Abundance and Distribution of Benthic Foraminifera in the Northern Oman Sea (Iranian Side) Continental Shelf Sediments
Abundance and distribution of benthic Foraminifera,
in the Northern Oman Sea (Iranian side) continental shelf sediments was
studied. Sediment samples were gathered in Winter 2006, from eight stations
ranging in depth from 30 to 103 m. Environmental conditions including
water depth, temperature, dissolved oxygen, salinity, pH, grain size,
total organic matter and calcium carbonate concentration were measured
and their relationship with the distribution of benthic foraminifera was
discussed. Forams were the most abundant meiobenthic group in nearby all
the stations. The suborder ROTALIINA was dominant in the northern region
while LAGENINA, MILIOLINA and TEXTULARIINA, were being abundant in the
northwest region too. LAGENINA were being very abundant in the stations
with higher depth. From the total 40 species belonging to 24 genera, Ammonia
beccarii was common in the whole research region. Water depth, salinity
and substrate seemed to be the most important environmental factors controlling
the distribution of benthic foraminifera. None or rare structural abnormal
and oil polluted individuals, in nearby all stations, leading to the clean
benthic environment of the Oman Sea benthic zone.
Foraminifera are the worldwide tested protozoa, living almost in all
of the aquatic environments including marine, brackish or fresh water
and are distributed in all latitudes especially in tropics. Forams are
benthic or planktonic (holoplanktonic or meroplanktonic) in the mode of
life. Marine species are reported from coastal regions to the very deep
oceanic zone. Benthic forams live on the substrate surface (epifaunal)
or in a few centimeters (6-7 cm) deep layer in the sediments (infaunal).
Water temperature, salinity and the structure of the sediments are the
important factors controlling the distribution of the benthic foraminifera.
Fossil forams are one of the most important instruments using in the geology
and paleontology sciences. Several researches on the fossil forams, compared
with the recent forms, leading to the understanding of the paleo-environmental
conditions of the earth. Forams are useful in the exploration of the petroleum
and natural gas sources. They are the well-known indicators for the benthic
environmental conditions and the appearance of the pollutants.
The distribution of the benthic foraminifera is affected highly with the environmental
conditions of the benthic zone. The diversity of the foraminifera is higher
in the tropical and temperate zone compared with the higher latitudes and decreased
in higher depths (Fairbridge and Jablonski, 1979).
Because they have a preservable shell and are abundant, benthic foraminifera
are the best meiofaunal group for giving a proxy record of past marine and marginal
marine environments (Murray, 2000). The structure of the
benthic zone, including the grain size of the sediment particles, pH and concentration
of nutrients and organic carbon has an important role on the structure of the
benthic foram populations. The patterns of distribution of benthic foraminifera
are controlled by those environmental factors that have reached their critical
thresholds. For each species, in variable environments, different factors may
be limiting distributions both temporally and spatially. For a species or an
assemblage to be useful as a proxy its abundance must show a strong correlation
with the chosen factor (Murray, 2001). The thickness
of the infaunal foram`s layer in the sediments is depended on the concentration
of dissolved oxygen (Murray, 1979). The study of recent
benthic foraminifera on the Guadiana shelf, showed that the spatial distribution
of benthic forams is closely associated with sea-bottom sedimentary environments
and bathymetry, the number of benthic foram tests and the distribution of several
shallowest nearshore species are clearly influenced by the outflow of the estuary
and by local hydrodynamic conditions and the deeper water assemblages are more
related to low levels of tidal energy and low oxygen environments associated
to fine-grained sediments and cold-water filaments related to seasonal upwelling
(Mendes et al., 2004). Recent calcareous benthic
foraminifera in surface sediment samples from the Northern North Sea area are
affected by strong current activity and coarse-grained sediments leads to a
decrease in the oxygen content in the bottom-waters during part of the year
(Kristensen, 2002). From the total 67 species belonging
to 31 genera reported in the Eastern side of the Chabahar Gulf, Northeast of
the Oman Sea, 3 species were distributed only in sandy sediments, 16 species
only in silt-sand and 1 only in sand-silt sediments and the others were distributed
in the whole research area (Rahmati, 1997). In the coastal
area of the Gheshm island, from the total 40 reported species of 21 genera,
3 species were distributed in sandy floors, 2 in sand-silt, 7 in silt-sand,
2 in mud and the others were distributed in the whole research area without
a specific substrate (Farahani, 1998).
The main aim of the present research is to identifying the structure
of benthic foram communities of the Northern Oman Sea and relation to
the environmental conditions of the benthic zone.
MATERIALS AND METHODS
Sediment samples were collected during winter 2005, from 8 stations in
the Northern Oman sea (Table 1, Fig. 1),
ranging in depth from 30 to 103 m. Bottom sediments were gathered using
a 0.1 m2 Van Veen Grab. Three samples were gathered in each
station by a 6.15 cm2 area slender sampler. All the sediment
samples were being mixed with formalin (4% concentrated solution) in plastic
boxes. The benthic environmental factors including water depth, temperature,
dissolved Oxygen, salinity and pH were measured by a CTD system during
the sampling time.
For the grain size analysis, 25 g of each dried sediment sample (70 °C-8
h) was mixed with tap water to a total volume of 250 and 10 mL of sodium
hexametaphosphate (6.2 g L-1) to separate the sediment particles.
The sediments were then stirred mechanically (15 min), allowed to soak
(overnight), stirred mechanically again (15 min), washed in a 0.063 mm
sieve with tap water and dried again (70 °C - overnight). The remaining
dried material on the sieve transferred into the uppermost of a stacked
series of graded sand sieves (aperture 4, 2, 1, 0.5, 0.25, 0.125 and 0.063
mL), the remaining material on each sieve was then carefully removed and
weighted. At the end, the percentage of each particle fraction was calculated.
|| Position of sampling stations
|| Location of sampling stations
The Total Organic Matter (TOM) in each sample was measured by calculating
the loss of weight during combustion. The sediment samples, each crucible
was weighed (C) and half filled with wet sediment and dried in an oven
(70 °C) to constant weight (about 24 h). After removing from the oven,
it was allowed to cool and reweighed (A). It was then placed in a Muffle
furnace (550 °C-overnight), removed, cooled and reweighed again (B).
The total content of organic matter (TOM) was determined by the loss of
weight on ignition at this temperature.
Calcium carbonate concentration was measured based on the reaction with
HCl 25 g (W1) of dried sediment (70 °C, 8 h) was mixed
with HCl (0.1 N) and stirred (until no CO2 bobbles appearing)
and allowed to soak (24 h). The upper liquid phase was discharged and
the remaining sediments were filtered (by paper), dried (70 °C, 8
h) and reweighed again (W2). Calcium carbonate percentage was
measured by the following formula:
CaCO3 (%) = 100 (W1-W2)/W1
For determining foraminifera, sediment samples were being washed in the sieve
(aperture 0.063 mm) with tap water, dried (75 °C, 8 h), floated by the heavy
liquid CCl4 and the upper layer of the liquid (consist of floated
forams and other tested specimens) were filtered by paper and allowed to dry.
A stereomicroscope and several studies (Cushman, 1969;
Loeblich and Tappan, 1988; Pinxian, 1985,
1997) were used for determining and counting of foraminifera.
RESULTS AND DISCUSSION
The quantities of the environmental factors, including water depth, temperature,
dissolved oxygen, salinity, pH, grain size, total organic matter and calcium
carbonate concentration, are shown in the Table 2 and
3. Water temperature in the benthic zone was nearby
similar in all stations (22.13 to 23.72 °C). The dissolved oxygen
concentration, decreased with the increase of the depth, was higher in
the station N1 with the lowest depth. Salinity was nearby similar in the
hole sampling region but in M2 it was considerably higher (related to
the dense hypersaline current comes from the Persian Gulf, threw the Strait
of Hormoz). The pH was equal in nearby all stations (7.9). The total organic
matter percentage was very high in the station M1 (24.53%) and decreased
from West to East.
The grain size analysis of the sediments showed that the sedimentary
structure of benthic zone, in the most of the sampling stations, consists
of very fine sedimentary particles including silt, clay and fine sand.
As the most of sedimentary particles of the Oman sea benthic zone (such
as silt and clay) are carried by the winds coming from the deserts around
the sea and also sinking of a huge value of small particles (such as calcium
carbonate and destroyed forams testes) originated in the Oman sea water
column, the sedimentary structure of the research region includes very
fine sedimentary particles such as clay, silt and very fine sand.
The total organic matter concentration of the Oman sea sediments increases
from East to West. That is related to the higher organic matter productivity
in the Western parts of the sea because of the higher concentration of
the nutrients available for photosynthesis and primary production. Accumulation
of the organic particles by the action of currents or sinking is guessed
to be the reason for the higher concentration of the total organic matter
in the station M1.
||Water depth, temperature, Dissolved Oxygen (DO), salinity,
pH, Total Organic Matter (TOM) and calcium carbonate (CaCO3)
of the Oman Sea benthic environment
|| The grain size analysis and percentage of each size
group of the sediment particles
The cosmopolitan Foraminifera, Ammonia beccarii, was common in
the all the stations but several rare species (such as Bolivina variabilis,
Reophax texana and Uvigerina pigmea) were being found in
only one or two stations. The forams Epodines sp., Dentalinoides
sp., Quinqueloculina contorta, Spiroloculina dentata, Spiroloculina
sp. I and Triloculina sp., were being found only in the station
M2. There were also some deposited empty testes of pelagic foraminifera
in all the stations, especially in the deepest station M2. There were
none or rare structural abnormal or oil polluted individuals in nearby
all the sediment samples except in the stations M2 (2 abnormals and 5
oil polluted) and N1 (1 abnormal and 2 oil polluted). The abundance of
benthic foram species in the sediment samples are shown in Table
|| Abundance of foraminifera in the sediment samples of
the Northern Oman Sea
By the results of the present research, the environmental conditions
of the Oman sea benthic zone (including warm water, enough dissolved Oxygen
and fine Silt-sand structure of the sediment particles) seems to be very
suitable for the benthic foraminifera compared with the other meiobenthic
communities. None or rare structural abnormality or oil polluted individuals,
in nearby all the stations, leading to the clean benthic environment of
the Oman Sea. It is the result of the absence of large cities, dense human
communities, pollutant industries and important rivers compared with the
huge amount of the Oman sea water volume and its water exchange with the
Indian Ocean too. It is difficult to select one of the environmental factors
as being of overall importance in the distribution pattern of the benthic
foraminifera of the Oman sea benthic zone, but it seems that the composition
and distribution of the benthic forams of the Oman Sea is affected by
the local combination of the total environmental factors all together.
The huge number of empty tests of the benthic and planktonic forams and
also the empty shells of Bivalvia and Gastropoda in the deep station M2
is related perhaps to the sinking or transportation of this empty tests
and small shells from other regions to the area. The large amount of the
Bivalvia and Gastropoda empty shells in the stations M1 and M2 is related
to the accumulation of the shells too. The deep water current coming from
the Strait of Hormoz brings small particles and fine empty shells downward
the area. This dense hyper saline and warm current originated in the Persian
Gulf, due to increasing of the benthic water salinity and temperature
in the station M2 compared with the other ones (Table 2).
The station N4 has probably the best conditions for the foram populations.
The highest foram abundance and diversity in the station N4 (Compared
with the N1 to N6) is also related to the fine structure of the sedimentary
particles in addition to the higher concentration of total organic matter
and calcium carbonate. Composition of foram communities studied in the
present research area shows some differentiations with that of the coastal
communities. The reason is that the environmental conditions of the coastal
zone are more alternating compared with deeper zones. So, Steno species
(stenothermal, stenohaline etc.) that are not adapted with the alternating
conditions of the environment can not bear it as well as Eury species
such as the foraminifer Ammonia beccarii.
The cosmopolitan foraminifer Ammonia beccarii, is a common foraminifer
in the whole of the present research area and is reported in earlier researches
in the Persian Gulf and the Oman sea coastal zones (Rahmati,
1997; Farahani, 1998) A. beccarii is distributed
from coastal zone to the depths up to 100 m. Some species such as Biloculinella
globula, Epodines sp., Pyrgo sarsi, Quinqueloculina contorta,
Spiroloculina excavate, Spiroloculina dentata, Spiroloculina
omata, Spirophthalmidium acutimargo, Textularia sagittula,
Tetrataxis palaeotrochus and Triloculina sp., are found only in
the station M2 (up to 100 m deep) seems to be distributed in the higher depths.
The species of the genera Quinqueloqulina, Spiroloculina and Textularia
and the species Septammina bradyi, Lagena perlucida, Lagena
spicata, Septammina bradyi, are distributed only in the Western stations
of the research region. As the suborders ROTALIINA and TEXTULARIINA had been
reported that are abundant in the estuaries and hypersaline lagoons and MILIOLINA
are dominant in the environments which have more interaction with the sea environment
(Murray, 1968), Considering the abundance of the four
major suborders ROTALIINA, LAGENINA, MILIOLINA and TEXTULARIINA (Fig.
2) in the foram communities of the stations N2 to N6, shows that the suborder
ROTALINA is dominant in the Northern Oman Sea benthic zone. But, in the northwest
region, it is not absolutely dominant and the foram individuals belonging to
the other suborders such as LAGENINA, MILIOLINA and TEXTULARIINA are abundant
too. The abundance of the suborder LAGENINA is very low in the Northern stations
N1 to N6, but is highly increased in the station M1, perhaps in the reflection
to the increasing of the depth in the suitable salinity. The depth of the station
M2 is the highest one but there is the higher salinity too so it could be a
limiting factor for the suborder LAGENINA. LAGENINA had been rare forams in
the Northern coastal region of the Oman Sea. TEXTULARIINA is the main suborder
in the deepest station M2 at the Northwest region of the Oman Sea sediments.
The higher salinity of the benthic environment can be the reason for the higher
percentage of the suborder TEXTULARIINA in the station M2 and the suborder ROTALIINA
in the whole research region.
||Composition of benthic foram communities, depended on
four suborders ROTALIINA, LAGENINA, MILIOLINA and TEXTULARIINA, in
the Northern Oman Sea
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