Geochemical characteristics of deep-sea sediments from the Arabian Sea

34 Pages·2000·1.26 MB·English

DSRII=702=KG Mahashankar=Venkatachala=BG
Deep-SeaResearchII47(2000)2735}2768
Geochemical characteristics of deep-sea sediments
from the Arabian Sea: a high-resolution study
B. Schnetger!,*, H.-J. Brumsack!, H. Schale!, J. Hinrichs!,
L. Dittert"
!InstituteofChemistryandBiologyoftheMarineEnvironment(ICBM),
CarlvonOssietzky-UniversityOldenburg,P.O.Box2503,D-26111Oldenburg,Germany
"DepartmentforGeosciences,UniversityofBremen,P.O. Box330440,D-28334Bremen,Germany
Received11April1999;receivedinrevisedform15December1999;accepted20December1999
Abstract
Fivedeep-seacoresfromtheArabianSea,coveringatimeintervalof170ky,weresampledat
highresolutionandanalysedformajorandtraceelements.Thestratigraphyofthecoreswas
obtainedbycomparingtheBa/AlratiowiththeSPECMAPdata.Thismethodisbasedonthe
closecorrelationbetweenBaconcentrations(orBa/Alratios)andoxygenisotoperatios(d18O)
of foraminifera in Arabian Sea sediments. Ba/Al ratios, used as a productivity proxy, are
variablebuthighinalldeep-seacoresoftheArabianSea,indicatingabasin-widein#uenceof
nutrient-richwatermasses.Comparedtoglacialintervalsthewarmerperiodsarecharacterised
by higher element/Al ratios of proxies, which are directly (Ba, Ca, Sr, P, i.e. hard parts of
organisms) or indirectly (U, 230Th ) related to biological productivity. 230Th provides
%9 %9
evidence for intense boundary scavenging caused by high productivity due to enhanced
upwellingand terrigenousinputfrom theIndus fan. The Mn distributionin a corefrom the
western Arabian Sea shows enrichments during interglacial periods and may indicate Mn
exportowingtothepresenceofastrongeroxygen-minimumzone.Severalprocesseslimitthe
applicabilityofproxies.Intercalatedturbiditesinthedeep-seacorescanbeidenti"edbymeans
ofSi/Al,Ti/AlandZr/Alratiosversusdepthduetotheenrichmentofquartz,Ti-mineralsand
zirconinthebasallayer.Theseelementsarealsoimportantforthereconstructionofthehistory
andextentoftheeoliandustinput.Diageneticredistributionofredoxsensitiveelementsoccurs
at the boundaries between turbidites and ‘normala pelagic sedimentation and may mask or
destroyprimarysignals. ( 2000ElsevierScienceLtd.Allrightsreserved.
*Correspondingauthor.Tel.:#[phone];fax.:#[phone].
E-mailaddress:[email protected](B.Schnetger).
0967-0645/00/$-seefrontmatter ( 2000ElsevierScienceLtd.Allrightsreserved.
PII: S0967-0645(00)00047-3
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1. Introduction
Marine sediments provide important information on past oceanographic and
climaticchanges,suchas#uctuationsinbiologicalproductivity,redoxstateofbottom
water, seawater and sediments, tectonic activity, wind strength and volcanic, hy-
drothermal,hydrogeneous,eolianand #uvialsources.Becausethese changescannot
bedetermineddirectly,geochemicalproxiesofclimaticvariationsandpaleoenviron-
mental conditions have been developed during the last decades (e.g., Boyle, 1983;
Calvert and Pedersen, 1993; Goldberg and Arrhenius, 1958; Lao et al., 1992; Kyte
etal.,1993;Shimmield,1992;Schneideretal.,1997;WehausenandBrumsack,1999).
Most of the paleoproxies have restrictions that con"ne their application to well-
de"nedenvironments.Theselimitations,andthein#uenceofsecondaryprocessesthat
may destroy or mask the primary signals, are only partly known (e.g., Shaw et al.,
1990). High-resolution studies of geochemical proxies in di!erent environmental
settingsand on the behaviourof diagnostic elementsin the marine environmentare
necessary to improve the applicability of such tracers.
The Arabian Sea, predominantly its northwestern section,is known to be an area of
active oceanic upwelling, which is coupled to the intensity of the Southwest Monsoon
(Naidu, 1998). The seasonal rain of organic matter and its microbial consumption are
responsible for the presence of an intense oxygen minimum zone (OMZ) in the entire
ArabianSea,whichoccursinwaterdepthsrangingfrom 150to1500m(Wyrtki,1971).
Thus,thesedimentsoftheArabianSeacontainanimportantarchiveofclimaticchange
thatpermitsthereconstructionofthehistoryoftheIndianOceanmonsoonalcirculation.
A large number of biological and geochemical proxies, including major and trace
elements,isotoperatiosofoxygenandcarboninplanktonicandbenthicforaminifera,and
pollen records, have been used to reconstruct the history of the dust and productivity
(upwelling) record. These studies also show the relationship between subtropical mon-
soon and high-latitude climate (Shimmield and Mowbray, 1991a; Sirocko et al., 1996;
Tribovillardetal.,1996;Reichardetal.,1997;Naidu,1998).Sincetheoceanographicand
climatic conditions in the Arabian Sea during the last 250 ky are well established, the
deep-seasedimentsrepresentauniquearchivethatallowustotestchemicalpaleoproxies.
In this paper we present high-resolution geochemical sediment data from four
deep-sea sites located in the western, northern, southern and central sector of the
Arabian Sea. To our knowledge geochemical studies to date have been carried out
only in the western and northeastern regions. Our data sets were combined with
resultsobtainedfromsitesclosetothecoastofSomalia(Tribovillardetal.,1996),on
Owen Ridge (Shimmield and Mowbray, 1991a,b), and from the Pakistan Margin
(Murray Ridge; Reichard et al., 1997).
First,wewillgiveageneralcharacterisationofthesediments,theassociationofthe
chemical elements and their geochemical relationship. We use the high-resolution
data to identify turbidites in the investigated cores and consider the diagenetic
redistributionsoccurringat the seawater/sedimentinterface and at turbidite bound-
aries.AstratigraphywasappliedusingthecloserelationshipbetweentheBa/Alratio
andthed18OvaluesofforaminiferaintheArabiandeepseasediments.Fortwocores
weidentifylateralsedimenttransportusingthe230Thmethod.Finally,wewillinterpret
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somegeochemicalpaleoproxieswithrespecttoglacial/interglacialperiodswithspecial
reference to the export of Mn from the shelf regions into the deep Arabian Sea.
2. Materials and methods
Five sites in the Arabian Sea (WAST, WAST-T: west, NAST: northeast, CAST:
central, SAST: south) were sampled during two cruises with FS Sonne in 1997 and
1998(Fig.1).WAST-Tislocatedonatopographichigh,whereasthenearbystation
Fig.1. LocationsofcoresitesNAST,WAST,WAST-T(onatopographichigh),SAST,CAST.
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WASTisfromadeeperlocation.GeneralinformationforthesitesisgiveninTable1.
Multicorer cores were sampled at 0.5-cm intervals for the upper 10 cm and at 1-cm
intervalsbelow.Gravitycoresweregenerallysampledat2-cmintervals.Sampleswere
kept frozen in PE bags until further processing in our Oldenburg laboratory. After
freeze-drying,the samples were ground and homogenised in an agate ball mill.
Table1
Chemicalcompositionandelement/aluminium-ratiosofsedimentsfromtheArabianSea
(a)WAST,WAST-TandNAST
Station WAST WAST-T NAST
Position 16313,0N 16310,7N 20300,0N
60316,0E 59345,7E 65335,0E
Waterdepth(m) 4047 1918 3186
Multiple-corer(MC);cruise MC05;So118 MC01;So118 MC41;So118
Corelength(cmbsf) 0}44 0}13 0}32
Gravity-corer(SL);cruise SL2;So118 SL6;So118 SL4;So118
Corelength(cm) 214 475 500
Age(ky) 170 125
Sedimentationrate(cm/ky) 2.8 3.9
DepthcorrectionSL(cm) Upto44 0 4
Meanvalues: Sediment Turbidite Sediment Sediment
Numberofsamples 33 46 160 165
Conc. Ratios Conc. Ratios Conc. Ratios Conc. Ratios Conc. Ratios
SiO2 (%) Si/Al 22.3 4.1 25.7 4.5 15.1 4.2 33.2 3.4
TiO2(%) Ti/Al 0.300 0.069 0.328 0.073 0.215 0.076 0.474 0.062
Al2O3 (%) 4.89 5.10 3.23 8.66
Fe2O3 (%) Fe/Al 2.46 0.67 2.61 0.67 1.63 0.68 4.06 0.62
MnO(%) Mn/Al 0.148 0.059 0.164 0.065 0.026 0.012 0.138 0.025
MgO(%)! Mg/Al 3.13 0.73 3.98 0.89 2.21 0.78 3.07 0.41
CaO(%) Ca/Al 31.0 9.4 26.9 7.2 38.5 19 22.0 3.5
Na2O(%)! Na/Al 0.68 0.20 0.69 0.19 0.56 0.26 0.93 0.15
K2O(%)! K/Al 0.968 0.31 0.952 0.29 0.638 0.31 1.73 0.31
P2O5 (%) P/Al 0.118 0.021 0.164 0.027 0.105 0.029 0.151 0.015
S(%)! S/Al 0.068 0.026 0.318 0.12 0.036 0.022 0.070 0.015
Cmin(%) Cmin/Al 6.93 2.9 6.12 2.3 8.47 5.8 4.83 1.1
TOC(%) TOC/Al 0.61 0.25 1.80 0.66 0.62 0.41 0.43 0.096
As(lg/g) As/Al 3.3 1.4 6.0 2.2 2.6 1.7 5.7 1.3
Ba(lg/g) Ba/Al 559 242 420 160 321 233 512 115
Co(lg/g) Co/Al 14 5.9 14 5.2 8.0 5.0 19 4.1
Cr(lg/g) Cr/Al 100 38 150 57 73 43 97 21
Cu(lg/g) Cu/Al 44 19 32 12 15 9.6 40 8.8
Ni(lg/g) Ni/Al 97 41 99 37 41 24 96 21
Pb(lg/g) Pb/Al 8.5 3.5 6.3 2.3 3.8 2.3 14 3.0
Rb(lg/g) Rb/Al 45 17 42 15 31 19 80 18
Sr(lg/g) Sr/Al 953 410 848 318 1057 719 705 163
V(lg/g) V/Al 78 32 5.0 25 45 28 100 22
Y(lg/g) Y/Al 15 6.2 67 5.3 12 7.4 19 4.2
Zn(lg/g) Zn/Al 74 31 14 22 36 23 85 19
Zr(lg/g) Zr/Al 64 25 59 25 42 24 92 20
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(b)SASTandCAST
Station SAST CAST
Position 10302,0N 14325,0N
65300,0E 64334,0E
Waterdepth(m) 4426 3957
Multiple-corer(MC);cruise MC18;So118 MC34;So118
Corelength(cmbsf) 0}34 0}13
Gravity-corer(SL);cruise SL1;So118 SL3;So118
Corelength(cm) 135 130
Age(ky) 65 30
Sedimentationrate(cm/ky) 1.8 3.4
DepthcorrectionSL(cm) 10 6
Meanvalues: Sediment Turbidite Sediment Turbidite
Numberofsamples 47 8 32 12
Conc. Ratios Conc. Ratios Conc. Ratios Conc. Ratios Conc. Ratios
SiO2 (%) Si/Al 39.2 3.01 54.7 3.5 23.0 3.5 49.7 2.9
TiO2 (%) Ti/Al 0.624 0.062 0.658 0.055 0.343 0.067 0.706 0.053
Al2O3 (%) 11.6 13.6 5.87 15.1
Fe2O3 (%) Fe/Al 5.88 0.67 5.27 0.51 3.29 0.74 6.35 0.55
MnO(%) Mn/Al 0.166 0.022 0.081 0.009 0.087 0.023 0.082 0.008
MgO(%)! Mg/Al 3.71 0.37 3.07 0.26 2.47 0.49 3.56 0.27
CaO(%) Ca/Al 15.1 2.0 7.31 0.72 30.8 8.0 7.20 0.64
Na2O(%)! Na/Al 1.12 0.13 1.70 0.17 0.64 0.16 1.46 0.14
K2O(%)! K/Al 2.39 0.31 2.68 0.31 1.21 0.32 3.13 0.32
P2O5 (%) P/Al 0.137 0.0099 0.146 0.009 0.125 0.018 0.142 0.008
S(%)! S/Al 0.083 0.014 0.076 0.011 0.11 0.038 0.11 0.013
Cmin(%) Cmin/Al 3.20 0.60 1.52 0.21 6.71 2.4 1.51 0.19
TOC(%) TOC/Al 0.21 0.036 0.18 0.025 0.35 0.12 0.27 0.034
As(lg/g) As/Al 20 3.0 7.5 1.0 6.2 2.1 9.2 1.1
Ba(lg/g) Ba/Al 617 107 460 64 500 181 509 64
Co(lg/g) Co/Al 20 3.4 16 2.2 13 4.1 19 2.4
Cr(lg/g) Cr/Al 103 17 87 12 71 23 102 13
Cu(lg/g) Cu/Al 64 11 28 3.9 38 14 37 4.6
Ni(lg/g) Ni/Al 96 17 46 6.3 70 23 61 7.6
Pb(lg/g) Pb/Al 17 2.8 18 2.5 7 2.3 21 2.6
Rb(lg/g) Rb/Al 109 17 132 18 54 17 160 20
Sr(lg/g) Sr/Al 493 93 208 29 975 353 213 27
V(lg/g) V/Al 120 20 105 15 72 23 128 16
Y(lg/g) Y/Al 23 3.9 23 3.2 17 5.8 22 2.8
Zn(lg/g) Zn/Al 98 16 81 11 60 20 97 12
Zr(lg/g) Zr/Al 107 18 144 20 59 19 122 15
!Correctedforseasalt.
For X-ray #uorescenceanalysis (XRF), 600 mg of the sample powder were mixed
with3600mglithiumtetraborate,pre-oxidizedat5003CwithNH NO andfusedto
4 3
glassbeads.Thebeadswereanalysedbyaspectrometer(PhilipsPW2400),calibrated
with 29 carefully selected geostandards. Analytical precision, as checked by parallel
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analysisofinternationalreferencematerialandin-housestandards,isbetterthan1%
for Si, Ti, Al, Fe, Mn, Ca, K, 2% for Na, Mg, and better than 5% for the
followingminorandtraceelementsBa,Cr,Ni,Rb,Sr,V,Y,Zn,Zr,exceptP,Co,Cu,
As, Pb (5}10%). Selected samples were analysed by ICP-MS (Finnigan Element)
for the elements Sb, Cd, Mo, Re, Tl, Bi, REE, Th and U after acid dissolution
of the sediments according to the procedure described by Schnetger (1997). A salt
correctionwasperformedforNa,Ca,Mg,KandSbasedonthewatercontentandby
assuming a 35 g kg~1 seawater salinity. TOC (total organic carbon) contents were
determined as the di!erence of total carbon (IR analyser following combustion;
LECO SC444) and carbonate carbon (UIC Coulometer with acidi"cation module).
Analyticalprecisionisbetterthan2%exceptfor sampleswith(0.1%TOC.Sulfur
alsowas analysedby IRanalyserfollowingcombustionwith a precisionof (10%.
The quanti"cation of 230Th, 234U (for the determination of 230Th ) and total
%9#%44
UwasperformedwithanisotopedilutiontechniquebyICP-MS(FinniganElement)
after dissolution, separation of the matrix elements, and a pre-concentration
step (Hinrichs and Schnetger, 1999). The overall uncertainty introduced by the
entireseparationprocedureandtheICP-MSanalysesisabout5%(95%con"dence
interval).
Porewaterwascollectedinagloveboxunderargonatmosphereatatemperature
of 4}93C immediately after recovery. All samples were pressure-"ltered in Te#on
squeezers.Thesqueezerswereoperatedwithargonat amaximumpressureof 5bar.
Theporewaterwaspassedthrough0.2 lmcelluloseacetatemembrane"lters,which
were treated in argon bubbled water before using. The pore water samples were
acidi"edwithHNO (s.p.)toapH(2andfrozenuntilfurthertreatmentatBremen
3
University.
PorewaterconcentrationsofMnweredeterminedbyICP-AES(ICP6500XRand
Optima3300,bothPerkin-Elmer).Priortoanalysisporewatersampleswerediluted
9#1 with HNO -acidi"ed Milli-Q-water. Calibration standards deviated within
3
a limit of (2%, whereas the precision of each triple-measurement was better
than 5%.
3. Results, comparison with previous investigations and discussion
3.1. General chemical characterisation of the sediments
Average chemical composition of the sediments, the element to aluminum ratios
(E/Al) from the di!erent sites and general information are given in Table 1. The
chemicalcompositionofthecoresincomparisontoaverageshale(Wedepohl,1971)is
presented in Fig. 2. We use E/Al ratios instead of absolute concentrations to reveal
subtlechangesinthechemicalcompositionofthesediments,whicharenotcausedby
dilution with CaCO .
3
Astatisticalanalysis (correlationcoe$cientmatrix)oftheentiredataset(allsites)
wasperformedtoexaminethe"rst-orderrelationshipsand controlsonthechemical
composition of the Pleistocene deep-sea sediments (silicate, biogenic, diagenetic
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Fig.2. AverageE/Alratiosofsedimentsamplesfrom"vedeepseacoresoftheArabianSea.Turbidite
sectionsareexclucedfromthecalculationofthemeans(datainTable1).
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779299230768 5465959444855
Balg/g) 0.20.20.20.30.0!0.20.30.20.3!0.60.00.1 0.5!0.00.70.50.40.50.20.2!0.10.30.40.50.2
(
11662267132 86948404332434
Aslg/g) 0.50.50.50.60.3!0.50.30.50.4!0.2!0.2 0.10.30.10.30.00.00.20.40.6!0.50.50.40.40.4
(
9554551543 261350308686334
org%) !0.2!0.3!0.3!0.3!0.00.2!0.0!0.30.10.2 !0.20.0!0.1!0.0!0.10.20.10.0!0.2!0.30.2!0.2!0.30.1!0.3
C
(
426734336 3373294674416142
444414442 2265053244434554
%) !0.!0.!0.!0.!0.0.!0.!0.!0. 0.!0.!0.!0.0.!0.!0.!0.!0.!0.!0.0.!0.!0.!0.!0.
S(
26774376 64109092898542855
5 76666766 21436544065667676
O2%) 0.0.0.0.0.!0.0.0. !0.0.0.0.0.0.0.0.0.0.0.0.!0.0.0.0.0.
P
ns) (
o
ditesecti KO2(%) 0.980.981.000.980.77!0.980.75 0.66!0.43!0.350.570.230.740.530.590.190.030.580.800.99!0.960.940.880.840.92
bi
dingtur NaO2(%) 0.790.740.740.730.69!0.81 0.750.67!0.43!0.010.360.320.730.480.540.400.070.680.660.720.760.790.700.830.73
u
cl
ex 0098989685 8198734425522980616331116979959795918995
ST, aO%) !1.!0.!0.!0.!0. !0.!0.!0.0.0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.!0.
A C(
S
T,
CAS MgO%) 0.830.820.770.75 !0.850.690.770.64!0.13!0.050.320.090.650.730.490.350.160.660.580.69!0.840.760.740.730.81
T, (
S
A
N O3 969797 75967398674734663275516424116078969393908589
T-T, Fe2%) 0.0.0. 0.!0.0.0.0.!0.!0.0.0.0.0.0.0.0.0.0.0.!0.0.0.0.0.
S (
A
W
able( AlO23%) 0.980.99 0.970.77!0.980.741.000.67!0.46!0.350.560.290.770.540.630.230.070.610.800.98!0.950.940.900.860.93
t (
nt
coe$cie TiO2(%) 0.98 0.990.970.82!0.980.740.980.66!0.42!0.350.510.270.780.600.640.250.090.640.780.95!0.960.930.920.860.95
n
o
correlati SiO2(%) 0.980.980.960.83!1.000.790.980.72!0.44!0.290.510.270.800.610.610.280.080.670.790.95!0.970.950.900.880.96
Table2Interelement SiO(%)2TiO(%)2AlO(%)23FeO(%)23MgO(%)CaO(%)NaO(%)2KO(%)2PO(%)25S(%)Corg(%)lAs(g/g)lBa(g/g)lCo(g/g)lCr(g/g)lCu(g/g)MnO(%)lMo(g/g)lNi(g/g)lPb(g/g)lRb(g/g)lSr(g/g)lV(g/g)lY(g/g)lZn(g/g)lZr(g/g)
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6539153252445648584483892
Zrlg/g) 0.90.90.90.80.80.90.70.90.6!0.4!0.30.40.20.70.70.50.20.00.60.70.80.90.80.80.8
(
866539345433579972452028 2
ng/g) 0.80.80.80.80.7!0.80.80.80.7!0.5!0.10.40.50.80.40.70.40.20.80.70.8!0.80.90.8 0.8
Zl
(
02004108813481387152448 89
g/g) 0.90.90.90.90.70.90.70.80.6!0.5!0.30.40.40.80.50.70.30.20.70.70.8!0.80.8 0.80.8
Yl
(
5343659426624938891820 828
g/g) 0.90.90.90.90.7!0.90.70.90.7!0.4!0.20.50.30.70.50.60.20.00.70.70.9!0.9 0.80.90.8
Vl
(
765347664183415025663 0403
g/g) !0.9!0.9!0.9!0.9!0.80.9!0.7!0.9!0.60.30.2!0.5!0.1!0.7!0.6!0.5!0.2!0.0!0.5!0.7!0.9 !0.9!0.8!0.8!0.9
Srl(
55869529546342877039 32428
bg/g) 0.90.90.90.90.60.90.70.90.6!0.4!0.30.60.20.70.40.50.10.00.50.7 !0.90.90.80.80.8
Rl
(
9808896084849714225 968254
bg/g) 0.70.70.80.70.5!0.70.60.80.5!0.4!0.20.40.20.60.40.50.20.00.5 0.7!0.70.70.70.70.7
Pl
(
741069889700544539 5361544
Nilg/g) 0.60.60.60.60.6!0.60.60.50.6!0.40.00.20.50.80.40.70.60.3 0.50.5!0.50.70.70.80.6
(
89716173863494281 92059128
Molg/g) 0.00.00.00.10.1!0.10.00.00.0!0.20.10.00.40.20.00.30.7 0.30.00.0!0.00.00.20.20.0
(
8534510924085510 132728775
MnO%) 0.20.20.20.20.3!0.30.40.10.4!0.30.20.00.50.50.20.5 0.70.60.20.1!0.20.20.30.40.2
(
143493499954646 0854708898
ug/g) 0.60.60.60.60.4!0.60.50.50.4!0.5!0.10.30.70.70.2 0.50.30.70.50.5!0.50.60.70.70.5
Cl
(
10413183023946 61241853394
Crlg/g) 0.60.60.50.50.7!0.60.40.50.50.0!0.00.1!0.00.4 0.20.20.00.40.40.4!0.60.50.50.40.7
(
0875503493165 645447219176
og/g) 0.80.70.70.70.6!0.80.70.70.6!0.5!0.10.30.5 0.40.70.50.20.80.60.7!0.70.70.80.80.7
Cl
(
SiO(%)2TiO(%)2AlO(%)23FeO(%)23MgO(%)CaO(%)NaO(%)2KO(%)2PO(%)25S(%)Corg(%)lAs(g/g)lBa(g/g)lCo(g/g)lCr(g/g)lCu(g/g)MnO(%)lMo(g/g)lNi(g/g)lPb(g/g)lRb(g/g)lSr(g/g)lV(g/g)lY(g/g)lZn(g/g)lZr(g/g)
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Fig.3. TriangularplotofrelativeproportionsofAl2O3’5,SiO2,andCaO’2intheArabianSeasediments
(includingturbidites).Sampleswhichplotonthelineaverageshale}CaO’2representbinarymixtures
betweenclayandcarbonate.
fractions) of the Arabian Sea (Table 2). Turbidite sections were excluded from the
statistical procedure, but mean values and the E/Al ratios are given separately in
Table 1.
3.1.1. Major components
Most deep-sea sediments contain variable mixtures of quartz (detrital quartz or
opal),clayandcarbonate.Avariationbyafactoroftwointhemeanconcentrationof
SiO ,CaOandAl O isobservedinthecompositionofthesedimentsfromdi!erent
2 2 3
sites (Table 1). To compare the relative proportions of the major components SiO
2
(quartz/opal,silicates),Al O (clayminerals)andCaO(mostlycarbonates)atallsites,
2 3
they are plotted in triangular diagrams (Brumsack, 1989), including the turbidites
(Fig.3).Mostofthesamplesplotonamixinglinebetweenaverageshaleand(marine)
carbonate. Opal contents in the sediments are low, as samples with signi"cantly
higherbiogenicsilicacontentthanaverageshalewouldplotclosertotheSiO corner.
2
This"nding contrasts with coastal upwelling areas, where high contents of opal are
present in the sediments (Brumsack, 1989; Calvert, 1983). In the Arabian Sea opal