Experimental determination of cadmium uptake in shells of the planktonic foraminifera Orbulina universa and Globigerina bulloides: Implications for surface water paleoreconstructions

Geochimicaet CosmochimicaActa, Vol. 61, No. 19, pp. 4053-4065, 1997 Copyright© 1997 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97) 00206.8

Experimental determination of cadmium uptake in shells of the planktonic foraminifera Orbulina universa and Globigerina bulloides: Implications for surface water paleoreconstructions TRACY A. MASHIOTTA,1 DAVID W. LEA, 1 and HOWARD J. SPERO 2 ~Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, California 93106, USA ZDepartment of Geology, University of California, Davis, California 95616, USA (Received July 25, 1996; accepted in revised form May 28, 1997) A b s t r a c t - - L a b o r a t o r y culturing is a direct means of determining Cd uptake in shells of planktonic foraminifera. We employed a new stable isotope technique using both 11°Cd and mCd to assess uptake in the symbiont bearing species, Orbulina universa, and the nonsymbiont bearing species, Globigerina bulloides. In certain experiments with G. buUoides the three endmember isotope dilution method was combined with a recently described 48Ca labeling technique. Shells of Orbulina universa cultured under a 12 h high light: 12 h dark cycle are found to incorporate very little Cd. Foraminifera can be induced to take up slightly more Cd by growth under 24 h darkness or under a 12 h high light:12 h dark cycle with exposure to the photosynthesis inhibitor, DCMU. These results demonstrate that O. universa under-represents the Cd concentration of seawater in which the shell is precipitated. Additionally, the results suggest a previously unknown influence of symbiotic dinoflagellates on foraminiferal shell chemistry. There are two different mechanisms by which symbionts might play a role in influencing Cd uptake in O. universa: (1) algal sequestration and removal of Cd from the foraminiferal calcification microenvironment or (2) photosynthetic enhancement of calcification rate, leading to Cd exclusion. If these results apply generally to bioactive trace metal uptake by dinoflagellate-bearing planktonic foraminifera, they suggest that shells of species such as O. universa only record qualitative changes in surface water metal concentrations. The response for Cd uptake in nonsymbiont bearing G. bulloides (cultured under a 12 h high light:12 h dark cycle) appears linear within and slightly above the range of Cd concentrations found in the modem ocean, with an effective partition coefficient equal to 1.9 _+ 0.2 (95% C.I.). The Cd partition coefficient determined for G. bulloides falls within the range of Dcd previously found for fossil benthic foraminifera but is twenty times higher than that for O. universa (Dcd = 0.095 _ 0.021 (95% C.I.)) grown under identical culture conditions. Because G. bulloides appears to reliably record seawater Cd concentrations, it should be suitable for accurate paleoreconstructions of surface water Cd and PO43- concentrations. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

(Boyle, 1988; Shen and Sanford, 1990). For Cd this relationship can be expressed through the exchange reaction

Boyle and others have successfully used the trace metal chemistry of fossil benthic foraminifera to reconstruct changes in the circulation and chemical inventory of the deep ocean through the Quaternary (e.g., Boyle, 1988, 1990; Boyle and Keigwin, 1982, 1985/1986; Lea, 1993; Lea and Boyle, 1990a,b). These studies are based on modem covariance between trace metals and seawater parameters such as nutrients and alkalinity, parameters which are not directly preserved in marine sediments. By extension of the modem relationship to past oceans, the distribution of nutrients and alkalinity can be inferred from the trace metal record obtained from fossil foraminifera. In particular, Cd is known to covary with phosphate (PO43-) in the modem ocean and based on the Cd content of fossil foraminifera, both Cd and PO~- concentrations in paleoseawater can be inferred. Because the foundation of all trace metal studies is the assumption of compositional proportionality between shell and seawater, reconstructions of seawater Cd and PO 3- concentrations can only be as accurate as the calibration of the relationship between seawater Cd and foraminiferal Cd. Thermodynamic equilibrium dictates a simple compositional relationship between seawater and precipitated calcite

CaCO3 + Cd 2÷ ~ C d C O 3 + Ca 2+

(1)

where D =

[ CdCO3 ] [ Ca 2+ ] [ CaCO3 ] [Cd 2--]

\ ~ / ........

(2)

(3)

For an ideal solid solution between Cd and Ca the partition coefficient, D, would be equal to the ratio of the solubility products of CaCO3 to CdCO3. Because solid solution between calcite and otavite (CdCO3) is known to occur and both assume a trigonal structure, for an ideal inorganic solid solution of Cd in CaCO3 precipitated from seawater, Dcd would be equal to 7 (Boyle, 1988). The rapid precipitation of biogenic carbonates, dominated by biochemical and kinetic processes, shifts the trace metal composition away from that predicted by equilibrium thermodynamics (Boyle, 1988; Shen and Sanford, 1990). Despite the fact that biogenic carbonates are not in thermo4053

4054

T.A. Mashiotta, D. W. Lea, and H. J. Spero

dynamic equilibrium with seawater, incorporation of Cd into foraminiferal calcite probably occurs by the same solid solution mechanism that is observed in inorganic calcite. Cadmium and calcium are identical in charge and have similar ionic radii in the trigonal calcite structure (Cd = 0.95 A and Ca = 1.00 ,~,) (Shannon, 1976), so steric incompatibility does not limit the extent of metal incorporation. The inorganic precipitation experiments of Lorens ( 1981 ), however, demonstrate that Cd incorporation in calcite is a function of precipitation rate. Because decreased Cd uptake occurs at higher precipitation rates, it is possible that the rapid precipitation of biogenic carbonates could limit metal incorporation. Experimental determination of Dcd in biogenic carbonates is, therefore, necessary. The assumption of compositional proportionality between shell and seawater can be tested by comparing the metal content of foraminifera shells obtained from core-top sampies with the estimated metal concentration of the water column at or near the core site. Although there are certain limitations (Boyle, 1988), this method has been used to calibrate Cd (Hester and Boyle, 1982) and Ba (Lea and Boyle, 1989) in benthic foraminifera and forms the basis of numerous thermohaline circulation studies (Boyle, 1986; Boyle and Keigwin, 1982; Lea and Boyle, 1990a). The application of core-top calibration to planktonic foraminifera is more problematic. Measurements on core-top samples demonstrated that foraminiferal Cd/Ca ratios increase with increasing depth habitat, consistent with increasing Cd concentrations with depth in the water column (Boyle, 1981). Estimating the metal concentration of seawater in which the foraminifer secreted its shell is difficult because (1) bioactive metals like Cd are characterized by very low surface water concentrations which rapidly increase through the thermocline (Boyle, 1976) and (2) spinose planktonic foraminifera migrate vertically over the upper water column (Br, 1960; Berger, 1969; Boltovskoy, 1973; Deuser et al., 1981) where sharp metal concentration gradients occur. A second major obstacle in core-top calibrations is purification of shells recovered from sediments because residual phases (clays, organic matter, oxide coatings, and carbonate overgrowths) present on fossil planktonic shells have trace metal concentrations several orders of magnitude higher than the calcite lattice (Boyle, 1988). Culturing experiments with living foraminifera are a more precise means of determining the proportionality of Cd uptake in planktics; the Cd concentration of culture seawater is known and controlled over the duration of the experiments, and cultured foraminifera shells are essentially free of contaminating phases, which minimizes sample cleaning and preparation. Finally, environmental factors which could influence metal uptake, such as light level, temperature, and salinity, are systematically regulated. We have examined the proportionality of Cd uptake in two species of spinose planktonic foraminifera, the symbiont bearing, Orbulina universa, and the nonsymbiont bearing, Globigerina bulloides. O. universa exhibits a wide oceanic distribution, from tropical to subpolar regions, and the ubiquitous occurrence of this species makes it an obvious choice for paleoreconstructions of surface water chemistry. Although G. bulloides occurs over a narrower oceanic range

from transitional to subpolar regions, it is particularly useful for determining upwelling loci and chemistry in past oceans because of its association with upwelling. An additional difference between the two species is shell morphology; O. universa secretes a large spherical terminal chamber encompassing the small juvenile form, while G. bulloides grows a succession of small chambers. Among spinose planktonic foraminifera, the vast majority are known for their symbiotic association with algae (typically dinoflagellates). The importance of this symbiotic relationship has been demonstrated by a number of culturing studies where symbiotic dinoflagellates were found to influence foraminiferal calcification rate (B6 et al., 1982; Jcrgensen et al., 1985; Lea et al., 1995) and final shell size and mass (B6 et al., 1982; Spero and DeNiro, 1987) in both Orbulina universa and Globigerinoides sacculifer, as well as carbon isotopic composition in O. universa (Spero and DeNiro, 1987; Spero and Williams, 1988). Based on these observations it is possible that symbionts might influence other shell characteristics, such as trace metal chemistry. This is particularly likely for bioactive trace metals where sequestration by algae has been documented (Gekeler et al., 1988; Morel et al., 1994; Price and Morel, 1990). A previous study of Cd uptake in cultured planktonic foraminifera relied on the use of radioisotopes to assess the proportionality of metal incorporation (Delaney, 1989). Three different foraminiferal species with symbiotic dinoflagellates (Globigerinoides sacculifer, O. universa, and Globigerinoides ruber) were cultured in seawater with additions of ~°gCd. The partition coefficient for Cd uptake was estimated to be between 2 and 4; this range of Dcd for planktics encompasses the coexisting Dcd for benthics which was estimated to be 2.9 _+ 0.5 based on core-top calibration (Boyle, 1988). To better assess Cd uptake in cultured foraminifera, we developed a three endmember isotope dilution method using two stable isotopes of Cd, one as a field spike (H~Cd) and the other as a laboratory spike (H°Cd). Foraminiferal lUCd uptake is determined by plasma mass spectrometry and assessed through two discrete calculated ratios, m C d / C a and Cd/Ca. This method enables precise quantification of ~ C d uptake during laboratory culture and independent assessment of potential sample contamination by natural, environmental Cd. A portion of the culturing experiments with G. bulloides were conducted using a combination of the three endmember isotope dilution method presented here and a recently described 48Ca label technique (Lea et al., 1995 ). The combination of these two techniques greatly simplifies determination of U~Cd uptake in G. bulIoides by making chamber separation unnecessary; based on the measured ratio of 48Ca/44Ca in the sample, it is possible to determine what percentage of the total calcite analyzed was added under culture conditions (Lea et al., 1995) and apply this percentage to the U~Cd/Ca ratio. The use of stable cadmium isotopes, paired with inductively coupled plasma mass spectrometry (ICP-MS) offers additional advantages over the radioisotope method: ( 1 ) the total metal content of culture solutions can be controlled and remains constant during the experiments; (2) ~UCd and U°Cd can be measured precisely in single O. universa shells; (3)

Uptake of Cd by planktonic forminifera H ~Cd/Ca and C d / C a ratios can be determined directly, based on simultaneous m e a s u r e m e n t of Cd and Ca; and ( 4 ) the handling of radioactive chemicals is avoided. Preliminary culturing experiments examining Cd uptake in O. universa using stable isotopes (D. W. Lea and H. J. Spero, unpubl, data, 1991) revealed that very little Cd was incorporated in cultured shells. This result was quite different from those of culturing experiments investigating B a uptake in the same species, in w h i c h foraminifera incorporated B a with a partition coefficient consistent with core-top studies (Lea and Spero, 1992, 1994). In addition, B a uptake in foraminifera g r o w n under different light levels was not significantly different, suggesting that s y m b i o n t photosynthesis does not influence Ba uptake ( L e a and Spero, 1992). W e considered the possibility that the unusual b e h a v i o r of Cd could be explained by a cation pool, but an investigation of the pool size suggests it is too small to play a significant role ( L e a et al., 1995; see Elderfield et al., 1996 for a detailed discussion of cation pools). G i v e n the preliminary Cd uptake restdts, a large matrix of experiments was designed in which environmental parameters thought to influence symbiont activit:y could be modified.

2. EXPERIMENTAL METHODS 2.1. Foraminifera Collection and Culturing Tae experimental procedure is similar to basic protocols established for Ba uptake experiments as described in Lea and Spero (1992, 1994). Presphere Orbulina universa and small Globigerina bulloides were hand collected by scuba divers between June and August, 1992-1994, from surface waters of the San Pedro Basin approximately 2 km NNE of the Wrigley Institute for Environmental Studies, Santa Catalina Island, California, USA. Surface seawater for culturing was collected at the dive site in acid-leached polyethylene bottles using standard precautions against contamination (Bruland, 1983). In the laboratory, seawater was filtered using 0.45 #m polycarbonate membrane filters and an acid-leached polysufone filter holder. Sample handling was done under trace metal clean conditions in a class 100 laminar flow bench whenever possible. After collection, individual foraminifera were checked under an inverted light microscope for identification of species and general condition and then transferred to acid-leached 120 mL Pyrex bottles (O. universa) or 29 mL borosilicate scintillation vials (G. bulloides) containing experimental seawater (see seawater preparation). Extreme care was taken to avoid contamination of culture water during transfer of specimens; transfers were done using an acid-leached glass pipette. All culture bottles were maintained at a constant temperature in a 22 4: 0.1oc water bath, the approximate maximum summer SST at the collection site. Orbulina universa were grown under the following conditions: ( 1 ) a 12 h high light: 12 h dark cycle where light levels were adjusted to above Pmax( >386 #Einsteins m -2 s 1) (Spero and Parker, 1985 ) ; (2) 24 h darkness; and (3) a 12 h high light:12 h dark cycle with 72 h exposure to a 10 5 M seawater solution of the photosynthesis inhibitor DCMU [3-(3,4 dichlorophenyl)l,l-dimethylurea]. All Globigerina bulloides were grown under a 12 h high light: 12 h dark cycle. During the seven to ten day culture period O. universa secretes and calcifies a spherical chamber (Spero, 1988), whereas G. bulloides secretes between two and four new chambers. Foraminifera were fed a one-day-old Artemia sp. nauplius (brine shrimp) every other day, and either checked under an inverted light microscope for size and general condition or visually inspected through the culture bottle, Upon termination of the experiment (gametogenesis) empty shells were archived for laboratory analysis. In addition, a sample of the culture solution was acidified and archived to check for any change in t~iCd concentration over the course of the experiment. Because

4055

the amount of calcite precipitated by foraminifera does not require more than 0.1% of the initial Ca present in the culture seawater, Ca concentrations were constant over the duration of the experiments.

2.2. Seawater Preparation Culture solutions were prepared by adding known quantities of aqueous mCd spike (mCdClz, 95.06% enrichment, pH = 2) to ambient trace element (TE) filtered seawater. The ambient Cd concentration at the culture site is about 0.05 nM. Additions of stable 1~~Cd were calculated to yield four different 1~~Cd concentrations: ambient, 1 nM, 2 nM, and 5 nM. In all cases addition of the spike has a negligible effect on both CI content and pH of the seawater. For experiments where O. universa were exposed to DCMU, stock solutions of 10 4 M DCMU seawater were prepared as outlined in B6 et al. (1982), using TE filtered seawater and an acid-leached reflux condenser. A 10 -5 M DCMU solution was prepared from the stock solution and then spiked with ~lCd. Presphere O. universa treated with DCMU were exposed for 72 h to a 10 -5 M DCMU seawater solution spiked with mCd and then transferred to mCd spiked seawater for the remainder of the culture period. The concentration and length of exposure to the DCMU solution have been previously demonstrated to be optimal for symbiont elimination, without negative effects on the foraminifer (B6 et al., 1982). 4SCa labeled seawater used in experiments with G. bulloides was prepared from an enriched calcium isotope spike (see Lea et al. ( 1995 ) for discussion of spike preparation). A small amount of 48Ca spike was added to TE filtered seawater to double the 48Ca/44Ca ratio (natural ratio = 0.09) of the culture seawater. 48Ca labeled seawater was then spiked with IHCd. Because seawater is prepared and equilibrated to the culture temperature several hours in advance of foraminiferal transfer, cadmium isotopic equilibration is assumed to have occurred. Additionally, isotopic equilibrium between the foraminifer and the prepared seawater is assumed because foraminifera typically spend several hours (G. bulloides) or several days (O. universa) in the culture seawater before secreting additional chambers or the final sphere, respectively.

2.3. Sample Preparation Whole foraminifera shells (both species) were transferred to Pyrex well plates (one foraminifer per well) and remnant cytoplasm was removed by oxidation, using either 1% H202 buffered in 0.1 N NaOH (1992-1993), or 5% NaOC1 (1993-1994), and rinsing several times with ultrapure water (Gaffey and Bronnimann (1991) demonstrates that NaOC1 is a more effective oxidant). Individual Orbulina universa shells were then weighed on a microbalance; shell masses for individuals grown under a 12 h high light:12 h dark cycle were between 20 and 45 #g, while foraminifera grown under 24 h darkness or under a 12 h high light:12 h dark cycle with DCMU exposure typically weighed between 10 and 30 izg (Appendix B). Shell masses for individual cultured Globigerina bulloides shells were generally between 7 and 10/zg. Shell calcite secreted in the laboratory under controlled conditions was then separated from calcite precipitated prior to collection and laboratory culture. Shells of O. universa were cracked open using a disposable scalpel, and the juvenile chamber (if present) was removed with a small brush. Sphere fragments were then transferred to well plates again and oxidized a second time. The fragments were then transferred to acid-leached 0.5 mL polyethylene centrifuge vials. Individual cultured chambers were amputated from G. bulloides using a disposable scalpel, after cytoplasm had been removed by oxidation. Chambers were pooled and transferred to acid leached 0.5 mL polyethylene centrifuge vials where they were oxidized a second time. Fragments were then transferred to new acid-leached centrifuge vials. For the experiments where G. bulloides were grown in 48Ca labeled seawater, whole foraminifera were pooled and oxidized. The cleaned shells were then weak acid leached (0.001 N HNO3) to remove any Cd adsorbed onto the shell during oxidation. Finally shell fragments were simultaneously dissolved and spiked in 500 #L of spike containing known amounts of 45Sc and ml°Cd in 0.1 N HNO3 (Fisher optima). 45Sc was added to the samples as an internal

4056

T.A. Mashiotta, D. W. Lea, and H. J. Spero

standard for Ca with reference to 44Cain order to quantify Ca. "°Cd was added to quantify mCd uptake, using isotope dilution. 2.4. Sample Analysis Samples were injected as an isolated 0.5 mL plug into a Fisons/ VG PlasmaQuad 2+ Turbo inductively coupled plasma mass spectrometer (ICP-MS) coupled to a Cetac ultrasonic nebulizer. Data were collected on six peaks (44Ca, 45Sc, 11°Cd, mCd, 114Cd, and ~2°Sn)in peak jumping mode. Dwell times were set to 10,240 #sec for 44Ca, 4SSc, ~°Sn, and 20,480 #sec for "°Cd, mCd, and 114Cd. For G. bulloides grown in 4SCalabeled seawater, data was collected on an additional peak, 48Ca, and dwell times were adjusted to 44,960 #sec for ~mCd, mCd, "4Cd, and 48Ca. Total scan times of 25 sec were achieved using a peristaltic pump rate of 1 mL/min. This method yielded count rates of between 200 and 5,000 counts per second (CPS) for mCd and between 6,000 and 11,000 CPS for "°Cd, compared to backgrounds of 10-40 CPS. Accuracy of measurements was controlled by repeatedly determining the "°Cd/mCd ratio on a mixture of spike and standard and applying a small mass bias correction (about 2.5%) based on the measured ratio to all samples (Lea and Martin, 1996). Inter-run precision was determined by analysis of the mCd/Ca ratio in a matched consistency standard. Thirty-eight analyses of the consistency standard CSlh matched in concentration to an average O. universa sample (0.168/zmol Ca; 15.7 fmol mCd) have a standard deviation of 3.4% for the mCd/Ca ratio. The three endmember isotope dilution method is unique because it enables complete and independent determination of two different ratios in the samples, mCd/Ca and Cd/Ca. These ratios are determined by a three endmember isotope dilution calculation derived from three unique isotopic signatures of Cd (110, 111, and 114) in the laboratory spike (110), field spike ( 111 ), and naturally occurring Cd. The ratios take into account the natural "°Cd/mCd and u4Cd/ mCd ratios, the known l~°Cd/mCd and H4Cd/mCd ratios in both cadmium isotope spikes, and the measured ratios in the spiked foraminiferal samples (Appendix A). The mCd/Ca ratio is determined to assess the extent of mCd uptake; this ratio accounts only for added mCd which has the isotopic signature of the enriched "~Cd spike. Although ambient seawater has a small amount of mCd (12.80% of Cdrot~), the calculated mCd/Ca ratio does not include naturally occurring reCd. Therefore, foraminifera cultured in ambient seawater should have a mCd/Ca ratio equal to zero because no mCd spike was added. The Cd/Ca ratio is determined to evaluate potential sample contamination by natural Cd; this ratio accounts only for Cd having the isotopic signature of naturally occurring Cd. Because these two calculated ratios are completely independent of each other, low-level contamination of samples by natural Cd cannot influence the mCd/Ca ratio. One of the largest potential errors in metal culturing experiments is a change in metal concentrationover the course of the experiment, either due to loss of Cd by adsorption on culture bottle walls or addition of Cd due to contamination. The amount of mCd incorporated in foraminifera shells is less than 0.5% of the total Cd present in the culture solutions. Analysis of initial and final culture solutions indicates that the mCd concentration was constant over the course of the experiments. 3. RESULTS 3.1. Orbulina universa Comparison of shell mCd/Ca with culture seawater mCd/ Ca for Orbulina universa exposed to different culture conditions reveals that very little Cd is incorporated in this species (Fig. 1 ). Data are only from individuals that underwent gametogenesis within a laboratory culture period of 15 days (Appendix B). Each point represents the average "~Cd/Ca of five to eleven individually analyzed spheres. All regressions have been constrained to pass through the origin because mCd/Ca of ambient foraminifera should be zero; in

each case forcing the regression through zero does not have a significant influence on the slope. The slope of each line represents the relative partitioning of Cd into shell calcite; partition coefficients less than one generally suggest discrimination against the metal during calcification. Among the three different experimental groups, foraminifera grown under a 12 h high light:12 h dark cycle incorporated the least Cd. This very low Cd uptake is consistent with preliminary culturing results (D. W. Lea and H.J. Spero, unpubl, data, 1991 ). A linear regression through the points yields a slope of 0.095 _ 0.021 (95% confidence interval). Slightly increased Cd uptake is found in individuals grown under conditions of 24 h darkness. A linear regression of these points yields a slope of 0.26 _ 0.01 (95% C.I.). Foraminifera grown under a 12 h high light:12 h dark cycle with 72 h exposure to DCMU demonstrate a slight increase in " C d uptake over individuals grown in 24 h darkness; a linear regression through these points yields a slope of 0.37 ___0.01 (95% C.I.). The determination of Cd in individual O. universa shells allows assessment of the potential importance of intraspecific variability in Cd uptake. Differences in measured mCd/Ca ratios within a single experimental group are larger than the analytical error involved in measuring the mCd/Ca ratio. This suggests that heterogeneity in Cd uptake occurs within the foraminiferal population. 3.2. Globigerina buUoides mCd]Ca measured in G. bulloides compared with mCd/ Ca of culture seawater reveals that this species takes up significantly more Cd than O. universa (Fig. 2). Each data point represents several foraminifera that were pooled together for analysis, either a pool of chambers amputated from four to nine foraminifera (UlCd tracer), or a pool of four to nine whole foraminifera shells ( m C d and 48Ca tracers; Appendix B). The response for Cd incorporation in the nonsymbiont bearing G. bulloides appears linear within and slightly above the range of Cd concentrations found in the modem ocean. The regression is forced through the origin to correct for slightly negative mCd/Ca ratios in foraminifera grown in ambient seawater. The Cd partition coefficient for G. bulloides is equal to 1.9 _ 0.2 (95% C.I.). Comparison of Dcd for cultured G. bulloides with other planktonic species is difficult in light of the limited data available. Based on a site for which both water column Cd and core-top foraminifera data are available, Dcd for Neogloboquadrina pachyderma, a nonspinose foraminifer common from temperate to polar regions, is estimated to be about 2 (Keigwin and Boyle, 1989; Saager, 1994). Dcd for cultured G. bulloides also falls within the range of partition coefficients determined for Cd in core-top fossil benthic foraminifera. Boyle (1992) estimated that Dcd is about 1.3 in benthics from depths shallower than 1,150 m and rises to 2.9 in benthics from deeper than 3,000 m, suggesting that Dcd in benthics is a function of depth. 4. DISCUSSION

Comparison of Cd uptake in O. universa and G. bulloides reveals a distinct difference in Cd incorporation between the

Uptake of Cd by planktonic forminifera

4057

12 h Light: 12 h Dark + DCMU - - ~ - - 24 h Darkness - - o - - 12 h Light: 12 h Dark 0.25

Orbulina universa

o

M o

0.20 Dcd = 0 . 3 7 _+ 0 . 0 1

::L

R 2 = 0.99 t.

0.15 D c a = 0 . 2 6 __. 0 . 0 1

. m

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o.os

•I" .-'""

. . . - """"". .

°

.

-

.

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.

0.99

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R 2 = 0.98 wl¢

0.00 '

0.0

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0.1

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0.3 seawater

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(~tmol/mol)

Fig. 1. H~Cd/Ca of cultured Orbulina universa vs. H'Cd/Ca of culture seawater for three different experimental groups: 12 h high light:12 h dark, 24 h darkness, and 12 h high light:12 h dark with 72 h DCMU (photosynthesis inhibitor) exposure. Each point represents the average of five to eleven individually analyzed single spheres (see Appendix B ). The data from each group is fitted with a least squares regression forced through the origin (see text), where the slope of the line equals the Cd partition coefficient(Dcd). Standard error bars are shown for each experimental group. Foraminifera grown under a 12 h high light:12 h dark cycle incorporatedthe least '~Cd, while slightly increased Cd uptake occurs when foraminifera are grown under 24 h darkness or grown under a 12 h high light: 12 h dark cycle with exposure to DCMU. The data suggest that Cd uptake in O. universa is influencedby its symbiotic association with the din.flagellate, Gymnodinium beii.

two species (Fig. 3). It is clear that O. universa grown under a 12 h high light:12 h dark cycle incorporated less Cd than foraminifera grown under 24 h darkness or under a 12 h high light:12 h dark cycle with DCMU exposure (see Fig. 1 ). This suggests that Cd uptake in O. universa is influenced by its symbiotic association with the din.flagellate Gymnodinium beii. If symbionts are sequestering and removing Cd from the calcifying microenvironment of the foraminifer, this might explain the very low Cd partition coefficients which are generally considered to represent Cd discrimination during calcification. If the symbiont sequestration mechanism is photosynthesis, Dcd for O. universa grown under 24 h darkness (no symbiont photosynthesis) should be similar to Dc~ for G. bull.ides• Because Dcd is significantly less than that determined for G. bull.ides, there must be another metabolic pathway for Cd uptake in the symbionts which is independent of photosynthesis. The O. universa DCMU results, which show a slight increase in Dc~ compared with foraminifera grown in darkness, further confirm that the mechanism for symbiont Cd uptake is not symbiont photosynthesis. Because DCMU inhibits photosystem-II in the symbionts, the din.flagellates are unable to photosynthesize but are still present in the foraminif-

eral cytoplasm. After about 72 h of DCMU exposure, the symbionts have been eliminated and can no longer influence foraminiferal Cd uptake. The fact that Cd uptake in O. universa exposed to DCMU is higher than Dcd for the same species grown in 24 h darkness suggests that when symbionts are present but not photosynthesizing, they are still able to remove Cd from the foraminiferal microenvironment; therefore, once the symbionts are eliminated more Cd is incorporated in the foraminiferal shell• Additionally, because Dcd for O. universa treated with DCMU is significantly lower than Dcd for G. bulloides, there could be inherent, physiological differences between symbiont bearing and nonsymbiont bearing foraminifera that influence partitioning of Cd into shell calcite. It is interesting to note that among O. universa exposed to DCMU, there is no clear difference in Cd uptake between those foraminifera that grew a portion of their sphere in the DCMU solution and those that grew their sphere entirely out of the solution (see Appendix B). Lower ~l~Cd uptake might be expected in foraminifera that secreted their shells in DCMU while symbionts were still present, based on the hypothesized role that symbionts play in Cd uptake. It is possible that interruption of the photosynthetic pathway in

4058

T.A. Mashiotta, D. W. Lea, and H. J. Spero

•

tllCd

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] t ICd and ~ C a

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Dcd = 1.9 ~" 0.2 R 2 = 0.96

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0.$

(l~mol/moi)

Fig. 2. mCd/Ca of cultured Globigerina bulloides vs. H1Cd/Ca of culture seawater. Each point represents a pool of four to nine foraminifera, either chambers amputated from foraminifera cultured in seawater with only the ~HCd tracer (diamonds) or whole foraminifera cultured in seawater with both H~Cd and 48Ca tracers (circles) (Appendix B and text). The data are fitted with a least squares regression forced through the origin (see text). HICd uptake in G. bulloides appears linear within and slightly above the range of Cd in the modem ocean, with a partition coefficient equal to 1.9 ___0.2 (95% C.I.). the dinoflagellate, Gymnodinium beii, results in disturbance of other metabolic functions which are related to Cd uptake. The culturing results presented here are unique because they suggest a previously unknown influence of symbiotic dinoflagellates on trace metal chemistry of Orbulina universa; this influence was not observed in previous Cd uptake experiments (Delaney, 1989) or Ba uptake experiments with O. universa (Lea and Spero, 1992). The results also are consistent with previous culturing results demonstrating that symbiotic dinoflagellates influence many shell characteristics of O. universa by creating and maintaining a chemically distinct microenvironment within the foraminiferal spines (B6 et al., 1982; Jergensen et al., 1985; Lea et al., 1995; Spero and DeNiro, 1987; Spero and Williams, 1988). Our results appear consistent with the creation of a chemically distinct microenvironment around O. universa, which influences Cd uptake. We hypothesize two different mechanisms: ( 1 ) direct sequestration and removal of Cd from the foraminiferal microenvironment, leading to Cd depletion in the shell or (2) symbiont enhancement of calcification rate through photosynthesis (B6 et al., 1982; Jergensen et al., 1985; Lea et al., 1995), resulting in limited Cd incorporation in shell calcite.

dently determined; however, Cd uptake has been documented in a number of algal species and occurs by complexation with phytochelatins (Ahner et al., 1995; Gekeler et al., 1988; Grill et al., 1985; Karez et al., 1990; Price and Morel, 1990). Based on the known sequestration of metals by algae, a net change in 111Cd concentration in the foraminiferal microenvironment resulting from the balance between symbiont removal and molecular diffusion of 111Cd can be estimated (Fig. 4). A Cd mass balance for foraminifera grown under high light levels is based on a one-dimensional, steadystate spherical diffusion model, with a constant source term and conservation of species (Crank, 1956). The equation in simplified form is C-

K 6D

(R ~ - r 2) + CR

(4)

where

4.1. Mechanism 1: Symbiont Sequestration

C= K= D = R= r = CR =

H1Cd concentration in microenvironment symbiont HICd consumption rate constant diffusion coefficient microenvironment radius distance from center of microenvironment mCd concentration at r = R

Cadmium uptake in isolated cultures of the symbiotic dinoflagellate Gymnodinium beii has not yet been indepen-

The rate of Cd removal used in the model is based on the

Uptake of Cd by planktonic forminifera

4059

. I

0 o

1°0"

G. b u l l o i d e s

Dcd = 1.9 + 0.2

0.8-

em

0.6-

t.., 0.4-

O. universa

0.2-

(12 h Light: 12 h Dark) • Dcd = 0.095 + 0.021

.I "

0

T [' . . '. . ' .

0.0

' ~I

. ' . '. . ' . '.

0.1

I

0.2

'

'

'

'

I

0.3

'

'

'

'

I

0.4

'

'

'

'

I

0.5

11 I C d / C a c u l t u r e s e a w a t e r ( g m o l / m o l ) Fig. 3. n :Cd/Ca of cultured Orbulina universa (symbiont bearing) and Globigerina bulloides (nonsymbiont bearing) vs. n :Cd/Ca of culture seawater. Both species were grown under a 12 h high light:12 h dark cycle. Standard error bars are shown for each experimental group. When data from the two species is compared, it becomes clear that the mechanisms controlling Cd uptake in each species are very different; Dcd for G. bulloides is twenty times higher than Dcd for O. universa grown under identical conditions. O. universa under-represents the mCd concentration of the seawater in which the shell is precipitated.

rate of Cd uptake determined for the diatom, Thalassiosira weissflogii, and is a linear function of the Cd concentration in the culture medium (Lee et al., 1995). Although there are important differences between diatoms and dinofiagellates and between free-living organisms and those living symbiotically, diatom Cd uptake rates are used as a starting point in the mass balance. The Cd uptake rate in T. weissflogii is approximately 0.08 x 10 -18 mol Cd cell -~ min i at a Cd concentration equal to 1 nM, and 0.18 X 10 -j8 mol Cd cell -1 rain -j at a Cd concentration equal to 5 nM (Lee et al., 1995). Cadmium uptake rates are converted to symbiont ren~oval rates by multiplying by the average number of symbiolic dinoflagellates (Spero and Parker, 1985) in association with an adult foraminifer (approximately 5,000). Cadmium removal rates, equal to 400 x 10 ,8 mol Cd min -1 and 900 x 10 ~8mol Cd rain -t, respectively, are then normalized to the volume occupied by a foraminifer (calculated as a sphere with a radius of 2 mm; see Fig. 4), yielding removal rates equal to approximately 200 x 10 -~5 mol Cd L -~ sec -~ and 500 x 10 -~5 mol Cd L ~ sec-l. Removal of Cd from the microenvironment is countered by inward diffusion of Cd ions. (in seawater at 22°C) at a rate of about 6.7 x 10 -t° m 2 sec -~ (Li and Gregory, 1974). Model calculations suggest that in the 1 nM experiments ~ C d is depleted by about 20% adjacent to the shell surface; in the 5 nM experiments, by about 10%. These depletions are insufficient to explain experimental results for both the 1 nM and 5 nM experimental groups; this is true regardless

of which Cd partition coefficient is used for O. universa with out symbionts (a partition coefficient closer to DCO for O. universa treated with DCMU, 0.37, or to Dcd for G. bulloides, 1.9). Because the model is an oversimplification of the foraminiferal microenvironment and the complex interactions that occur within, the model results do not rule out the role of symbionts in Cd sequestration. There can be considerable individual variability among O. universa in both sphere size and symbiont density (Spero and Parker, 1985 ), two factors which are critical in the diffusion model. In addition, mechanisms of Cd transport within the foraminiferal microenvironment could be considerably more complex than simple diffusive processes. Furthermore, the mechanisms and processes controlling Cd uptake in the symbionts, Gymnodinium bell, could be quite different from those controlling Cd uptake in other algal species, as suggested by a recent study (Ahner et al., 1995). Despite these uncertainties, the culturing results clearly suggest that symbionts play a role in Cd uptake in Orbulina universa, and further elucidation of Cd sequestration by the symbiotic dinoflagellates is necessary.

4.2. Mechanism 2: Calcification Rate Lorens (1981) observed that for inorganic calcite, DCd is an inverse function of precipitation rate. By analogy, precipitation rate could influence the degree of Cd incorporation in biogenic carbonates. Lea et al. (1995) determined that the average calcification rate of cultured O. universa is 7.2 #g/ day when foraminifera are grown under a 12 h high light: 12

4060

T.A. Mashiotta, D. W. Lea, and H. J. Spero partition coefficients of 10 and 30 are associated with the Lorens (1981) fast and slow inorganic precipitation rates, respectively, while the biogenic partition coefficients are 0.095 (fast precipitation--high light O. universa) and 1.9 ( slow precipitation-- G. bulloides). 4.3. Resolving C a d m i u m U p t a k e in Cultured Planktonic Foraminifera

\

/

tltCd remove by symbiotic algae

/ 1liEd2+ diffusion

Fig. 4. The foraminiferal microenvironmentof Orbulina universa as defined in the spherical diffusion model is a sphere having a radius of approximately 2 mm, the distance from the center of the foraminiferal shell to the end of the spines. The diffusion model is used to determine a Cd mass balance in the foraminiferal microenvironment, which results from net effects of inward diffusion of lJ~Cd2+ ions and H~Cdremoval by symbiotic dinoflagellates.

h dark cycle at 22°C. Although calcification rates for cultured G. bulloides have not been determined experimentally, a rate between 1 and 1.4 #g/day is estimated based on average shell mass and length of time in culture. Comparison of biogenic and inorganic calcite precipitation rates necessitates normalization of rates to shell surface area. Shell surface area is calculated as a smooth, solid sphere for O. universa (see Carpenter and Lohmann, 1992 for discussion); this yields an average surface normalized calcification rate equal to about 3,000/zmol m -2 h -~ (Lea et al., 1995). The estimated surface area of G. bulloides, based on the diameter of individual cultured chambers, is approximately 24 × 10 -8 m 2. This yields surface normalized calcification rates between about 1,700 #mol m -2 h -~ and 2,400/zmol m -2 h-~; lower than the rate determined for O. universa grown under identical culture conditions (Lea et al., 1995). Calcification rates for both O. universa and G. bulloides fall within inorganic precipitation rates used by Lorens (1981), ranging from 75/zmol m -2 h -1 to 7,500 #tool m 2 h -~ (Fig. 5). The difference in Dcd between G. bulloides (which calcifies more slowly) and O. universa might appear to be consistent with trends observed for Dcd in inorganic calcite. Despite what appear to be comparable precipitation rates and trends, Dcd are not at all similar between the Lorens (1981) experiments and cultured foraminifera. Cadmium

Further work is needed before the mechanisms controlling Cd uptake in Orbulina universa and Globigerina bulloides can be completely understood. The culturing results suggest that the differences are related to the presence of symbiotic dinoflagellates in O. universa and could be more than just a function of the role that symbionts play in sequestering Cd or enhancing calcification rate. Although their nature remains elusive, physiological differences between the two species probably exist, aside from the presence or absence of symbiotic dinoflagellates. Further explanation of the culturing resuits awaits direct determination of Cd sequestration in isolated cultures of Gymnodinium bell. The results of the present study differ from those of previous culturing experiments conducted to determine Dcd in symbiont bearing planktonic foraminifera, where the effective partition coefficient was found to be between 2 and 4 (Delaney, 1989). Although we cannot say with certainty why the results differ, the use of radiotracers in the earlier study might be a factor. Radiotracers are subject to certain inherent limitations: ( 1 ) the total metal content of the culture solution is not controlled; (2) the radiotracer additions must be large enough to yield detectable activities in cultured

Inorganiccalcite (Lorens, 1981) 1 Culturedforaminifera I Calculatedthermodynamic value

o • 35 30

20 Dcd 15 10

{7. bulloides S

O. universa o

0

'

'

'

~

' Q '

7,000

I

'

4000

'

'

I

'

6000

'

'

i

'

SO00

'

'

L

'

10000

Precipitation rate (lzmol m" 2 h" l)

Fig. 5. Comparison of the Lorens (1981) inorganic precipitation data and cultured foraminifera data shows that the difference in Dcd between G. bulloides (which calcifies more slowly) and O. universa appears consistent with the observed trend for inorganic calcite. Despite what appear to be comparable precipitation rates, however, Dcd in cultured foraminifera is not at all similar to Dca determined by Lorens (1981). The calculated thermodynamic Dcd is placed at a precipitation rate equal to zero for equilibrium.

Uptake of Cd by planktonic forminifera shells and, therefore, might not be representative of the natural growth environment; and, (3) the simultaneous quantification of Ca and Cd is not possible.

4.4. Paleoeeanographic Implications Because Globigerina bulloides appears to reliably record seawater Cd concentrations with a partition coefficient equal to 1.9, this species is more suitable than Orbulina universa for quantitative reconstructions of surface water Cd and pO34- concentrations. The limited distribution of G. bulloMes, however, necessitates the use of other species for surface water reconstructions in tropical and subtropical regions. Although the majority of spinose planktonic foraminifera are symbiont bearing, a few foraminifera are nonsymbiont bearing, and others bear nondinoflagellate symbionts. If oui culturing results apply generally to other planktics, they suggest that nonsymbiont bearing or nondinoflagellate bearing species might be suitable for Cd and PO 3- paleoreconstructions. While the ubiquitous distribution of O. universa makes it a more favorable species for paleoreconstructions, our results suggest that shells of this foraminifer under-represent seawater Cd concentrations. This result might extend to other planktonic species including Globigerinoides sacculifer, Globigerinoides ruber, and Globigerinoides conglobatus, which also have dinoflagellate symbionts. Despite this limitation, dinoflagellate-bearing foraminifera might be useful for qualitative assessment of surface water Cd and PO43concentrations, because Cd uptake appears linear. Once the influence of symbionts on Cd uptake can be quantified through culturing experiments with isolated G. beii, it might be possible to use dinoflagellate-bearing species for quantitative reconstructions. Additionally, our results might extend to foraminiferal uptake of other biologically active metals, such as Zn, Cu, Ni, and Co. If foraminiferal uptake of other bioactive metals is similar to Cd uptake, it is possible that shells of dinofagellate-bearing planktonic foraminifera might not accurately reflect seawater concentrations of other biologically active trace metals, elements which are known to be sequestered by algae. This implication is particularly important for Zn, which might be a potentially useful paleoceanographic tracer (Boyle, 1981 ). The possible role of symbiotic dinoflagellates in foraminiferal uptake of Zn and Mn is currently being examined through culturing experiments with both O. univer,~a and G. bulloides. 5. CONCLUSIONS We have assessed the proportionality of Cd uptake in the symbiont bearing planktonic foraminifer, Orbulina universa, and the nonsymbiont beating planktonic foraminifer, Globigerina bulloides, using a new three endmember isotope dilution method. Cadmium uptake in O. universa is infuenced by the presence of the symbiotic dinoflagellate, Gymnodinium beii. Foraminifera grown under a 12 h high light: 12 h dark cycle recorded lower ll~Cd/Ca ratios than individuals grown under 24 h darkness and grown under a 12 h high light:12 h dark cycle with 72 h exposure to the photosynthe-

4061

sis inhibitor, DCMU. Symbiotic dinoflagellates could influence foraminiferal incorporation of Cd by sequestering Cd from the calcifying microenvironment of O. universa or by photosynthetically enhancing calcification rate leading to Cd exclusion. If the mechanism for Cd depletion in O. universa grown under high light levels is algal metal sequestration, it appears that metal sequestration by the symbionts is independent of photosynthesis. Combining the three endmember isotope dilution method with a recently described 48Ca labeling technique (Lea et al., 1995), we find that Cd is incorporated in shells of G. bulloides grown under a 12 h high light:12 h dark cycle. The effective partition coefficient is equal to 1.9 ___ 0.2 (95% C.I.). This Cd partition coefficient is twenty times higher than Dc~ determined for O. universa grown under identical culture conditions. Additionally, Dcd for G. bulloides falls within the range of Cd partition coefficients determined for benthic foraminifera, where Dcd is between 1.3 and 2.9 (Boyle, 1992). The ability to accurately reconstruct the chemistry and circulation of the surface ocean in the past is dependent upon the reliability of foraminiferal trace metal records that are used as proxies. Our results suggest that shells of dinoflagellate-bearing planktonic foraminifera under-represent the metal content of seawater in which they calcified and are only suitable for qualitative paleoreconstructions. However, the results also demonstrate that G. bulloides, which lacks symbionts, faithfully records the Cd content of the seawater in which it calcifies. Acknowledgments We thank the Wrigley Institute for Environmental Studies for providing laboratory facilities and gratefully acknowledge the field assistance of Bryan Bemis, Jelle Bijma, Luis Cafiedo, Dave Chan, Chris Hamilton, Evonne Mochon, and Maria Uhle. Discussions with Alain Trial, Pamela Martin, and Frank Spera were helpful in developing the diffusion model. We also thank Ed Boyle and two anonymous reviewers for their insightful comments about the manuscript. Finally, we are indebted to our development engineer, Howard Berg, for whom no problem is unsolvable. This research was supported by NSF OCE-9415991 (DWL) and NSF OCE9416595 (HJS). This is WIES contribution #193. Editorial handling: R. H. Byrne

REFERENCES Ahner B. A., Kong S., and Morel F. M. M. (1995) Phytochelatin production in marine algae. 1. An interspecies comparison. Limnol. Oceanogr. 40, 649-657. BE A. W. H. (1960) Ecology of Recent planktonic foraminifera: Part 2--Bathymetric and seasonal distributions in the Sargasso Sea off Bermuda. Micropaleo. 6, 373-392. B6 A. W. H., Spero H.J., and Anderson O.R. (1982) Effects of symbiont elimination and reinfection on the life processes of the planktonic foraminifera Globigerinoides sacculifer. Mar. Biol. 70, 73-86. Berger W. H. (1969) Ecologic patterns of living planktonic foraminifera. Deep-Sea Res. 16, 1-24. Boltovskoy E. (1973) Daily vertical migration and absolute abundance of living planktonic foraminifera. J. Foram Res. 3, 89-94. Boyle E. A. (1976) On the marine geochemistry of cadmium. Nature 263, 42-44. Boyle E. A. ( 1981 ) Cadmium, zinc, copper, and barium in foraminifera tests. Earth Planet. Sci. Lett. 53, 11-35. Boyle E. A. (1986) Paired carbon isotope and cadmium data from benthic foraminifera: Implications for changes in oceanic phos-

4062

T.A. Mashiotta, D. W. Lea, and H. J. Spero

phorous, oceanic circulation, and atmospheric carbon dioxide. Geochim. Cosmochim. Acta 50, 265-276. Boyle E.A. (1988) Cadmium: Chemical tracer of deepwater paleoceanography. Paleoceanogr. 3, 471-489. Boyle E. A. (1990) Quaternary deepwater paleoceanography. Science 249, 863-870. Boyle E. A. (1992) Cadmium and 6 ~3C paleochemical ocean distributions during the stage 2 glacial maximum. Annu. Rev. Earth Planet. Sci. 20, 245-287. Boyle E. A. and Keigwin L. D. (1982) Deep circulation of the North Atlantic over the last 200,000 years: Geochemical evidence. Science 218, 784-787. Boyle E. A. and Keigwin L. D. ( 1985 / 1986) Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: Changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135-150. Bruland K.W. (1983) Trace elements in seawater. In Chemical Oceanography (ed. J. P. Riley and R. Chester), Vol. 8, pp. 157220. Academic Press. Carpenter S. J. and Lohmann K. C. (1992) Sr/Mg ratios of modern marine calcite: Empirical indicators of ocean chemistry and precipitation rate. Geochim. Cosmochim. Acta 56, 1837-1849. Crank J. (1956) The Mathematics of Diffusion. Clarendon Press. Delaney M.L. (1989) Uptake of cadmium into calcite shells by planktonic foraminifera. Chem. Geol. 78, 159-165. Deuser W. G., Ross E. H., Hemleben C., and Spindler M. (1981) Seasonal changes in species composition, numbers, mass, size, and isotopic composition of planktonic foraminifera settling into the deep Sargasso Sea. Palaeogeogr. Palaeoclimatol. Palaeoecol. 33, 103-127. Elderfield H., Bertram C. J., and Erez J. (1996) A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate. Earth Planet. Sci. Lett. 142, 409-423. Gaffey S. J. and Bronnimann C. E. ( 1991 ) Effects of bleaching on organic and mineral phases in biogenic carbonates. J. Sediment. Petrol. 63, 752-754. Gekeler W., Grill E., Winnacker E. L., and Zenk M. H. ( 1988 ) Algae sequester heavy metals via synthesis of phytochelatin complexes. Arch. Microbiol. 150, 197-202. Grill E., Winnaker E. L., and Zenk M. H. (1985) Phytochelatins: The principal heavy-metal complexing peptides of higher plants. Science 230, 674-676. Hester K. and Boyle E. (1982) Water chemistry control of cadmium content in Recent benthic foraminifera. Nature 298, 260-262. Jc~rgensen B.B., Erez J., Revsbach N.P., and Cohen Y. (1985) Symbiotic photosynthesis in a planktonic foraminifera, Globigerinoides sacculifer (Brady), studied with microelectrodes. Limnol. Oceanogr. 30, 1253-1267. Karez C. S., Allemand D., De Renzis G., Gnassia-Barelli M., Romeo M., and Puiseux-Dao S. (1990) Ca-Cd interaction in the prymnesiophyte Cricosphaera elongata. Plant Cell Environ. 13, 483-487. Keigwin L.D. and Boyle E.A. (1989) Late Quaternary paleochemistry of high-latitude surface waters. Palaeogeogr. Palaeoclimatol. Palaeoecol. 73, 85-106. Lea D. W. (1993) Constraints on the alkalinity and circulation of glacial Circumpolar Deep Water from benthic foraminiferal barium. Global Biogeochem. Cycles 7, 695-710. Lea D. W. and Boyle E. A. (1989) Barium content of benthic foraminifera controlled by bottom water composition. Nature 338, 751-753. Lea D. W. and Boyle E. A. (1990a) A 210,000-year record of barium variability in the deep northwest Atlantic Ocean. Nature 347, 269272. Lea D.W. and Boyle E.A. (1990b) Foraminiferal reconstruction of barium distributions in water masses of the glacial oceans. Paleoceanogr. 5, 719-742. Lea D.W. and Martin P.A. (1996) A rapid mass spectrometric method for the simultaneous analysis of barium, cadmium, and strontium in foraminifera shells. Geochim. Cosmochim. Acta 60, 3143-3149. Lea D. W. and Spero H. J. (1992) Experimental determination of barium uptake in shells of the planktonic foraminifera Orbulina universa at 22°C. Geochim. Cosmochim. Acta 56, 2673-2680.

Lea D. W. and Spero H. J. (1994) Assessing the reliability of paleochemical tracers: Barium uptake in the shells of planktonic foraminifera. Paleoceanogr. 9, 445-452. Lea D. W., Martin P. A., Chan D. A., and Spero H. J. (1995) Calcium uptake and calcification rate in the planktonic foraminifer Orbulina universa. J. Foram. Res. 25, 185-206. Lee J. G., Roberts S. B., and Morel F. M. M. (1995) Cadmium: A nutrient for the marine diatom, Thalassiosira weissflogii. Limnol. Oceanogr. 40, 1056-1063. Li Y.-H. and Gregory S. (1974) Diffusion of ions in seawater and in deep-sea sediments. Geochim. Cosmochim. Acta 38, 703-714. Lorens R.B. (1981) Strontium, cadmium, manganese, and cobalt distribution coefficients in calcite as a function of calcite precipitation rate. Geochim. Cosmochim. Acta 45, 553-561. Morel F. M. M., Reinfelder J. R., Roberts S. B., Chamberlain C. P., Lee J. G., and Yee D. (1994) Zinc and carbon colimitation of marine phytoplankton. Nature 260. Price N. M. and Morel F. M. M. (1990) Cadmium and cobalt substitution for zinc in a marine diatom. Nature 344, 658-660. Saager P. M. (1994) On the relationships between dissolved trace metals and nutrients in seawater: Implications for the use of cadmium as a paleoceanographic tracer. Ph.D. dissertation, Free Univ. Shannon R. D. (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A32, 751-767. Shen G. T. and Sanford C. L. (1990) Trace element indicators of climate variability in reef-building corals. In Global Ecological Consequences of the 1982-1983 El Niao-Southern Oscillation (ed. P. W. Glynn), pp. 255-283. Elsevier. Spero H. J. (1988) Ultrastructural examination of chamber morphogenesis and biomineralization in the planktonic foraminifer Orbulina universa. Mar. Biol. 99, 9-20. Spero H.J. and DeNiro M.J. (1987) The influence of symbiont photosynthesis on the 6 ~80 and 6 ~3C values of planktonic foraminiferal shell calcite. Symbiosis 4, 213-228. Spero H. J. and Parker S. L. (1985) Photosynthesis in the symbiotic planktonic foraminifer Orbulina universa and its potential contribution to oceanic primary productivity. J. Foram. Res. 15, 273281. Spero H.J. and Williams D.F. (1988) Extracting environmental information from planktonic foraminiferal 6 ~3C data. Nature 335, 717-719. Webster R. K. (1960) Mass spectrometric isotope dilution analysis. In Methods in Geochemistry (ed. A.A. Smales and L.R. Wagner), pp. 202-246. Interscience. APPENDIX A Three Endmember Isotope Dilution The three endmember isotope dilution technique employed in this study is unique because it enables complete and independent determination of two different ratios in the samples H1Cd/Ca and Cd/ Ca. The calculations are based on simple isotope dilution (Webster, 1960), using three cadmium isotopes from three different sources, each source having a unique and known isotopic signature: natural Cd 11°Cd spike HICd spike

110 12.49% 93.63% 0.67%

lll 12.80% 2.66% 96.05%

114 28.73% 1.21% 0.57%

Generally, when using isotope dilution, the isotope of highest natural abundance is measured in the sample and quantified with respect to a spike isotope having a lower natural abundance. For the culturing experiments, H°Cd was used as the laboratory spike, mCd as the field spike, and 114Cd to assess contamination. The use of three cadmium isotopes makes foraminiferal Cd uptake distinguishable from Cd contamination. The equations used in the three endmember isotope dilution are outlined below. (llO).~p~,, = (110 \ ' i ~ ] a d , j (lll).dd + \ 1 1 1 ] .... (111) ....

(A1)

4063

Uptake of Cd by planktonic forminifera

( 111Lpk

( 111hpike

(AZ)

( 111 )ln,x

(A3)

(114),0., con,

= add

(lll),ike

(A4)

(A14)

Isolate ( 111 )con,in both Eqns. Al 3 and Al4 and divide by ( 111 )spike (A5)

(A6) The subscripts used in the equations indicate the source of Cd: sample represents Cd in the foraminiferal sample, add represents Cd taken up by the foraminifera, cant represents contamination by natural Cd, spike represents Cd added to the sample as spike, and mix represents Cd in the spiked sample. In basic isotope dilution, ( 1 10),i, and ( 11l)m,x are a function of two isotopic signatures, the sample and the spike, as indicated by the following two equations: ( 1lohllix = ( 1~Ohlple+ (1 lohpike

(A7)

( 111 ),i. = ( 111 )ramp,e+ ( 111 Lpire

(A8)

(A15)

11LO”, = 111spike

C-J

Substituting Eqns. Al, A2, and A3 into Eqn. A7 and Bqns. A4, A5, and A6 into Fqn. A8 yields

Set Eqns. Al5 and Al6 equal and solve for ( 111 )addto determine foraminiferal “‘Cd uptake

(1

( 111)a&?

iii 110

(

113

=

( 111 )add+ ( 111 110 1 ca”t( 111),0”,+ ( ‘IT;i 110 1 spire( 111 )rpiix add

(ill).,+

(K

1

1

(ill),,,,

+

CO”t

add

=

(

114 111

(111Apike

1 sp,lie

(1141

(lll),i.

z

(AlO)

rmi

Because the Cd in the spiked samples is a combination of both added Cd and contamination Cd, Eqns. A7 and A8 can be written as (llO),i,

= (110)&d + (llO),,“, f (llO),ik~

( 111 ),ix = ( 111 )a&!+ ( 111),,“, + (111 )spilre

(All)

x (111&e (A17) Set Eqns. Al5 and Al6 equal and solve for ( 111 ),,,t to determine foraminiferal “‘Cd contamination (ill),,,

(A12)

Substitute for (11 l)mir in Bqns. A9 and Al0 using Bqns. All and Al2 to express that the Cd in the spiked sample is a function of added, contamination, and spike Cd

--111) (lllhdd 11 ’ add

( )

+

110

-

( 111Lm,

( 111) con,

(A13)

Divide Eqn. Al8 by the natural abundance of “‘Cd (12.80%) to determine contamination by Cd (Cd,,,,,).

4064

T.A. Mashiotta, D. W. Lea, and H. J. Spero APPENDIX B mCd/Ca of Cultured Orbulina universa and Globigerina bulloides

Species

Sample

Experiment

TE33 TE21 TE34 TE42 TE20

O. O. O. O. O.

universa universa universa universa universa

24 24 24 24 24

h h h h h

TD19 TE41 TE30 TD17 TD15 TEl9 TEl8 TD16 TD14 TD18

O. O. O. O. O. O. O. O. O. O.

umversa untversa untversa umversa unlversa untversa unlversa unlversa untversa umversa

12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h

TA17 TD12 TA16 TE02 TC07 TE01 TE03 TC06 TD04 TC11 TD01 TD05

O. O. O. O. O. O. O. O. O. O. O. O.

umversa umversa umversa umversa umversa umversa umversa umversa umversa unlversa unlversa umversa

TA20 TB12 TA21 TE40 TD49 TB11 TA19 TEl4 TE29 TB13 TD50

O. u m v e r s a O. O. O. O. O. O. O. O. O. O.

TA24 TE24 TD24 TB17 TA22 TB18 TE23 TD23

Shell mass (mg) or N

"ICd/C a foram (~zmol/mol)

0 0 0 0 0

0.018 0.023 0.022 0.023 0.020

-0.007 -0.007 -0.005 -0.004 -0.001

0 0 0 0 0 0 0 0 0 0

0.028 0.021 0.032 0.029 0.027 0.032 0.023 0.022 0.037 0.030

-0.004 -0.003 -0.003 -0.002 -0.002 -0.002 -0.002 -0.002 -0.001 -0.001

+DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU

0 0 0 0 0 0 0 0 0 0 0 0

0.021 0.014 0.022 0.018 0.024 0.023 0.023 0.024 0.035 0.033 0.010 0.010

-0.010 -0.009 -0.006 -0.005 -0.005 -0.004 -0.004 -0.002 -0.002 -0.002 0.004 0.019

umversa umversa untversa umversa unlversa untversa umversa untversa umversa unlversa

24 24 24 24 24 24 24 24 24 24 24

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.026 0.021 0.024 0.017 0.013 0.027 0.017 0.025 0.017 0.008 0.013

0.007 0.007 0.008 0.008 0.011 0.014 0.025 0.030 0.031 0.052 0.067

O. O. O. O. O. O. O. O.

universa universa universa universa universa universa universa universa

12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.031 0.033 0.033 0.031 0.027 0.028 0.028 0.028

0.005 0.009 0.010 0.011 0.014 0.021 0.031 0.032

TB01 TD07 TB02 TB06 TD02 TE08 TE04 TD06 TE07

O. O. O. O. O. O. O. O. O.

untversa untversa umversa untversa untversa umversa umversa umversa umversa

+DCMU + DCMU + DCMU +DCMU +DCMU +DCMU +DCMU +DCMU +DCMU

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.029 0.032 0.029 0.024 0.010 0.030 0.024 0.042 0.031

0.014 0.015 0.015 0.016 0.020 0.029 0.034 0.079 0.114

TE26 TE43 TE36 TD33 TE25

O. O. O. O. O.

universa universa universa universa universa

24 24 24 24 24

0.5 0.5 0.5 0.5 0.5

0.016 0.028 0.023 0.036 0.023

0.020 0.033 0.035 0.044 0.077

h h h h h h h h h h h

h h h h h

darkness darkness darkness darkness darkness

"ICd/C a seawater (~mol/mol)

light:12 light:12 light:12 light:12 light:12 light:12 light:12 light:12 light:12 light:12

h h h h h h h h h h

dark dark dark dark dark dark dark dark dark dark

darkness darkness darkness darkness darkness darkness darkness darkness darkness darkness darkness light:12 light:12 light:12 light:12 light:12 light:12 light:12 light:12

h h h h h h h h

darkness darkness darkness darkness darkness

dark dark dark dark dark dark dark dark

Notes

*, in in in out in in in in in out R *, in

in in in in in out *, in in in

Uptake of Cd by planktonic forminifera

4065

APPENDIX B Continued

Species

Sample

Experiment

, iCd/C a seawater (#mol/mol)

Shell mass (rag) or N

I11Cd/C a foram (#mol/mol)

0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.025 0.024 0.016 0.014 0.018 0.012 0.025

0.078 0.092 0.119 0.264 0.298 0.346 9.288

0.5 0.5 0.5 0.5 0.5 0.5

0.045 0.042 0.040 0.043 0.044 0.044

0.016 0.016 0.019 0.031 0.088 0.108 0.104 0.122 0.161 0.176 0.216 0.342

Notes

TD32 TA06 TD34 TD29 TA04 TE37 TA05

O. O. O. O. O. O. O.

universa untversa untversa unlversa umversa untversa unlversa

24 24 24 24 24 24 24

h h h h h h h

darkness darkness darkness darkness darkness darkness darkness

TA27 TCl3 TC12 TC14 TC16 TD20

O. O. O. O. O. O.

universa universa universa universa universa universa

12 12 12 12 12 12

h h h h h h

light:12 light:12 light:12 light:12 light:12 light:12

TD26 TC18 TE38 TC17 TD27 TA07

O. O. O. O. O. O.

untversa umversa untversa ttntversa untversa umversa

+DCMU +DCMU +DCMU +DCMU +DCMU +DCMU

0.5 0.5 0.5 0.5 0.5 0.5

0.015 0.018 0.011 0.014 0.026 0.020

TC41 TC43 TC42 EF11 EF10

G. G. G. G. G.

bulloides bulloides bulloides bulloides bulloides

l llCd l llCd lllCd IlICd llICd

0 0 0 0 0

7 4 7 9 9

-0.004 -0.003 -0.002 0.000 0.001

TF02 TF05 TF08 TF12

G. G. G. G.

bulloides bulloides bulloides bulloides

mCd 1HCd i~JCd tllCd

0.1 0.1 0.1 0.1

5 5 5 5

0.159 0.177 0.383 0.404

TG03 TG08 TGI7 TG05 TG10 TG13

G. G. G. G. G. G.

bulloides bulloides bulloides bulloides bulloides bulloides

HICd H~Cd t llCd I]lCd JllCd i liCd

0.2 0.2 0.2 0.2 0.2 0.2

7 6 6 6 5 6

0.180 0.266 0.290 0.298 0.430 0.459

R

TG12 TG04 TG07 TG01

G. G. G. G.

bulloides bulloides bulloides bulloides

IlICd l]]Cd mCd lIICd

0.5 0.5 0.5 0.5

4 4 4 5

0.744 0.923 0.971 1.100

*, R

TF06 TF10 TF13

G. bulloides G. bulloides G. bulloides

HICd and 48Ca lllCd and 48Ca lllCd and 4SCa

0.5 0.5 0.5

5 5 5

0.631 0.910 1.079

and and and and

h h h h h h

48Ca 48Ca 4SCa 48Ca

dark dark dark dark dark dark

* outlier

out out *, in *, in *, in in

R

*

All O. universa data represent single shells. O. universa shell mass does not include remnant cytoplasm. N indicates number of G. bulloides pooled for analysis, either amputated chambers or whole foraminifera (see text). Light levels are high light and are greater than 386 ~E m -1 s -]. ~ C d / C a of shells is determined on purified calcite (see text). To convert seawater NICd/Ca to m C d concentration in nmoles/L multiply by 10 mM. * Indicates final shell mass analyzed was less than 10 #g. In/out specifies whether sphere formed in or out of D C M U seawater solution. R indicates rejection based on suspected sample contamination by natural Cd; data is not included in figures. O. universa data rejected based on Cd/Ca ratio; G. buUoides rejected based on HICd/Cd ratio.