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High metal inputs to closed seas: the New Caledonian lagoon
JOURNAL OF EAPLURATION
ELSEVIER
Journal of Geochemical Exploration 59 (I 9Y7) 59-74
High metal inputs to closed seas: the New Caledonian lagoon P. Ambatsian
‘, F. Fernex ‘,*, M. Bernat ‘, C. Parron ‘, J. Lecolle
’
Received I5 October 1945: accepted 18 June 1996
Abstract The islands of New Caledonia are largely composed of ultrabasic rocks (peridotites). severily weathered. rich in Fe, Mn and Co, and where several ore deposits of Ni and Cr are extensively mined. Sediment cores from the bay of Dumbea, in the south of the main island, and from the northern part of the lagoon (Belep Islands), less affected by the mining activities, were analyzed for their mineral composition and metal concentrations. In the surficial sediments, oxides and silicates, including Fe serpentine and smectites, undergo rapid transformation or neoformation in a short time, in particular in the confined bay of Dumbea. Fe is largely present as goethite, and in deeper layers (60-100 cm) as hematite and magnetite. Chromite can be identified at each horizon. The metal concentrations decrease from the near shore areas. in particular the vicinity of the Dumbea river mouth to the open part of the lagoon. This trend is more important for Ni than for Fe or Cr. Fe ranges from 3.5 to 9% (dry weight), Ni from 200 to 2000 pg,/g, Cr from 700 to 2000 pg/g, Co from 20 to I.50 pg/g and Mn from 130 to 900 pg/g; yet the concentrations are lower than concentrations found in the ultrabasic rocks or laterites of the watershed. To try to understand the behavior of metals during the sedimentation-diagenesis events. we evaluated the sediment accumulation rate, and used different sequential leaching procedures. Fe, Mn, Ni, Cr and Co are mainly present as, or bound to, oxides or oxyhydroxides, even in the deeper layers (> 100 cm) where the organic content is relatively high (about 6% of organic C). Metals are mainly transported from the land to the lagoon as oxides and dispersed in the lagoon sediments, where they are diluted with a large amount of carbonaceous sediment. During diagenesis, a significant part of Mn, Co and Ni are dissolved; but, unlike Mn and Co, which seem to coprecipitate with carbonate, most of the Ni is released into the waters of the lagoon. Apparently no horizon of the sediment has undergone significant in-situ metal enrichment. Keyord.vt
kqoon
sediments: early diagenesis; nickel; chromium; sequential extractions; New Caledonia
1. Introduction
New Caledonia is a group of island located Southwest Pacific Ocean (Fig. 1). An important reef isolating a shallow lagoon (mean depth surrounds the islands. Large andesitic basalt
* Corresponding O375-6742/97/$17.00
author. Fax: + 33-93-529965
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PII SO375-6742(96)00020-Y
in the coral 20 m) flows
and overthrusting peridotites, weathered to laterite, are found in the south of the main island (Grande Terre) and in the Belep islands in the northern part of the lagoon. Deposits of Ni and Cr ores which have been extensively mined are common in the southern part of the main island. The petrography of the rocks is well documented (Trescases, 1975; Paris, 1981; Latham. 1985). and.
1997 Elsevier Science B.V. All rights reserved.
and S3 were taken from water 1 I m deep in the inner part of the Dumbea Bay, near the outlet of the Dumbea-Coulev6e River, whose watershed (220 km’) is largely made up of the southern peridotitic mountains. The main tlow of the river is 10 &/a. but varies from I to 1000 m’/s. Core S9 was collected at a depth of 19 m. in an area close to the peridotic Belep islands in the northern part of the lagoon, which is less influenced by the mining activities (Fig. I). The cores were sent off by plane to Nice (France). where they were stored at 4°C till sampling.
to a lesser extent, the characteristics of the lagoon sediments (Launay. 1972: Dugas and Debenay, 1982). Weathering of the ultrabasic rocks results in production of laterites enriched in metals. in particular Fe, Mn. Co, Cr and Ni. The lagoon was expected to trap a significant proportion of the material derived from the land. In particular, the Dumbea Bay sediments may trap metals like Cr and Ni transported there by River Dumbea-CoulevCe whose watershed comprises ore deposits of these metals. To try to determine the behavior and fluxes of metals during their transfer from land to the lagoonal sediments and during diagenesis, we investigated the mineral composition, metal concentrations, and deposition rates of sediments. Sequential extractions were used to determine various operationally defined chemical forms of the metals (species extractable at pH 5 or carbonate phase, reducible phases. oxidizable phase. etc...).
2. Materials
2.2. Minrrulo~~~. Grain-size frequency analysis was performed bctween 0.05 mm and 1000 mm with a laser granulometer. The mineralogical composition of the bulk sediment and clay fraction (< 2 pm) was determined by X-ray diffractometry using a Co anode. The clay fraction was separated by sedimentation in deionised water according to Stocke’s law, and subjected to different treatment before being analysed: (a) treatment with ethylene-glycol; (b) treatment with hydrazine hydrate; and (c) heated at 490°C during 4 hours. Semi-quantitative metal analysis at selected points of minerals of some samples was performed
and methods.
2. I. Sampling Three 1.2-m cores designated as S2. S3 and S9 were collected with a gravity corer in 1988. Cores S2
ultrabastcrocks
Fig.
\ coral
1.Location
, Dumbea
reiG
of the sampling
site\
Ba!
using a scanning electron microscope (SEMI equipped with an energy dispersive spectrometer (EDS). 2.3. Chemical utnulvses Redox potential was measured with a pH-meter equipped with a Pt electrode in the sediment immediately after opening the core. Carbonate content was measured with a Bernard calcimeter using concentrated HCI as the dissolving agent, and the total amount of organic carbon by the reducing ability of the samples using a sulfochromic solution. Total sulfur content was determined by the infrared analysis of SO, after oxidation at 1800°C. Metal concentrations were estimated by atomic absorption spectrophotometry. To the classical sequential extraction of metals (Tessier et al., 1979) we added the following steps: successive extractions with two reducing reagents to try to distinguish
PH 1
3
5
easily reducible oxides and hydroxides from less reducible ones (Segalen et al., 1971; Fijrstner et al.. 1986), and extraction with NaOH 0. I N with the purpose of isolating humic substances with complex metals. The following procedure was therefore used for the sequence with 7 steps. (1) - Digestion with 100 ml of a CH,COOHNa IN + CH,COOH solution at pH 5 (if necessary CH,COOH was added to maintain the pH below 5.5). This first step is assumed to extract chiefly metals of the exchangeable fraction and those bound to carbonates, although some poorly crystallized oxides may be also solubilised (Fiirstner et al.. 1986). (2) - After centrifugation under a nitrogen atmosphere. the residue of (I) was submitted 3 to 4 hours to leaching by 50 ml of a NH,OH-HCI 0.1 M solution adjusted at pH 3.5 by CH,COOH. This should solubilize principally the easily reducible fraction, i.e. Mn oxides and hydroxides and Fe hydroxides (Fig. 2); some poorly crystallized sulfides
7
9
Fig. 2. Eh-pH diagram showing the redox equilibrium lines of some geochemical species which are in discussion in the present paper. Lines corresponding to the reducing reagents used during the sequential extraction procedures are also represented. The lines are established for a
I
molar activity of the ions when no other indication.
would also be solubilized by this solution (Rapin et al., 1986). (3) - 50 ml of a (NH&c~o~ 0.25 M solution adjusted at pH 3 by means of HZC201 should dissolve the moderately reducible phase which is mainly constituted by Fe oxides. (4) - 100 ml of NaOH 0.1 M was used to extract humic substances. After centrifugation, the supernatant was acidified to pH 2 by HZSO, to precipitate humic acids; the leachable fraction is assumed to contain fulvic acids. (5) - The precipitated humic acids were digested by HZOZ adjusted to pH 1 with HNO,. There is no evidence that the amounts of each metal solubilized during steps (41 and (51 correspond actually to the metal proportion in the humic and fulvic fractions; metals extracted during steps (4) and (5) should be gathered in a unique humic fraction. (6) - To remove the remaining organic matter and poorly crystallized sulfides, the residue of (41 was oxidized by H202 and progressively heated; afterwards HNO, was added till pH reached I. (7) - The residue of (6) was introduced in 30 ml of a concentrated acid solution (2/3 HNO, ION + l/3 HCIO, ION), which was then evaporated to dryness. Redissolution by HN03 at pH I. Most of metals bound to clay minerals, remaining sulfides and oxides proof against the former leachings are expected to be removed by this attack. As such sequential processes have been criticized for low specificity (Jouanneau et al.. 1983; Rapin et al., 1986; Nirel and Morel. 19901, we compared the metal amounts extracted at various steps to the results of mineralogical analysis on residues from various steps of the sequence (X-ray diffractometry and magnetic susceptibility measurements). We also performed a 15-stage extraction sequence on sediments from two levels. O-5 cm and 25-30 cm, with the aim of better defining the efficiency of two reducing reagents: hydroxylamine-hydrochloride and hydrosulfite (dithionite) (Fig. 2). This process differed from the former 7-step sequence procedure by the repeated use of moderate reagents. (1) to (4) - Acetic acid leaching (I 00 ml each): pH 5.6 for (I 1, pH 4.7 for (2); pH 5 for (3) and (4). (5) to (8) - Leaching with NH,OH-HCI (50 ml); 0.04 M solution at pH 2.8 to 3 for (5) and (6); 0.2 M solution at pH 2 for (8).
(91 - NaOH 0. I M (100 ml). (IO) to (121 - A 40 g/l hydrosulfite solution at pH 4.7 to 5 (50 ml), which permits Fe oxides like hematite to be progressively removed (Segalen et al., 19711. ( I31 - Oxidation by H ?02 + HNO, (pH I ) heated. with a view to destroying organic matter and sulfides; oxides not totally removed during the former steps were leached during this step. (141 - In order to dissolve oxides which could have been formed during the oxidation of the 13th step, a new leaching by dithionite was used (40 g/l; 50 ml). (15) - Heated concentrated acids: 2/3 HNO, + l/3 HCIO,. In both processes, we started with 2 g of wet sediment. Immediately after the core was open, the samples were put in a IOO-ml beaker filled with the acetic acid solution until the effervescence stopped. The following steps, including centrifugation, were conducted under a nitrogen atmosphere until the oxidizing step with HZ02. Taking into account the water content, we express the results for metals with regard to the dry sediment. To control the mineralogical transformations induced by the leaching solutions, we analyzed by X-ray diffractometry and magnetic susceptibility (Poutiers. 1973) the successive residues obtained by the leaching of 12 g of wet sediment from three levels (O-5. 30-35, 65-70 cm). 2.4. Natural rudionuclides Concentration and isotopic composition of ura nium and thorium were determined by isotopic dilution and alpha spectrometry. The samples were pro cessed chemically to separate and purify the chemical species. Percent recovery was evaluated with two tracers, 23’Th and ‘.i’U. The purified species were subjected to electro-plating and then alpha counting. “‘Pb was determined for the same sample. A known amount of common lead was added with the alpha tracer. The common lead, which served as a carrier. was also used to calculate the chemical yield: it was precipitated as lead sulfate, dried, weighed, and then analysed for the beta activity of ““Bi, the ““Pb I’ daughter I! The sediment accumulation rate was evaluated
P. Amhatsian
et al. /.lounzal
of Gewheminll
through the decay of the atmospheric excess ““Pb activity within the sediment column (half-life 21.4 years). The fraction produced in the sediment (the supported ““Pb), is assumed to be equal to *jOTh the last radioactive antecedent analysed.
3. Results and discussion 3.1. Mineralogical
characteristics
of the sediments
The sediments are light brown down to the bottom (1.2 m) in core S9 (Belep). In cores S2 and S3 (Dumbea Bay) the color is darker below 90 cm. The sediment includes,chiefly in the Belep area, numerous coral fragments and other organic bioclasts (great foraminifers, siliceous spicules, etc.). Granulometry is coarser close to the Belep islands, in the northern, open part of the lagoon (core S9) than in the confined Bay of Dumbea (cores S2 and S3)(Fig. 3). According to X-ray diffractometry the carbonate fraction is 50 to 55% calcite and aragonite (CaCO,), the rest being magnesian calcite with approximately 12 mole percent of MgCO,. and a much less abundant Ca-Fe carbonate (1 to 5%). The presence of this latter mineral was confirmed under SEM examina-
Exploration
63
59 I19971 59-74
tion and EDS analysis: Ca/Fe ratios of rhombohedral crystals suggest an iron/calcium carbonate solid solution; Ti. Cr. Mn and Cu occur as minor elements in the iron richest samples (Fig. 4). Quartz and to a lesser extent feldspar represent almost half the carbonate-free fraction. In the Bay of Dumbea, the fine fraction ( < 2 km) is mainly smectite (65 to 75%). kaolinite (10%). talc (Mg silicate: 5-10%). According to EDS spectra (Fig. 51, clay minerals are essentially Si-Fe; the ratio ranges from 1 to 3, respectively compatible with (1: 1) phyllosilicate structure (serpentine) and (2: 1) structure (smectite). A small proportion (10%) of the clay fraction is characterized by X-ray peaks in the range 7.3-7.35 A. These peaks. which do not disappear after heating, characterize minerals of the serpentine type; but they decrease after leaching with reducing reagents (hydroxylamine-hydrochloride and oxalate)(Fig. 6). Therefore, a major proportion of the serpentine minerals in the sediments probably consists of Fe(III) type, which is uncommon. Such a mineral was described by Odin et al. (19871, and found by Parr-on and Amouric ( 1988) in some old sediments in association with green phyllosilicates (Fe-smectite and glauconite). On treatment with ethylene-glycol. the main
100
so
.fJl
.I
1
io
100
: coo
grain size (jlm) FiF. 3. Granulometry
cumulative
curve?l t’or sediments of the Dumbea Bay (S2 and S3) and the Belep area (SW
63
400
in@ and goethite
are better represented
levels.
is present in the upper levels.
Magnetite
core S2 it was separable minerals
Y 5 e
in the upper
magnetically
from
below 50 cm. In conformity.
the bulk mag-
netic susceptibility
increases from the 40 cm level to
the bottom
8). Pyrite.
(Fif.
characterized
X-ray peaks at I .74 and 2.7 I A, is clearly
by
the
detectable
below 60 cm; these peaks only vanish after HNO,
200
In
other
HCIO,
+
leaching.
Although
the organic
content
the measured redox potential
is relatively
remained
high.
positive
from
0.35 V on top of core S2 to 0 V at about 80 cm. The organic
C content
upper level 0
12
3
4
5
6
(I 10-I 14 cm.
8
matter oxidation.
crystal\
reduction ohherwd
S3). The main peaks are proportional ratios
910
(keV)
Energy Fig. 3. EDS hpectra of rhomboedral
7
bug@
with SEM to the ele
ment concentrations.
Ca/Fe
an iron-calcium
bonate solid wlution.
Ti. Cr. Mn and Cu occur a mmor element\
car-
in the iron richest wnples.
mentation
higher
becomes
sharper
and
in samples from deeper levels (Fig. 7). indi-
cating that crystallinity
improves
with depth. Accord-
approximately
carries great amounts
concentrations to develop. Hematite
of disleads to
that are high enough to allow smectite
C was annually
disappeared
after oxalate
a significant
low potential
for
loss was
I m depth
(corrc-
transformed
to CO,
in the
decline.
transformations
such as Mn(IV)
to Mn(lll)
with and
Fe-Mn oxides to Fe oxides. and also hematite However.
the development
of mag-
netite and pyrite below 60 cm indicates
lower poten-
tial.
to micro-en-
-0.2
V (Fig. 2). perhaps restricted (e.g. little
shells).
Near the Belep islands (S9). the mineral
leach-
b)
0
012345678910 Energy i’ig. 5. a and h: EDS spectra of the clay fraction
in-
thickness corresponds
by means of reactions involving
vironments (which
to slight
that the sedi-
upper levels. Organic matter could be partly oxidized
to magnetite.
Dumbea
I m sediment
the
organic
to at least 300 years). less than 0.1% of the
(1974).
river
Assuming
IO m&/kg
Mn(II).
silica. In the delta area. evaporation
slow
which seems to correspond
rate has not undergone
ing to Baltzer and Le Ribault (1971) and Tardy et al. solved
decreases from
to 300 or 400 years. As the total organic sponding
peak of smectite
slightly
to I m. indicating
of the sediment.
crease recently.
total diffraction
only
down
12
3
4
5
6
7
8
910
(keV)
(< 2 p.m: S3. 60-65
cm). The Si/Fe
ratio is
I
m (a) and 3 in (hb.
composi-
P. Ambatsian
et al. /Journal
of Geochemical
tion resembles that of the Dumbea area. However, the clay mineral proportions are different, 30-35% is smectite, 25530% kaolinite, 20-25% chlorite-
DUMBEA
Exploration
59 (1997) 59- 74
65
serpentine, 10% illite, and 5% talc. There, the fine particles partly originate from the schists of the northern area of the main island (Grande Terre).
S2 0-5
cm
, ’
HNO,
+
/
HC104
Fig. 6. Core S2. X-ray diffractometry. (A) (above) O-5 cm: after successive leachings with 1) the acetic acid solution (pH 5). 2) NH20H HCI (0.2 M), 3) oxalate. (B) (below) 65-70 cm; after leaching with (1) acetic acid. (2) NH,OH HCI 0.05 M, (3) NH,OH HCl 0.2M, (4) oxalate, and (5) heated concentrated acids. The main peaks of some minerals are located. Go: goethite; Serp: serpentine group; Br: pyroxene (bronLite-enstatite); Q: quartz. Mn indicates only the position of main peaks of Mn or (Mn-Fe) oxides or hydroxides.
I’. Amhntsiun
66
et al. / Journul
of GeochrmicalExploration
5Y 1 lYY7)
SY-73
BELEP
DUMBEA S3
DUMBEA 52
0 3-6 20 -:;
40-‘_
_v 3
-1 7
0
10
5
Fig. 7. Glycolated C:
IS-20
cm;
cm.
6
6
.
230Thi234U
.
234Ui238U
(Co Kn,
clay fraction (core X3); A: O-5 D: 6%68
4
20
15
a Z theta
2
cm:
H: 3-S
cm:
The smectite retlection near 17 i
narrows and becomes more intensive with depth.
U concentrations in biogenic carbonate debris. mainly coral fragments, are 2-4 p&/g. The propor-
Fig. 9. Concentrations and activity ratios of II and Th in core S2. S3 (Dumbea Bay) and S9 (Belep).
20
__._____ A
,
0
20
40
60
80
100
120
depth (cm) Fig. 8. Magnetic susceptibility profile of core S2 measured on (A) hulk
sediment
sample (5 g of wet sediment).
and after the
sequential extraction process: (B) acetic acid attack (pH 5). (C) hydroxylamine
0.2 M, (D) acidified hydrogen peroxide (pH 2).
(E) oxalate 0.25 M (pH 3). The magnetic susceptibility became stronger after the first attack, which allowed to get rid of the carbonates: the particles with higher magnetic susceptibility were then more concentrated in the measurement coil. The magnetic susceptibility vanishes after step E.
tion of carbonates being the same at both areas, Dumbea Bay and Belep, higher U concentrations in Belep sediments (Fig. 9) result from the composition of the continental contribution, richer in shales originating from the northern mountains of the main island. The cores show a ‘30Th radioactive deficit compared to ‘3’U owing to the high proportion of recent coral debris. whose radiogenic ‘3”Th content is very low. The cores, moreover show no significant trend in the ‘30Th/ 23’U ratio, the time needed to deposit this thickness of sediment being too short compared to the half life of ‘3”Th. Relatively high values of the “‘U/ ‘jxU ratio as that found in the upper levels of core S2 (about I.3 to 1.4) are perhaps due to some recent change in the arrangement of the most eroded area which could be locally enriched in ‘31U (Banner et al.. 1990; Miekeley et al., 1992). If “6Ra and
P. Ambnrsian
210Pb (dpmig)
Fig.
IO. ““Ph
vertical
et al././oumal
ofGeochemiccll
in S2
(Dumhea
bay)
5Y (19Y7J
61
59-74
“‘Rn are not mobile in the sediment, the supported (produced in situ) “‘Pb activity is equal to the ‘30Th activity. The excess activity, due to the natural atmospheric fallout. decreases with depth following the half life of “‘Pb (21.4 y)(Fig. 10). The computed average sediment accumulation rates are 0.9 mm/y (0.07 g/cm2/y) in the Belep area and 2.9 to 3.2 mm/y (0.22 to 0.25 g/cm’/y> in the Dumbea Bay. From the moisture content of fresh sediment (50%) and the density of dry sediment of 2.5, we calculated that the 2’0Pb influx to the surficial sediment is 0.65 to 0.9 dpm/cm’/year, which is higher than the reported atmospheric fluxes in the South pacific: 0.2 to 0.5 dpm/cm”/y (Turekian and Cochran. 1981). Flux to sediments is the sum of the “‘Pb from the direct (vertical) atmospheric flux and the “‘Pb brought with settling material originating from the watershed.
210Pb (dpmlg)
distribution
Ex$orurion
and S9
(Belep).
Table
1
Major
and minor
Depth
Fe
Ni
Cr
Mn
co
Zn
cu
taco,
org.
(cm)
(c/I,)
(/@EVE)
( k%/&)
( I-%/g)
( P&/S)
( /G/g)
( k&k?)
(%)
‘i;
‘i;
41
6.0
0.45
6.7
0.45
6.7
0.52
6.3
0.70 0.97
element
concentrations
in sediments
-
c
Tot S
S2 (Dumbea) o-5
4.5
1121
781
440
74
97
16
5-10
5.1
1402
764
96
76
38
44
IS-20
4.6
1212
708
h3X 454
62
55
IO
47
2s -30
5.3
I284
711
596
IO7
83
24
SO
30-3s
3.Y
1210
699
573
96
85
?I
SO
40-4s
6.S
1508
796
653
109
88
52
48
50-5s
5.3
1312
718
580
93
106
26
35
60-6.S
6.4
1526
847
642
I03
81
20
34
70-7s
6.7
I S98
735
708
104
79
75
4s
80-85
4.9
I372
594
536
YO-YS
x.7
I960
883
870
I00
82
IS
44
6.0
I-II
92
30
39
5.0
85
I OS
9.0
195x
904
866
I41
80
31
35
5.0
I4
7.x
3096
816
949
15.5
85
31
37
5.6
I IO-I
S9 (Belep) o-5
2.70
300
5.50
137
IS
I0
7
47
IO-l.5
3.00
290
650
145
17
47
7
49
20-25
3.60
400
795
177
21
57
IS
SO
30.-3s
3.40
390
700
168
20
16
IO
30
40-45
3.20
340
720
165
I8
13
7
48
SO--S5
3.50
410
750
I80
I7
13
6
52
60-65
2.90
320
740
I60
IY
IX
7
49
70.-75
3.90
480
805
20s
24
IS
IO
49
80-X.5
3.50
395
750
185
23
13
9
45
3.50
430
825
I95
24
26
I4
46
3.20
380
660
I76
I9
22
Ii
49
2.30
240
515
130
I6
IO
90-9.5 1oo-
I OS
110-114
9
48
5.0 4.6
1.2 5.5
4.3
68
P. Ambatsiun
3.3, Metal concentrations
and sequential
of Geochemical
et al. /Journal
Exploration
59
f 1997159-
NI. Cr. Mn
extractions
1ccc
Except for Cr, metals are generally more concentrated in the Dumbea Bay sediments than in the sediments near the Belep islands, which support little weathered rocks (Table 1). In cores S2 and S3 and in cores from the same area of the lagoon described by Launay (197X2), relatively high concentrations of Fe, Mn and Ni were found between 1 m and 1.3 m. There are correlations between Fe, Co, Mn and Ni (Table 2). On the contrary, concentrations are lower in deeper layers. The metal enrichment could be assumed due to ionic diffusion from the deeper, reducing levels and precipitation at the redox boundary (at I to 1.3 m>. But the high sedimentation rate like in Dumbea Bay opposes consequent enrichment by ionic ascending diffusion. The decline in metal concentrations from 1.2 m to the surface (Table 1 and Fig. 11) is more probably due to changes in the composition of the eroded material caused by processes such as deforestation followed by mining activities. Lower concentrations in the deeper levels (> I .2 m) were
X00
74
@gig) X00
. : . Mn
I! ‘,
/
:oc 4
i :2c
.
’ 5
Fe?/,
:c
15
distributions of carbonate-free
Fig. 1 I. Vertical
basis metal con-
centrations in core S2.
perhaps related to more reducing conditions in the lagoon at the settling moment. Concentrations of Co and particularly Cr and Ni in the lagoon sediments are higher than those reported for most areas of other continental shelves. We may compare this to the Ligurian continental
Table 2 Correlation matrix showing relations between constituents of whole sediment samples. with 95’% validity for r > 0.55 in the case of S2 and r > 0.58 in the case of S9 Fe
Mn
Cr
Ni
Zn
co
CU
Th
U
S2 (Lhtmbea). n = 13 Ni
0.96
Cr
0.17
0.69
Mn
0.93
0.97
0.68
CO
0.91
0.93
0.68
Zn
0.09
0.06
0.14
0.12
0.24
CU
0.46
0.42
0.44
0.50
0.52
0.23
?32Th
0.38
0.60
0.3x
0.58
0.58
0.04
0.06
0.13
0.16
0.15
0.30
0.54
- 0.22
- 0.74
- 0.63
- 0.66
13xU
-0.14
taco,
- 0.66
0.98
-0.61
- 0.60
-0.15
0.27 - 0.52
~ 0.30
S9 (Belep). n = 12 Ni
0.94
Cr
0.88
0.85
Mn
0.91
0.97
0.89
co
0.77
0.8 I
0.8 I
Zn
0.28
0.09
0.3
I
0.09
0.16
cu
0.39
0.46
0.05
0.47
058
0.53
ziLrh
0.26
0.24
- 0.01
0. I2
0.1 I
0.0
23xU
0.10 0.08
0.30 0.02
0.27 0.05
0.43
0.50
- 0.05
- 0.34
- 0.06 0.35
taco,
0.88
I
0.01 0.54
- 0.09
- 0.13
- 0.46
- 0.30
P. Ambatsian et al. /Journal
of Geochemical Exploration 59 (1997159- 74
shelf in the Savona area (Northern Italy), where the neighboring mountains contain ultrabasic rocks with Cr ore deposits. There the near shore sediments have Cr concentrations of 150 to 1800 pg/g and Ni concentrations of 80 to 600 pug/g (Cosma et al., 1979). But generally the total Co, Cr and Ni concentrations do not exceed 100 pg/g in little or unpolluted sediments of the Western Mediterranean continental shelf (Damiani et al., 1986; Added et al., 1980), as well as, for instance, in sediments of the continental margin of NW Africa (Schiittle, 1978). The proportions of metal extractable at each step during the sequential procedures varied slightly with depth (core S2; Figs. 12 and 13). This does not mean that no important mineralogical transformations occurred with depth. Some observations should be pointed out: (1) Mn extracted during the first step (acetic solution at pH 5.6) of the 15-step procedure partly originates from the adsorbed metal. Only a few amount of Mn was extracted during this step (at pH 5.61, as well as during steps 3 and 4 (pH 51, indicat-
69
ing that almost the whole Mn bound to the carbonate phase was released after step 2. No consequent amount of Mn expected to belong to the following phases was therefore extracted by means of the acetic solution at pH 5 in both the 7- or 15-step procedures. A large fraction Mn (along with Co and Cu) appears to be bound to carbonates as is frequently the case in carbonaceous sediments (Femex et al., 1986; Wartel et al., 1990). As the hydroxylamine solution (second step) extracted more Mn from the upper levels than from the deeper ones, and the Mn oxide X-ray peaks disappeared after this step (Fig. 6), the proportion of easily reducible Mn oxides was greater in the upper levels. In the sediments, a transfer of Mn (and Co and Cu) occurred therefore from oxides not only to sulfides but also to carbonates. The decline in the amount of Mn extracted with the hydroxylamine solution from step 4 to 8 in the 15-step procedure confirms the efficiency of this reagent to solubilize Mn oxides. (2) During the three successive extractions with identical hydrosulfite (dithionite) solution (in the fif-
100 60
S
60
IL” 40 20
s
60
6
40 20 0,
60
$
60
z
40
/
20 0 0
20
40
60
60
100
120
0
20
40
4-2
I
I
I
80
100
120
depth (cm)
depth (cm) -1
60
-&-
3
l
4.5
--t
6
-C
7
Fig. 12. Percentages of metal extracted from core S2 (Dumbea Bay) during the 7-step leach procedure, with: (1) the acetic solution at pH 5; (2) the hydroxylamine-hydrochloride solution at pH 3.5; (3) the oxalate solution at pH 3 (bars indicate this step); (4) and (5) NaOH 0.1 M (humic substances); (6) H,O, + HNO, (extraction of remaining organic matter and sulfides); (7) concentrated HNO, + HCLO,.
70
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
400 a
2
3oo 200 -
I
Ni Dumbea
100 -
0
i T -i I / 7 : ‘: n 7 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
200 5
150
s
100
EZI
Mn Antibes
50 0 1
2
3
4 lIII m
5
6
7
8
9 10 11 12 13 14 15
Zn Dumbea Zn Antibes
0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
10
3 G 3
I
8
!
Cu Dumbea
6 4 2 0
W
Cu Antibes
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
steps_ (AC. AC.)
1(NH20H.HCI)I
$1
&04=)
1 (O.ph.)l
(A.)
z
Fig. 13. Metal fractions extracted during the 15-step sequential procedure. The results for the sample of core S2, O-5 cm. are compared to those from a sediment of the northwestern Mediterranean continental shelf near Antibes (France) (20 m in water depth). The reagents used during the successive steps were for (I) to (4) acetic solutions. at pH 5.6 for (I), 4.7 for (2), 5 for (3) and (4); for (5) to (8) NH20H+HC10.04 M, at pH 2.8 for (5) and (6), 0.2 M at pH 2 for (7) and (8); for (9) NaOH 0.1 M; for (IO) to (12) hydroaulfite (40 g/l). at pH 5 for (10) and (1 I), and pH 4.7 for (12); for (13) H,O, _ fHN0, at pH I (not only organic matter and sulfides but also remaining oxides were probably removed): (14) hydrosulfite used here with the aim of solubilizing oxides assumed to have been formed during step (13); (I 5) HNO, + HCLO,. The low proportion of Fe and Mn extracted from the Antibes sediment during the 3rd hydrosulfite leaching (step 12) indicates that the main part of the metal was solubilized before. Ac.Ac.: acetic acid solutions. O.ph.: oxidizable phase. A.: concentrated acids.
teen-step procedure). Fe and Ni were not less and less extracted from the 10th to the 12th step, as expected (Fig. 13); in contrast Fe was almost totally removed from a coastal sediment of the Western Mediterranean Sea after two leachings with hydrosulfite. The less easy solubilisation of Fe from the moderately reducible phase in the tropical area is perhaps due to better crystallinity of oxides brought to sediment from a watershed rich in eruptive rocks and laterite. (3) Compared to Fe or Ni, very little Cr was extracted with the hydroxylamine-hydrochloride solution, and even with hydrosulfite; Cr oxides are scarcely reducible (Fig. 2). The correlation between Ni and Cr, during the steps 10 to 13 suggests however that part of the Cr was present in oxides more readily reduced than chromite. The oxalate solution (3rd step of the seven-step process), which removed about 50% of the whole extracted Fe and 40% of the Ni, is a strong complexing reagent for C&II) and removed 60% of the Cr. (4) Fe was less extracted during the sixth step (oxidizable phase) below SO-85 cm; since the concentration of total S increases with depth, this is not due to a decline in sulfide concentration, but probably to the transformation of poorly crystallized monosulfides into pyrite, or at least to more highly crystalline monosulfides, more resistant to chemical attack (Bemer, 1971; Femex et al., 1986). (5) About 20% of the Cu was extracted by the NaOH solution, which presumably leached humic substances simultaneously to Cu complexed by these substances. As the Cu profile parallels the Fe one in the sixth step, Cu probably coprecipitated with Fe during Fe sulfide formation. .?.4. Minerulogicul ior
tran~fi~rrnations and metal brhtrl,-
Iron in ultrabasic rocks is principally borne by the spinel-chromite, magnetite and, to a lesser extent, olivine and pyroxene minerals. The Fe concentrations in the basic rocks do not reach IO%, and in the ultrabasic 8%. During weathering, olivine is transformed into ME-Fe smectite and limonite (FeOOH) (Pelletier, 1983). The Fe content of laterites on the peridotitic rocks is as high as 50%. Laterites represent the intensively weathered part of the exposed
P. Ambatsian rt al. /Journal
oj’Geochemicu1 Exploration 5Y ClYY7) 59-74
rocks and should comprise a major part of the suspended load of the river, but the composition of this load is closer to that of the rocks of the watershed than to the laterites (Baltzer and Trescases, 1971; Paris, 1981). As a matter of fact laterites are thick only on flat areas, but scarce on slopes (Podwojewski and Bourdon, 1996). In the upper part of the laterite profiles, goethite and hematite are the main Fe-bearing minerals, as is also the case in the surficial sediments of the lagoon. The proportions of goethite and hematite decrease with depth in the cores, while the proportions of magnetite and sulfides increase. Serpentine of the Fe(II1) type is not known in continental rocks, where serpentines are of Mg-Fe(I1) or Mg-Ni type. As siliceous organism fragments (radiolaria, sponge spicules) are very abundant in the surficial sediments of the Dumbea Bay, the H,SiO, concentration of the pore water may be high enough to allow Fe-serpentine to develop (Tardy et al., 1974). Using sedimentation rates and water content, we estimated the fluxes of Fe from the watershed of river Dumbea-Coulevte (220 km’) to the sediments: 0.015 g/cm2/year for S2, 0.002 for S9 and 0.001 for the core C9 (taken in the center of the lagoon, as discussed by Launay, 1972). That is to say, the flux from the Dumbea-CoulevCe River watershed to the sediments in the open lagoon is at most one fifth those in the bay; and at least 1800 t Fe originating from the watershed deposes every year in sediments. Assuming that Fe represents at most 30% of the solid material transported to the lagoon (Baltzer and Trescases, 197 1), the total solid transported would be at least 6000 t/y. This estimation is higher than that of Launay (1972) who estimated the average solid load at 4000 t/y and the soluble at 10000 t/y. Part of Fe in the lagoon sediments results perhaps from precipitation processes at the sediment-lagoon water interface. On a carbonate-free basis, Mn, Co, Cr and Ni concentrations are generally lower in the sediments of the lagoon than in the peridotic bedrock or in laterites. On an average, the Cr concentrations are 3 to 4 mg/g in the peridotitic rocks. Close to the Dumbea river mouth, the Cr concentrations locally exceed 4 mg/g, but they are generally < 2 mg/g in the
71
sediments of the lagoon. Chromite can be decomposed under intense weathering, with release of Cr, which may then be taken up by iron oxides or clay minerals (Latham, 1985); nevertheless most of chromium transported from the Dumbea river watershed to the lagoon remains as chromite which in great part deposes near the mouth of the river. Mn and Co, in the form of oxides during weathering and transport, are reduced and dissolved during the first stage of sedimentation and then may coprecipitate with calcium carbonate (Wartel et al., 1990) Unlike Cr, Ni undergoes important mineralogical transformations. In the peridotitic rocks, Ni is mainly found in olivine. At the base of the weathered profiles it is bound to limonite or incorporated into silicates (serpentine or talc) (Trescases, 1975); in the surface it is more closely associated with Mn hydroxides (Manceau and Calas, 1987). A large proportion of this metal must be carried to the lagoon bound to Mn oxides. However, in the sediments it seems to be mainly linked to iron oxides (Figs. 12- 14). Ni concentrations in the surficial sediments decrease from the vicinity of the river mouth to the centre of the lagoon (Launay, 1972). Whereas the Ni concentration decreases by a factor of 20, Fe, Cr and Mn concentrations are reduced by a factor of less than 4. Therefore, the decrease in the Ni concentrations must be due not only to a diluting or sorting effect but also to solubilization, and release must occur after deposition. Gerringa (1990) showed that during aerobic degradation of organic matter there is a correlation between dissolved Ni and dissolved organic matter; Ni can be readily released from sediment by an increase in the concentration of dissolved organic ligands. Lower rates of sedimentation, as in the central region of the lagoon, allow the sediment at the sediment-water interface to be subjected to a longer exposure time, leading to greater release of metal into the overlying water. In the river flows, the Fe/Ni ratio in suspended matter is 33 (Paris, 1981). If we assume that the Fe concentration variations in the surficial sediments depend essentially on the dispersion of Fe bearing particles in a carbonaceous matrix poor in Fe, and iron minerals (oxides) do not suffer important dissolution within surficial sediment, we can use Fe as a normalizing element. In that case, the surficial sediment from site S2 (Dumbea), where Fe is 5%, would have an
72
P. Ambatsiun
I
I
1
peridot
et al. / Journul
r,f Geochemicul Exploration
4. Conclusions
(olivine)
I
Fe
I
orthapyroxene
59 0997159-74
n
clinopyroxene
Fig. 14. EDS spectra of freshly broken detrital grains (I. 3, 5), and at the surface of the same grains (2, 4, 6) from the magnetic mineral fraction of core S2 (30-35 cm). Si/Mg ratio and Ca content differentiate: peridot (olivineX1). orthopyroxene (3) and clinopyroxene (5). Ni is present in the Fe coating at the surface of the grains.
adjusted Ni concentration of about 50000/33 = 1650 pg/g. The Ni concentration in the top levels is 1245 pg/g. The difference between the measured concentration and the calculated value is about 400 pg/g. In core C9, where the Ni concentration is 53 pg/g, the difference is (32000/33) - 53 = 916 pg/g. Considering the water content (about 50% w/w.) and the sedimentation rates (3.2 mm/y for S2, and 0.55 mm/y for C9), respectively 64 and 33 pg/cm’ of nickel should be released each year from the surface layer. The lagoon sediments do not act as efficient ‘I sink” for this metal that may have harmful effect on some living species (Alikahn. 1989; Wood, 1989).
Compared to sediments of other coastal areas, sediments of the New Caledonia lagoon have relatively high Ni, Cr and Co concentrations, but low compared with the peridotitic rocks or weathering products from the island. In the Dumbea Bay. the vertical profiles of Fe, Mn, Ni and Co present a metal maximum at the 1 to 1.2 m level, the concentrations being in particular lower in the deeper and more reducing levels. If the sedimentation rate was not lower than the actual (and if there is no ascending water flow from the deeper levels through the redox boundary), the enrichment could not be due to mere ionic migration from the deeper, reducing levels, but more likely to changes in the geomorphology of the eroded area or moving of the mouth of river Dumbea-CoulevCe. Even in the richer levels the metal concentrations remain too low to allow mining exploitation. In the lagoon sediments. in particular in the confined bay of Dumbea, the transformation rates varied from one deposited species to the other. On the one hand, the organic C concentration only little decreased from the upper layers to I m depth. The mineralogical species which carry most Cr did not undergo important variations even at levels deeper than I m. Iron oxides were found in similar proportion from the top to the bottom of the core. However, in deeper levels, (below 40 or 50 cm) goethite and also hematite transform to magnetite and Fe sulfides. On the other hand, silicates are transformed in surficial sediments. Smectite shows progressive improvement in its degree of crystallinity from the top to the bottom of core S2 (Dumbea Bay). Moreover, at the upper horizons of the sediments we detected Fe(III)-serpentine; as such mineral was not observed in the rocks of the islands, we assume it has an authigenic origin. Mn, Co and Ni undergo more important transformations in surficial sediments than Fe and Cr. Ni is transported from land with Mn oxides but the sequential extraction showed a close relationship between Ni and Fe oxides in the sediment of the bay. A great part of Mn is dissolved in the first stage of the diagenesis. Co and Ni linked to Mn oxides are freed but follow two different geochemical pathways. Co,
like Mn, coprecipitates with CaCO,. Whereas Ni is mostly released to the lagoon watkrs, with the remainder Ni being fixed to Fe oxides.
W..
19X6. Mobility
Springer Gerringa.
Vcrlag.
L.J..
\lurries.
The authors wish to thank the referees for helpful] comments and help with the english language text. and Drs G. Boillot and J. Mascle (Lab. GCodynaVillefranche/Mer) for laboramique sous-marine, tory facilities.
Latouche.
critique
de\
extractions
quelques constituants
metals in the superficial
marine
sediments
the Rhone mouth. Prog. Wat. Tech., Alikahn,
M.A..
Vernet (Editor),
Heavy Metal\
aultants, Edinburgh. Baltler.
t’orwllio
crustacean, Vol.
F. and Le Ribault.
hanca sCdimentaire\
in the vicinity
of
: X9-96.
12/I
19X9. Copper and nickel
a terrestrial
of heavy
Say.
in the Environment.
by
In:
J.P.
CEP Con-
I, pp. 475-478.
M..
Luunay.
d’un delta tropical.
C.R. Acad. Sci. Pa&.
273: 1083-1086. Bnltrer.
J..
de l’trosion
sous le climat
tropical
et de la skdimentation
de la Nile Caltdonie.
du hilan
sur piridotitc
C.R. Acnd.
Sci.
J.L.,
IV,’ I :
1990.
G.J.,
“‘U-““U-“‘Th
systematics
from central Missouri. Berner,
R..
1971.
Graw-Hill. Cosma,
Chen,
and Moore.
in haline
Earth Planet. Sci. Lett..
Principles
New York.
of
NY,
Chemical
C.H..
groundwater\ 101: 296-3
Sedimentology.
1979. Heavy metals in Ligurian
12. Mc
G. and Tucci.
Sea sediment\.
in surficial
sediments.
Distribution Mar.
Chem..
S..
0..
V.. Amhroso,
the input dispersal environment. Interactions. Dugas.
E.. De Rosa, S.. De Simonc.
In:
and settling P.G.
du
Sly
Springer-Verlag,
F. and Debenay,
annexe
G., 1986. Sediment\
Lagon
J.P..
de Nlle
NoumCa). O.R.S.T.O.M..
Sedimenta
and Water
NY. pp. 13-25.
mine and Mom) Nircl.
1982. Carte
sCdimentologlqur
Caltdonie
Paris-Bondy.
i
l/.iOOOo
ct
(Feuille
Notice explicative.
sediments of the Northwest
ranean continental
In: P.G. Sly (Editor),
Interactions.
\heIf.
95:
Springer
Verlag.
New
York.
of
Mediter-
Sediments NY.
and pp.
353-370. U.. Ahlf.
Odin.
W.. Ker\ten,
ultra-
331 pp.
M..
Paris-Bondy.
(Nile
S&r. C&l.
from
of cobalt and
New
Caledonia.
Acta. S I: IOS- I 13. P. and Osmond J.K.,
1992. Uranium
in groundwaters
I’rom the Osamu
do Ferro natural
analogue
P.M..
G.S.. Dehenay.
and
Utsumi
sites. Poc;os de
Explor.,
35: 345-363.
1990. Pitfalls
of sequential
J.P.. &get.
Icrdlne:
\uhrCcil’al.
d’unc
extractions.
cn
milieu
2: 17-4 I.
Orltans-La
Pal-ran. C. and Amouric.
P.. 19X7. Le
phyllitc
Lcy rcssources mi&rale<
B.R.G.M..
chemical
C. and Rigolot,
nCotormation
G&)dynamique.
Pari\, J.P.. IYXI.
M..
de la Nllc
Source,
Caledonie.
113: 2lY-123.
19X8. Phase transformation
process as revealed hy HRTM
durmg
nnd micro-
study. Inst. Phys. Conf. Ser., 93. 2: 557-558.
B.,
IYX3.
Localisation
“,garnt&itiques” Podwojcwski. tion
du nickel
de Nile CalL:donie.
P. and Bourdon.
de\
Pout&.
oxvdlsol:,
dans lc\
Sci. C&l..
mintraux
M:m..
Rapin.
Stra\-
5s
M. and Salomons.
Rev.
I:., Tessicr.
In%
permettant
magnttique franq.
A.. Campbell,
artifact\
d‘enregistrer
les
le long d‘une carotte
P&role
(Ryeil-Malmaisonl,
P.G.C. and Carigan,
in the determination
by a sequential
Trchnol..
d’indura-
en Nouvelle
I-562.
\edimenth
R., 1986.
of metal partitioning
extraction
procrah.
Environ.
in Sci.
20: 55 I-562.
P., Robin.
E‘.. IchtertL.
I97 I. La determination
M.N.,
Fusil. G. and Parrot. J.F..
du fcr libre dans les sols h sehquioxy-
de\. Cah. O.R.S.T.O.M.,
Paris-Bondy.
SL:r. Ptdolosic.
IX/I:
I. M..
197X. Geochemischc
Sedimenten Schriften, Y..
Untersuchungen
des Kontinentalrandcs
Kandrihchen Tardy.
sur pCridotite
C.R. Acad. Sci. Paris, 322. \t-r. IIa: 453-459. de Ia \usceptibilite
Potential
4-l
E.. 1996. Lc procesus
goethitiques
J.. 1973. Sur un appareil
I’enthalpie W., Calmano.
Univ.:
de la haie de Dumbda
and asholane
J. Geochem.
R. and Morel.
Schiittle.
some metals in surt’icial
sur roches
These Etat, Dijon
Water Reh., 2-I: lO5S-1056.
Scsalen.
45 p.
Fiirstner.
isotope\
de &diments.
R., Fcrretti.
Femex, F.. Span. D.. Flatau. G. and Renard. D.. 19X6. Behavior
Water
N.. Linsalata.
thorium
8:
as a record of
New York,
et pCdog&Ce
R.S T.0
Cosmochim.
variation\
processes in a coastal marine
(Editor),
0
In lithiophorite
29/A:
Izzo. G. and Zurlini,
de
Tech-
25-5 I.
of
125-142. Damiani,
(‘ah.
Cal2donie.
M.. Scarponi,
I’Ctide
Envlronm.
haurg. 73: 17% 183.
240 pp.
B., Drago. M.. Picazzo,
Cr. Cu. Ni and Mn
J.H.
travers
A. snd Cltlas. G., 19X7. Crystal chemistry
Pelletier.
Wassenburg,
matter and
F.. 198.3. Analyse
i
des residue d’attaque\.
Alt&ation
the glauconitisatlon
Paris, 273: 2034-2037.
s6quentielles
1977. Sedimentation
M&moire
F. and Treacases, J.J., 1971. Premiere estimation
de I’alt&ation
Banner.
lYX5.
Cal&donicI.
l’aci&
L.. 1971. NCogenPse de quartz dam le\
C. and Pautrirel.
hasiques cn Nile Caltdome.
Caldas. Brazil.
uptake. accumulation ~~pinicornu
of organic
nol. Lett.. 1: 500&S II. Latham.
Mlekrlcy. F. and Rapin. F., 19X0. Diatrihution
pp. 37 l-380.
Mar. Chem.. 2: 355-373.
Geochim.
A., Femex.
NY,
degradation
of Cu, Cd, Ni, Zn, Fe and Mn in marine sediment
J.M..
nickel
Added.
New York,
Jouanneau.
Manceau.
References
metals in dredged harbor sedi-
Sediments and Water Interactions.
1990. Aerobic
the mobility
Acknowledgements
of heavy
ments. In: P.G Sly (Editor),
und
Kapverdishen
17. Lenz-Verlag. Treacase\, lihre
an rezenten
van NW-Africa Insrln.
rwischen
Giesscner
Geol.
I88 pp.
J.J. and Fritz.
de formation
B..
1974.
Evaluation
de montmorillonitc
C.R. Acad. Sci.. Paris, (D). 27X: 1665-166X.
dc
i’erriferes.
74
P. Amhutsicrn et ~1. / Journul
Tessier, A..
Campbell.
P.G.
and Bison.
M..
ofGeochemrco1
lY7Y. Sequential
Explorution
Wartel,
M..
59 C1997) 50-74
Skirer.
M.,
Auger,
Y.
and Boughriet,
extraction process for the separation of particulate trace met-
Interaction
als. Anal. Chem.. 51: 844-850.
aase\sment of the solubility product of MnCO,
Trescases, J.J., basiques
en
nickClifere.\ K.K.
Enewetak 522-524.
zone de
Paris-Bondy. Turekian,
1975.
L’&olution tropicale;
super&e
des roches ultra-
formation
Nouvelle-CalCdonie.
de.\
M6m.
gisemenr\
O.R.S.T.O.M..
1981. ““Pb
in \&ace
and the Asian dust to the Pacific.
Nature,
A..
cles. Mar. Chem.. 29: 90-l Wood.
J.M..
lY89.
Transport.
and Mn distrl-
(Editor), Heavy Met&
292:
Edinburgh. pp. I - 12.
parti-
17. bioaccumulation
under environmental
air LLI
IYYO.
with carbonates in sea water:
bution coefficient between the liquid phase and CaCO,
microorganisms
7X: 259 p. and Cochrdn, J.K.,
of manganese II
and toxicity
111
stress. In: J.P. Vemct
in the Environment. CEP Consultanta.
ELSEVIER
Journal of Geochemical Exploration 59 (I 9Y7) 59-74
High metal inputs to closed seas: the New Caledonian lagoon P. Ambatsian
‘, F. Fernex ‘,*, M. Bernat ‘, C. Parron ‘, J. Lecolle
’
Received I5 October 1945: accepted 18 June 1996
Abstract The islands of New Caledonia are largely composed of ultrabasic rocks (peridotites). severily weathered. rich in Fe, Mn and Co, and where several ore deposits of Ni and Cr are extensively mined. Sediment cores from the bay of Dumbea, in the south of the main island, and from the northern part of the lagoon (Belep Islands), less affected by the mining activities, were analyzed for their mineral composition and metal concentrations. In the surficial sediments, oxides and silicates, including Fe serpentine and smectites, undergo rapid transformation or neoformation in a short time, in particular in the confined bay of Dumbea. Fe is largely present as goethite, and in deeper layers (60-100 cm) as hematite and magnetite. Chromite can be identified at each horizon. The metal concentrations decrease from the near shore areas. in particular the vicinity of the Dumbea river mouth to the open part of the lagoon. This trend is more important for Ni than for Fe or Cr. Fe ranges from 3.5 to 9% (dry weight), Ni from 200 to 2000 pg,/g, Cr from 700 to 2000 pg/g, Co from 20 to I.50 pg/g and Mn from 130 to 900 pg/g; yet the concentrations are lower than concentrations found in the ultrabasic rocks or laterites of the watershed. To try to understand the behavior of metals during the sedimentation-diagenesis events. we evaluated the sediment accumulation rate, and used different sequential leaching procedures. Fe, Mn, Ni, Cr and Co are mainly present as, or bound to, oxides or oxyhydroxides, even in the deeper layers (> 100 cm) where the organic content is relatively high (about 6% of organic C). Metals are mainly transported from the land to the lagoon as oxides and dispersed in the lagoon sediments, where they are diluted with a large amount of carbonaceous sediment. During diagenesis, a significant part of Mn, Co and Ni are dissolved; but, unlike Mn and Co, which seem to coprecipitate with carbonate, most of the Ni is released into the waters of the lagoon. Apparently no horizon of the sediment has undergone significant in-situ metal enrichment. Keyord.vt
kqoon
sediments: early diagenesis; nickel; chromium; sequential extractions; New Caledonia
1. Introduction
New Caledonia is a group of island located Southwest Pacific Ocean (Fig. 1). An important reef isolating a shallow lagoon (mean depth surrounds the islands. Large andesitic basalt
* Corresponding O375-6742/97/$17.00
author. Fax: + 33-93-529965
Copyright 0
PII SO375-6742(96)00020-Y
in the coral 20 m) flows
and overthrusting peridotites, weathered to laterite, are found in the south of the main island (Grande Terre) and in the Belep islands in the northern part of the lagoon. Deposits of Ni and Cr ores which have been extensively mined are common in the southern part of the main island. The petrography of the rocks is well documented (Trescases, 1975; Paris, 1981; Latham. 1985). and.
1997 Elsevier Science B.V. All rights reserved.
and S3 were taken from water 1 I m deep in the inner part of the Dumbea Bay, near the outlet of the Dumbea-Coulev6e River, whose watershed (220 km’) is largely made up of the southern peridotitic mountains. The main tlow of the river is 10 &/a. but varies from I to 1000 m’/s. Core S9 was collected at a depth of 19 m. in an area close to the peridotic Belep islands in the northern part of the lagoon, which is less influenced by the mining activities (Fig. I). The cores were sent off by plane to Nice (France). where they were stored at 4°C till sampling.
to a lesser extent, the characteristics of the lagoon sediments (Launay. 1972: Dugas and Debenay, 1982). Weathering of the ultrabasic rocks results in production of laterites enriched in metals. in particular Fe, Mn. Co, Cr and Ni. The lagoon was expected to trap a significant proportion of the material derived from the land. In particular, the Dumbea Bay sediments may trap metals like Cr and Ni transported there by River Dumbea-CoulevCe whose watershed comprises ore deposits of these metals. To try to determine the behavior and fluxes of metals during their transfer from land to the lagoonal sediments and during diagenesis, we investigated the mineral composition, metal concentrations, and deposition rates of sediments. Sequential extractions were used to determine various operationally defined chemical forms of the metals (species extractable at pH 5 or carbonate phase, reducible phases. oxidizable phase. etc...).
2. Materials
2.2. Minrrulo~~~. Grain-size frequency analysis was performed bctween 0.05 mm and 1000 mm with a laser granulometer. The mineralogical composition of the bulk sediment and clay fraction (< 2 pm) was determined by X-ray diffractometry using a Co anode. The clay fraction was separated by sedimentation in deionised water according to Stocke’s law, and subjected to different treatment before being analysed: (a) treatment with ethylene-glycol; (b) treatment with hydrazine hydrate; and (c) heated at 490°C during 4 hours. Semi-quantitative metal analysis at selected points of minerals of some samples was performed
and methods.
2. I. Sampling Three 1.2-m cores designated as S2. S3 and S9 were collected with a gravity corer in 1988. Cores S2
ultrabastcrocks
Fig.
\ coral
1.Location
, Dumbea
reiG
of the sampling
site\
Ba!
using a scanning electron microscope (SEMI equipped with an energy dispersive spectrometer (EDS). 2.3. Chemical utnulvses Redox potential was measured with a pH-meter equipped with a Pt electrode in the sediment immediately after opening the core. Carbonate content was measured with a Bernard calcimeter using concentrated HCI as the dissolving agent, and the total amount of organic carbon by the reducing ability of the samples using a sulfochromic solution. Total sulfur content was determined by the infrared analysis of SO, after oxidation at 1800°C. Metal concentrations were estimated by atomic absorption spectrophotometry. To the classical sequential extraction of metals (Tessier et al., 1979) we added the following steps: successive extractions with two reducing reagents to try to distinguish
PH 1
3
5
easily reducible oxides and hydroxides from less reducible ones (Segalen et al., 1971; Fijrstner et al.. 1986), and extraction with NaOH 0. I N with the purpose of isolating humic substances with complex metals. The following procedure was therefore used for the sequence with 7 steps. (1) - Digestion with 100 ml of a CH,COOHNa IN + CH,COOH solution at pH 5 (if necessary CH,COOH was added to maintain the pH below 5.5). This first step is assumed to extract chiefly metals of the exchangeable fraction and those bound to carbonates, although some poorly crystallized oxides may be also solubilised (Fiirstner et al.. 1986). (2) - After centrifugation under a nitrogen atmosphere. the residue of (I) was submitted 3 to 4 hours to leaching by 50 ml of a NH,OH-HCI 0.1 M solution adjusted at pH 3.5 by CH,COOH. This should solubilize principally the easily reducible fraction, i.e. Mn oxides and hydroxides and Fe hydroxides (Fig. 2); some poorly crystallized sulfides
7
9
Fig. 2. Eh-pH diagram showing the redox equilibrium lines of some geochemical species which are in discussion in the present paper. Lines corresponding to the reducing reagents used during the sequential extraction procedures are also represented. The lines are established for a
I
molar activity of the ions when no other indication.
would also be solubilized by this solution (Rapin et al., 1986). (3) - 50 ml of a (NH&c~o~ 0.25 M solution adjusted at pH 3 by means of HZC201 should dissolve the moderately reducible phase which is mainly constituted by Fe oxides. (4) - 100 ml of NaOH 0.1 M was used to extract humic substances. After centrifugation, the supernatant was acidified to pH 2 by HZSO, to precipitate humic acids; the leachable fraction is assumed to contain fulvic acids. (5) - The precipitated humic acids were digested by HZOZ adjusted to pH 1 with HNO,. There is no evidence that the amounts of each metal solubilized during steps (41 and (51 correspond actually to the metal proportion in the humic and fulvic fractions; metals extracted during steps (4) and (5) should be gathered in a unique humic fraction. (6) - To remove the remaining organic matter and poorly crystallized sulfides, the residue of (41 was oxidized by H202 and progressively heated; afterwards HNO, was added till pH reached I. (7) - The residue of (6) was introduced in 30 ml of a concentrated acid solution (2/3 HNO, ION + l/3 HCIO, ION), which was then evaporated to dryness. Redissolution by HN03 at pH I. Most of metals bound to clay minerals, remaining sulfides and oxides proof against the former leachings are expected to be removed by this attack. As such sequential processes have been criticized for low specificity (Jouanneau et al.. 1983; Rapin et al., 1986; Nirel and Morel. 19901, we compared the metal amounts extracted at various steps to the results of mineralogical analysis on residues from various steps of the sequence (X-ray diffractometry and magnetic susceptibility measurements). We also performed a 15-stage extraction sequence on sediments from two levels. O-5 cm and 25-30 cm, with the aim of better defining the efficiency of two reducing reagents: hydroxylamine-hydrochloride and hydrosulfite (dithionite) (Fig. 2). This process differed from the former 7-step sequence procedure by the repeated use of moderate reagents. (1) to (4) - Acetic acid leaching (I 00 ml each): pH 5.6 for (I 1, pH 4.7 for (2); pH 5 for (3) and (4). (5) to (8) - Leaching with NH,OH-HCI (50 ml); 0.04 M solution at pH 2.8 to 3 for (5) and (6); 0.2 M solution at pH 2 for (8).
(91 - NaOH 0. I M (100 ml). (IO) to (121 - A 40 g/l hydrosulfite solution at pH 4.7 to 5 (50 ml), which permits Fe oxides like hematite to be progressively removed (Segalen et al., 19711. ( I31 - Oxidation by H ?02 + HNO, (pH I ) heated. with a view to destroying organic matter and sulfides; oxides not totally removed during the former steps were leached during this step. (141 - In order to dissolve oxides which could have been formed during the oxidation of the 13th step, a new leaching by dithionite was used (40 g/l; 50 ml). (15) - Heated concentrated acids: 2/3 HNO, + l/3 HCIO,. In both processes, we started with 2 g of wet sediment. Immediately after the core was open, the samples were put in a IOO-ml beaker filled with the acetic acid solution until the effervescence stopped. The following steps, including centrifugation, were conducted under a nitrogen atmosphere until the oxidizing step with HZ02. Taking into account the water content, we express the results for metals with regard to the dry sediment. To control the mineralogical transformations induced by the leaching solutions, we analyzed by X-ray diffractometry and magnetic susceptibility (Poutiers. 1973) the successive residues obtained by the leaching of 12 g of wet sediment from three levels (O-5. 30-35, 65-70 cm). 2.4. Natural rudionuclides Concentration and isotopic composition of ura nium and thorium were determined by isotopic dilution and alpha spectrometry. The samples were pro cessed chemically to separate and purify the chemical species. Percent recovery was evaluated with two tracers, 23’Th and ‘.i’U. The purified species were subjected to electro-plating and then alpha counting. “‘Pb was determined for the same sample. A known amount of common lead was added with the alpha tracer. The common lead, which served as a carrier. was also used to calculate the chemical yield: it was precipitated as lead sulfate, dried, weighed, and then analysed for the beta activity of ““Bi, the ““Pb I’ daughter I! The sediment accumulation rate was evaluated
P. Amhatsian
et al. /.lounzal
of Gewheminll
through the decay of the atmospheric excess ““Pb activity within the sediment column (half-life 21.4 years). The fraction produced in the sediment (the supported ““Pb), is assumed to be equal to *jOTh the last radioactive antecedent analysed.
3. Results and discussion 3.1. Mineralogical
characteristics
of the sediments
The sediments are light brown down to the bottom (1.2 m) in core S9 (Belep). In cores S2 and S3 (Dumbea Bay) the color is darker below 90 cm. The sediment includes,chiefly in the Belep area, numerous coral fragments and other organic bioclasts (great foraminifers, siliceous spicules, etc.). Granulometry is coarser close to the Belep islands, in the northern, open part of the lagoon (core S9) than in the confined Bay of Dumbea (cores S2 and S3)(Fig. 3). According to X-ray diffractometry the carbonate fraction is 50 to 55% calcite and aragonite (CaCO,), the rest being magnesian calcite with approximately 12 mole percent of MgCO,. and a much less abundant Ca-Fe carbonate (1 to 5%). The presence of this latter mineral was confirmed under SEM examina-
Exploration
63
59 I19971 59-74
tion and EDS analysis: Ca/Fe ratios of rhombohedral crystals suggest an iron/calcium carbonate solid solution; Ti. Cr. Mn and Cu occur as minor elements in the iron richest samples (Fig. 4). Quartz and to a lesser extent feldspar represent almost half the carbonate-free fraction. In the Bay of Dumbea, the fine fraction ( < 2 km) is mainly smectite (65 to 75%). kaolinite (10%). talc (Mg silicate: 5-10%). According to EDS spectra (Fig. 51, clay minerals are essentially Si-Fe; the ratio ranges from 1 to 3, respectively compatible with (1: 1) phyllosilicate structure (serpentine) and (2: 1) structure (smectite). A small proportion (10%) of the clay fraction is characterized by X-ray peaks in the range 7.3-7.35 A. These peaks. which do not disappear after heating, characterize minerals of the serpentine type; but they decrease after leaching with reducing reagents (hydroxylamine-hydrochloride and oxalate)(Fig. 6). Therefore, a major proportion of the serpentine minerals in the sediments probably consists of Fe(III) type, which is uncommon. Such a mineral was described by Odin et al. (19871, and found by Parr-on and Amouric ( 1988) in some old sediments in association with green phyllosilicates (Fe-smectite and glauconite). On treatment with ethylene-glycol. the main
100
so
.fJl
.I
1
io
100
: coo
grain size (jlm) FiF. 3. Granulometry
cumulative
curve?l t’or sediments of the Dumbea Bay (S2 and S3) and the Belep area (SW
63
400
in@ and goethite
are better represented
levels.
is present in the upper levels.
Magnetite
core S2 it was separable minerals
Y 5 e
in the upper
magnetically
from
below 50 cm. In conformity.
the bulk mag-
netic susceptibility
increases from the 40 cm level to
the bottom
8). Pyrite.
(Fif.
characterized
X-ray peaks at I .74 and 2.7 I A, is clearly
by
the
detectable
below 60 cm; these peaks only vanish after HNO,
200
In
other
HCIO,
+
leaching.
Although
the organic
content
the measured redox potential
is relatively
remained
high.
positive
from
0.35 V on top of core S2 to 0 V at about 80 cm. The organic
C content
upper level 0
12
3
4
5
6
(I 10-I 14 cm.
8
matter oxidation.
crystal\
reduction ohherwd
S3). The main peaks are proportional ratios
910
(keV)
Energy Fig. 3. EDS hpectra of rhomboedral
7
bug@
with SEM to the ele
ment concentrations.
Ca/Fe
an iron-calcium
bonate solid wlution.
Ti. Cr. Mn and Cu occur a mmor element\
car-
in the iron richest wnples.
mentation
higher
becomes
sharper
and
in samples from deeper levels (Fig. 7). indi-
cating that crystallinity
improves
with depth. Accord-
approximately
carries great amounts
concentrations to develop. Hematite
of disleads to
that are high enough to allow smectite
C was annually
disappeared
after oxalate
a significant
low potential
for
loss was
I m depth
(corrc-
transformed
to CO,
in the
decline.
transformations
such as Mn(IV)
to Mn(lll)
with and
Fe-Mn oxides to Fe oxides. and also hematite However.
the development
of mag-
netite and pyrite below 60 cm indicates
lower poten-
tial.
to micro-en-
-0.2
V (Fig. 2). perhaps restricted (e.g. little
shells).
Near the Belep islands (S9). the mineral
leach-
b)
0
012345678910 Energy i’ig. 5. a and h: EDS spectra of the clay fraction
in-
thickness corresponds
by means of reactions involving
vironments (which
to slight
that the sedi-
upper levels. Organic matter could be partly oxidized
to magnetite.
Dumbea
I m sediment
the
organic
to at least 300 years). less than 0.1% of the
(1974).
river
Assuming
IO m&/kg
Mn(II).
silica. In the delta area. evaporation
slow
which seems to correspond
rate has not undergone
ing to Baltzer and Le Ribault (1971) and Tardy et al. solved
decreases from
to 300 or 400 years. As the total organic sponding
peak of smectite
slightly
to I m. indicating
of the sediment.
crease recently.
total diffraction
only
down
12
3
4
5
6
7
8
910
(keV)
(< 2 p.m: S3. 60-65
cm). The Si/Fe
ratio is
I
m (a) and 3 in (hb.
composi-
P. Ambatsian
et al. /Journal
of Geochemical
tion resembles that of the Dumbea area. However, the clay mineral proportions are different, 30-35% is smectite, 25530% kaolinite, 20-25% chlorite-
DUMBEA
Exploration
59 (1997) 59- 74
65
serpentine, 10% illite, and 5% talc. There, the fine particles partly originate from the schists of the northern area of the main island (Grande Terre).
S2 0-5
cm
, ’
HNO,
+
/
HC104
Fig. 6. Core S2. X-ray diffractometry. (A) (above) O-5 cm: after successive leachings with 1) the acetic acid solution (pH 5). 2) NH20H HCI (0.2 M), 3) oxalate. (B) (below) 65-70 cm; after leaching with (1) acetic acid. (2) NH,OH HCI 0.05 M, (3) NH,OH HCl 0.2M, (4) oxalate, and (5) heated concentrated acids. The main peaks of some minerals are located. Go: goethite; Serp: serpentine group; Br: pyroxene (bronLite-enstatite); Q: quartz. Mn indicates only the position of main peaks of Mn or (Mn-Fe) oxides or hydroxides.
I’. Amhntsiun
66
et al. / Journul
of GeochrmicalExploration
5Y 1 lYY7)
SY-73
BELEP
DUMBEA S3
DUMBEA 52
0 3-6 20 -:;
40-‘_
_v 3
-1 7
0
10
5
Fig. 7. Glycolated C:
IS-20
cm;
cm.
6
6
.
230Thi234U
.
234Ui238U
(Co Kn,
clay fraction (core X3); A: O-5 D: 6%68
4
20
15
a Z theta
2
cm:
H: 3-S
cm:
The smectite retlection near 17 i
narrows and becomes more intensive with depth.
U concentrations in biogenic carbonate debris. mainly coral fragments, are 2-4 p&/g. The propor-
Fig. 9. Concentrations and activity ratios of II and Th in core S2. S3 (Dumbea Bay) and S9 (Belep).
20
__._____ A
,
0
20
40
60
80
100
120
depth (cm) Fig. 8. Magnetic susceptibility profile of core S2 measured on (A) hulk
sediment
sample (5 g of wet sediment).
and after the
sequential extraction process: (B) acetic acid attack (pH 5). (C) hydroxylamine
0.2 M, (D) acidified hydrogen peroxide (pH 2).
(E) oxalate 0.25 M (pH 3). The magnetic susceptibility became stronger after the first attack, which allowed to get rid of the carbonates: the particles with higher magnetic susceptibility were then more concentrated in the measurement coil. The magnetic susceptibility vanishes after step E.
tion of carbonates being the same at both areas, Dumbea Bay and Belep, higher U concentrations in Belep sediments (Fig. 9) result from the composition of the continental contribution, richer in shales originating from the northern mountains of the main island. The cores show a ‘30Th radioactive deficit compared to ‘3’U owing to the high proportion of recent coral debris. whose radiogenic ‘3”Th content is very low. The cores, moreover show no significant trend in the ‘30Th/ 23’U ratio, the time needed to deposit this thickness of sediment being too short compared to the half life of ‘3”Th. Relatively high values of the “‘U/ ‘jxU ratio as that found in the upper levels of core S2 (about I.3 to 1.4) are perhaps due to some recent change in the arrangement of the most eroded area which could be locally enriched in ‘31U (Banner et al.. 1990; Miekeley et al., 1992). If “6Ra and
P. Ambnrsian
210Pb (dpmig)
Fig.
IO. ““Ph
vertical
et al././oumal
ofGeochemiccll
in S2
(Dumhea
bay)
5Y (19Y7J
61
59-74
“‘Rn are not mobile in the sediment, the supported (produced in situ) “‘Pb activity is equal to the ‘30Th activity. The excess activity, due to the natural atmospheric fallout. decreases with depth following the half life of “‘Pb (21.4 y)(Fig. 10). The computed average sediment accumulation rates are 0.9 mm/y (0.07 g/cm2/y) in the Belep area and 2.9 to 3.2 mm/y (0.22 to 0.25 g/cm’/y> in the Dumbea Bay. From the moisture content of fresh sediment (50%) and the density of dry sediment of 2.5, we calculated that the 2’0Pb influx to the surficial sediment is 0.65 to 0.9 dpm/cm’/year, which is higher than the reported atmospheric fluxes in the South pacific: 0.2 to 0.5 dpm/cm”/y (Turekian and Cochran. 1981). Flux to sediments is the sum of the “‘Pb from the direct (vertical) atmospheric flux and the “‘Pb brought with settling material originating from the watershed.
210Pb (dpmlg)
distribution
Ex$orurion
and S9
(Belep).
Table
1
Major
and minor
Depth
Fe
Ni
Cr
Mn
co
Zn
cu
taco,
org.
(cm)
(c/I,)
(/@EVE)
( k%/&)
( I-%/g)
( P&/S)
( /G/g)
( k&k?)
(%)
‘i;
‘i;
41
6.0
0.45
6.7
0.45
6.7
0.52
6.3
0.70 0.97
element
concentrations
in sediments
-
c
Tot S
S2 (Dumbea) o-5
4.5
1121
781
440
74
97
16
5-10
5.1
1402
764
96
76
38
44
IS-20
4.6
1212
708
h3X 454
62
55
IO
47
2s -30
5.3
I284
711
596
IO7
83
24
SO
30-3s
3.Y
1210
699
573
96
85
?I
SO
40-4s
6.S
1508
796
653
109
88
52
48
50-5s
5.3
1312
718
580
93
106
26
35
60-6.S
6.4
1526
847
642
I03
81
20
34
70-7s
6.7
I S98
735
708
104
79
75
4s
80-85
4.9
I372
594
536
YO-YS
x.7
I960
883
870
I00
82
IS
44
6.0
I-II
92
30
39
5.0
85
I OS
9.0
195x
904
866
I41
80
31
35
5.0
I4
7.x
3096
816
949
15.5
85
31
37
5.6
I IO-I
S9 (Belep) o-5
2.70
300
5.50
137
IS
I0
7
47
IO-l.5
3.00
290
650
145
17
47
7
49
20-25
3.60
400
795
177
21
57
IS
SO
30.-3s
3.40
390
700
168
20
16
IO
30
40-45
3.20
340
720
165
I8
13
7
48
SO--S5
3.50
410
750
I80
I7
13
6
52
60-65
2.90
320
740
I60
IY
IX
7
49
70.-75
3.90
480
805
20s
24
IS
IO
49
80-X.5
3.50
395
750
185
23
13
9
45
3.50
430
825
I95
24
26
I4
46
3.20
380
660
I76
I9
22
Ii
49
2.30
240
515
130
I6
IO
90-9.5 1oo-
I OS
110-114
9
48
5.0 4.6
1.2 5.5
4.3
68
P. Ambatsiun
3.3, Metal concentrations
and sequential
of Geochemical
et al. /Journal
Exploration
59
f 1997159-
NI. Cr. Mn
extractions
1ccc
Except for Cr, metals are generally more concentrated in the Dumbea Bay sediments than in the sediments near the Belep islands, which support little weathered rocks (Table 1). In cores S2 and S3 and in cores from the same area of the lagoon described by Launay (197X2), relatively high concentrations of Fe, Mn and Ni were found between 1 m and 1.3 m. There are correlations between Fe, Co, Mn and Ni (Table 2). On the contrary, concentrations are lower in deeper layers. The metal enrichment could be assumed due to ionic diffusion from the deeper, reducing levels and precipitation at the redox boundary (at I to 1.3 m>. But the high sedimentation rate like in Dumbea Bay opposes consequent enrichment by ionic ascending diffusion. The decline in metal concentrations from 1.2 m to the surface (Table 1 and Fig. 11) is more probably due to changes in the composition of the eroded material caused by processes such as deforestation followed by mining activities. Lower concentrations in the deeper levels (> I .2 m) were
X00
74
@gig) X00
. : . Mn
I! ‘,
/
:oc 4
i :2c
.
’ 5
Fe?/,
:c
15
distributions of carbonate-free
Fig. 1 I. Vertical
basis metal con-
centrations in core S2.
perhaps related to more reducing conditions in the lagoon at the settling moment. Concentrations of Co and particularly Cr and Ni in the lagoon sediments are higher than those reported for most areas of other continental shelves. We may compare this to the Ligurian continental
Table 2 Correlation matrix showing relations between constituents of whole sediment samples. with 95’% validity for r > 0.55 in the case of S2 and r > 0.58 in the case of S9 Fe
Mn
Cr
Ni
Zn
co
CU
Th
U
S2 (Lhtmbea). n = 13 Ni
0.96
Cr
0.17
0.69
Mn
0.93
0.97
0.68
CO
0.91
0.93
0.68
Zn
0.09
0.06
0.14
0.12
0.24
CU
0.46
0.42
0.44
0.50
0.52
0.23
?32Th
0.38
0.60
0.3x
0.58
0.58
0.04
0.06
0.13
0.16
0.15
0.30
0.54
- 0.22
- 0.74
- 0.63
- 0.66
13xU
-0.14
taco,
- 0.66
0.98
-0.61
- 0.60
-0.15
0.27 - 0.52
~ 0.30
S9 (Belep). n = 12 Ni
0.94
Cr
0.88
0.85
Mn
0.91
0.97
0.89
co
0.77
0.8 I
0.8 I
Zn
0.28
0.09
0.3
I
0.09
0.16
cu
0.39
0.46
0.05
0.47
058
0.53
ziLrh
0.26
0.24
- 0.01
0. I2
0.1 I
0.0
23xU
0.10 0.08
0.30 0.02
0.27 0.05
0.43
0.50
- 0.05
- 0.34
- 0.06 0.35
taco,
0.88
I
0.01 0.54
- 0.09
- 0.13
- 0.46
- 0.30
P. Ambatsian et al. /Journal
of Geochemical Exploration 59 (1997159- 74
shelf in the Savona area (Northern Italy), where the neighboring mountains contain ultrabasic rocks with Cr ore deposits. There the near shore sediments have Cr concentrations of 150 to 1800 pg/g and Ni concentrations of 80 to 600 pug/g (Cosma et al., 1979). But generally the total Co, Cr and Ni concentrations do not exceed 100 pg/g in little or unpolluted sediments of the Western Mediterranean continental shelf (Damiani et al., 1986; Added et al., 1980), as well as, for instance, in sediments of the continental margin of NW Africa (Schiittle, 1978). The proportions of metal extractable at each step during the sequential procedures varied slightly with depth (core S2; Figs. 12 and 13). This does not mean that no important mineralogical transformations occurred with depth. Some observations should be pointed out: (1) Mn extracted during the first step (acetic solution at pH 5.6) of the 15-step procedure partly originates from the adsorbed metal. Only a few amount of Mn was extracted during this step (at pH 5.61, as well as during steps 3 and 4 (pH 51, indicat-
69
ing that almost the whole Mn bound to the carbonate phase was released after step 2. No consequent amount of Mn expected to belong to the following phases was therefore extracted by means of the acetic solution at pH 5 in both the 7- or 15-step procedures. A large fraction Mn (along with Co and Cu) appears to be bound to carbonates as is frequently the case in carbonaceous sediments (Femex et al., 1986; Wartel et al., 1990). As the hydroxylamine solution (second step) extracted more Mn from the upper levels than from the deeper ones, and the Mn oxide X-ray peaks disappeared after this step (Fig. 6), the proportion of easily reducible Mn oxides was greater in the upper levels. In the sediments, a transfer of Mn (and Co and Cu) occurred therefore from oxides not only to sulfides but also to carbonates. The decline in the amount of Mn extracted with the hydroxylamine solution from step 4 to 8 in the 15-step procedure confirms the efficiency of this reagent to solubilize Mn oxides. (2) During the three successive extractions with identical hydrosulfite (dithionite) solution (in the fif-
100 60
S
60
IL” 40 20
s
60
6
40 20 0,
60
$
60
z
40
/
20 0 0
20
40
60
60
100
120
0
20
40
4-2
I
I
I
80
100
120
depth (cm)
depth (cm) -1
60
-&-
3
l
4.5
--t
6
-C
7
Fig. 12. Percentages of metal extracted from core S2 (Dumbea Bay) during the 7-step leach procedure, with: (1) the acetic solution at pH 5; (2) the hydroxylamine-hydrochloride solution at pH 3.5; (3) the oxalate solution at pH 3 (bars indicate this step); (4) and (5) NaOH 0.1 M (humic substances); (6) H,O, + HNO, (extraction of remaining organic matter and sulfides); (7) concentrated HNO, + HCLO,.
70
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
400 a
2
3oo 200 -
I
Ni Dumbea
100 -
0
i T -i I / 7 : ‘: n 7 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
200 5
150
s
100
EZI
Mn Antibes
50 0 1
2
3
4 lIII m
5
6
7
8
9 10 11 12 13 14 15
Zn Dumbea Zn Antibes
0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
10
3 G 3
I
8
!
Cu Dumbea
6 4 2 0
W
Cu Antibes
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
steps_ (AC. AC.)
1(NH20H.HCI)I
$1
&04=)
1 (O.ph.)l
(A.)
z
Fig. 13. Metal fractions extracted during the 15-step sequential procedure. The results for the sample of core S2, O-5 cm. are compared to those from a sediment of the northwestern Mediterranean continental shelf near Antibes (France) (20 m in water depth). The reagents used during the successive steps were for (I) to (4) acetic solutions. at pH 5.6 for (I), 4.7 for (2), 5 for (3) and (4); for (5) to (8) NH20H+HC10.04 M, at pH 2.8 for (5) and (6), 0.2 M at pH 2 for (7) and (8); for (9) NaOH 0.1 M; for (IO) to (12) hydroaulfite (40 g/l). at pH 5 for (10) and (1 I), and pH 4.7 for (12); for (13) H,O, _ fHN0, at pH I (not only organic matter and sulfides but also remaining oxides were probably removed): (14) hydrosulfite used here with the aim of solubilizing oxides assumed to have been formed during step (13); (I 5) HNO, + HCLO,. The low proportion of Fe and Mn extracted from the Antibes sediment during the 3rd hydrosulfite leaching (step 12) indicates that the main part of the metal was solubilized before. Ac.Ac.: acetic acid solutions. O.ph.: oxidizable phase. A.: concentrated acids.
teen-step procedure). Fe and Ni were not less and less extracted from the 10th to the 12th step, as expected (Fig. 13); in contrast Fe was almost totally removed from a coastal sediment of the Western Mediterranean Sea after two leachings with hydrosulfite. The less easy solubilisation of Fe from the moderately reducible phase in the tropical area is perhaps due to better crystallinity of oxides brought to sediment from a watershed rich in eruptive rocks and laterite. (3) Compared to Fe or Ni, very little Cr was extracted with the hydroxylamine-hydrochloride solution, and even with hydrosulfite; Cr oxides are scarcely reducible (Fig. 2). The correlation between Ni and Cr, during the steps 10 to 13 suggests however that part of the Cr was present in oxides more readily reduced than chromite. The oxalate solution (3rd step of the seven-step process), which removed about 50% of the whole extracted Fe and 40% of the Ni, is a strong complexing reagent for C&II) and removed 60% of the Cr. (4) Fe was less extracted during the sixth step (oxidizable phase) below SO-85 cm; since the concentration of total S increases with depth, this is not due to a decline in sulfide concentration, but probably to the transformation of poorly crystallized monosulfides into pyrite, or at least to more highly crystalline monosulfides, more resistant to chemical attack (Bemer, 1971; Femex et al., 1986). (5) About 20% of the Cu was extracted by the NaOH solution, which presumably leached humic substances simultaneously to Cu complexed by these substances. As the Cu profile parallels the Fe one in the sixth step, Cu probably coprecipitated with Fe during Fe sulfide formation. .?.4. Minerulogicul ior
tran~fi~rrnations and metal brhtrl,-
Iron in ultrabasic rocks is principally borne by the spinel-chromite, magnetite and, to a lesser extent, olivine and pyroxene minerals. The Fe concentrations in the basic rocks do not reach IO%, and in the ultrabasic 8%. During weathering, olivine is transformed into ME-Fe smectite and limonite (FeOOH) (Pelletier, 1983). The Fe content of laterites on the peridotitic rocks is as high as 50%. Laterites represent the intensively weathered part of the exposed
P. Ambatsian rt al. /Journal
oj’Geochemicu1 Exploration 5Y ClYY7) 59-74
rocks and should comprise a major part of the suspended load of the river, but the composition of this load is closer to that of the rocks of the watershed than to the laterites (Baltzer and Trescases, 1971; Paris, 1981). As a matter of fact laterites are thick only on flat areas, but scarce on slopes (Podwojewski and Bourdon, 1996). In the upper part of the laterite profiles, goethite and hematite are the main Fe-bearing minerals, as is also the case in the surficial sediments of the lagoon. The proportions of goethite and hematite decrease with depth in the cores, while the proportions of magnetite and sulfides increase. Serpentine of the Fe(II1) type is not known in continental rocks, where serpentines are of Mg-Fe(I1) or Mg-Ni type. As siliceous organism fragments (radiolaria, sponge spicules) are very abundant in the surficial sediments of the Dumbea Bay, the H,SiO, concentration of the pore water may be high enough to allow Fe-serpentine to develop (Tardy et al., 1974). Using sedimentation rates and water content, we estimated the fluxes of Fe from the watershed of river Dumbea-Coulevte (220 km’) to the sediments: 0.015 g/cm2/year for S2, 0.002 for S9 and 0.001 for the core C9 (taken in the center of the lagoon, as discussed by Launay, 1972). That is to say, the flux from the Dumbea-CoulevCe River watershed to the sediments in the open lagoon is at most one fifth those in the bay; and at least 1800 t Fe originating from the watershed deposes every year in sediments. Assuming that Fe represents at most 30% of the solid material transported to the lagoon (Baltzer and Trescases, 197 1), the total solid transported would be at least 6000 t/y. This estimation is higher than that of Launay (1972) who estimated the average solid load at 4000 t/y and the soluble at 10000 t/y. Part of Fe in the lagoon sediments results perhaps from precipitation processes at the sediment-lagoon water interface. On a carbonate-free basis, Mn, Co, Cr and Ni concentrations are generally lower in the sediments of the lagoon than in the peridotic bedrock or in laterites. On an average, the Cr concentrations are 3 to 4 mg/g in the peridotitic rocks. Close to the Dumbea river mouth, the Cr concentrations locally exceed 4 mg/g, but they are generally < 2 mg/g in the
71
sediments of the lagoon. Chromite can be decomposed under intense weathering, with release of Cr, which may then be taken up by iron oxides or clay minerals (Latham, 1985); nevertheless most of chromium transported from the Dumbea river watershed to the lagoon remains as chromite which in great part deposes near the mouth of the river. Mn and Co, in the form of oxides during weathering and transport, are reduced and dissolved during the first stage of sedimentation and then may coprecipitate with calcium carbonate (Wartel et al., 1990) Unlike Cr, Ni undergoes important mineralogical transformations. In the peridotitic rocks, Ni is mainly found in olivine. At the base of the weathered profiles it is bound to limonite or incorporated into silicates (serpentine or talc) (Trescases, 1975); in the surface it is more closely associated with Mn hydroxides (Manceau and Calas, 1987). A large proportion of this metal must be carried to the lagoon bound to Mn oxides. However, in the sediments it seems to be mainly linked to iron oxides (Figs. 12- 14). Ni concentrations in the surficial sediments decrease from the vicinity of the river mouth to the centre of the lagoon (Launay, 1972). Whereas the Ni concentration decreases by a factor of 20, Fe, Cr and Mn concentrations are reduced by a factor of less than 4. Therefore, the decrease in the Ni concentrations must be due not only to a diluting or sorting effect but also to solubilization, and release must occur after deposition. Gerringa (1990) showed that during aerobic degradation of organic matter there is a correlation between dissolved Ni and dissolved organic matter; Ni can be readily released from sediment by an increase in the concentration of dissolved organic ligands. Lower rates of sedimentation, as in the central region of the lagoon, allow the sediment at the sediment-water interface to be subjected to a longer exposure time, leading to greater release of metal into the overlying water. In the river flows, the Fe/Ni ratio in suspended matter is 33 (Paris, 1981). If we assume that the Fe concentration variations in the surficial sediments depend essentially on the dispersion of Fe bearing particles in a carbonaceous matrix poor in Fe, and iron minerals (oxides) do not suffer important dissolution within surficial sediment, we can use Fe as a normalizing element. In that case, the surficial sediment from site S2 (Dumbea), where Fe is 5%, would have an
72
P. Ambatsiun
I
I
1
peridot
et al. / Journul
r,f Geochemicul Exploration
4. Conclusions
(olivine)
I
Fe
I
orthapyroxene
59 0997159-74
n
clinopyroxene
Fig. 14. EDS spectra of freshly broken detrital grains (I. 3, 5), and at the surface of the same grains (2, 4, 6) from the magnetic mineral fraction of core S2 (30-35 cm). Si/Mg ratio and Ca content differentiate: peridot (olivineX1). orthopyroxene (3) and clinopyroxene (5). Ni is present in the Fe coating at the surface of the grains.
adjusted Ni concentration of about 50000/33 = 1650 pg/g. The Ni concentration in the top levels is 1245 pg/g. The difference between the measured concentration and the calculated value is about 400 pg/g. In core C9, where the Ni concentration is 53 pg/g, the difference is (32000/33) - 53 = 916 pg/g. Considering the water content (about 50% w/w.) and the sedimentation rates (3.2 mm/y for S2, and 0.55 mm/y for C9), respectively 64 and 33 pg/cm’ of nickel should be released each year from the surface layer. The lagoon sediments do not act as efficient ‘I sink” for this metal that may have harmful effect on some living species (Alikahn. 1989; Wood, 1989).
Compared to sediments of other coastal areas, sediments of the New Caledonia lagoon have relatively high Ni, Cr and Co concentrations, but low compared with the peridotitic rocks or weathering products from the island. In the Dumbea Bay. the vertical profiles of Fe, Mn, Ni and Co present a metal maximum at the 1 to 1.2 m level, the concentrations being in particular lower in the deeper and more reducing levels. If the sedimentation rate was not lower than the actual (and if there is no ascending water flow from the deeper levels through the redox boundary), the enrichment could not be due to mere ionic migration from the deeper, reducing levels, but more likely to changes in the geomorphology of the eroded area or moving of the mouth of river Dumbea-CoulevCe. Even in the richer levels the metal concentrations remain too low to allow mining exploitation. In the lagoon sediments. in particular in the confined bay of Dumbea, the transformation rates varied from one deposited species to the other. On the one hand, the organic C concentration only little decreased from the upper layers to I m depth. The mineralogical species which carry most Cr did not undergo important variations even at levels deeper than I m. Iron oxides were found in similar proportion from the top to the bottom of the core. However, in deeper levels, (below 40 or 50 cm) goethite and also hematite transform to magnetite and Fe sulfides. On the other hand, silicates are transformed in surficial sediments. Smectite shows progressive improvement in its degree of crystallinity from the top to the bottom of core S2 (Dumbea Bay). Moreover, at the upper horizons of the sediments we detected Fe(III)-serpentine; as such mineral was not observed in the rocks of the islands, we assume it has an authigenic origin. Mn, Co and Ni undergo more important transformations in surficial sediments than Fe and Cr. Ni is transported from land with Mn oxides but the sequential extraction showed a close relationship between Ni and Fe oxides in the sediment of the bay. A great part of Mn is dissolved in the first stage of the diagenesis. Co and Ni linked to Mn oxides are freed but follow two different geochemical pathways. Co,
like Mn, coprecipitates with CaCO,. Whereas Ni is mostly released to the lagoon watkrs, with the remainder Ni being fixed to Fe oxides.
W..
19X6. Mobility
Springer Gerringa.
Vcrlag.
L.J..
\lurries.
The authors wish to thank the referees for helpful] comments and help with the english language text. and Drs G. Boillot and J. Mascle (Lab. GCodynaVillefranche/Mer) for laboramique sous-marine, tory facilities.
Latouche.
critique
de\
extractions
quelques constituants
metals in the superficial
marine
sediments
the Rhone mouth. Prog. Wat. Tech., Alikahn,
M.A..
Vernet (Editor),
Heavy Metal\
aultants, Edinburgh. Baltler.
t’orwllio
crustacean, Vol.
F. and Le Ribault.
hanca sCdimentaire\
in the vicinity
of
: X9-96.
12/I
19X9. Copper and nickel
a terrestrial
of heavy
Say.
in the Environment.
by
In:
J.P.
CEP Con-
I, pp. 475-478.
M..
Luunay.
d’un delta tropical.
C.R. Acad. Sci. Pa&.
273: 1083-1086. Bnltrer.
J..
de l’trosion
sous le climat
tropical
et de la skdimentation
de la Nile Caltdonie.
du hilan
sur piridotitc
C.R. Acnd.
Sci.
J.L.,
IV,’ I :
1990.
G.J.,
“‘U-““U-“‘Th
systematics
from central Missouri. Berner,
R..
1971.
Graw-Hill. Cosma,
Chen,
and Moore.
in haline
Earth Planet. Sci. Lett..
Principles
New York.
of
NY,
Chemical
C.H..
groundwater\ 101: 296-3
Sedimentology.
1979. Heavy metals in Ligurian
12. Mc
G. and Tucci.
Sea sediment\.
in surficial
sediments.
Distribution Mar.
Chem..
S..
0..
V.. Amhroso,
the input dispersal environment. Interactions. Dugas.
E.. De Rosa, S.. De Simonc.
In:
and settling P.G.
du
Sly
Springer-Verlag,
F. and Debenay,
annexe
G., 1986. Sediment\
Lagon
J.P..
de Nlle
NoumCa). O.R.S.T.O.M..
Sedimenta
and Water
NY. pp. 13-25.
mine and Mom) Nircl.
1982. Carte
sCdimentologlqur
Caltdonie
Paris-Bondy.
i
l/.iOOOo
ct
(Feuille
Notice explicative.
sediments of the Northwest
ranean continental
In: P.G. Sly (Editor),
Interactions.
\heIf.
95:
Springer
Verlag.
New
York.
of
Mediter-
Sediments NY.
and pp.
353-370. U.. Ahlf.
Odin.
W.. Ker\ten,
ultra-
331 pp.
M..
Paris-Bondy.
(Nile
S&r. C&l.
from
of cobalt and
New
Caledonia.
Acta. S I: IOS- I 13. P. and Osmond J.K.,
1992. Uranium
in groundwaters
I’rom the Osamu
do Ferro natural
analogue
P.M..
G.S.. Dehenay.
and
Utsumi
sites. Poc;os de
Explor.,
35: 345-363.
1990. Pitfalls
of sequential
J.P.. &get.
Icrdlne:
\uhrCcil’al.
d’unc
extractions.
cn
milieu
2: 17-4 I.
Orltans-La
Pal-ran. C. and Amouric.
P.. 19X7. Le
phyllitc
Lcy rcssources mi&rale<
B.R.G.M..
chemical
C. and Rigolot,
nCotormation
G&)dynamique.
Pari\, J.P.. IYXI.
M..
de la Nllc
Source,
Caledonie.
113: 2lY-123.
19X8. Phase transformation
process as revealed hy HRTM
durmg
nnd micro-
study. Inst. Phys. Conf. Ser., 93. 2: 557-558.
B.,
IYX3.
Localisation
“,garnt&itiques” Podwojcwski. tion
du nickel
de Nile CalL:donie.
P. and Bourdon.
de\
Pout&.
oxvdlsol:,
dans lc\
Sci. C&l..
mintraux
M:m..
Rapin.
Stra\-
5s
M. and Salomons.
Rev.
I:., Tessicr.
In%
permettant
magnttique franq.
A.. Campbell,
artifact\
d‘enregistrer
les
le long d‘une carotte
P&role
(Ryeil-Malmaisonl,
P.G.C. and Carigan,
in the determination
by a sequential
Trchnol..
d’indura-
en Nouvelle
I-562.
\edimenth
R., 1986.
of metal partitioning
extraction
procrah.
Environ.
in Sci.
20: 55 I-562.
P., Robin.
E‘.. IchtertL.
I97 I. La determination
M.N.,
Fusil. G. and Parrot. J.F..
du fcr libre dans les sols h sehquioxy-
de\. Cah. O.R.S.T.O.M.,
Paris-Bondy.
SL:r. Ptdolosic.
IX/I:
I. M..
197X. Geochemischc
Sedimenten Schriften, Y..
Untersuchungen
des Kontinentalrandcs
Kandrihchen Tardy.
sur pCridotite
C.R. Acad. Sci. Paris, 322. \t-r. IIa: 453-459. de Ia \usceptibilite
Potential
4-l
E.. 1996. Lc procesus
goethitiques
J.. 1973. Sur un appareil
I’enthalpie W., Calmano.
Univ.:
de la haie de Dumbda
and asholane
J. Geochem.
R. and Morel.
Schiittle.
some metals in surt’icial
sur roches
These Etat, Dijon
Water Reh., 2-I: lO5S-1056.
Scsalen.
45 p.
Fiirstner.
isotope\
de &diments.
R., Fcrretti.
Femex, F.. Span. D.. Flatau. G. and Renard. D.. 19X6. Behavior
Water
N.. Linsalata.
thorium
8:
as a record of
New York,
et pCdog&Ce
R.S T.0
Cosmochim.
variation\
processes in a coastal marine
(Editor),
0
In lithiophorite
29/A:
Izzo. G. and Zurlini,
de
Tech-
25-5 I.
of
125-142. Damiani,
(‘ah.
Cal2donie.
M.. Scarponi,
I’Ctide
Envlronm.
haurg. 73: 17% 183.
240 pp.
B., Drago. M.. Picazzo,
Cr. Cu. Ni and Mn
J.H.
travers
A. snd Cltlas. G., 19X7. Crystal chemistry
Pelletier.
Wassenburg,
matter and
F.. 198.3. Analyse
i
des residue d’attaque\.
Alt&ation
the glauconitisatlon
Paris, 273: 2034-2037.
s6quentielles
1977. Sedimentation
M&moire
F. and Treacases, J.J., 1971. Premiere estimation
de I’alt&ation
Banner.
lYX5.
Cal&donicI.
l’aci&
L.. 1971. NCogenPse de quartz dam le\
C. and Pautrirel.
hasiques cn Nile Caltdome.
Caldas. Brazil.
uptake. accumulation ~~pinicornu
of organic
nol. Lett.. 1: 500&S II. Latham.
Mlekrlcy. F. and Rapin. F., 19X0. Diatrihution
pp. 37 l-380.
Mar. Chem.. 2: 355-373.
Geochim.
A., Femex.
NY,
degradation
of Cu, Cd, Ni, Zn, Fe and Mn in marine sediment
J.M..
nickel
Added.
New York,
Jouanneau.
Manceau.
References
metals in dredged harbor sedi-
Sediments and Water Interactions.
1990. Aerobic
the mobility
Acknowledgements
of heavy
ments. In: P.G Sly (Editor),
und
Kapverdishen
17. Lenz-Verlag. Treacase\, lihre
an rezenten
van NW-Africa Insrln.
rwischen
Giesscner
Geol.
I88 pp.
J.J. and Fritz.
de formation
B..
1974.
Evaluation
de montmorillonitc
C.R. Acad. Sci.. Paris, (D). 27X: 1665-166X.
dc
i’erriferes.
74
P. Amhutsicrn et ~1. / Journul
Tessier, A..
Campbell.
P.G.
and Bison.
M..
ofGeochemrco1
lY7Y. Sequential
Explorution
Wartel,
M..
59 C1997) 50-74
Skirer.
M.,
Auger,
Y.
and Boughriet,
extraction process for the separation of particulate trace met-
Interaction
als. Anal. Chem.. 51: 844-850.
aase\sment of the solubility product of MnCO,
Trescases, J.J., basiques
en
nickClifere.\ K.K.
Enewetak 522-524.
zone de
Paris-Bondy. Turekian,
1975.
L’&olution tropicale;
super&e
des roches ultra-
formation
Nouvelle-CalCdonie.
de.\
M6m.
gisemenr\
O.R.S.T.O.M..
1981. ““Pb
in \&ace
and the Asian dust to the Pacific.
Nature,
A..
cles. Mar. Chem.. 29: 90-l Wood.
J.M..
lY89.
Transport.
and Mn distrl-
(Editor), Heavy Met&
292:
Edinburgh. pp. I - 12.
parti-
17. bioaccumulation
under environmental
air LLI
IYYO.
with carbonates in sea water:
bution coefficient between the liquid phase and CaCO,
microorganisms
7X: 259 p. and Cochrdn, J.K.,
of manganese II
and toxicity
111
stress. In: J.P. Vemct
in the Environment. CEP Consultanta.