azharuddin et al., 2016

Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx–xxx

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Monsoon-influenced variations in productivity and lithogenic flux alongoffshore Saurashtra, NE Arabian Sea during the Holocene and YoungerDryas: A multi-proxy approach

Syed Azharuddin a, Pawan Govil a,⁎, A.D. Singh b, Ravi Mishra c, Shailesh Agrawal a,A.K. Tiwari c, Kamlesh Kumar a

a Birbal Sahni Institute of Palaeosciences, 53-University Road, Lucknow 226 007, Indiab Department of Geology, Banaras Hindu University, Varanasi 221 005, Indiac National Centre for Antarctic and Ocean Research, Vasco-da-Gama, Goa 403 804, India

⁎ Corresponding author.E-mail address: [email protected] (P. Govil).

http://dx.doi.org/10.1016/j.palaeo.2016.11.0180031-0182/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Azharuddin, S., et aArabian Sea during the Holocene an..., Palaeo

a b s t r a c t
a r t i c l e i n f o
Article history:Received 30 May 2016Received in revised form 17 October 2016Accepted 11 November 2016Available online xxxx

A sediment core from NE Arabian Sea was studied to reveal the variations in shelf environment of western con-tinental margin of India. Amulti-proxy analysis based on planktonic foraminifera abundance (PFA), benthic fora-minifera abundance (BFA), planktonic-benthic foraminiferal ratio (P/B), CaCO3 percentage (wt%), Oxygen (δ18OG.

ruber) and Carbon (δ18OG. ruber) isotopes of foraminifera as well as sediment X-ray studies were carried out on3.4 m long core SK-240/485 recovered from offshore Saurashtra, NE Arabian Sea. The study aims to understandthe monsoon variability in the NE Arabian Sea offshore Saurashtra since the late Pleistocene-Holocene transitionand howmonsoon climate coupled with sea-level change influenced paleo-productivity pattern. Six AMS radio-carbon dates place the core in the time interval 12–1 Ka. Our study reveals that there have been three periods ofmajor changes in the monsoonal climate, productivity and sea level; viz. 12–8 Ka (Younger Dryas and early Ho-locene); 8–4 Ka (middle Holocene) and 4–1 Ka (late Holocene). The studied time period was also marked byweakened south west monsoon (SWM) during the cold events such as Younger Dryas (YD), Abrupt Event (AE)centring around 9.8 Ka as well as 8.2 and 4 Ka cold events. The Younger Dryas and early Holocene shows thelow sea level stand and less Indus river discharge. Thereafter during middle and late Holocene the monsoonstrengthened which resulted in raised sea level and increased Indus river discharge. The proxy records broadlysuggest relatively low foraminiferal productivity at the core location during the Younger Dryas and early Holo-cene. We record a moderate foraminiferal productivity during the middle Holocene between 8 and 4 Ka,which suggest the mesotrophic condition. The foraminiferal productivity however was relatively high since4 Ka suggesting the eutrophic conditions at the core site.

© 2016 Elsevier B.V. All rights reserved.

Keywords:PalaeoclimateNE Arabian SeaOxygen isotopePlanktonic foraminiferaOffshore Saurashtra

1. Introduction

In the northern Indian Ocean, the surface circulation is controlled byseasonal monsoon wind system, with a strong and humid South WestMonsoon (SWM) during summer (June – September) (Nair et al.,1989; Haake et al., 1993) and a moderate and dry North East Monsoon(NEM) during winter (December to February) (Böll et al., 2014 and ref-erences therein). Both SWM and NEM are the result of differentialheating of land and ocean during summer and winter seasons respec-tively. SWM is the result of development of low surface atmosphericpressure over Asia (centred at Tibet and northern India) and high sur-face atmospheric pressure develops over the southern Indian ocean(Shea, 1986). Therefore, a strong south to north air flow rises which

l., Monsoon-influenced variageogr. Palaeoclimatol. Palaeo

blows steadily along SW at 15 m/s over the region of Arabian Sea(Hastenrath and Lamb, 1979). During NEM the direction of windreverses due to high pressure over Asian continent while low pressuredevelops over Indian Ocean.

The Arabian Sea is one of themost productive basinsworldwide, be-cause of SWM induced nutrient upwelling (Ryther and Menzel, 1965)and winter NEM related mixed layer nutrient input (Madhupratap etal., 1996) by virtue of wind circulation pattern during summer andwinter seasons respectively. In the western and central Arabian Sea,most sediment trap studies indicate maximum biological productivityduring the SWM (Broerse et al., 2000; Wakeham et al., 2002), whereasin the NE Arabian Sea, maximum particle fluxes are observed duringthe winter monsoon season (Goes et al., 1992; Andruleit et al., 2000;Schulz et al., 2002; Matondkar et al., 2006). This is because in the NEArabian Sea the particle flux is determined by re-suspension of localsediment and winter productivity rather than by summer monsoonal

tions in productivity and lithogenic flux along offshore Saurashtra, NEecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.11.018

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upwelling, which results in a non-upwelling environment, in contrast tothe summer upwelling regime offshore Oman, Somalia and southernIndia (Schulz et al., 2002).

Planktonic foraminifera have been recognised as potential proxy forpaleoceanographic studies since a very long time. Due to their highabundance and extensive presence in almost all marine environments,foraminifera are widely used to reconstruct paleoclimate from marinearchives (Saraswat, 2015). The absolute abundance of foraminifera insediments mainly depend on 2 factors viz. 1) Variations of planktonicforaminiferal population in the surface layer and benthic foraminiferalpopulation on the seafloor resulting in change in rate of flux of the spec-imen which incorporates in the sediments, 2) the effect of dilutioncaused by other materials, such as terrigenous influx (Kroon et al.,1993). At present the most accurate and widely important proxy usedfor reconstruction of paleo-sea level are of micropaleontological nature(Van Der Zwaan et al., 1990). Therefore in the present study we usedPFA, BFA and P/B ratio supported by mineralogical (Quartz, Calcite andAragonite) abundance for the sea level reconstruction. The idea startedwith the pioneer study of Phleger (1951)whichwas followed by the at-tempt of Grimsdale and Van Morkhoven (1955) to use P/B ratio andplanktonic foraminiferal percentage (Nigam and Henriques, 1992) inpaleodepth reconstruction and found that the proportion of planktonicforaminifera in the total foraminiferal population increase towardsdeeperwater. This increase in the planktonic foraminiferawith distancefrom the coast is due to the decrease in turbidity (Berger and Diester-Haass, 1988). However BFA follows a slightly different pattern fromPFA. It increases from near shore to continental edge and furtherdecreases seawards (Douglas and Woodruff, 1981).

The productivity of an ocean mainly depends on source of nutrientsupply. The source of nutrient may be from the outside supply of nutri-ents (river influx) or the internal cycling (upwelling and/ormixed layercirculation) of nutrients within the water column (Berger et al., 1989).The uniqueness of the present study is that the core location belongsto such a shallowwater depth (88m)which is above oxygenminimumzone (OMZ). Only few limited attempts (Singh et al., 2007; Nisha andSingh, 2012) were made in the past to study the evolution of such ashallow shelf environment of the Arabian Sea, where both the upwell-ing as well as mixed layer nutrient input does not have significant rolein the productivity of the area. In such a case, the paleoproductivity ofthe area mainly depends on the fresh water nutrient input. Previousstudies from the NE Arabian Sea (Milliman et al., 1984) reveal that theIndus river has been the only significant source of freshwater dischargein the area during Holocene. Hence it can be concluded that thepaleoproductivity along the core sitemainly depends on freshwater nu-trient input from Indus river. Therefore, the present study aims to get ahigh resolution monsoonal record to understand Indus river discharge,sea level fluctuation and productivity variations in the NE Arabian Seaoffshore Saurashtra during the Younger Dryas and Holocene using amulti-proxy approach.

1.1. Previous studies

Despite widespread concern, limited attempts were made to studythe paleoceanographic and paleoclimatic variations over shelf regionoffshore Saurashtra of theNEArabian Sea using amulti-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 etal., 1995), organic carbon (Corg) distribution (Babu et al., 1999),210Pbxs, 137Cs and 14C (Somayajulu et al., 1999) to decipher the sedimentdeposition 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

Please cite this article as: Azharuddin, S., et al., Monsoon-influenced variaArabian Sea during the Holocene an..., Palaeogeogr. Palaeoclimatol. Palaeo

decipher carbonate preservation as well as monsoon wind inducedhydrographic changes and mudflat (Banerji et al., 2015) to study themid Holocene land sea interactions.

There are numerous evidences which explain the lowering of sealevel during early Holocene along the western continental margin ofIndia. The geomorphic features and associated sediments on the outershelf 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 onthe outer continental shelf of the north-western margin of India (Raoet al., 2003). The FFF consist of relic sediments which are mainly ooliticand palletal aragonite sands, having tan colour landward and shinywhite towards the offshore (Rao et al., 2003). The age of these sandsrange from 14.3 to 7.6 Ka (Rao et al., 2003). Other important studies in-clude the earlyHolocene evidence of relict sand zone, coated grains, pal-lets and benthic foraminifera of shallowwater origin (Hashimi andNair,1976). On the basis of these evidences, a Holocene sea level curve for thewestern continental margin of India was prepared (Hashimi et al.,1995). The curve indicated the lowering of sea level during earlyHolocene. During middle and late Holocene the sea level raised rapidlyand took a nearly stable mark up to the recent times.

1.2. Regional oceanographic settings

The studied sedimentary core is located in the shelf offshoreSaurashtra, 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 thewest and north and by coastal highlands of western India tothe east. Three major river (Indus, Narmada and Tapti) discharge theirenormous fresh water and sediments into the AS. The Indus fan beingthe second largest fan (Clift et al., 2002) is the most extensive physio-graphic feature of the AS in the Northern Indian Ocean covering anarea of approximately 1.1–1.25 million km2 with a length of 1500 kmand a maximum width of 960 km (Govil and Naidu, 2007). The IndusFan is bounded by continental margin of India-Pakistan and Chagos-Laccadive Ridge on the east, by the Owen and Murray Ridges on thewest and north, and by Carlsberg Ridge on the south. The sediments inthe Indus fan are mainly brought by Indus river (Govil and Naidu,2007). The Narmada and Tapti rivers drain the peninsular shield ofIndia also contribute sediments to eastern AS whereas the Indus riverdrains the northern AS including offshore Pakistan, Kutch andSaurashtra coast. The deposited sediments in to the ocean basin havealways been used to infer paleoclimatic variations at different timescales from decadal to centennial to millennial. A strong precipitationgradient exists across the AS due to the effect of SWM (Staubwasser etal., 2003). Indus river has been the only significant source of freshwater 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 observedin the sea surface temperature (SST) and mixed layer depth of NEASby 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 NationalCentre of Antarctic and Ocean Research (NCAOR), Goa, India. Core SK240/485 (Fig. 2) was collected during Sagar kanya cruise no. 143rdfrom the continental margin of Northeastern Arabian Sea offshoreSaurashtra region (Lat 21°16′N Long 68°55.99′E) at a water depth of88 m. The core is 340 cm long sub sampled at 2 cm interval to obtaina high temporal resolution. Six sediment samples including core topand bottom were dated using 14C accelerated mass spectrometry(AMS) technique at Radiocarbon Laboratory, Institute of Physics, Centre



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 earlyHolocene as described earlier (Rao et al., 1994) (B)&(C) Similar aragonite pallets found in our samples during Younger Dryas and early Holocene.

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for Science and Education, Konarskiego, Gliwice, Poland and NSFArizona AMS Facility, University of Arizona, U.S. The ages wereconverted to calendar year BP (1950) by applying appropriate reservoircorrection (ΔR) of 129 ± 35 years for the Saurashtra coast (Dutta et al.,

Fig. 2.Map showing core location SK 240/485 (Lat 21°16′N Long 68°55.99′E) collec


2001) and calibrated using Calib 7.1 version (Stuiver and Reimer, 1993)(Fig. 3).

10 g of sediment samplewas dried keeping in an oven at 60 °C. Driedsediment sample weighed and soaked in distilled water overnight.

ted from offshore Saurashtra, NE Arabian Sea during 143rd Sagar kanya cruise.



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.


Water was decanted from top without disturbing the bottom settledsediment. To remove organic matter and clay lumps, 10ml of 10% sodi-umhexa-meta-phosphate (NaPO3)6 and 5ml of 10% hydrogen peroxide(H2O2) solutions were added respectively and kept overnight. Subse-quently these samples were wet sieved through a 63 μm size sievewith enough care to prevent the breakage of foraminifer shells.N63 μm fraction (coarse fraction) retained on the sieve was transferredin to 25 ml beaker and dried in the oven at 50–60 °C. The dried fractionwas again sieved through 250 μmand150 μmsieves,weighed (to obtainsand percentage in the sediment) and used for census counts of Plank-tonic (total planktic foraminiferal abundance: PFA) and Benthic (totalbenthic foraminiferal abundance: BFA) abundance according to thestandard procedure (CLIMAP, 1976) of foraminiferal counting. 25–30clean 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 samplewas taken in Carbonate bomb jar and 5 ml 12% HCl was inserted inthe cylinder. This was carefully done to avoid spilling of acid over sedi-ment sample while the jar is unscrewed. Once the jar was screwedtightly by the manometer head, the acid was spilled and allowed toreact with sediment sample. The CO2 thus produced created pressurein the jar which was recorded by attached manometer giving theCaCO3 (uncorrected) reading. Before running a batch of sample everytime, CaCO3 standard reading was recorded using Sigma-Aldrich 100%CaCO3 standard powder in order to calibrate the recorded uncorrectedmanometer reading. Calibration of scale was done using the followingequation (Müller and Gastner, 1971) -

CaCO3%sample ¼CaCO3 manometer reading of sampleð Þ � 100CaCO3 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 PowderXRD instrument at XRD laboratory, Birbal Sahni Institute ofPalaeosciences (BSIP), Lucknow, India. X-rays were allowed to passthrough fine powdered sediments placed on sample holder stage. Theanalysis was targeted to obtain 2θ values ranging between 20° and40°. The diffracted X-rays give signature peaks at specific 2θ angle (bysatisfying Bragg's law) of the corresponding minerals. The peak countswere noted for quartz, calcite and aragonite in order to obtain the min-eralogical abundance.


Formeasuring δ18O and δ13C values, ~15–20 specimens of plankton-ic foraminifera surface water species G. ruber, were put into individualscrew capped glass vials. The vials were systematically kept in the GasBench along with three carbonate standards i.e. NBS 18, Merk andMMB (n=5 in each run). Subsequently, the vials were flushedwith ul-trapure He gas for about 10min per sample. After flushing, 100% ortho-phosphoric acid (H3PO4) was injected into each vial which was kept at72 °C temperature bath for 2 h. The evolved CO2 was purified by Naflontube and Pora pack column in Gas Bench and allowed into ContinuousFlow Isotope Ratio Mass Spectrometer (CFIRMS, MAT 253) for analysis.Each measurement comprised of three pulses of reference followed bysix pulses of sample CO2 gas. The tank reference gas was calibrated byusing NBS-18. All samples including internal standard MMBwere 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 δ18Oand δ13C values on daily basis. These samplesweremeasured at the sta-ble isotope facility at BSIP, Lucknow, India.

3. Results

The rate of sedimentation at the core site ranges between 18.3 and47.9 cm/Ka. The maximum rate of 47.9 cm/Ka has been recorded atthe core top between 1.4 and 2.7 Ka (0-62 cm) which decreased to35 cm/Ka to the next date interval i.e. 4.2 Ka (114 cm) and decreasedfurther 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 resolutionof ~40–110 years per sample which is first of its type from the offshoreSaurashtra region.




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 last12.65 Ka During Younger Dryas and early Holocene the CF % maximaandminima lies at 8.2% at 8.38 Ka and 1.5% at 9.76 Ka respectively. Dur-ingmiddle Holocene, themaxima increases to 43.1% at 5.86Ka andmin-ima lies on 4.5% at 4.33 Kawhile the late Holocenemarked amaxima of21.4% at 1.56 Ka and minima of 4.1% at 3.13 Ka.

Planktonic foraminifera abundance (PFA) (Fig. 5) is the number ofundistorted intact planktonic foraminifera shells present per gram ofthe sample. In our dataset, PFA ranges between 1413 and 191 intactshells per gram at 9.7 and 8.38 Ka respectively during the YoungerDryas and earlyHolocene. DuringmiddleHolocenePFA ranged between6464 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 intactshells per gram at 2.85 and 1.86 Ka respectively.

Benthic foraminifera abundance (BFA) (Fig. 5) is the number ofundistorted intact benthic foraminifera shells present per gram of thesample. The fluctuation in BFA is the measure of nutrient availabilityin the benthic environment (Herguera and Berger, 1991) which inturn depends on the water depth. Generally in the continental shelf re-gion BFA is directly proportional to water depth. In our record, the BFAranges between 2228 and 535 intact shells per gram at 10.2 and10.45 Ka respectively during the Younger Dryas and early Holocene.

Fig. 5. Productivity and sea level variations in the core SK-240/485 ( bottom to top)Planktonic foraminifera Abundance per gram (PFA), Benthic foraminifera Abundanceper gram (BFA), Planktonic/Benthic foraminifera (P/B Ratio), CaCO3%, δ13CG.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 Kacooling) (Warming event – PB = Preboreal). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)


During middle Holocene PFA ranged between 2343 and 1000 intactshells per gram at 4.22 and 7.99 Ka respectively, while during lateHolocene BFA ranged between 7669 and 1906 intact shells per gramat 1.61 and 3.99 Ka respectively.

The planktic/benthic foraminifera (P/B) (Fig. 5) ratio is the ratiobetween the PFA and BFA at a particular interval of sample. The P/Bratio generally follows a directly proportional trend towards increasingwater depth (Van Der Zwaan et al., 1990). In our results the P/B ratioranges between 1.59 and 0.2 at 8.12Ka and 12.52 Ka respectively duringYounger Dryas and early Holocene. While during middle Holocene, itranges 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 at4.84 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 foraminiferaldevelopment due to low water depth and we get high CaCO3% by thevirtue of relict carbonates which are result of Halimeda bioherm fecalpallets. Singh et al. (2006) also suggested the down-core variation ofCaCO3 attributed to aragonite preservation in the eastern Arabian Sea.The recorded CaCO3% minima and maxima values in our results are92.27 at 11.85 Ka and 99.01 at 8.58 Ka corresponding to the YoungerDryas and early Holocene. However for middle Holocene it rangesbetween 38.89 and 95.37 at 4.05 and 7.99 Ka respectively. During thelate Holocene the values range between 32.41 and 47.69 at 2.96 and1.98 Ka respectively.

X-Ray Diffraction (XRD) is the measure of mineralogical abundanceby using their 2θ angle tomeasure the absolute count of the correspond-ingminerals at their signature peaks. In the present studywemeasuredthe counts through the peaks of Calcite, Aragonite and Quartz (Fig. 4).During the Younger Dryas and early Holocene maxima and minima ofquartz 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 Karespectively. 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 at5.1 and 7.66 Ka respectively, while the calcite count lie between 3345and 1056 at 5.64 and 7.99 Ka respectively. The aragonite count duringmiddle Holocene ranged between 3782 and 1209 at 7.99 and 4.11 Karespectively. During the late Holocene XRD values for quartz countrange between 4781 and 3610 at 2.69 and 1.69 Ka respectively, whereascalcite range between 3349 and 2838 at 2.27 and 3.82 Ka respectively.The aragonite counts during late Holocene range between 1205 and1125 at 1.69 and 3.53 Ka respectively.

Carbon isotopes (δ13C) (Fig. 5) of foraminifera gives an idea of thecirculation pattern as well as nutrient content in the water mass(Kroopnick, 1985). To study the surface ocean circulation pattern andnutrient content, δ13C of planktonic foraminifera serves as a reliableproxy (Ravelo and Fairbanks, 1995). During Younger Dryas and earlyHolocene δ13C (‰ 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 and8.51 Ka. However during the middle Holocene δ 13C minima andmaxima lies between 0.7 and 1.7 (‰) at 8.12 and 6.19 Ka. During lateHolocene the minima values 0.7 (‰) at 1.52 Ka, while maxima values1.3 (‰) at 3.53, 3.19, 2.73, 2.36, 2.19. 1.82, 1.65 and 1.61 Ka.

Oxygen isotopes (δ18O) (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). Inthe NE Arabian Sea, the only significant source of freshwater dischargeis Indus river (Milliman et al., 1984). This freshwater from Indus riverconsist of meltwater discharge (by virtue of snow and glacial melting)as well as monsoon precipitation run-off, indicating a regime which ishighly sensitive to temperature changes (Miller et al., 2012). DuringYounger Dryas and early Holocene the δ18OG. ruber (‰ VPDB) maximavalues −0.7 (‰) at 11.73 Ka whereas minima values −2.2 (‰) at 9.7



Fig. 6. Temporal variation of δ18OG.ruber (‰ VPDB) showingwet and dry oscillations due tomonsoonal 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).


and 8.77 Ka. During middle Holocene the maxima and minima rangebetween−1.8 and−2.6 (‰) at 7.59 and 4.11 Ka respectively, whereasduring late Holocene the maxima and minima ranges between −2.0and −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 sedimentsupply from the fresh water river discharge around the core location(ex-situ) as well as the in-situ supply of sedimentmaterial due to local-ised production. The sedimentation observed at the core location re-veals a high depositional rate during the Younger Dryas and earlyHolocene. Whereas, the low percentage of coarse fraction (N63μm)along with low quartz and calcite abundance points towards low riverdischarge during that time. The high abundance of aragonite in theformof pallets and relictmass suggest thehigh 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 andfluctuating percentage of coarse fraction (N63μm) indicates high energyterrigenous input whereas sudden decrease in aragonite abundancesuggest the lack of in-situ sediment accumulation in the area. Howeverthe rate of in-situ sediment supply during Younger Dryas and earlyHolocene was much higher than the ex-situ terrigenous sedimentsupply during middle Holocene, therefore the area experienced overalldecrease in sedimentation rate during the middle Holocene.

During the late Holocenemaximum sedimentation rate has been re-corded with less fluctuations in the coarse fraction (Fig. 4). Maxima ofQuartz 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 inpercentage of coarse fraction (N63μm) indicate stable deposition condi-tions. Therefore, it can be concluded that thehigh sedimentation rate re-corded during the Younger Dryas and early Holocene is by the virtue ofhigh in-situ aragonite deposition and negligible ex-situ river inputwhereas the high sedimentation rate during late Holocene is by the


virtue of ex-situ river discharge from Indus as well as localised seasonalrivers with negligible in-situ contribution.

4.2. Paleoproductivity and sea level

The nutrient properties in shallow water depths are controlled byriver inputs (Macdonald et al., 1987) due to limited nutrient circulationwithin the water column. Therefore the productivity and sea level vari-ations in the core location of present study directly depend on fluvialrun-off in the area which ultimately depends on SWM precipitation aswell asmelt water contribution in the Indus river as it is themost signif-icant fresh water source in NE Arabian Sea (Milliman et al., 1984).

The primary productivity ofmarine organisms is an important factorfor climate studies as it accounts the partitioning of CO2 gas between theatmosphere and ocean (Broecker, 1982). Since long time, a wide varietyof chemical and micropaleontological proxies have been applied to in-terpret sediment properties for paleoproductivity studies (Müller andSuess, 1979; Berger et al., 1989 and references therein). The presentstudy used chemical (CaCO3%) proxies to decipher paleoproductivityof the area. However PFA and BFA are used to decipher foraminiferalproductivity in the area which also in turn depends on the nutrientavailability. On the other hand, one of the major factor which controlsthe productivity in the coastal (20 - 120 m water depth) areas is ba-thymetry, which is of great importance in the field of oceanographysince very long time. Themain objective of paleobathymetry analysis in-clude the study of upliftment and subsidence of sedimentary basin aswell as in the preparation of sea level fluctuation curve.

During Younger Dryas and early Holocene, comparatively low PFAand BFA (Fig. 5) has been observed which correspond to lower forami-niferal productivity in the area. On the other hand, high values ofCaCO3% (N95%) have been observed. Generally high CaCO3% corre-sponds to high PFA and BFA but in this case the available CaCO3 in thearea was not utilised for the foraminiferal flourishment. X-ray studiesreveal that the high CaCO3 is by the virtue of aragonite abundance(Fig. 4) in the formof relict hardmass and pallets. Thedominance 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 marginof India during early Holocene due to increasedHalimeda growth. Thesealgae grow and accumulate the CaCO3 in the form of carbonate debriscalled as Halimeda bioherm (Raees et al., 2006). Earlier it was reportedto confined only with the Great Barrier reef, Java Sea and NicaraguanRise areas of the world (Rao et al., 1994 and reference therein). Forthe first time Rao et al. (1994) reported the growth of Halimeda alongthe western continental margin of India during 13,700–8300 years be-fore present. However, the reason of abundant carbonate debris wasnot explained. Later, Raees et al. (2006) published the optimumflourishing water depth of Halimeda as around 40–50 m from the SWCaribbean waters. Therefore, in order to have a better understandingof the study area we have divided the paleoproductivity into two differ-ent types viz. 1) Algal productivity 2) Foraminiferal productivity. Thisdivision is based on the algal and foraminiferal abundance which inturn depends on their flourishing environments. The algal productivityin the area is due to the algae Halimedawhich favours low water depthfor itsflourishment. The remains ofHalimeda carbonate debris consist ofaragonitic 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. Thesemainly depend on the flux rate of incorporating sediments as well asthe dilution caused by terrigenous inputs. However there could be sev-eral reasons for the variation of planktonic and benthic foraminiferalpopulation in the ocean. Their distribution is strongly linked to severalocean water properties such as chemistry, temperature, turbidity etc.Foraminifera are very specific to their survival range when it comes tothe abundance. The optimum range of their survival which definestheir absolute abundance is typically narrow and distinct from other




proxies. Therefore, any change in the ocean conditions can distinctly beobserved in the correspondingPFA andBFA of thedepositing sediments.

The lower values of PFA, BFA and P/B ratio (Fig. 5) during YoungerDryas 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 areawas deprived of riverine input as well as the nutrient supply fromIndus River. Therefore the biological along with the mineralogicalproxy record suggest the lowered sea level during the Younger Dryasand early Holocene. It should be noted that the present day waterdepth of the core site is 88 m. Therefore, if the Halimedal signatureswere dominant during Younger Dryas and early Holocene, it suggestat least ~40–50 m lowered sea level during that time (as discussedabove) at the present core location. Furthermore, the more evidencefrom 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. Withinthe YoungerDryas and early Holocene, Hashimi et al. (1995) document-ed ~70m rise in sea level along the western continental margin of Indiawhereas Smith et al. (2011) marked up to ~60 m rise in sea levelglobally up to ~7 Ka This remarkable increase in sea level is attributedto 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 levelfluctuations may have played a major role on the present site duringYounger Dryas and early Holocene which was the governing factor forthe productivity signatures, less foraminiferal abundance and lownutri-ent supply from the Indus river discharge. Therefore, during YoungerDryas and early Holocene when the water depth was low at the corelocation and the conditions were not favourable for the foraminiferalgrowth, hence, recorded a low foraminiferal productivity.

The early-middle Holocene transition (8–7 Ka) is recorded by drasticvariations in biological and chemical proxies. However the biologicalproxy responded ~500 years before the chemical proxy response. Themiddle Holocene recorded comparatively higher PFA and BFA (Fig. 5)which demarcates the increased foraminiferal productivity. Also thehigh and fluctuating percentage of coarse fraction (N63μm) indicatehigh energy depositional environment of biogenic and terrigenous ma-terial which increased the turbulence of surface and intermediate waterresulting in high nutrient supply due to mixing. Naik et al. (2014) also

Fig. 7. Planktonic foraminifera percentage in total foraminifera population. Rectangular bands ievent. Productivity regenerated between YD and AE i.e. PB = Preboreal, HSLM = Holocene Se


recorded similar increased productivity in the middle to late Holocenegeochemical proxy record from the Core AAS9/19. Although theCaCO3% during middle Holocene is recorded lower than that duringYounger Dryas and earlyHolocene, still it favoured foraminiferal growthdue to the dominance of Calcite and by the virtue of raised sea level (asdiscussed later in the section). This led to the cessation of algal growthin the area which in turn decreased the aragonite abundance duringmiddle Holocene. Decreased aragonite abundance during middle andlate Holocene have also been recorded earlier from marine sedimentcores 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 theOMZ, so the decreased aragonite abundance could be attributed toceased algal productivity in the area. A remarkable increase in PFA,BFA and P/B ratio (Fig. 5) during themiddleHolocene (8–4Ka) plausiblybe related to the raised sea level after 8 Ka. The sudden increase inQuartz and calcite (Fig. 4) with simultaneous decrease in aragoniteabundance between 7.5 and 7 Ka demarcates a major variation in sealevel, nutrient supply and riverwater dischargewithin themiddle Holo-cene. Also the increased δ13C during middle Holocene points towardsincreased productivity after early Holocene. This is due to the fact thatprimary producers utilise 12C during photosynthesis thereby increasing13C/12C ratio in the surface water and hence increased δ13C signatures(Fig. 5). The simultaneous increase in calcite abundance along with in-creased PFA and BFA points towards raised sea level which wouldhave resulted in the increased water depth of the area providing suit-able ambient environment for the calcification and foraminiferalgrowth. Also the decreased aragonite with increased quartz abundancesuggests the onset of riverine flux in the area due to raised sea levelwhich ceased the Halimeda growth during middle Holocene. The highP/B ratio as well as percentage of coarse fraction (N63μm) around 6 Kais probably related to Holocene sea level maxima (HSLM). The sealevel curves from western continental margin of India (Hashimi et al.,1995) and Singapore (Bird et al., 2010) also recorded the HSLM around6 Ka After HSLM the minor drop in sea level has been recorded withinthe middle Holocene by P/B ratio (Fig. 5) and planktonic foraminiferapercentage (Fig. 7).

Maximum values of PFA and BFA are recorded during the lateHolocene (Fig. 5) as compared to early and middle Holocene and thiscorresponds to Holocene foraminiferal Productivity Maxima (HPM).However, the values of CaCO3% are lower during the late Holocenethanearly and middle Holocene. It is intriguing that calcite predominatesover aragonite and thus CaCO3 favoured foraminiferal flourishment

ndicate productivity collapse during YD= Younger Dryas; AE= Abrupt event and 8.2 Kaa Level Maxima.




during late Holocene. During the onset of late Holocene, high values ofthe P/B ratio suggest the slight lowering of sea level which is also sup-ported by PFA and BFA records (Fig. 5). Similar observations havebeen recorded in oyster bed from Saurashtra coast (Banerji et al.,2015) which marked the records of ~2 m elevated sea level around3.5 Ka.

4.3. South-west monsoon (SWM)

The 2 °C variation in sea surface temperature during the Holocene ineastern Arabian Sea has been recorded (Govil and Naidu, 2010). OtherSST studies from eastern Arabian Sea include 2.5 °C variation sincemid-Holocene in the SE Arabian Sea (Tiwari et al., 2015), 2 °C variationsince mid-Holocene from 800 m water depth (Kessarkar et al., 2013)and 1.5 °C SST variability in the deep water (1245 m) offshore Malabarcoast, SW Arabian Sea (Saraswat et al., 2013). Other than SST, mainlyevaporation, precipitation and river run-off during SWM influence theδ18O of sea water and hence the δ18O of calcite. Recent studies fromsouth-eastern as well as central Arabian Sea (Kumar and Ramesh,2016 and references therein) indicate that the salinity/δ18O of theseareas depend on NEM precipitation run-off from Western Ghats aswell as the effect of NE monsoon driven West Indian Continental Cur-rent (WICC), which brings low salinity/δ18O water from Bay of Bengalto the Arabian Sea. However the effect of WICC is not significant in theNEArabian Sea. The salinity/δ18O of NEArabian Sea depends significant-ly on Indus river freshwater run-off (both glacial and snowmelt aswellas SWM precipitation) which is ultimately driven by SW Monsoon. Ingeneral, the Indus river system consist of a variety of climate regimeswhich 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 bysucceeding summer flow in themid altitudes and themonsoon inducedrainfalls in the foothill areas (Archer, 2003). Hydrological regime of theIndus basin is primarily controlled by snow and glacial melts(Mukhopadhyay and Dutta, 2010). Immerzeel et al. (2010) modelledthe upper Indus basin and concluded that the total stream flow in thearea include 34% snow melt and 26% glacial melt indicating a regimewhich 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 isdue to the seasonalmonsoonal rains in lower parts of the basinwhereasSingh and Bengtsson (2005) reported 49% snow and glacial melt contri-bution in the Chenab river at Akhnoor. Hence, it can be concluded thatthe Indus river system consist of glacial and snow melt as the primarydriver of downstream flows. Therefore, the impact of declining glacialmasses on river run-off due to climate change is more substantial toIndus system due to high melt water contribution to the total run-off.Singh and Bengtsson (2005) suggested that with increase intemperature of ~1–3 °C, there would be a decrease of 11–23% snowmelt contribution in Sutlej basin but also indicated 16–50% increase inglacial melt, resulting an overall increase in melt water flux due toincrease in temperature. Therefore δ18O values in the present studymay also be attributed to the fluvial discharge magnitude from Indusriver. G. ruber are surface dwelling planktonic foraminifera which livesthroughout the year (Guptha et al., 1997; Govil and Naidu, 2011) andhence these are expected to record riverine flux induced changesaround the core location. Therefore we have evaluated δ18OG. ruber (Fig.6) variability down core in order to understand themonsoonal variabil-ity in the offshore Saurashtra region of NE Arabian Sea.

SWM is themajor source of moisture over the Indian sub-continent.It is also responsible for the riverineflux aswell as salinity of the ArabianSea. In the Arabian Sea, every 1 p.s.u. (practical salinity unit) change inSea Surface Salinity (SSS) are found to be related to 0.33‰ of δ18O(Duplessy et al., 1981; Sarkar et al., 2000). Our data documents an


average ~1.1‰ difference in δ18OG. ruber between early and lateHolocene. Higher values of δ18OG. ruber during Younger Dryas and earlyHolocene as compared to middle and late Holocene corresponds toweakened SW monsoonal conditions leading to arid climate. SimilarYounger Dryas and early Holocene weakening of SWM have beendocumented from paleolimnological studies of Thar Desert lake whichrecorded low lake levels and high evaporation rate (Prasad and Enzel,2006).

Heavier incursions of ~0.7‰ and ~0.5‰ have been observed in δ18Ovalues around 11.5 and 8.2Ka (Fig. 6) respectivelywhich corresponds toYounger Dryas (YD) and 8.2 Ka cooling events. These cooling eventspoint towards further weakening of monsoon during YD and 8.2 Ka.Similar incursions have also been recorded in other parts of Indianocean i.e. δ18Oc of benthic foraminifera C. wuellerstorfi in the core ABP-25.02 from NE Arabian Sea (Gupta et al., 2011), δ18Osw of cores SK218/1 from Bay of Bengal (BOB) (Govil and Naidu, 2011), RC12–344from Andaman Sea (Rashid et al., 2007), SK-17 from eastern ArabianSea (Anand et al., 2008) and δ18Oc of KL126 from northern BOB(Kudrass et al., 2001)which reveals theweakening of SWM rainfall dur-ing YD. Terrestrial recordswhich also documentweakening of SWmon-soon during YD include speleothems from India (Sinha et al., 2005) andChina (Wang et al., 2001). However, thewestern Arabian Sea upwellingindices record do not show any signs of reduced upwelling strengthduring YD (Naidu and Malmgren, 1996; Overpeck et al., 1996; Guptaet al., 2003). The YD coolingwas a typical event of NorthernHemisphere(Ohkushi et al., 2016). For example, the δ18O records of planktonic andbenthic 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 Blunierand 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 lowerIndus river discharge on the core site further leading to reduced produc-tivity. After 8.3 Ka we observed another sudden increase in δ18O valueswhich correspond to 8.2 Ka cooling which also resulted in decreasedproductivity and Indus discharge due toweakened SWM. Similar obser-vations have also been documented from δ18O records of bulk carbon-ates from paleolake Riwasa, North India (Dixit et al., 2014) andradiometrically-dated speleothems (Cheng et al., 2009; Liu et al.,2013) which point towards the short term but pronounced weakeningof SWM at 8.2 Ka.

Lighter incursion of ~0.7‰ in δ18O value is observed around 11.2 Kasoon after the YD coolingwhich corresponds to Preboreal (PB)warmingperiod. Kessarkar et al. (2013) reported similar abrupt event centringaround11.2Ka suggestinghuge freshwater input due to high variabilityof rainfall during that period. The lower values of δ18OG. ruberduringmid-dle Holocene and subsequentlymore lower values during late Holocenesuggests the amelioration in the magnitude of SWmonsoon after 8 Ka.Therefore it is concluded, that during Younger Dryas and early Holoceneoverall SWMwasweak and further underwent threemain rapid coolingphases viz. Y.D., AE and 8.2 cooling event as well as one short warmingevent i.e. PB. (Fig. 6).

Middle Holocene recorded comparatively lower (~0.5‰) averageδ18OG. ruber values which corresponds to strengthening of SWM. Similarstrengthening of SWMafter 8 Kahas beendocumented in the foraminif-eral SST records from western Arabian Sea (Saher et al., 2007).

Further strengthening of monsoon took place during late Holocenewhich increased Indus riverine input at the core location. However, aheavier incursion of ~0.4‰ δ18OG. ruber (Fig. 6) is observed during 4.1–3.2 Ka which corresponds to short term weakening in the strength ofSWM. 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 Seaaround 4.5 Ka as well as δ18O and salinity records of core AAS62/1from SE Arabian Sea (Kessarkar et al., 2013) due to weakening ofSWM. This weakening of SWM is associated to major low latitude




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 incold tolerant evergreen oak-pollen during 4000–3500 years B.P. whichalso suggest progressive cooling. During that time, the reduction infresh water flux from Indus river has been reported (Staubwasser etal., 2003) which could be a result of weakened SWM and glaciationsin Karakoram and Western Himalaya.

4.4. Relationship between productivity collapse, sea level and Indus riverdischarge

The productivity of present core location, being at shallower depthand outside OMZ regime, directly depends on the nutrient availabilityfrom the Indus river run-off. Hence any fluctuation in the run-offwould have directly affected the productivity of the area. The intensityof Indus river run-off mainly depends on the SWM precipitation(Milliman et al., 1984) as well as the melt water input from the glaciersand snowfields of Karakoram and Western Himalayas (Wake, 1989).Therefore deglaciation in the Karakoram and central Himalaya wouldaccount for high nutrient availability from the Indus river dischargeand vice-versa. The δ18OG. ruber (Fig. 6) record suggest the lowmonsoonintensity during YD, AE, 8.2 as well as Late Holocene cooling between4.1 and 3.2 Ka which marks the cooling events (discussed in the abovesection). These events are also reflected in lower planktonic foraminif-era percentage (Fig. 7) which demarcate the foraminiferal productivitycollapse during YD, AE and 8.2 cooling whereas improved foraminiferalproductivity during PB warming. Moreover the remarkable decrease inthe value of δ13C G. ruber during these events also point towards pro-ductivity decline. These results are consistent to the hypothesis thatcolder periods result in year round productivity collapse due to weak-ened monsoon (Singh et al., 2011b) in the eastern Arabian Sea. TheIndus river discharge (which was already at the low during YoungerDryas and early Holocene) further declined considerably pronouncingthe 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 lateHolocene cooling probably due to raised sea level and sufficient calciteabundancewhich prevented the late Holocene cooling effect on forami-niferal flourishment.

5. Conclusions

The present multi-proxy study over offshore Saurashtra NE ArabianSea sediment core reveals that the area underwent three major phasesof 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. aboveOMZ as well as possess negligible effect of SWM nutrient upwellingand NEM mixed layer. This implies that the area strongly depends onIndus river discharge for maintenance of productivity.

Younger Dryas and early Holocene records comparatively low sealevel and productivity signatures. The area was deprived of Indus riverdischarge possibly due to weakened SWMwhich resulted in arid condi-tions. YD, AE and 8.2 Ka cooling as well as PB warming events have alsobeen identified during Younger Dryas and early Holocene.

Early-middle Holocene transition reveals abrupt climatic variationsbetween 8 and 7 Ka. Biological proxy responded ~500 years earlierthan chemical proxy response to the transition. The middle Holocenerecorded comparatively raised sea level and higher productivity signa-tures. HSLM has been recorded around 5.5 Ka. Improved SWM strengthhave been recorded which promoted Indus river discharge around thecore location.

Late Holocene recorded slight lowered sea level. Productivity im-provedmarginallywhichmarked theHPMaround3.5 Ka SWMstrengthfurther intensified which resulted in increased run-off from Indus river


increasing fresh water nutrient supply in the area. Moreover, 4 Kacooling 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 andpermitting us to publish the same. Extended thanks to the shipboardteam of 143rd Sagar kanya cruise for sampling core location SK-240/485. SA is also thankful to SERB-DST Project No. SR/FTP/ES-53/2013for the award of Junior Research Fellowship. This work was supportedby fastrack grant of SERB-DST under project No. SR/FTP/ES-53/2013.

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