Chemical Geology 165 Ž2000. 243–266
www.elsevier.comrlocaterchemgeo
Increase in mercury contamination recorded in lacustrine
sediments following deforestation in the central Amazon 1
M. Roulet
a
a,)
, M. Lucotte a , R. Canuel a , N. Farella a , M. Courcelles a ,
J.-R.D. Guimaraes
˜ b, D. Mergler c , M. Amorim d
Chaire de Recherche en EnÕironnement, H-Q r CRSNGr UQAM and GEOTOP, UniÕersite´ du Quebec
´ a` Montreal,
´ B.P. 8888, succ.
Centre Ville, Montreal, Quebec, Canada H3C3P8
b
Laboratorio
Inst. de Biofısica,
UniÕersidade Federal do Rio de Janeiro, Bloco G-CCS, Rio de Janeiro,
´ de Radioisotopos,
´
´
CEP 21949-900, Brazil
c
CINBIOSE, UniÕersite´ du Quebec
´ a` Montreal,
´ Montreal, Quebec, Canada
d
UniÕersidade Federal do Para,
´ Belem,
´ Brazil
Received 19 October 1998; accepted 11 August 1999
Abstract
The geochemical study of surface sediments Žvertical profiles of 30–80 cm. from lentic ecosystems of the Tapajos
´ River
permit the observation of environmental changes responsible for the mercury contamination of aquatic systems exploited by
the human riverine population. The Arapiuns and Amazon rivers are compared. Measurements of mercury, textural
indicators Žwater content and dry density., mineralogic indicators Žiron and aluminum associated with oxyhydroxides and
aluminosilicates., and organic indicators Žcarbon, nitrogen, atomic CrN ratio. were performed over the full length of the
cores. The results demonstrate that soil erosion is responsible for an increase in surficial sediment mercury concentrations in
the different aquatic systems of the Tapajos
´ and Arapiuns rivers. This increase is the result of the relative enrichment of the
sedimentary deposits in fine particulates rich in aluminosilicates, oxyhydroxides and mercury, transported in suspension in
the water column. The oxyhydroxides of iron and aluminum associated with fine, clayey particles seem to control the
accumulation of heavy metals in the sediments of the Tapajos,
´ Arapiuns and Amazon rivers. Overall, the mercury levels in
the sediments studied have the same relationship with the aluminosilicates and the texture of the sediments. The quantity of
aluminosilicates permits the evaluation of diagenetic effects, the influence of the clay content and the matrix effect on the
levels of mercury in sediments. The activity of lead-210 measured in two cores suggests that the superficial sediments
originate from eroded soils. A preliminary dating using the constant initial concentration model indicates that the
environmental changes recorded in the Tapajos
´ River sediments would have been initiated some time between the 1950s and
1970s. They would then coincide with the important colonization of the Brazilian Amazon during this period. The results
presented have important implications for the geochemical interpretation of anthropogenic disturbances in the Amazon. They
demonstrate that the recent colonization of the drainage basins and the growing exploitation of new parcels of land in the
)
Corresponding author. Tel.: q1-514-987-3000 4080a; fax: q1-514-987-3635; e-mail: [email protected]@er.uqam.ca
The present investigation is part of an ongoing study, the CARUSO project ŽCRDI-UFPa-UQAM., initiated to determine the sources,
fate and health effects of the presence of MeHg in the area of the Lower Tapajos.
´
1
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 7 2 - 2
244
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
central Amazon disturb the mineral and organic matter cycles, as well as that of mercury. The ensuing result is increased
exportation of fine particulate mercury from the surface of soils to drainage waters that transport them to fluvio-lacustrine
systems where they finally settle out. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Mercury; Amazon; Deforestation; Sediment; Erosion; Normalization
1. Introduction
In the Tapajos
´ River numerous studies have shown
that higher than normal levels of mercury ŽHg. are
found in fish and hair ŽCleary et al., 1994; Akagi et
al., 1995; Malm et al., 1995, 1997a; Lebel et al.,
1996, 1997; Kehrig et al., 1997.. The concentrations
are sufficiently elevated to reveal early signs of
sensory and motor deficiencies ŽLebel et al., 1996;
Lebel et al., 1998. in the riverine populations as a
result of exposure to methylmercury ŽMeHg. from
the consumption of fish products ŽLebel et al., 1997..
The presence of MeHg was initially attributed to Hg
wastes produced by gold mining activities along the
upper Tapajos
´ River ŽAkagi et al., 1994, 1995; Cleary
et al., 1994; Malm et al., 1995, 1997a.. More recently, however, a series of detailed studies on forest
ecosystems, soils and the water column along the
Tapajos,
´ from the valley to the lower reaches, indicated ongoing leaching and erosion of Hg present in
the soils ŽRoulet et al., 1996, 1998a,b, 1999a,b..
Along the river no Hg concentration gradient was
observed with increasing distance from the gold
mining regions ŽRoulet et al., 1998a.. The levels of
Hg in the pedologic cover are, however, relatively
elevated ŽRoulet et al., 1998b.. The soils constitute
an important reservoir of naturally accumulated Hg.
In comparison, the inputs from gold mining would
represent less than 3% of the Hg present in the first
20 cm of the soil ŽRoulet et al., 1999a.. These
studies suggest that throughout the river valley, erosion of fine particles from the soil cover is responsible for the increased export of terrestrial Hg to
aquatic ecosystems.
The principal objective of the present investigation is to provide temporal evidence of the effect
erosion has on Hg deposited on the bottom of lentic
systems of the Tapajos
´ River. High resolution vertical profiles of the sediments were produced for
quantitative and qualitative analysis of geochemical
indicators of the mineral and organic matter. The
importance of the sediment geochemistry on the
accumulation and behaviour of Hg is discussed.
2. Materials and methods
2.1. Region, sites and the context of the study
The Tapajos
´ is one of the principal tributaries of
the Amazon River ŽFig. 1.. It is a clear water river
that drains the central Brazilian shield on its upper
part and the plateau of the Cretaceous-aged ŽPutzer,
1984. Alter-do-Chao
˜ sedimentary formation on its
lower part. In the region of Santarem
´ Ž02825X S,
X
54842 W. the maximum seasonal water level variation is 6 m. The average annual precipitation is 2096
mm with regular temperatures of 21.9 to 33.18C for
an annual average of 268C ŽInemet, 1979, in Salati
and Marques, 1984.. The climate is equatorial with a
marked dry season.
The section of the Tapajos
´ studied is composed of
two distinct aquatic ecosystems, corresponding to
two successive physiographic zones in the lower part
of the river ŽFig. 1.. The section upstream from
Aveiro Ž rio . is formed by a river–floodplain system.
The river is relatively narrow Ž2–4 km wide. and
characterized by a strong, central advection current.
The section immediately downstream from Aveiro,
where the Tapajos
´ suddenly widens and the current
slows, constitutes a large sedimentation zone characterized by numerous islands and channels ŽSioli,
1984.. From this zone until its meeting with the
Amazon, a river lake Ž ria or mouthbay-lake. 8–15
km wide and having a weak current and is characteristic of several of the major tributaries of the Amazon. As a means of comparing the Hg geochemistry
in the sediments of different drainage basins, the ria
of the Arapiuns River and a varzea
´ of the Amazon
River were also studied ŽFig. 1.. The Arapiuns River,
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
245
Fig. 1. Map of the study region and sampling sites.
the major tributary of the Tapajos
´ and characterized
by very clear water, is located upstream from the
point where the Tapajos
´ flows into the Amazon.
In the Tapajos,
´ four lakes were chosen in the
upstream zone. They represent two lake types characteristic of Amazonian floodplains ŽHamilton and
Lewis, 1990; Sippel et al., 1992..
The channel lakes, such as the lago Piranga ŽAM43, on the edge of the river. and the lago da Ilha
Itapena ŽAM-60, in the middle of an island., are
small, elongate lakes Ž; 0.02–0.04 km2 . that are fed
directly from advection of the river. Plankton development is negligible during all seasons. During the
rainy season, these lakes are completely flooded by
high water flow in the river. The low residence time
of the water in this type of lake implies that the
water column is directly influenced by inputs of
material from the rio. In contrast, the channel lakes
are completely isolated during the dry season. Wind
action at the surface of these lakes is seriously
restricted by their size as well as by the canopy of
the flood plain forest Ž igapo´ . along the river banks.
No resuspension of bottom sediments occurs during
the dry season.
The lago Enseada Grande ŽAM-32. and the lago
do Tavio ŽAM-65. are the largest lakes Ž; 0.3–1
km2 . and are more or less circular Žsimilar to the
dish-lakes as described by Hamilton and Lewis,
1990.. They are continuously linked to the river by
access canals but are less directly influenced by
water of the Tapajos.
´ Water inflow is principally via
the canal, even during the rainy season. The current
is very weak and the residence time is longer than
that of the channel lakes. Inundation due to overflow
of the river banks occurs only when maximum water
levels are attained. These lakes are always less turbid
than the river during the rainy season and are practically without current, permitting plankton development. They represent large, wind exposed surfaces
susceptible to sediment resuspension Že.g., in Enseada Grande during the dry season..
The lago do Arrozalzinho ŽAM-102., situated on
Ituqui Island downstream from Santarem
´ ŽFig. 1.,
can also be included in this last lake category, although much larger Ž3–4 km2 .. It is directly and
uniquely influenced by the Amazon during the rainy
season, but remains completely isolated during the
dry season.
246
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
The Tapajos
´ River ria is crossed, season long, by
waters coming from upstream. In the ria, two stations were chosen in deep regions Ž10–20 m., the
first one lying just beyond the sedimentation zone
ŽAM-73. and the second located in the middle of the
ria ŽAM-80.. In the Arapiuns, only the downstream
section was sampled. The rias are characterized by a
weak current and significant phytoplankton and zooplankton development ŽSchmidt, 1982; Sioli, 1984..
At the inlet of the ria, the Tapajos
´ loses approximately 50% of its fine particulate load and nearly all
of the coarse material ŽRoulet et al., 1998a..
Since the beginning of the century, the drainage
basins of the Amazon have been affected by numerous anthropogenic inputs. During the 1960s and
1970s, the initiation of large-scale development projects in the Amazonian frontier of Brazil promoted
unprecedented colonization and exploitation in the
region ŽKohlhepp, 1984.. In the Tapajos
´ Valley,
these projects comprise trans-Amazonian routes
ŽTransamazonica
BR 163, Cuiaba-Santarem
ˆ
´
´ PA 150.,
installation of large agricultural colonies, the colonization of the river banks, mining and logging
activities ŽKohlhepp, 1984.. From the end of the
1970s until the beginning of the 1990s, gold rush
have greatly affected the region upstream from the
river ŽBidone et al., 1997; Malm et al., 1997a.. In the
late 1980s, the region above the Tapajos
´ was the
largest gold mining center but between 1990 and
1992 experienced a considerable decrease in activity
ŽMalm et al., 1997b.. In comparison, the Arapiuns
river is practically free of mining activities and its
banks are relatively unpopulated.
2.2. Sampling methods
All of the sediment samples were collected during
1995. Two sampling methods were employed. Short
cores Ž25–30 cm. and floc from the sediment–water
interface were collected manually by scuba divers.
The cores were retrieved by gently inserting a PVC
tube Ž30 cm long and 15 cm in diameter. into the
sediments. The floc samples were collected directly
from the interface with 50 cc plastic syringes. Long
cores Ž80–100 cm. were collected using a Mackereth
corer connected to a scuba tank and controlled from
the surface.
The two coring methods allow for sampling with
a minimum of compaction or disturbance to the
sediments. Following retrieval, a jack is used to
extract the sediments from the corer at 0.5 cm intervals. Consecutive slices are carefully cut and transferred to polyethylene centrifuge tubes and then immediately placed in a freezer. Prior to sub-sampling
the sediments, redox potential Ž E h . profiles were
determined by inserting a small platinum electrode at
numerous depths along the length of the core.
2.3. Laboratory analyses and calculation
All of the sediment samples were freeze dried.
The samples were weighed before and after drying to
determine water content Ž% water.. The dry densities
Ž d . were obtained by measuring the volume of a
known weight of sediment. The volumes were measured directly in the tubes following centrifugation
of the unfrozen samples.
The percent water content and the dry density
measurements ŽFig. 3. were used to evaluate textural
variations of the sediments and changes in the deposition history ŽSmith and Walton, 1980; Eakins et al.,
1983; Flower et al., 1984; Droppo and Stone, 1994;
Foster and Walling, 1994..
The Hg analyses are performed using cold vapour
atomic fluorescence spectrometry ŽCVAFS. following a modification ŽPichet et al., 1999. of the method
developed by Bloom and Fitzgerald Ž1988.. Briefly,
the sediments were digested in glass test tubes with a
10:1 mixture of nitric acid and hydrochloric acid
ŽHNO 3rHCl. and heated at 1208C.
Carbon ŽC. and nitrogen ŽN. were determined in
all of the samples using a Carlo-Erba elemental
analyzer.
The iron and aluminum oxyhydroxides were measured using the protocol developed by Lucotte and
d’Anglejan Ž1985., a chemical extraction with a
citrate–dithionite–bicarbonate Žcdb. buffer. Total
iron ŽFe tot. . and total aluminum ŽAl tot. . were extracted from samples in Teflon bombs in a 6:1
mixture of aqua regia and concentrated hydrofluoric
acid ŽHF. heated in a microwave. The concentrations
of Fe cdb , Al cdb , Fe tot., and Al tot. were determined by
atomic absorption spectrometry. The fraction associated with the silicate matrix ŽFe si. and Al si. . was
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
calculated by subtraction of the cdb fraction from the
corresponding total quantity ŽFontes, 1995; Jonsson,
1997..
The cdb extraction is specific for Fe oxyhydroxides and corresponds to crystalline and amorphous
forms ŽJonsson, 1997.. Al cdb represents Al substituted for Fe in Fe oxyhydroxides, as well as amorphous Al. The cdb method also seems to be able to
extract Al and Fe associated with organic matter
ŽParfitt and Childs, 1988; Ross and Wang, 1993..
Al si. corresponds to Al atoms that are incorporated in
clay minerals of the sediments, while Fe si. represents
Fe atoms substituted for Al in the same minerals
ŽJonsson, 1997..
The analysis of lead-210 Ž210 Pb. by alpha spectrometry is based on the postulate of a secular radioactive equilibrium between 210 Pb and its daughter, polonium-210 Ž210 Po.. The samples were analyzed more than 6 months following their collection
in order to respect this equilibrium. Approximately 1
g of dry sediment was initially treated according to a
protocol developed by Courcelles Ž1998.. Sample
deposits on a silver disk are dried and then placed in
one of the vacuum detection chambers of the alpha
spectrometer ŽEGG-Ortec, type 576A..
The excess 210 Pb activities Ž210 Pb u . by the parent
226
Ra are calculated by assuming that the activity of
210
Pb in equilibrium is uniform in each core. In this
case, the constant activity of 210 Pb with depth represents the 210 Pb in equilibrium with 226 Ra. The 210 Pb u
can also be estimated by subtracting the total stable
activity measured at the base of the core profile from
the activity measured closer to the surface.
The two alternative geochronological models used
are the constant initial concentration ŽC.I.C.. model
and the constant rate of supply ŽC.R.S.. model
ŽGoldberg, 1963; Appleby and Oldfield, 1978; Robbins, 1978; Appleby and Oldfield, 1983.. If the
C.I.C. conditions are satisfied, the 210 Pb u activities
vary with depth according to the following equation:
C s C0 eyk t , where C0 is the sediment 210 Pb u activity at the sediment–water interface and k is the
radioactive decay constant for 210 Pb Ž0.03114 yry1 ..
The age of sediment activity C is given by: t s 1rk
lnŽ C0rC .. If the C.R.S. conditions are satisfied, it
can be presumed that the cumulate residual 210 Pb u
activity, A, below sediments of age, t, varies according to the following formula: A s A 0 eyk t , where A 0
247
is the total accumulated residual activity in the sediment column. A and A 0 are calculated by the direct
numerical integration of 210 Pb u profiles as a function
of the cumulate mass. The age at a particular sediment depth is given by: t s 1rk lnŽ ArA 0 ..
3. Results
3.1. Vertical profiles in the sediments
In all of the cores, the Hg profiles ŽFig. 2. show a
more or less marked increase at the surface. For
several centimeters below this increase, the concentration profiles show less elevated and more stable
Hg levels. The increases in Hg concentrations correspond to the changes in the physical quality of the
sediments ŽFig. 3.. Aside from the AM-80 core, all
of the profiles for C also show a superficial increase.
Several of the profiles delimit, more or less, a leveling off at the surface ŽFig. 4.. The CrNatom ratio
varies inversely with C. In all of the profiles Žexcept
AM-102., the cdb fractions for Fe and Al follow the
variation of the silicate-associated Al fraction ŽFig.
5..
In the floodplain lakes ŽAM-60, AM-43, AM-32
and AM-65., the surface concentrations vary little
from one core to another, ranging from 200 to 250
ngrg. The cores AM-43 and AM-60, corresponding
to lakes more directly influenced by the river Ži.e.,
channel lakes ., demonstrate a drastic change from
the general Hg concentration profiles, with maximum increases of 5–20 times greater than basal
concentrations. The marked density variations in the
sediments do not appear to be the result of simple
progressive compression under the weight of overlying sediments. Visually, we noted important variations in the proportion of fine material at the surface
as compared to a preponderance of silty andror
sandy material mixed with fine matter Žand, hence,
denser. with depth. These observations suggest a
change in the relative quality of deposited sediments
and corresponds to an increase in flocculated fine
material. The amplitude of the superficial C and
CrNatom variations in vertical profiles is significant.
The quantities of Fe cdb and Al cdb increase in the
surficial zone where Eh show lower values.
248
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 2. Profiles of mercury concentration Žngrg dry weight. in sediments.
The core profiles from the two large circular lakes
Ž dish-lakes AM-32 and AM-65. show a progressive
and less pronounced increase in Hg concentration
than in channel lakes, with the surface sediments
being only 1.2–2 times greater than those at depth.
The variations in water content and density with
depth are less evident and, with visual inspection, do
not appear to correspond to changes in the granulometry. The surface sediments are more flocculated
than those at depth where they become clayey,
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
249
Fig. 3. Profiles of water content and dry density in sediments.
clumped and compact. In superficial sediments, the
Fe cdb and Al cdb profiles do not follow the Eh variations. The AM-32 profile, however, demonstrates a
much greater increase in oxyhydroxides as compared
to aluminosilicates.
In floodplain lakes, the Fe cdb and Al cdb contents
more closely follow aluminosilicate contents than the
redox conditions. The increase of oxyhydroxides at
the surface does not appear to be the effect of
remobilization and reprecipitation of these redox
250
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 4. Profiles of carbon content and atomic CrN ratio in sediments.
sensitive elements. In following the siliceous fraction
we can account for the increase in the quantity of
clay and associated oxyhydroxides in the sediments.
In the Tapajos
´ ria ŽAM-73 and AM-80., the
superficial increase of Hg levels is greater at the inlet
ŽAM-73. of the system than in the middle ŽAM-80.
where the change in concentration is more progressive for the same sediment thickness. The surface
concentrations correspond to increases of 3 and 1.5
times those of the basal levels for AM-73 and AM-80,
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
respectively. The marked density variations at the
inlet of the Tapajos
´ ria profile ŽAM-73. illustrates
the presence of sediments that are finer and richer in
water than those for channel lakes ŽAM-43 and
AM-60.. The AM-80 core shows very weak and
progressive changes at the surface. The sediments of
the core comprise fine, flocculated material which,
over the length of the core, do not appear to have
been affected by compression. At the beginning of
the Tapajos
´ ria the surface levels of C increase
gradually but distinctly. The AM-80 core, which
shows an exceptional decrease in C at the surface,
has relatively high C Ž) 3%. in comparison to other
cores. The two cores from the ria show lower Eh
values just below the surface. The general shape of
the Fe cdb profile is not changed but a net impoverishment in oxyhydroxides, as compared with Al si., is
observed at this depth. It is possible that a portion of
the Fe oxyhydroxides is remobilized in this reducing
zone of the sediments. The effect of this reduction on
the vertical profiles does not, however, alter the
absolute increase in concentrations of Fe cdb and Al cdb
corresponding to an enrichment of clayey material,
as indicated by the increase in Al si. concentrations.
At the end of the Arapiuns ria ŽAM-94., the Hg
profile shows a net progressive increase of 1.3 times
for the surface sediments. For more than 60 cm
below this increase, the Hg levels are relatively
stable. The sediments of the core are particularly rich
in water Ž85%–95%. and show imperceptible variations in the density. These sediments have C contents
of ) 4.8% and show the least variability with depth.
In the Arapiuns ŽAM-94., the levels of oxyhydroxides progressively increase at the surface where the
sediments have lower E h values. The sediment–water
interface clearly shows a favourable effect on the
precipitation of oxyhydroxides in the floc. As was
observed for the sediments of the Tapajos
´ rio, the
progressive increase in the cdb fraction follows that
of the silicate fraction for over 20 cm below the
interface.
The depth profile of Hg in a Õarzea
lake of the
´
Amazon ŽAM-102. shows the lowest concentrations
with two distinct levels. Sediments from the surface
contain more Hg, with a maximum of 130 ngrg
between 8 and 12 cm, while below this, for more
than 25 cm, the Hg data do not vary. The surface
result indicates an increase of 1.3 times, with a
251
maximum just below of 2 times the basal concentrations. Although, the dry densities for the sediments
of the Õarzea
´ lake are not available, the % water
results are similar to those of the circular lakes of the
Tapajos.
´ This core is the only one to yield highly
modified oxyhydroxide profiles, indicative of diagenetic effects. The profiles demonstrate significant
Fe cdb and Al cdb precipitation at a depth corresponding to the base of surficial C enrichment ŽFig. 5..
The sediments have higher Eh values at this depth.
The C increase corresponds to a slight decrease in
Al si. and Fe si. in the superficial part of the profiles
and appears to be independent of the oxyhydroxides.
There seems to be a relationship between the superficial accumulation of C, the oxyhydroxide geochemistry and the redox potential in these sediments.
3.2. Hg geochemistry of the sediments
The Hg concentrations measured in all of the
cores collected in the basins of the Tapajos
´ and
Amazon Rivers are compiled according to different
geochemical and textural indices ŽFig. 6.. Inter-indicator relationships in the sediments were investigated
by linear regression analysis, with the resulting correlation matrix shown in Table 1. The core from the
Arapiuns ŽAM-94. was excluded from the calculation of the regression parameters because the interindicator relationships for this core always indicate it
to be an outlier as compared to the other sediments.
The sediments from the end of the Arapiuns ria are
clearly distinguished from those of the Tapajos
´ River
by their relatively low densities despite similar concentrations of Hg, their lower Hg levels as compared
with C and oxyhydroxides and their slight enrichment of Hg as compared to Al si.. In the sediments
from the Tapajos
´ and Amazon rivers, Hg is significantly correlated with all indicators, particularly with
Al si., water content and N.
Although a strong relationship exists between C
and N, the correlation between Hg and N is better
than that with C. The relationships observed are in
contrast to those previously noted for soils of the
region where no correlation was found between Hg
concentrations and C and N ŽRoulet et al., 1998b..
The most marked deviations in C as compared to Hg
are noted in core AM-43 and appear to correspond to
the presence of coarse plant debris observed in sedi-
252
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 5. Profiles of the different fractions of iron and aluminum, and redox potentials in sediments.
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 5 Žcontinued..
253
254
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 6. Relationships between Hg concentration and water content, dry density, carbon, nitrogen, iron and aluminum oxyhydroxides and
aluminosilicate concentrations in the different sediments.
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Table 1
Correlation matrix of Hg concentrations, geochemical and textural
indicators for the Tapajos
´ and Amazon river sediments
p- 0.0001 except U ps 0.0285; UU ps 0.0937; UUU ps 0.8127;
UUUU
ps 0.0025; UUUUU ps 0.0083.
% eau d
Hg
0.81
% eau
d
C
N
Fe cdb
Fe si
Al cdb
C
e
N
0.65 0.63 0.78
0.71 0.73 0.77
0.63 d 0.67 c
0.92
Fe cdb Fe si
0.52
0.59
0.60 b
0.23
0.38
Al cdb Al si
UUUUU
0.05
0.078UUUU
0.18
0.00UUU
0.02UU
0.30
0.58
0.57
0.39
0.31
0.42
0.60
0.04U
0.87
0.74
0.62 a
0.39
0.55
0.46
0.15
0.40
Note: best fit with 2nd degree polynom.
a 2
r s 0.71, p- 0.0001.
b 2
r s 0.66, p- 0.0001.
c 2
r s 0.75, p- 0.0001.
d 2
r s 0.74, p- 0.0001.
ments Žarrows in Fig. 6c.. The debris probably originated from the Igapo´ Žseasonally flooded forest.
surrounding this little lake. C and N were only
weakly correlated with the mineral matter indices
ŽFe cdb , Al cdb and Al si . but show stronger relationships with textural indices Ž d and % water..
Inter-relationships between Hg, d, % water, Al cdb ,
Fe cdb and Al si show that all of these indices are
significantly correlated with each other ŽTable 1..
Between the different cores and over their lengths,
the Hg concentration profiles demonstrate the same
relationship with the % water and d. The only noted
255
deviation is with the deep sediments collected from
the circular lakes of the Tapajos
´ plain which, below
the flocculated surface, have a clayey, lumpy and
compact texture.
The relationships of Hg with mineral matter
demonstrate that the Al si. inputs coincide with those
of Hg over the length of the profiles and among all
sediments of the Tapajos
´ and Amazon rivers. Weaker
relationships between Hg and Fe cdb and Al cdb are
observed. They present several deviations that are
probably related to diagenesis, as well as the specific
oxyhydroxide mineralogy Ži.e., quality and size of
the crystals, Al substitution, interaction with organic
matter.. All of these factors greatly influence the
adsorption capacity and the availability of adsorption
surfaces. The quantity of oxyhydroxides in the clayey
matrix are equally variable, depending on the texture
of the particles and the type of soil in the Tapajos
´
Valley ŽRoulet et al., 1998b.. The profiles of the
Õarzea,
showing a decoupling between the cdb and
´
siliceous fractions, demonstrate a similar relationship
between Hg and the quantity of Al si..
3.3. Profiles of
210
Pb and geochronological models
Profiles of the total 210 Pb activity as a function of
the dry material accumulated in the long cores from
the flood plain, lago Piranga ŽAM-43., and the ria
ŽAM-73. seem to follow the increase of aluminosilicates, oxyhydroxides, C, as well as the textural
variations noted at the surface ŽFig. 7.. The profiles
Fig. 7. Profiles of 210 Pb activities in AM-43 and AM-73 sediments as a function of cumulative dry matter weight.
256
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Fig. 8. Relationships between time and depth in AM-43 and AM-73 sediments calculated with C.R.S. and C.I.C. geochronological models.
of excess 210 Pb Ž210 Pb u . are not regular and therefore
indicate that the classic radioactive decay conditions
are not respected. In such a case, the geochronology
cannot be established using a simple constant fluxconstant sedimentation rate model ŽCF:CS, Robbins,
1978..
The C.I.C. model ŽGoldberg, 1963; Robbins,
1978. was developed for systems where an increase
in sedimentation is associated with an increase in
210
Pb u inputs, be they the result of erosion of surface
material containing appreciable quantities of 210 Pb u
or due to a significant adsorption of available 210 Pb
by particles in the water column. Another model, the
C.R.S. model is applicable to systems where, even
before a perturbation event, the transfer of 210 Pb, via
particulate matter, from the water column to the
sediments is an efficient route. For this model, an
increase in particulate inputs cannot provoke a depositional increase in the isotope. The levels of residual
210
Pb are comparable to the atmospheric flux as well
as to those of other cores from the same region. The
210
Pb flux is considered to be constant. The application of the two models to the surface sediments
yields different chronologies ŽFig. 8..
4. Discussion
4.1. Normalization of the quantities of Hg in the
sediments
It has already been demonstrated that even if
clays do not play a direct role in the sequestration of
trace metals, they may act as a physical substrate for
the transport, precipitation and flocculation of organic matter and oxyhydroxides which more strongly
absorb metals ŽWindom et al., 1989; Ravichandran et
al., 1995.. Numerous studies have demonstrated that
As, Co, Cr, Cu, Fe, Pb, Mn, Ni, and Zn significantly
correlate with Al tot., Fe tot. and grain size, suggesting
that natural aluminosilicates are the support of natural trace metals ŽBruland et al., 1974; Goldberg et
al., 1979; Trefry et al., 1985; Sinex and Wright,
1988; Windom et al., 1989; Schropp et al., 1990;
Alexander et al., 1993; Morse et al., 1993; Daskalakis
and O’Connor, 1995; Shine et al., 1995.. The Fe tot.
and Al tot. concentrations are also frequently used for
the normalization of naturally occurring metals to
their anthropogenic counterparts. In these studies,
however, Cd and, especially, Hg were generally
found to not correlate with these parameters as a
result of the greater natural contribution of organic
matter to their adsorption by sediments.
In the present study, since the mineral indicators
are linearly inter-correlated, they can be normalized
to each other ŽTable 2.. Where data permitted, the
same normalizations for soils ŽRoulet et al., 1998b.
and suspended particulate matter ŽRoulet et al.,
1999b., as well as the mean HgrAl atomic ratios for
the earth’s crust, sediments and soils ŽBowen, 1979;
Salomons and Forstner,
1984. were also compiled.
¨
HgrAl si. atomic ratios in the Tapajos
´ sediments are
higher than the mean for the earth’s crust but similar
to overall mean for sediments, shells and soils. Values for soils from the Tapajos
´ Valley are higher than
the mean earth soils. Along the Tapajos
´ Valley,
Table 2
Elemental atomic ratios in sediments, suspended particles and soils from the lower Tapajos
´ Valley and in the earth’s surface
HgrAl si.
Ž=10 6 .
surf.
0.26
0.21
0.23
0.22
0.18
0.2
0.33
0.18
Soils
Oxisol
AM-78 a
Oxisol
AM-114a
Oxisol
AM-25a
Ultisol
AM-09 a
Organic horizons c
deep
0.35
0.16
0.19
0.19
0.15
0.2
0.32
0.12
0.74
1.31
1.21
1.16
1.20
1.00
1.03
1.50
1.14
Fe cdb rAl si.
Ž=10.
surf.
0.68
0.84
0.84
0.82
0.79
0.71
2.1
0.85
deep
1
0.76
0.48
0.73
1.3
0.71
1.9
1
EF
0.68
1.11
1.75
1.12
0.61
1.00
1.11
0.85
1.03
Al cdb rAl si.
Ž=10.
surf.
0.18
0.22
0.25
0.16
0.26
0.15
0.72
0.1
deep
0.45
0.23
0.16
0.13
0.3
0.14
0.65
0.14
EF
0.40
0.96
1.56
1.23
0.87
1.07
1.11
0.71
0.99
HgrFe cdb
Ž=10 6 .
surf.
3.82
2.65
2.73
2.65
1.29
2.82
1.52
2.15
deep
3.63
2.15
4.72
2.65
1.25
2.79
1.65
2.13
EF
1.05
1.23
0.58
1.00
1.03
1.01
0.92
1.01
0.98
HgrAl cdb
Ž=10 6 .
surf.
15.4
9.98
9
13.9
7.14
13.7
4.56
18.4
deep
9.54
7.06
13
14.8
4.69
13.7
4.7
10.1
EF
1.61
1.41
0.69
0.94
1.52
1.00
0.97
1.82
1.25
HgrC
surf.
2.97
2.31
3.39
3.59
3.53
4.45
1.54
1.99
EF
deep
3.74
1.88
2.98
3.7
2.87
3.01
13.7
3.33
0.79
1.23
1.14
0.97
1.23
1.48
0.11
0.60
0.94
AlrAlqFe cdb CrN
surf.
0.2
0.21
0.23
0.16
0.16
0.17
0.29
0.11
deep
0.28
0.24
0.28
0.15
0.22
0.17
0.26
0.15
surf.
13.7
16.2
12.9
12.9
13.0
15.1
12.5
14.2
deep
16.2
27.9
13.4
13.8
19.1
15.9
14.9
16.5
0.25
0.66
0.42
3.74
6.02
5.42
0.39
16.8
0.33
2.26
0.49
1.40
6.27
6.50
0.18
20.76
0.51
1.47
0.75
3.59
7.09
8.10
0.34
23.52
0.68
2.14
1.11
3.18
6.57
5.24
0.27
13.24
4.6–17.8
5.9–34.5
0.19–0.79
0.30–0.48
17.0–21.8
3.13
6.26
6.67
49.05
3.75
1.72
0.33
0.12
12.2
14.3
Suspended particles
FPM
CPM
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Sediments
AM-60
AM-43
AM-32
AM-65
AM-73
AM-80
AM-94
AM-102
Mean
EF
Mean earthb
Crust
0.082
Sediment 0.20–0.35
Shale
0.28–0.30
Soils
0.11–0.20
a
Mineral horizons.
Ž1984..
HgrAl after data compiled by Bowen Ž1979. and Salomons and Forstner
¨
c
Range of mean values.
b
257
258
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
HgrAl si., HgrFe cdb , HgrAl cdb , and HgrC atomic
ratios in sediments are similar or slightly lower than
those of soils and fine particulate matter ŽFPM, - 63
mm.. This indicates that sediments have the tendency to be less enriched in Hg than soils and
suspended particles. This apparent impoverishment
of Hg could be due to a desorption during transport
or sedimentation, or early diagenesis, but could also
be the result of a difference in the quantity of
oxyhydroxides relative to aluminosilicates, as shown
by the variation of Fe cdbrAl si. and Al cdbrAl si. atomic
ratios ŽTable 2.. HgrFe cdb , HgrAl cdb and HgrC
atomic ratios in soils and FPM are similar, indicating
the pedological source and the natural geochemical
equilibrium of the Hg associated with particles in the
river ŽRoulet et al., 1999b..
An enrichment factor ŽEF. for Hg and Fe cdb and
Al cdb in the surface Žsurf.. relative to the subsurface
Ždeep. sediment normalized to Al si. can be calculated
according to the formula ŽHorovitz, 1991.: EF s ŽHg
surf.rAl si. surf..rŽHg deeprAl si. deep. ŽTable 2..
EFs for Hg relative to Fe cdb , Al cdb and C are also
given. In all cores, EFs are close to 1 indicating the
absence of an anthropogenic component in the sediment Hg load. When EF values for Hg relative to
Al si. are slightly higher than 1, a similar trend of the
EF values for Fe cdb and Al cdb relative to Al si. is
noted in most of the cores. EFs of Hg relative to
Fe cdb are very close to 1 in the majority of the cores
suggesting that along sediment profiles, as with soils
ŽRoulet et al., 1998b., Hg is in equilibrium of with
the Fe oxyhydroxide phase. The variation of these
ratios, and hence the variation of EFs, may be influenced by diagenesis, change in oxyhydroxide adsorption surfaces, as well as, changes in the quality and
type of oxyhydroxides. Another factor of variation
could be the presence of more or less Al and Fe
linked to organic matter in the cdb extract. These
geochemical factors do not sufficiently alter the relationship between Hg and textural and aluminosilicate
indicators in the Tapajos
´ sediments.
In the soils of the Tapajos
´ Valley, Hg follows
Al cdb and, in a less pronounced manner, Fe cdb ŽRoulet
et al., 1998b.. The profiles of these elements in the
Tapajos
´ sediments show the same trend. The relationship between Hg and C and N is principally
attributed to the effect of a clayey matrix. In the
profiles, Hg does not follow C but the concentrations
increase with the different mineral indicators considered, and C follows textural indicators rather than
mineral indicators. As in the soils of the region
ŽRoulet et al., 1998b., as well as the suspended
matter of the Tapajos
´ river ŽRoulet et al., 1999b., it
is the oxyhydroxides incorporated into the aluminosilicate matrix that appear to be responsible for the
adsorption of Hg by the sediments ŽGibbs, 1977..
However, even if oxyhydroxides are excellent absorbents of Hg in soils, they are not the best candidates for the normalization of natural Hg levels
Žoriginating from the alteration and erosion of the
soil cover. as compared to other Hg inputs Žanthropogenic contamination. to the sediments. They,
themselves are subject to redox andror geochemical
transformation processes during their transport and
following their deposition. The fact that cdb also
extracts Al and Fe associated with organic matter
introduces another possible factor of variation. These
processes can significantly modify the initial or apparent adsorption capacity of the material of concern.
Aluminum is the element most commonly used to
trace the natural and anthropogenic origin of metals
accumulated in sediments ŽRavichandran et al.,
1995.. Even if it is preferable to use Al si., the use of
Al tot. yields a satisfactory estimation of the quantity
of aluminosilicates since the cdb-extracted fraction
of Al is often low as compared to the siliceous
fraction Ž- 10%.. Al si. compensates for the effects
of grain size and composition variations, redox processes and sedimentation on the concentration of
metals in the sediments. The Al si. concentrations
represent the quantity of aluminosilicates; the natural
adsorption matrix for Hg originating from the alteration andror surficial erosion of soils in the drainage
basin of the Tapajos
´ River.
4.2. Change of sediment source
In the case of colonized drainage basins such as
that of the Tapajos,
´ it is important to take into
consideration numerous possible ground cover alterations and their effects on the quantity and quality of
the drained material. The development of mines,
forestry practices, crop and pasture land, and roads
promotes the loss of forest cover and is followed by
the progressive exploitation of the underlying soils.
As a result the soils are subjected to increased and
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
more invasive erosion. The bottom sediments of
aquatic systems may also be subjected to resuspension as a result of dredging carried out by gold
miners during the dry season.
Aluminum concentrations and size of the deposited particles are often used as indicators of erosion of fine clayey particles from cultivated soils
ŽMackereth, 1966; Binford, 1983; Engstrom and
Wright, 1983; Engstrom et al., 1985; Foster and
Walling, 1994; De Boer, 1997.. An increase in the
proportion of fine sediments andror suspended particles may lead to increased flocculation ŽDroppo and
Stone, 1994; Milligan and Loring, 1997.. The increase in Al si. in the surface sediments is a probable
indication of such effects on the finer, less dense and
more water-rich surface sediments. Additional evidence of soil erosion processes was observed by the
variation of organic matter quality in some of the
cores ŽFarella et al., 1996.. These researchers observed that variations in the quantity of C and the
CrNatom ratio correspond to a complete change in
lignin-derived phenols as a result of erosion processes affecting the organic and mineral horizons of
the drainage basin soils.
The indicators suggest that the geochemical and
textural changes observed in the surface sediments
are the result of an increase in fine particles from the
soils of the drainage basin. These changes correspond, in turn, to increased overland flow as a result
of clear cutting. The absence of forest cover and the
development of agriculture encourages the surface
erosion of fine, mineral particles rich in Hg ŽRoulet
et al., 1998b.. Erosion during the rainy season increases the quantity of fine terrigenous suspended
particles transported by the river ŽRoulet et al.,
1999b.. The subsequent sedimentation of these fine
particles increases the Hg concentrations measured
in the surface sediments of the flood plain and ria of
the Tapajos
´ river.
4.3. Sedimentation of Hg in the flood plains and rias
The turbidity of the Tapajos
´ in the zone upstream
from Aveiro ŽFig. 1. is always relatively elevated in
comparison to the ria ŽIrion, 1984; Roulet et al.,
1998a; Roulet et al., 1999b.. In the turbid systems,
the water of the alluvial flood plains undergoes
progressive decanting as a function of current and
259
geomorphology ŽBrown, 1983; Sioli, 1984; Pizzuto,
1987; Marriott, 1992; Asselman and Middelkoop,
1995; Walling et al., 1996.. The sedimentation is
generally greater, and the sediments deposited
coarser, in the zones close to the channel of the river,
as compared to those further away ŽSioli, 1984..
These conditions explain the variations in texture
and quantity of aluminosilicates observed between
the different cores of the flood plain. The small
channel lakes ŽAM-60 and AM-43., as compared to
the circular lakes ŽAM-32, AM-65 and AM-102., are
characterized, at depth, by coarser sediments relatively poor in Al si. and Hg. Similar effects are
anticipated downstream ŽHe and Walling, 1997.. In
the ria, the current determines the sedimentation at
the inlet Žsedimentation zone. as well as along the
length of the system via its influence on the load of
organo-mineral material transported and progressively decanted ŽIrion, 1984; Roulet et al., 1998a,
1999b.. The sedimentation in the middle of the ria
ŽAM-80. is richer in Hg and fine Al si. than at the
inlet ŽAM-73.. The greater levels of C in the Tapajos
´
ŽAM-80. and Arapiuns ŽAM-94. rias suggest coagulation of the mineral particles with the colloidal
organic matter ŽChauvel et al., 1996; Roulet et al.,
1999b., particularly in the Arapiuns. This effect is
suggested by suspended particles which appear to be
enriched in C, Al cdb , Fe cdb and Hg ŽRoulet et al.,
1999b.. In the AM-94 core, the relationships between the different indicators are visibly modified as
compared to those of the other sediment types ŽFig.
6..
The effect of erosion on Hg geochemistry in
sediments depends upon the sedimentation conditions prevailing prior to the system perturbation.
Increased sedimentation of fine material provokes a
significant increase in superficial Hg concentrations
in sediments of the channel lakes and at the beginning of the ria. In the circular lakes and at the end of
the ria, the sediments are already richer in fine,
clayey material as evidenced by Al si. at the base of
the cores. In these lakes, erosion has a relatively less
pronounced effect on the variation of Hg concentrations and on the quality of surface sediments. The
resuspension of bottom sediments in the circular
lakes during the dry season may also contribute to
the mixing or homogenization of the sediments. In
the long term, these perturbations attenuate the effect
260
Table 3
Comparative data on the use of
210
Pb in tropical America and the world
Western Europe
core satisfying
the C.R.S. criteria
Bolivian Ande Valley,
C.R.S. sediment with
detritic input
bog
Total residual
unsupported
210
Pb content
A 0 Ždpm cmy2 .
11.1–95
6.7–15.5
0.54
Direct atmospheric
flux Ž198–208S.
Titicaca Lake, Bolivia
C.R.S. low depth- 5 m
Unsupported
Pb concentration
at surface
C 0 Ždpm gy1 .
210
5.2–81.5
28.3–31.2
210
Pb flux
equivalent to
210
Pb residual
Ždpm cmy2 yry1 .
Pb flux
equivalent to
210
Pb at surface
Ždpm cmy2 yry1 .
References
0.27–2.95
Appleby and Oldfield, 1983
0.88–0.97
Pourchet et al., 1995
7.8
0.25
6.1–7.4
0.19–0.23
12.1
210
Krishnaswami and Lal, 1978
0.34
0.66
Pourchet et al., 1994
2.3
Forsberg et al., 1989
Brazilian Amazon
Floodplain lake,
Jamari River, C.R.S.
9.9
85.4
2.5
2.66
AM-43
AM-73
2.48
4.33
160.84
120.15
18.63
12.84
5.01
3.74
this study
this study
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
Mean
concentration
of supported
210
Pb
Ždpm gy1 .
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
of variations on the accumulation of sediments in
this type of lake.
As indicated by higher concentrations of Hg and
Al si. in surface sediments, erosion promotes an increase in Hg contamination in aquatic environments,
but the effect of sedimentation on the texture of
sediments dominates the apparent amplitude of this
increase.
4.4. History of mercurial contamination in the lower
Tapajos
´
Few studies have used 210 Pb or other such dating
methods in the central Amazonian basin. Forsberg et
al. Ž1989. employed the C.R.S. model to interpret the
effect of colonization of a drainage basin on the
sedimentation rates in Rondonia
ˆ Žsouthern Amazon..
While they estimated the residual 210 Pb, they did not
compare numerous cores, thereby greatly limiting the
validity of the model used ŽBinford et al., 1993..
In the AM-43 and AM-73 cores, the accumulated
residual 210 Pb activities of 120 and 160 dpm cmy2
are elevated compared to other studies and the observed atmospheric fluxes below those for low latitudes ŽTable 3.. The AM-43 sediments show the
greater accumulated residual activity and correspond
to greater sedimentation of eroded material in com-
261
parison to the AM-73 core, as defined by the physiography of the river ŽRoulet et al., 1998a.. The
increase in 210 Pb can be explained by an increase in
superficial erosion of soils containing appreciable
quantities of excess 210 Pb ŽEakins et al., 1983; McCall et al., 1984..
For our case, the C.I.C. model is the most appropriate for surface sediments ŽEakins et al., 1983. but
it necessitates an elimination of sedimentation zones
where the activities increase. The efficient modeling
of such a system implicates, however, an evaluation
of the relative importance of inputs derived from the
surface of soils as compared to those coming from
the atmosphere, as well as the behaviour and fate of
210
Pb in the water column ŽOldfield and Appleby,
1984; He and Walling, 1996..
In view of the considerable incertitude in
chronologies determined via theoretical modeling of
210
Pb fluxes, Oldfield and Appleby Ž1983; 1984.
proposed an empirical approach in which the theoretical model is tested using geochemical, magnetic,
palynological and textural factors. If the C.I.C. model
is valid, the increase in Hg at the surface would,
according to the AM-73 core, be dated at least 25
years Ži.e., since 1960–1970. and at more than 30
years Ži.e., before 1960. for the AM-43 cores. The
AM-73 core is, however, irregular below the superfi-
Fig. 9. Population growth of the Santarem,
and Cuiaba Municipalities.
´ Itaituba, AveirorRuropolis
´
262
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
cial increase and so it is difficult to determine if the
profiles are comparable in terms of chronology. As a
means of testing the chronology we can attempt to
use historical or geographical indicators of colonization in the region. Fig. 9 shows the population
growth in Santarem,
´ Itaituba and AveirorRuropolis
´
Municipalities in the lower Tapajos
´ region and another Amazonian Municipality, Cuiaba ŽMato
Grosso.. The growth between 1940 and 1991 is
perfectly exponential. The most important increase
occurred since the beginning of the 1970s and corresponds to the heavy colonization in the region following development of the trans-amazonian highway
ŽWalker et al., 1997.. The observed erosion typically
corresponds to the effects of colonization accompanied by exponential population growth on the
drainage basins ŽMcCall et al., 1984; Forsberg et al.,
1989; De Boer, 1997.. The history of the region
indicates that the increased transport and deposition
of terrigenous Hg was significantly accelerated during the 1950s, 1960s and 1970s.
5. Conclusions
Our sediment results indicate, for the first time,
an excellent relationship between Hg and Al si. and
other geochemical parameters associated with aluminosilicates. The relationship between Al si. and Hg
over the length of, and between, all of the profiles
for a particular physiographic zone raises numerous
questions regarding interpretations documented for
previous studies in the Amazon region which did not
take into consideration mineral or organic matter
geochemistry Že.g., Lacerda and Salomons, 1991;
Lacerda et al., 1991; Pfeiffer et al., 1993; Hylander
et al., 1994; Gomes et al., 1996; Filho and Maddock,
1997.. The use of independent geochemical indicators appears to be indispensable for verifying whether
the increase of Hg levels in surface sediments is
indeed the result of gold mining activities in the
Amazon. The systematic use of Al si. concentrations
opens an interesting route toward the normalization
of Hg derived from the alteration andror erosion of
terrestrial regions and measured in the sediments
since, in one determination, it accounts for granulometry and the concentration of clays and oxyhydroxides.
The geochronology is imprecise and the geo-chronological indicators insufficient for determining, with
certainty, the perturbation history observed in the
lacustrine sediments of the Tapajos
´ basin. In all of
the aquatic environments studied in the Tapajos,
´ the
Arapiuns and the Amazon, a significant increase
Ž1.2- to 25-fold depending on the type of sediment
and sedimentation. in Hg concentrations in recent or
superficial sediments is observed. This increase is
significantly correlated with a change in the quality
of deposited sediments and is related to a relative
modification of the source of surface sediments.
They are finer and richer in Hg, oxyhydroxides,
aluminosilicates, and C than deeper sediments.
The variations in concentrations of the different
elements studied are similar in all of the lentic
systems and suggest an overall enrichment of the
matrix of recent sediments by fine clays. These
observations provide geochemical evidence for the
effects of the mechanical erosion of the surface soils
of the Tapajos
´ Valley ŽRoulet et al., 1998b. on the
increased transport Žduring the rainy season. of fine
particles rich in Hg ŽRoulet et al., 1999b.. The origin
of recently accumulated Hg demonstrates that continual colonization, deforestation and agricultural exploitation of virgin Amazonian forest are responsible
for a major perturbation of the cycling of Hg in soils
of the central Amazon. This perturbation, in turn,
results in increased Hg contamination of lentic systems of the lower Tapajos.
´
The bioavailability of sediment-bound Hg would
be expected to be low given its strong association
with sediment particles. Due to the importance of
MeHg in the trophic transfer and toxicity of Hg,
inorganic Hg associated with sediment particles also
needs to be methylated before ingestion. The
bioavailability of MeHg to filter-feeders is generally
greater than that for inorganic Hg. Gagnon and
Fisher Ž1997. show that the adsorption of sedimentbound MeHg by mussels is rapid and efficient, even
from uncoated inorganic particles with low nutritional value. They conclude that MeHg is assimilable
from all major sedimentary components. Favourable
methylation conditions were observed in the Tapajos
´
flood plain ŽGuimaraes
˜ et al., 1996; Roulet et al.,
1999c.. We need now to evaluate the influence of
erosion on nutrients ŽDorich et al., 1984. and organic
matter availability and to consider its effects on the
M. Roulet et al.r Chemical Geology 165 (2000) 243–266
production and food transfer of MeHg in the Tapajos
´
aquatic ecosystems.
Acknowledgements
Financial support for this project was provided by
Canada’s federal government through the International Development and Research Center ŽIDRC.
and as part of the CARUSO project Žcontract a961052-01r001300-01 and scholarship to the first author.. Supplementary financial assistance, in the form
of a doctoral scholarship, was provided to the first
author by the Quebec
´ government via the Fond pour
la formation de Chercheurs et l’Aide a` la Recherche
ŽFCAR.. We would like to thank the Universidade
Federal do Para´ ŽUFPa. and the UFPa campus in
Santarem
´ for their assistance with financial, logistical and technical aspects. The project would never
have seen the light of day without the collaboration
of Aldo Queiroz Gomes, coordinator of the Santarem
´
campus and Cristovan Waverly Diniz, rector of
UFPa. We would also like to thank Louise
Cournoyer, Gerson da Costa da Oliveira, Jadson
Dezincourt, Erinaldo de Jesus da Silva, Jean Lebel,
Everaldo Portela, Isabelle Rheault, Sewbert Rodrigues Jati, Gil Serique, Delaine Sampaio da Silva,
Persio Scavone de Andrade, Nicolas Soumis, Silmara Sousa de Morales, Carlos-Jose´ Sousa Pasos,
Sophie Tran, and Ronilson Vasconcelos Barbosa for
their help in the field and laboratory. The analytical
methods used for Hg were developed by Professor P.
Pichet for the laboratory of the Chaire de Recherche
en Environnement H-QrCRSNGrUQAM and
GEOTOP. Special thanks to Shelagh Montgomery
for help in the english translation of the manuscript. [JD]
References
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