chapter 4 seawater dynamics, seafloor and core...
TRANSCRIPT
116
CHAPTER 4
SEAWATER DYNAMICS, SEAFLOOR AND CORE
SEDIMENT CHARACTERISTICS AND THEIR SPATIAL
AND VERTICAL VARIATIONS ALONG THE
CONTINENTAL SHELF, EAST COAST TAMILNADU.
4.1 EAST COAST TAMILNADU CONTINENTAL SHELF
The continental slope along the East Indian coast from Chennai to
Nagapattinam, Tamilnadu, supports a unique marine environment (Pradhan
et al 2009) due to the incursions of the easterly flowing peninsular rivers such
as Palar, Uppanar, Ponnaiyar, Vellar and many distributaries of the Kaveri
river and depositing large volumes of sediments into the Bay of Bengal. The
region between Chennai and Nagapattinam receives both the monsoonal rains
(the SW and NE rains), the NE rains being dominant. In the near-shore
coastal marine waters and estuaries, the biogeochemical parameters exhibit
considerable seasonal variations depending on the local conditions, such as
rainfall, tidal incursions, various abiotic and biotic processes, and quantum of
fresh water inflow that affects the nutrient cycle of the coastal environment
(Choudhary and Panigrahy 1991). Subsequent to the changes in the current
patterns, alterations in the coastal water quality have been reported
(Somayajulu 1987, Ramaraju et al 1992, Babu 1992, Saravanane et al 2000).
Determination of the physico-chemical parameters of seawater (vertical and
horizontal sea surface water profile) such as pressure, temperature, pH,
117
conductivity, salinity, turbidity, oxygen, density, and nutrient composition
(such as nitrite, nitrate, ammonium and phosphate) thus becomes important
to assess the status of the near coastal environment for marine productivity
and food chain.
The sediment and the fresh water influx during the SW and NE
rains influence the coastal ecosystem and cause a substantial change in the
biogeochemistry along the continental slope along this coastal stretch. The
primary objective of this study is to quantify the distribution of
biogeochemical parameters spatially and through vertical profiles along the
continental slope amid Chennai and Nagapattinam to a vertical depth of
nearly 150 m. The study was also carried out to correlate nutrient distribution
with changing monsoon pattern along the coastal stretch, which is heavily
influenced by various land based inputs. In this chapter an integration of the
results and data generated is presented.
4.2 SEASONAL, SPATIAL AND DIURNAL VARIATION OF
CONTINENTAL SHELF IN SURFACE WATERS.
The biogeochemistry data generated on sea water collected
spatially and vertically reveals that the physico-chemical properties of sea
water are not homogenous; neither spatially nor vertically. During the
monsoon periods (NE and SW), the salinity, temperature, oxygen and
nutrients show a distinct variation increasingly with depth towards the deeper
sea water column following the continental slope along the East Coast. This is
probably due to the seasonal monsoon reversal of the wind which is a unique
feature of Indian Ocean that results in the consequent change of the
circulation pattern (Anonymous 1952, La Fond 1957, Wirtki 1973a).
118
4.2.1 Spatial Variation Of Physico-Chemical Parameters During Post
NE And SW Monsoon
The present study shows that physico-chemical parameters in SW
and Post NE monsoon periods varied accordingly with depth. Sea surface
water temperature along the continental shelf of East Coast Tamilnadu
showed slight marginal variations. Physical parameters indicate that the sea
surface temperature remained fairly constant, increasing slightly during the
Post NE monsoon period. The spatial surface water temperature marginally
showed an increasing value during the Post NE monsoon by ~ 2oC than
during the SW monsoon period. This slight increase in the temperature along
the surface waters is due to the influence by the intensity of solar radiation,
evaporation, freshwater influx and cooling and mix up with ebb and flow
from adjoining neritic waters (Ajithkumar et al 2006, Saravanakumar et al
2008). In the study conducted by Cromwell (1953) in the Pacific Ocean the
average temperature ranged between 26°C-29°C at the surface and (12-24oC)
at a depth of 150-200 m. Prasanna Kumar et al (2002) studied the water
masses between the Arabian Sea and the Bay of Bengal and suggested that the
surface water along the western shelf in the South were warmer and less
saline compared to the North.
Satpathy et al (2009) opined that the upwelling, a well known
phenomenon during SW monsoon period was also responsible for relatively
low temperature. With the change of current from north to south during the
post SW monsoon or post transition period, the depth of mixed layer increases
due to the massive mixing of riverine fresh water (559×109 m
3) from rivers of
northern Bay of Bengal, resulting in the low saline surfaced water exposed to
increased solar radiation as the cloud coverage reduced during October. This
explanation is strengthened by the fact that the heat potential in the Bay of
119
Bengal during the October (16 k cal/cm2) is relatively high as compared to the
SW monsoon period (10-18 k cal/ cm2); (Saduram et al 2004).
The salinity along the continental slope varies due to the influence
of freshwater inflow into the coastal waters. Salinity acts as a limiting factor
in the distribution of living organisms and its variation caused by dilution and
evaporation and these factors are most likely to influence the faunal
distribution of the coastal ecosystems (Gibson 1982). A marginal decrease in
the salinity in the southern stretch around Puducherry, Marakkanam,
Cudallore and Nagapattinam area is due to the inflow of freshwater from the
major perennial rivers. The spatial variation of the physical parameters during
post NE and SW monsoon periods along the continental shelf from Chennai
to Nagapattinam has been given in (Table 4.1) (Figure 4.1 and 4.2).
The productivity of phytoplankton causes an increase in the pH of
the surface water (Satpathy et al 2009). The pH values (7.3-8.1) during NE
and (7-10) SW monsoon period are due to the biological activities. The
observed insignificant variation in pH is attributed to the terrestrial runoff and
the precipitation during this period coupled with the extensive buffering
capacity of the seawater that causes the change of pH within a very narrow
limit (Riley and Chester 1971).
Oxygen plays a vital role in the chemistry and biology of coastal
waters, and its concentration is a major indicator of sea water quality. The
variation of dissolved oxygen in the coastal waters is a function of physico-
chemical properties of water that alters its solubility (Aston 1980) and also as
a result of photosynthesis and degradation of organic matter and re-aeration
(Granier et al 2000). All these factors cause variations in the physico-
chemical properties of seawater along the continental shelf.
120
Table 4.1 Spatial variations of salinity and temperature for post NE and
SW monsoon period along the continental shelf from Chennai to
Nagapattinam.
Samples ID. Location Depth (m) Latitude Longitude Salinity Temperature
Post Northeast (NE)
CH 1 Chennai 75 m N12 °10.90 E 80° 16.61 34.3 28.42
CH 2 Chennai 150 m N12°11’23.34 E 80°16’6417 34.2 27.73
MR 1 Marakkanam 75 m N12°14'.099'' E80°15'1978'' 33.8 29.41
MR 2 Marakkanam 150 m N12°14'2260'' E80°19'5779'' 33.9 29.55
PN 1 Puducherry 75 m N10 ° 39.2104 E 80 °07.5824 33.4 28.37
PN 2 Puducherry 150 m N10 ° 39.4100 E 080° 3.8161 34.5 28.25
CD 1 Cudallore 75 m N11 °47.1750 E 080 12.9400 34.0 24.17
CD 2 Cudallore 150 m N11 ° 45.0200 E 80° 23.4581 34.0 28.77
TR 1 Tharangampadi 75 m N13 ° 02.3800 E 80° 25.620 33.3 28.55
TR 2 Tharangampadi 150 m N11 ° 01.5104 E 80° 34.7843 33.4 28.56
NP 1 Nagapattinam 75 m N10°46'3796'' E80°05'4436'' 33.7 28.51
NP 2 Nagapattinam 150 m N10°46'709'' E80°10'356'' 33.7 28.39
Southwest (SW)
CH 1 Chennai 75 m N 13º04.3109’ E 80°29.0144’ 32.96 28.42
CH 2 Chennai 150 m N 13º04.1065 ’ E 80°29.0144’ 33.74 27.73
MR 1 Marakkanamn 75 m N 12º14.2260’ E 80°19.5779’ 34.39 29.38
MR 2 Marakkanamn 150 m N 12º20.24’ E 80°20.24’ 34.40 29.41
PN 1 Puducherry 50 m N 11º53.29’ E 79°58.8440’ 34.35 29.55
PN 2 Puducherry 75 m N 11º53.3915’ E 80°02.8763’ 34.52 27.98
PN 3 Puducherry 150 m N 11º52.83’ E 80°02.8763’ 34.47 28.37
CH 1 Chidambaram 50 m N11°19'6097'' E79°56'853'' 34.52 28.25
CM 2 Chidambaram 75 m N11°19'633'' E79°59'3924'' 34.64 28.68
CM 3 Chidambaram 150 m N11°20'0618'' E80°00'5289'' 34.54 28.86
TR 1 Tarangampadi 50 m N 11º31.009’ E 79°54.774’ 34.51 28.14
TR 2 Tarangampadi 75 m N 11º30.4950’ E 79°56.9264’ 34.40 28.45
TR 3 Tarangampadi 150 m N 11º29.9460’ E 79°58.2951’ 34.40 28.55
CD 1 Cudallore 50 m N 11º42.4196’ E 79°57.9927’ 34.59 28.56
CD 2 Cudallore 75 m N 11º42.3730’ E 79°59.0663’ 34.45 28.77
CD 3 Cudallore 150 m N 11º42.5241’ E 79°59.0663’ 34.27 24.17
NP 1 Nagapattinam 50 m N 10º 46.3796' E 80°05.4436’ 34.52 28.77
NP 2 Nagapattinam 75 m N 10º46.709’ E 80°10.356’ 34.67 28.51
NP 3 Nagapattinam 150 m N 10º46.256’ E 80°13.178’ 34.74 26.44
121
The influx of the river runoff into the sea causes a decrease in
the salinity and an increase in the solubility of dissolved oxygen concentration
during the SW monsoon period. At the same time, it is reasonable to assume
that, during the monsoon period, the riverine outflow brings with it an
increased load of organic matter which could deplete O2, with utilization
being greater than O2 produced by photosynthesis causing a hypoxic
condition (Kemp et al 2009).
Oxygen concentration was elevated towards the southern
continental slope covering a large area but showed a decline at Chennai,
Marakkanam, and Puducherry areas during the Post NE period. However, an
increase in the oxygen concentration was observed in the SW monsoon period
in all the stations. A distinct positive correlation between precipitation and
oxygen concentration and a negative correlation with salinity was obvious.
This is due to the cumulative effect of higher wind velocity coupled with
heavy rainfall and the resultant freshwater mixing as suggested by Mitra et al
(1996).
Figure 4.1 Average spatial distribution of Temperature, salinity and
DO at 75m and 150 m depths along the continental shelf
from Chennai to Nagapattinam during post NE monsoon.
CH 1 CH 2 MR 1MR 2 PN 1 PN 2 CD 1 CD 2 TR 1 TR 2 NP 1 NP 2
0
5
10
15
20
25
30
35
Concentr
ations
Stations
TemperatureNE
SalinityNE
Dissoved oxygen (DO) NE
122
Figure 4.2 Average spatial distribution of Temperature, salinity and
DO at 75m and 150 m depths along the continental shelf
from Chennai to Nagapattinam during SW monsoon.
4.2.2 Vertical Distribution Of Physico-Chemical Parameters
In the present study, it is observed that the thermocline (Figures 3.8
- 3.11) becomes thicker during the SW monsoon period which could be
probably due to the vertical mixing of fresh waters from the hinterland and
also due to the upwelling processes. The presence of halocline during was
observed between 40 m and 70 m during both the monsoon periods. The
average vertical distribution of temperature and salinity during the post NE
and SW monsoon periods is shown the (Figure 4.3). The vertical profiles of
the physical parameters shows that the mixed layer depth (MLD) exists
between 20 m and 60 m seawater column depth during during both the
monsoon periods. The dynamic variations could be probably due to the
complex interaction between physical and biogeochemical processes
involving riverine and fresh water input, vertical and horizontal seawater
CH 1
CH 2
MR 1
MR 2
PN 1
PN 2
PN 3
CM
1CM
2CH 3
TR 1
TR 2
TR 3
CD 1
CD 2
CD 3
NP 1
NP 2
0
5
10
15
20
25
30
35
Stations
TemperatureSW
DOSW
SalinitySW
Co
ncentr
atio
n
123
mixing, air water exchange, photosynthesis, sediment and seawater column
physico-chemical demand.
Figure 4.3 Average vertical distributions of salinity and temperature
during post NE and SW monsoon periods.
4.2.3 Distribution Of Nutrients And Oxygen
Distribution of nutrients is mainly based on the monsoonal
variations, tidal conditions and terrestrial freshwater inflow (Kumar et al
2006). The surface concentration of nutrients (Nitrite, Nitrate, Ammonium,
and Phosphate) showed a significant variation during the post NE and SW
monsoon period. The N: P ratio reflects the productivity of nutrients that are
abundant during the SW monsoon in the southern region of the coast, which is
not reflected in the northern region along the Tamilnadu coast. Freshwater
input, land runoff and horizontal advection of deep water masses are
responsible for the higher productivity in the southern region of the East
160
140
120
100
80
60
40
20
0
26 27 28 29 33 34 35
Concentration
De
pth
(m
)
SalinityNE
SalinitySW
TemperatureSW
TemperatureNE
124
Coast continental shelf. Along the East Coast, high N:P ratio reflects high
quantity of nitrogen produced as a result of the existence of chemical
industries, and intense urbanisation near Chennai that have resulted in lower
concentration of phytoplankton during both the SW and post NE monsoons.
However, an increase in the nutrients concentration was found during the SW
monsoon period.
Several studies (Gilbert et al 1982, Choudhary and Panigrahy 1991)
carried out on seawater environments have established that the growth of
phytoplankton is controlled to a large extreme by the limitation of nutrients.
The increased concentration of phosphate and nitrate values recorded during
the SW monsoon period may be attributed due to heavy rainfall, land runoff,
its autochthonous origin and weathering of rocks liberating soluble alkali
metal phosphates, the bulk of which are carried into the continental shelf
waters (Das et al 1997, Gowda et al 2009). Another possible way of nitrate
entry is through oxidation of ammonia form of nitrogen to nitrite and then
consequently to nitrate (Rajasegar 2010).
The low values recorded during post the NE monsoon period may
be due to the utilization by phytoplankton as evidenced by high
photosynthetic activity and the dominance of neritic seawater having
negligible amount of nitrate (Das et al 1997). The higher value of nitrite
recorded during the SW monsoon season may be due to variation in
phytoplankton excretion, oxidation of ammonia and reduction of nitrate and
by recycling of nitrogen and bacterial decomposition of planktonic detritus
present in the environment (Govindasamy et al 2000) and also due to
denitrification and air-sea interaction exchange of chemicals (Mathew and
Pillai 1990). An increase in the nutrients concentration and dissolved oxygen
values at each station has provided clues to the understanding of productivity
change along East Coast of Tamilnadu.
125
Oxygen plays a vital role in the chemistry and biology of coastal
waters, and its concentration is a major indicator of water quality. It is
observed from the data that the physico-chemical properties of the sea water
are not homogenous either horizontally or vertically, and it changes
increasingly with depth towards the deeper sea water column and continental
slope during both SW monsoon and post NE monsoon periods.
An appreciable hypoxic condition exists at a depth between 25 m-
80 m which acts a photic zone along the East Coast continental shelf. A
shallow hypoxic condition at 25 m - 35 m during the post NE monsoon period
reveals substantial river runoff with less vertical mixing with the deeper
seawater layers. Hypoxic conditions observed during both the monsoon
periods shows that though there exist riverine influx into the continental shelf
region, stratification of the seawater column with the fresh water lens and the
pollutants causes a decrease in vertical mixing of seawater body causing the
restriction of oxygen supply to the deeper water column levels (Nixon et al
1996).
4.2.4 Isotopic Variations In The Seawater Along The Continental
Shelf With A Reference To 18
O -S Relationship.
Many investigators (Deshpande et al 2012, Achyuthan et al 2013
and references therein) studied the correlation of 18
O and salinity in surface
as well as deep seawaters of Bay of Bengal and inferred that the 18
O-S
relationship is dependent on the physico-chemical properties of seawater,
precipitation, river influx, upwelling and mixing of different water masses
(Bigg and Rohling 2000, Ferronsky and Brezgunov 1989, Singh et al 2010,
Achyuthan et al 2013). Since 18
O and salinity vary spatially and temporally,
their relationship provides useful informations about operating the physical
processes (Benway and Mix 2004).
126
Figure 4.4a-n 18
O-S trend and the variations in the R2 values during
the post NE as well as SW monsoon periods.
(a) (b)
(d)(c)
(e)(f)
CH1 SW monsoon
127
Figure 4.4a-n (Continued)
(k) (l)
(J)(i)
(g) (h)
128
Figure 4.4a-n (Continued)
Table 4.2 Variations in the R2 values during the post NE as well as SW
monsoon periods
Sl.NoPost Northeast (NE)
and Southwest (SW)Sample ID Station R
2 value
1 Post Northeast (NE) CH1 Chennai 0.26
2 Southwest (SW) CH2 Chennai 0.26
3 Post Northeast (NE) MR1 Marakkanam 0.96
4 Southwest (SW) MR2 Marakkanam 0.68
5 Post Northeast (NE) PN1 Puducherry 0.22
6 Southwest (SW) PN2 Puducherry 0.47
7 Post Northeast (NE) CD1 Cudallore 0.70
8 Southwest (SW) CD2 Cudallore 0.10
9 Post Northeast (NE) TN1 Tarangampadi 0.67
10 Southwest (SW) TN2 Tarangampadi 0.25
11 Post Northeast (NE) CM1 Chidamparam 0.57
12 Southwest (SW) CM2 Chidamparam 0.16
13 Post Northeast (NE) NP1 Nagapattinam 0.57
14 Southwest (SW) NP2 Nagapattinam 0.80
The spatio-temporal variations in 18
O and salinity data of the
seawater samples along the continental shelf has been studied to understand
the influence of monsoonal (post NE and SW) precipitation and evaporation
(m) (n)
129
patterns as these parameters are affected by the fractionation and mixing
processes. The low values are observed during the SW monsoon and a mild
increase during the post NE period. Vertical distribution of 18
O and salinity
follows a general trend up to 20 m water column depth and then fluctuates
towards the deeper seawater levels and this is due to the mixing of fresh water
and occurrence of thin fresh water lens following the continental slope.
A 18
O-Salinity (S) relationship during the post NE as well as SW
monsoon periods is presented in Figure 4.4a-n. The 18
O-S relationship in
different parts of the East Coast continental shelf indicates a multi-end
dynamic system the fresh water-saltwater mixing processes. From Figure
4.4a-n and Table 4.2, it is noted that in all the stations R2 value are higher for
the post NE monsoon than the SW monsoon period. R2 Value was found to be
high at Marakkanam (R2= 0.96) during the post NE monsoon and low at
Cudallore (0.10) during the SW monsoon period. While there is no
relationship observed in Chennai. The high and low values of the regression
slope in all the stations reveal strong influence of precipitation over the East
Coast Tamilnadu. The decrease in the regression slope value at various
stations are due to the seawater mixing processes, industrial influxes and
anthropogenic influence of adding water to the sea causing fractionation in
18O isotope (Achyuthan et al 2013).
Chidambaram et al (2009) studied the stable isotopic signatures in
precipitation during SW monsoon of Tamilnadu coast and inferred that the
surface water of Bay of Bengal along the Tamilnadu coast is lighter in 18
O
due to the large influx of surface run-off from the surrounding continental
area. This influx of freshwater into the BOB reduces the salinity and enriches
the lighter isotope concentration. Overall in the present study, during both the
monsoon periods there exist a fair 18
O-S relationship through the vertical
seawater columns across the continental shelf East Coast Tamilnadu.
130
One of the possible mechanisms to account for the higher 18
O
offset along the East Coast includes the contribution from the atmospheric
vapour which gets depleted due to rainout over the land area and low 18
O of
the surface water of the BOB during the SW monsoon period (Gupta et al
2005). Post NE monsoon is a transitional period from winter to summer
monsoon and oceanic currents are feeble. During this monsoon period, water
discharges from both the peninsular rivers are low. At the same time the air
temperatures and SST progressively rise (Shenoi et al 2002) and evaporation
from the BOB becomes high leading to enrichment of both salinity and 18
O
in the entire BOB. This is consistent with the highest values of both salinity
and18
O observed during this monsoon when compared to other monsoons
(Achyuthan et al 2013). The lower values of 18
O and salinity in the surface
waters along the East Coast continental shelf is a clear indication of enhanced
contribution from the easterly flowing rivers and distributaries
(Achyuthan et al 2013).
4.3 SEDIMENT BIOGEOCHEMISTRY
4.3.1 Textural Sediment Quality Characteristics
Silty sand dominates in the East Coast continental shelf region
during both the monsoon periods with an average sand content of 65.44%
during the post NE and 80.13% during the SW monsoon period. The inflow
of the riverine input has changed the sediment nature during both the
monsoon periods. The higher sand content in the sediments indicates a higher
wave energy regime due to the local bathymetry that prevents sedimentation
of fine grained particles (Satpathy et al 2010). The increase in the sand with
minor amounts of clay content as observed in several stations during both the
monsoon periods could be due to the influence of littoral drift and the current
from the abrasion zones that transport finer sediments once they reach the
coastal zone (Szefer and Skwarzec1988).
131
The difference in the textural parameters indicate that they are
mainly dependent on the dynamic process that affects the shallow region in
the study area rather than the deeper region which acts in an opposite way
during the two different monsoon periods (Gopalakrishna and Sastry 1985,
Yu et al 1991). In addition, the distribution pattern also indicates that the finer
fractions are rapidly transported to the deeper regions due to the active coastal
currents acting in this region (Pichaimani et al 2008). The increase in sand
content along the continental shelf during both the monsoons indicates that
the currents from the abrasion zone transports the finer sediments once they
reach the coastal zone (Szefer and Skwarzec1988). Average distribution and
variation in the textural parameters for post NE and SW monsoon periods at
various seawater column depths along the East Coast continental shelf is
shown in the Figure (4.4a-b).
4.3.2 Distribution of CaCO3 and OM
Calcium carbonate content was observed to be higher in the central
sites along the continental slope during the SW monsoon period. At few
stations such as Chennai, Cudallore and Tarangampadi, the accumulation of
CaCO3 was comparatively higher. Relatively moderate values of carbonate at
few depths may be due to the sturdy current leading to the non-deposition of
terrigenous material (Rao 1978). The high CaCO3 content is of biogenic
origin and the input may also be from the adjacent land mass where Tertiary
limestone and calcareous sandstones extend in the southern region (Szefer
et al 1988, Rao 1978, Ray 1990, Armstrong Altrin Sam 1998). Sea surface
samples off Puducherry coast reveal low CaCO3 content (1.6 to 1.8 %) and are
attributed to the fine nature of sediments that are not of biogenic origin.
132
Figure 4.5 a-b Distribution of textural parameters at various seawater
column depths along the East Coast continental shelf
from Chennai to Nagapattinam, Tamilnadu.
Generally, there is no preferential trend in the distribution of
CaCO3 and OM content at these depths (50 m, 75 m and 150 m) in sea surface
sediments. The low and high alternate carbonate values observed during both
the monsoons with alternate stations attributed with increase in sand could be
due to the waste water influx from various coastal industries, which brings the
noncarbonated materials, and enhances the formation of low and high
alternate values (Rao 1978). Relatively moderate values of carbonates at few
depths may be due to the sturdy currents leading to non-deposition of
terrigenous materials (Rao 1978).
The organic matter content was found to be lower in the southern
regions and it exhibited a higher concentration in the northern regions
including Chennai during the SW monsoon period. The surfer diagrams show
that the organic matter and CaCO3 are inversely correlated. The variations in
the organic matter values in all the stations indicates that they are moderately
influenced by the minor input and deposition of organic debris from local
industrial sources. This is due to a higher terrestrial and anthropogenic input
(b)
CH
1C
H 2
MR
1M
R 2
PN 1
PN 2
PN 3
CH
1C
M 2
CM
3TR 1TR
2TR
3C
D 1C
D 2C
D 3
NP 1
NP 2
NP 3
0
10
20
30
40
50
60
70
80
90
100
Co
nc
en
tra
tio
n
Stations
Sand % SW
Mud % SW
CH
1C
H 2M
R 1M
R 2
PN 1PN 2PN 3C
H 1
CM
2C
M 3
TR 1
TR 2
0
10
20
30
40
50
60
70
80
90
100
Concentartion (%)
Co
nc
en
tart
ion
(%
)
Sand % NE
Mud % NE
Stations
(a)
133
from the minor rivers that drain into the Bay of Bengal (Pichamani 2005). The
average distribution and concentration of CaCO3% and OM % content in the
seafloor sediment samples during both the monsoon periods are presented in
the Figure 4.6a-b.
Figure 4.6a-b average distribution and concentration of CaCO3% and
OM % content in the seafloor sediment samples during
both the monsoon periods
4.3.3 Geochemistry And The Spatial Distribution Of Major Oxides
And Trace Metals
The spatial distribution of SiO2 in sea surface sediments reveals
that the detritus quartz is not evenly distributed along the continental shelf
(Figure 3.46a-l) and this is probably due to the long shore drift current and the
wave energy. The distribution pattern shows that except CaO and MgO, other
oxides exhibit a higher concentration towards the middle region along the
continental shelf. The depletion of CaO in seafloor sediments during the SW
monsoon period, compared to the PAAS values reflects strong weathering,
recycling of sediments and their removal during transportation (Nesbitt et al
l982, Condie l993). Low content of SiO2, MnO, MgO, K2O, and P2O5
concentrations in the sea floor sediments with higher percentage of TiO2 and
CH 1
CH 2
MR 1
MR 2
PN 1
PN 2
CD 1
CD 2
TR 1
TR 2
NP 1
NP 2 --
0
2
4
6
8
10
12
14
16
18
Co
ncen
trati
on
%
Stations
Organic matter NE
CaCO3 NE
CH 1
CH 2
MR 1
MR 2
PN 1
PN 2
PN 3
CH 1
CM
2C
M 3
TR 1
TR 2
TR 3
CD 1
CD 2
CD 3
NP 1
NP
2NP
3
0
1
2
3
4
5
6
7
8
9
10
Co
nc
en
tra
tio
n %
Stations
Organic matter SW
CaCO3 SW
Figure 4.6a Figure 4.6b
134
Fe2O3 when compared with PAAS and UCC values during the post NE
monsoon period (Taylor and Mclennan, l985) (Table 3.2) indicates
terrigeneous influx of a variety of differentailly weathered bedrock source of
sediment detritus and a strong chemical weathering.
Metals like Cr, Cu and Ni display quiet similar pattern of
distribution and these elements are used as markers of metal industry
contribution (Kumar et al 2001). Elevated values of Cu is exhibited at
Tarangampadi and Chennai (41.9ppm, 41.5ppm) indicating anthropogenic
origin, and controlled by the sediments, organic matter and grain size
(Harbison et al 1984). Co, Pb and Zn are elements with similar distribution
pattern are known as markers of paint industry (Lin et al 2002). The striking
feature of the trace metal concentration in the study area indicating the higher
concentration (especially at stations 1, 4, 5 and 6) is due to the anthropogenic
activities and minor inputs of rivers in the study area. Moreover, it is also due
to the recent development of major industries (in the coastal areas and
offshore drilling) and minor harbour activities where heavy movement of
naval vessels throughout the year for regular surveillance in the coastal region
(Hershelman et al 1981, Luoma et al 1988, Fukushima et al 1992, Jonathan
and Ram Mohan 2004). In addition, many distributaries of Cauvery drain in to
continental shelf through the agricultural belts in the Cauvery delta region
(Pichamani 2005). These inferences are very well supported by the high
values of Cr, Pb, Zn, Cu, which are all components of the fertilizers used in
agricultural activities. The geochemical elements delivered in the continental
slopes are not only anthropogenic but also reflect natural flux of elements
from the catchment areas (Sundararajan and Natesan 2010).
The average concentration of trace metals (except Mn) shows
higher content at Chennai and Nagapatinam during the post NE and SW
monsoon periods (Figure 3.47 a-n). It is well established that iron and
135
manganese are excellent scavengers for trace metals (Tessier et al 1979). This
would lead the co-precipitation of other metals in the seafloor sediments and
so increase the concentration of many metals in sediments (Muthuraj and
Jayaprakash 2008). The higher metal concentration found in seafloor
sediments during both the monsoon periods is due to the differences in
hydrodynamics, churning, bioturbation and navigation activities. During the
post NE monsoon period, low concentration of Cr and Cu suggest that the
continental shelf environment is less affected by the industrial, domestic
wastes and sewage wastes (El Nemr et al 2006).
4.3.4 Correlation matrix
The results of the correlation matrix (R2) are represented in the
(Table 3.4 and 3.5). Most of the minor metals such as Cr, Co, Ni, Co exhibit
mutual coherency and also show strong affinity towards Fe and Mn (R > +8).
Significant fraction of trace metals are found co-precipitated with or adsorbed
on to Fe and Mn geochemical phases controlling the trace metals in the
sediments. These characters are due to the large area, extensive cation
exchange capacity and wide spread availability (Szefer et al 1988).
Correlation coefficient between metal concentration shows that all the metals
correlate significantly with other parameters except sand, mud and CaCO3
duirng both monsoon periods. Strong correlation relationship of Cr with
Al(r=0.86), Cu with Ni (r=0.9), Co with Pb (r=0.6) and Mn with Al(r=0.86)
exhibits their similarity and existence between them and localized
contamination during SW monsoon period (Turner 2000). Fe has strong
positive correlation with all the other metals (r=0.5 to 0.8). Significant
correlation of Fe with Mn, Ni, Zn, Co, Cr (0.59-0.9) for both the monsoon
periods suggests important association between oxide-Oxyhydroxides of
Fe-Mn and other elements (Praysers et al 1991). Notably, the correlation and
geochemical association of trace metals reveal a significant source of
136
contamination reflecting a common origin of similar nature existing probably
from the industrial effluents (Turner 2000).Similar observations were also put
forward by Muthu Raj and Jayaprakash (2008).
4.3.5 Geochemical Normalisation And Enrichment Factor (EF)
In an attempt to compensate for the natural variability of major and
trace elements in the sediments, normalisation of the major elements using Al
was carried out to detect and quantify the anthropogenic metal contributions.
Aluminium remains the most widely and successfully used element for
normalisation although recent studies have shown a significant seawater
scavenged component of Al in marine sediments. It also represents the
quantity of alumina silicates, which are the most important carrier phase of
adsorbed metals (Chen et al 2007). Loring (1991) indicated that the natural
mineralogical and granular variability is best compensated by the
geochemical normalisation of major and trace metal data. Therefore in the
present study, normalisation was done with Al content to calculate enrichment
factors (EF) which is calculated by using metal/Al values of upper continental
crust. Element /Al ratio has been graphically represented (Figure 3.48a-n).
The higher concentration of Ca/Al in the continental shelf is mainly
due to the higher CaCO3 content and calcium-rich shell fragments
(Sundararajan and Natesan 2010). According to Zeller and Ray (1956), the
high Ca/Al ratio could be due to the precipitation and extraction of CaCO3
separated by the organisms in warm water in the tropical regions (Zeller et al
1956). This is well supported by the high Ca/Al ratio in the present study. The
abundance of major elements and their ratios relative to Al is found to be
higher at Chennai and Cudallore during SW monsoon period. Except Ti/Al all
other estimated values of almost all elements during the SW monsoon period
were high at Chennai (Figure 3.48b, d, f, h, j, l and n). Fe/Al ratio showed a
maximum of 5.81% at Tarangampadi. Higher Ti/Al along Chidamparam and
137
Tarangampadi indicates major transition to dominance of carbonate dilution
over insoluble residue. Relatively low P/Al ratio suggest that organic
contribution of P in the sediment samples is of minor importance and that
detrital phases mainly control the P content (Sundararajan and Natesan 2010).
The major concentrations of Al normalised metals can be summarised in the
decreasing order as Ca>Si>Mg>Fe>Na>K>Ti>P.
The Enrichment Factor (EF) is a useful indicator reflecting the
status of environmental contamination, which differentiates metals originating
from human activities or from natural weathering. In the present study the
enrichment of the trace metals over the crustal values is in the following order
Zn>Pb>Cr>Ni>Co>Cu>Mn for the post NE monsoon period and
Pb>Cr>Co>Zn>Ni>Cu>Mn during SW monsoon period indicating the high
values are due to anthropogenic activities (Pichamani 2005). The high
proportion of Pb could be due to the contamination by off shore drilling,
atmospheric deposition of finer particles, domestic effluent discharges and use
of paints (Pichamani 2005). It could be also due to the atmospheric deposition
of Pb from atomic power plant operation and other industries and also from
constant movement of fishing boats along the study area (Huntzicker et al
1975, Chatterjee et al 2007, Stephen-Pichaimani et al 2008).
Higher Cr, Co, Zn could be due to the corrosion of iron and chrome
alloy which are used by a number of industries (Danielsson et al 1980,
Ford et al 1990, Jonathan et al 2004). High values of Co could be due to the
steel industry discharges and harbour activities (Jonathan et al 2003).
Enrichment of Cu could be from different non-point sources. Rainfall can
wash the Cu from impervious surfaces (combustion of diesel, and lubricating
oils) directly into riverine systems which are ultimately carried to the
continental shelf.
138
Trace metal contamination in coastal sediment has been associated
not only with effects on benthic community but also with effects on water
column species. The trace metal accumulation in the continental shelves is
primarily controlled by the pollutant sources and their discharge into the
drainage basin of the rivers. East Coast Tamilnadu continental shelf has a
high energy dynamic system with comparatively high pollution input but does
not show higher trace metal concentrations and this could be due to high
discharge rate of water and considerable amount of dilution in the system.
Increase in the abundance of studied trace metals in both the systems
indicated a general association with sediment-seawater interaction, major
elements and organic matter. Municipal waste, agricultural run-off, aqua-
cultural waste, frequent dredging of navigation channels, port activities,
mechanized boat movements and fishing activities are the present day
potential sources of trace metals. Monsoonal variations cause the variations in
the concentration of sediment geochemistry along the continental shelf.
4.3.6 Phytolith Distribution in the Seafloor Sediments
From the Phytolith study conducted on the seafloor samples
collected during post NE and SW monsoon periods, it is clearly evident that
there was a substantial change in the concentration of Phytolith and
microfossil concentration during both the monsoon periods (Figure 4.7a-v).
The very reason for the variation in the distribution of phytoliths is the wash
of vegetation from the outflow of rivers into the continental shelf regions.
Here it is noted that the post NE monsoon samples had a less concentration of
Phytolith morphotyes as compared to the SW monsoon Phytolith types.
Diatoms were present in post NE and SW monsoon seafloor sediment
samples while Sponge spicules were observed only in SW monsoon samples
specifically in (SW9) Trangampadi at 150 m seawater column depth, (SW10),
Cudallore at 75 m seawater column depth and (SW11) Cudallore 150 m
depth.
139
Silica, SiO2, in dissolved and particulate form, is both a major
product of continental weathering. The study estimates of the spatial
distribution of riverine silica fluxes under natural conditions, to segments of
the coastal zone. It may be noted that some part of the phytoliths is not
recycled and may be eroded, as the soil organic matter, and carried by rivers.
Freshwater diatoms, living and as detritus, are also considered as biogenic
particulate silica. All silica-containing components, including phytoliths are
part of the particulate silica being carried by the easterly flowing rivers.
Each locality yielded abundant and distinctive Phytolith
assemblages. It should be noted that Cyperaceae, produce primarily dumbbell,
small cross, and Cyperaceae phytoliths. The Pooideae subfamilies of grasses,
which are generally common along the maritime forests, produce Elongate
phytoliths and long saddle phytoliths.
The retention of silica in river systems is linked to their tropic state.
Besides riverine inputs, atmospheric deposition of phytoliths contributes to
the surface oceanic system. Phytolith analysis of the grasses from different
coastal vegetation communities shows significant variation in shapes and
assemblages.This study also demonstrates that the coastal environments
produce distinctive phytolith assemblages that can be used as a supplementary
tool in coastal paleoenvironmental reconstructions.
4.4 SEDIMENT CORE OFF CUDALLORE
4.4.1 Sediment Textural Characteristics And Deposition
The sediments of the continental shelf, East Coast Tamilnadu, Bay
of Bengal, consists of calcareous sand and gravels, derived from coral /algal
reefs and benthic macro fauna and other terrigenous detritus depending on the
local sites from where the sediment core was retrieved (Abbas et al 2009).
140
Figure 4.7a-v Photomicrographs of phytoliths for the sediment samples along
the East Coast Tamilnadu (Plane light photomicrographs of
different types of phytoliths, scale= 10µm)
141
Figure 4.7a-v (Continued)
142
Figure 4.7a-v (Continued)
The sediment core reveals a heterogeneous mixture of detritus mainly
composed of quartz sand, biogenic carbonate and shell fragments. The
relative abundance of sand, silt and clay content with the sediment type
inferred is given in the trilinear diagram (Figure 4.8). Consequently, the
increase in coarse sediments along the shelf indicates a flux of fresh water
input with coarser particles that settle to the bottom when current and wind
speed reduced (Thomson Becker and Luoma 1985). Occurrence of higher
percentages of very fine silt and clay corresponding to lower sand content
throughout the core probably indicates less fresh water influx into the
continental shelf which facilitated the deposition of suspended fine grained
particles during those periods (Selvaraj et al 2003). The increased percentage
of sand in the surface indicates more fresh water runoff from the rivers in the
late Holocene and thus reflecting the intensification of the Northeast
monsoonal rains. Analyses of the sediment core supported by parameters such
as organic matter, CaCO3 content and radiocarbon dates suggest a late
Holocene NE monsoon high precipitation and slow sedimentation.
143
Figure 4.8 Trilinear diagram showing the core sediment texture
4.4.2 Distribution Of CaCO3 And OM In Cudallore Core Sediments
An accurate explanation of marine sediment records demand the
processes controlling the preservation of CaCO3 (Milliman et al 1999).
Calcium carbonate from the shelf sediments are often derived as carbonate
materials and particulate matter from adjacent landmass and through
inorganic and organic precipitation from water column (Sundararajan and
Natesan 2009) and the major sources of carbonate in the sediments from the
continental shelf is mainly from the shell fragments of organisms, molluscs
and due to dilution of biogenic calcite by detrital material in the sediments
(Sundararajan and Natesan 2009).
144
Similar observations were made by Sebastian et al (1990), in their
study on sediments of Mahe estuary, West Coast of India. The association of
sand particles with CaCO3% indicates a major contribution of shell fragments
in the sand fraction. Variation in organic matter content has been attributed to
changes in either the productivity or preservation or both (Paropkari et al
1987, 1992, 1993). Low content of carbonate in the surface sediments is due
to the processes of dissolution (Oenema et al 1988).
The oxidation of organic matter and release of metabolic acids
which are probably neutralized by the dissolution of sedimentary CaCO3
produces CO32-
(Martin and Sayles 1996) have estimated that 40-60% of
CaCO3 dissolution is linked to organic matter oxidation. Few peak
concentration in middle of the core is probably due to the reprecipitation of
carbonates in the reduced sediment layers due to increase in alkalinity which
may be generated, in general, by sulphate reduction at these depths (Oenema
et al 1988, Gaillard et al 1989).
The distribution of the OM increases up to 25 cm and then
decreases till 40 cm and further shows a steady profile in the down core layers
without any prominent peaks. The concentration of CaCO3 in the core
sediments are inverse of OM % and Fe which is due to the precipitation of
carbonate through the oxic layers of water column. The higher concentrations
of OM in top layers reveal the adsorption and incorporation of organic
materials from the overlying polluted water column (Selvaraj et al 2003).
Moreover, OM values above 1% suggest the adsorption of OM by fine
fraction of sediments (Selvaraj et al 2003). Higher concentration of OM
indicates a high productivity during the late Holocene (depth 6-10cm). Our
observations also indicate that organic matter distribution is influenced by
various complex oceanographic and geologic processes rather than surface
productivity alone. The down core variation of the textural parameters and the
biogenic parameters of the Cudallore core sediments is given in the
Figure 3.53.
145
4.4.3 Major Oxide, Major And Trace Elements Distribution In
Cudallore Core Sediments.
Large amounts of trace metals are bound in the fine-grained
fraction (< 63 µm) of the sediment, mainly because of its high surface area-to-
grain size ratio and humic substance content (Horowitz and Elrick 1987,
Moore et al 1989). The metals in the fine-grain fraction are more likely to be
biologically available than those in the bulk sediments (Bryan and Langston
1992, Everaart and Fischer 1992). The major elements (Si, Al, Na, K, Ca, Mg
and P) in the sediment core samples off shore Cudallore, Bay of Bengal, are
compared with the values from other regions of the Bay of Bengal, Upper
Continental Crust (UCC) and Post-Archean Australian Shale (PAAS) (Taylor
and Mclennan 1985) and the North American Shale Composite (NASC)
(Gromet et al 1984) (Table 4.3). Chemical composition is dependent on
quartz and clay fraction in the sediments related to grain size, with Al2O3
increasing towards the finer sediments (clays) and SiO2 towards sand. This
strongly reflects the mineralogical composition, in which the clay
concentration dominates the finer fraction, and quartz and feldspar, the
coarser fractions (Das and Birgit 2003). In the (Figure 4.9) the moderate trend
of Fe2O3, K2O, MgO, MnO, SiO2 can be observed against Al2O3 with
variations which is due to the clay minerals and grain size differences. CaO,
Na2O, P2O and TiO2 showed a negative trend against Al2O3. The Positive
trend of Fe2O3 against Al2O3 reflects that the sediments are of terrigenous
origin. All the major elements show lower concentrations when compared to
UCC, PAAS and NASC (Table 4.3) that further substantiates the coarse
grained, detrital nature of the sediments. This is supported by the observations
of Rao and Sarma (1993) who concluded that the geochemistry of sediments
in the Bay of Bengal is dominated by the grain texture irrespective of their
origin, which are mostly detritus in nature.
146
Table 4.3 A comparison of major and trace element composition of
sediment core from Bay of Bengal with PAAS, UCC and NASC.
ElementAverage
Core valuePAAS UCC NASC
SiO2 % 48.8 62.8 66.00 64.8
TiO2 % 0.99 1.0 0.5 0.78
Al2O3 % 15.13 18.9 15.2 16.90
Fe2O3 % 7.84 7.23 5.00 6.33
MnO % 0.09 0.11 0.08 0.06
MgO % 3.51 2.20 2.20 2.85
CaO% 3.89 1.30 4.20 3.56
Na2O % 3.76 1.20 3.90 1.15
K2O % 1.81 3.70 3.40 3.99
P2O5 % 0.13 0.16 ----- 00 0.11
Sr ppm 304.09 200.00 350.00 142.00
Y ppm 23.80 27.00 22.00 35.00
Zr ppm 250.14 210.00 190.00 200.00
Cr ppm 152.77 110.00 35.00 125.00
Ni ppm 86.09 55.00 20.00 58.00
Cu ppm 39.99 50.00 25.00 ------
Zn ppm 77.82 85.00 71.00 ------
Pb ppm 18.37 20.00 00 20.00 -------
Th ppm 9.68 14.60 10.70 12.00
U ppm 2.53 3.10 2.80 2.66
Ba ppm 339.50 650.0 -------- 636.00
Si % 22.81 62.80 66.00 64.80
Al% 8.02 18.90 15.20 16.90
Fe% 6.10 7.22 5 5.65
Na% 2.79 1.20 3.90 1.14
K% 1.50 3.70 3.40 3.97
Ca% 2.78 1.30 4.20 3.63
Mg% 2.12 2.20 2.20 2.86
P % 0.06 0.16 ------- 0.13
CIA 56.47 75.30 56.92 65.91
CIW 60.74 88.32 65.24 77.99
147
Figure 4.9 Down core variation of major and trace element composition
4.4.4 Provenance Of Chemical Elements In Cudallore Core
Sediments
Si forms one of the major elements of rock forming minerals and its
association is usually found in silicate minerals such as quartz, feldspars, mica
and clay minerals. Al is often associated with the broad classification of
minerals called alumino silicates which includes the common sedimentary
minerals phyllo-silicates, feldspars and amphiboles. Buckley and Cranston
(1991) inferred that the variation of Al in sediments is associated with
changes in the clay mineral content or with the content of feldspars in the
sediment. The increase in the alumina content could be due to the contribution
from adjacent rivers in addition to the precipitation of their dissolved solids in
the marine environment by coagulation in the coastal areas that which is
common (Coonley et al 1971, Holiday and Liss 1976, Sholkovitz 1976, Boyle
et al 1977). Ferro-magnesium minerals are the main contributors for Fe and
Al, which has been derived from the continental margin (Sundararajan and
Srinivasalu 2010) and is attributed from detrital marginal grains supplied
through adjacent rivers.
148
In the present study Cu content is high, and is mainly of the
anthropogenic origin, which is probably controlled by the sedimentary
features such as organic matter and grain size. Ca is normally present in
silicate minerals at very low levels and its contribution to sediment
composition from detrital silicate minerals is relatively minor. The minor
source of CaO in the sediments is from calcium rich shell material contributed
by marine life (Sundararajan and Natesan 2010). Ca shows a decreasing trend
towards the lower layers of the sediment core (Figure 4.10). Na and K largely
reflect the distribution of K and Na-Ca feldspar in the continental shelf
sediments.
The variation in the P content along with organic matter suggests
that it is partly controlled by the organic supply (Sundarajan et al 2010) and is
mainly controlled by the detritus phases. Even though the overall
geochemistry shows detritus nature of the sediments, the different layers
within the sediment core shows some variations in their chemical signature
and fractionation due to burial, diagenesis and the pressure of the water
column.
Figure 4.10 Distribution of major elements of the Cudallore sediment
core
149
The higher concentration of Cu, Cr, Ni, Fe, Co and Zn is observed
in the upper (25-30 cm) Holocene (3935±60 yrs BP) sediment layers, while
the lower concentration is found at the lower part of the sediment core (40-50
cm depth). The depth profile of Mn shows a decrease in its concentration in
the surface layers and increases slightly downwards. The occurrence of Cr
and Cu in the industry and domestic waste affects the area (El Nemr et al
2006). The higher Ni concentration (52-112 ppm) could be probably due to
the petroleum related activities (El Nemr et al 2006). Co and Zn are
recognised as the markers of paint industry (Lin et al 2002) and they had a
higher concentration at 20-30 cm of the sediment core.
It is well established that Fe and Mn are excellent scavengers for
trace metals (Tessier et al 1979). Under a variety of natural environmental
conditions, Fe also changes its oxidation state steadily with changes in the
amount of oxygen and variations in pH conditions in the aqueous phases.
Hence, this element provides useful information on the present and past
depositional and environmental conditions (Buckley and Hargrave 1989). The
behaviour of Fe is mainly governed by the distribution of ferro-magnesium
minerals and dispersed oxy-hydroxides (Sundararajan and Srinivasalu 2010).
Fe enrichments from (5.91-7.98 %) start at a depth 20 cm and further
downwards due to higher stability of Fe oxyhydroxides under mildly reducing
conditions and the faster oxidation kinetics of Fe2+
) (Zwolsman et al 1993).
The Fe2+
gets precipitated in the sediment due to change in the pH or near the
oxic-suboxic interface (Froelich et al 1979, Santschi et al 1990, Janaki-Raman
et al 2007) also indicating the scavenging of Fe and Mn oxyhydroxides that
are deposited as metal sulphides with a common source of origin (Sugirtha
and Patterson 2009). Down core variation of Cu, Ni and Co content indicate
moderate removal in the suboxic layers till the zone of oxygen penetration.
Down profile distribution pattern of Cu and Ni is similar to Fe and Mn
indicating that they are cycled along with Fe-Mn oxides in the redox
150
boundaries and are precipitated as iron sulphides (Klinkhammer 1980).The
higher concentration of Ni and Cu reveals the anoxic condition and addition
of these is due to the scavenging of Fe and Mn oxides (Ayyappaperumal et al
2006).
Zn enters in to the aquatic environment from a number of sources
including sewage effluent, runoff and from various input of organic wastes
(Boxall et al 2000, Alagarsamy 2006). Higher concentration of Pb (19.1 -24
ppm) in the down core profile is attributed to the local redox conditions which
allows Pb to co-precipitate with Mn during Mn oxide formation in the
superficial segment (Sugirtha and Patterson 2009). A comparison of trace
element of the present study with other coastal regions around the world is
presented (Table 4.4).
4.4.5 Paleo Weathering And Weathering Indices (CIA And CIW
Values) In Cudallore Core Sediments.
The (A-CN-K) ternary diagram is often widely used to reflect
silicate weathering trends (Nesbitt and Young 1982). This parameter
quantifies more precisely the effect of chemical weathering on the sediments
by loss of labile elements. In the presence of chemical weathering, labile
grains like feldspar will be prone to progressive decay and will eventually be
transformed into clay minerals and a marked compositional difference
between source and sediment will become evident (Johnsson et al 1988). The
ternary plot of Al2O3–CaO +Na2O–K2O [compared to data of UCC and PAAS
and NASC] shows that all the sediments fall on a trend parallel to the
Al2O3–CaO+Na2O join. On both the A-CN-K diagram (Figure 4.11a), and the
A-CNK-FM diagram (Figure 4.11b), all the sediments display a moderate
weathering history.
15
1
Table 4.4 Comparison of trace elements of the present study with other coastal regions around the world
Sl.No. Locations Zn ppm Mn ppm Cr ppm Cu ppm Ni ppm Co ppm Pb ppm
1 Present study core range 53.7-89.9 0.05-0.10 112.5-177.80 20.3-59.2 52.9-112.5 15.7-112.5 15-24
2 Gulf of Mannar 71-128 83-379 49.6-512 29-69 34-149 4-11 5-30
3 Gulf of Aquaba (Red Sea) 31-260 53-655 15-186 7-27 19-76 21-56 83 -225
4 Palos Verdes Peninsula,
Southern California
54-2,880 - 74-1,480 14-937 16-134 - 19-578
5 Halifax Bay 33 - - 7 12 7.6 17
6 China Shelf Sea 65 530 61 15 24 12 20
7 Tokyo Bay 322 1,098 77.3 53.47 32.63 - 50.68
8 Bombay Coast 96.2 1192 103 100.9 52 38.2 16.4
9 Tuticorin coast 73 305 177 57 24 15 16
10 Kalpakkam, Bay of Bengal 71 356 57 20 30 9 16
11 Shallow cores, Bay of Bengal - 529 84 26 64 - -
15
2
Figure 4.11a-b The major oxide data for core sediments are plotted in Al2O3–CaO +Na2O–K2O (A–CN–K) and
Al2O3–CaO +Na2O+K2O–Fe2O3+MgO (A-CNK-FM) compositional space (molecular proportions).
Figure 4.11 a representing A-CN-K diagram Figure 4.11 b representing A-CNK-FM diagram
153
The CIA values in the upper surface sediments of the core indicates
moderate weathering pattern (56.47) and mean CIW (60.74) value, compared
to UCC, PAAS and NASC in the core sediments. In the present study, the
sediment data falls along the feldspar line indicating moderate or partial
weathering (Figure 4.10a-b). This also implies that the sediments represent
weathered products from granite and charnockite sources (Fedo et al 1996,
Nath et al 2000). The geochemical data of the present sediments when plotted
in Al2O3–CaO +Na2O+K2O–Fe2O3+MgO ternary diagram, the plots fall in the
proximity of Fe2O3+MgO phase indicating the occurrence of ferromagnesian
minerals (likely the pyroxenes) in the sediments. The CIA values reflect
changes in the proportion of feldspar and various clay minerals in the
weathering product (Nesbitt and Young 1982). The CIA values also exhibit a
general increase of CIA values at the depth 25-45 cm ranging from 65.15 to
66.76 indicating the occurrence of highly weathered clay (Figure 4.12).
Figure 4.12 Down core variation of the CIA values in the Cudallore core
sediments
154
4.6 PHYTOLITH STUDIES IN CUDALLORE SEDIMENT
CORE
Phytolith analyses reveals three zones that have a very low
frequency of diatoms; as typically diatoms are expected to decrease with
climate warming because of reduced nutrient redistribution and increasing
sinking velocities. The Cudallore sediment core reflects this phenomenon
since 4000 yrs BP. The phytolith assemblage supports the terrestrial
palynomorph and this is due to several factors, including dense source
vegetation, a slower sedimentation rate causing slight concentration of
microfossils or increased proportions of re-worked specimens at these levels.
In summary, the Phytolith assemblages suggest a very gradual decrease in
temperature upward through the core. The mildest temperatures can be
interpreted with the occurrence of short panicoid Phytolith morphotypes. The
phytoliths imply localized mild conditions during deposition of the lower part
of the core, and may represent remnants of temperate vegetation interpreted as
represented by silicified woody elements and leaf phytolith assemblages. The
Phytolith assemblage from the Cudallore core suggests full forest conditions
along the East Coast, Tamilnadu with a humid oceanic climate since the last
4000 yrs BP.
4.7 SUMMARY
4.7.1 An Integration of Geochemical Parameters of Seawater,
Seafloor Sediments and the Sediment Core from Cudallore
Collected along the Continental Shelf East Coast Tamilnadu
The inferences drawn from the various physico-chemical
parameters of water samples at various seawater column depths reveals that
during the monsoon periods (post NE and SW), the salinity, temperature,
oxygen and nutrients shows a distinct variation increasingly with depth
155
towards the deeper seawater column following the continental shelf and slope
across the East Coast (Chennai to Nagapattinam). The main aim of this study
was to understand the sediment water interaction along the near coastal
continental shelf environment. The objectives put forward in this study was
partially accomplished and there are still gaps that need to be filed in. This is
mainly due to the complex interaction of terrestrial and oceanic processes.
This present study gives a tentative interpretation of biogeochemistry of
sediments of seawater collected spatially and vertically from continental shelf
East Coast Tamilnadu to infer biogeochemical processes that occur during
post NE and SW monsoons. The value or importance of the study lies in its
potential to demarcate that geochemistry of the seawater and sediments
deposited on seafloor and sediment core for deciphering the dynamic
interaction between seawater and detritus as it is responsible for the chemical
environment of seawater for productivity and resources.
A detailed geochemical parameter and their distribution in
seawater, seafloor sediments and sediment core studied along the East Coast
continental shelf Tamilnadu reveal a fluctuating distribution pattern during
both the post NE and SW monsoons. Monsoon and its allied phenomenon like
upwelling and mixing processes are the main causative factors (Kumar et al
1998). Changes in the physico-chemical parameters of seawater and the
sediment geochemical characteristics are evident with respect to the
increasing water column depths and latitudes (Qasim 1982). However, in the
deeper seawater column (150 m) the variations are less pronounced. Previous
studies conducted in different parts of the world revealed that the benthic
realm and the monsoonal variations influences the hydrographic and sediment
parameters such as salinity, temperature, dissolved oxygen, sediment texture
and geochemistry (Joydas and Damodaran 2009, Jayaraj et al 2008).
156
The river runoff influences the nutrient balance and the
biogeochemistry of the seafloor sediments through circulation and mixing
processes along the continental shelf (Milliman and Meade 1983). 18
O values
reflect a positive gradient with other biogeochemical parameters, dissolved
oxygen, salinity, pH and temperature. It is very obvious that there exists a
dynamic and kinetic interaction between the saline waters and the sediment
passing through the seawater column at different depths and settling on the
seafloor. Shallow the seawater column, coarser the sediment depositing on the
seafloor. Thicker the seawater column and away from the coastline, finer the
sediment deposition also allowing a thin film of fresh water lens to exist
following the topography of the continental shelf.
This study also reveals that each geochemical parameters of the
seawater and sediment along the East Coast continental shelf Tamilnadu are
individual in perspective and there exist little or no one to one sediment-water
relation. The variations in water and sediment characteristics are mainly due
to the significant monsoonal impacts, diagenitic processes and distinct
stratification formed due to the fresh water runoff, thermal inversion and
mixed layers along the continental shelf region. The impact of sediment burial
has not been taken in to account. The results of this study demands a further
detailed investigation in the nature of resources along the coastal area as well
as the adjacent regions bordering the continental shelf followed by deeper sea
water chemical analysis collected from continental shelf using high resolution
techniques such as stable isotope of water, sediment and mineral assemblages
on sediments. It is also essential to study deep sea cores collected from deeper
seawater column depths. This would help in understanding the
biogeochemical processes occurring at deeper seawater column levels which
are vital in addressing the dynamics of productivity of waters, anthropogenic
impacts and the evolution of the continental shelf along the East Coast
Tamilnadu.
157
Although seafloor sediments has been collected from Chennai to
Nagapattinam covering the East Coast Tamilnadu at 150 m seawater column
depth, the data generated and inferences drawn based on one core in order to
understand the monsoonal variation characteristics and sediment water
interaction is meagre. Hence future work further demands retrieving several
marine cores along the continental shelf correlating the sediments supported
by detailed isotopic and micropaleontological work could throw enough light
to understand the variation caused by the anthropogenic influxes in the paleo
continental shelf environment and sediments. Hence the present study with all
its constraints forms a primary baseline for future work.