Calcitic foraminiferal data confirmed by cadmium in aragonitic Hoeglundina : Application to the Last Glacial Maximum in the northern Indian Ocean

PALEOCEANOGRAPHY,

VOL. 10, NO. 5, PAGES 881-900, OCTOBER 1995

Calcitic foraminiferal data confirmed by cadmium in aragonitic Hoeglundina: Application to the last

glacial maximum in the northern Indian Ocean Edward A. Boyle Departmentof Earth,Atmospheric,andPlanetarySciences, Massachusetts Institfiteof Technology,Cambridge

LaurentLabeyrie and Jean-ClaudeDuplessy Centre desFaiblesRadioactivites,CNRS-Commissariata l'EnergieAtomique, Gif-sur-Yvette France

Abstract. A core-topstudyof cadmiumuptakeinto the shellsof the aragoniticbenthic foraminiferaHoeglundinaelegansdemonstrates thatthe HoeglundinaCd/Ca partition coefficientis closeto 1.0 throughoutthe ocean.Cd uptakeby Hoeglundinais far less depthdependentthanthatof calciticbenthicforaminifera.Furthermore,manganese carbonatedoesnot precipitateon thesearagoniticshells,allowingfor therecoveryof Cd estimatesin somesampleswherecalciticspeciesare spoiledby contaminating overgrowths.BecauseCd incorporation intoHoeglundinashowslittle depthdependence, a comparison of calciticandHoeglundinaCd datacanbe usedto verifythe assumption thatthedepthdependence observedfor calciticbenthicforaminiferais time invariant. This comparison hasbeenundertakenin downcoreandlastglacialmaximum(LGM) samplesfrom thenorthernIndianOcean.Aragoniticandcalciticforaminiferalestimates for Cd in the LGM oceanare in excellentagreement.This resultindicatesupperocean estimatesof LGM Cd arereliabledespitea factor-of-twovariationin Cd uptakeby

calciticspecies overthisdepthrange. Cdandfi13Cdataindicate thattheuppermost watersof the LGM ArabianSeawere stronglynutrientdepletedrelativeto todaybut that thedeepestwatersof thenorthernBay of Bengalhadnutrientconcentrations at leastas highasor higherthanmodernlevels.A strongverticalgradientof increasing nutrients with increasing depthexistedin bothbasinsduringtheLGM. A westto eastgradientof increasing nutrientconcentrations alsois evidentat all depths.In theBay of Bengal,there is evidencefor an enhancedgradientof nutrientsincreasingfrom southto noah. The LGM deepArabianSeaandintermediate-depth Bay of Bengalwerenutrientdepleted

compared tothemodern ocean; however, Cdand15•3C dataindicate thatnutrient depletionin the northernIndian Oceandid not exceedthat of the deepeasterntropical Pacificexceptduringthe latestpart of oxygenisotopestage2 and(accordingto Cd data only) the early stagesof deglaciation.During oxygenisotopestage3 andearly stage2, Cd in the northernIndian OceanandCd in the easterntropicalPacificwere indistinguishable, althoughbothregionshavelower Cd at thesetimescomparedto the Holocene.

Introduction

Geochemicaltracersin benthicforaminiferaare a key to

unravelling changesin the deep water circulationand behavior of the oceanic carbon system during late Quaternaryglacial cycles. Continuousscrutiny of the accuracyand reliability of tracersis an importantpart of paleoclimatic reconstructions. With expansion of thecoretop databasefor benthicforaminiferalCd into shallower waters,it only recentlybecameevidentthat uptakeof Cd

into the shells of calcitic

benthic foraminifera

and it is the same in both the Atlantic

Copyright1995by theAmericanGeophysical Union. Papernumber95PA01625. 0883-8305/95PA-01625510.00

at 1100 m ,

and shallower water depth was about a factor of 2 lower thanin coresgreaterthan 2500-3500 m deep[Boyle, 1992]. Therefore it is necessary to apply a depth dependent correctionwhen usingcalciticbenthicforaminiferalCd/Ca data to estimatepast bottomwater Cd. The mechanismof this depthdependenceis not understood,althoughit seems more likely that it is a functionof changesin foraminiferal physiologyrather than thermodynamicfactors.The depth dependenceis similar in all of the calcitic speciesstudied, and Pacific oceans. It

seems likely that pressure or some tightly correlated property is the master variable governing this depth dependenceratherthan changesin water columncarbonate

882

BOYLE ET AL.' CADMIUM

IN HOEGLUNDINA

IN NORTHERN

INDIAN OCEAN

ion chemistry, which differ substantiallybetween the

believe that these analysesare affected by the artifact. In any event, the major point of the paper will be that LGM generatingthisdepthdependence, however,it is reasonable Cd is low compared to the modern ocean, and CdS to be concernedabout whetherthis depthdependencehas contamination cannot have caused values to be too low. remained constantin time. Therefore it would be of great value to have an independentmeasure of past water Cd/Ca in Hoeglundina as a Tracer column Cd with which to compare the estimates from of Bottom Water Cadmium calcitic benthic foraminifera. The first part of this study uses core-top data to The aragoniticbenthicforaminiferalgenusHoeglundina demonstrate that Cd uptakeinto the aragoniticshellsof the (consisting today solely of the species H. elegans) is benthicforaminiferaHoeglundinaelegansis governedby a potentially of great use in tracing intermediate-water differentpartitioncoefficientthanfor calciticspecies,and chemical composition becauseit is common in samples that this partition coefficient is not strongly depth dependent.Cd reconstructions usingHoeglundinado not through this depth zone. However, relatively little paleochemicalwork on this specieshas been undertaken, require the constant depth dependence assumption for two reasons:(1) its •513C is considerably morepositive necessary for calcitic foraminifera. For this reason, a than observedfor bottom water or Cibicidoides species, comparisonof Cd data from Hoeglundina with calcitic speciesin the upper ocean is an excellent test for the and (2)it does not appear to maintain a systematic constancyof depthdependentCd uptakeby calciticspecies relationship between shell and bottom water •5•3C [Grossman, 1984; W. Curry, personal communication, in the past. 1994]. For thesereasons,Hoeglundinahasbeen considered The secondpart of this paper examinesdowncoreand of dubious reliability for •513Creconstructions.The lastglacialmaximum(LGM) Cd datafor Hoeglundinaand aragonitic mineralogy of this genus indicates that the three speciesof calciticbenthicforaminiferain the depth molecularbiology of its carbonateprecipitationbiologyis range 1254-2442 m from the last glacial maximum quite different than other benthic foraminifera. By northern Indian Ocean. The northern Indian Ocean is an extension, it would not be surprisingto discoverthat its ideal place to make this comparisonbecauseHoeglundina chemical behavior for trace elements is different from and three species of Cd-calibrated calcitic benthic calcitic foraminifera as well. As shall be seen, Cd foraminifera coexistin most Holocene and LGM samples. Furthermore, it has been reported that major glacial- incorporationinto Hoeglundina is significantly different from that of calcitic foraminiferal species.Nevertheless, interglacialchangesin the verticalnutrientgradientshave the evidence also will indicate that in contrast to its carbon occurredin this region, with upper water being nutrient depletedrelative to today [Boyle, 1992; Kallel et al., 1988]. isotopebehavior,Cd in Hoeglundinais a reliableindicator But due to disagreementsbetween GeochemicalOcean of bottomwaterCd with someuniqueadvantages. The first publishedcore top Cd data for Hoeglundina are Sections Study (GEOSECS) and core-top Cibicidoides given in Table 1, along with estimatesfor bottom water •513C data in this region, these findings have been phosphorus (P) and cadmium (Cd) at these sites. The P •ontroversial[Naqviet al., 1994]andcanbenefitfrom Atlantic

and Pacific.

Given an unknown

mechanism

for

furtherdetailedexamination.This studywill showthat independent estimatesof Cd from Hoeglundinaandthree speciesof calcitic benthic foraminifera are in good agreement with most of the •513Cdata, given the assumption thattheglobaloceanic phosphorus inventory is constant.

estimates

are derived

from

the nearest

GEOSECS

or

Transient Tracers in the Ocean (TTO) stations,and the Cd estimatesare derived from the "global"Cd-P relationship given by Boyle [1988] as Cd = 0.208P for P 1.34, exceptfor the shallow stations on the Bahama Banks, where a local Cd-P

regressionwas used(basedon dataof Bruland and Franks Methods

[1983] and Sakamoto-Arnold et al. [1987]. This data set

includesall Hoeglundina measurementsobtainedup until Cd analyses of benthicforaminiferafollowedprocedures July, 1994. Comparisonof foraminiferalCd and estimated standardin our laboratoryfor many years [Boyleand bottom water Cd values (Figure 1) show that the Cd/Ca Keigwin, 1985/1986]. Recently,we havenotedthat a small ratio of Hoeglundina appearsto record bottom water Cd changein the procedure(reversingthe orderof treatments concentrations with an implied average partition to have the reductive cleaning before the oxidative coefficient [MorseandBender, 1990]Dp--l:

cleaning)eliminates anartifactdueto CdSprecipitates that

occurin a small percentageof samples[Rosenthal,1994]. (Cd/Ca)foram Almostall of the analysesin thispaperfollowedthe old procedure.The only exceptionsare three out of the four (Cd/Ca)water glacial maximumHoeglundinasamplesfrom RCl1-147 (wherethereis no differencebetweenspecimens cleaned Bottom water Cd concentrationsin the core-top data set by thetwotechniques) andtheearlydeglacial Hoeglundina tend to cluster around two central values with a gap insample from MD77-183 (which is consistentwith the

nearbyglacial maximumsamplescleanedby the older technique).Givenevidencefor lowerproductivity in the glacial Arabian Sea [Altabet et al., 1995], we do not

between; this clustering is caused by Atlantic/Pacific chemicalcontrastand the absenceof Hoeglundinafrom the deepest samples (therefore eliminating bottom water conditions with intermediate Cd concentrations). The

BOYLE ET AL ßCADMIUM

IN HOEGLUNDINA

IN NORTHERN

INDIAN OCEAN

883

Table 1. HoeglundinaCore-TopMeasurements Water Sample Depth, Depth, P estimate, Cd estimate, Core

Latitude

Lon•gitude

m

cm

!.tmol/k•

nmol/kg

Cd/Ca,

Partition

•tmol/mol Coefficient Dp

Bahamas Bank (westernNorth Atlantic) OC205 79BC

26ø13.63'N

77ø39.20'W

301

0-2

0.19

0.06

0.007

1.08

OC205 76BC

26ø13.93'N

77ø39.89'W

529

0-2

0.71

0.20

0.019

0.92

OC205 48BC OC205 52BC

26ø14.22'N 26ø14.21'N

77ø41.05'W 77ø41.54'W

580 668

0-2 0-2

0.88 1.26

0.23

0.012

0.51

0.29

0.017

0.57

OC205 69BC OC205 51BC

26ø13.69'N 26ø13.55'N

77ø41.49'W 77ø42.11'W

735 830

0-2 0-2

1.48 1.97

0.33

0.030

0.89

0.41

0.028

0.67

OC205 OC205 OC205 OC205 OC205 OC205

26ø13.49'N 26ø11.69'N 26ø09.89'N 26ø08.53'N 26ø09.98'N 26ø07.45'N

77ø42.71'W 77ø42.46'W 77ø43.32'W 77ø44.11'W 77ø44.47'W 77ø44.81'W

908 1043 1243 1312 1477 1585

0-2 0-2 0-2 0-2 0-2 0-2

1.54 1.37 1.27 1.26 1.23 1.22

0.34

0.027

0.78

0.31

0.024

0.75

0.29

0.019

0.63

0.29

0.025

0.84

0.29

0.021

0.71

0.28

0.022

0.75

0.24

0.050

2.02

0.28

0.040

1.39

0.22

0.036

1.59

0.31

0.037

1.16

0.32

0.039

1.18

0.31

0.046

1.44

0.25

0.028

1.09

0.28

0.047

1.63

0.25

0.025

0.97

0.32

0.037

1.12

0.32

0.031

0.94

0.25

0.022

0.85

0.33

0.040

1.18

0.33

0.047

1.38

0.34

0.029

0.83

0.34

0.033

0.94

2.72 2.69 2.74 2.90 2.70 2.69 2.51 2.25

0.83

0.084

0.98

0.82

0.071

0.84

0.84

0.070

0.81

0.90

0.076

0.82

0.82

0.063

0.75

0.82

0.058

0.69

0.74

0.081

1.06

0.64

0.080

1.21

72BC 54BC 57BC 60BC 59BC 61BC

Other Atlantic

V23-6 V23-81 AII94 22PC EN66 38GGC EN66 16GGC

42ø29'N 54ø02'N 55ø59'N 4ø55'N 5ø28'N

61ø48'W 16ø08'W 29ø08'W 20ø30'W 21ø08'W

2246 2393 2768 2931 3152

6-10 39-41

0-1

1.16 1.32 1.07 1.43 1.44

V22-197 CHN82 11 PC V22-26tw CHN82 4PC EN66 44GGC EN66 10GGC RC10-288 V22-196 RC13-205 EN66 21GGC EN66 36GGC

14ø10'N 42ø22'N 8ø43'N 41ø43'N 5ø16'N 6ø39'N 35ø32'N 13ø50'N 2ø17'S 4ø14'N 4ø19'N

18ø35'W 31 ø48'W 41ø15'W 32ø51'W 21ø43'W 21ø54'W 73ø25'W 18ø58'W 05ø11'W 20ø38'W 20ø13'W

3167 3209 3270 3427 3428 3527 3678 3728 3731 3995 4270

14-17 11-15 0-3 1-4 0-1 0-1 3-7 6-8 8-12 3-5 2-3

1.43 1.19 1.32 1.19 1.45 1.45 1.19 1.48 1.47 1.48 1.48

V21-30 V21-29 V 19-27 MW88 9GGC ERDC 92BX CH84-04 V19-30 RC15-61

00ø57'N 00ø57'N 00ø28'N 11ø38'N 02ø13'S 34ø46'N 03ø23'S 40ø37'S

89ø21'W 89ø21'W 82ø04'W 118ø38'E 57ø00'E 142ø13'E 82ø21'W 77ø12'W

617 712 1373 1465 1598 2630 3071 3771

28-29 3-4

Pacific 9-11 3-6 5-7 5-11 0.5-5 14-16 22-25 8-11 Indian

1004 1357

3-6 0+10

2.40 2.73

0.70

0.067

0.93

0.83

0.056

0.65

13ø05'N 08ø21'N

117ø57'E 93ø06'E 73ø11 'E 75ø12'E

1712 1878

0+10 0+10

2.81 2.59

0.86

0.068

0.76

0.78

0.069

0.86

14ø16'N 16ø31'N 19ø08'N

50ø19'E 09ø32'E 60ø25'E

1895 2100 2427

40-42 6-9 19-20

2.39 2.66 2.69

0.70

0.075

1.04

0.80

0.076

0.92

0.82

0.062

0.73

V28-345 MD77-176

17ø40'S 14ø30'N

MD76-128 MD76-125 MD76-135 V34-88 MD77-202

This tableincludesHoeglundina datacollectedthroughJuly, 1994

partition coefficient clearly doesnot show a strongdepth Cdwater(nmol/kg) = Cd/Ca (gmol/mol) / [0.72+0.08z (km)] dependenceas observedfor calcitic benthic foraminifera In the depthinterval studiedin the samplesconsideredhere (Figures1, 2, and 3). A slightdepthdependence cannotbe (1254-2442 m), thisrepresents a rangeof Dp of 0.82to excludedbasedon the presentdata set.A linear regression 0.91. At worst, this would representa systematicbias of

of Dp versus bottomdepthfor samples withDp