Suspended matter and bottom deposits in the Mahury estuarine system (French Guiana): Environmental consequences

191

Netherlands Journal of Sea Research 21 (3): 191-202 (1987)

S U S P E N D E D M A T T E R A N D B O T T O M D E P O S I T S IN T H E M A H U R Y E S T U A R I N E SYSTEM (FRENCH GUIANA): ENVIRONMENTAL CONSEQUENCES

J.M. JOUANNEAU and M. PUJOS Department of Geology and Oceanography, Aquitaine Basin Geology Institute, Avenue des Facult#s, 33405 Talence Cedex, France

ABSTRACT As part of a study carried out on the major rivers in French Guiana to identify and estimate the respective share of local rivers in the sedimentation of estuaries and the inshore continental shelf, results are reported on the Mahury after 2 periods of fieldwork carried out during one dry and one rainy season. The specific characteristics of the whole Mahury fluvio-estuarine system can be summed up as follows: the sands have invariable annual characteristics; they show evidence of a long history, first marine, then continental and finally fluviatile. They play little part in sedimentation on the continental shelf, and what part they do play is limited to the beaches. The fine sediments too have invariable annual characteristics. They fall, if we consider their mineralogical and elemental composition, on either side of a limit separating the fluviatile and estuarine zones. Here, the sediment stocks are respectively authigenic and allothogenic, while the clay association and elemental composition in the latter zone are evidence of an Amazonian origin. The contribution of the Mahury (Comte-Orapu) to fine estuarine sedimentation thus appears at present to be a very limited one, the Amazon be. ing predominant over vast coastal areas, including the French Guiana estuaries.

1. INTRODUCTION The coastline and inshore continental shelf of French Guiana are subject to cycles of silting and unsilting due to the fine particles suspended

in the water. Authors have shown that this silt was of Amazonian origin, carried northeast by the Guianas current (GIBBS, 1976; Hydraulics Laboratory Delft, 1962; NEDECO, 1968; PuJos & ODIN, 1986). While not denying the predominant role played by that river, we shall endeavour to assess the influence of the rivers of French Guiana on sedimentation in the estuary and inshore area. For the purposes of this study, we carried out twice-yearly sampling (in the dry and rainy seasons) at points between the mouth and the area upstream of the waters affected by dynamic tide, thus covering the specific characteristics of the whole fluvio-estuarine system, to assess the respective influence of the Mahury and the Amazon in estuarine and coastal sedimentation. Acknowledgements.--This work forms part of a programme of research into the marine environment in the Caribbean Sea and French Guiana. The CORDET Commission has encouraged this research with its support. We wish to thank the Gendarmerie forces of Apatou and Cacao for the sampling and filtering of water samples over one year. Without their participation, this work could not have been completed. We should also like to thank the Maritime Division of the French Guiana Department of Works, whose perfect familiarity with the problems of silting was extremely useful, and who very kindly made their transport facilities on both the Maroni and the Mahury available to us. Finally, our thanks go to Mile Eliane Gonthier and Mr Phillippe Legigan, who carried out the heavy mineral counts and the quartz exoscopy examinations, respectively.

192

J.M. JOUANNEAU & M. PUJOS

2. METHODS

2.2. FIELD MEASUREMENTS

2.1. SAMPLING

Salinity and temperature were determined with a Cambridge in situ salinometer (in November) and a Ysi S.C.T. meter (in May). Part of the water sampled was immediately filtered on board with a Nalgene hand-operated vacuum pump, through 0.45 ~zm Whatman filters, oven dried and weighed in the laboratory for suspended matter (S.M.). 100 cm 3 of the filtered water was then acidified with Prolabo Normatum HCl (1 cm 3 of acid in 100 cm 3 of filtered water) for subsequent determination of the quantities of dissolved Fe and Mn. A further flask of untreated water was kept for reference purposes.

Two field studies were undertaken; the first in November 1984, during the dry season, and the second in May 1985, during the rainy season. Both campaigns were carried out on the area between the mouth and the upper limit of the zone affected by dynamic tide (Fig. 1). Positioning of stations downstream of Roura (on the Mahury) was carried out by radiolocation (using the Syledis system), and in the upstream part by topographical positioning using the French National Geographical Institute's 1:50000 scale maps. Bottom samples were taken either with an Eckmann bucket or a Berthois cone. Samples of water and suspended matter were taken with a horizontal Ottmann type bottle - 5 0 cm below the surface.

2.3. LABORATORY MEASUREMENTS 2.3.1. MAJOR AND TRACE ELEMENTS The bottom sediments, and especially the muddy

~6 e

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Fig. 1. Location maps with numbered stations sampled in two seasons.

SUSPENDED MATTER IN THE MAHURY

sediments, were analysed for major and trace elements. Concentrations were measured by Xray fluorescence. Quantities of dissolved Fe were determined by orthophenantroline colorimetry, and of dissolved Mn by atomicabsorption spectrometry (the electrothermal method). Carbonate concentrations were measured by the volumetric method. Particulate organic carbon (P.O.C.) content was determined by wet oxidation using the oxidoreduction method derived from that of STRICKLAND & PARSONS (1972) and adapted by ETCHEBER (1981). Results are expressed as a percentage of dry weight. A leaching technique was used for determining the "removable" manganese and iron content of some deposited sediments. 1 g of the sample was crushed and then agitated with 100 cm 3 of HCI 0.1N for 24 hours. After centrifugation, the supernatant etching liquid was stored in polyethylene flasks for subsequent flame atomic-absorption spectrometry.

193

hydrogen peroxide. The raw granulometric data in weight of each size group in relation to the whole sample were processed by computer.

3. RESULTS 3.1. DISTRIBUTION OF SEDIMENTARY FACIES The sediments on the bottom fell into distinctive categories --mainly sandy upstream and muddy downstream, the boundary between the two lying roughly downstream of the point where the Comt~ and Orapu join, near station HU16 (Fig. 1). There were, however, a few exceptions with sandy muds on the Comt~ (My 19/22) and muds on the edges downstream of Roura. Samples taken several months apart, in the rainy and dry seasons (November 1984 and May 1985), show that there was no noticeable seasonal variation in grain size. 3.1.1. SAND

2.3.2. MINERALOGICAL ANALYSIS X-ray diffraction analysis was used to identify clay minerals. This analysis was performed on a previously decarbonated oriented paste of the fraction below 2 #m. Quantity determinations and estimations were based on the 3 classical diagrams --normal, glycol and oven (450°C) - at an angle of between 2 and 32 ° (2e), using a K(xCU ray and rear monochromator. 2.3.3. GRANULOMETRY - - Microgranulometry: Microgranulometric analysis of the fraction below 63 #m was carried out by sedigraphy, using a Coutronics 5000 sedigraph. After straining through a 63 #m screen and eliminating organic matter with hydrogen peroxide (20 vol.), the sediment was dispersed by ultrasound action for 15 min. The addition of sodium hexametaphosphate (1%o
An examination of the various granulometry graphs shows that the stocks of sand were fairly homogenous for all the samples dredged in the centre of the river. The graphs for the stations My 21 and HU 5, from samples taken near the banks, were noticeably different from the others (Fig. 2). The following granulometric analysis is based on a study of the various distinctive parameters (Table 1). In the Mahury/Comt6, the sands were characterized by: --an average of between 0.48 and 0.93 (in (/) units) for all the samples from near the centre of the river. At the edges, the average was around 2, resulting from an increase in the finer elements. --very good classification (inclusive graphic standard deviation, Trask's coefficient), especially around the average grain size (sharpness). --very good classification of the finest elements (skewness and inclusive graphic skewness). The relationships between the various parameters showed, irrespective of the season: --that the good classification remained more or less constant whatever the variations in average grain size and whatever the tendency towards positive or negative skewness. --that the average increased in proportion to the skewness value. This resulted from good classification of the fine grains when the average

194

J.M. JOUANNEAU & M. PUJOS

TABLE 1 Granulometric parameters as determined by two different methods from samples collected in November 1984 and May 1985 in the Mahury river. Except for My 21 and Hu 5 (marked by x), all samples were taken near the centre of the river. moment method

graphic method

means station

means

o units

microns

Standard Skewness Deviation

Kurtosis

November My 21x My 19 My 2 My 4 My 5

2.12 0.48 0.65 0.91 1.03

231 716 637 532 490

0.59 0.60 0.83 0.47 0.50

-0.44 -0.28 - 0.09 - 0.32 - 1.12

May Hu 1 Hu 2 Hu 2 bis Hu 2 ter Hu 3 Hu 5x Hu 19 Hu 18 Hu 16

0.77 0.58 0.06 0.37 0.09 1.93 0.96 0.93 0.65

597 669 962 774 938 262 515 526 636

0.76 0.36 0.59 0.83 0.50 0.33 0.59 0.43 0.50

-0.75 -0.10 0.05 0.71 0.10 - 0.32 -0.66 -0.79 -0.58

::i~i~i!~iii

November 1984

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,

MY

2t

o

Moy 1985

H/

60

HU5

Inman

Folk et Ward

Inclusive Inclusive standard graphic deviation skewness

Graphic Kurtosis

TRASK Coeff.

3.40 2.78 2.64 3.24 4.77

2.13 0.47 0.64 0.94 1.03

3.22 0.75 0.99 t.40 1.61

0.37

0.38 0.46 0.37 0.35

-- 0.18 -0.19 - 0.08 0.02 - 0.41

1.01 0.93 0.95 1.18 1.17

1.42 1.47 1.57 1.34 1.32

3.99 3.57 2.66 4.41 2.69 2.85 3.54 4.10 3.11

0.77 0.53 0.02 0.28 0.07 1.93 0.97 0.95 0.64

130 0.86 0.06 0.46 0.12 2.93 1.50 1.44 1

0.15 0.31 0.39 0.44 0.34 0.29 0.39 0.33 0.36

-0.24 -0.34 -0.07 0.04 0.01 - 0.22 -0.31 -0.19 -0.33

1.19 1.14 0.94 1.09 0.85 0.98 1.05 1.12 1.15

1.42 1.29 148 1.46 1.46 1.32 1.41 1.31 1.34

w a s low, and of c o a r s e g r a i n s w h e n the a v e r a g e w a s high. The s a n d s f r o m t h e C o m t e ( s a m p l e My 4) w e r e s u b j e c t e d to e x o s c o p i c e x a m i n a t i o n of t h e i r q u a r t z by s c a n n i n g m i c r o s c o p y . An e x a m i n a t i o n of 21 g r a i n s s h o w e d m i c r o s t r u c t u r e s w h i c h corr e s p o n d to a h i g h - e n e r g y d y n a m i c i n f r a t i d a l env i r o n m e n t . This w e l l - m a r k e d m a r i n e f a c i e s is p r o b a b l y of very a n c i e n t origin. S u b s e q u e n t l y , this s t o c k u n d e r w e n t i m m o b i l i z a t i o n , w i t h e m e r sion and p e d o g e n e s i s (seen in t h e p r e s e n c e of s p e c i f i c c h e m i c a l l y e t c h e d V-forms). Its final e v o l u t i o n s h o w s a return of f l u v i a t i l e c o n d i t i o n s , c h a r a c t e r i z e d by the p r e s e n c e of s i l i c a c o a t i n g s (Fig. 3) and r e c e n t i m p a c t f e a t u r e s , e v i d e n c e of the c u r r e n t h y d r o b i o l o g i c a l c o n d i t i o n s in the rivers of French G u i a n a . An a n a l y s i s of the heavy m i n e r a l s s h o w e d t h e m e a n c o m p o s i t i o n of t h e heavy f r a c t i o n in t h e river, in an area near its d e p a r t u r e f r o m t h e c r y s t a l l i n e base. The a s s o c i a t i o n of m i n e r a l s found remained constant, with a predominance of e p i d o t e s (30 to 6 0 % of t h e heavy f r a c t i o n ) , amp h i b o l e s ( a c t i n o t e ) and m a g n e t i t e (10 to 3 0 % of t h e heavy fraction).

20 ~0

o

,

,

,

,

,

,

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,

,

Fig. 2. Granulometric curves of the sandy samples taken in the centre of the river (within shaded area) and near the banks (2 samples indicated by station numbers).

SUSPENDED MATTER IN THE MAHURY

195

3.2. TURBIDITY, SALINITY, DISSOLVED IRON AND MANGANESE 3.2.1. TURBIDITY AND SALINITY

Fig. 3. Detail of a grain of quartz (magnified 4000 times). Geometrical chemically-etched V-forms are present only in the impact feature depressions and show a pedological development. The slight polishing of the edges of these V-forms shows a return to aquatic conditions of a medium-energy fluviatile type, as is also evidenced by the presence of silica coatings on the flat surfaces and near the edges. At the centre of the picture, traces of rubbing in the form of chattermarks show previous high-energy fluviatile evolution of coarse heterogranular material. (photo: Carme, La Teste).

3.1.2. FINE SEDIMENTS The fine sediments were made up of silts and clays (Table 2). The graphs obtained from the fraction with grain size <63 #m could be grouped into two different types according to the granulometric criteria proposed by RIVIERE (1977). They enabled us to characterize a hydraulic process of deposit settling. Most of the graphs were in the form of a hyperbolic curve, with the concavity facing downwards. They included all samples taken downstream of the meeting point of the Comt~ and the Orapu (irrespective of the season). The facies is characteristic of calm sedimentation by decantation of fine particles. Other graphs were logarithmic, and roughly straight. These were found in the Comt~ samples --in November 1984-- but the absence of fine sediments in the samples taken in May 1985 means that this result cannot be extended to cover the whole of the year measured. In any event, these were fairly developed sediments deposited by excess load.

An area of hyper-concentrated suspended material with concentrations reaching 0.8 g.dm -3 in surface waters was located in the middle estuary during the dry season. This turbidity maximum was found in the lower estuary and the mouth itself during the rainy season (Fig. 4). This kind of distribution of turbidity has been described in several temperate estuaries (GLANGEAUD, 1938; POSTMA, 1967; MEADE et al., 1979; ALLEN, 1972; GALLENNE, 1974) and in the Orinoco (EISMA et al., 1978). The distribution of salinity in the surface layer followed the river discharge situation. During the dry season, the saline intrusion reached the middle estuary (5 km upstream of Degrad des Cannes) and during the rainy season this limit (S= 1) was found at about 2 km downstream of the same point. 3.2.2. DISSOLVED IRON AND MANGANESE The dissolved Fe and Mn contents were practicaly constant in the fluviatile part of the estuary, increasing strongly at the upstream edge of the turbidity maximum, then decreasing markedly downstream as the saline intrusion appeared (Figs 5 and 6). In the marine waters, the dissolved Fe and Mn values were lower than in the fluvial part. This decrease in the dissolved values was greater for Fe (Fig. 6) than for Mn (Fig. 5), and appears to be independent of the season. On the other hand, the highest Fe values in the fluvial part were found during the dry season. 3.2.3. PARTICULATE ORGANIC CARBON IN SUSPENDED MATTER An examination of the longitudinal variation in the P.O.C. concentrations in the S.M. of the fluvio-estuarine system shows the following (Fig. 7): --Upstream of the mud plug, below 100 mg S.M..dm-3, the P.O.C. content was at its highest (over 2% and even up to 14%). --In the mud plug itself, the P.O.C. content was remarkably constant (1 _+0.2%). - - A very slight increase in P.O.C. content appeared to be detectable in the outermost samples (from the mouth). This tendency ap-

196

J.M. JOUANNEAU & M. PUJOS

TABLE 2 Granulometric analysis of the fine-grained deposits of the Mahury, sampled in November 1984 and in May 1985. Samples arranged in upstream-downstream order (see Fig. 1) (indicated by My and Hu numbers). November 1984 station % fraction no >63 w'n

My 22 My 19b My 23 My 1 My 3 My 6 My 7 My 8 My 9 My 10 My 11 My 11b My 12 My 18 My 17 My 16b My 16 My 15b My 15 My 14 My 13

% fraction <63 W'n

21,5

79.5

50.5 11.0 3.5 17,0 5.5 0.5 0.5 13 0 0 0 0 1.5 0.5

% fraction < 2 #m

May 1985 median grain (#m)

station % fraction no >63 ~rn

52

1,4

Hu 4b

49.5 89.0 96.5 83.0 94.5 99.5 99.5 87 100 100

39.5 60.5 63.5 57.0 67 67 74 64.5 66 66

100 0.8 <1 <1 <1 <1 <1 <1 <1 <1

Hu Hu Hu Hu Hu Hu Hu Hu Hu Hu

100 100 98.5 99.5

68.5 68.5 65 71.5

<1 <1 <1 <1

Hu 7 Hu 6

1 1

99 99

68 66

<1 <1

1 0.5 5 1.5

99 99.5 95 98.5

67 69 66 65.5

<1 <1 <1 <1

peared to be more pronounced in the period with a low fluvial flow rate (in November) and might in that case be attributable to an increase in organic matter of marine origin (plankton). This type of P.O.C. distribution in the S.M. in the Mahury is very similar to that described in other estuaries with a high turbidity load (JOUANNEAU, 1982).

20 17 15 14 13 12 11 10 9 8

1.5

% fraction <63 ~n

98.5

% fraction <2 #

median grain (~m)

66.5

<2

17 0 0 0 0 0 0 1 0 0

83 100 100 100 100 100 100 99 100 100

46 68 68.5 70 70 68 70 67 70.5 68.5

2.8 <2 <2 <2 <2 <2 <1 <1 <1 <2

0 0

100 100

66.5 68

0.6 <2

and illite 45%. Thus, a clear boundary exists between the riverine and estuarine environments with regard to the clay mineral spectrum of their muds. 3.4. ELEMENTARY COMPOSITION 3.4.1. MAJOR AND TRACE ELEMENT CONCENTRATIONS

3.3. CLAY MINERALOGY The clay minerals (Table 3) were in the < 2 ~m fraction of the muds of the Mahury estuarine system were illite and kaolinite (25 to 40%), smectites (15 to 25%), and chlorites (13 to 18%). In the riverine part of the system, the composition of the clay mineral spectrum was distinctly different. Upstream of the junction of the Comte and Orapu rivers, we found: in the Comte (My 22, 19) essentially kaolinite with a great deal of illite, but with no smectites or chlorites, and in the Orapu only two clay minerals - - k a o l i n i t e s 55%,

Some 40 samples of deposited sediments (mud) were analysed for major and trace element concentrations (Table 4). Despite the fact that the concentrations of Fe, Cr, Mg, Zn, Pb, and, to a lesser extent, Sr were positively correlated to the alumina content, it is possible to distinguish riverine from estuarine mud, as we could from clay mineral composition. We have not been able, however, to distinguish any significant variation in the composition of the estuarine muds (downstream of Roura), neither spatially nor according to the season.

SUSPENDED MATTER IN THE MAHURY

197

I

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Fig. 4. Distribution of turbidity (in mg.dm - 3 and salinity (in %°) in surface water in two seasons.

To provide a meaningful comparison between our results and those previously published, element concentrations have been normalized with regard to alumina (Table 4).

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Removable Fe and Mn were analysed in 7 samples. Values are given in Table 5. About 50%

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3.4.2. REMOVABLE IRON AND MANGANESE

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Fig. 5. Longitudinal distribution of dissolved manganese and salinity in surface water in two seasons.

198

J.M. JOUANNEAU & M, PUJOS

TABLE 3 Composition of clay minerals (in %). The samples are arranged in upstream-downstream order (see Fig. 1). Tr = trace; IC = interlayered illite-chlorite; CM = interlayered chlorite-smectites minerals (%) river

Sample

Comt~

Smectites

Illites

Kaolinites

Chlorites

My 22 My 19

Tr 4

100 96

Tr

Orapu

Boulanger Fourgassie Marguerite

45 42 43

55 58 53

Mahury

My My My My My My My My My My My My My My My My My My My

31 29 37 39 36 38 42 35 38 37 36 33 35 35 42 35 36 38 37

33 39 28 31 30 30 28 27 27 26 27 27 29 27 28 27 29 28 27

23 1 3 6 7 8 9 10 11 11b 12 18 17 16 16b 15 15b 14 13

23 17 17 16 18 15 15 20 20 24 26 20 20 20 17 23 20 18 22

Interlayered

13 15 18 14 16 17 15 18 15 13 13 14 16 18 13 15 15 16 14

IC + CM CM +IC CM + IC CM CM + IC CM CM CM CM CM CI CM CM CM CM CM CM + IC CM

TABLE 4 Elementary composition of muddy deposits of the Mahury estuarine system: average values, ranges, and numbers of samples analysed. rivers Corn te 4

Orapu 3

Middle 17

Outlet 13

50.6 46.0 -56.7 ) 23.9 20.7 -29.3 ) 1.19 1.98- 1.30) 0.25 0.13- 0.39)

40.0 35.7 -45.7 27.0 - 28.9 t.71 1.52- 1.91 0.12 0.07- 0.18

51.1 47.6 - 57,0 ) 19.5 17.5 -21.2 ) 1.10 1,00- 1.20) 0.42 0.37- 0.50)

51.6 49.3 -53.8 ) 18.8 - 20.0 ) 1.08 1.00- 1.14) 0.46 0.40- 0.55)

0.53 0.46- 0.59) 0.12 0.07- 0.16) 0.14 0.10- 0.16) 13.53 11.85 - 15.52) 9.40 7.27 - 10,15) 7.24 6.27- 7,82) 4.14 3.19- 5.76) 2.22 1.7t- 2.64) 2.39 2.07 - 3.15) 1.88 1.33- 2.32)

0.54 0.13 0.15 14.07 9.86 7.31 3.72 2.32 2.45 1.73

number SIO2(% ) AI203(% ) TIO2(% ) CaO(%) Fe203/AI203 MgO(AI203 K2OIAI203 Sr/AI203 Rb/AI203 Zn/AI203 Cr/AI203 Cu/AI203 Ni/AI203 Pb/AI203

estuary

0.38 0.05 0.05 8.55 5.19 4.82 3.98 2.54 2.39 0.99

0.17 0.01 0.03 5.22 2.32 2.97 0.66 2.28 2.180.48-

0.75) 0.11) 0.11) 11.35) 8.01) 6.42) 6.20) 2.75) 2.63) 1.78)

0.44 0.01 0.06 7.01 ( 3,21 ( 2.58( 8.82( 2.58 ( 2.23 ( 0.20 (

0.430.010.066.613.012.278.122.402.000.07-

0.46 0.02 0.06 7.29 3.53 3.19 9.51 2.77 2.40 0.41

0 . 5 1 0.61) 0.09 0.16) 0.14 0.18) (13.25 15.46) ( 9.40 10.75) ( 6.75 8.04) ( 3.42 4.02) ( 1.97 3.01) ( 2,10 3,04) ( 1,22 2.49)

SUSPENDED MATTER IN THE MAHURY

199

TABLE 5 Concentrations of total and removable Fe and Mn in samples deposited sediment Mn (l~g.g- 1) Sample

Hu Hu Hu Hu Hu Hu Hu

2 (Comt6) 17 (Mahury) 16 (Mahury) 11 (Mahury) 10 (outlet) 8 (outlet) 7 (outlet)

Fe (%)

Total content (MnO)

Removable Mn

Total content (Fe203)

Removable Fe

100 1 700 500 1 400 1 300 1 400 1 500

5 760 225 675 575 775 785

0.3 10.1 2.2 9.5 10.0 9.9 10.2

0.03 1.32 0.10 1.21 1.18 1.30 1.24

of th Mn content and 10% of the Fe content were generally leachable. Only 1 sample (Comtd river) showed a lower percentage for Mn.

of the same type as that for suspended matter, i.e. the highest values were found in the upstream part of the system (in the fluvial zone and the upper estuary). In the middle and lower estuary, levels were practically constant at -1%.

3.4.3. CARBONATES The sediment samples never contained more than 1% of CaCo 3.

4. DISCUSSION AND CONCLUSIONS

3.4.4. PARTICULATE ORGANIC CARBON

The above data provide information on the origin of the estuarine sediments and on the mechanics of their distribution. The river sands showed invariable annual characteristics, pointing to a long marine and then

The values for the P.O.C. content of the sediments (arranged in Table 6) showed that the change observed as one moves downstream was

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Fig. 6. Longitudinal distribution of dissolved iron and salinity in surface water in two seasons.

200

J.M. JOUANNEAU & M. PUJOS S,M. mg/l

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0

o

Fig. 7. Relationship between solid matter concentration (S.M.. in mgdm-3) and particulate organic carbon (P.O.C., in %) in surface water in two seasons. continental history prior to the present period with dominant fluviatile characteristics. These sands, like those found in other Guianese rivers, now only contribute to the formation of beaches on the coastline near the river mouths (LAFOND, 1967; BOUYSSE et al., 1977). Their contribution to the sedimentation of the continental shelf has not, however, always been so limited. For example, the association of heavy minerals from the Mahury (epidotes/amphiboles/magnetites) found beyond the present shoreline, is the surface trace of a submarine

palaeovalley off the Mahury and Approuague estuaries (PuJOS et al., ms.). This confirms the existence of a "Mahury-Approuague" mineralogical province, and the part played by the Mahury in the sedimentation of the shelf during periods when the sea level was lower. The importance of these periods, shown by one of the present authors (PuJos & ODIN, 1986), is seen in the deepening of the hydrographic network and the return of fluvial erosion leading to a phase of intense evacuation of waste sediment discharge on to the continental shelf.

SUSPENDED MATTER IN THE MAHURY

TABLE 6 Particulate organic carbon (P.O.C.) contents in selected samples of bottom sediments. The samples are arranged in an upstream-downstream order.

samples My My My My My My My My

23 1 6 9 11 18 16 14

201

source, the c o n t r i b u t i o n of the Mahury is a small one, as concluded earlier for Guiana rivers by EISMA & VAN DER MAREL (1971).

P.O.C. (%) 1.45 3.93 1.47 0.99 1.12 0.98 1.06 1.08

The fine sediments, in their mineralogical and elementary composition, show the existence of a very clear boundary between the fluvial part (Orapu and Comt~ rivers) on the one hand, and the estuarine part and mouth on the other. As with sands, there appears to be no seasonal variation in their composition, neither in minerals nor in major or trace elements. These observations lead to the supposition that there are two stocks of differing origin: - - a u t h i g e n i c in the Orapu and Comt6 rivers - - a l l o t h o g e n i c , with a clay mineral association and elementary composition similar to the Amazonian (EISMA et al., 1978; BOWLES & FLEISCHER, 1985; REYNE, 1961). It therefore appears that practically all the muddy sedimentation found in the Mahury estuary is allothogenic in origin (from the Amazon). These results confirm LAFOND'S (1967) observations, but contradict BERTHOIS & HOORELBECK (1968), who attributed major importance to the contribution of suspended matter of local origin. In fact, it is now clear that the impact of the Guianese sediments brought down by the Mahury is relatively unimportant as they are only found in the fluviatile parts of the estuarine system. These sediments are easily identifiable both mineralogically and geochemically (major and trace elements). These findings are corroborated by estimates we have carried out on the amount of suspended solid matter contributed by the Mahury, w h i c h do not exceed 5.106T.y -1 (JOUANNEAU & PUJOS, in ms.) - - a very modest figure compared with that for the Amazon estimated at 500.106T.y -1 (GIBBS, 1967) to even 1300.106T.y- 1 (MEADE et al., 1985). A s i g n i f i c a n t fraction of this Amazon material is transported westward along-coast. Compared to the sediments originating from this

5. REFERENCES ALLEN, G.P., 1972. I~tude des processus s~dimentaires dans I'estuaire de la Gironde. Th~se Doct. ~sSciences, Univ. Bordeaux 1" 1.310. BERTHOIS, L. & J. HOORELBECK,1968. I:ttude dynamique de la s6dimentation clans trois cours d'eau de la Guyane fran~:aise. La riviere Mahury, la rivi~re de Cayenne et le fleuve Maroni.--M~moires ORSTOM 26: 1-128. BOUYSSE, PH., H.R. KUDRABS& F. LE LANN, 1977. Reconnaissance s~dimentologique du plateau continental de la Guyane fran~:aise (mission Guyamer 1975).--Bu11. BRGM 4: 141-179. BOWLES, F.A. & P. FLEISCHER, 1985. Orinoco and Amazon river sediment input to the eastern Caribbean Basin.--Mar. Geol. 68: 53-72. EISMA, D & H.W. VAN DER MAREL, 1971. Marine muds along the Guyana coast and their origin from the Amazon Basin.--Contr. Mineral. Petrol. 31: 321-334. EISMA, D., S.J. VAN DER GAAST, J.M. MARTIN & A.J. THOMAS, 1978. Suspended matter and bottom deposits of the Orinoco delta: turbidity, mineralogy and elementary composition.--Neth. J. Sea Res. 12: 224-251. ETCHEBER, H., 1981. Comparaison de diverses m~thodes d'evaluation des teneurs en mati~res en suspension et en carbone organique particulaire des eaux marines du plateau continental aquitain.--J. Res. Oceanogr. 6: 37-42. GALLENNE, g., 1974. Les accumulations turbides de I'estuaire de la Loire. I~tude de la cr6me de vase. Th~se 3(~me cycle, Univ. Nantes: 1-323. GIBBS, E.J., 1967. The geochemistry of the Amazon river system; part 1: the factors that control the salinity and the composition and concentration of the suspended solids.--Bull, geol. Soc. Am. 78: 1023-1232. GIBBS, R.J., 1976. Amazon river sediment transport in the Atlantic Ocean.--Geology 4: 45-48. GLANGEAUD, L., 1938. Transport et s~dimentation dans I'estuaire et ~ I'embouchure de la Gironde, caracteres p~trographiques des formations fluviatiles, saum&tres, littorales et n6ritiques.-Bull. Soc. G~oi. Fr. 5" 599-630. HYDRAULICS LABORATORY DELFT, 1962. Demerara coastal investigation. Delft Hydraulics Lab. 1-240. JOUANNEAU, J.M., 1982. Mati~res en suspension et oligo-61~ments m~talliques dans le syst~me estuarien girondin: comportement et flux. Th~se Doct. ~s-Sciences, Univ. Bordeaux 732: 1-150. JOUANNEAU, J.M. & M. PUJOS, ms. Variations annuelles des concentrations en mati~res en suspension et estimations des d6bits solides des fleuves Maroni et Mahury (Guyane fran?aise).

202

J.M. JOUANNEAU & M. PUJOS

LAFOND, L.R., 1967. I~tudes littorales et estuariennes en zone intertropicale humide.--These Doct. esSciences, Orsay, 3 tomes: 1-836 MEADE, R.H., C.F. NORDIN JR., W.F. CURTIS, F.M.C. RODRIGUES, C.M. DO VALE & J.M EDMOND, 1979. Sediment loads in the Amazon river.--Nature 278: 161-163. MEADE, R.H., T. DUNNE, J.E. RICHEY, U. DE M. SANTOS & E. SALATI, 1985. Storage and remobilization of suspended sediment loads in the Amazon r i v e r . Science 226: 488-490. N EDECO, 1 9 6 8 . Surinam transportation study. Netherlands Engineering Consultants, The Hague: 1-293. POSTMA, H., 1967. Sediment transport and sedimentation in the estuarine environment. In: G.H. LAUFF. Estuaries. Publ. no. 83 A.A.A.S. Washington D.C.: 158-179. PuJos, M. & G.S. ODIN, 1986. La sedimentation au

Quaternaire terminal sur la plate-forme continentale de la Guyane fran~aise.--Oceanol. Acta 9: 363-382. PuJos, M., PH. BOUYSSE & J.C. PONS, ms. Les mineraux Iourds du plateau continental guyanais: inter~t de I'etude pour les reconstitutions paleogeographiques du Quaternaire terminal. REYNE, A., 1961. On the contribution of the Amazon river to accretion of the coast of the Guianas.-Geologie Mijnb. 40: 218-226. RIVIERE, A., 1977. Methodes granulometriques.-Techniques et interpretation. Ed. Masson, Paris: 1-164. STRICKLAND, J.D.H. & T.R. PARSONS, 1972. Determination of particulate carbon. In: A practical handbook of sea water analysis.--Fish. Res. Bd Can. Bull. 167: 207.211. (received 10-4-1987; revised 23-7-1987)