recent metal distribution in surface deposits and sedimentary

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OVERVIEW OF THE METAL DISTRIBUTION IN SURFACE DEPOSITS
AND SEDIMENTARY RECORDS IN THE SUVA LAGOON
1J.M.
Fernandez, 1G. Cadiou, 1B. Moreton, 1R. Legendre, 2R. Fichez and 3C. Badie
1IRD-Noumea,
2IRD-COM,
Nouméa, NEW CALEDONIA
Station Marine d'Endoume, Marseille, FRANCE
3IRSN/LESE-Papeete,
FRENCH POLYNESIA
ABSTRACT
Human activities have impacted the natural environment, especially since the middle of the
19th century. In tropical regions the history of recent or past environmental conditions is
partly recorded in the layers of the sediment reservoirs of lagoons. Geochemical approaches
applied to sediments can provide information on past and present changes in heavy metal
concentrations and other environmental tracers that are indicative of specific environmental
modifications. These studies are combined with dating techniques such as the 210Pb method.
This paper presents all the data collected from the coral reef environment of the Suva
Lagoon. Superficial sediments have been studied in terms of heavy metal concentration
(sequential extractions) and cores provided complementary 210Pb unsupported radioactivity.
In Laucala Bay, the results showed that the inputs of terrigeneous particulate matter
originating from the Rewa River has remained high and constant over time, resulting in an
extremely limited bioturbation layer, in contrast with the more disturbed Suva Harbour
profiles.
INTRODUCTION
During the past 150 years, terrestrial and coastal environments have been strongly modified
by human activities. In the tropics, population growth and economic development have
imposed serious constraints on lagoon ecosystems (Hatcher et al., 1989). Deforestation and
mining, which are primarily responsible for hyper-sedimentation and metal pollution, are two
of the major causes of disturbance in coral-lagoon environments jointly with industrial reef
dredging (Carey, 1981; Naidu and Morrison, 1994; Zann, 1994, Morris et al., 2003). Evidence
of these upheavals may be preserved in the sediment layers that gradually build up a history
of the various successive events affecting the environment.
Interpretation of these ‘sedimentary records’ is generally based on the dating of deposits,
using natural clocks, such as, radionuclides, to establish a geochronology of sediment
deposition. Most of the studies dealing with recent sedimentary records of environmental
changes have been based on 210Pb determinations, the decrease in unsupported 210Pb
permitting age determinations back to about 100 years before the present (Faure, 1986).
However, deciphering both the geochemical and sedimentological data collected from these
layers is not always straightforward, with results often leading to misinterpretations due to a
lack of converging information.
This article reports on studies of sediment samples collected during the BULA-1 cruises
carried out by the IRD with the O/V ALIS in July 1998.
MATERIALS AND METHODS
Samples were collected in the barrier lagoon surrounding Suva, the capital of Fiji (Figure 1).
Sixty one surface sediment samples were collected using a light Van-Veen grab of small
capacity (approximately 1.8 L) made of stainless steel and able to sample the surface
sediments with a minimum of disturbance of the water/sediment interface. The upper surface
layer consisting of oxidized sediments was stored in one use only plastic bags (Whirl-pack),
frozen then freeze-dried. Two aliquots were stored for each sampling site for the
sedimentiological and geochemical analyses respectively.
Two cores were also sampled in Suva Lagoon (Figure 1). The first was collected in Suva
Harbour (S14) and the second in Laucala Bay (S31). Suva Harbour is known to have
experienced a large increase in industrial and urban development in the coastal zone over the
past 30-40 years (Naidu et al, 1991). Laucala Bay, where the other core was sampled, is a
coral reef barrier lagoon acting as a major recipient of the discharge from the 2900 km 2
catchment of the Rewa River, the largest in Fiji.
The sediment cores were sampled using a specially designed PVC corer operated by SCUBA
divers (Harris et al., 2001). The corer consisted of a 1.2 m long PVC tube, 25 cm in diameter,
which had been cut in half from top to bottom. The two halves were clamped together during
coring and transportation to keep the core intact until sampling. The corer was forced down
into the sediment by hammering on a cap placed on the top of the corer to about 0.5 m deep.
The sediment surrounding it was then pumped away until the bottom could be sealed with a
second cap. The corer was then removed and kept vertical.
Once onboard, the core was allowed to settle vertically, the top was then removed and the
overlying water carefully pumped out. The first fluid layers were sampled using a spatula and
the core was then laid horizontal, and 2 cm slices taken for geochemical and sedimentological
analysis. A sub-sample of each slice was used for 210Pb measurements.
For the geochemical study, each sediment sample was grain sized by wet sieving in order to
recover the fine material fraction (particles < 40 µm of the sediment). This fine fraction
contains more than 90% of the sorbed on particles as a result of reactions taking place on their
very large specific surface (e.g., Mayer and Fink, 1979; Ackerman, 1980; Deely and
Fergusson, 1994; He and Walling, 1996). The sediment grain size fractionation was carried
out using 25 to 30 g of bulk sediment weighted in polyethylene flasks, homogenized in 100
mL of ultra-pure water (Milli-Q, 18.6 µS.cm-1) and gently shaken during 1 hour. The samples
were then sieved through a 40 µm mesh and both fractions dried at 40°C, weighed and stored
in vinyl bags.
Finally, to obtain water content, additional 10 mL sub-samples were taken and weighed
before and after drying, in an oven at 110°C until constant weight. Results are expressed as a
percentage of the initial sediment dry weight. This was subsequently used to calculate
sediment accumulation in g cm-2.
• 61 surface sampling sites
• 2 cores (S14 & S31)
Figure 1: Location
of the sampling
stations during the
IRD Bula-1 Cruise
in Suva Lagoon in
July 1998
Kinoya sewage discharge
Nasinu River
Ndraunimbota
Bay
Rewa
River
Vunindawa River
Lami
Samabula River
Ndaveta
Levu
Laucala
Point
Suva
Point
Nukulau Island
Ndaveta Nukulau
Ndaveta Nukimbutho
PACIFIC OCEAN
Sequential extraction of metals
Analysis of the total metal concentrations contained in the sediments is nowadays considered
a too vague method of quantification, as it includes the entire geochemical phenomena
occurring in the water column. Much more useful information can be extrapolated by
studying metals partitioned in different geochemical phases which can be chemically defined
in the sediments and of the metal concentrations which they contain. This information relates,
in particular, to the source terms of metals and the origin of the particles which transport them
(geochemical signatures).
To highlight the anthropogenic and terrigenous sources of metals, a sequential extraction
protocol was used in this study for the determination of Ca, Cr, Mn, Fe, Co, Ni, Cu and Zn
(Fernandez et al., 2005). Despite some criticism associated with sequential extraction
techniques in the literature (e.g., Jouanneau et al., 1983; Nirel and Morel, 1990), this type of
protocol makes it possible to assess the various inputs impacting the environmental medium
(see Cornu and Clozel, 2000); the first two metal extractions (in the sequence below), in
particular, reflect the conditions of sedimentation and the anthropic input influences while the
two last can be comparable with geochemical signatures of the terrigenous contributions. The
protocol used was based on the technique developed for river sediments (Tessier et al., 1979;
Meguellati, 1982), and subsequently adapted to carbonate environments. Briefly, two grams
of fine sediment were subjected to a 4 phase chemical extraction protocol to obtain metal
concentrations associated with 4 geochemical fractions.
The various extractions were completed as follows:
 Oxidisable forms - this fraction represents the metals linked to the organic matter in the
sediment. This extraction was completed using strong oxidants (Oxygen peroxide) in an
acidic medium (nitric acid);
 Acid-soluble forms - these are relatively easily extracted using a weak buffered acidic
solution (acetic acid / ammonium acetate). The forms either occur in the carbonate matrix of
the sediment, or are adsorbed on the particle surfaces or coprecipitated (e.g., Chester and
Hugues, 1967; Perhac, 1974; Gupta and Chen, 1975). In lagoonal deposits, these metals
come from the aragonitic skeleton of organisms which, when alive, ingest or assimilate
metals (Brocero, 1998). Due to a lack in the chemical selectivity of the reactants, this
extraction step also potentially includes the easily leachable metal, initially bound by
adsorption on the poorly crystallized manganese and iron oxides (Förstner et al., 1986) and
clays (Posselt et al., 1968).
 Reducible forms - these forms are associated with oxyhydroxides of manganese, aluminium
and iron, and are extracted using reducing conditions such as hydroxylammonium chloride
in acidic medium.
 Refractory forms - these are the metals that are strongly held by the particle matrix, e.g., in
silicates and aluminosilicates. These were extracted using aqua regia mixture. In this final
step of the protocol, samples were placed in Teflon vessels and subjected to high pressure
and temperatures conditions (Anton Paar/Perkin Elmer mircowave oven).
After each of the 4 steps of the sequential extraction protocol, the loss of mass was
determined by weighing.
The reagents and chemicals used were Merck, ProAnalysis grade. The containers used for the
sequential extractions were made of HDPE, or Teflon. All containers were decontaminated by
soaking in nitric acid (5%) for 24 hours and rinsed in ultra-pure water (Milli-Q). After each
use, the Teflon vessels of the microwave oven underwent a cycle of decontamination with a
strong acid solution.
The quantification of Ni, Cr, Zn and Ca in each geochemical phase was carried out by ICPOES (Optima 3300 VD, Perkin Elmer). The concentrations are expressed, in mass of metal
vs. mass of fine sediment, (CPel, in mg/kg) and in concentration per gram of dissolved
refractory phase, (CRef, in mg/kg), defined as :
CPel = (C . D . V) / Mi
(1)
where C is the concentration in solution (mg/L), D the dilution factor of the sample, V its
volume (l) and Mi is the mass of initial fine fraction (kg),
CRef = (C . D . V) / (Ma - Mb)
(2)
where Ma is the mass of the fine fraction before attack of the geochemical phase (kg) and Mb
the mass of this fraction after solubilization of the geochemical phase (kg). The juxtaposition
of concentrations CPel and CRef ensures in particular a good differentiation of the metal
sources and makes it possible to specify the origin of the contribution.
The analyses were carried out on triplicate to check the reproducibility of the protocol and to
estimate the variability of the sample. In addition, certified reference materials (SD-M-2/TM,
NRCC-BCSS-1) were analyzed for total elements content to check for total extraction
efficiency.
Geochronology
Sediment accumulation rates were determined from the decrease in excess 210Pb activity.
210
Pb was analysed on triplicate samples by measuring its granddaughter 210Po, considered to
be in secular equilibrium with 210Pb (Teksöz et al., 1991). Each sample was spiked with 208Po
in order to appraise possible losses incurred during application of the digestion protocol. The
210
Po measurement was performed in a NUMELEC gridded-chamber (NU 114B model) by
alpha counting, following the standardised methods of Flynn (1968), modified by Nittrouer et
al. (1979) and further adapted to carbonate rich sediments by Serra et al. (1991). Excess 210Pb
was determined as total 210Pb minus 226Ra supported 210Pb. Sediment supported 226Ra activity
was measured using gamma spectrometry. Sediment accumulation rates were determined
according to Faure (1986). Mass accumulation of sediment has been commonly used to derive
independent depth scale based upon the cumulative weight per unit area (g.cm-²) and plotting
excess 210Pb activity versus mass accumulated sediment (g.cm-2) rather than simple depth
(cm) therefore compensating for compaction effects (Bollhöfer et al., 1994). Mass
accumulation was determined according to Buesseler and Benitez (1994) after measuring
particle density on each sediment sub-sample according to Boyd (1995). The age of each
sediment level was derived from the linear regression of excess 210Pb versus accumulated
sediment mass that allowed an average accumulation rate to be calculated (Robbins and
Herche, 1993). However, carbonate, which contains little or no 210Pb, may be responsible for
a significant dilution of the 210Pb signature. In order to compensate for this bias, the linear
regression was conducted on non-carbonate accumulated sediment mass instead of bulk
accumulated sediment mass. In doing this, only the accumulation rate of terrestrial sediment
was calculated, from which a more reliable age-depth relationship could be established and
subsequently was used to calculate bulk sediment accumulation rates and interpret changes in
sediment deposition and composition.
An additional determination of 137Cs was attempted despite the low activity levels commonly
recorded in the southern hemisphere (Hancock et al., 2002).
Direct 210Pb determination using gamma spectrometry at 46.54 keV, was also conducted on
the two Fijian cores. These measurements were carried out on an ORTEC X beryllium
window Diode with a relative efficiency of 80%. The unsupported 210Pb results were
subsequently plotted for the 2 cores.
RESULTS
Superficial sedimentary layer
The distribution of the fine terrigeneous fractions (Figure 2) makes it possible to demonstrate
the extreme influence of the Rewa River as mentioned by Morrison et al. (2001); the
sediments found in the eastern half of Laucala Bay containing also more than 80% of fine
material. A second source of fine particulate material can be highlighted in the sheltered
Draunibota Bay. The lowest values (4%) were recorded in the channel which separates
Laucala Bay from Suva Harbour. This spatial distribution was remarkably consistent with the
distribution of the refractory geochemical phase that constitutes a characteristic of the
terrigenous contributions (Figure 3). In addition, particularly in the eastern part of Laucala
Bay, the sediments contained a considerable proportion of oxyhydroxides that are good
tracers of the terrigenous contributions resulting from the natural inputs (Rewa River).
Conversely, Figures 4, 5 and 6 underline the existence of sediments of marine origin which
are prevalent in the fringing reef area of Suva Point with more than 40% of carbonate and/or
of organic matter.
2
2
2
Figure 2: Map of
the fines content
(%<40µm) in the
Suva Lagoon
sediments.
Figure 3: Map of
the refractory
forms extracted
from fines
(%<40µm) in the
Suva Lagoon
sediments (% of
total mass).
Figure 4: Map of
the reducible
forms extracted
from the fines
(%<40µm) in the
Suva Lagoon
sediments (% of
total mass).
2
2
Figure 5: Map of
the acid-soluble
forms extracted
from the fines
(%<40µm) in the
Suva Lagoon
sediments (% of
total mass).
Figure 6: Map of
the oxidisable
forms extracted
from the fines
(%<40µm) in the
Suva Lagoon
sediments (% of
total mass).
Terrigenous and Anthropogenic Influences
The analysis of the labile forms of metals extracted in acidic soluble fractions (carbonates)
and oxidisable fractions (organic matter) made it possible to highlight various sources with
respect to the different metals. These results lead to detail the different contributions of
anthropogenic metals suspected in a previous study (Morrison et al, 2001). Actually, the
commercial activities of Suva Harbour contribute in an obvious way to the introduction of
metals, in particular zinc (Figure 7), copper, nickel, chromium and cobalt (Figures 8 and 9), in
considerable quantities, the latter being also present in front of the mouth of the Rewa River
in slightly lower levels of concentrations.
2
2
2
Figure 7: Map of
the Zn extracted
from the
oxidisable forms
of the fines
(%<40µm) in the
Suva Lagoon
sediments
(mg/kg).
Figure 8: Map of
the Cu extracted
from the
oxidisable forms
of the fines
(%<40µm) in the
Suva Lagoon
sediments
(mg/kg).
Figure 9: Map of
the Cr and Ni
extracted from the
oxidisable forms
of the fines
(%<40µm) in the
Suva Lagoon
sediments
(mg/kg).
The other potential sources of metals identified were the Lami rubbish dump, the Tamavua
River (principally cobalt and zinc), the Nasinu River (principally cobalt), as well as the liquid
wastes discharged from the Kinoya sewage treatment plant which introduces metals such as
copper, nickel, chromium and cobalt into the Lagoon.
The lowest metal contaminated sediments were located in the channel zone at the end of Suva
Peninsula which separates the two bays (Suva Harbour and Laucala Bay) of the Suva Lagoon.
Sedimentary records
From the point of view of granulometry, the fine fraction (70-75 %) remains constant
throughout the entire length of the core S31 extracted in the zone of influence of the Rewa
River (Figure 10). This profile shows that the particulate material supply has not undergone
major modifications during the past 100 years as the mean rates of accumulation is estimated
at 0.37 g/cm²/year (210Pb) even though two sedimentary events could be distinguished (Figure
11). Moreover, the values of 234Th and 40K showed a light inflection in the top 20-25 cm of
the sediments, suggesting the possibility of a sudden modification in the particulate supply.
The two sedimentary layers identified in the core contain on average 20 4 Bq/kg in the lower
layer and 15 3 Bq kg-1 in the upper layer for the 234Th and approximately 297 25 Bq kg-1
then 281 26 Bq Kg-1 for the 40K.
The reverse was observed in the zone of the Tamavua River discharge and in the vicinity of
the commercial port (Suva Harbour), where the contribution of fine particles seems to have
appreciably changed during about the last 20 years since they constitute 70% in the deepest
layers and approximately 60% in the top 20 cm of the core S14 (Figure 12). The rates of
accumulation decreased in parallel by approximately 0.29 g/cm²/year to 0.17 g/cm²/year in the
most recent levels of the core (210Pb); the values of 234Th also underlined a possible change in
the nature of the contributions since the measured radioactivity is notably more important in
the top layer (14 3 Bq Kg-1) than in the underlying layer of the core (10 2 Bq Kg-1).
The profiles of the geochemical phase content along the core did not show any real gradients
except for the refractory phase (residue) where an important decrease in the top 20 cm of the
sediment was observed.
The metal concentrations were constant in the sediments sampled in the area of influence of
the Rewa River (Figure 11). In Suva Harbour, a low gradient of increasing metal
concentrations towards the top of the core for chromium in the reducible phase and almost the
all of the metals analyzed in the refractory phase (residue) was noted. In general, calcium was
more highly concentrated in the top of the core, and, in particular, a strong gradient was noted
in the top 10 cm (S14) highlighting a recent input to the Lagoon (Figure 12).
Core S14
0
20
Core S31
Pelitic fraction (%)
40
60
80
50
0
0
10
10
20
20
60
70
0
80
20
40
60
80
0
5
10
30
Depth (cm)
Depth (cm)
Depth (cm)
15
30
Figure 10: Profiles
of the fines
fraction and
geochemical
phases in cores
S14 and S31.
20
25
30
35
% Org Matter
40
40
S31
S14
%Carbonate
40
% Org Matter
% Oxide
% Residue
50
%Carbonate
45
50
%Oxide
% Residue
50
[Ni] in
oxidable
forms (ppm)
0,0
0,5
[Cr] in acidosoluble
forms (ppm)
1
1,0
3
[Cr] in
reducible
forms (ppm)
5
0
5
[Cr] in pelites
(ppm)
10
20
40
60
Pelitic fraction
%(<40 µm)
80
60
70
80
90
Ln(Po-210)
(mBq/g)
-5
-3
-1
0
0
0
0
0
0
5
5
5
5
5
5
10
10
10
10
10
10
15
15
15
15
15
15
20
20
20
20
20
20
25
25
25
25
25
25
30
30
30
30
30
30
Co, Cr, Fe, Zn !
Mn : 15 -47 ppm
Zn : 2.2-5.9 ppm
Mn : 139-259 ppm
[Co] in
oxidable
forms (ppm)
0
5
[Ca] in acidosoluble forms
(ppm)
10
0
Mn : 80-112 ppm
0
10000
[Cr] in
reducible
forms (ppm)
20000
0
2
[Cr] in pelites
(ppm)
4
0
0
[Cr] in
residue (ppm)
0
10
Accumulation rate:
R = 0.37 g/cm²/y
Pelitic fraction
%(<40 µm)
20
0
Figure 11: Metals
profiles in different
geochemical phases
of the core S31.
Accumulation rates
assessment
40
60
80
Ln(Po-210)
(mBq/g)
-5
-4
-3
-2
0
0
= 0,9686
RR22 =
0,97
5
5
5
5
5
10
10
10
10
10
10
15
15
15
15
15
15
5
2 =2 0,9471
RR
= 0,95
20
20
20
Other metal :
Same gradient
Other metals :
No gradient
20
[Ca]: 4600-26500
Other metals :
No gradient
[Cr] in residue
(ppm)
20
20
Accumulation rates:
R1 = 0.17 ; R2 = 0.29 g/cm²/y
Figure 12: Metals
profiles in different
geochemical phases
of the core S14.
Accumulation rates
assessment
CONCLUSIONS
The two constituent bays of the Suva Lagoon, Laucala Bay and Suva Harbour, are different in
the nature and the amounts of particulate matter which has been deposited. The first core
(Laucala Bay) is clearly influenced by the Rewa River inputs which are distributed over the
whole of the eastern half of Laucala Bay. The second area (Suva Harbour), of more reduced
size, seems to be principally influenced by the anthropogenic activities, centring mainly on
the commercial port and the industrial areas, but also the rivers bordering Suva Harbour.
These various influences as regards sources are found in the distribution of metals which
provide geochemical signatures in the recent sedimentary layers. In Suva Harbour, evidence
of significant anthropogenic activities in recent times is visible through profiles of metal
concentrations, but this is not so clear in Laucala Bay. In addition, the two sedimentary
systems are characterized, respectively, by a constant particulate matter input in Laucala Bay
and a modified particulate matter flux involving a decrease in the accumulation rates in Suva
Harbour.
ACKNOWLEDGMENTS
The authors are extremely grateful to University of the South Pacific Marine Studies
Programme for their technical contribution during the cruise and special thanks are addressed
to Dr Garimella Sitaram for his additional radiological studies. This work was supported both
scientifically and financially by the IRD Camélia Research Unit.
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