region (400 km) because of the positive curl in the wind stress ... on a submarine plateau. ... coccolithophorids of the class Haptophytes [Volkman et al.,.
PALEOCEANOGRAPHY,
VOL. 15, NO. 3, PAGES 307-321, JUNE 2000
Late Quaternary insolation forcing on total organic carbon and alkenone variations in the Arabian Sea
• Frauke Rostek, 2PeterJ.Mfiller,• D6rteBudziak, • RalphR. Schneider, Edouard Bard, 2andGerold Wefer • Abstract. We here presentrecordsof total organiccarbon(TOC) and C37alkenones,usedas indicatorsfor past primaryproductivity, fromthewestem(WAS) andeastemArabianSea(EAS). New datafrom an openoceansite of the WAS upwellingareaare comparedwith similarrecordsfrom OceanDrilling Program(ODP) Site 723 from thecontinental marginoff OmanandMD 900963 fromthe EAS. Theserecordstogetherwith otherproxiesusedto reconstruct upwellingintensity,indicateperiodsof high productivityin tune with precessional forcing.On the basisof their phaserelationshipto borealsummerinsolationthey can be dividedinto threegroups:in the WAS differencesbetweenmonsoonalproxies(1) and productivity(2) documenta combinedsignalof moderateSW monsoonwinds and of strengthened and prolongedNE monsoonwinds,whereasin the EAS phasingindicates maximumproductivity(3) at timesof strongerNE monsoonwindsassociated with precession-related maximain ice volume.
1. Introduction
Deep-seasedimentrecordsfrom the ArabianSeadocument the historyof the monsoonalcirculationin the Indian Ocean. Various studieson wind-transportedmatehal such as landderivedpollen spectra[Van Campo et al., 1982] and dust input [Clemensand Prell, 1990, 1991; Sirockoet al., 1993] from adjacentlandmasses, aswell asproxyrecordsof surface waterproductivity[e.g.,Prell andKutzbach,1987;Naidu and Malmgren,1996;Reichartet al., 1998] revealeda stronglink between precessionallydriven insolation changes and monsoonintensity.This is in agreementwith resultsfrom generalcirculationmodels(GCM) [e.g.,Prell and Kutzbach, 1987, 1992; DeMenocal and Rind, 1993] pointing to an intensification
of
SW
monsoon
winds
and
enhanced
upwelling with increased boreal summer insolation. Additionally,in the precessional frequencyband, Clemenset al. [1991] documented a time lag of• 8 kyr (about-120ø on average) between maximum boreal summer insolation (referencedto June 21) and various indicators of SW monsoon winds and related upwelling intensity. Thus radiativeforcingalonecannotaccountfor the timingof strong monsoons.Clemenset al. [ 1991] concludedthat this time lag is phase-locked to the cross-equatorial transportandreleaseof latent heat over the Tibetan Plateau resulting in intensified summer monsoon winds.
In the northernArabian Sea,Reichart et al. [ 1998] useda varietyof productivityproxies,e.g.,variationsof totalorganic carbon(TOC), to examinevariationsin the Indian monsoon
system,applyingan age model independentof the Spectral MappingProject(SPECMAP) referencestack[Imbrie et al., 1984]. Their results indicate that precession-related productivity maximalagborealsummerinsolation maximaby - 6 kyr (about-94ø) on average.This lag is attributedto a prolongedsummermonsoonseasonlinked to late insteadof earlysummerinsolation[Reichartet al., 1998]. In
contrast, sediments from
shallower sites at the
continentalslopeoff Oman seemto be more influencedby changesin climatic boundary conditions associatedwith glacial-interglacialcycles due to sea level changesand
resulting sea-landward shiftsof thecoastal upwelling cells [Erneis, 1993].Accordingly, datafrom OceanDrilling Program (ODP) Site723 suggested thattheSW monsoon was
weakenedduring glacial times as comparedto the interglacials [Niitsuma et al., 1991;Anderson and Prell, 1993].Thiswasinterpreted to resultfroma glacialreduction of the pressuregradientbetweenthe ice-coveredTibetan
Plateau andtheIndian Ocean [Prell,1984].Consequently, the authors concluded thatupwelling andtheresulting primary production werereduced duringcoldclimate periods. In addition,paleoproductivity recordsrevealeda different phasingwith respectto borealsummerinsolationbetweenthe
western (WAS)andeastern Arabian Sea(EAS).Erneis [1993] usedTOCaccumulation ratestoreconstruct paleoproductivity at ODPSite723.Theprecessional component in thisrecord, thoughof minor importanceat this site, indicatesa rather
opposite phasing +10.7kyr (+167ø) [Erneis,1993],whilein
the EAS a paleoproductivity reconstruction basedon the coccolithophorid Florisphaeraprofundashoweda lead of •Fachbereich Geowissenschafien, Universit•it Bremen,Bremen, +7.7 kyr (+ 120ø)with respectto maximumsummerinsolation Germany. 2Centre Europ6en deReserche et d'Enseignement enG6osciences[Beaufortet al., 1997]. Thereforetheseformerstudiesraise
twoquestions: (1) Howimportant isthe100kyrcycle,and(2) whatisthereason forthedifference in thephasing ofprimary
de l'Environment (CEREGE), Universit6 d'Aix-Marseille III et CNRS-UMR 6635, Europ61ede l'Arbois, Aix-en-Provence,France.
productivityrecordsrelative to precession and insolation
Copyfight2000 by theAmericanGeophysical Union.
changes?
In orderto reassess thedifferentphasing of TOC variations and relatedpaleoproductivity in the ArabianSea, we here
Papernumber1999PA000433. 0883-8305/00/1999PA000433512.00
307
308
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY
presentnew dataof TOC andC37alkenone(C37)variationsof core GeoB 3005, an open ocean site from within the upwellingareaof the WAS (14ø58.3'N,54ø22.2'E;2310 m), in combination with ODP Site 723 (18ø03.079'N, 57ø36.561'E;816 m) [Emeis,1993;Emeiset al., 1995] and MD 900963 from the EAS (05ø04'N, 73ø53'E; 2450 m) [Rosteket al, 1994, 1997].
On average,highestbiogenicflux ratesoccurin the WAS, and lower fluxes occur in the CAS and EAS. Rixen et al.
[1996] observedthat at leastin the WAS, particlefluxesare controllednot only by the intensityof the upwellingsystems but additionallyby the amountof cold, nutrient-poor water masseswhich are transportedfrom southof the equatorinto the upwellingregion off Oman, where highestfluxes occur duringSW monsoonswith moderatewind speeds. During the summerseason,coastalupwellingdevelopsoff the Somali and Oman coasts [Wyrtki, 1973] because of Ekman transport. As the SW monsoon winds, blowing Setting parallelto the coastwith the coaston its left (Figure 1), drive Thedominant component of tropicalclimatevariability in the oceancirculation,surfacewater is transportedoffshore the ArabianSea is the seasonal reversalof atmosphericandreplacedby colder,nutrient-richsubsurface watermasses circulationand precipitationassociatedwith the Asian within a narrow band (100 km) along the coast. Farther monsoonsystem.The distribution of a continentallandmass offshore,an open ocean-typeupwelling developsin a broad in the north and a large oceanin the southleads to this region(400 km) becauseof the positivecurl in the wind stress reversalof air flow [Cadet, 1979]. Duringthe Northem [Swallow, 1984; Luther et al., 1990; Brock and McClain, Hemisphere's Summer thesouthern Asiancontinent heatsup 1992]. Along the west coastof India, weak upwellingmay morethantheIndianOcean.Thisextraheatingcauses a low- occurunder favorableconditions[ Wyrtki, 1973]. Long-time pressure cell overthe TibetanPlateauanda pressure high sedimenttrap studiesreport an increase in biogenic and abovetheIndianOcean.Theformation of a strong, low-level lithogenic particle fluxes with the onset of the summer jet stream knownastheFindlater Jet[Findlater,1969]is the monsoon in correlation with higher wind speeds and result.Latentheat,absorbed overthe southern subtropical decreasingsea surfacetemperaturesin the WAS and EAS ocean,transported northwardacrossthe equator,andreleased [Nair et al., 1989;Ittekkotet al., 1992;Haakeet al., 1993]. by precipitationover the Indian continent,controlsthe With the cooling of the Tibetan Plateauthe atmospheric intensity of thisjet [e.g.,Webster, 1987]. pressuregradientreversesandinitiatesthe wintermonsoon.A The strongcouplingbetweenatmosphere and oceanand moderate,cool NE monsoonwind (Figure 1) compensates the the seasonalchangesin wind direction as well as wind pressuregradientbetweena now persistinghigh-pressure cell intensitiescausea completesemiannualreversalof surface abovethe snow-coveredTibetan Plateau and a pressurelow currentsin the Arabian Sea basin affectingprimary over the Indian Ocean [e.g., Hastenrath and Lamb, 1979]. productivity. Haakeet al. [1993] described enhanced fluxes During the 1995 U.S. Joint Global Ocean Flux Study duringboth the SW and NE monsoonseasons.Lower fluxes (JGOFS) Arabian Sea ProcessStudy, Hansell and Peltzer were observedduringthe intermonsoon seasons in the WAS [1998] examinedTOC concentrationsin the upper ocean. and centralArabian Sea (CAS), whereaspeak fluxes in the They describedhighest TOC concentrationsin the mixed EAS do not startuntil the late SW monsoonperiod,which layer during the NE monsoonperiod remainingthroughto
2. Monsoon Meteorology andHydrographic
lastsuntil the end of October[Haake et al., 1993]. Here particlefluxesare more variableduringthe rest of the year.
I
I
mid summer, while lowest TOC concentrationsin the mixed
layer occurredin late summer.
I •d•_idrl
I
I
I
I
20 ø N
ODP
Site
aeo soo$
10 øN'
•
/ SMSoWns ß oon
0ø N
MD 900963
ß
40 ø E
50 ø E
60 ø E
70 ø E
80 ø E
Figure1. MapoftheIndianOcean north oftheequator. Indicated arethelocations ot'cores GeoB3005.Ocean Drilling Program (ODP)Site723,andMD 900963.An'ows showmajorwinddirections of theIndianMonsoon system during summer(SW monsoon)andwinterseasons (NE monsoon).
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY 3. Material 3.1.
and Methods
Material
309
To avoid dilution effects causedby changesin the sedimentation ratedueto varyinginputof terrigenous detritus and othermajorbiogeniccomponents (e.g., carbonatewhen consideringthe TOC and alkenone fluctuations),mass accumulation ratesof TOC (TOC MAR) and C37(C37MAR)
During R/I/ Meteor cruise M31/3 a gravity core. (GeoB3005-1, water depth 2316 m), a piston core (GeoB 3005-2, water depth 2309 m), and a multicorercore were calculated for all cores: The concentrations of TOC and C37weremultipliedwith the dry bulk densityof the sediments (GeoB 3005-3, water depth 2316 m) were recoveredfrom a and the linearsedimentation ratesbetweenstratigraphic tiestationwithin the upwelling area in the WAS (14ø58.3'N, points. 54ø22.2'E,site 108 in the work of Hemlebenet al. [1996]). To correlatethe last 300 kyr with a recordfrom the EAS, To minimize possible effects caused by lateral sediment transportfrom the continentalmargin,the sitewaspositioned core samplingof MD 900963 was extendedto a depthof 1720 cm. For the extendedpart of MD 900963, TOC was on a submarineplateau.GeoB 3005-1 was sampledover its usinga NA 1500 (FISONS)elementalanalyzer. entirelength(1098 cm) at 5 cm intervalswith 10 mL syringes determined weretreatedusingthemethoddescribed by Verardo and GeoB 3005-2 only between 748 and 1948 cm. These Samples et al. [ 1990]. Samplepreparationandtechnicaldetailsusedat sampleswere storedat 4øC. GeoB 3005-3 was sampledin Centre Europren de Reserche et d'Enseignementen 1 cm sliceson board,andthe sampleswere storedfrozen. 3.2.
Methods
One sampleseriesof the coresfrom stationGeoB 3005 was wet sievedto obtainthe coarsefraction(> 63 gm) which was used to pick the planktonic foraminiferal species Neogloboquadrinadutertrei (d'Orbigny) for stable oxygen
isotope (•lSo)measurements. The samples wereanalyzed usinga FinniganMAT 251 micromassspectrometer coupled with a Finniganautomatedcarbonatedevice.The carbonate was reactedwith 100% orthophosphoric acid at 75øC. The reproducibility (1 •) basedon replicatemeasurements of a laboratoryinternalcarbonatestandard(Solnhofenlimestone)
G6osciences de l'Environment(CEREGE)for extendingthe alkenonerecord of core MD 900963 are given elsewhere [Sonzogniet al., 1997]. Sampletreatmentand determination
of TOC andalkenonedataof ODP Site 723, providedby K.C. Emeis, were previouslydescribedin detail by Emeis [•93]. The softwareprogramSPECTRUM developedby Schulz and Stattegger [1997] was used to identify the dominant
frequencies intherecords of15•So, TOC,TOCMAR,C37, and
C37MAR and to carryout cross-spectral analyses.Becauseof an internalalgorithmthis softwaredoesnot need equidistant time series. To prove the statistical significance, a Fischer/Siegel test was employed in the subroutine is+/- 0.07ø/oo. A secondsampleserieswas freeze-dried,groundin an ,,Harmonic". The same parameters were used in these agatemortar,and used for carbonand alkenoneanalyses. calculation procedures (level of significance is 0.05, Organiccarbonwas determinedon alecalcifiedsamplesby OversamplingFactor (OFAC) is 4, and High Frequency combustionat 1050øC using a Heraeus CHN-O-Rapid Factor (HIFAC) is 1). For a more precise evaluation of betweenboreal summerinsolation elementalanalyzer[Miiller et al., 1994]. The precisionof the temporalcorrespondence analyses wereperformed. measurements was betterthan 3% on the basisof duplicates andTOC timeseries,cross-spectral This method does not only include an estimation of anda laboratoryinternalreferencesediment(WS2). coherency, a linear correlation coefficient in the spectral The concentrations of C37 alkenones (long-chain domain,but also estimatesthe lead (+) or lag (-) reportedas unsaturated ketones, which are biosynthesized by
coccolithophorids of the classHaptophytes[Volkmanet al., 1995]) were determinedin 2 g aliquotsof freeze-dried sediment samples. The samples were extracted with a UP200H ultrasonicationdisruptor probe (S3 micropoint, amplitude0.5, and pulse 0.5) and successively less polar solventmixtures(MeOH, MeOH/CH2CI2(1:1), and CH2CI2), each for 3 min. The extracts were combined, washed with demineralizedwater to remove sea salt and methanol, dried
over Na2SO4,concentratedunderN2, and finally taken up in 25 I.tL of a 1:l (volume)MeOH/CH2CI2mixture.
3 I.tL aliquotsof the final extractswere analyzedby capillarygaschromatography usinga HP 5890 SeriesII gas chromatograph (GC) equippedwith a 50 m x 0.32 mm inner diameter(ID) HP Ultra 1 (cross-linked methylsilicone)fused silica column,a split injector(l:10), and a flame ionisation detector. Helium was used as career gas. The GC was programmed from 50ø to 150øCat 30øC/min,150ø to 230øC at 8øC/min,and 230ø to 320øC at 6øC/min, followed by an isothermal periodof 45 min. Quantification of C37alkenones was achieved by an internal standard method using octacosaneacid methyl ester (OCSME) and the relative response factorof theC38n-alkaneasinternalstandards.
phaseanglesbetween+ 180ø and-180ø,respectively, between records sharing a similar variance at a given frequency [Jenkinsand Watts,1968;Imbrie et al., 1989]. Three recordsof Northern Hemispheresummerinsolation weregenerated for June21 at 30øN afterBerger [ 1978], with time resolutionsof 2.5, 1.6, and 0.75 kyr in accordancewith the mean time resolutions of the TOC records from ODP Site
723, MD 900963 and GeoB 3005, respectively. The calculationprocedureof the cross-spectral analyseswas also kept constantfor all records(level of significanceis 0.1, OFAC is 4, HIFAC is 1; window type is Welch, and 2 segments with 50% overlap;seeSchulzand Stattegger[ 1997] for furtherdetails).
4. Stratigraphy The /5•O recordsof the planktonicforaminifera N. dutertrei of GeoB 3005-1 (gravity core), GeoB 3005-2
(pistoncore),and GeoB3005-3 (multicorercore)wereused to combine these three cores and to build a stratigraphic flamework. The gravity and piston cores were brought togetherwithin an overlappingsectionfrom 938 to 1158 cm
310
BUDZIAK
ET AL.: ARABIAN
SEA VARIATIONS
IN PALEOPRODUCTIVITY
MC 3005-3
-0.
0
1
2
3
4
5
-2- /
SL 3005-1
KL 3005-2 a
2
0
100
200
300
10-
•
m
b 0
100
200
300
Age (ka) Figure2. (a) Combination of GeoBcores3005-1and3005-2withinan overlapping section (119 to 156kyr).(;cob 30053 wassetontop.Isotopicevents(Table1) wereusedasstratigraphic controlpoints.(b) Linearsedimentation ratesderived t?omthis age model.
(originalcoredepth)by peakto peakcorrelation. On thebasis
5. Results
of both the isotopicand TOC records,GeoB 3005-3 was set
5.1. Western Arabian Sea, Open Ocean: on top of the stackedcores(Figure2a). Zero ageis assumed GeoB 3005 for the multicorer surface. Afterward, for the sake of
homogeneity with a commontimescale,the stableoxygen isotoperecordof N. dutertreiwas correlatedto the widely usedmarineoxygenisotopestages(MIS) of the SPECMAP
Theb•80signalof theplanktonic foraminifera N. dutertrei
(Figure3) reflectsthetypicallateQuaternary patternof global glacialto interglacialclimatechanges. In contrast,the records b•SOstack(Table1) [Imbrieet al., 1984]usingthesoftware of TOC and C37 (Figure 3), parallelingeach other, reach programAnalySeties1.0a7[Paillardet al., 1996]. maximumvaluesevery 20 to 25 kyr. Exceptfor the high Accordingto our age model, the compositesediment valuesduringthe Holocene,probablybecauseof lacking recordat site GeoB 3005 resolvesthe last 307 kyr, showing diageneticequilibrium,TOC rangesbetween0.4 and 2.1%. an averagesedimentation rate of- 7 cm/kyr. The linear The absolute concentrationof C37 shows pronounced sedimentation rateswere calculatedbetweenstratigraphic tievariationsbetween400 and 6900 ng/g dry sediment.During pointsandrangefrom 5 cm/kyrin MIS 7 up to 10 cm/kyrin theHolocenethe alkenonecontentremainsrelativelylow. MIS 2, as shownin Figure2b. Thusthe meantime resolution of samplestakenat intervalsof 5 cm lies between500 and
between 0.2and1.31g/m2 peryear,and14and356gg/m 2
1000 years.
per year, respectively.The pronouncedperiodicity with
TOC MAR and C37 MAR of GeoB 3005 show variations
Theb•SOsignals of theplanktonic foraminiferal species minimumand maximumvaluesoccurringduringglacial as Pulleniatinaobliquiloculataand Globigerinoidestuber were usedto establishthe stratigraphic frameworkof ODP Site 723
[Niitsumaet al., 1991;Emeis,1993;Emeiset al., 1995] and of coreMD 900963 [Bassinotet al., 1994],respectively.
well as interglacialstagesevery20 to 25 kyr are still obvious in the TOC MAR and C37MAR of GeoB 3005.
In thevariance powerspectrum theb180signal (Figure 4) exhibitspeaksat periods(1/frequency)of 112,40, and23 kyr
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY
311
Table 1. IsotopicEventsUsed as Stratigraphic
Emeiset al., 1995]. Variationsof TOC (Figure5) range
Control Points From Correlation of the Timescale
between 0.5 and 7.5%. At ODP Site 723, TOC values are
of the SpectralMappingProject(SPECMAP) With
consistently higherduringinterglacial periods[Emeis,1993;
GeoB 3005
Emeis et al., 1995] (Figure 5). Superimposedon these fluctuations in the low-frequencydomainarevariationswhich indicate,similar to GeoB 3005, periodic increasesat time intervalsof-• 20 to 25 kyr. Theseare still evidentwithin the
Event
Time, kyr
Depth,cm
Sediment Surface
0
0.5
2
12
98
2.2
19
163
3.1
28
253
3.3
53
453
4.2
65
558
5.1
80
673
5.2
87
728
5.3
99
808
5.4
107
858
5.5
122
958
6.2
135
1033
6.4
151
1128
6.5
171
1238
6.6
183
1313
7.1
194
1383
7.2
205
1438
7.4
228
1563
7.5
238
1633
8.2
249
1718
8.4
269
1883
8.5
287
1998
8.6
299
2078
End of core
307
2138
SPECMAP is from Irnbrie et al. [1984], and GeoB 3005 is in centimeters.
TOCMAR (0.8to 13 g/m2 peryear)(Figure5), although ODP Site 723 exhibits intervals of very high TOC accumulation around10 to 20 kyr, 60 to 80 kyr, and 130 to
150kyr corresponding to theglacialmaximain MIS 2, 4, and 6, respectively [Emeiset al., 1995]. The differencebetween glacialandinterglacial sedimentation ratesseemsto be less pronounced in sediments olderthanMIS 6, whenTOC and TOC MAR show a more parallel progress[Emeis, 1993; Emeiset al., 1995]. At ODP Site 723 the concentrations of C37(Figure5) range
between47 and4508 ng/g dry sediment,andthusare rather low comparedto the other sites.In contrast,C37 MAR
(Figure 5) (5 to 1158gg/m 2 peryear)is higher thanin the WAS and EAS open oceansites.Variationsof C37and C37MAR are in good accordance with each other,while higheramounts of C37arerevealedin glacialstages8 to 4, followingmorecloselythepattemof TOC MAR. The resultsof the spectralanalysesof TOC (Figure7) showthemajorpeakin the eccentricity bandanddecreasing variancetowardthe higherfrequencies, while TOC MAR is dominated by variancein the obliquityband.Althoughof minorimportance at this site,variancein the precessional frequency bandis stillpresent. In contrast to ODP Site723, theproductivity records of GeoB3005havevariedin tune withprecessional forcing buthavenoclear41 kyrcyclicity. 5.3. Eastern Arabian Sea, Open Ocean:
in the eccentricity,obliquity,and precessional bands, MD 900963 respectively, with decreasing variancetowardthe higher frequencies. Periodicities > 100 kyr haveto be interpreted At site MD 900963, TOC varies between 0.2 and 0.9 % with caution because of the relative shortnessof the time
duringthe last 330 kyr. Thus highestvaluesare reachedoff
series of GeoB3005,buttheyclearly indicate thatthe•5•SO Oman, and intermediateand lowest values are reachedin the record reflects theglobal changes inseawater •5•SO duetothe WAS and EAS, respectively.In the EAS, highest TOC buildupandretreatof continental icemasses andchanges in
(Figure6) valuesareassociated withglacialb•80maxima,
seasurface temperatures duringthelast307 kyr.
and lower values are associatedwith interglacialminima every20 to 25 kyr [Rosteket al., 1994]. The TOC MAR (0.08
A different distribution of variance is revealed in the time
seriesof TOC and C37(Figure 4). Dominantvarianceis concentrated withintheprecession bandof bothrecords. Only
to 0.33g/m2peryear)followthepattemof theTOCrecord,
record.The overalldominanceof the 23 kyr periodicityin the
TOC MAR (Figure7) are in goodaccordanceto GeoB 3005.
while the concentrations of C37alkenones(243 to 6696 ng/g
andC37MAR (6 to 242 gg/m 2 per year) TOC revealsa significantpeak in the obliquityband. dry sediment) Variancein the low-frequency domainof eccentricityis (Figure6) alsoparallelthesevariations[Rosteket al., 1994]. The results of the spectral analyses on TOC and present in C37butisof onlysecondary importance in theTOC at the 23 kyr period TOC andC37records indicates a precessionally relatedforcing Again,dominantvarianceis concentrated ratherthanchanges in the glacialboundary conditions. The [Rosteket al., 1997] in both records,while TOC MAR, resultsof the spectral analyses of TOC MAR andC37MAR additionally, reveals a peak in the frequency band of
(Figure4) are iv accordance with TOC and C37,which eccentricity.Thus the 23 kyr periodicityis revealedin the indicatesthat changesin the sedimentation rate do not significantly influence thesesignals atthissite.
WAS and EAS and at the continentalmargin off Oman, althoughwith muchlowerimportanceat the coastalsite.
5.2. Western Arabian Sea, Continental Slope off Oman:
5.4. Cross-SpectralAnalysis
ODP Site 723
The resultsof the cross-spectral analyses(Figure 8) reveal differencesin the frequencydomainas well as in the phase relationshipto boreal summerinsolationfor June21 at 30øN
Forcomparison with a coastalupwellingsitein the WAS
weherereconsider results fromODPSite723[Emeis, 1993;
312
BUDZIAK
ET AL.: ARABIAN
Western
SEA VARIATIONS
Arabian
IN PALEOPRODUCTIVITY
Sea' GeoB 3005
0
1 ,
0
8000
600
4000
400 0
2OO
"',"
,
i ....
0
,
, .... ; ...... ,....... ,"
100
,
, ..... ; ...... ; ........
200
0
300
Age (ka)
Figure3. Timeseries ofthe•5180 record oftheplanktonic foraminifera Neogloboquadrma dutertre•, totalorganic carbon (TOC) concentrations andmassaccumulation rates(TOCMAR), andC.•7alkenone concentrations (C;,) and mass accumulation rates(Cs7MAR) for coreGeoB 3005.
of all three coresin the WAS (GeoB 3005), at the continental slope off Oman (ODP Site 723), and in the EAS off the Maldives (MD 900963).
6. Discussion
evident
importance to the timingandwind strength thanthe glacialinterglacialclimatevariability.Additionally,$irocko et al. [1993] discussed variationsin the recordsof marineoxygen isotopesand carbonateand aeolian dust supply, which originatedfrom Arabiaand Mesopotamia by northwesterly winds,in relationwith insolationchanges in theWAS. Recent studies suggesta coupling between increased biogenic as well as lithogenicparticle fluxes, higher
With the availability of longer proxy records the precessional componentof the Earth's parametersbecame The 23 kyr cyclicityof GeoB 3005 (Figure8a) is coherent evident in productivitychangesall over the Arabian Sea, (k = 0.9931) with boreal summer insolation but reveals a althoughwith varyingintensity[e.g., Prell 1984; Clemens phase relationshipof +11.3 kyr (+177ø) (Table 2). This and Prell, 1990, 1991;Shimmield et al., 1990;Murray and oppositephasing between summer insolation and TOC in Prell, 1992;Andersonand Prell, 1993;Emeis, 1993;Bassinot GeoB 3005 is shownmoreillustrativelyin Figure9. et al., 1994;Rosteket al., 1997;Altabetet al., 1995;Reichart ODP Site 723 (Figure 8b) exhibitsstrongvariancesin the et al., 1998;Schubert et al., 1998].Onthebasisof biological, low-frequencydomain,which links TOC variationsmainlyto biogeochemical,and lithogenic evidence, Clemenset al. the glacial-interglacial cycles dominatedby the 100 kyr [ 1991] concludedthat the responseof the summermonsoon period.However,a strongcoherency(k = 0.9906) is still windsoverthe ArabianSeato insolationchangesis of more between
boreal
summer
insolation
and TOC
time
serieswithin the precessionband. The phaserelationshipof -11.4 kyr (-178ø) is similar to that calculatedfor GeoB 3005 (Figure9, Table 2). The coherence between TOC
of MD
900963
and boreal
summerinsolation(Figure 8c) is strong(k = 0.9609) in the precessionband, but phase relationshipreveals a 7.7 kyr (+ 120ø) lead of TOC.
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS
0.3
100 41
IN PALEOPRODUCTIVITY
313
23
-i::i::: i•i•i •?::
•5•8 0
0.0
0.00
0.2
i
100
41
:!:i
:.:.
0.05
23
i:
TOC
0.10
0.2
100 41 23
.
0.1
0.1
0.0
0.0
0.00
0.05
0.10
100 4!
0.00
0.05
0.10
_ 100 4.1 ii::::i ::i::::23 ?:11 C37MAR
0.1
0.0
0.00
0.05
0.10
Frequency (1/kyr)
0.00
0.05
0.10
Frequency(1/kyr)
Figure4. Results ofharmonic analysis calculated ontheills(),TOC,TOCMAR,C•7andC•, MARrecord.• of GeoB3005 usingthe softwareprogramSPECTRLIM [Schulzand Stattegger.1997]. A Fischer/Siegeltest(dashedline) for compound periodicitywasemployed,andsignificantpeaksoutsidethe eccentricity(100), obliquity(41). andprecession band(23) are labeledseparately.Horizontalbar marks6 dB bandwidthfbr eachcalculation.
SWmonsoon wind speeds, and decreasingsea surface 1993;Altabet et al., 1995; Beaufortet al., 1997; Reichartet temperatures in theWAS andEAS [Nair et al., 1989;Ittekkot al., 1998]. This indicatesthat maxima in wind strength, and upwellingsignificantlydeviatefrom et al., 1992; Haake et al., 1993]. Thus, if the SW monsoon paleoproductivity, wind strengthrespondsdirectlyto insolationchanges,then maxima in boreal summer insolation at June 21. None of
biological,biogeochemical, and lithogenicproxies for paleoupwelling andwind intensityshouldbe in phasewith boreal summer insolation. However, numerous studies
them evenhas a maximumwithin the half cycle of enhanced insolation.On the basis of their phase anglesthese proxies used to reconstruct monsoonal climate can be divided into
concerning the reconstruction of upwelling-related three groups (Table 2, Figure 9) reflecting (1) summer paleoproductivity in the ArabianSeareveala widerangeof monsoonwind strengthandrelatedupwellingintensityin the in the phaseangles(-6.6 to +7.7 kyr) betweenthe variousproxies WAS (Group1:-6.4 to -9.6 kyr), (2) paleoproductivity in the andborealsummerinsolation[e.g., Clemensand Prell, 1990, WAS (-10.9 to +9.6 kyr), and (3) paleoproductivity 1991;Shimmieldet al., 1990;Murray and Prell 1992;Emeis,
EAS (+8.3 to +6.4 kyr).
314
BUDZIAK
ET AL.: ARABIAN
SEA VARIATIONS
IN PALEOPRODUCTIVITY
ContinentalSlopeoff Oman' ODP Site 723
o I
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MAR Figure for 5.ODP Time Site series 723of [Niitsuma •SO ofthe etal.. planktonic 1991'Emets. t•)raminit•ra 1993:Emets !•ullemat•na etal., 1995]. obhqudoculata. Datawaskindly T()C.provided TOCMAR. byK.-C. C•,. and Emeis. œ'3,
6.1. Group 1: SW MonsoonWind Strengthand Upwelling Intensity in the Western Arabian Sea (-6.4 to-9.6 kyr) .
Clemensand Prell [ 1990] and Clemenset al. [ 1991] used marineandterrestrialproxies,givingindependent information on marinebiologicalprocesses and the timing of maximain summer monsoon wind intensity. The marine proxies, consisting of the concentration of the planktonicforaminifera Globigerina hulloides, barium, and biogenic opal fluxes, reveal a strongcoherency with boreal summer insolation but showphaselagsof-8.7 (-121ø),-6.6 (-104ø),and-7.2 kyr (-113ø), respectively,in the precessionband. A grain size record was used as terrestrial indicator for the SW monsoon
intensity, reveaiing aphase lagof-9.5kyr(-148ø)[Clemens et al., 1991]. Togethertheseproxiesforminggroup 1 reveal a time lag of roughly 6 to 9 kyr, which was attributed to variations in the cross-equatorialtransport of latent heat originatingfrom the southernIndian Ocean and its release over the Asian Plateau, which accordingto the model of Clemens et al. [1991], determinesthe strength of the SW monsoon.
Altabet etal. [1995]usedthesedimentary •SN/•4N ratios to reconstruct denitrificationintensity,the processby which nitrateis reducedto gaseous nitrogenspecies,an important limitingfactorfor marineproductivity [Altabetand Curry,
1989].In the precessional frequency bandthe b•,,l record from the OwenRidgeis coherentandnearlyin phasewith G. hulloides(percent)suggestinga strong link between denitrificationand upwelling.Similar to the otherrecords
described above, blSNlagsboreal summer insolation by9 kyr (Table2) [Altabetet al., 1995]andthusbelongs to group1. Reichartet al. [ 1998]appliedan agemodelindependent of the SPECMAPtimescale,which reducedthe phaselag between monsoon proxies andsummer insolation by 2 kyr.In spite of this alternativechronology tuned to the bl80 timescaleof MD 84641 [Lourenset al., 1996] from the Mediterranean a time lag betweenthe productivity proxies G. hulloides (percent) andTOC withrespect to earlysummer insolationof 6 to 7.5 kyr (Table2) is still evident.This was attributed by the authorsto a prolonged upwellingseason causedby an increasedinsolationduring Augustand September, suggesting thatthelengthof the summermonsoon
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY Eastern Arabian
315
Sea: MD 900963
1.0 0.8
0.6
0.4
0.4 0.2 0.2
8000
4000
400
200• •
0
!
!
0
!
i
!
!
100
!
!
200
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Figure6. Timeseries of 8•aOoftheplanktonic foraminifera Globigermotdes rnber,TOC.TOCMAR.C•: andC_•7 MAlt for MD 900963 [Rosteket al., 1994, 1997].
is more importantto annualfluxesof G. bulloidesand TOC reflectspreservationchangesdue to enhancedsedimentation thanthe wind strengthat the time of early summerinsolation ratesor theproduction of organiccarbonis notdirectlylinked maximum. to monsoonal upwelling. 6.2. Group 2: Paleoproductivityin the Western Arabian Sea (-10.9 to +9.6 kyr) Group 2, consistingonly of records of TOC and C37 alkenones,reveals opposite phasing between -10.9 and +9.6kyr to boreal summer insolation maxima in the precessional frequencyband(Table2, Figure9). Murray and Prell [1992] describedsimilar oppositerelationshipsof +10 kyr (--- +157ø) (Table 2) between maxima of TOC, TOC MAR, and estimatesof paleoproductivity, inferredfrom
Variations in TOC and the C37 alkenone concentrations
may be attributedto changesin either productivityor preservation. Today,the ArabianSea is characterized by a pronouncedoxygenminimum zone (OMZ) between150 and -• 1500 m [ Wyrtki,1971], initiatinga controversialdiscussion about its influence on proxies used to reconstruct
paleoproductivity,especially sedimentaryTOC. Higher sedimentary TOC contentsmay be the result of generally higher export productionrates and a probably somewhat better preservationof organic matter settling through a the sedimentary TOC contentafterMiiller andSuess[ 1979], strongerOMZ. Paropkari et al. [1992] comparedthe and maximain NorthernHemispheresummerinsolation.In sedimentary TOC contentson the continentalslopesof the their studythe phaseof TOC and productivityrecordslie Indianmarginandthe ArabianPeninsula. Theyinferredthat directly between the phasing of indicatorsfor summer bottom water anoxia plays an important role on the In contrast,Pedersenet al. monsoonstrengthat about -7.7 kyr (-120ø) on average sedimentaryTOC concentrations. (group1) andthat of high sedimentation ratesassociated with [1992] examinedthe 0 to 1 cm depth interval from 14 lithogenicinput at 5.8 kyr (+90ø) in phasewith maximumice undisturbed box corescollectedfrom the outershelf-upper volume.Fromthis they concludedthat eitherthe TOC signal continental slopeareaoff Omanfromwaterdepths< 1650m.
316
BUDZIAK ET AL.' ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY
ld041 23
0.2 ii i::iiii?:i
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Figure7. Results of harmonic analysis calculated on(a)TOCand(b)TOC MAR of OI)P Site723 [t;met.•'. 1993:Pjmet,• • et
al., 1995]and(c)TOCand(d)TOCMARof coreMD 900963[Rostek etal., 1994.1997 ! using thesoftware program SPECTRtlM [Schulz andStattegger, 1997].A Fischer/Siegel lesl(dashed line)lbrcompound periodicily wasemployed. andsignificant peaksoutside theeccentricity (100).obliquity (41).andprecession band(23) arelabeled scparalcly. Horizontal bar marls 6 dB bandwidth lbr each calculation.
Their findingssuggested that there is little relationship coccolithophorid species F. profundaas a proxyto estimate variationsin surfaceproductivity.High TOC contentsand alkenone concentrations are accompanied by a low ratioof margin. F. profunda to total coccoliths,indicatinga shallow Reichartet al. [1998] comparedTOC patternsin cores thermocline andenhanced surface waterproductivity. Thisis fromdifferent waterdepths in thenorthern ArabianSea.They corroborated bytransfer functions based oncoccolithophorids betweenthe bottom water oxygen concentrationand the sedimentary contentof marineorganicmatteron the Oman
found that TOC fluctuations in cores from within and below
the OMZ can be correlatedone by one with each other, although TOC contents arelowerin thedeepwatercores,and
concludedthat variationsin TOC are not causedby fluctuations in the OMZ but are primarilycontrolledby changes in surfacewaterproductivity. Additionally, Schubert et al. [1998]examined severalorganicbiomarker compounds, such as alkenones in relation with variations of TOC over the
past200 kyr. Theyconfirmedthe useof alkenones, TOC, and other organic biomarkers as qualitative indicators for paleoproductivity changesin ArabianSeasediments. While Paropkariet al. [ 1992] suggestthat bottomwater
anoxiaplays an importantrole on the sedimentary TOC
[Beaufort etal.,1997]andplanktonic foraminifera [Cayre et al., 1999]atsiteMD 900963.Additionally, different kindsof
organic biomarker compounds [Schulte et al., 1999]and redox-sensitive tracemetals [Pailleret al., 1998]revealed thatthebottom waters remained oxygenated during thelast 330kyratsiteMD900963. Intense bioturbation throughout thewholecoreindicates oxygenated bottomwaterconditions
at siteGeoB3005[Hernleben et al., 1996].qZhe good agreement betweenthe TOC and TOC MAR, as well as betweenC37and C37MAR recordsin both GeoB3005 and MD 900963 indicatesthat variationsin theserecordsreflect
changes in surface waterproductivity rather thanpreservation changes dueto varyingsedimentation rates[Miillerand
concentrations, several other studies indicate that variable
Suess,1979].
bottomwateroxygenconcentrations in the OMZ haveonly littleor no effectonTOC variations in theArabianSea[e.g.,
In contrast, dilutioneffectsdue to highersedimentation ratesof terrigenous lithogenic material andpreservation asan additionalcontrollingfactoron TOC and C37cannotbe excluded at ODP Site723. The higherTOC concentrations
Pedersenet al., 1992; Reichart et al., 1998; Schubertet al., 1998]. Rosteket al. [ 1994, 1997] assumedthat variationsof
organic matterarerelatedto changes in marineproductivity in the WAS and EAS. They used the distribution of the
during interglacial times atODPSite723arecorroborated by a G. bulloides record[Erneis et al., 1995]alsoindicating
BUDZIAKET AL.: ARABIANSEAVARIATIONSIN PALEOPRODUCTIVITY 100kyr
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Figure8. Results ofthecross-spectral analyses between boreal summer insolation (June21,30øN)calculated afterPIerger [1978]using theAnalySeries software package [Pa•llard et al.. 1996]andTOCrecords of (a) GeoB3005,(b) ODPSite 723[Erneis, 1993;Erneis et al., 1995],and(c) MD 900963[Rostek et al., 1994,1997]applying thesortware program SPECTRUM [Schulz andStattegger, 1997].Horizontal barmarks 6 dBbandwidth foreachcalculation.
higher productivitylevels during interglacialtimes. Thus Emeis et al. [1995] suggestedTOC to be a more suitable indicatorof productivity thanaccumulation ratesat thissite. The resultsof the cross-spectral analysesof GeoB 3005 and ODP Site 723 describedin this study(Table 2, Figure9) are in goodagreementwith Emeis [1993] and Murray and Prell [1992], who calculatedthe paleoproductivity ratesafter Sarntheinet al. [1992] or M•iller and Suess[1979] using TOC MAR at ODP Site 723 and RC27-61, respectively. These calculationsrevealed a phase relationshipto boreal summerinsolationof about+ 10.5 kyr (+ 165ø) (Table 2). We thereforeassumethat variationsof TOC in the precessional frequencydomain in the WAS can be attributedto
productivity ratherthanto preservation changes. To explain the oppositephasing,we proposethat variationsof paleoproductivity indicated by TOC, alkenones, andother organicbiomarkers are not directlylinkedto maxinmm monsoonal upwelling intensity andwindstrength in theWAS. 6.3. Group 3: Paleoproductivityin the Eastern Arabian Sea (+8.3 to +6.4 kyr)
Group3 summarizes phaseanglesbetween+8.3 (+130ø) and +6.4 kyr (+100ø) (Figure 9) in phasewith maximum precessional ice volume.The leadingof productivity records of siteMD 900963 of +7.7 kyr (+ 120ø) on average(Table2)
318
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY
Table 2. Summaryof PhaseAnglesBetweenTime Seriesof DifferentCoresandPrecessional Insolation(June21) Author(s)
Core
Time Series
Precession(June21) Phase,deg.
Group1
Clemenset al. [ 1991]
ODP Site 722 Opal MAR
RC27-61
Lead/Lag,kyr
- 113 +/-21
7.2 +/- 1.3
Ba flux
- 04 +/-10
6.6 +/-0.6
G. bulloides(%)
-121 +/-23
8.7 +/-1.3
LithogenicGrain Size
-148 +/-12
9.5 +/-0.6
Groupl Altabetet al. [1995]
RC27-61
d15N
-141 +/-20
9.0 +/-1.3
Groupl
NIOP 464
G. bulloides(%)
-103 +/-31
6.6 +/-2.0
TOC (%)
- 116 +/-29
7.4 +/- 1.9
TOC (%)
+ 170 +/-25
10.9 +/- 1.6
TOC MAR
+155 +/-15
PPestimates
+162+/-28
10.4+/-1.8
+167+/-27
10.7+/-1.7
+177 +/-4
11.3 +/-0.3
TOC MAR
+ 173 +/-4
11.1 +/-0.3
C37(ng/g)
+ 172 +/-7
11 +/-0.4
C37MAR
+ 168 +/-6
10.7 +/-0.4
ODP Site 723 TOC (%)
-178 +/-5
11.4 +/-0.3
+ 153 +/-6
9.8 +/-0.4
Reichartet al. [1998]
Group2 Murray & Prell [ 1992]
RC27-61
[M•iller and Suess,1979]
Group2
Emeis[ 1993]
ODPSite723 PPestimates
Group2
thisstudy
GeoB 3005
Group2
[Sarntheinet al., 1992] TOC (%)
TOC MAR
Group3 Beaufortet al. [1997] Group3
this study
MD 900963
PP (F. profunda)
9.9 +/-1
+ 118
7.5
TOC (%)
+120 +/-10
7.7 +/-0.6
TOC MAR
+ 129 +/-8
8.2 +/-0.5
C37(ng/g)
+108 +/-2
6.9 +/-0.1
C•7 MAR
+ 110 +/-4
7 +/-0.3
Positivevaluesindicatea lead with respectto insolationmaximum,whereasnegativevaluesindicatea lag. A
detailed description of theagemodelsandcross-spectral analyses of MD 900963,NIOP464,ODP Site723,RC27-61, andODP Site722 aregivenby Bassinotet al. [ 1994](agemodel)andBeaufortet al. [1997] (cross-spectral analysis), Reichartet al. [1998], Emeis[1993], and Clemensand Prell [1990] and [1991], respectively. The proxieswere dividedintothreegroupsreflectingsummermonsoon wind strength andrelatedupwellingintensityin the western ArabianSea(WAS) (-6.4 to -9.6 kyr) andpaleoproductivity in the WAS (-10.9 to +9.6 kyr) andeasternArabianSea (+8.3 to +6.4 kyr).
is consistent with the previouslypublishedwork of Beaufort et al. [1997]. They used the relative abundanceof the coccolithophorid F. profunda as a marker for variationsin primaryproductivityfor the past910 kyr at site MD 900963. Beaufortet al. [ 1997] suggested thatproductivityin the EAS is coherentand in phase with the early spring equatorial insolationand thereforeis relatedto the wind intensityof the
intothe photiczone[Rao et al., 1989]. Accordingto Emeiset al. [1995], an expandedand thickenedsnowcoverover the TibetanPlateaumayhaveat leastprolongedthe NE monsoon season.Consequently, in the EAS, paleoproductivity seemsto be predominantlylinked to variationsin the NE monsoon winds,which are probablyrelatedto maximumprecessional ice volume.
westerlies.
On the other hand, Rosteket al. [1994, 1997] inferreda link between precession-dominated paleoproductivityand deeper mixing due to stronger and predominating NE monsoonsduringglacialtimesin the EAS on the basisof higher TOC and C37 alkenone contents during glacial interstadials.Additionally,recent studiessuggested that the NE monsoonwinds are responsiblefor deepeningof the mixed layer,therebyinjectingnutrient-richsubsurfacewaters
6.4. Implications for the Sedimentary TOC and C37 Aikenone Signal in the Arabian Sea
AlthoughClemenset al. [1991] attributedthe 6 to 9 kyr time lag betweenproxiesbelongingto group 1 and boreal summer insolation to variations in the cross-equatorial transportof latent heat, the TOC and C37signalsfrom the WAS
and EAS indicate that the SW monsoon and related
upwelling intensityalone cannotaccountfor the different
BUDZIAK
ET AL.: ARABIAN
SEA VARIATIONS
Precession
(June 21) 0o
IN PALEOPRODUCTIVITY
319
1995] and to rather moderate SW monsoonintensity, as suggested by recentstudiesof Rixenet al. [ 1996]. In the EAS the TOC and alkenonemaxima are in phasewith maximum ice volume,indicatingan additionalinfluenceof strongerand prolongedNE monsoonwinds associatedwith cold climates, asproposedby Rosteket al. [ 1994, 1997]. 7. Conclusions
Variationsin the sedimentary contentof TOC and C37at
sitesGeoB3005andMD 900963reveala dominating 23 kyr cyclicity, indicating a precession-related insolation forcingon paleoproductivity in the WAS andEAS.It is worthnoting thatwhilein the EAS variancein the frequency domainof eccentricity is present in TOC MAR associatedwith
interstadials in glacial/slSomaxima [Rostek et al., 1994, +/-180 ø
Group 2 Figure 9. Precessional phasewheel summarizingproxiesof group 1 (-6.4 to -9.6 kyr) usedto reconstructSW monsoonwind intensityand relatedupwellingin the WAS [Clemensand Prell, 1990; Clemenset al., 1991; Altabet et al., 1995] referenced to maximum insolation corresponding to June21. Groups2 (-10.9 to +9.6 kyr) and 3 (+8.3 to +6.4 kyr) indicateproductivityproxies (TOC, TOC MAR, C37, and C37MAR) fromcoresGeoB3005 (solidline) as well as from ODP Site 723 (long-and short-dashed line) and MD 900963 (dashedline) in the WAS andEAS, respectively,calculatedin this study.
phasingof groups1, 2, and 3. The explanations of group1, cross-equatorial transport of latentheat[Clemens et al. 1991], and prolongedsummermonsoonseason[Reichartet al., 1998]cannotbe resolvedhere.The timingof groups2 and 3 betweenmaximain the monsoonindicesof group 1 and maximumice coverleadinginsolationat June21 by about +6.4 kyr (+ 100ø) (Figure9) impliesa pronounced influenceof the NE monsoonwinds on TOC and C37fluxes in the WAS
and EAS. Accordingto Prell [ 1984], a strongersnowcover overAsia delaysthe onsetand shortensthe summermonsoon season.Thus a prolongedNE monsoondue to minimum summerinsolationandincreased ice volumemayhaveleadto
a deepeningof the mixed layer enhancingthe primary productivity andparticlefluxesin the WAS andEAS [Nair et al., 1989; Rao et al., 1989] rather than inducingstronger upwelling. A strengtheningand prolongation of the NE monsoonseason,increasingthe vertical mixing during borealwinter at times oppositeto maxima in precessional summerinsolation,werepreviouslyproposed by Van Campo et al. [1982] andEmeiset al. [1995]. Additionally,recentstudiesof Rixenet al. [1996] suggest
1997],the recordfrom the westernregionsis exclusively characterized by theprecession component. In contrast, ODP Site 723 exhibitsstrongvariancein the eccentricity band [Emeis,1993],showing higherTOC valuesduringinterglacial periodsat the continentalslopeoff Oman [Emeiset al., 1995]. Superimposedon these fluctuations in the low-
frequencydomainare variationsin the precession band at ODP Site 723.
According to their phasing related to precessional variationsin borealsummerinsolation,referencedto June21, monsoonal indicesaswell aspaleoproductivity proxiescanbe dividedinto threegroupsreflectingsummermonsoonwind
strength andrelatedupwellingintensityin theWAS (group1: -6.4 to -9.6 kyr) andpaleoproductivity in the WAS (group2: -10.9 to +9.6 kyr), and EAS (group3:+8.3 to +6.4 kyr). Cross-spectral analyses reveala coherent but oppositephase relationshipof TOC recordsof GeoB 3005 and ODP Site 723 with respectto Northern Hemispheresummer insolation.
Theseproductivity-related proxyrecordslie directlybetween the indicatorsfor SW monsoonwind strengthand related upwellingintensityas described by Clemenset al. [1991] belongingto group 1, and maximum ice volume, which probablyprolongsand strengthensthe NE monsoonwinds.
TOC of MD 900963 leadssummerinsolationby +7.7 kyr (group 3) thus is very close to maximum ice volume in the precession band. We interpret the maxima in TOC and C37 alkenone variationsto be a combinedsignalof moderateSW monsoon
wind strength,as suggested by Rixen et al. [1996], and additionallystrengthened and prolongedNE monsoonwinds
enhancing primaryproductivity dueto deepermixingin the WAS, while paleoproductivity seemedto predominantly correspond to the NE monsoonwinds in the EAS. Acknowledgments.We thankHella Buschhoff,Dietmar Grotheer,
andRalphKreutzfor technical assistance in thehomelaboratory; the that during periods of strong SW monsoon winds, cold, officers,crew,andscientists aboardR/V Meteorfor theirhelpwith nunlent-depleted, southequatorialwateris transported to the coringandsamplingoperations; theOceanDrillingProgram(ODP) andKai-ChristianEmeisfor providingdatafromODP Site723. This north into the WAS, where these water masses reduce the and amountof nutrients,therebydiminishingproductivityrelated researchwas fundedby the DeutscheForschungsgemeinschaft the Bundesminister far Bildung und Forschung(BMBF), Bonn. biogenicparticle fluxes. Thus highestflux rates associated Paleoclimate workat CEREGEis supported by CNRS(PNEDC),the with upwelling occur during SW monsoonsof minor to EuropeanCommunity(projectsENV4-CT97-0564, FMRX-CT96intermediatestrength[Rixen et al., 1996]. Consequently, 0046, and ENV4-CT97-0659), and the IFCPAR (project1809-1). variationsin paleoproductivity in the WAS are probably Previousversionsof this manuscriptgreatlybenefitedfrom reviews
linkedto deepermixingof surfacewatersthroughthe action of strongerand prolongedNE monsoonwinds[Emeiset al.,
by M. Delaney,G.J. Reichart,and an anonymous reviewer.The data of GeoB3005presented in thispaperwill be archivedin thePangaea database (www.pangaea.de/Projects/SFB261/DBMziak et al 2000/).
320
BUDZIAK ET AL.: ARABIAN SEA VARIATIONS IN PALEOPRODUCTIVITY
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