2. upwelling, which results in a non-upwelling environment, in contrast to
the summer upwelling regime offshore Oman, Somalia and southern
India (Schulz et al., 2002).
Planktonic foraminifera have been recognised as potential proxy for
paleoceanographic studies since a very long time. Due to their high
abundance and extensive presence in almost all marine environments,
foraminifera are widely used to reconstruct paleoclimate from marine
archives (Saraswat, 2015). The absolute abundance of foraminifera in
sediments mainly depend on 2 factors viz. 1) Variations of planktonic
foraminiferal population in the surface layer and benthic foraminiferal
population on the seafloor resulting in change in rate of flux of the spec-
imen which incorporates in the sediments, 2) the effect of dilution
caused by other materials, such as terrigenous influx (Kroon et al.,
1993). At present the most accurate and widely important proxy used
for reconstruction of paleo-sea level are of micropaleontological nature
(Van Der Zwaan et al., 1990). Therefore in the present study we used
PFA, BFA and P/B ratio supported by mineralogical (Quartz, Calcite and
Aragonite) abundance for the sea level reconstruction. The idea started
with the pioneer study of Phleger (1951) which was followed by the at-
tempt of Grimsdale and Van Morkhoven (1955) to use P/B ratio and
planktonic foraminiferal percentage (Nigam and Henriques, 1992) in
paleodepth reconstruction and found that the proportion of planktonic
foraminifera in the total foraminiferal population increase towards
deeper water. This increase in the planktonic foraminifera with distance
from the coast is due to the decrease in turbidity (Berger and Diester-
Haass, 1988). However BFA follows a slightly different pattern from
PFA. It increases from near shore to continental edge and further
decreases seawards (Douglas and Woodruff, 1981).
The productivity of an ocean mainly depends on source of nutrient
supply. The source of nutrient may be from the outside supply of nutri-
ents (river influx) or the internal cycling (upwelling and/or mixed layer
circulation) of nutrients within the water column (Berger et al., 1989).
The uniqueness of the present study is that the core location belongs
to such a shallow water depth (88 m) which is above oxygen minimum
zone (OMZ). Only few limited attempts (Singh et al., 2007; Nisha and
Singh, 2012) were made in the past to study the evolution of such a
shallow shelf environment of the Arabian Sea, where both the upwell-
ing as well as mixed layer nutrient input does not have significant role
in the productivity of the area. In such a case, the paleoproductivity of
the area mainly depends on the fresh water nutrient input. Previous
studies from the NE Arabian Sea (Milliman et al., 1984) reveal that the
Indus river has been the only significant source of freshwater discharge
in the area during Holocene. Hence it can be concluded that the
paleoproductivity along the core site mainly depends on fresh water nu-
trient input from Indus river. Therefore, the present study aims to get a
high resolution monsoonal record to understand Indus river discharge,
sea level fluctuation and productivity variations in the NE Arabian Sea
offshore Saurashtra during the Younger Dryas and Holocene using a
multi-proxy approach.
1.1. Previous studies
Despite widespread concern, limited attempts were made to study
the paleoceanographic and paleoclimatic variations over shelf region
offshore Saurashtra of the NE Arabian Sea using a multi-proxy approach.
Existing records from the north-west Indian margin include geomor-
phologic studies (Rao et al., 1994, 2003; Rao and Wagle, 1997) to ex-
plain the role of carbonate platform, sea level fluctuations (Hashimi et
al., 1995), organic carbon (Corg) distribution (Babu et al., 1999),
210
Pbxs, 137
Cs and 14
C (Somayajulu et al., 1999) to decipher the sediment
deposition rates, foraminiferal isotope records (Sarkar et al., 2000;
Gupta et al., 2011) for monsoon studies, Corg, CaCO3 and C/N records
(Bhushan et al., 2001) to study the concentration and burial fluxes, tem-
perature and salinity data (Balachandran et al., 2008) to study the phys-
ical oceanographic parameters during present day winter monsoon,
pteropod studies (Singh and Singh, 2010; Singh et al., 2011a) to
decipher carbonate preservation as well as monsoon wind induced
hydrographic changes and mudflat (Banerji et al., 2015) to study the
mid Holocene land sea interactions.
There are numerous evidences which explain the lowering of sea
level during early Holocene along the western continental margin of
India. The geomorphic features and associated sediments on the outer
shelf of western India were studies by several workers (Nair, 1975;
Wagle et al., 1994; Rao et al., 1994). There is a unique carbonate plat-
form reported also known as Fifty Fathom Flat (FFF) carbonate platform
(Fig. 1), which extends between 60 and 100 m water depth, occurs on
the outer continental shelf of the north-western margin of India (Rao
et al., 2003). The FFF consist of relic sediments which are mainly oolitic
and palletal aragonite sands, having tan colour landward and shiny
white towards the offshore (Rao et al., 2003). The age of these sands
range from 14.3 to 7.6 Ka (Rao et al., 2003). Other important studies in-
clude the early Holocene evidence of relict sand zone, coated grains, pal-
lets and benthic foraminifera of shallow water origin (Hashimi and Nair,
1976). On the basis of these evidences, a Holocene sea level curve for the
western continental margin of India was prepared (Hashimi et al.,
1995). The curve indicated the lowering of sea level during early
Holocene. During middle and late Holocene the sea level raised rapidly
and took a nearly stable mark up to the recent times.
1.2. Regional oceanographic settings
The studied sedimentary core is located in the shelf offshore
Saurashtra, North Eastern Arabian Sea (NEAS). The Arabian Sea (AS)
covers an area of about 3,863,000 km2
, and is surrounded by arid land-
mass to the west and north and by coastal highlands of western India to
the east. Three major river (Indus, Narmada and Tapti) discharge their
enormous fresh water and sediments into the AS. The Indus fan being
the second largest fan (Clift et al., 2002) is the most extensive physio-
graphic feature of the AS in the Northern Indian Ocean covering an
area of approximately 1.1–1.25 million km2
with a length of 1500 km
and a maximum width of 960 km (Govil and Naidu, 2007). The Indus
Fan is bounded by continental margin of India-Pakistan and Chagos-
Laccadive Ridge on the east, by the Owen and Murray Ridges on the
west and north, and by Carlsberg Ridge on the south. The sediments in
the Indus fan are mainly brought by Indus river (Govil and Naidu,
2007). The Narmada and Tapti rivers drain the peninsular shield of
India also contribute sediments to eastern AS whereas the Indus river
drains the northern AS including offshore Pakistan, Kutch and
Saurashtra coast. The deposited sediments in to the ocean basin have
always been used to infer paleoclimatic variations at different time
scales from decadal to centennial to millennial. A strong precipitation
gradient exists across the AS due to the effect of SWM (Staubwasser et
al., 2003). Indus river has been the only significant source of fresh
water discharge in the NEAS during the Holocene (Milliman et al.,
1984). In the western AS the low salinity is observed due to SWM relat-
ed upwelling whereas no such effect has been observed in NEAS
(Levitus et al., 1994). However strong seasonality has been observed
in the sea surface temperature (SST) and mixed layer depth of NEAS
by virtue of seasonal reversing of SWM and associated air temperatures
(Rao et al., 1989).
2. Material and methods
Samples were obtained from the ORV core repository at National
Centre of Antarctic and Ocean Research (NCAOR), Goa, India. Core SK
240/485 (Fig. 2) was collected during Sagar kanya cruise no. 143rd
from the continental margin of Northeastern Arabian Sea offshore
Saurashtra region (Lat 21°16′N Long 68°55.99′E) at a water depth of
88 m. The core is 340 cm long sub sampled at 2 cm interval to obtain
a high temporal resolution. Six sediment samples including core top
and bottom were dated using 14
C accelerated mass spectrometry
(AMS) technique at Radiocarbon Laboratory, Institute of Physics, Centre
2 S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
3. for Science and Education, Konarskiego, Gliwice, Poland and NSF
Arizona AMS Facility, University of Arizona, U.S. The ages were
converted to calendar year BP (1950) by applying appropriate reservoir
correction (ΔR) of 129 ± 35 years for the Saurashtra coast (Dutta et al.,
2001) and calibrated using Calib 7.1 version (Stuiver and Reimer, 1993)
(Fig. 3).
10 g of sediment sample was dried keeping in an oven at 60 °C. Dried
sediment sample weighed and soaked in distilled water overnight.
Fig. 1. (I) Map showing Fifty Fathom Flat (FFF) Carbonate Platform Extension along the western continental margin of India (after Rao et al., 2003). (II) (A) Aragonite pallets dating early
Holocene as described earlier (Rao et al., 1994) (B)&(C) Similar aragonite pallets found in our samples during Younger Dryas and early Holocene.
Fig. 2. Map showing core location SK 240/485 (Lat 21°16′N Long 68°55.99′E) collected from offshore Saurashtra, NE Arabian Sea during 143rd
Sagar kanya cruise.
3S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
4. Water was decanted from top without disturbing the bottom settled
sediment. To remove organic matter and clay lumps, 10 ml of 10% sodi-
um hexa-meta-phosphate (NaPO3)6 and 5 ml of 10% hydrogen peroxide
(H2O2) solutions were added respectively and kept overnight. Subse-
quently these samples were wet sieved through a 63 μm size sieve
with enough care to prevent the breakage of foraminifer shells.
N63 μm fraction (coarse fraction) retained on the sieve was transferred
in to 25 ml beaker and dried in the oven at 50–60 °C. The dried fraction
was again sieved through 250 μm and 150 μm sieves, weighed (to obtain
sand percentage in the sediment) and used for census counts of Plank-
tonic (total planktic foraminiferal abundance: PFA) and Benthic (total
benthic foraminiferal abundance: BFA) abundance according to the
standard procedure (CLIMAP, 1976) of foraminiferal counting. 25–30
clean white G. ruber specimens were picked for the isotope analysis.
CaCO3 percentage (wt%) was determined using “Karbonat-Bombe”
method (Müller and Gastner, 1971). 1 g dried and powdered sample
was taken in Carbonate bomb jar and 5 ml 12% HCl was inserted in
the cylinder. This was carefully done to avoid spilling of acid over sedi-
ment sample while the jar is unscrewed. Once the jar was screwed
tightly by the manometer head, the acid was spilled and allowed to
react with sediment sample. The CO2 thus produced created pressure
in the jar which was recorded by attached manometer giving the
CaCO3 (uncorrected) reading. Before running a batch of sample every
time, CaCO3 standard reading was recorded using Sigma-Aldrich 100%
CaCO3 standard powder in order to calibrate the recorded uncorrected
manometer reading. Calibration of scale was done using the following
equation (Müller and Gastner, 1971) -
CaCO3%sample ¼
CaCO3 manometer reading of sampleð Þ Â 100
CaCO3 manometer reading of standardð Þ
X-Ray powder diffraction (XRD) was done to obtain relative abun-
dance of Quartz, Calcite and Aragonite on PANalytical X'Pert3
Powder
XRD instrument at XRD laboratory, Birbal Sahni Institute of
Palaeosciences (BSIP), Lucknow, India. X-rays were allowed to pass
through fine powdered sediments placed on sample holder stage. The
analysis was targeted to obtain 2θ values ranging between 20° and
40°. The diffracted X-rays give signature peaks at specific 2θ angle (by
satisfying Bragg's law) of the corresponding minerals. The peak counts
were noted for quartz, calcite and aragonite in order to obtain the min-
eralogical abundance.
For measuring δ18
O and δ13
C values, ~15–20 specimens of plankton-
ic foraminifera surface water species G. ruber, were put into individual
screw capped glass vials. The vials were systematically kept in the Gas
Bench along with three carbonate standards i.e. NBS 18, Merk and
MMB (n = 5 in each run). Subsequently, the vials were flushed with ul-
trapure He gas for about 10 min per sample. After flushing, 100% ortho-
phosphoric acid (H3PO4) was injected into each vial which was kept at
72 °C temperature bath for 2 h. The evolved CO2 was purified by Naflon
tube and Pora pack column in Gas Bench and allowed into Continuous
Flow Isotope Ratio Mass Spectrometer (CFIRMS, MAT 253) for analysis.
Each measurement comprised of three pulses of reference followed by
six pulses of sample CO2 gas. The tank reference gas was calibrated by
using NBS-18. All samples including internal standard MMB were mea-
sured with respect to the calibrated tank gas. The isotopic data are re-
ported against VPDB with a precision of ±0.1‰ (1σ) for both δ18
O
and δ13
C values on daily basis. These samples were measured at the sta-
ble isotope facility at BSIP, Lucknow, India.
3. Results
The rate of sedimentation at the core site ranges between 18.3 and
47.9 cm/Ka. The maximum rate of 47.9 cm/Ka has been recorded at
the core top between 1.4 and 2.7 Ka (0-62 cm) which decreased to
35 cm/Ka to the next date interval i.e. 4.2 Ka (114 cm) and decreased
further to 18.3 cm/Ka up to 7.4 Ka (172 cm). An increase in sedimenta-
tion rate was observed reaching the value of 30.4 cm/Ka up to 10 Ka
(250 cm) and thereafter further increased to 32.7 cm/Ka until 12.6 Ka
(core bottom). The results yield a high sub-centennial scale resolution
of ~40–110 years per sample which is first of its type from the offshore
Saurashtra region.
Fig. 3. Calibrated Age-Depth Model of the core SK-240/485 and sedimentation rates (cm/
Ka).
Fig. 4. Variation in depositional environment in the core SK-240/485 (bottom to top)
Mineralogical counts of Quartz, Calcite & Aragonite and Coarse fraction (N63μm CF) %.
Dark gray bands indicate abrupt change in depositional environment.
4 S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
5. The variation in coarse fraction N63 μm percentage (CF %) (Fig. 4)
defines contribution of sandy sediments in the core at particular inter-
val. In our record, the CF % ranges between 43.1% to 1.5% during last
12.65 Ka During Younger Dryas and early Holocene the CF % maxima
and minima lies at 8.2% at 8.38 Ka and 1.5% at 9.76 Ka respectively. Dur-
ing middle Holocene, the maxima increases to 43.1% at 5.86 Ka and min-
ima lies on 4.5% at 4.33 Ka while the late Holocene marked a maxima of
21.4% at 1.56 Ka and minima of 4.1% at 3.13 Ka.
Planktonic foraminifera abundance (PFA) (Fig. 5) is the number of
undistorted intact planktonic foraminifera shells present per gram of
the sample. In our dataset, PFA ranges between 1413 and 191 intact
shells per gram at 9.7 and 8.38 Ka respectively during the Younger
Dryas and early Holocene. During middle Holocene PFA ranged between
6464 and 1315 intact shells per gram at 4.05 and 7.99 Ka respectively,
while during late Holocene PFA ranged between 10,561 to 5850 intact
shells per gram at 2.85 and 1.86 Ka respectively.
Benthic foraminifera abundance (BFA) (Fig. 5) is the number of
undistorted intact benthic foraminifera shells present per gram of the
sample. The fluctuation in BFA is the measure of nutrient availability
in the benthic environment (Herguera and Berger, 1991) which in
turn depends on the water depth. Generally in the continental shelf re-
gion BFA is directly proportional to water depth. In our record, the BFA
ranges between 2228 and 535 intact shells per gram at 10.2 and
10.45 Ka respectively during the Younger Dryas and early Holocene.
During middle Holocene PFA ranged between 2343 and 1000 intact
shells per gram at 4.22 and 7.99 Ka respectively, while during late
Holocene BFA ranged between 7669 and 1906 intact shells per gram
at 1.61 and 3.99 Ka respectively.
The planktic/benthic foraminifera (P/B) (Fig. 5) ratio is the ratio
between the PFA and BFA at a particular interval of sample. The P/B
ratio generally follows a directly proportional trend towards increasing
water depth (Van Der Zwaan et al., 1990). In our results the P/B ratio
ranges between 1.59 and 0.2 at 8.12 Ka and 12.52 Ka respectively during
Younger Dryas and early Holocene. While during middle Holocene, it
ranges between 5.13 and 1.12 at 5.31 Ka and 7.85 Ka respectively. Dur-
ing the late Holocene the P/B ratio have a maximum value of 3.44 at
4.84 Ka and a minimum value of 1.17 at 1.46 Ka.
The CaCO3 percentage (%) (Fig. 5) is the measure of the total CaCO3
present in the deposited sediments at the corresponding time interval.
The water depth plays a major role in the development of foraminifera.
Sometimes, the available CaCO3 does not get used up in foraminiferal
development due to low water depth and we get high CaCO3% by the
virtue of relict carbonates which are result of Halimeda bioherm fecal
pallets. Singh et al. (2006) also suggested the down-core variation of
CaCO3 attributed to aragonite preservation in the eastern Arabian Sea.
The recorded CaCO3% minima and maxima values in our results are
92.27 at 11.85 Ka and 99.01 at 8.58 Ka corresponding to the Younger
Dryas and early Holocene. However for middle Holocene it ranges
between 38.89 and 95.37 at 4.05 and 7.99 Ka respectively. During the
late Holocene the values range between 32.41 and 47.69 at 2.96 and
1.98 Ka respectively.
X-Ray Diffraction (XRD) is the measure of mineralogical abundance
by using their 2θ angle to measure the absolute count of the correspond-
ing minerals at their signature peaks. In the present study we measured
the counts through the peaks of Calcite, Aragonite and Quartz (Fig. 4).
During the Younger Dryas and early Holocene maxima and minima of
quartz count is 1437 and 1057 at 12.57 and 9.89 Ka respectively, where-
as the calcite counts range between 1058 and 631 at 8.31 and 8.64 Ka
respectively. The aragonite count during Younger Dryas and early Holo-
cene ranges between 4272 and 3637 at 8.64 and 9.89 Ka respectively.
During middle Holocene the quartz range between 3369 and 1186 at
5.1 and 7.66 Ka respectively, while the calcite count lie between 3345
and 1056 at 5.64 and 7.99 Ka respectively. The aragonite count during
middle Holocene ranged between 3782 and 1209 at 7.99 and 4.11 Ka
respectively. During the late Holocene XRD values for quartz count
range between 4781 and 3610 at 2.69 and 1.69 Ka respectively, whereas
calcite range between 3349 and 2838 at 2.27 and 3.82 Ka respectively.
The aragonite counts during late Holocene range between 1205 and
1125 at 1.69 and 3.53 Ka respectively.
Carbon isotopes (δ13
C) (Fig. 5) of foraminifera gives an idea of the
circulation pattern as well as nutrient content in the water mass
(Kroopnick, 1985). To study the surface ocean circulation pattern and
nutrient content, δ13
C of planktonic foraminifera serves as a reliable
proxy (Ravelo and Fairbanks, 1995). During Younger Dryas and early
Holocene δ13
C (‰ VPDB) values in our record marks a minima of 0.2
(‰) at 11.61 Ka, and maxima of 1.3 (‰) at 11.97, 11.3.11.12, 9.83 and
8.51 Ka. However during the middle Holocene δ 13
C minima and
maxima lies between 0.7 and 1.7 (‰) at 8.12 and 6.19 Ka. During late
Holocene the minima values 0.7 (‰) at 1.52 Ka, while maxima values
1.3 (‰) at 3.53, 3.19, 2.73, 2.36, 2.19. 1.82, 1.65 and 1.61 Ka.
Oxygen isotopes (δ18
O) (Fig. 6) of planktonic foraminifera are a ro-
bust proxy to predict long term variability of SWM in the Arabian Sea
(Govil and Naidu, 2010; Saraswat et al., 2013; Tiwari et al., 2015). In
the NE Arabian Sea, the only significant source of freshwater discharge
is Indus river (Milliman et al., 1984). This freshwater from Indus river
consist of meltwater discharge (by virtue of snow and glacial melting)
as well as monsoon precipitation run-off, indicating a regime which is
highly sensitive to temperature changes (Miller et al., 2012). During
Younger Dryas and early Holocene the δ18
OG. ruber (‰ VPDB) maxima
values −0.7 (‰) at 11.73 Ka whereas minima values −2.2 (‰) at 9.7
Fig. 5. Productivity and sea level variations in the core SK-240/485 ( bottom to top)
Planktonic foraminifera Abundance per gram (PFA), Benthic foraminifera Abundance
per gram (BFA), Planktonic/Benthic foraminifera (P/B Ratio), CaCO3%, δ13
CG.ruber (‰
VPDB) (red line = 5 point average). Dark gray bands indicate abrupt climate variations
(Cooling events - YD = Younger Dryas; AE = Abrupt event; 8.2 Ka event and 4 Ka
cooling) (Warming event – PB = Preboreal). (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
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Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
6. and 8.77 Ka. During middle Holocene the maxima and minima range
between −1.8 and −2.6 (‰) at 7.59 and 4.11 Ka respectively, whereas
during late Holocene the maxima and minima ranges between −2.0
and −3.4 at 1.77 and 1.44 Ka respectively.
4. Discussions
4.1. Depositional environment at the core location
The environment of deposition mainly depends on the sediment
supply from the fresh water river discharge around the core location
(ex-situ) as well as the in-situ supply of sediment material due to local-
ised production. The sedimentation observed at the core location re-
veals a high depositional rate during the Younger Dryas and early
Holocene. Whereas, the low percentage of coarse fraction (N63μm)
along with low quartz and calcite abundance points towards low river
discharge during that time. The high abundance of aragonite in the
form of pallets and relict mass suggest the high in-situ deposition of sed-
iments at the core location during Younger Dryas and early Holocene.
The increased quartz and calcite abundance along with high and
fluctuating percentage of coarse fraction (N63μm) indicates high energy
terrigenous input whereas sudden decrease in aragonite abundance
suggest the lack of in-situ sediment accumulation in the area. However
the rate of in-situ sediment supply during Younger Dryas and early
Holocene was much higher than the ex-situ terrigenous sediment
supply during middle Holocene, therefore the area experienced overall
decrease in sedimentation rate during the middle Holocene.
During the late Holocene maximum sedimentation rate has been re-
corded with less fluctuations in the coarse fraction (Fig. 4). Maxima of
Quartz and Calcite abundance indicate maximum terrigenous and bio-
logical flux whereas low aragonite with no pallets and relict mass indi-
cate negligible in-situ sediment supply. However the stability in
percentage of coarse fraction (N63μm) indicate stable deposition condi-
tions. Therefore, it can be concluded that the high sedimentation rate re-
corded during the Younger Dryas and early Holocene is by the virtue of
high in-situ aragonite deposition and negligible ex-situ river input
whereas the high sedimentation rate during late Holocene is by the
virtue of ex-situ river discharge from Indus as well as localised seasonal
rivers with negligible in-situ contribution.
4.2. Paleoproductivity and sea level
The nutrient properties in shallow water depths are controlled by
river inputs (Macdonald et al., 1987) due to limited nutrient circulation
within the water column. Therefore the productivity and sea level vari-
ations in the core location of present study directly depend on fluvial
run-off in the area which ultimately depends on SWM precipitation as
well as melt water contribution in the Indus river as it is the most signif-
icant fresh water source in NE Arabian Sea (Milliman et al., 1984).
The primary productivity of marine organisms is an important factor
for climate studies as it accounts the partitioning of CO2 gas between the
atmosphere and ocean (Broecker, 1982). Since long time, a wide variety
of chemical and micropaleontological proxies have been applied to in-
terpret sediment properties for paleoproductivity studies (Müller and
Suess, 1979; Berger et al., 1989 and references therein). The present
study used chemical (CaCO3%) proxies to decipher paleoproductivity
of the area. However PFA and BFA are used to decipher foraminiferal
productivity in the area which also in turn depends on the nutrient
availability. On the other hand, one of the major factor which controls
the productivity in the coastal (20 - 120 m water depth) areas is ba-
thymetry, which is of great importance in the field of oceanography
since very long time. The main objective of paleobathymetry analysis in-
clude the study of upliftment and subsidence of sedimentary basin as
well as in the preparation of sea level fluctuation curve.
During Younger Dryas and early Holocene, comparatively low PFA
and BFA (Fig. 5) has been observed which correspond to lower forami-
niferal productivity in the area. On the other hand, high values of
CaCO3% (N95%) have been observed. Generally high CaCO3% corre-
sponds to high PFA and BFA but in this case the available CaCO3 in the
area was not utilised for the foraminiferal flourishment. X-ray studies
reveal that the high CaCO3 is by the virtue of aragonite abundance
(Fig. 4) in the form of relict hard mass and pallets. The dominance of ara-
gonite over calcite in the form of pallets and relict hard mass (Fig. 1)
suggest the growth of algae Halimeda in the area. Rao et al. (1994)
reported similar aragonite pallets from the western continental margin
of India during early Holocene due to increased Halimeda growth. These
algae grow and accumulate the CaCO3 in the form of carbonate debris
called as Halimeda bioherm (Raees et al., 2006). Earlier it was reported
to confined only with the Great Barrier reef, Java Sea and Nicaraguan
Rise areas of the world (Rao et al., 1994 and reference therein). For
the first time Rao et al. (1994) reported the growth of Halimeda along
the western continental margin of India during 13,700–8300 years be-
fore present. However, the reason of abundant carbonate debris was
not explained. Later, Raees et al. (2006) published the optimum
flourishing water depth of Halimeda as around 40–50 m from the SW
Caribbean waters. Therefore, in order to have a better understanding
of the study area we have divided the paleoproductivity into two differ-
ent types viz. 1) Algal productivity 2) Foraminiferal productivity. This
division is based on the algal and foraminiferal abundance which in
turn depends on their flourishing environments. The algal productivity
in the area is due to the algae Halimeda which favours low water depth
for its flourishment. The remains of Halimeda carbonate debris consist of
aragonitic CaCO3 (Rao et al., 1994). In contrast, the foraminiferal pro-
ductivity is the function of absolute abundance of planktonic foraminif-
era on ocean surface and benthic foraminifera on ocean bottom. These
mainly depend on the flux rate of incorporating sediments as well as
the dilution caused by terrigenous inputs. However there could be sev-
eral reasons for the variation of planktonic and benthic foraminiferal
population in the ocean. Their distribution is strongly linked to several
ocean water properties such as chemistry, temperature, turbidity etc.
Foraminifera are very specific to their survival range when it comes to
the abundance. The optimum range of their survival which defines
their absolute abundance is typically narrow and distinct from other
Fig. 6. Temporal variation of δ18
OG.ruber (‰ VPDB) showing wet and dry oscillations due to
monsoonal variation. Dark gray bands indicate abrupt climate variations (Cooling events -
YD = Younger Dryas; AE = Abrupt event; 8.2 Ka event and 4 Ka cooling) (Warming event
– PB = Preboreal).
6 S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
7. proxies. Therefore, any change in the ocean conditions can distinctly be
observed in the corresponding PFA and BFA of the depositing sediments.
The lower values of PFA, BFA and P/B ratio (Fig. 5) during Younger
Dryas and early Holocene also indicate the coastal sub-tidal to inter-
tidal conditions at the core location. The low quartz abundance (Fig.
4) during Younger Dryas and early Holocene indicates that the area
was deprived of riverine input as well as the nutrient supply from
Indus River. Therefore the biological along with the mineralogical
proxy record suggest the lowered sea level during the Younger Dryas
and early Holocene. It should be noted that the present day water
depth of the core site is 88 m. Therefore, if the Halimedal signatures
were dominant during Younger Dryas and early Holocene, it suggest
at least ~40–50 m lowered sea level during that time (as discussed
above) at the present core location. Furthermore, the more evidence
from the Holocene sea level curve for western Indian continental mar-
gin (Hashimi et al., 1995) as well as the global sea level fluctuation re-
cord (Thompson and Goldstein, 2006; Smith et al., 2011) also reported
~50–70 m lowered sea level during onset of early Holocene. Within
the Younger Dryas and early Holocene, Hashimi et al. (1995) document-
ed ~70 m rise in sea level along the western continental margin of India
whereas Smith et al. (2011) marked up to ~60 m rise in sea level
globally up to ~7 Ka This remarkable increase in sea level is attributed
to the melt water release from decaying ice masses (Smith et al.,
2011). More evidently, planktonic foraminifera percentage over total fo-
raminiferal abundance (Fig. 7) is lower during the abrupt cold events
(discuss in the later section), hence it can be inferred that the sea level
fluctuations may have played a major role on the present site during
Younger Dryas and early Holocene which was the governing factor for
the productivity signatures, less foraminiferal abundance and low nutri-
ent supply from the Indus river discharge. Therefore, during Younger
Dryas and early Holocene when the water depth was low at the core
location and the conditions were not favourable for the foraminiferal
growth, hence, recorded a low foraminiferal productivity.
The early-middle Holocene transition (8–7 Ka) is recorded by drastic
variations in biological and chemical proxies. However the biological
proxy responded ~500 years before the chemical proxy response. The
middle Holocene recorded comparatively higher PFA and BFA (Fig. 5)
which demarcates the increased foraminiferal productivity. Also the
high and fluctuating percentage of coarse fraction (N63μm) indicate
high energy depositional environment of biogenic and terrigenous ma-
terial which increased the turbulence of surface and intermediate water
resulting in high nutrient supply due to mixing. Naik et al. (2014) also
recorded similar increased productivity in the middle to late Holocene
geochemical proxy record from the Core AAS9/19. Although the
CaCO3% during middle Holocene is recorded lower than that during
Younger Dryas and early Holocene, still it favoured foraminiferal growth
due to the dominance of Calcite and by the virtue of raised sea level (as
discussed later in the section). This led to the cessation of algal growth
in the area which in turn decreased the aragonite abundance during
middle Holocene. Decreased aragonite abundance during middle and
late Holocene have also been recorded earlier from marine sediment
cores SK-17 (Singh, 2007) and AAS9/19 (Naik et al., 2014) due to the in-
crease in OMZ intensity. Since the present core location falls out of the
OMZ, so the decreased aragonite abundance could be attributed to
ceased algal productivity in the area. A remarkable increase in PFA,
BFA and P/B ratio (Fig. 5) during the middle Holocene (8–4 Ka) plausibly
be related to the raised sea level after 8 Ka. The sudden increase in
Quartz and calcite (Fig. 4) with simultaneous decrease in aragonite
abundance between 7.5 and 7 Ka demarcates a major variation in sea
level, nutrient supply and river water discharge within the middle Holo-
cene. Also the increased δ13
C during middle Holocene points towards
increased productivity after early Holocene. This is due to the fact that
primary producers utilise 12
C during photosynthesis thereby increasing
13
C/12
C ratio in the surface water and hence increased δ13
C signatures
(Fig. 5). The simultaneous increase in calcite abundance along with in-
creased PFA and BFA points towards raised sea level which would
have resulted in the increased water depth of the area providing suit-
able ambient environment for the calcification and foraminiferal
growth. Also the decreased aragonite with increased quartz abundance
suggests the onset of riverine flux in the area due to raised sea level
which ceased the Halimeda growth during middle Holocene. The high
P/B ratio as well as percentage of coarse fraction (N63μm) around 6 Ka
is probably related to Holocene sea level maxima (HSLM). The sea
level curves from western continental margin of India (Hashimi et al.,
1995) and Singapore (Bird et al., 2010) also recorded the HSLM around
6 Ka After HSLM the minor drop in sea level has been recorded within
the middle Holocene by P/B ratio (Fig. 5) and planktonic foraminifera
percentage (Fig. 7).
Maximum values of PFA and BFA are recorded during the late
Holocene (Fig. 5) as compared to early and middle Holocene and this
corresponds to Holocene foraminiferal Productivity Maxima (HPM).
However, the values of CaCO3% are lower during the late Holocenethan
early and middle Holocene. It is intriguing that calcite predominates
over aragonite and thus CaCO3 favoured foraminiferal flourishment
Fig. 7. Planktonic foraminifera percentage in total foraminifera population. Rectangular bands indicate productivity collapse during YD = Younger Dryas; AE = Abrupt event and 8.2 Ka
event. Productivity regenerated between YD and AE i.e. PB = Preboreal, HSLM = Holocene Sea Level Maxima.
7S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
8. during late Holocene. During the onset of late Holocene, high values of
the P/B ratio suggest the slight lowering of sea level which is also sup-
ported by PFA and BFA records (Fig. 5). Similar observations have
been recorded in oyster bed from Saurashtra coast (Banerji et al.,
2015) which marked the records of ~2 m elevated sea level around
3.5 Ka.
4.3. South-west monsoon (SWM)
The 2 °C variation in sea surface temperature during the Holocene in
eastern Arabian Sea has been recorded (Govil and Naidu, 2010). Other
SST studies from eastern Arabian Sea include 2.5 °C variation since
mid-Holocene in the SE Arabian Sea (Tiwari et al., 2015), 2 °C variation
since mid-Holocene from 800 m water depth (Kessarkar et al., 2013)
and 1.5 °C SST variability in the deep water (1245 m) offshore Malabar
coast, SW Arabian Sea (Saraswat et al., 2013). Other than SST, mainly
evaporation, precipitation and river run-off during SWM influence the
δ18
O of sea water and hence the δ18
O of calcite. Recent studies from
south-eastern as well as central Arabian Sea (Kumar and Ramesh,
2016 and references therein) indicate that the salinity/δ18
O of these
areas depend on NEM precipitation run-off from Western Ghats as
well as the effect of NE monsoon driven West Indian Continental Cur-
rent (WICC), which brings low salinity/δ18
O water from Bay of Bengal
to the Arabian Sea. However the effect of WICC is not significant in the
NE Arabian Sea. The salinity/δ18
O of NE Arabian Sea depends significant-
ly on Indus river fresh water run-off (both glacial and snow melt as well
as SWM precipitation) which is ultimately driven by SW Monsoon. In
general, the Indus river system consist of a variety of climate regimes
which affect water input from various stratified climate zones. These in-
clude temperature induced run-off in high altitudes catchments cov-
ered by large areas of glaciers, winter precipitation dominated by
succeeding summer flow in the mid altitudes and the monsoon induced
rainfalls in the foothill areas (Archer, 2003). Hydrological regime of the
Indus basin is primarily controlled by snow and glacial melts
(Mukhopadhyay and Dutta, 2010). Immerzeel et al. (2010) modelled
the upper Indus basin and concluded that the total stream flow in the
area include 34% snow melt and 26% glacial melt indicating a regime
which is highly sensitive to temperature. The run-off generated by gla-
cier and snow melting is 151% greater than flow generated in down-
stream areas due to monsoon precipitation (Immerzeel et al., 2010).
However a variable pattern is suggested by Winiger et al. (2005)
reporting 70% of the annual run-off which is entering in the plain is
due to the seasonal monsoonal rains in lower parts of the basin whereas
Singh and Bengtsson (2005) reported 49% snow and glacial melt contri-
bution in the Chenab river at Akhnoor. Hence, it can be concluded that
the Indus river system consist of glacial and snow melt as the primary
driver of downstream flows. Therefore, the impact of declining glacial
masses on river run-off due to climate change is more substantial to
Indus system due to high melt water contribution to the total run-off.
Singh and Bengtsson (2005) suggested that with increase in
temperature of ~1–3 °C, there would be a decrease of 11–23% snow
melt contribution in Sutlej basin but also indicated 16–50% increase in
glacial melt, resulting an overall increase in melt water flux due to
increase in temperature. Therefore δ18
O values in the present study
may also be attributed to the fluvial discharge magnitude from Indus
river. G. ruber are surface dwelling planktonic foraminifera which lives
throughout the year (Guptha et al., 1997; Govil and Naidu, 2011) and
hence these are expected to record riverine flux induced changes
around the core location. Therefore we have evaluated δ18
OG. ruber (Fig.
6) variability down core in order to understand the monsoonal variabil-
ity in the offshore Saurashtra region of NE Arabian Sea.
SWM is the major source of moisture over the Indian sub-continent.
It is also responsible for the riverine flux as well as salinity of the Arabian
Sea. In the Arabian Sea, every 1 p.s.u. (practical salinity unit) change in
Sea Surface Salinity (SSS) are found to be related to 0.33‰ of δ18
O
(Duplessy et al., 1981; Sarkar et al., 2000). Our data documents an
average ~1.1‰ difference in δ18
OG. ruber between early and late
Holocene. Higher values of δ18
OG. ruber during Younger Dryas and early
Holocene as compared to middle and late Holocene corresponds to
weakened SW monsoonal conditions leading to arid climate. Similar
Younger Dryas and early Holocene weakening of SWM have been
documented from paleolimnological studies of Thar Desert lake which
recorded low lake levels and high evaporation rate (Prasad and Enzel,
2006).
Heavier incursions of ~0.7‰ and ~0.5‰ have been observed in δ18
O
values around 11.5 and 8.2 Ka (Fig. 6) respectively which corresponds to
Younger Dryas (YD) and 8.2 Ka cooling events. These cooling events
point towards further weakening of monsoon during YD and 8.2 Ka.
Similar incursions have also been recorded in other parts of Indian
ocean i.e. δ18
Oc of benthic foraminifera C. wuellerstorfi in the core ABP-
25.02 from NE Arabian Sea (Gupta et al., 2011), δ18
Osw of cores SK
218/1 from Bay of Bengal (BOB) (Govil and Naidu, 2011), RC12–344
from Andaman Sea (Rashid et al., 2007), SK-17 from eastern Arabian
Sea (Anand et al., 2008) and δ18
Oc of KL126 from northern BOB
(Kudrass et al., 2001) which reveals the weakening of SWM rainfall dur-
ing YD. Terrestrial records which also document weakening of SW mon-
soon during YD include speleothems from India (Sinha et al., 2005) and
China (Wang et al., 2001). However, the western Arabian Sea upwelling
indices record do not show any signs of reduced upwelling strength
during YD (Naidu and Malmgren, 1996; Overpeck et al., 1996; Gupta
et al., 2003). The YD cooling was a typical event of Northern Hemisphere
(Ohkushi et al., 2016). For example, the δ18
O records of planktonic and
benthic foraminifera from continental slope, Offshore Hokkaido, Japan
(Ohkushi et al., 2016) show cooling signatures during YD. Whereas,
no YD signatures were observed in Antarctic ice-core record of Blunier
and Brook (2001). Another abrupt cooling event (AE) (Figs. 6 & 7)
centring around 9.8 Ka have been observed in our record which corre-
sponds to weakening of SWM which could have resulted in lower
Indus river discharge on the core site further leading to reduced produc-
tivity. After 8.3 Ka we observed another sudden increase in δ18
O values
which correspond to 8.2 Ka cooling which also resulted in decreased
productivity and Indus discharge due to weakened SWM. Similar obser-
vations have also been documented from δ18
O records of bulk carbon-
ates from paleolake Riwasa, North India (Dixit et al., 2014) and
radiometrically-dated speleothems (Cheng et al., 2009; Liu et al.,
2013) which point towards the short term but pronounced weakening
of SWM at 8.2 Ka.
Lighter incursion of ~0.7‰ in δ18
O value is observed around 11.2 Ka
soon after the YD cooling which corresponds to Preboreal (PB) warming
period. Kessarkar et al. (2013) reported similar abrupt event centring
around 11.2 Ka suggesting huge fresh water input due to high variability
of rainfall during that period. The lower values of δ18
OG. ruber during mid-
dle Holocene and subsequently more lower values during late Holocene
suggests the amelioration in the magnitude of SW monsoon after 8 Ka.
Therefore it is concluded, that during Younger Dryas and early Holocene
overall SWM was weak and further underwent three main rapid cooling
phases viz. Y.D., AE and 8.2 cooling event as well as one short warming
event i.e. PB. (Fig. 6).
Middle Holocene recorded comparatively lower (~0.5‰) average
δ18
OG. ruber values which corresponds to strengthening of SWM. Similar
strengthening of SWM after 8 Ka has been documented in the foraminif-
eral SST records from western Arabian Sea (Saher et al., 2007).
Further strengthening of monsoon took place during late Holocene
which increased Indus riverine input at the core location. However, a
heavier incursion of ~0.4‰ δ18
OG. ruber (Fig. 6) is observed during 4.1–
3.2 Ka which corresponds to short term weakening in the strength of
SWM. Similar cooling trend has also been observed around 4600–
3300 years B.P. in the temperature and salinity record of core SO90-
56KA (Rolinski et al., 2001) from offshore Pakistan NE Arabian Sea
around 4.5 Ka as well as δ18
O and salinity records of core AAS62/1
from SE Arabian Sea (Kessarkar et al., 2013) due to weakening of
SWM. This weakening of SWM is associated to major low latitude
8 S. Azharuddin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx
Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE
Arabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018
9. drought event and southward movement of ITCZ (Mayewski et al.,
2004). Palynological record from central Himalaya (Phadtare, 2000)
observed sharp decrease in conifer pollen whereas relative increase in
cold tolerant evergreen oak-pollen during 4000–3500 years B.P. which
also suggest progressive cooling. During that time, the reduction in
fresh water flux from Indus river has been reported (Staubwasser et
al., 2003) which could be a result of weakened SWM and glaciations
in Karakoram and Western Himalaya.
4.4. Relationship between productivity collapse, sea level and Indus river
discharge
The productivity of present core location, being at shallower depth
and outside OMZ regime, directly depends on the nutrient availability
from the Indus river run-off. Hence any fluctuation in the run-off
would have directly affected the productivity of the area. The intensity
of Indus river run-off mainly depends on the SWM precipitation
(Milliman et al., 1984) as well as the melt water input from the glaciers
and snowfields of Karakoram and Western Himalayas (Wake, 1989).
Therefore deglaciation in the Karakoram and central Himalaya would
account for high nutrient availability from the Indus river discharge
and vice-versa. The δ18
OG. ruber (Fig. 6) record suggest the low monsoon
intensity during YD, AE, 8.2 as well as Late Holocene cooling between
4.1 and 3.2 Ka which marks the cooling events (discussed in the above
section). These events are also reflected in lower planktonic foraminif-
era percentage (Fig. 7) which demarcate the foraminiferal productivity
collapse during YD, AE and 8.2 cooling whereas improved foraminiferal
productivity during PB warming. Moreover the remarkable decrease in
the value of δ13
C G. ruber during these events also point towards pro-
ductivity decline. These results are consistent to the hypothesis that
colder periods result in year round productivity collapse due to weak-
ened monsoon (Singh et al., 2011b) in the eastern Arabian Sea. The
Indus river discharge (which was already at the low during Younger
Dryas and early Holocene) further declined considerably pronouncing
the present site oligotrophic resulting in foraminiferal productivity col-
lapse. However no such productivity decline in the planktonic forami-
nifera percentage record have been found corresponding to late
Holocene cooling probably due to raised sea level and sufficient calcite
abundance which prevented the late Holocene cooling effect on forami-
niferal flourishment.
5. Conclusions
The present multi-proxy study over offshore Saurashtra NE Arabian
Sea sediment core reveals that the area underwent three major phases
of climatic variations viz. Younger Dryas and early Holocene (12–
8 Ka), Middle Holocene (8–4 Ka) and Late Holocene (4 Ka to recent).
The core location belongs to shallow water depth (88 m) i.e. above
OMZ as well as possess negligible effect of SWM nutrient upwelling
and NEM mixed layer. This implies that the area strongly depends on
Indus river discharge for maintenance of productivity.
Younger Dryas and early Holocene records comparatively low sea
level and productivity signatures. The area was deprived of Indus river
discharge possibly due to weakened SWM which resulted in arid condi-
tions. YD, AE and 8.2 Ka cooling as well as PB warming events have also
been identified during Younger Dryas and early Holocene.
Early-middle Holocene transition reveals abrupt climatic variations
between 8 and 7 Ka. Biological proxy responded ~500 years earlier
than chemical proxy response to the transition. The middle Holocene
recorded comparatively raised sea level and higher productivity signa-
tures. HSLM has been recorded around 5.5 Ka. Improved SWM strength
have been recorded which promoted Indus river discharge around the
core location.
Late Holocene recorded slight lowered sea level. Productivity im-
proved marginally which marked the HPM around 3.5 Ka SWM strength
further intensified which resulted in increased run-off from Indus river
increasing fresh water nutrient supply in the area. Moreover, 4 Ka
cooling event has been observed due to weakened SWM.
Acknowledgement
The authors are thankful to Prof. Sunil Bajpai, Director, BSIP,
Lucknow for providing necessary facilities to carry out this work and
permitting us to publish the same. Extended thanks to the shipboard
team of 143rd Sagar kanya cruise for sampling core location SK-240/
485. SA is also thankful to SERB-DST Project No. SR/FTP/ES-53/2013
for the award of Junior Research Fellowship. This work was supported
by fastrack grant of SERB-DST under project No. SR/FTP/ES-53/2013.
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