaba,” and “Likouala.” For the pK61.5. pK87, pK577, and Likouala samples, two sand samples were collected in the middle of the main channel and near the banks in order to check for lateral variability. 3. ANALYTICAL 3.1.

Suspended

METHODS

Load

Trace element concentrations were determined by instrumental neutron activation analysis (INAA) and y-spectrometry at Laboratoire Pierre Sue (CEA Saclav) followinc the method described bv Chayla et al. ( 1973). The solid phase (suspended sediments and sands) concentrations were calculated using an external standard (GSN) method. Lead, rubidium, potassium, strontium, neodymium, and samarium concentrations of suspended material were determined by isotope dilution using thermal-ionisation mass spectrometry (TIMS) using the analytical procedures described in Allegre et al. (1996). 3.2.

Dissolved

Load

For the major components, Si02 was determined by spectrophotometric measurement of the molybdenum blue complex, Cl- and SO:- by ion-chromatography, and Ca, Na, and Mg by conventional atomic absorption procedures. Rubidium, potassium, and strontium concentrations were determined by isotope dilution. The analytical precision is close to I % for isotopic dilution (TIMS) and better than 10% for the other techniques. Blanks and technical details are given

by Negrel et al. ( 1993). Other trace elements concentrations were determined by INAA and/or ICP-MS to check the accuracy. Two INAA techniques were used. Concentrations in the dissolved phase were determined using 250 mL of water. The sample was evaporated in a Teflon container, the residue dissolved in concentrated HNO,, diluted, and transferred to a small ultrapure quartz vial and analysed using a purely instrumental technique (Ko technique) (Moens et al., 1984; De Corte et al., 1986; Picot, 1987). These two techniques were checked using the geostandards ACE (granite) and BE-N (basalt) and GSN granite (for the Ko technique) and compared to the values given by Govindaraju (1984; Table 2). Concentrations obtained by the external standard technique are consistent with the standard values, within 10% of analytical uncertainty. For the Ko technique, standard measurements are in good agreement with the certified values or the values measured by the external technique within 20% of analytical uncertainty. Europium, tantalum, and zirconium were not measurable using this technique. The ratio of the Ba concentration obtained by the two INAA techniques is constant for all the standards that we analysed and has been used to correct for any experimental bias. The blanks of the whole procedure applied to the filtered samples (including potential contamination during filtration, evaporation or due to the containers) were analysed and are negligible for all elements. Trace elements in the dissolved phase were also analysed by ICPMS at Toulouse. ICP-MS analyses are performed on a PerkinElmer ELAN 5000 equipped with a standard pneumatic nebulizer and an automatic sampler (AS-80). Indium was chosen as an internal standard and corrections for oxide and hydroxide-ions were made for the REE elements. The river samples were analysed without preconcentration. During the course of an analysis, standards BE-N and SLRS-2 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) were repeatedly measured. The mean values for BE-N are reported in Table 2. Our values for SLRS-2 are in a good agreement (< 10%) with the certified values. ICP-MS and INAA data for the river samples of this study show an excellent consistency as shown in Table 4. 3.3. Bedload In the sand samples, SiOz, AlzOi, and Ni were determined by X-Ray des analyses des roches et mineraux Analytical precision is ?lO%. The tions were determined using the same phase.

Ba, K, Sr, Zr, Na, Ca, Fe, Cr, fluorescence at the “Service du CNRS” at Nancy, France. other trace element concentraprocedure as for the suspended

4. RESULTS

4.1. Suspended Concentrations different Congo

Load Concentrations measured Basin rivers

(Cp)

in the suspended phase for the are listed in Table 3. The scar-

1304

B. Duprk

et al.

Table 2. Analyses of international standards and comparison between the two INAA methods (External standards and Ko) and ICP-MS. External

stan

rd method

I

Ko method

12 rerGcates BE-k SD

7 replicates K-E SD

Ba Ca%

ce

co Cr

cs Eu Fe% Hf La Na% Nd Ni

Rb SC Sm Ta

Tb l-h U

Yb zr

154 61 358 0.68 3.80 8.8 5.8 81 2.50

5

154 0.3 11 2.7 2.2 1.67 28 57 4.8

1 11 0.04

0.18 0.2 0.3 2

0.16

7 0.2 7 0.1

0.1 0.04

1 2 0.2

Ce co Cr cs Eu Fe% Hf La Na%

144 0.25 23 7.5 4.3 17.6 4.4 17 814

0!6 0.4

0.1 0.03 0.3

0.12 0.14 41

4 0.04

Ni Rb SC

1

Sm

0.2

replicate!

3-N

SD

BE-N

AC-E

GS-b

1261

48 2 I 1 0.2

1025 9.9 152 61 360 0.8 3.6 9 5.4 82 3.1 70 267 47 22 12 5.5 1.3 11 2.4 1.8 265

55 0.24 154 0.2 3.4 3 2 1.75 28 59 4.9 92 1.5 152 0.1 24 6.4 4.8 18.5 4.6 17.4 780

1380 1.76 140 65 55 5.43 1.7 2.62 6.5 70 2.19 47 34 180 1.3 7.2 2.58 0.54 40.8 7.7 1.43 250

5 2 29 0.06

154

5

3.2

0.1

138 67 54 5.6

9.3 5.4 81

0.2 0.2

1.73 26 59

0.05 I 2

2.5 6.3 71

0.1 0.2 2

267

7

50 36

2 5

32 11

1 1

0.34 23

0.04

6.9 1.4

0.2 0.3

1.20 10.1 2.2 1.8

0.07 0.6

0.1

4.6 17 4.2

0.1 I 0.2

0.03 1.5 0.7

0.2

15

2

cl.50 10.3 7.6 1.37

1

154 66 336 0.08 3.71

4 2 57 0.05 0.06

87

2

68.0 233 50

1.4

12.0

0.6

1.15 11.0 2.56 1.85

0.07 0.5 0.09 0.03

20 2

Ta

0.1

Tb

0.6 0.2 I 72

‘I% u Yh Zr

values (1)

6

149 61 355 0.85

1

I certified

ICP-MS

42

Nd 12

218 41 25.0 11.5 6.6 1.13 10.4 2.40 1.78 280

11

0.15

ce co Cr Cs l51 Fe % Hf La Na % I Nd Ni Rb SC Sm Ta -I% Th U I Yb zr

(1) The certified values are from Govindaraju (1984). SD: standard deviation.

city of concentration data in the suspended load of Congo rivers in the literature makes it difficult to compare our results with previous measurements. Martin et al. ( 1978) reTable 3. Major sample

and trace element concentrations

Oobaoguil

Lobaye 4.7 68 2.8 17.8 31.4 8660 236 55.1 118.7 1.8 3.9 57 183 18

co Cr Ni

4.4 a7 3 17.6 32.5 10677 302 57.4 143.2 1.6 3.1 56 139 72 2520 9.7 2.1 1.1 3.3 26400 85960 20.4 21.8 133.2 99.6

10.1 2.3 1.2 3.7 32300 119530 21.1 23.8 141.5 80.3

RblSr Sm/Nd U/Pb ThN

1.2 0.17 0.092 5.9

SM (2)

CS

Rb U n Pb K Ba la Ce Ta Hf Nd zr Sr Na Sm Eh Tb Yb ca Fe SC

Oubangui34

in suspended sediments of the Congo Rivers expressed in ppm Zaire.

Congo 42 5.8

2075 10.8 2.2 1.1 3.71 39100 89150 23.6 33.7 147.7 76.8

6.9 111 4.3 15.6 31.7 16370 454 55.6 109.3 2.0 4.2 49 166 18 3260 8.4 1.8 0.86 3 26100 68940 17.2 21.2 113.8 13.7

0.87 0.18 0.075 6.3

0.12 5.8

1.42 0.17 0.14 3.6

4.3

5.55

3.71

4.62

6.81 29.7

6.56 16.2

6.81 32.9

5.98 30.6

5.1 3.3 19.2 21.6 416 61.7 134.6 1.9 4.9 64 184

ported data for two samples of the Congo river at Brazzaville, which are in general agreement (within 20%) with the data of this study except for Ba (our value is about 2 times

3.5 18.3

442 51.4 125.8 1.9 3.9 55 180 2445 9.6 2.1 1 3.3 29000 98320 21.0 34.2 140.1 66.5

Likouala 1.4 33 0.9 4.7 8610 123 17.0 41.8 0.73 1.6 12 72 66 2.1 0.6 0.28 0.9 9600 38470 6.5 6.0 71.28 129.8 0.5 0.17

Sangha

Alima

Kasai

Congo 64

upper crust

4.6 42 2.5 15.1 24.6 8000 346 54.6 107.9 1.2 3.0 46 145 40 2590 8.2 1.9 0.88 2.9 22400 109350 18.2 25.3 147.1 141.4

1.9 38 0.9 6.9 68.1 5750 125 16.9 31.9 0.56 1.4 14 59 51

--L. I 49 1.7 13.3 28.3 9480 312 48.0 98.2 1.4 3.1 43 152 51

3.7 112 2.8 10.7 20

2.3 0.75 0.28 0.98 7700 52230 8.1 8.1 66.02 53.4

7.1 1.7 0.69 2.6 27400 76010 17.7 17.0 131.1 13.8

4.4 60 2.4 13.8 34.1 9110 339 41.7 94.8 1.6 4 39 156 52 2590 6.7 1.6 0.72 2.6 25000 78420 16.1 23.9 119 65.6

4.5 0.88 0.64 2.2 30000 35ooo 11 10 35 20

0.75 0.17 0.013 1.62

0.96 0.17 0.061 1.72

1.15 0.17 0.069 5.1

0.32 0.17 0.14 3.82

5.2

5.5

1.1 0.18 0.1 6.1

13.6

10.1

6.78

4.95

5.1

6.53 19.8

4.65 5.9

5.5 18.6

4.98 13.8

6.35 16.8

6.4 20.7

AlI concentrations were determined by INAA exept K, Rb, Sr, Sm. Nd and Pb which were determined by isotope dilution. (1) Particulate Organic Carbon expressed in weight % of suspended sediments. After Barreau (1992). (2) Suspended sediments (SIkl) concentrations in mg/l, determined in the field. Upper crust abundances from Taylor and McLennan (1985). Primitive mantle abundances from Hofmann (1988).

550 30 2: 5.8 26 190 350

primitive mautle 0.008 0.535 0.02 0.08 1 0.175 258.2 6.049 0.614 1.601 0.035 0.368 1.189 9.714 18.21 2460 0.387 0.146 0.094 0.414 22942 58598 14.88 104 3000

0.03 0.32 0.11 4.1

Transport

of sediment

and dissolved

ions in rivers

1305

o Oubanguil 0 Zaire q Lobaye n Likouala

a

A Sangha

+ Oubangui 34 x Congo 42

b n

l7QL-i

-

O Amazon Y kJY&sippi v Ohio

\ w

I

I

LaCePr

I

I

I

II

III

III

I

NdSmEuGdTbDyHoErTmYbLu

FIG. 2. Upper crust normalized suspended REE (Cp) patterns for (a) all the rivers value (calculated using all samples except Likouala and Alima rivers) in comparison patterns from Goldstein and Jacobsen ( 1988)

lower), Ca (our value is 3 times higher), Cr (30% difference), Cs (30%)) K (2.5%), and Ta (45% ) Their Pb concentration value, measured by X-ray fluorescence analysis is one order of magnitude higher than our value measured by isotope dilution. As one of the important purposes of this study is to get information on the upper continental crust composition over a large African drainage basin, our data have been normalized to the mean upper crust composition proposed by Taylor and McLennan ( 1985 ) . Before examining the whole set of elements, we will first focus on the REE patterns. suspended REE patterns

The upper crust normalized REE patterns are plotted in Fig. 2a. The absolute REE concentrations of the suspended load strongly fluctuate among the rivers. As an example, Ce concentrations range from 38 ppm in the Alima sediments to 143 ppm in the Oubangui 34 sediments. All samples, except Likouala and Alima river sediments, are enriched relative to the Taylor and McLennan (1985) upper crust. These rivers have significantly lower concentrations: Likouala and Alima are 1.5 to 2 times depleted relative to the continental crust. In spite of these absolute variations, all the upper crust normalized suspended REE patterns (Fig. 2a) are remarkably similar. The patterns show a slight depletion in HREEs (Tb and Yb) with respect to the continental crust and a small positive Eu anomaly. The patterns of Likouala and Alima rivers are flat with a more pronounced positive Eu anomaly. The mean pattern for the Congo rivers (Alima and Likou-

of this study and (b) the mean to the river suspended material

ala are excluded from this calculation) is compared to the suspended patterns for other large rivers studied by Goldstein and Jacobsen ( 1988) in Fig. 2b. The slight upward convexity of REE patterns observed in this study also appears for the Amazon, Mississippi, and Ohio rivers, but the Ohio and Amazon rivers are slightly more depleted in HREEs than the Mississippi and the Congo rivers. The Indus river shows more pronounced convexity. Thus, it seems that, like other large rivers, the Congo rivers do not have flat upper crust normalized REE patterns. 4.1.2.

4.1. I. Upper crust normalized

Congo mean

Upper crust normalized

extended patterns

The upper crust normalized patterns for the whole set of elements are plotted in Fig. 3a (for the whole set of rivers) and in Fig. 3b (for the average of Oubangui, Lobaye, Zaire, Sangha, Kasai, and Congo 64 samples). The order of elements of all the normalized diagrams used in this paper is taken from Hofmann ( 1988) and results in a monotonic decrease of continental abundances (using the upper crust model of Taylor and McLennan, 1985) normalized to the primitive mantle of the Earth (or to chondrites) . The primitive mantle concentrations are from Hofmann ( 1988 ) (Table 3). The most incompatible elements are on the left, the most compatible are on the right. The continental crust concentrations given by Condie ( 1993) would lead to the same sequence of elements. The observations inferred from the REE patterns can be extended to the whole set of elements. In spite of variations in absolute concentrations, all the patterns for the suspended load normalized to the upper crust (Fig. 3a,b) are surpris-

1306

B. Dupre

et al

a

10

0 l

1

cl w . A

~

v . + x

I

I

Cs

I

U Rb

10

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I1

I

I

Pb Ba Ce Hf Zr Na Eu Yb SC Co Th K La Ta Nd Sr Sm ‘lb Ca

I

Oubangui 1 Zaire Lobaye Likouala Sangha Kasai Congo 64 Alima Oubangui 34 Congo 42

I

Ni q

mean suspended pattern

n

mean dissolved pattern

1

i

Cs

Rb

U

Th

Pb Ba Ce Hf Zr Na Eu Yb SC Co Ni K La Ta Nd Sr Sm Tb Ca Fe Cr

FIG. 3. Extended patterns normalized to the upper crust for the Congo Basin rivers suspended (Cp) and dissolved (Cw) loads. The order of elements is designed in order to obtain a monotonic decrease of crustal abundances normalized to primitive mantle concentrations (Hofmann, 1988) using the upper crust model of Taylor and McLennan ( 1985) upper crust (Table 3). Figure 3a presents the whole set of samples, Fig. 3b only shows the mean patterns. Dissolved concentrations are expressed in lO-5 g/L water (Table 4).

ingly similar, but they are neither flat nor regular. From the observation of the suspended and dissolved phases patterns (see next section), we can clearly separate the elements into two groups. (1) The first group comprises Rb, U, K, Ba, Hf, Zr, Sr, Na, and Ca. These elements show important negative anomalies relative to their neighbouring elements. The most depleted elements are Na and Sr. For example, the Na/Sm of the Oubangui sediments (200) is 32 times lower than the upper crust ratio (6400). The least depleted elements are U and Ca. Note that these negative anomalies are less important in the Alima and Likouala rivers. In the Alima river, the suspended Na/Sm is only 16 times lower than the Na/Sm ratio for the upper crust. (2) The second group of elements consists of Cs, Th, Pb, Ta, REEs, SC. Fe, Co, Cr, and Ni. Their patterns show a trend of increasing enrichment from incompatible elements

(Cs, Th) to compatible elements (Fe, Co, Cr, Ni), except for Tb, Yb, and SC which are slightly depleted. If we disregard the Likouala and Alima rivers, which have abnormally high suspended Ni concentrations, the mean Ni/Th ratio in the suspended load is three times higher than that of the continental crust. Note that the irregularities of Tb, Yb, and SC might well be the result of uncertainties in the composition of the continental crust. Figure 3a and Table 3 show that the Likouala and Alima rivers exhibit the lowest abundances in the suspended load (except for the elements of the first group: Rb, U, K, Ba, Sr, Na, and Ca). The Alima and Likouala rivers have concentrations in their suspended loads that are 1.5 to 2 times lower than the upper crust. In all the other rivers, the elements of the second group (Cs, Th, Pb, Ta, REE, SC, Fe, Co, Cr, and Ni) are systematically enriched in the riversuspended sediments relative to the continental crust. This

1307

Transport of sediment and dissolved ions in rivers n

0

mm n

n

#

n n

lb

2b SM.

3b

M@)

i

7h

lb

1215

Particulate Organic Carbon (%)

FIG. 4. Plot of Th suspended load concentrations, Cp(Th) in ppm vs. Particulate Organic carbon (POC in ‘%), pH. and river Suspended Matter content (SM in mg/L). The three relationships are not independent because good correlation coefficients are observed between POC and pH (0.84) and between POC and SM (0.76). Similar relationships are observed for all other elements except Pb, Ni, K, Sr, and Na.

enrichment is the most important for the Oubangui river which is 1.6-fold enriched with respect to the crust. More generally, the suspended concentrations of the elements of the second group are well correlated if we exclude Ni (which is enriched in the Alima and Sangha rivers). As an example the correlation coefficient is 0.97 between Cp(Th) and Cp(La), and 0.91 between Cp(Th) and Cp( Cr) In Fig. 4, we plotted Cp( Th) as a function of other river parameters: particulate organic Carbon (POC in weight %), river pH, and the river suspended sediments concentration (SM in mg/L). The POC concentrations were determined by Barreau (1992) for the same sampling location. but on different aliquots using a 0.45 pm filtration, while the suspended sediments analysed in this study were sampled using a 0.2 pm filtration pore size. These proportions of POC (Table 3) are in good agreement with previous measurements of Mariotti et al. ( 1991), and Nkounkou and Probst ( 1987) in the Congo waters and confirm the global inverse correlation between POC and SM reported at a world scale by Meybeck ( 1982). The most POC-rich rivers are the Black rivers (Alima, Sangha, and Likouala). Figure 4 indicates that a trend of increasing Th suspended concentrations is observed with increasing pH (r = 0.90, n = 10) and SM (Y = 0.76, n = 10) and a trend of decreasing concentrations with POC. (r = 0.72, n = 10). These three relations are not independent, because good correlation coefficients are observed for the pairs pH-POC (0.84), pH-SM (0.7 1) , and POC-SM (0.76). Such a correlation of suspended concentrations with pH, POC, and SM is also observed for a number of other elements, but not for Pb, K, Sr, Na. and Ni. 4.2. Dissolved 4.2.1.

Major

Phase Concentrations

(Cw)

elements

Major element concentrations in the dissolved phases are shown in Table 4. The total cation charge (2 + in peq/L), the total anion charge (C in p.eq/L), the Normalized Inorganic Charge Balance (NICB) defined as (z’ - X-/X’; Stallard and Edmond, 1983), and the total dissolved load TDS in

mg/L are also given. Dissolved Ca, Na, Sr, Mg, Cl, Rb, HCO?, and SO, concentrations have already been published in NCgrel et al. ( 1993). They are in good agreement (within 10%) with previously reported analyses for the Congo waters (Meybeck, 1979; Nkounkou and Probst, 1987, and references therein). Three main characteristics of the Congo waters are apparent from Table 4: 1) All the rivers have low pH, ranging from 4.65 to 6.81, with a mean value of 5.9. The three Black rivers have the lowest pH: Likouala (4.65), Alima (4.98), and Sangha (5.5). 2) The Congo rivers are very dilute with Z +-values ranging from 202 to 523 peq/L. The most dilute rivers are Likouala and Sangha (Z + = 200-220 peq/L) and the least dilute are the Oubangui and the Zaire. A significant imbalance between negative and positive charges is observed with a deficit of negative charges. The NICB index increases from 6% in the Oubangui water to 37% in the Sangha waters. The imbalance of charges is attributed to the presence of organic acids whose dissociation produces H’ ions and organic anions, not included in the negative charges (Berner and Bemer, 1987). This interpretation is supported by the Congo rivers since the Black rivers (which are the most acidic) have the largest NICB values. 3) The last striking feature of the Congo rivers is that the dissolved load is dominated by Si (OH), . Dissolved SiO? concentrations range from 13.6 mg/L in the Oubangui river to 9.04 mg/L in the Likouala river. The H$iO,/ (Na + K + Mg + Ca) mass ratio ranges from 1 to 3 and does not exhibit a Si enrichment in the Black rivers. 4.2.2. Upper crust-normalized

dissolved REE patterns

The REE concentrations are listed in Table 4. Upper crust normalized REE patterns are shown in Fig. 5a. None of these patterns is flat and a slight upward convexity centered on the intermediate REEs (Eu and Gd) is observed. [LulLa], ratios (dissolved L&La ratio normalized to the upper crust

1308

B. DuprC et al. Table 4. Major and trace elements concentrations ramp’e CS ” nl Ba La ce

Oubmgui I INAA 0.008 0.055 0.051 17.6

0.055 0.042 16.8

0.2m 0.5

0.249 0.492

Sm F.u Gd Tb m

HO !a 2

INAA ICP-MS 0.0017 0.034 0.042 13.5 0.208 0.449

0.069

Pr Nd

ICI-MS

0.343 0.057

o.ow5

0.020

0.277 0.06 0.016 0.051 0.007 0.043

0.39 0.076

0.01

0.009 0.029 0.004 0.024 0.033

LA!

Hf SC Fe CO Cr Ni Al

0.004 0.0042 0.055

60 0.077 0.533 1.15

Rb

2.7 (I)

K Sr

IIco(I) IS(I)

NaW caca

2.99

17.1

2 782 II

0.023 L4.1 0.297 0.55 0.088 0.363 0.079 0.021 0.071 0.012 0.064 0.014 0.039 0.006 0.036 0.005

0.1 0.55 29 2.5 12

zait INAA n W26 0.086 0.056 24.8 0.354 0.679 0.395 0.075

0.0088

0.022 0.0057 0.067 202 0.075 0.386 1.02

3.9 ZIP0 2,

ICP-MS 0.071

0.349 0.661 0.093 0.36 0.082 0.022 0.063 0.01 0.053 0.01 I 0.033 0.005 0.027 0.004

0.07 0.96 46 3.8 24

Likcuals WAA “075 DO,, 0.078 17.0 0.437 1.08 0.530 0.101

0.0115

0.031 0.012 0.153 1097 0.462 0.838 I.200

sangha

ICFMS “mfi1 ““24 17 0.38 0.85 0.1 0.39 0.08 0.016 0.066 0.008 0.047 0.01 0.024 00% 0.026 0.0028

0.52

INAA DOlR “069

Alima INAA

0.0191

0.059 0.0093 0.16 273 0.272 0.78,

3.6 1486 13

3.8 1330 9.6

16cQ 948

782 2520 753

782 152n 899

1152 630

0.88 280

923

864 315

22875 2784 1245

768 560

7930 960 630

13.61

12.54

II.38

9.04

12.57

10.05

6.81

6.56 215

5.98 523

4.65 202

5.5 259

4.98 217

HC03(4) SW5) CK5)

14335 1248 770

Sio2(3) PH z++me4n

303

1920

0.027 0.023

0.46

4.1

299

0.025 0.031 24.0 0.205 0.463

0.173 0.697 0 143 0.033 0.123 0.015 0.104 0.019 0.059 0.01 I 0.047 0.006

I .23

9.3

hi24

I.44

0.8 384

3.4 1173 7.9

ICP-MS 0.04

ICP-MS

20.7 0.587

0.775 0.14

KW.i INAA 0009

0.101

0.128 28.0 0.6 I.4

3100 2720 2381

2600 9%

x+-zNICB

I.13 12

0.0348 0.062 80 0.09 0.33 0.738

529

Mg(2)

z-mqn TDs OW)

0.07

in the dissolved phase of the Congo Rivers, expressed in ppb. SiO2 is expressed in mgil.

L&aye

3.9 10

0.291 0.047

0.0081

0.019 00038 0.062 108 0.058 0.4 0.41

0.189 0.452 0.052 0.241 0.047 0.014 0.047 0.006 0.038 0.008 0.025 0.003 0.019 0.003

0.05

congo 64 INAA

ICP-MS

0.016 0.049

0.01

0.065 19.7 0.372 0,739 0.416 0.075

O.OOV85

0.032 0.0067 0.087 179 0.0594 0.501 0.934

51 2.7 1290 10.5

12

,219 1760 923 9455

1248 910

6.35 208

164

3.1 1408 II.5

1.15 3.2 16

1361 11224 1056 1155

6.4 301 239

2a3 36.1 20

48.68 43

27.6 95

44

62

6.6%

8%

37%

21%

21%

ratio) are > 1. They range from 1 in the Zaire river to 1.5 in the Oubangui and Lobaye rivers. Only the Likouala and Alima rivers have [Lu/LalN < 1, (0.7 and 0.9, respectively ) These two Black rivers also show irregular patterns for the HREEs. The dissolved patterns of other major rivers, Amazon, Indus, Ohio, and Mississippi (Goldstein and Jacobsen, 1988) are plotted in Fig. 5b for reference. All these rivers have lower concentrations than that of the Congo rivers and a more pronounced LREE depletion. Compared to the Congo rivers, the most similar pattern (and concentrations) is that of the Amazon. The REE patterns of the Congo rivers are similar to the type 2 in the classification defined by Elderfield et al. ( 1990) for the rivers of North American and the UK (Dove River for example). Note finally that the negative Ce anomaly found in some rivers (Mississippi, Ohio, Indus from Fig. 5b and Elderfield et al.. 1990) is not observed in the tropical rivers of this study. Absolute concentrations fluctuate widely (with La concentrations ranging from 0.19 ppb, for the Kasai river to 0.59 ppb for the Alima river), but the order of increasing concentrations from the Kasai River to the Alima River is the same for most of the REEs (Fig. 5a). Consequently, REE concentrations in the dissolved phase are well correlated, with a majority of interelement correlation coefficients greater than r = 0.9. The lowest correlation coefficients (about 0.7) are obtained for pairs involving Lu. A worldwide comparison of dissolved REE concentrations shows that the

0.017

1127 2080

164

(4): detamkd

0.689 0.089 0.35 0.062 0.017 0.066 0.0097 0.0058 0.012 0.033 0.0035 0.029 0.0045

76

480

(I): daumiwd by i%Xcpedilution: (2): detamincd by flame atomic absorpion; (3): dctmnined by spccm@aaneq:

19.6 0.319

by acid tiuation; (5): &ermin& by ion chmmata~y.

dissolved REE concentrations for the Congo Basin rivers are among the highest. Considering for example Ce, our rivers (Ce concentrations range from 0.49 to 1.4 ppb) have concentrations similar to the Isua (Greenland) and Great Whale (North America) waters analysed by Goldstein and Jacobsen (1988) (respectively, 0.34 and 1.2 ppb) and close to the Connecticut (0.76 ppb), the Mullica (0.70 ppb), the Swale (0.67 ppb), and the Hodder (0.44 ppb) rivers analysed by Elderfield et al. ( 1990), despite the fact that these authors used a 0.45 ym pore size filtration. As pointed out by the latter authors, high levels of REEs in the dissolved phase are often associated with high concentrations of dissolved organic carbon (DOC). DOC concentrations of 9 mg/L in the Congo at Brazzaville have been reported by Cad&e ( 1982). The data from this study (in particular, from the Alima, Sangha, and Likouala rivers which are typical Black rivers) support this observation. 4.2.3. Dependence

of REE dissolved

concentrations

on pH

As an example, the dissolved La concentrations are plotted against river pH in Fig. 6. The data from other world rivers (Keasler and Loveland, 1982; Goldstein and Jacobsen, 1987; Elderfield et al., 1990) are also included for comparison. Such a relationship would also be valid by considering any other REEs because of their good intercorrelations. Our data are consistent with the global inverse correlation of riverdissolved REEs with pH, already pointed out by Keasler and

Transport of sediment and dissolved ions in rivers

0.1

1309

o Oubangui 1 l Lobaye q Kasai

a

3

o Likouala 0 Alima

0 Amazon l

10-54

,

,

,

[

,

(

,

,

La Ce Pr NdSmEuGdTbDyHo

,

,

(

[

[

Indus

,

ErTmYbLu

FIG. 5. REE patterns normalized to the upper crust in the Congo river dissolved loads (a) for the rivers of this study and (b) compared to the rivers of Goldstein and Jacobsen ( 1988). Dissolved concentrations are in ppb (Table 4). crustal concentrations in ppm (Table 3)

Loveland ( 1982) and Goldstein and Jacobsen ( 1987 ) for Nd and Goldstein and Jacobsen (1988) and Elderfield et al. ( 1990) for the whole set of REEs. They also support the “whole river pH titration” experiments carried out by Sholkovitz (1995) on unfiltered Connecticut and Hudson river waters. In these experiments, river water pH is controlled between pH I and pH 8 and dissolved (<0.2 pm and <0.025 pm phases) REE concentrations show a strong inverse proportionality with pH. Note, however, that the field data for the Congo rivers reported in this paper show a much greater range of REE dissolved concentrations between pH 4.5 and pH 6.8 than that reported in experiments by Sholkovitz ( 1995). More precisely, Fig. 6 shows a weak dependence of La concentration with pH for pH < 6-6.5 but a stronger dependence with higher pH. This change may be evidence of the increasing importance of adsorption processes on suspended sediments with pH greater than 6-6.5. Below this critical value, other mechanisms must dominate.

4.2.4. Extended

upper crust normalized

diagrams

Very few trace element concentrations data exist for the Congo river waters. Only Fe and Al data have been published (Nkounkou and Probst, 1987 1 for the Congo waters at Brazzaville. With 250 ppb for Fe and 36 ppb for Al, our data are in good agreement with published values. No dissolved concentrations are available in the literature for the other trace elements. The dissolved patterns normalized to the upper crust, extended to the whole set of elements measured by neutron activation are compared to the suspended patterns in Fig. 3, for the mean sample (Fig. 3b) and the whole set of rivers (Fig. 3a). This allows the following general observations. The two groups of elements deduced from the observation of the suspended patterns are also evident in the dissolved patterns. The elements Rb, U. K, Ba, Sr, Na, and Ca from the first group are all enriched in the dissolved phase relative

1310

B. Dupre et al

A

l

I

l AMississippi

‘0

Elderfield et al. (1990) a GoldsteinandJacobsen(1988) l Keaslerand Loveland(1982) I 5 ’’ pH

1-1 I I n

1 -1 4

~

I 77

8

9

FIG. 6. Global log-log inverse correlation between dissolved La, Cw(La) in ppt and pH in rivers. The large open squares represent the data from this study (shaded area). The other data are from the literature. Data from Keasler and Loveland (1982) and Elderfield et al. (1990) were measured after filtration through 0.45 pm and 0.4 ym filters respectively. Data from Goldstein and Jacobsen ( 1988) were measured after filtration through 0.2 pm filters as in this study. The linear correlation shown by our data, deviates slightly for pH greater than 6.5-7. Similar global relationships are obtained for all the other REEs.

to the neighbouring elements. Their negative anomalies in the suspended loads complement the positive anomalies in the dissolved phases. For example, the NalSm in the dissolved load of the Zaire river is 41300, i.e., 6.5 times the upper crust ratio. Exceptions to this rule appear for Na in the Likouala and Sangha rivers and for U in the Likouala River. Hafnium is the only element to have a different behavior. Hafnium is highly depleted in the dissolved phase relative to the upper continental crust. The ratio Nd/Hf in the dissolved load is about 20 times the continental ratio. Zirconium was not measurable. The patterns of elements of the second group (Cs, Th, Pb, Ta, REE, SC, Fe, Co, Cr, and Ni) display a striking firstorder similarity with the corresponding suspended patterns. A slight upward convexity centered on the intermediate REEs together with an enrichment in the most compatible elements is apparent from the dissolved phase patterns on Fig. 3. As an example, Ni/Th normalized ratios range from 5 to 12 and Cr/Th normalized ratios from 1.8 to 3.2. For these elements, the Likouala and Sangha rivers also differ from other rivers because they have higher values for Cs, Fe, and Co dissolved concentrations. In order to investigate the interrelations between the elements in the dissolved phases, the correlation coefficients between each pair of elements have been systematically calculated and classified using cluster analysis with complete linkage (CA). This technique (Velleman, 1988) provides a geochemical classification of the elements by grouping data progressively more distant together. in a statistical sense. Prior to CA. the variables were centered and reduced [( Cw - Cw,,,,,)/a]. The results are displayed on Fig. 7 in the form of a dendrogram. From this figure, and from Fig. 3, three categories of elements can be then distinguished: 1) The elements

U, Rb, Ba, K, Sr, Na, and Ca (which

we

call elements of the first set). The best correlation coefficients are observed between U and Sr (r = 0.9, n = 7). It is clear from the spidergrams of Fig. 3 that the main feature of this set is a general enrichment in the dissolved phase and a general depletion in the suspended phase when normalized to the upper continental crust. None of them correlate with pH. 2) The second set consists of the REEs and elements whose behavior in the Congo waters is very close to that of the REEs. These elements are Th, SC, and Cr (r[Th, Ce] = 0.96, r[Sc, Ce] = 0.96), r[Cr, Ce] = 0.87). Graphically, the same idea is expressed by the fact that the REE dissolved patterns (Fig. 5) are similar but that absolute abundances fluctuate from Lobaye and Oubangui samples (the least enriched samples) to Likouala and Alima (the most enriched samples). Although Hf is part of the REE group according to the dendrogram, it is not include in the second set because of its strong depletion in the dissolved phase (Fig. 3). All the elements of the second set display relations with pH, similar to the relations between pH and REE reported above. 3) Finally, the elements Cs, Co. Hf, Fe, and Al define the third set. These elements (as for that of the previous set) are not enriched in the dissolved phase with respect to the continental crust (Fig. 3). The elements Hf, Fe, and Al show a strong depletion in the dissolved phase. For example, the mean Fe/La (if we exclude Likouala river) and Al/La dissolved ratios are 2.7 and 8.8 times lower than the continental ratio, respectively. All the elements of the third set are well correlated with pH (from 0.89 to 0.94 for Hf) and except Hf, and to a lesser extent Co, they are not correlated to the REE group. In Fig. 3, dissolved Ni appears to be slightly enriched relative to the continental crust. For example, the Ni/Cr dissolved

FIG. 7. Dendrograms for cluster analysis (single linkage) of the concentrations in the dissolved phase of the Congo rivers of this study. The cluster analysis allow three sets of elements to be distinguished

(see text).

1311

Transport of sediment and dissolved ions in rivers

pK615, and Likouala). In order to compare the chemical composition of river bottom sediments with the composition of suspended river sediments, patterns of the sand samples normalized to the Zaire suspended load concentrations are shown on Fig. 10. Zirconium and hafnium exhibit very large positive anomalies in sands with respect to suspended sediments. This enrichment of Zr and Hf in the sandy phase shows a large range from pK158 to Motaba. All other elements are uniformly depleted in the sands relative to the river suspended sediments. The factor of depletion of sands relative to suspended sediments ranges from 0.02 (Motaba sand sample) to 0.3 (Likouala sand sample). Cesium and rubidium are the most depleted elements in sands relative to suspended sediments.

0.14

0.12

. 1

\

5. DISCUSSION 5.1. Particulate

5

PH

6

FIG. 8. Filtered concentrations of Th (element of the second set) and Al (element of the third set) vs. pH. Similar relationships could be shown for all the other elements from the second and third sets of elements defined in Fig. 7.

ratios range from 1 to 2.6. while this ratio in the continental crust is 0.6. The dependence of Al and Th concentrations solved phase on pH are shown in Fig. 8. 4.3. Bottom

Sediments

in the dis-

(Cs)

The concentrations in the different sand samples are listed in Table 5. Macroscopically. the bottom sediments of the Congo Basin rivers are made of clear quartzitic sands. XRD studies by Jouanneau et al. ( 1990) have confirmed the importance of quartz (90%), the presence of very few clay minerals, and the presence of minor minerals: alkali feldspars and heavy minerals such as disthene, epidote, rutile, zircon, staurotide, hornblende, and tourmaline. The most important geochemical feature of bottom sediments is the overall heterogeneity of the concentrations, in contrast with the homogeneity of suspended sediments concentrations. Most of the trace element concentrations range over one order of magnitude, the most enriched sample being pK158 and the most depleted one being Motaba (Fig. 9). This large variation appears to be controlled by SiO, concentration (Fig. 9, correlation coefficient 0.87, n = 13). No clear relationship is observed between the concentrations in the bedloads and the geographical location of the sampling point. The variability observed in the concentrations for the whole set of samples is of the same order of magnitude as the variability observed for the four traverses (pK87, pK577,

Phases

The mineralogy of the particulate phases in the Congo Basin rivers have been investigated by Jouanneau et al. ( 1990) using X-ray diffraction analysis. and by Muller et al. ( 1994). The mineralogy of suspended sediments is overall homogeneous with typical distributions of kaolinite (60%) and associated Fe oxyhydroxides, illite ( 15%)) smectite, plagioclase, and quartz (5%) for the crystallised phases. These rivers are characterized by relatively high proportions of amorphous phases, with an average of 20% in the different rivers. These proportions are higher for the Likouala river where the amorphous phases account for 40% of the total. The most striking result of the present study is that, for numerous elements, independent of the surface area of the different drainage basins, the suspended sediments of the tributaries of the Congo river all have similar patterns. This feature illustrates the idea that large rivers smooth out the lithological and chemical diversity of their drainage basin, as stated in the introduction. However, important deviations from the upper crust defined by Taylor and McLennan ( 1985) are observed. The observation of discrepancies between the mean continental composition and the river suspended sediments, combined with the analyses of the dissolved and sandy river phases presented above, suggest the following conclusions. (I ) The strong depletion of the elements U, Rb, K, Ba, Na, Sr, and Ca (elements of the first set) in the suspended sediments relative to the mean upper crust complements their enrichment in the dissolved phase. It is well known that these elements are mobile in weathering processes, owing to their high hydratation energies (see for example Nesbitt et al., 1980; Kronberg et al., 1987; Cullers, 1988). One exception is Cs, which does not follow the behavior of the other alkali elements in the Congo Basin rivers. This observation disagrees with recent observations of Edmond et al. ( 1995) for the Orinoco Basin. This contrast with the Orinoco river may be explicable in terms of differences in clay mineralogy because Cs is known to be taken up by clay minerals (Cremers et al., 1988). Mass balance considerations imposes that the removal of the most mobile elements from the bedrock will concentrate all the remaining elements in the weathered phases of soils (elements of the second and third set: REEs, SC, Th, Cr, Cs, Hf, Fe, Co, and Ni). This explains

1312

B. DuprC et al

Table 5. Major and trace elements concentrations in the Congo river bedloads fluorescence. Other concentrations are determined by INAA.

sample CS

Rb U n Ba* K* la Ce Ta Sr* Hf zr’ Na* Sm Tb Yb Ca* SC Fe* Cr’ Ni* SiO2* At203*

L&aye

pK87 East bank 0.1 4.3 0.34 1 67 1410 3.1 6.4 0.07

0.08 3.2 0.24 1.2 59 996 3 6.2 0.07 8 1.1 61 148 0.52 0.07 2930 0.6 3360 7 5 97.17 0.85

pK158

pK203

Mot&

0.22 11.5 0.84 3.5 141 3400 8.8 19.3 0.78 25 11.4 473 816 1.33 0.16

0.14 7.5 0.28 1 132

0.4 0.13 0.6 22

276 58 148 0.47 0.06

pK87 west bank 0.18 13 0.65 3.5 161 4230 7.9 18.7 0.13 26 2.8 63 890 1.15 0.13

0.7 6020 9 66 97.06 0.59

2860 1.7 21140 26 17 92.56 1.83

2790 4.2 19880 36 12 92.06 2.16

3.2 8.8 0.07 21 54 816 0.51 0.08 1 15680 21 1s 93.94 1.56

expressedin ppm exceptSD2

and Al203 in ‘&The star * means measured by X-Ray

pK300

pK345

pK520

pK577 m

pK577 bank

pK580

0.14 7.9 0.25 0.99 98 2410 2.8 6.1 0.17 15 1.7 80 223 0.42 0.06

0.13 7.5 0.52 1.1 100 2490 3.3 8 0.19 15 3.1 98 297 0.49 0.09 0.39 3290 1.6 11900 24 9 94.97 1.58

0.09 4.7 0.29 1.4 70 1660 3.6 7.5 0.21 11 2.5 115 223 0.45 0.06

0.48 8.6 0.65 2.7 89 2070 6.9 14.1 0.3

0.1 5.8 0.21 1.2 60 1330 3.9 8.3 0.04 ii 1.2 32

0.1 5.4 0.57 4.2 75 1830 10.3

3143 1.1 4900 10

2860 2.7 10010 20 7 93.1 2.2

1.5 2.8 0.05 7 1.2 30 0.19

2860 0.3 770

3070 1.2 7280 15 5 96.12 1.39

97.28 0.58

96.03 1.08

:.t 150 371 0.95 0.13

0.48 0.05 0.5 3570 8 98.43 0.5

0.42 12 8.2 312 74 0.33 0.13 3290 1.8 4480 15 5 96.43 1.31

pK615 North bank 0.19 9 0.35 0.9 76 2320 2.4 5 0.13 14 2.2 68 371 0.34 0.05 0.18 3640 0.5 196il 5 96.4 1.04

pK615 m

Likwala m

Liiouala bank

0.2 9.1 0.43 1.6 50

0.05 1.8 0.27 0.7 35 495 1.7 3.7 0.11

0.67 10.9 1.34 4.3 109 2410 9.6 19.1 0.58 14 8.3 391

3.9 7.2 0.08

4 146 0.55 0.08

0.2 0.03

0.4

1930 0.7 560 97.54 0.64

1.5 0.31 1.13 3643 3.5 3710 17 5 93.72 2.59

The sand samples collected along the Oubangui and Congo mainstreams are referenced by their distance from Bangui to the sampling site (Fig. 1). For pK 87, pK 577 pK 615 and Likouala locations, tw separate samples were collected in the mainstream (m) and (or) near the river banks. in order to check for lateral variability.

the observation that suspended sediments (removed mechanically from soils) are enriched for these elements with respect to the upper continental crust. The complementarity between suspended and dissolved phases for U, Rb. K. Ba, Na, Sr, and Ca can be used to model the weathering processes and to determine erosion rates (Gaillardet et al., 1996). (2) The same kind of complementarity is suggested by the present data between suspended and sandy phases regarding the elements Zr and Hf. In contrast to U, Rb, Ba, K, Sr, Na, and Ca, their low dissolved levels (at least for Hf, Zr was below detection limits) shows that these elements may not be mobile in the weathering processes. Therefore, their partitioning between the suspended and sandy phases is pro-

pK158

duced by a mechanical sorting of zircons in which these elements are incorporated. The positive anomalies of Hf and Zr in sands relative to suspended sediments (Fig. 10) reflect their presence in the bedloads of rivers. (3) Lastly, the compositional discrepancies between the suspended phase and the mean upper crust of Taylor and McLennan ( 1985 ) for the other elements (Cs, Th, Ta, REEs, Fe, Ni, Cr, and Co) can be attributed to local chemical heterogeneities in the composition of the source material. The enrichment of the suspended sediments for the most compatible elements may be evidence of a mafic component in the local upper crust. This crustal heterogeneity will be discussed in more detail using isotopic arguments in a related paper (Allkgre et al., 1996). We now focus on the variations in absolute abundance in the suspended sediments and sands. To a first order, the negative correlation of suspended load concentrations with Particulate Organic Carbon suggests a simple dilution effect of particulate matter by organic material. In fact. a visual observation of the filters shows that suspended sediments consist of a mixture between inorganic and organic material. If CP,,“, and Cp,,,, respectively, are the true (without any organic material, i.e., the concentration of the inorganic part of the suspended sediments) and measured concentrations in the suspended sediments, then CP,,,,

=

cpt,., i

93

95

97

99

Cs(SiO2) % FIG. 9. Plot of Ce concentrations Cs( Ce) in ppm vs. SiO: concentrations. Cs(SiOz) in percent in the river sand samples of the Congo

Basin. Extreme samples are indicated. The linear correlation coefticient is 0.87. Similar linear relationships are also obtained for each element in the sandy phase.

SM - OSM SM

= (1 - POM)Cpt,,,

where SM is the amount of suspended sediments (organic and inorganic) per liter of river water (Tables 1 and 3 ) and OSM the amount of the organic part of the suspended sediments. POM is the weight fraction of particulate organic material, where POM = 2POC (Thurman, 1986). This equation can be applied to determine POC in the Likouala river using, for example, Th concentrations. If one assume that of the CPwlE is close to 18 ppm (the average concentration

Transport

of sediment

and dissolved

1313

ions in rivers

o Lobaye l pK87E q pK87W w pK 158 A pK 203 A Motaba v pK 300 v pK 345

a

0.1

0.01

\

J

0.001

3

10-i

I

1 I

Cs

U

_

Rb

I

I

/

I

I

I I

I I

I

I I

I

I

I I

I

I

I

I

I

Pb Ba Ce Hf Zr Na Eu Y?J SC Co Ni Th K La Ta Nd Sr Sm Tb Ca Fe Cr

o pk 520 pK577m q pK577 bank m pK580 A pK 615 bank A pK615m v Likouala m I Likouala bank l

0.001

1 ,

, ,

Cs

U Rb

,

,

, ,

,

,

,

,

/ /

,

, ,

,

,

, ,

,

/

/ ,

,

Pb Ba Ce Hf Zr Na Eu Yb SC Co Ni Th K La Ta Nd Sr Sm Tb Ca Fe Cr

FIG. 10. Sand samples concentrations (Cs in ppm) normalized to (Cp in ppm). This graph clearly shows the enrichment of Zr and suspended sediments. Sample locations are given Fig. 1. For pK87, samples were collected across the river in order to check for lateral traverses is of the same order of magnitude as that observed for Brazzaville.

rivers of lowest organic carbon content), then Cp,,, = 5 ppm (Table 3) leads to POC = 0.36 which is significantly higher than the measured value in the Likouala suspended sediments (13.6%, Table 3). This discrepancy can be interpreted by the difference in filter pore size of the sample collected for POC analysis (0.45 pm) and collected for trace and major analysis (0.2 pm). This dilution by organic material can not explain why Ni, Pb, K, Sr, and Na do not behave like the other elements. Nickel is positively correlated with POC and an additional input of this element by organic material is likely because this element is known to be complexed by organic matter (Sholkovitz and Copland. 198 1) Lead is also known to have a great affinity for organic substances but this is not apparent in this dataset. The behavior of Sr, Na, and K is different from the other trace elements in the suspended phase because they are depleted relative to the other elements and because this depletion is highly variable. No correlation between their

the suspended concentrations of the Zaire sample Hf in the river sands with respect to the river pK577, pK6 15, and Likouala locations, two sand variability. The variability observed for the four the whole set of samples between Bangui and

concentration in the suspended phase and POC is expected, because they are controlled by variable weathering intensity from one drainage basin to an other. Finally, the correlations between suspended concentrations and pH or SM can be explained in light of the correlation of POC with SM, already reported by Meybeck (1982) and with pH (correlation coefficient of 0.74). One possibility for the existence of this latter correlation could be that rivers rich in POC are rich in dissolved organic carbon (DOC) as well. A constant mass ratio POC/DOC = 2 is given by Richey et al. (1980). This DOC mostly consists of organic acids (humic and fulvic) that account for the low pH of such diluted and weakly buffered waters (Bemer and Bemer, 1987). As shown on Figs. 9 and 10, the important variations of bottom sands concentrations can also be explained by a variable dilution of other substances by quartz. To

conclude,

it must

be emphasised

that

only

elemental

1314

B. Duprt

ratios should be used to avoid the effect of dilution of river suspended sediments by organic material in making elemental mass budget in rivers. Such a use is exemplified in Gaillardet et al. (1995) or Allbgre et al. (1996). Table 3 shows some elemental ratios of geochemical interest (Sm/Nd, Rb/ Sr, U/Pb, and Th/U). The Sm/Nd ratio (0.17 5 0.09) is the only one close to the ratio of the upper crust given by Taylor and McLennan ( 1985). All other ratios (RblSr = 0.99 + 0.29, U/Pb = 0.084 2 0.039, and Th/U = 5.9 2 1.2) show important discrepancies with average crustal values. These discrepancies are due to the larger mobility of U, Rb, and Sr relative to Th and Pb during weathering. 5.2. Dissolved Phase 5.2.1, Major

Phase and its Relations

with Suspended

elements, major processes

The total cation charge (Z’) and total dissolved solids (TDS) (Table 4) allow a comparison of the rivers analysed here with other world rivers. With Z’ ranging from 202 PeqlL (Likouala) to 523 peq/L (Zaire), the Congo Basin rivers are comparable to the lowland rivers of the Amazonian Equatorial Basin defined by Stallard and Edmond ( 1983) as being influenced by both atmospheric inputs and the weathering of siliceous terrains and therefore enriched in silica relative to other species. On a more global scale, our data can be compared to the compilation of Bemer and Bemer ( 1987) and to the mean Earth values proposed by Meybeck ( 1979). It clearly appears that the Congo rivers are among the most diluted in the world (world average Z’ = 1210 peq/L), the most acidic (world average pH = 7.5 ), and the most enriched in Si relative to other species. Another feature of the Congo Basin rivers is their high proportion of dissolved organic carbon (Meybeck, 1982; Thurman, 1986) in particular for the Likouala and Alima rivers. These characteristics have already been noticed for all wet tropical rivers (Berner and Bemer, 1987; Edmond et al., 1995). In a recent paper, NCgrel et al. ( 1993) focused on the major element chemistry of the Congo rivers of this study. The systematics of an extensive set of monolithological waters allowed them, using elemental ratios (involving Ca, Na, Mg, Sr, and Cl) and Sr isotopic ratios, to show that the dissolved load of each Congo river tributary results from the mixing of atmospheric and rock weathering components. The proportions of Ca, Na, Mg, and Sr coming from the different reservoirs (rain, carbonates, silicates, and evaporites) were then computed. Keeping in mind these results for major ion chemistry in the Congo Basin rivers, we will now examine whether the same formalism can be used to explain the distribution of the other trace elements in the dissolved phases. 5.2.2. REE normalized

Cw/Cp patterns

Numerous studies on trace elements in rivers have shown that dissolved elemental concentrations are dependent upon the filtration pore size used to separate the suspended and dissolved loads (Fig&es et al., 1978, for Fe; Moore et al., 1979, for Mn; Stordal and Wasserburg, 1986, for Nd). Focusing on the REEs, Goldstein and Jacobsen ( 1988) (us-

et al.

ing a 0.2 pm filtration protocol), Elderfield et al. (1990) (using a 0.45 pm filtration protocol), and Sholkovitz ( 1992) (using 0.45, 0.2 and 0.02 pm filters), confirmed this result and showed that what is called dissolved load may, in fact, be the mixture of a colloidal fraction ( <0.2 pm or 0.45 pm) and a true dissolved fraction. Following Stumm and Morgan ( 1981) and the above authors, colloids are the particles that bypassed a 0.2 or 0.45 pm pore size filter. Recently, the use of tangential flow ultrafiltration techniques has proven useful in isolating and fractionating river water colloids. Sholkovitz ( 1995) for the Connecticut and Hudson rivers or Viers et al. (1995), for organic-rich rivers of South Cameroon confirmed that the <0.22 pm, <50000 MW and <.5000 MW (molecular weight) river water fractions have progressively decreasing concentrations. These studies also highlight the close association between dissolved organic matter and colloids. Therefore, if a significant part of the dissolved material in the Congo rivers studied here is actually made of fine colloids, the normalisation of the dissolved concentrations to the suspended ones allows a comparison between the composition of the colloidal phase and that of the suspended phase. Another advantage of the normalization of the dissolved concentrations to the suspended ones is to remove the variations of the absolute abundances due to variations in the source materials abundances. We first focus on the REE patterns before considering the extended patterns. The REE normalized CwlCp patterns are shown on Fig. 11 and compared to the data of Goldstein and Jacobsen ( 1988). None of the patterns are flat. All of the rivers in this study show a gradual enrichment from La to Eu in the dissolved load relative to the suspended load. Europium, Tb, and Yb patterns are flat. No Ce depletion is observed, and the Likouala and Alima rivers show a slight negative Eu anomaly. The Congo rivers have distinctive REE patterns compared to all the others rivers analysed by Goldstein and Jacobsen (1988), but all the rivers analysed by Goldstein and Jacobsen (1988) show a variable enrichment of HREEs with respect to the suspended load. The enrichment of dissolved HREEs relative to the suspended sediments seems therefore to be a general feature. Following Goldstein and Jacobsen (1988), an index of this enrichment is Cw(La)

&am =

Cp(La) aCw(Yb‘, Cp(Yb)

Cw(La)

= CwO’b) CDcLa) I\

I

.

Cp(Yb)

The Black rivers are the less fractionated with respect to suspended sediments as indicated by their values of KLalYbr 0.78 and 0.73 respectively, similar to the enrichment factor of the Isua Lake. In the dataset of Goldstein and Jacobsen ( 1988), the greater KLalYb value ( KLalYb = 1) is obtained for the Great Whale River. In the other Congo rivers, the KLaI Yh value fluctuates from 0.52 in the Kasai River to 0.70 in the Zaire River, which is comparable to the enrichment factor for the Pampanga River in the Philippines. The rivers of lower pH tend to have the highest KLalYh values but no clear relationship exist between KLalYb and pH for the rivers of this study and those from Goldstein and Jacobsen ( 1988).

Transport of sediment and dissolved ions in rivers

1315

o Oubangui 1 l Lobaye 0 Zaire n Likouala A Alima A Kasai v Congo 64

b

o Amazon Great Whale q Indus n Isua A Mississippi A Ohio v Pampanga v Shinano l

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu FIG. I I. REE abundances in dissolved loads (Cw in ppb) normalized (Cp in ppm) (a) for the rivers of this study. The Black rivers display set of rivers from Goldstein and Jacobsen (1988)

The fractionation of REEs between the true dissolved and colloidal fractions in rivers has been documented by several pioneering studies. In North American and UK rivers, Elderfield et al. (1990) recognised from plots of REE ratios vs. REE concentrations, the existence of a colloidal phase having a shale-like or slightly LREE-enriched La/Lu ratios. The ultrafiltration experiments carried out by Sholokovitz (1992, 1995) on the Connecticut and Hudson rivers and Viers et al. ( 1995) in West African rivers confirms the similarity between colloidal and shale REE patterns, but leads to contrasting conclusions concerning the fractionation of REEs between the true dissolved and colloidal phases. On one hand, the rivers analysed by Sholkovitz (1995) show that dissolved REEs are more HREE enriched (with respect to the continental crust) when the filter pore size decreases. On the other hand, the tropical river waters filtered by Viers et al. ( 1995) do not indicate a fractionation of REEs in filtrates with decreasing pore size filter. A shale like pattern is obtained for the <5000 MW filtrates of the organic-rich (DOC = 20 mg/L) rivers of their study. Although ultrafiltration experiments would be necessary to complete the following interpretation, all these studies strongly suggest that the high dissolved REE levels reported here for the rivers of the Congo Basin are due to the presence of a colloidal phase. Our results (Fig. 11) are consistent with the existence in the <0.2 pm dissolved river fractions, of colloidal particles having approximately the same REE patterns as the suspended sediments. Noticeably, the most-or-

to the REE abundance of suspended materials the most enriched and flat patterns (b) for the

ganic rivers (Likouala and Alima rivers) have the highest dissolved REE concentrations; this supports the close association of colloids and dissolved organic carbon already pointed out by the above studies. However, the slight HREE depletion in the less organic rivers (Fig. 11 a) may have several origins, that are discussed below. ( 1) In the less organic rivers, the colloidal particles that account for the REE dissolved concentrations are slightly HREE enriched with respect to suspended sediments. (2) The slight HREE enrichment of the less organic rivers indicates the contribution of a true dissolved pool of REEs. According to this hypothesis, the true dissolved pool must have HREE-enriched patterns similar to those described by Sholkovitz ( 1992) or Sholkovitz ( 1995 ) For example, in Sholkovitz ( 1995 ). the true dissolved pool of the Connecticut river (<5000 MW) is characterized by KLalYb = 0.05. This strong enrichment in HREEs for the true dissolved pool is consistent with the tendency of REEs to form inorganic complexes with CO:-, OH ~, SO:-, and Cl (Turner et al., 1981; Wood, 1990) and with the gradual increase of the surface adsorption stability constants from LREEs to HREEs (Sholkovitz, 1992, and references therein). A simple mass balance equation of REEs in river water can be written (REEL where

= (REEL,,

+ (REEhis,.

( REE I,+, ( REE hoi, and (REE),,,,

denote

the total

B. DuprC et al.

1316

(measured). colloidal, and true dissolved mass of any REEs in I L of river water. Applied to La and Yb:

(x)*,=(it),,,(y+H,,,. (l - a)T where Ly=- (Yb LI (Ybh, is the proportion of Yb present in a colloidal form in the river. Then, dividing by the La/Yb of the suspended phase we get

As discussed earlier, if we assume that K&,h = 1 (colloidal phase patterns are similar to suspended sediments, Sholokovitz. 1995; Viers et al.. 1995) and K&, = 0.05 (Sholkovitz, 1995)) then the proportion of colloidal Yb (or of any REEs) can be estimated. The proportion of colloidal La can be deduced from the proportion of Yb by dividing by K,.,I,Yh, This approximate calculation applied to the river set of the Congo Basin gives proportions of colloidal Yb close to SO60% in the nonorganic waters and close to 70-7X in the Black rivers. The proportion of colloidal La is close to 9598% for all the rivers investigated. The true dissolved level of La is then estimated to be = IO ppt in the nonorganic rivers (similar to the Mississippi and Ohio measured levels, after Goldstein and Jacobsen, I988 ), and 20 ppt in the Black rivers. (3) Conversely, we can consider that the true dissolved REEs have a flat pattern with respect to the continental crust, in agreement with the observations of Viers et al. ( 1995) in small tropical rivers of Cameroon. In these diluted and organic-rich streams, whose aquatic chemistry is close to that of the Congo rivers, the complexation of true dissolved REEs by humic acids should predominate. In this case. the slight enrichment of HREEs observed in the less organic rivers with respect to the suspended sediments could be due to the preferential adsorption of LREEs on suspended particles. Note that the most organic-rich rivers (Alima and Likouala) have the lowest pH values and the lowest suspended sediments concentrations (=5- 10 mg/L) compared to the other rivers. These two features tend to reduce the importance of adsorption processes. Finally, the inverse correlation between pH and dissolved REE concentrations reported in Fig. 6 for the Congo rivers and more generally at a global scale, can be interpreted in several ways. ( 1) These trends may indicate the decreasing importance of colloidal particles with increasing pH. A process of colloid coagulation and agglomeration onto suspended sediments with increasing pH (or/and ionic strength like in estuaries) can be invoked. With increasing size, colloids are filtered with increasing efficiency. (2) Many reasons can be invoked to suggest that true dissolved REE (e.g., free La” ) concentrations are likely to vary with pH. Acidic rivers are generally organic rich and so the ability of REEs to be complexed by organic acids increases. However. in rivers with high pH. carbonate ion

concentration is important, as a result carbonate complexation is high and this tend to enhance the dissolved REE concentrations. ( 3) Finally, when pH > 7, both adsorption processes and (or) coprecipitation by Fe-Mn oxyhydroxides scavenge REEs from solution, preferentially the LREEs (Sholkovitz, 1995). It is worth noting that Fig. 6 is consistent with the surface-complexation adsorption models of Schindler and Stumm ( 1987 ) The decrease in REE dissolved concentrations with pH could therefore be a complex combination of a number of mechanisms. The difficulty in understanding the role of pH on the REE dissolved abundances resides in the fact that acidic rivers tend to have high DOC concentrations and low suspended sediment concentrations. It is clear from the present results, that all future investigations of dissolved REEs in the Congo Basin rivers will have to report ultrafiltration techniques in order to confirm the conclusions proposed in this section, to better constrain the fractionation of REEs between the dissolved and colloidal fractions, and finally to assess more precisely the role of pH on dissolved REE concentrations. 5.2.3. Extended normalized

Cw/Cp

diagrams

The extended normalized Cw/Cp patterns are plotted on Fig. 12. Figure 12a displays all the samples, Fig. 12b is a simplified version, comparing a typical Black river (Likouala) to average of less organic-rich rivers. Elements of the jirst set (U, Rb, Bu, K, Na, Ca, and Sr). On Fig. 12. the elements of the first set show strong positive anomalies relative to the other elements, confirming their enrichment in the dissolved phase relative to the suspended phase. This enrichment fluctuates from one sample to another but is not the highest for the Black rivers, in contrast to what is observed for the other elements. As stated above, these elements show no correlation with pH. Their behavior is determined by atmospheric inputs and mixing between different rock sources and does not seem to be influenced by the presence of colloidal material that bypasses the filters. The high correlation coefficients observed between (U, Ca), (Ba, K), (U, Sr), and (K, Na) supports the idea that U, Ba, and K may also be controlled by the mixing of reservoirs required to explain the distribution of Na, Ca, Sr, and Mg. Uranium is known to be partially controlled by dissolution of carbonate (Palmer and Edmond, 1993). The quantification of these mixing processes requires knowledge of endmembers. This in turn requires a systematic study of monolithological waters (Nkgrel et al., 1993) and will be reported elsewhere. An order of “solubility” can be deduced by considering Fig. 12. The most enriched element is Na which has C,/C,, ratios ranging from 0.36 (Lobaye) to 0.95 (Zaire). Then, in decreasing order of enrichment: Sr ( C,/C, = 0.14 in the Lobaye to 0.32 in the Sangha), K (C,/C, = 0.09 in the Lobaye to 0.23 in the Sangha), Ca (G/C,, = 0.06 in the Kasai to 0.17 in the Likouala), Ba (C,/ C, = 0.06 in the Zaire to 0.14 in the Likouala), Rb (C,/C, = 0.03 in the Lobaye to 0.1 in the Alima), U (C,l C, = 0.01 in the Lobaye to 0.03 in the Sangha). Naturally, this order of “solubility” depends on atmospheric inputs

Transport of sediment and dissolved ions in rivers

1317

a

o Oubanguil l Lobaye 0 Zaire w Likouala A Sangha A h%li v Congo 64

0 average pattern Likouala

l

FIG. 12. Diagram showing the dissolved load concentrations (Cw in ppb) normalized to the suspended load concentrations (Cp in ppm). All the samples are plotted on (a). On (b) the average of Oubangui, Lobaye, Zaire, Kasai, and Congo 64 is compared to the Likouala river pattern. The most soluble elements exhibit strong positive anomalies while the other elements have convex patterns centered on the HREEs. Hafnium and iron are strongly depleted in the dissolved load relative to the suspended load.

and on the proportions and erosion rates of carbonate, evaporites, and silicate rocks on the drainage basin and has therefore little to do with solid/solution partition coefficients of these elements during the erosion processes. Elements of the second set (REE, SC, Th, and Cr). The observations and interpretations inferred from the REE Cwl Cp patterns can be extended to the other elements of the second set because they are well correlated. Figure 12 shows that the patterns of these elements show a slight upward convexity centered on the HREEs. Chromium, scandium, thorium, and lanthanium have flat patterns in both nonorganic influenced rivers and Black rivers. The normalized dissolved Cr/Th ratios (KcrlTh) range from 0.62 (Likouala) to I .38 in the Oubangui (the mean value is I), whereas KYblCr ratios range from 1.5 (Oubangui) to 3.8 (Lobaye). As inferred from the REE Cw/Cp patterns, there is a possibility that the presence of REEs in the dissolved loads is due to a mixture between a colloidal pool with a composition similar to the suspended sediments and a true dissolved pool. This interpretation can be extended to Th, SC, and Cr. In this scheme, their presence in the dissolved phase could be mainly due to a colloidal component with a composition similar to that of suspended phase. This hypothesis is in agreement with the very low levels of dissolved Th concentrations in fresh waters (cf. Langmuir and Herman, 1980,

for a review). The discussion of the role of pH on REE dissolved concentrations also applies to Th, SC, and Cr but ultrafiltration experiments would be necessary to refine this interpretation. Elements of the third set (Cs, Hj Fe, Co, and Ni). In contrast to the elements of the second set, these elements show more irregular patterns ranging from 6. 10 -’ (Fe, Lobaye) to 8 * lo-’ (Co, Likouala) with no systematic order of enrichment from one element to another (Fig. 12a). Figure 7 shows that they are not correlated with the elements of the previous set in the dissolved phase (whereas good correlations coefficients exist in the suspended phase) and all the elements of this group display good correlation coefficients with pH. Among them, Fe, Al, and Hf are clearly depleted in the dissolved load relative to the suspended load. ( 1) Although Fe is slightly enriched in suspended sediments with respect to the continental crust (as all the compatible elements, Fig. 3), a strong Fe depletion, with respect to suspended sediments characterizes all the rivers of this study, except for the Likouala river (Fig. 12). The Fe/La dissolved ratios (except Likouala) range from 231 to 580 and are globally similar to the Fe/La ratios reported by Sholkovitz ( 1995) in the Connecticut and Hudson rivers, despite the fact that Fe concentrations in the Connecticut and Hudson rivers are one to two orders of magnitude lower than

1318

B. Dupri et al.

those of the Congo River. By contrast, the Fe/La in suspended sediments is close to 1500. (2) Another element that is depleted in the colloidal phase is Al. This element was not represented in Fig. 12 because its concentration in the suspended sediments was not measured, due to insufficient amounts of suspended material for analysis. However. an average of 30% for A&O3 is reported for the Congo suspended load at Brazzaville by Nkounkou and Probst (1987), which leads to Al/La ratios in the suspended load close to 3800 at Brazzaville. By contrast, the Al/La ratio in the dissolved phase ranges from 25 in the Sangha River to 880 in the Likouala River (the Zaire value is 130). A striking deficit of Al in the dissolved phase relative to the other trace elements is thus observed. (3) The last element to be depleted is Hf. As previously noticed, Hf is depleted in both the suspended and dissolved phases (Fig. 3). Figure 12 also shows a strong depletion of Hf in the dissolved phase with respect to the suspended sediments for all the rivers of this study. As shown, for example, by Elderfield et al. (1990) and Sholkovitz ( 1995), dissolved Fe (as REEs) concentrations in rivers are strongly affected by the pore size of the filter used to separate the dissolved and suspended fractions, the dissolved concentrations decreasing with decreasing pore size. Such observations support the idea that a fraction or the totality of Fe is in a colloidal form. For Co and Ni (and generally, all transition elements) complexation by organic matter has been reported by Sholkovitz and Copland ( 198 1) and Lee ( 1983). Iron organic complexation by organic colloids of low molecular weight has also been reported by several authors: Perdue et al. (1976), Sholkovitz et al. ( 1978)) Moore et al. ( 1979), and Eyrolle (1994). In this latter study, high correlation coefficients between DOC and Fe dissolved concentrations are reported for small tropical rivers draining podzols in Central South America. For the tropical rivers of the Congo basin, such arguments suggest that Fe (and by extension the other elements of the third set) is present in a colloidal form. This conclusion could definitely be confirmed by the use of ultrafiltration techniques, but if this proves to be correct, then the present dataset show that these colloids are strongly depleted in Al, Fe, and Hf compared to the elements of the second set (REE, Th, SC, Cr) It is interesting to note that, because Fe and Al are major components in the suspended solids, their depletion in the dissolved phase indicates that the colloidal material that bypassed the filters can not have a composition similar to that of suspended solids and does not consist, for example, of fine (<0.2 pm) suspended-like material. The ultimate cause of this Al, Fe, and Hf depletion is not clear from the present dataset. Aluminum, iron, and hafnium may be depleted in the dissolved phase because these element are controlled by weathering-resistant (primary or secondary) minerals. This is likely the case for Hf, preferentially incorporated into zircons. The incorporation of Fe and Al in insoluble iron and aluminium hydroxides (of laterites) could be a similar mechanism that would account for the depletion of Fe and Al in waters. Alternatively, the Fe and Al depletion in the dissolved phase could also reflect the lower affinity of colloids to complex Fe and Al, compared to the other trace elements. Moreover, the high correlation coefficients between the elements of the third set and pH, seems to

indicate that different colloid complexing sites are involved in the fixation of the elements of the second set and that of the third set. Another possibility is that two types of colloids exist. In such a scheme, the colloids that complex the elements of the third set would have to be more pH dependent than colloids which complex the elements of the second set. Finally, the possibility that the majority of Fe and by extension, all the elements of the third set, can be in a true dissolved form can not be ruled out by the present results. The good correlations with pH could reflect complexation of these elements by organic acids such as humic and fulvic acids. These macromolecules, which are responsible for the black color of the Congo Basin rivers are known to account for 60% of the dissolved organic carbon (DOC) and their concentration increase with decreasing pH (Thurman, 1986). 6. DISSOLVED

AND SOLID

TRANSPORT

One of the principal aims of this paper is to compare the transport fluxes (mass of element per liter of water or per year) in the “dissolved,” suspended, and sandy phases of the Congo rivers. To estimate the particulate fluxes, the amount of suspended sediments per liter of water (SM) measured during the cruise has been used (Table 1) An estimate of 10% of SM for the amount of transported sands is consistent with the proportion admitted in the literature (Gibbs, 1967; Milliman and Meade, 1983; Pinet and Souriau, 1988). The choice of mean values for concentrations of major and trace elements in the bedload sands is not straightforward because of their great variability (Table 5, Figs. 9 and 10). In the calculations presented here, the maximum values of sand concentrations have been used (corresponding to sample pK158). Figure 13 illustrates in the form of stack columns the proportion of each element transported by the three river phases, in the non-organic rivers (Fig. 13a) and in the Black rivers (Fig. 13b). 6.1. Non-organic-Dominated

Rivers

(Fig. 13a)

These rivers (Oubangui, Lobaye, Zaire, Kasai, and Congo 64) have typical SM values of 20-30 mg/L. The transport of the elements of the first set (U, Rb, K, Ba, Na, Sr, and Ca) is partitioned between the suspended and dissolved phases and for all of them, the sandy flux is negligible. The proportions of dissolved transport ranges from 95% for the most soluble element (Na) to 40-50% for the least soluble element (U) . The proportions of the least insoluble elements (the second and third set) in the “dissolved” phase ranges from lo-20% (Th) to 20-40% (for HREEs) , these proportions increasing from the Oubangui river at Bangui to the Congo at Brazzaville. For example, 18% of Th is exported in the <0.2 pm phase in the Congo river at Brazzaville. If the suspended sediment concentrations measured in the river at the time of the cruise is taken as a mean annual value, 80 tons/y of Th is transported in the dissolved phase and 440 tons/y in a suspended form. As emphasised earlier, these proportions and exportation fluxes are most probably the proportions and exportation rates of colloidal transport rather than true dissolved transport. Finally, Zr and Hf are mostly transported by the sandy and suspended phases (70%).

Transport

of sediment

2 70 D .e 60 ‘1

50

3

40

g

30 20 10

cs

u Rb

(b)

K Th

La Ba

Hf Ce

Sr Nd

Sm Na

Tb

Ca

Eu

Yb

Fe SC

Cr Co

Ni

and dissolved

1319

ions in rivers

of a high content of colloids. All other “soluble” element proportions are higher than in the non-organic-dominated rivers. In conclusion, the Congo Basin rivers are an example of a very simple and extreme chemical transport. All the elements analysed in this study are transported significantly by the suspended sediments (consisting mainly of kaolinites) The exceptions are Hf and Zr which are predominantly carried in zircons and U, Rb, Ba, K, Na, Sr, and Ca, which are transported in the dissolved load. Owing to high concentrations in “dissolved” trace elements, probably related to the presence of a colloidal phase (especially for the Black rivers) a significant part of these elements is transported in the soluble phase. This is in spite of their very low solubility.

100 90

7. CONCLUSIONS

80

e

a .z .u

70

60

B M

5o

8

40 30

cs

u Rb

K Th

La Ba

Hf Ce

Sr Nd

Sm Na

soluble

Tb Eu

Ca Yb

Fe SC

Cr Co

Ni

phase

m

suspended

q

sandy

phase

phase

FIG, 13. Elemental stack column diagram showing the elemental proportions which are transported by the sands, the suspended sediments and the <0.2 pm filtered load (a) for the less organic rivers (Oubangui, Lobaye, Zaire, Kasai, Congo 64) and (b) for the Black rivers, Likouala and Sangha rivers. The amount of sand transported per liter of water is taken as 10% of the suspended sediment concentration (SM). Among the elements analysed. only a few elements are significantly partitioned between particulate and dissolved loads: U, Rb, Ba, K, Na, Ca, and Sr. Zirconium and hafnium are partitioned between the sands and suspended sediments. The dissolved flux for the most insoluble element (e.g., Th) is close to 10% of the total flux in the non-organic rivers, but increases up to 50% in the Black rivers.

6.2.

Black

Rivers

(Fig.

13b)

The distinction between black and non-organic dominated rivers is purely arbitrary. Rather than a clear distinction, a gradual transition exists from one river to the next, as indicated by pH values. Likouala. Alima. and Sangha rivers represent the rivers the most influenced by organic material coming from the degradation of continental biomass. They exhibit high trace element content in the so-called “dissolved phase” and very low SM values. As a results, these rivers have trace element transport rates in the “dissolved” phase always greater than in the suspended phase. As an example, the fluxes of the less insoluble elements in the Likouala river range from 70% (for Th) to 8.5% (for HREEs) of the total transport flux. For the most soluble elements, only the transport of Na is not affected by the presence

This paper presents a trace element systematics of the Congo Basin river and several of its tributaries, in both dissolved and solid phases. The general conclusions of this study are: ( 1) The three river-borne phases of the Congo Basin (dissolved, suspended, and bedloads) have very contrasting chemical signatures. Our study shows the chemical complementarity between the three river-borne phases relative to the mean continental crust composition of Taylor and McLennan ( 1985 ). Suspended sediments and dissolved phases are complementary reservoirs for the most soluble elements (U, Rb, Ba, K, Na, Sr, and Ca) which are depleted in the former and enriched in the latter. Suspended sediments and bottom sands are complementary reservoirs for Hf and Zr owing to the high concentration of zircon in the sands. All other species are enriched in the suspended sediments relative to the mean continental crust and their patterns show a slight enrichment in the most compatible elements. These chemical complementarities are used in a mass budget model in Gaillardet et al. (1995). (2) A major difference is observed between the Likouala, Alima, and Sangha rivers and the other rivers. These rivers flow entirely under the tropical rainforest and are typical Black rivers. They display very low suspended sediments concentrations (5 mg/L) and their high amount of particulate organic matter dilutes all the major and trace element concentrations in their suspended phases (except Sangha). These rivers are the most colored (they have high dissolved organic matter), have the lowest pH values, and exhibit the highest dissolved concentrations in REEs and associated elements. In these rivers, 80% of the elements that are normally classified as insoluble are transported in the “dissolved” phase as a result of their high “dissolved” concentrations and of the low river suspended material concentrations. (3) One of the most obvious observations of this study is the decoupling between the most soluble elements (U, Rb, Ba, K, Ca, Na. and Sr) and the other trace elements. At a global scale, the Congo waters are among the most diluted (with respect to the dissolved concentrations of U, Rb, Ba, K, Ca, Na, and Sr) but conversely. are among the most concentrated rivers with respect to the dissolved concentrations of the other trace elements, such as REEs. Although the concentrations of Ca, Na, Sr, and by extension U, Rb, Ba, and K, can be accounted for by mixing of

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B. Dupre

rainwater and waters draining the major lithologies (carbonate and silicate), the concentrations of the other trace elements rather seems to be dominated by the existence of a colloidal phase that bypassed filters during the filtration step. The physicochemical characteristics of these colloids cannot be precisely determined from the results of this study and future works including physical isolation and characterization studies of the colloidal material for the Congo rivers or other large tropical rivers are clearly necessary. Several investigations of rivers in the industrial countries of the northern hemisphere emphasised the importance of the colloidal fraction in the transport of trace elements in natural waters (Salbu et al.. 198.5; Tanizaki et al. 1992). The results of these studies, which apply size fractionation methods to the dissolved phase, are consistent with our conclusion that the solubilities of a number of trace elements are very limited and mainly controlled by a colloidal phase. One particular aim of these investigations will be to understand the relations between the colloidal phase (its abundance and composition) and the river chemistry parameters (such are TDS, pH, E ’ ) , which are ultimately controlled by weathering conditions and rock types. (4) The conclusions of this paper (concerning trace element concentrations) and of the previous paper of NCgrel et al. ( 1993 ) (concerning soluble elements) indicate that caution is in order when partition coefficients between water and suspended solids or between water and the upper crust (e.g., Whitfield and Turner, 1979) are calculated, at least for the tropical rivers. On the one hand, the most soluble elements in the dissolved phase are controlled by the dissolution in various proportions of different rock types (mainly carbonate, evaporite, and silicate) and by atmospheric inputs. On the other hand, the abundance of insoluble elements is controlled by the presence of colloids whose concentration seems to be related to river chemistry. It is therefore obvious that, in such a scheme, apparent partition coefficients between the water and the suspended solids or between the water and the upper continental crust, being dependent on filtration pore size, have no real physical meaning. Acknowledgments-This work was supported by the INSU/PIRAT/ DBT program. We thank the scientists of ORSTOM (Office de Recherche Scientihque et technique d‘Outre Mer) in Bangui and Brazzaville and especially J. C. Olivry. 3. P. Bricquet, and M. Thiebaux for their technical assistance during the sampling. F. Capmas, H. Chazaly. G. Michard, N. Lefol. G. Pinte. J. L. Joron, B. Reynier. and M. Valadon are acknowledged for their analytical assistance. P. NCgrel, M. Roy Barman. and S. Roy are thanked for their helpful comments. Eric Lewin is acknowledged for its statistical assistance. We are also grateful to the reviews of .I. Schott. E. Sholkovitz, D. Turner, R. H. Byrne, and two anonymous reviewers who contributed to substantial improvement of the manuscript. B. Bourdon is acknowledged for English corrections. This is IPGP contribution no. 1408, CNRS/INSU contribution no. 7.56. Editorial

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