Did the Indo-Asian summer monsoon decrease during the Holocene following insolation?

JOURNAL OF QUATERNARY SCIENCE (2010) 25(7) 1179–1188Copyright � 2010 John Wiley & Sons, Ltd.Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jqs.1398
Did the Indo-Asian summer monsoon decreaseduring the Holocene following insolation?MANISH TIWARI,1,2* RENGASWAMY RAMESH,2 RAVI BHUSHAN,2 MADAVALM S. SHESHSHAYEE,3

BAMMIDIPATI L. K. SOMAYAJULU,2 A. J. TIMOTHY JULL4 and GEORGE S. BURR41 National Centre for Antarctic and Ocean Research, Vasco-da-Gama, Goa, India2 Geosciences Division, Physical Research Laboratory, Ahmedabad, India3 Department of Crop Physiology, University of Agricultural Sciences, Bangalore, India4 NSF Arizona AMS Facility, University of Arizona, Tucson, Arizona, USA

Tiwari, M., Ramesh, R., Bhushan, R., Sheshshayee, M. S., Somayajulu, B. L. K., Jull, A. J. T. and Burr, G. S. 2010. Did the Indo-Asian summer monsoon decrease duringthe Holocene following insolation? J. Quaternary Sci., Vol. 25 pp. 1179–1188. ISSN 0267-8179.

Received 16 July 2009; Revised 9 January 2010; Accepted 29 January 2010

ABSTRACT: A few studies from the western Arabian Sea indicate that the Indian summer (orsouthwest) monsoon (ISM), after attaining its maximum intensity at ca. 9 ka, declined during theHolocene, as did insolation. In contrast, earlier and later observations from both the eastern and thewestern Arabian Sea do not support this inference. Analysis of multiple proxies of productivity in a newsediment core from the western Arabian Sea fails to confirm the earlier, single-proxy (e.g. abundance ofGlobigerina bulloides) based, inference of the Holocene weakening of ISM, following insolation. Thereason for the observed decreasing trend in foraminiferal abundance – the basis for the earlier inference
– could be the favouring of silicate rather than carbonate productivity by the increased ISM windstrength. Although ISM exhibits several multi-millennial scale fluctuations, there is no evidence fromseveral multi-proxy data to conclude that it declined during the Holocene; this is consistent with thephase lag analysis of longer time series of monsoon proxies. Thus, on sub-Milankovitch timescales, ISMdid not follow insolation, highlighting the importance of internal feedbacks. A comparison with EastAsian summer monsoon (EASM) records suggests that both ISM and EASM varied in unison, implyingcommon forcing factors on such longer timescales. Copyright # 2010 John Wiley & Sons, Ltd.
Supporting information can be found in the online version of this article.

KEYWORDS: monsoon; Arabian Sea; productivity; foraminifera; Holocene.

Introduction

Significant amount of palaeomonsoon data derives from thewestern Arabian Sea because of the strong monsoon-inducedupwelling signal recorded by foraminifera (e.g. Prell and Curry,1981; Emeis et al., 1995; Saher et al., 2007, and referencestherein). A few studies concluded that the Indian summer orsouthwest monsoon (ISM hereafter) steadily declined during theHolocene, as did insolation, based on a single proxy: either theabundance of Globigerina bulloides, which abounds in coolerwaters due to a strong monsoon-induced upwelling (Gupta etal., 2003, 2005) or d18O of speleothems from Oman (Neff et al.,2001; Fleitmann et al., 2003), which is at the western extremityof the monsoon region and receives very little precipitationrelative to the Indian west coast. On the other hand, earliermulti-proxy studies (employing d18O, d15N and other chemicalproxies) from the eastern Arabian Sea, a region stronglyinfluenced by 18O-depleted monsoon runoff from the westcoast of India, do not clearly support a declining monsoon

* Correspondence to: M. Tiwari, National Centre for Antarctic and OceanResearch, Headland Sada, Vasco-da-Gama, Vasco, Goa 403804, India.E-mail: [email protected]

during the Holocene (Sarkar et al., 2000; Thamban et al., 2001;Agnihotri et al., 2003). Furthermore, studies based on otherproxies such as varve thickness (von Rad et al., 1999), d13C oflake organic matter (Enzel et al., 1999), dolomite (Sirocko et al.,1993), d15N (Altabet et al., 2002), d18O and percentage of G.bulloides (Naidu and Malmgren, 1996; Overpeck et al., 1996;Naidu, 2004; Tiwari et al., 2006) did not show a monotonicdecreasing trend of ISM, which was also noted by Morrill et al.(2003) and Ji et al. (2005). Recently Clemens and Prell (2003,2007) observed that over the past 350 ka the Arabian Seamonsoon record shows that ISM lags precession-drivenNorthern Hemisphere insolation maxima by ka 8 ka. Moreover,climate models that simulate the Holocene monsoon revealthat, in addition to direct insolation forcing, other factors suchas ocean–atmosphere interactions also control monsoondynamics (Liu et al., 2003, 2004). Thus it appears that long-term insolation decline is not directly coupled with monsoonreduction.

We report here results of a multi-proxy analysis of a sedimentcore – SS4018G – from the western Arabian Sea; we find thatCaCO3 abundance decreases during the Holocene, but otherproductivity proxies do not favour a reduction in ISM strength.We therefore suggest an alternative interpretation for theobservation consistent with the multi-proxy results.

1180 JOURNAL OF QUATERNARY SCIENCE

Modern oceanography of the core site

The surface circulation at the core site (above Socotra Island inthe western Arabian Sea) is controlled by the seasonal reversalof monsoon winds. During the ISM, the East African CoastCurrent (EACC) feeds the northward-flowing Somali Current(SC), which develops into various clockwise-rotating cells andgyres such as the Southern Gyre (SG), Great Whirl (GW) andSocotra Eddy (SE) (Schott et al., 1990). It induces intenseupwelling along the Somalian and Omanian coasts, withupwelling transport of 1.5–2 Sv in the upper 50 m (Smith andBottero, 1977; Shi et al., 2000). The typical temperature of theupwelled water is 19–248C: a decline of up to 88C in sea surfacetemperature (SST) (Schott and McCreary, 2001). The reasonsattributed for such intense coastal upwelling is the Ekmandivergence due to the flow of strong winds parallel to the coast.The offshore upwelling takes place due to the strong positivewind stress curl to the northwest of the axis of the Somali jet,which is a low-level cross-equatorial jet at a height of 1–1.5 km,with wind speeds up to 15 m s�1 (Smith and Bottero, 1977;Swallow, 1984).

During the winter monsoon or northeast monsoon the winddirection reverses completely, in turn reversing the direction ofthe Somali Current, which now flows southward and meets theEACC at 2–48 S, which supplies the eastward-flowing SouthEquatorial Countercurrent (Schott and McCreary, 2001). Thisreversal in wind direction inhibits upwelling. Cooler winterwinds cause convective overturning, leading to reducedproductivity relative to summer.

Materials and techniques

Core SS4018G was collected from the mouth of the Gulf ofAden, Western Arabian Sea (138 12.80 N, 538 15.40 E; waterdepth 2830 m; core length 130 cm; Fig. 1) during ORV SagarSampada cruise no. SS164 in 1998.

Calcium carbonate was measured using the UIC coulometer(model 5012), employing the principle of coulometric titration.The standard used was sodium carbonate (Na2CO3). Totalcarbon and total nitrogen were measured using a Fisons NA1500 NC Elemental Analyzer employing flash combustion ofthe sample, passage of evolved gases through the reductionchamber and ultimately detection using a thermal conductivity

Figure 1 Location of core SS4018G. Inset: age–depth model. Sedimentatioradiocarbon dates are 1s (standard deviation)

Copyright � 2010 John Wiley & Sons, Ltd.

detector. We used Deer River Black Shale as a standard, having2.53% carbon and 0.12% nitrogen. The values are expressed inweight percent by dividing the concentrations obtained by theweight of the sediment taken. Organic carbon values wereobtained by deducting inorganic carbon values from the totalcarbon values. The precisions of CaCO3, Corg and C/Nmeasurement were 3%, 5% and 8%, respectively. Errorsquoted are 1s (standard deviation). The burial flux of variousgeochemical proxies (CaCO3, Corg) has been calculated bymultiplying concentration by dry bulk density (DBD) andsedimentation rate. The DBD has been calculated using theempirical equation derived by Clemens et al. (1987), whichutilises the correlation between DBD and percent CaCO3, insamples from the southeast Indian Ocean that are free of ice-rafted debris.

The d13C values of foraminifera were measured by duel-inletisotope ratio mass spectrometry using a PDZ-Europa 20-20. Thetotal CO2 production assembly was maintained at 808C forrapid acid digestion of the carbonates. In order to checkprecision, a lab standard known as Z-Carrara (ZC-2002) wasrun at least three times daily: at the start of the measurement, inthe middle and at the end of the day’s measurement. Based onrepeat measurements of ZC-2002, the external reproducibilityfor d13C measurements was better than 0.1% (1s, relative to V-PDB). d15N was measured using a Finnigan Delta Pluscontinuous-flow mass spectrometer interfaced with an Elemen-tal Analyzer (Flash EA 1112 Series, CE Instruments) via ConfloIII. The precision of d15N measurement was 0.38 %, obtainedby making multiple measurements of ammonium sulphatestandard, IAEA-N-2 ((NH4)2SO4, no. 342). Further, care wastaken to avoid instrumental drift by the frequent measurementof two isotopic standards: USGS 32 KNO3 and USGS 26(NH4)2SO4. A nonlinear correction factor was applied forcorrecting instrumental nonlinearity when percent N was small(Higginson and Altabet, 2004). Detailed experimental pro-cedures for stable isotopes and chemical proxies are discussedelsewhere (Bhushan et al., 2001; Kumar et al., 2004; Rameshand Tiwari, 2007).

Results and discussion

The age–depth model (Fig. 1, inset) is based on 15 radiocarbondates (Table 1) obtained on selected species of plankticforaminifera (G. ruber, G. sacculifer, O. universa and

n rates (underlined) in cm ka�1 are marked; error bars shown for the

J. Quaternary Sci., Vol. 25(7) 1179–1188 (2010)

Table 1 Radiocarbon and calibrated ages for core SS4018G. Errorsquoted are 1s (standard deviation)

Depth(cm below sea floor)

Radiocarbonage (a BP)

Calibratedage (a BP)

3 1116�38 540�3011 3097�58 2720�3021 5227�45 5440�8033 7375�74 7660�7043 8734�49 9000�16051 9558�55 10120�220

61� 10364�55 11000�23061� 10543�60 10930�38073 11383�58 12860�13081 12036�67 13220�30093 13170�91 14720�570

103 13613�75 15580�220113 14719�77 16850�250123 15098�83 17290�260129 16660�120 19020�330

� Duplicate analyses (see text for details).

ASIAN SUMMER MONSOON DURING THE HOLOCENE 1181

N. dutertrei; size range 250–500mm) from the NSF acceleratormass spectrometer (AMS) facility, University of Arizona, USA(see methods in Linick et al., 1986). AMS radiocarbon ageshave been calibrated to calendar ages using Calib 4.1 (INTCAL

Figure 2 Geochemical proxies for ISM strength. (A) G. bulloides record from(Berger and Loutre, 1991; Berger, 1992). (C) CaCO3 concentration (wt%). (D)carbon burial flux. (G) Linear sedimentation rate. (H) C/N ratio in SS4018G. G


98; Stuiver et al., 1998) with a reservoir age correction of563� 30 a (DR¼ 163� 30; Dutta et al., 2001; Southon et al.,2002). One depth, i.e. 61 cm, was repeated (by picking anotherset of the same species of planktic foraminifera) to checkreproducibility and both samples gave similar results, whichplaces high confidence in our chronology when coupled withthe high number of accurate dates. For this particular sample,the two radiocarbon dates obtained are 10 364� 55 and10 543� 60 14C a BP, which after calibration gave calendarages of 11 000� 230 and 10 930� 380 cal. a BP, respectively,which are similar within the error range (errors given are 1standard deviation). An average of the calibrated ages, i.e.10 965� 222 a BP, has been used for the age–depth model (theerror has been calculated using standard error propagationtechniques). The core spans ca. 19 000 calendar years with anaverage sedimentation rate of �7 cm 10�3 a. The resolution is150 a cm�1 but as the sampling is done at every 2 cm theeffective resolution is ca. 300 a.

Figure 2 shows results of the down-core variations of burialfluxes and weight percent of CaCO3, Corg and C/N in coreSS4018G (data presented as supporting information in Table 2online). C/N ratio (Fig. 2) indicates the provenance of theorganic matter: recent marine sedimentary organic matter has atypical value of �8� 2 and older sediments yield organicmatter with a value of �12 to �15 (Mackenzie, 1980). Incontrast, the terrestrial organic matter has a C/N ratio of �20 to

the western Arabian Sea (Gupta et al., 2005). (B) June insolation at 308NCaCO3 burial flux. (E) Organic carbon concentration (wt%). (F) Organicrey bars in panels A and C depict long-term trends during the Holocene



�100, with an average value of �60 (Premuzic et al., 1982;Meyers, 1994). The average C/N value in this core (�9)unequivocally shows the marine origin of organic matter duringthe Holocene (0–11.5 ka). We first discuss the results pertainingto this period.

Productivity as manifested by CaCO3 and Corg

The core has been raised from well above the lysocline, whichpresently lies at �3800 m in the Arabian Sea (Peterson andPrell, 1985); hence CaCO3 can act as an overhead productivityindicator. CaCO3 percent shows a sharp increase at ca. 9 ka,indicating an early Holocene monsoon maximum (as observedby Sirocko et al., 1993). Subsequently, both CaCO3 percent andits burial flux decline thereafter, akin to the steady decline in theG. bulloides abundance shown by Gupta et al. (2003, 2005)(Fig. 2). If the decline in calcium carbonate content and percentG. bulloides (a calcium carbonate secreting microorganism)were indeed due to reduction in monsoon, then organic carboncontent and its burial flux should also have decreased, which isnot the case. Although the bottom waters were not anoxic, webelieve that organic carbon is well preserved and represents theoverhead surface productivity, as discussed below.

Organic carbon preservation is controlled by the availabilityof oxidising agents and its removal from the diageneticallyactive layer with a complex interplay of many variables, suchas organic matter composition, bioturbation rates, diffusiveopenness of the sediments to various oxidising agents andprotective adsorption of organic matter on the mineral surfaces(Hedges and Kiel, 1995). The core has been raised from a waterdepth of 2830 m that is overlaid by oxic waters, but the core siteexperiences a high sedimentation rate (�7 cm ka�1) due to theintense productivity occurring overhead (Nair et al., 1989).Owing to the high sedimentation rate the organic matter rapidlycrosses the diagenetically active layer, which is of the order of�20 cm. Thus organic matter not only escapes the effect ofbioturbation, which is more active near the sediment surface,but also dissolved oxidising agents such as O2, NO�

3 and SO2�4

(Heinrichs, 1992). Furthermore several workers have evenquestioned the effect of oxygen availability on diagenesis.Several laboratory and field studies of the relative mineralis-ation rates of bulk organic matter or specific biochemicalcompounds such as dissolved sugars and amino acids underoxic vs. anoxic conditions have indicated little or no effectof O2 concentration (Cowie and Hedges, 1991, 1992; Hansenand Blackburn, 1991; Lee, 1992; Hedges and Kiel, 1995).Moreover, lack of any relationship between sedimentaryorganic matter preservation and O2 concentrations (Calvertand Pederson, 1992; Pederson et al., 1992) or burialefficiencies (Heinrichs and Reeburgh, 1987; Betts and Holland,1991) has cast doubt on the importance attached to oxygenavailability. Thus many exceptions exist to the oxygen effectand there is no universal pattern (Pederson et al., 1992).

Schulte et al. (1999) studied different biomarkers/pro-ductivity proxies (alkenones, dinosterol, brassicasterol, n-alkanes) on a core near Maldives (raised from a water depthof 2450 m) that has experienced oxic conditions throughout itshistory. They found excellent correlation between theseproductivity proxies and Corg content; they noted that thevariation in organic carbon is due to overhead productivity andis not affected by the bottom water preservation characteristics.Similarly, Reichart et al. (1997) studied a core from the MurrayRidge in the northern Arabian Sea from a water depth of 1470 m(oxic waters) and concluded that Corg record is a manifestationof surface water productivity. Rostek et al. (1997) also obtained


a core from a location adjacent to that of SS4018G from a waterdepth of 2490 m (similar to the water depth in this study).The similar variations exhibited by Corg and total alkenoneconcentration showed that variations in organic matter arerelated to marine productivity.

Rixen et al. (2000a) combined the results from long-termsediment trap data with satellite-derived wind fields andreported a clear link between the ISM wind strength (Findlaterjet), organic carbon flux and its preservation in sediment. Thisled us to use Corg content and its burial flux as an overheadproductivity indicator, which do not show a declining trendduring the Holocene; in fact Corg shows a slightly increasingtrend, while its burial flux stays more or less uniform.

Carbon and nitrogen isotopes as productivityindicator

Figure 3 shows down-core variations of d13C of three species ofplanktic foraminifera and d15N of organic matter (datapresented as supporting information in Table 3 online). Thedetailed interpretation of d13C in upwelling regions could besomewhat tricky (Curry et al., 1992) because of the competinginfluences of intense productivity (enriched 13C in surfacewaters), carbonate chemistry of sea water (Spero et al., 1997;Zeebe and Wolf-Gladrow, 2001) and upwelling (which brings13C-depleted waters) (Kroopnick, 1985). It is also clear fromFig. 3, however, that the trends in d13C of all three species offoraminifera show a slight increase during the Holocene.Previous studies from the northwestern Arabian Sea haveindicated a close relationship between upwelling andd13C values of foraminifera (Kroon and Ganssen, 1989; Steenset al., 1992). Peeters et al. (2002) carried out a detailedinvestigation on the effect of seawater carbonate chemistry onthe d13C values of G. ruber and G. bulloides near the area ofpresent study in the northwestern Arabian Sea (upwelledwater had much lower carbonate ion concentration thansurface water; Peeters et al., 2002). They found out thatcarbonate ion effect and d13C of dissolved inorganic carbon(d13CDIC) act in the opposite direction (13C-depleted upwelledwater results in lowered d13C values of foraminifera, whereasless carbonate ion in upwelled waters results in enhancementof the d13C values of foraminifera). They concluded that sincethe effect of seawater carbonate chemistry has a much largermagnitude than the effect of d13CDIC, higher d13C values offoraminifera are possible during periods of upwelling.

It may be noted here that the slightly increasing trend ind13C values is shown by all three species in this study, whichindicates that upwelling probably increased during theHolocene.

Had the productivity indeed decreased during the Holocene,it should be manifested in the organic nitrogen isotopicsignatures: a decrease in the otherwise intense denitrification inthe water column at the site, and hence the organic mattersynthesised at the surface by the upwelling of such waters,would show a decrease in d15N (Naqvi and Noronha, 1991;Ganeshram et al., 1995; Altabet et al., 2002), and this too is notobserved in the d15N record (Fig. 3). Altabet et al. (2002) havereconstructed d15N variations for the past 60 ka in a core fromthe Oman margin, near the core site; they have argued thatd15N fluctuations in this region depict productivity variationsbased on comparison with other productivity indicators such astotal chlorins and percent nitrogen from the same core. Thepresent d15N (Fig. 3) record is very similar to that observed byAltabet et al. (2002); thus there is no evidence for a decline inproductivity during the Holocene.


Figure 3 Down-core variation of d13C of three species of planktic foraminifera, and d15N of organic matter in SS4018G, compared with thed15N record of Altabet et al. (2002) from near the core site and CH4 record from the GISP2 Ice Core (Brooks et al., 1996). Grey bars depict long-termtrends during the Holocene


Our observations based on carbon, nitrogen isotopes andmarine organic carbon variations clearly show the absence ofthe Holocene decline in the monsoon following insolation.What then caused the reported decrease in G. bulloidesabundance observed by Gupta et al., (2003), or the reduction inCaCO3 content observed by us (if both represent calcareousproductivity)? One plausible mechanism is suggested below.

Carbonate vs. silicate productivity

In the western Arabian Sea, when ISM becomes active,enhanced concentration of nitrate and phosphate is seen at�100 m water depth, after which it declines, whereas silicateconcentration increases at around �175–200 m and evenenhances thereafter (Haake et al., 1993; Morrison et al., 1998;Rixen et al., 2000b). During the onset of monsoon, upwellingtakes place from shallower depths, enhancing calcareousproductivity; during advanced stages of the monsoon, as windstrength increases further, upwelling is from the deeper levels(200–300 m; Brock et al., 1992; Rixen et al., 2000b), as evidentby a further decrease in SST. This injects sufficient silicate intothe photic zone, which leads to diatom blooms, causing highsiliceous fluxes (Rixen et al., 1996). Satellite observations tooshow that diatom blooms cover the whole northwesternArabian Sea during the later stages of the ISM (Brock et al.,1991; Brock and McClain, 1992). Thus increased wind strengthlikely enhances siliceous productivity at the cost of calcareousproductivity.

Naidu et al. (1993) found evidence for this in a sediment corefrom the western equatorial Indian Ocean (Somali basin):d13C of G. menardii exhibited enriched values (indicatingenhanced organic productivity) in contrast to CaCO3 percent,which was lower (indicating reduced calcareous productivity)during the interglacial. Murray and Prell (1991) also reportedhigher opal content during the interglacial, indicating increased


productivity due to upwelling, which would result in higherCO2 (via organic matter degradation) supply to the bottomwaters, resulting in enhanced dissolution of CaCO3. Similarly,Reichart et al. (1997) analysed a core from the northern ArabianSea spanning the past 225 ka and found that high calcareousproductivity does not always correspond to enhanced surfaceproductivity as evidenced by other productivity indicators suchas Corg. They argued that pelagic carbonate production(including planktic foraminifera) decreased and was replacedby organic walled and siliceous organisms during enhancedproductivity. The sediments underlying such regions areenriched in silica and organic carbon (Broecker and Peng,1982) and depleted in calcite. Thus biogenic calcareousproductivity might reduce during increased ISM wind strengthsand is compensated by enhanced biogenic siliceous pro-ductivity. This would explain the decline observed in carbonateproductivity proxies (such as percent G. bulloides and calciumcarbonate content) despite slightly increasing monsoon.

Link with the atmospheric CH4 concentration

A large body of evidence favours a strong link between theIndian (and East Asian) summer monsoon strengths and ice corerecords of temperature and atmospheric greenhouse gases(CH4, N2O etc.) during the past several tens of thousands ofyears (e.g. Schulz et al., 1998; Reichart et al., 1998, 2002;Leuschner and Sirocko, 2000; Wang et al., 2001; Altabet et al.,2002; Zhao et al., 2003; Yuan et al., 2004; Ivanochko et al.,2005; Tiwari et al., 2006; Rohling et al., 2009). Specifically,during warmer periods (times of increased atmosphericmethane) the ISM was stronger. This argues against theHolocene ISM decline. The CH4 record from the GISP2 IceCore (Brook et al., 1996) is compared with d15N records ofthe present study and the one by Altabet et al. (2002) (Fig. 3).Both records show similar variations: increase during the early



Holocene indicating monsoon maxima followed by a smalldecline and then a uniform, slightly increasing trend signifyingthe strengthening ISM.

Comparison with land- and marine-basedrecords of ISM and East Asian Summer Monsoon(EASM)

Figure 4 compares the present study with a few recent ISMrecords from Omanian speleothem (Fleitmann et al., 2003),western Arabian Sea (Sirocko et al., 1993; Ivanochko et al.,2005), northern Arabian Sea (Staubwasser et al., 2003), easternArabian Sea (Sarkar et al., 2000; Thamban et al., 2001;Agnihotri et al., 2003), equatorial Arabian Sea (Tiwari et al.,2006) and EASM records of loess/palaeosol magnetism/rainfalltransfer function from Duowa, north-central China (Maher andHu, 2006) and speleothem records from Dongge (Wang et al.,2005) and Heshang Caves (Hu et al., 2008), along with thedifference in contemporaneous speleothem d18O from Donggeand Heshang Caves (Hu et al., 2008). Recently, d18O variations

Figure 4 Comparing records of Indian summer (A–J) and East Asiansummer monsoons (K–N) using diverse proxies from different regions;AS, Arabian Sea; arrows show the direction of increasing monsoon (seetext for details)


of absolutely dated (U-Th dating) speleothems from caves fromArabia and southeast China have been used to generate severalhigh-resolution records of ISM (Fleitmann et al., 2003) andEASM variability during the Holocene (Wang et al., 2005; Huet al., 2008, and references therein). It is believed that theisotopic composition of water dripping in the cave, whichprecipitates speleothem, after seepage through thick bedrock/sediment cover, reflects the average isotopic composition of themeteoric water falling on top of the cave (McDermott, 2004). Ithas been observed that in low-latitude regions the so-called‘amount effect’ (the greater the amount of precipitation, themore depleted is the vapour and condensed phase in heavierisotopes of oxygen i.e., more negative d18O values) takesprecedence over the surface air temperature effect (d(d18O)/dT¼ 0.69% 8C�1) that dominates in high latitudes (Dansgaard,1964). Even in low-latitude caves, however, there are manyfactors that can disturb the oxygen isotope content inspeleothem such as (i) evaporation leading to kineticfractionation, (ii) rapid dripping, (iii) degassing rates (relatedto ambient cave CO2 partial pressure) without allowing forisotopic equilibrium and (iv) temperature variations inside thecave that would affect oxygen isotopes (lower d18O valuesimplying warmer temperature; d(d18O)/dT¼�0.24% 8C�1 at258C; O’Neil et al., 1969) etc. Even if a perfect speleothem isfound that appears to avoid the above-mentioned problems, theissue of variability in rainfall seasonality and rainfall sourceremains (Maher, 2008). The Indian and East Asian monsoonhave clear summer and winter monsoon seasons when themoisture-bearing wind reverses completely, with entirelydifferent d18O values in the ensuing precipitation. The EastAsian monsoon records from southeast China, particularly,receive abundant rainfall during winter season, making theirinterpretation as an indicator of summer monsoon ambiguous(winter monsoon comprises precipitation with d18O of (�3% to12%); summer monsoon has d18O values about 10% lower(�9% to �13%); Wang et al., 2001). This proportion of winterand summer monsoon may have changed in the past, bringingrainfall seasonality uncertainty into the picture (McDermott,2004; Cobb et al., 2007). On glacial/interglacial timescales,variations in oxygen isotope content of source (oceans, due toice-volume effect) will add another component of uncertainty(McDermott, 2004). More important in the context of theHolocene, however, are the long-term shifts in moisturesources or storm tracks along with changing proportion ofprecipitation from vapour supplied by re-evaporation of cycledcontinental waters instead of oceans (McDermott, 2004;Maher, 2008). Thus there are several site-specific variablesthat may complicate the speleothem d18O; hence long-termcave monitoring including continuous precipitation and dripwater analysis is required from several caves; such studies arestill, unfortunately, in their preliminary stages (Cobb et al.,2007; Fleitmann et al., 2008).

Fleitmann et al. (2003) (Fig. 4(A)) reconstructed the ISMprecipitation from Stalagmite Q5 collected from Qunf Cave insouthern Oman in the Arabian peninsula and noted thatmonsoon declined gradually after ca. 8 ka, following insola-tion. This site lies in a semi-arid region, which is at the edge ofthe monsoon precipitation belt, and it receives very littleprecipitation during the summer season (�40 cm) as comparedto the west coast of the India/Eastern Arabian Sea along theWestern Ghats (a low-lying hill chain along the west Indiancoast), where intense orographic precipitation takes place (upto �400 cm during summer months). The caves in the arid/semi-arid region pose another problem of long residence timeof water – up to decades (Ayalon et al., 1998) – in the dynamickarstic terrain. More importantly, intense evaporation inthe arid/semi-arid region may alter the d18O of precipitation



before infiltration and in the upper portion of the vadose zone(McDermott, 2004). Thus records from the eastern Arabian Seaare naturally a more robust recorder of ISM precipitation. Therecords from the eastern Arabian Sea (Sarkar et al., 2000;Thamban et al., 2001; Fig. 4(H), (I)), however, show manymultimillennial-scale fluctuations in precipitation intensity butthe overall trend during the Holocene is that of an increase insummer precipitation. Agnihotri et al. (2003) (Fig. 4(D))reported increasing denitrification intensity supported bysurface productivity (relatable to summer monsoon intensity)during the Holocene, indicating strengthening monsoon. Thestrength of the western and northern Arabian Sea records is thatthey store the dominant ISM signal; such records (Sirocko et al.,1993, Fig. 4(E); Altabet et al., 2002, Fig. 3; Staubwasser et al.,2003, Fig. 4(G); Ivanochko et al., 2005, Fig. 4(F)) also showeither flat or slightly increasing monsoon, as in the present study(Fig. 4(B), (C)) but none shows a decline as observed in thespeleothem records. Studies from the equatorial Arabian Sea,although very few exist, indicate strengthening of the ISM (e.g.Tiwari et al., 2006, Fig. 4(J)) during the study period.

Similar to the speleothem records of the South Asianmonsoon, speleothem records from southern China, specifi-cally from Stalagmite DA from Dongge and Stalagmite HS4from Heshang caves (Fig. 4(K), (L)), have been used toreconstruct high-resolution East Asian monsoon variability(Wang et al., 2005; Hu et al., 2008) during the Holocene. Theyreported a uniformly declining trend in d18O, indicatingweakening EASM intensity akin to that observed from theOmanian speleothem record. Studies in this region, however,have shown that the correlation between the d18O of rainfalland rainfall amount is spatially and temporally variable,making speleothem interpretation questionable (Johnson andIngram, 2004; Maher, 2008), which gets further compoundedby equal contribution from summer and winter monsoon,which is supported by the modern IAEA/WMO data. The GNIP(Global Network of Isotopes in Precipitation) stations nearDongge Cave (�258 N, 1088 E) and Heshang Cave (�308 N,1108 E) are Guilin (�258 N, 1108 E; station no. 5795700) andZunyi (�27.88 N, 106.88 E; station no. 5771300), respectively.The weighted monthly d18O at Guilin and Zunyi during winterprecipitation (January) is �3.54% and �3.70%, while thatof summer precipitation (August) is �9.58% and �11.17,respectively (IAEA/WMO, 2006). On the other hand, theweighted annual d18O is �6.13% and �8.40% for Guilin andZunyi, respectively, which is near the mid value for winter andsummer precipitation; clearly, the mean has approximatelyequal contributions from the two seasons in that region. Thisseasonality in the cave speleothem d18O in this regionprecludes its use as a rigorous proxy for summer monsoonstrength. Interestingly, when the difference between thecontemporaneous oxygen isotope values (Dd18O) of Donggeand Heshang speleothem are plotted (Fig. 4(M)), the starkdeclining trend in monsoon intensity disappears (Hu et al.,2008). The Heshang cave lies downwind of the Dongge cave,along the same moisture pathway. The authors argue that bydifferencing contemporaneous d18O of the two sites, secondaryeffects on oxygen isotopes due to natural variability in moisturesource, moisture transport, non-local rainfall, temperatureetc. can be avoided, which would otherwise ruin such areconstruction. Their records show EASM maximum centred atca. 6 ka and thereafter, despite the millennial-scale fluctu-ations, it stayed more or less flat. Maher and Hu (2006) andMaher (2008) reconstructed EASM intensity for the past ca.12 ka using the soil magnetism/rainfall transfer function inloess/palaeosol sequences from Duowa, north-central China(Fig. 4(N)). They found an early Holocene wet period centred atca. 10.5 ka, dry conditions from ca. 8 to 10 ka, followed by


wetter conditions from ca. 6 to 8 ka, after which EASM furtherintensified, which is also reported from multi-proxy analysisof lake sediments from northwest China (Chen et al., 2008).Thus the record of EASM variability during the Holocenecorresponds very well with the present study regarding ISM.

Comparison of strengthening EASM records with ISMrecords, particularly that of Oman speleothem or westernArabian Sea G. bulloides records, led a few studies to proposethat EASM and ISM exhibit an inverse relationship during theHolocene (Hong et al., 2005; Chen et al., 2008; Maher, 2008).The reason attributed is that although both the EASM and ISMrespond to continental warm, low-pressure cells and aregoverned by precession-induced insolation forcings (Wanget al., 2008), different land–ocean configurations introducedifferences, resulting in diverse behaviour. EASM is proposed tobe significantly altered by local and remote variabilities in SSTdue to its supposedly stronger interaction with ENSO occurringin the Pacific Ocean (Basil and Bush, 2001; Wang et al., 2003).A recent study related to SST variabilities in equatorial IndianOcean and the equatorial Pacific, spanning the past 137 ka,however, suggests a common mechanism controlling the SSTsin both regions (Saraswat et al., 2005), implying similar SSTvariabilities. Thus the proposed difference in ISM and EASMdue to SST variability may not be that large. The present study(as also evident from Fig. 4) shows that such an inverserelationship may not exist; both the ISM and EASM exhibitsimilar variation during the Holocene implying, on longsub-Milankovitch timescales, both are governed by the sameforcing factors, despite the geographical differences, as was thecase during the pre-Holocene (both ISM and EASM have beenreported to show similar correlation with Greenland ice corerecords; e.g. Schulz et al., 1998; Wang et al., 2001; Kelly et al.,2006).

In the pre-Holocene, CaCO3 burial flux shows distinct peaksat 10.5, 13, 15 and 17 ka (Fig. 2). Three of these are also shownby organic carbon flux. The sedimentation rates at these timeswere also quite high (11.8, 14.3, 11.6 and 22.7 cm ka�1,respectively; Fig. 1 inset and Fig. 2). As the CaCO3 fluxes werehigh, we infer that ISM winds began to strengthen gradually, butnot to their Holocene levels, when silicate productivity wasfavoured. As some of these peaks are also seen in the C/N ratio,there could be a terrestrial contribution to the organic matter inthis period. A possible scenario would be erosion of coastalsediments (which were exposed during peak glaciation) as thesea level rose. The broad peaks between 13 and 11 ka seen ind15N and d13C (Fig. 3) probably indicate the contribution offresh nutrients from land (as C/N ratio fluctuates widely duringthat period; Fig. 2), in the absence of upwelling, which couldhave enhanced the productivity.

Conclusions

Multi-proxy results from the western Arabian Sea do not supporta steadily decreasing productivity during the Holocene; i.e.productivity is not in phase with insolation. We explain thereduction in foraminiferal abundance/calcium carbonatecontent by increased winds favouring silicate rather thancarbonate productivity. During the early Holocene, an Indiansummer monsoon (ISM) maximum is observed at ca. 8.5–9 ka;thereafter ISM did not decline during the Holocene, or at bestshowed a slight increase, implying that the insolation (whichdeclined monotonically) was not the only controlling factor forthe monsoon strength and that other internal feedbackprocesses were equally important. Parallel correlation with



the EASM records suggests that ISM and EASM do not exhibit aninverse relationship and are governed by common forcingfactors on such long, sub-Milankovitch timescales. Theseresults corroborate the results from phase lag analysis of longermonsoon proxy time series showing that ISM lagged summerinsolation maxima by several thousand years (Clemens et al.,1991; Clemens and Prell, 1996, 2003, 2007).

Acknowledgements We thank the Indian Space Research Organiz-ation – Geosphere Biosphere Program for funding, Chris Turney foreditorial comments and Steven Clemens, along with an anonymousreferee, for constructive criticism that improved the presentation; M. G.Yadava for help in stable isotope measurements; and Anil K. Gupta,Thamban Meloth, Rajesh Agnihotri, Gideon Henderson and BarbaraMaher for sharing published data. Most of the data used for comparisonhave been obtained from data archived at the World Data Center forPaleoclimatology, Boulder, Colorado, USA. MT thanks Rasik Ravindra,Director NCAOR, for his support and encouragement. This is NCAORcontribution no. NCAOR-R 56.

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