Environmental Geochemistry and Health (2006) 28:509–518
Springer 2006

DOI: 10.1007/s10653-005-8619-7

Geochemical index of trace metals in the surficial sediments from the western
continental shelf of India, Arabian Sea

C.M. Laluraj1 & S.M. Nair1,2

1
Department of Chemical Oceanography, School of Marine Sciences, Cochin University of Science and
Technology, 682 016, Cochin, India
2
Author for correspondence (Fax: +91-484-2374164; e-mail: nairgowri@hotmail.com)

Received 3 December 2004; Accepted 7 June 2005

Key words: Arabian Sea, continental shelf, enrichment factor, geochemistry, trace metals

Abstract
The present study focuses on the determination and abundance of trace metals (viz. Cu, Ni, Zn, Cr, Co, Cd,
Mn and Fe) in the surficial sediments of west coast of Arabian Sea along the Indian subcontinent. Sediment
samples were collected from three transects along the western continental shelf of Arabian Sea. The
enrichment of Fe and Mn in coastal oxic-sediments indicates the precipitation of these redox sensitive
elements as Fe- and Mn-hydroxides and oxides, whereas the low Fe and Mn concentrations in the oxygen
deficient sediments of deeper stations reflects the dissolution of their hydroxides and oxides. Concentrations
of fairly redox insensitive trace metals like Cu, Ni, Zn, Cr and Cd (with the exceptions of Cr) showed higher
values at nearshore sediments, then it decreased towards seaward and again showed a slight increase at
oxygen minimum stations in all the three transects. This geochemical variability in their distributional
characteristics is mainly associated with the extent to which the precipitation or dissolution of Fe- and
Mn-oxides/hydroxides occur since the scavenging or releasing effects of Fe- and Mn-oxides/hydroxides act
as significant ‘sinks’ or ‘sources’ of heavy metals. The change in wind pattern, coastal upwelling and
increased productivity are also the reported factors which influence the biogeochemical cycling of trace
metals in the surface sediments of west coast of India. Enrichment factor generally showed a high gradient
accumulation from nearshore to shelf.

Introduction released to the water column by various processes

of remobilisation. Thus in aquatic system, sedi-
The determination of trace metals in recently ments may be both a carrier and a possible source
deposited sediments is a useful tool in the assess- of metals.
ment of status of environmental pollution as the The present study focuses on the determination
surficial sediments are potentially good indicators and abundance of trace metals (viz. Cu, Ni, Zn,
of the quality of overlying waters (Berrow 1991). Cr, Co, Cd, Mn and Fe) in the surficial sedi-
Once trace metals were discharged into coastal ments of west coast of Arabian Sea along the
waters, they rapidly become associated with par- Indian subcontinent. These trace metals all share
ticulates and are incorporated in bottom sediments a common trait of being authigenically enriched
(Forstner & Wittmann 1983; Hansen et al. 1993). in sediments under reducing conditions. The
The trace metals associated with sediments are not mechanism of authigic enrichment of trace metals
essentially sheltered permanently, and under in sediments, along with their behaviour has a
changing environmental conditions they may be vital role in the surficial distribution of trace
510 c.m. laluraj and s.m nair

metals in western continental shelf of Arabian coastal zone of western India experiences upwell-
Sea. ing in most of the seasons and intense local wind
stress (Banse 1959; Naqvi et al. 2000). Thus, high
overlying productivity and sluggish intermediate
Materials and methods water moments during northeast monsoon
(December–February) can lead to higher con-
Study area sumption of oxygen and more intense suboxic and
denitrifying conditions in Arabian Sea (De’Souza
Arabian Sea is a semi-enclosed sea bordered on the et al., 1996; Madhupratap et al. 1996). So the
northern, eastern and western sides by the land- intense suboxic condition, change in wind pattern
masses of Asia and Africa. The width of the con- and coastal upwellings are the major factors which
tinental shelf along the south west coast of India is influence the biogeochemical cycling of trace
found to vary from place to place. It is narrower metals in the surface sediments of west coast of
on southern side and gets widened towards the India.
north. This is an area of negative water balance
where evaporation exceeds precipitation and run Sampling
off, the high rate of evaporation results in the
formation of high salinity water masses (Venk- Surficial sediment samples were collected during
ateswaran 1956). The maximum production was the 192A cruise of FORV SAGAR SAMPADA of
reported nearer to the coasts, with in 50 m depths Department of Ocean Development, Government
and gradually decrease towards the open ocean of India from 12 stations off west coast of India
(Nair et al. 1973). between Cochin and Bombay during winter mon-
An outstanding feature of the Arabian Sea is the soon (February 2000). The area lies between lati-
development of its oxygen minimum layer. The tude 1027.25¢–1729.39¢ N and longitude
mid-depth oxygen defiency in the Arabian Sea is 7126.74¢–7551.45¢ E. Sediment samples were
perhaps the most severally observed anywhere in collected from three transects along the western
the ocean, as the concentrations within 150– continental shelf of Arabian Sea. One transect is in
1000 m are <0.2 mL L)1 in a large part of Ara- the northern zone (1729¢39 N) with intense
bian Sea and which narrower from north to south reducing condition, one in the central zone
(Naqvi & Jayakumar 2000). The compatibly high (1329¢52 N) and one in western zone (1028¢45 N)
photosynthetic activity in the Arabian Sea than of the western continental shelf of India and the
that of the average of all other ocean often leads to details are given in Figure 1. Sediment samples
a greater concentration of the dead cell and were collected from the sediment bed using Smith-
detritus at the thermal discontinuity layer, which Mc Intyre grab, which is 0.1 m2 and is of moderate
consume oxygen during decomposition (Sen weight of 45 kg. About 250 g of the samples were
Gupta et al. 1976). The supply of oxygen to the transferred to polythene bag and were kept shock
waters below the euphotic zone gets restricted by frozen to preserve their chemical integrity until
the intense strong density gradient and poor hor- analysis. The sampling reproducibility was
izontal advection due to semi-enclosed nature of checked by duplicate subsampling measurements.
Arabian Sea, results in severe depletion of oxygen
below the thermo cline and at intermediate depths Analysis
(Naqvi & Qasim 1983). The high organic carbon
content (2–6%) in the western continental shelf of Hydrographical parameters such as water salinity
Arabian Sea (Prakash Babu et al. 1999) has also and temperature were obtained from the CTD
enhanced the oxygen minimum nature of water instrument. The bottom-dissolved oxygen from
column. each station was also estimated chemically on-
The west coast of India is under the influence of board using Winkler’s method (Grasshoff et al.
changing wind patterns associated with almost all 1999).
seasons. The coastal currents are southerly from The sediment samples were dried at room tem-
April to September and reverse their direction to perature at constant weight (0.5 g) and were
northerly from November to February. The heated with perchloric acid, nitric acid and
 geochemical index of trace metals 511

were analysed in similar way to check the accuracy

of analysis, which was better than 6% for Cu, Co,
DABHOL
4 1 Fe & Mn and better than 10% for Zn, Ni, Pb and
INDIA
Cr (Table 1). Analytical reproducibility was
Shastri river checked by performing duplicate analyses of each
16˚ Study
area
of the duplicated subsamples. Thus, final analyti-
cal date are presented on a weighted-average bases
of quadruplet values.

COONDAPORE
AR

8 5 Results and discussion

A
BIA

Salinity, temperature and dissolved oxygen

12˚
NSEA

Ponnani river
Salinity, temperature and dissolved oxygen values
of the sampling stations are given in Table 2.
VADANAPALLY The coastal stations showed lowest values of
12 9
salinity, and increased towards seaward in all
Periyar river
transect. The low values at coastal stations are due
to the surface dilution. The bottom temperatures
were found to decrease from south to north. The
8˚ high values were observed at coastal stations and
decreased towards seaward. The variations of
salinity and temperature in all transects were
found to be a similar pattern. The farthermost
71˚ 78˚ seaward stations (4, 8 & 12) in all the three transect
were distinguished with comparatively very low
Fig. 1. Location of 12 sampling stations along the 3 transects
from the western continental shelf of Arabian Sea. temperature (16.0–17.0 C) and all the three
stations were significantly above 195 m deep.
The dissolved oxygen values were found to
hydrochloric acid (2:10:8) in closed beaker in a hot decrease from coastal to seaward. In higher depth
plate at 90 C for 20 h. The acidic solutions were stations (4, 8 &12), the values were very low
centrifuged in glass containers at 6000 rpm and the (0.1–0.2 mL L)1) which are very close to almost
final solution made upto 50 mL with 1 N HNO3. an anoxic condition. The very low values were
All the chemical treatments were carried out using found in the northern transect and increased
Milli Q 18 mW quality deionised water. Care was towards the south. These values were well sup-
taken during sample handling and analysis to ported by the earlier findings of Naqvi &
prevent the samples coming into contact with dust Jayakumar (2000). The dissolved oxygen was more
and metals. The metal concentrations were esti- in the surface layer due to high primary produc-
mated in Graphite furnace atomic absorption tivity. The observed variations in the distribution
spectrometer (Perkin Elmer model 3110). Proce- of dissolved oxygen may be due to the interplay
dural blank and a reference material (BCSS-1) between biochemical process, and those by which

Table 1. Comparison of standard reference material (BCSS-1) with the present study (lg g)1 dry weight expect for Fe, which is
in %).

BCSS-1 Cu Ni Mn Fe Pb Co Cr Zn Cd

Certified values 18.5 ± 2.7 55.3 ± 3.6 229 ± 15 5 22.7 ± 3.4 11.4 ± 2.1 123 ± 14 119 ± 12 0.59 ± 0.1
This studya 18 ± 2.0 49.8 ± 2.3 216 ± 17 4.7 20.4 ± 2.9 10.8 ± 2.5 111 ± 19 107 ± 9 0.53 ± 0.2
a
SD is for n = 4.
512 c.m. laluraj and s.m nair

Table 2. Sample location and analysis of primary parameters.

Transect Station No. Depth (m) Lat. (N) Long. (E) Temp. (C) Salinity D.O. (mL L)1)

1729¢39 1 35.0 1729¢39 7257¢22 26.70 35.68 3.87

2 57.0 1729¢39 7243¢09 26.66 35.60 3.72
3 96.0 1729¢39 7216¢25 25.69 35.68 2.03
4 192.0 1729¢39 7126¢74 16.18 35.47 0.10
1329¢52 5 34.0 1329¢52 7430¢04 28.48 34.60 3.87
6 54.0 1329¢52 7405¢81 27.71 35.75 3.11
7 104.0 1329¢52 7331¢63 25.90 35.83 2.26
8 193.0 1329¢52 7326¢55 16.04 35.30 0.11
1028¢45 9 38.0 1028¢45 7542¢59 28.47 34.66 3.30
10 53.0 1028¢45 7531¢57 28.27 35.27 3.13
11 103.0 1028¢45 7552¢53 25.42 35.62 2.46
12 197.0 1028¢45 7525¢52 17.18 35.24 0.20

the oxygen enters and is transported in the water. Sagar 1994). Mn and its compounds such as
But the ultimate factor causing the depletion of O2 manganese nodules are important constituents of
in deep stations were due to the oxidation of or- pelagic sediments and control the transition metal
ganic matter. concentration in marine sediments. Almost similar
trend of Fe was exhibited by Mn at all the stations
Trace metals and shows a high correlation between them

Analytical results and variation with depth of trace

metals in the surface sediments of western conti-
nental shelf of India are given in Figures 2 to 4.
Fe concentration was found to be the highest
among all metals at all stations studied. In all the
three transects, the highest concentrations of Fe
750
and Mn were present in the nearshore stations (1, 5 20
o
600
&12) and decreased with depth. Among the values, 450

the highest values were found at the northern 300 INDIA

150
transect. This was due to the high anthropogenic 0 DABHOL
4 3 2 1 4
activities from riverine input at the northern 750
1
o
transect. It is well known that Fe and Mn are 16 600
enriched in oxic sediments due to precipitation as 450
300
their corresponding oxyhydroxides (Klinkhammer
150
et al. 1982). The low Fe and Mn concentrations in 0 8 5
COONDAPORE
8 7 6 5
the sediments of deeper stations (4, 8 &12) were o
750
12 600
mainly due to the fact that development of
Conc. ( µ g/g)

450
reducing conditions around the sediment-water 300 VADANAPALLY
interfaces. Under these oxygen minimum condi- 150 12 9
0
tions these metals are released from sediments to 12 11 10
Stations
9

the surrounding waters due to the subsequent 8

o
Mn (µ g/g)
reduction of their oxyhydrides (Sullivan et al. 2
Fe ( x 10 µ g/g)
1997; Manford et al. 2001; Nameroff et al. 2002).
o o o
The reducing condition may be developed due to 70 74 78

the high productivity and high organic matter in

Fig. 2. Surface sediment concentrations of Fe and Mn at 12
the sediment, which is very much effective in stations along the 3 transects from the western continental
removing and adsorption of number of elements shelf of Arabian Sea (each value is an average of quadruplet
from the sediments (Calvert & Perderson 1993; sample analysis).
 geochemical index of trace metals 513

100
o 35
20 80 o
20 28
60
40 INDIA 21

20
14 INDIA
DABHOL
7
0
4 3 2 1 4 1 0 DABHOL
100 4 3 2 1 4 1
o
16 80 o
35
60 16 28
40 21
20 14
COONDAPORE
0 7
8 7 6 5 8 5
COONDAPORE
0 8 5
o 100 8 7 6 5
12
80 35
Conc. ( µ g/g)

o
60 12 28

Conc. ( µ g/g)
40 VADANAPALLY
12 9 21
20
14 VADANAPALLY
0 12 9
12 11 10 9 7
o
Stations
8 0
12 11 10 9
Stations
Ni Zn o
8 Co Cd
Cr Cu
o o o
Pb
70 74 78

o o o
70 74 78
Fig. 3. Surface sediment concentrations of Ni, Cr, Zn and Cu
at 12 stations along the 3 transects from the western conti-
Fig. 4. Surface sediment concentrations of Co, Cd and Pb at
nental shelf of Arabian Sea (each value is an average of qua-
12 stations along the 3 transects from the western continental
druplet sample analysis).
shelf of Arabian Sea (each value is an average of quadruplet
sample analysis).

(>0.91). So it could be viewed that Fe and Mn chemical variability in their distributional charac-
behaves as similar way in the marine environment. teristics was mainly attributed to the scavenging or
The concentrations of Co range from co-precipitating effect of Fe- and Mn-hydroxides
31.93 lg g)1 at station 1–4.41 lg g)1 at station 12. on these trace metals. The slight increase in the
The higher concentrations of Co were seen at concentrations of these redox sensitive metals at
nearshore stations and decreased with increasing oxygen minimum layers was due to the develop-
depth. Co was also highly correlated with Fe. The ment of reducing conditions at the end of conti-
adsorption of the clay minerals, iron and manga- nental shelf stations (4, 8 & 12). Under this
nese oxides may be contributing to the observed reducing conditions, these elements whose valency
concentration of Co in this study. The trace metal does not change but which form highly insoluble
studies in anoxic water revealed that the reduced sulphides such as Cu, Cd, Ni, Cr and Zn. They
species of Mn, Fe and Co are more soluble than exist as a network of interconnected sulphur
the oxidised one. A similar association of Co with complexes and sulphides and are removed from
the Mn cycle has been reported in many earlier seawater by diffusion into anoxic sediments
works in anoxic waters (Haraldsson & Westerlund (Calvert & Perderson 1993; Manford et al. 2001;
1988; Dyressen & Kremling 1990; Lewis & Land- Marcus et al. 2003). So an enrichment of Cu, Cd,
ing 1991). Zn, Cr and Ni in the upper slope stations (4, 8 &
Concentrations of Cu, Ni, Zn, Cr and Cd 12) suggests that a reducing condition do prevail in
showed higher values at nearshore sediments, then this environment, which was impinged by the
it decreased towards seaward and again showed a oxygen minimum zone in Arabian Sea. The values
slight increase at oxygen minimum stations (4, 8 & further revealed that the extend of enrichment of
12) in all the three transects. The maximum con- these metals were comparatively high in northern
centrations were observed in northern transect and transect. So that the geochemical behaviour of Cd,
minimum were in the western transect. This geo- Cu, Zn, Ni and Cr can be used as indicator of
514 c.m. laluraj and s.m nair

sedimentation under redox condition. Among content, a simple correlation has been used (Fig-
these metals, Cr showed higher values in all sta- ures 5 and 6). Salinity has not been considered as a
tions which is possibly due to their high enrich- function for the interpretation of the trace metal
ment in earth crust, especially in marine data of the present study since the values of
environment. salinities are in the range 34.64–35.83 (Table 2)
The concentrations of lead ranged from and hence the variations are insignificant. The
0.505 lg g)1 (station 12) to 3.04 lg g)1 (station 5). correlation coefficient (r) values which are signifi-
The higher concentrations were at central transect cant at ±1 levels are only considered here. For
and lower were at western transect. Lead also interpretation, we consider r > 0.70 as a most
showed coastal enrichment but the variation is not significant association which has very high realistic
significant. These low and less fluctuated Pb con- correlation and r values between 0.56 and 0.7 as
centrations indicated that anthropogenic inputs moderate association. These associations are in
were of minor importance, this in contrast to other agreement with nature of sediment and the relative
areas around the world. Lead is easily removed enrichment or depletion of elements in the zones
from the water column by adsorption to mineral (such as temperature, dissolved oxygen and Fe),
particles and subsequent sedimentation (scavenged suggesting well-defined carriers of metals to the
elements) with Fe- and Mn-hydroxide precipitate. sediments.
Anthropogenic deposition of trace metals is a The moderate positive correlation observed for
significant source for these metals in sediments. In Fe and Mn with temperature and dissolved oxygen
the case of nearshore sediments due to the con- (Figure 5) can be attributed to their redox-poten-
stant turbulent nature of water by wave action, tials (Sullivan et al. 1997; Russell & Manford
nearshore environment become more oxic which in 2001). The reducing condition may be developed
turn increases the deposition of precipitate with due to the high productivity and high organic
adsorption of different elements in the sediments. matter in the sediment, which is very much effec-
The coastal enrichment of these elements is con- tive in removing and adsorption of number of
sistant with previous studies in other areas of the elements from the sediments (Calvert & Perderson
world ocean (Klomp et al. 1986; Sanudo-Wil- 1993; Sagar 1994). Dissolution of these elements
helmy & Flegal 1996). Those elevated coastal from sediments to overlying water makes them
concentrations have been attributed to benthic impoverished at low temperature and oxygen
remobilisation, including particle resuspension and depleted environment. The supply of oxygen to the
diffusive fluxes, upwelling and continental weath- waters below the euphotic zone gets restricted by
ering (Martin & Gordon 1988; Westerlund & the intense strong density gradient and poor hor-
Ohman 1991). The coastal current in the coastal izontal advection due to semi enclosed nature of
zone of western India and the coastal upwelling Arabian Sea, results in severe depletion of oxygen
are the other important mechanism of metal below the thermo cline and at intermediate depths
transport in coastal waters and sediments in Ara- (Naqvi & Qasim 1983). The high organic carbon
bian Sea (Naqvi et al. 2000; Shankar 2000). The content (2–6%) in the western continental shelf of
samples analysed in this study were collected in Arabian Sea (Prakash Babu et al. 1999) has also
February, when coastal upwelling and primary enhanced the oxygen minimum nature of water
productivity were intensified. Thus, the most of the column.
metals enhanced sorption onto sinking particles. The oxygen deficiency in the bottom sediments
Several studies have provided evidence that these may lead to the dissolution of hydrous iron and
coastal processes can enhance trace metal con- manganese oxides and to a release of co-precipi-
centrations in coastal waters and sediments tated heavy metals (Forstner & Wittmann 1983).
(Bruland 1980; Jones & Murray 1984). It also follows that environment observed with
increase in temperature is possibly due to its
General features and correlations accelerated precipitation towards the coastal re-
gion, where the overlying waters were warmer and
In order to find the dependency of trace metal saturated. The impact of redox potential for Cd,
levels in surface sediments as a function of water Cu, Cr, Ni and Zn on temperature and dissolved
column depth/dissolved oxygen/temperature/iron oxygen is quite contrary, while on precipitation
 geochemical index of trace metals 515

700 35
100
600 30

80 25
500 r =0.34
r = 0.52
r =0.69
20 Co
------------------------Conc (µg/g)------------------------

400 60
r =0.62 Cr
r = 0.32 15
300 Fe r = 0.39
40 Ni
Mn
Zn 10
200 r = 0.22
20 Cu
5
100
Cd r =0.52
Pb r =0.34
0 0
0 16 18 20 22 24 26 28 30
16 18 20 22 24 26 28 30 16 18 20 22 24 26 28 30

-------------------------Temperature (˚C)--------------

35
700
100
30
600
80 25
500 r =0.68 r =0.52
r = 0.86

60 20 Co
400 r = 0.79
r = 0.56
Fe Cr r = 0.65 15
300 Mn 40
Ni r = 0.49
200 10
Zn
20 Cu
100 5
Cd r =0.50
Pb r =0.39
0 0 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

------------------------Dissolved Oxygen (ml/l)----------

Mn Cu Ni Cd Fe (102 µg/g)
Zn Co Pb Cr

Fig. 5. Correlation of trace metals in surficial sediments with temperature and dissolved oxygen (n = 12, and each reported value
is a weighted average of quadruplet sample analyses).

was the mechanism for their accumulation at low correlation between Fe and Mn is due to their
temperature and dissolved oxygen (Calvert & characteristics as scavenging elements (Lewis &
Perderson 1993; Marcus et al. 2003) for stations 4, Landing 1991). The geochemistry of sediments
8 & 12. The depth wise negative correlation shows that Fe and Mn form the principle lattice
(Figure 6) shows that trace metals burial progres- for adsorption of other elements due to their
sively decreased from nearshore to the shelf. The scavenging property, which leads accumulation by
uniform moderate correlation exhibited by ele- co-precipitation. Thus, the distribution and dige-
ments Cd, Cu, Cr, Ni and Zn probably indicates netic mobility of trace metals is largely controlled
their common sources. The compatibly high pho- by redox cycles of Mn and Fe. Many workers have
tosynthetic activity in the Arabian Sea than that of pointed out similar condition in other similar areas
the average of all other ocean often leads to a (Sullivan et al. 1997; Manford et al. 2001;
greater concentration of the dead cell and detritus Nameroff et al. 2002).
at the thermal discontinuity layer, which consume So the intense suboxic condition, change in wind
oxygen during decomposition (Sen Gupta et al. pattern and coastal upwellings are the major fac-
1976). tors which influence the biogeochemical cycling of
The bivarient analysis for different elements in trace metals in the surface sediments of west coast
the study showed their preferential burial mecha- of India.
nism in marine environment. Since Fe was the
most abundant element in the study region, its Geochemical normalisation
linear correlation with other elements varied in the
order Mn > Zn> Ni> Cu> Cr> Co > Cd > Pb Since the concentrations of metals from environ-
(Figure 6). It is evident that the high significant mental sediment samples depend on large number
516 c.m. laluraj and s.m nair

100 35
700
30
600 80
25
500
60 20
400
Co
Cr r =-0.50
Fe 15
300 40
Mn Ni
-----------------------Conc. ( µg/g)--------------------------

r = -0.69 10
200 Zn
r = -0.53
20 Cu
100 5
r = -0.60
r =-0.75 r = -0.42 Cd
Pb r = -0.45
r =-0.84 0
r = -0.38
0 0
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200

---------------------------------Depth (m)-----------------------------

35
700 100
r = 0.78 r = 0.77
600 30

r = 0.91 80
r = 0.87 25
500 Co
r = 0.83
60 Cr
400 r = 0.80 20

Mn
300 Ni 15
40 Zn
200 Cu 10
20
100 5
Cd r = 0.72
Pb r = 0.35
0 0
0 100 200 300 400 500 600 700 0
0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

-----------------------Conc. ( µg/g)--------------------------
Cd 2
Mn Cu Ni Fe (10 µg/g)

Zn Co Pb Cr

Fig. 6. Correlation of trace metals in surficial sediments with Fe content and water column depth (n = 12, and each reported
value is a weighted average of quadruplet sample analyses).

of factors, normalisation for metal contamination contaminations. Here, the procedure applied has
in surfacial sediments is very difficult and uncer- some change, instead of the global shale values, the
tainties are inherent to any methodology em- concentration of offshore values (depth  200 m)
ployed. The simplest and most frequently used from corresponding transect (stations 4, 8 & 12)
method of assessment of metal contamination is to has been taken for getting a more critical evalua-
compare the values with those of global shale of tion of the regional anomaly. The enrichment fac-
Turekian & Wedepohl (1961). The ratios however tor’s computed should be noted that many of these
suffers from errors due to mismatching of genesis values exceed the corresponding Global shale levels
of matrix, which play as a crucial role in metal (Table 3).

Table 3. Enrichment factor (EF) of metals analyseda in the sediments of western continental shelf of Arabian Sea.

Transect Fe Mn Cr Zn Cu Cd Ni Pb Co

1729¢39 11.93 5.23 3.38 2.88 2.46 2.20 1.65 1.51 1.45
10.41 3.36 1.94 2.87 2.34 1.78 1.53 1.46 1.43
4.32 2.42 0.84 0.39 0.41 0.79 0.84 1.18 1.11
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
1329¢52 4.11 3.81 1.85 4.50 2.68 1.09 2.42 1.22 1.18
2.30 2.57 1.07 1.27 1.56 0.94 1.01 1.12 1.15
1.78 1.69 0.79 0.94 0.90 0.92 0.96 1.00 1.06
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
1028¢45 5.19 2.05 1.82 1.50 1.86 1.22 1.45 3.96 1.84
4.46 1.99 1.64 1.50 1.26 1.15 1.38 2.12 1.32
2.06 1.91 0.86 0.65 0.59 0.75 0.97 1.64 1.06
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
a
Factors have been derived from the average values of quadruplet sample analyses.
 geochemical index of trace metals 517

Enrichment factor (EF) generally showed a high Haraldsson C, Westerlund S. 1988 Trace metals in the water
gradient accumulation from nearshore to shelf. An column of the Black Sea and Franvaren Fjord. Mar Chem 23,
417–424.
abnormality was observed in EF of Cd, Cu, Cr, Ni
Jones CJ, Murray JW. 1984 Nickel, cadmium, and copper in
and Zn, where they were unusually low (stations 3, the northeast Pacific off the coast of Washington. Limnol
7 & 11) compared to the farther stations (stations Oceanogr 29, 711–720.
4, 8 & 12) in all transects (Figure 7). As the normal Klinkhammer GP, Heggie DT, Graham DT. 1982 Metal
trend should give a lesser EF values at stations 4, 8 diagenesis in oxic marine sediments. Geochim Acta 39, 635–
647.
and 12, and since this was observed right through
Klomp R, pagee JAVan, Glas PCC. 1986 An integrated
the study area, there seems to be some mechanisms approach to analyze the North Sea ecosystem behaviour in
associated with low dissolved oxygen. It is widely relation to waste disposal In pp. 205–231, Kullenberg G.,
accepted that metal precipitation was favoured to Lewis BL, Landing WM. 1991 The biochemistry of manga-
elements exhibiting redoxpotential in oxygen de- nese and iron in the Black Sea. Deep Sea Res 38, 773–S 803.
Madhupratap M, Prasanna Kumar S, Bhattathiri PMA.,
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