Experimental determination of barium uptake in shells of the planktonic foraminifera Orbulina universa at 22°C

Geochimica

er Cosmochimica

Ada

Vol.

0016-7037/92/$5.00

56, pp. 2673-2680

+ .I0

Copyright 0 1992 Pergamon PressLtd.Printedin U.S.A.

Experimental determination of barium uptake in shells of the planktonic foraminifera Orbulina tiniversa at 22°C DAVID W. LEA' and HOWARD J. SPERO’ ‘Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93 106, U S A *Department of Geology, University of California, Davis, CA 95616, U S A (Received December 30, 199 1; accepted in revised form April 16, 1992)

Abstract-We have sought to directly test the assumption that the Ba/Ca ratio of foraminifera shells is proportional to the Ba/Ca ratio of the seawater in which the shells precipitated via experiments with living planktonic foraminifera. Specimens of the symbiont-bearing species, Orbulina universa, were grown in seawater with five different Ba concentrations at a temperature of 22°C. Individual foraminifera shells (spherical chambers) were then cleaned and analyzed for Ba/Ca. We found that the response for Ba incorporation is linear within and somewhat above the range of Ba concentrations found in the ocean. The calculated distribution coefficient for Ba incorporation lies between 0.14 and 0.17. The value determined previously by comparison of core-top fossil planktonic foraminifera and surface water compositions was 0.19 f 0.05. Foraminifera grown under low-light conditions to simulate reduced symbiont photosynthetic activity did not have significantly different Ba/Ca. This suggests that for this species, biogenic precipitation rates of calcite varying in response to changing light levels do not affect the degree of Ba discrimination. Earlier observations of Ba incompatibility and lack of substitution in inorganically precipitated calcite at higher Ba activities do not apply to coprecipitation of Ba in the planktonic foraminifera Orbulina over the range of Ba concentrations found in seawater. INTRODUCIION

it is common practice to lump the thermodynamic terms on the right-hand side of Eqn. (2) into a single distribution coefficient relating the ion ratios of the shell material to the total concentration ratios in seawater. Recently it has been suggested that this distribution coefficient be termed a partition coefficient so as to avoid confusion with a true thermodynamic distribution constant (MORSE and BENDER, 1990). For an ideal solid solution between Ba and Ca, the distribution coefficient D would be equal to the ratio of the solubility products of CaCOs to BaC03. However, since no solid solution is documented between Ca and Ba in the trigonal carbonate structure, and because pure BaC03 assumes an orthorhombic structure, it is not possible to calculate a thermodynamic value of D for Ba substitution in calcite. Therefore, one must determine D empirically by specifically comparing the Ba/Ca ratio of shell material to the Ba/Ca of the seawater in which the shell grew. Even in a system where true solid solution exists, metal to Ca ratios in biogenic carbonates often deviate markedly from the value expected from thermodynamic constants (BOYLE, 1988; SHEN and SANFORD, 1990). Incorporation of Ba into foraminifera shells might occur via substitution for Ca in the calcite lattice. Since Ba2+ is a large ion with an ionic radius in the 6-fold trigonal calcite coordination of 1.35 A vs. 1.OO8, for Ca2+ (SHANNON,1976 ) , it has been suggested that steric incompatibility should limit incorporation of Ba in calcite ( PINGITORE, 1986; PINGITORE and EASTMEN, 1984). On the other hand, inorganic precipitation experiments clearly indicate that Ba is incorporated into calcite, although the mechanism is debated ( KITANO et al., 197 1). KITANO et al. ( 197 1) found that under conditions of slow precipitation the distribution coefficient D for Ba in inorganic calcite is about 0.1. However, during rapid precipitation achieved by “vigorous stirring,” Ba distribution coef-

FORAMIN~FERALPALEOCHEMKALtracers have become an important means of reconstructing the circulation and chemistry of ancient oceans. Among these tracers the stable isotope ratios of oxygen and carbon as well as the Cd content of foraminifera shells have been most widely utilized (BOYLE, 1988, 1990; CURRY et al., 1988). Several recent studies have presented evidence demonstrating that incorporation of Ba in the shells of planktonic and benthic foraminifera can be used as a means of reconstructing Ba distributions in past ocean waters (LEA and BOYLE, 1989, 199 1). Because Ba covaries in seawater with silica and alkalinity, paleo-Ba distributions can be used to infer the distribution of these components in past ocean waters (LEA, 1990; LEA and BOYLE, 1990a,b). Use of foraminiferal tracers like Ba is dependent on the assumption that the trace element is incorporated in shell material in proportion to its seawater concentration (BOYLE, 1988; SHEN and SANFORD, 1990). If Ba is incorporated via the substitution reaction CaC03 + Ba2+ =+ BaCOS + Ca2+,

(1)

it follows that

( Ba/Cahoram w

=

D

-YBafea{Ba}

HYC.J& { Ca>

(2)

where DH defines a Henry’s Law distribution coefficient, y is the activity coefficient, f is the fraction of the elements occurring as a free, uncomplexed ion (both faa and f& = 0.8-0.9), and { Ba } or { Ca } are the total concentrations of the ions in solution. Because of the inherent difficulty in determining the true activity coefficients of ions in seawater, 2673

2614

D. W. Lea and H. J. Spero

ficients were as high as 3 during the early stages of precipitation and never dropped below 0.5 during the final stages of precipitation. PINGITORE ( 1986) published data suggesting that Ba was incorporated into calcite with a partition coefficient of 0.04 it 0.0 1 under a variety of precipitation conditions. However, these experiments were not conducted in seawater, and Ba concentrations were 2 to 1000 times greater than surface seawater concentrations. PINGI~ORE( 1986) argued that scatter in determination of the partition coefficients in the precipitation experiments suggests that Ba is not incorporated isomorphously for Ca2+ in calcite precipitates, but rather is trapped or adsorbed. Results of LEA and BOYLE (1989, 1990a,b, 199 1) indicating that foraminifera shells do take up Ba in proportion to seawater concentration suggest that Pingitore’s results are not applicable to biogenic coprecipitation at the low Ba concentrations present in seawater. The validity of using foraminifera metal to calcium ratios as a means of reconstructing past seawater metal concentrations depends on the assumption that metals are taken up into the shells in proportion to seawater concentrations (i.e., Eqns. 1 and 2). The value of D for a particular trace elementforaminifera pair can be determined by comparing the metal content of fossil foraminifera shells picked from deep-sea core tops to the estimated metal concentration of the water column at or near the sites of the core material. However, a reasonably large variation in ocean water chemistry is required to construct a convincing correlation. In addition, this approach assumes that factors such as temperature, salinity, pressure, and COz chemistry do not significantly affect the magnitude of the distribution coefficient over the range by which these factors vary in the ocean (BOYLE, 1988). Because the range of variation in metal concentrations in the deep ocean is large compared to the errors involved in foraminiferal metal determinations, comparison of core-top material to water column chemistry turns out to be a reasonably effective means of calibrating benthic foraminiferal trace element uptake, as demonstrated for Cd (HESTER and BOYLE, 1982) and Ba (LEA and BOYLE, 1989). Still, one of the limitations to this method is that fossil foraminifera from core tops range in age from Recent to several thousand years BP. Hence, comparisons of fossil foraminifera to ambient seawater are subject to a degree of uncertainty, and calibration of the magnitude of D is not as well constrained as it could be. The problems inherent in such calibrations are extensively discussed by BOYLE (1988). Verification of trace metal uptake in planktonic foraminifera by comparison of core-top material and water column chemistry has proved to be a more difficult problem (BOYLE, 1981; KEIGWIN and BOYLE, 1989; LEA and BOYLE, 1991). For elements like Cd, estimating the metal concentration of the seawater in which the foraminifera secreted its shell is generally not straightforward. For instance, the habitat range of a number of foraminiferal species used in paleoceanographic studies range between surface waters and the base of the thermocline, a depth region which exhibits up to lOOOfold change in Cd concentrations (BOYLE, 1981; BRULAND, 1983; KEIGWIN and BOYLE, 1989). Although Ba does not exhibit large variations in the upper part of the water column, the concentration range of Ba in the entire warm surface

ocean is only about 20%. Since the typical variability observed in splits of planktonic foraminifera Ba/Ca from a deep-sea core top is +5-lo%, it is difficult to construct a convincing calibration from core-top data (LEA and BOYLE, 199 1). We suggest that the best method of verifying proportional uptake of metals in foraminiferal shells is by growing planktonic foraminifera in seawater of known trace element concentration and temperature. This method eliminates all the uncertainties required in estimating the metal content of the water from which the foraminifera secreted its shell. In addition, the extensive problems involved in purifying the shells of residual sedimentary phases are avoided. Finally, the range of metal concentration in the experiments can be varied to a greater extent than is normally present in surface seawater, and external variables such as temperature and salinity can be held constant. Because the full reproductive cycle of planktonic foraminifera has not been established under laboratory conditions, juvenile specimens must be collected by SCUBA divers from the water column and transported to the laboratory where they can be cultured to maturity. For this reason some portion of the shell material is not grown in the laboratory and therefore has calcified under conditions different from those set in the controlled experiments. Fortunately, a surface-dwelling planktonic foraminifera, Orbulina universa, secretes a large terminal spherical chamber around the juvenile shell which comprises 90-95s of the calcite secreted by the foraminifera (SPERO, 1988; SPERO and PARKER, 1985; SPERO and WILLIAMS, 1988). Hence, pre-sphere Orbulina universa can be collected in the field after which the spherical chamber is secreted in the laboratory under defined conditions. Upon gamete release (completion of the foraminifer’s life cycle), the sphere can be collected and the early juvenile chamber removed so that the final shell material which is analyzed for metal content is grown entirely under controlled conditions (Be and ANDERSON,1976). Previous culturing experiments to determine metal uptake in planktonic foraminifera were conducted on two conservative elements in seawater, Sr and Li, as well as two important nutrient-like trace metals, Cd and Zn (DELANEY, 1983, 1989; DELANEYet al., 1985). In the case of the experiments on Cd and Zn, radiotracers were utilized. A limitation of radiotracer experiments is that total metal contents of the growth solutions are not controlled. Therefore, if added radioactive spikes are much lower than ambient seawater concentrations, nonideal solid solution effects due to greater degree of metal incorporation in the crystal lattice will not be observed; conversely, if higher concentrations than are typically found in surface seawater are used, results might not realistically represent natural conditions. Here we describe a series of Ba uptake experiments using living foraminifera in which we used stable Ba additions. Total Ba was held constant throughout the duration of each experiment, and between different experiments Ba concentrations were varied over the maximum possible range in seawater. External environmental parameters such as temperature, salinity, and light were carefully controlled. Finally, analytical developments have lowered detection limits sufficiently so that Ba/Ca of individual shells from repetitive experiments can be determined.

Ba2+ in foram shells at 22°C

2675

been documented in Orbulino and other symbiont-bearing foraminifera (CARON et al.. 1981:SPEROand DENIRO. 1987). oossiblv a function of different &kification rates, we grew O;buli& under three different light levels: low (5-7 pEinsteins m-* s-l), medium (300 rE), and high (500 rE). During the culture period of 7-10 days, the spherical chamber was secreted and calcified (SPERO, 1988). Individuals were fed one-day-old Artemia nauplias (brine shrimp) every other day and were examined under an inverted light microscope for size and general condition. During this protocol, the rinse procedure outlined above was always followed to minimize potential contamination. Upon termination of the experiment (5-10 days, generally aher the shells underwent gametogenesis), the empty spherical chambers were archived for later analysis. In addition, a portion of the culture solutions was acidified and archived to check for any change in Ba concentration over the course of the experiments. Because the amount of Ca precipitated as foraminiferal calcite was never more than 0.1% of the initial Ca present, Ca concentrations were constant over the course of the experiments.

EXPERIMENTAL Foramirdfera Collection and Culturing The basic experimental procedure follows established protocols for stable isotopic experiments (SPERO and WILLIAMS,1988). Presphere Orbuha universuwere hand collected by SCUBA divers from a depth of 3-5 m in the San Pedro Basin, approximately 2 km NNE of the Catalina Marine Science Center, Santa Catalina Island, Cahfomia. ABer collection of the foraminifera, surface water samples were collected in acid-cleaned polyethylene (PE) bottles using standard precautions against contamination (BRULAND, 1983). (Acidcleaned = 24 h in 60°C I N HCl, followed by five rinses of ultrapure deionized water.) Upon return to the laboratory, the seawater was filtered using acid-cleaned Nuclepore 0.45 hrn membrane filters and an acidcleaned Nalgene polysulfone filter holder. Whenever possible, handling of samples was done within a class 100 laminar flow bench. Individual foraminifera were checked under a microscope for identification of species and general condition and then transferred to clear polymethylpentene (PMP) acid-cleaned 125 mL bottles containing either ambient filtered seawater or the same with the addition of stable Ba ( 1 foram/bottle). Special care was taken in transferring the foraminifera specimens: the living specimens were rinsed in three successive baths of clean ambient seawater, transfers were accomplished using 5 mL acid-cleaned PE pipet tips with a Finn pipet. Surface seawater collected from the San Pedro Basin had a mean Ba concentration of 40 nmol L-’ and did not vary by more than t21 over the course of the experiments (see Fig. 1 and following text). Solutions of different Ba concentration were prepared by adding known quantities of an aqueous BaClz standard ( pH = 3) to filtered ambient seawater. The additions were calculated to yield Ba concentrations ranging from about 1.5times ambient seawater to just below the saturation limit of barite ( BaSO.,) in seawater, in this case about 6 times ambient seawater concentrations (CHURCH and WOLGEMUTH, 1972). In all cases additions of the standard have a negligible effect on both the Cl- content and pH of seawater. Bottles were placed in a temperature controlled water bath set at 22”C, the average surface temperature at the collecting site during the time of the experiments. Because light-enhanced shell growth has

Analytical Methods Individual foraminifera shells (spherical chambers) were weighed on a microbalance; shell masses for individuals grown at medium and high light varied from about 20 to 50 pg, while those grown under low light were typically between 5 and 20 @ Shells were gently cracked open with a fire-rounded Pasteur pipet, and the juvenile chamber (if present) was removed with a small brush. The remaining fragments were transferred to an acidcleaned polyethylene centrifuge tube. The cleaning protocol used for the shell calcite. is an abbreviated version of techniques developed for analysis of planktonic foraminiferal Cd, with the primary emphasis being to clean any remaining protoplasm in the shells (BOYLE, 198 I; MYLE and KEIGWIN, 1985/ 1986; LEA and BOYLE, 199 1). The cleaning steps include two ultrapure water rinses with ultrasonic agitation followed by a 0.1% H202 solution buffered with 0.1 N NaOH to remove remnant organic material. Reagent amounts were adjusted downward for the small sample size: 25 PL of the H202/NaOH solution per sample. Some of the

Ba in Cstallns

Surface

Waters,

1990

60 o

Upper limit of Pacific surface waters north of the Antarctic Polar Front

55-

XI-

Ba

(nmol/L)

45-

40-

T z

f

I

I I

r

8

se.7 f 0.7 (2%) f

I

350 30-r

Lower limff of Pacific gyre surface waters

I 9119

.

I 0119

.

I

0120

.

,

a122

Date

.

,

0127

.

,

,

8128

0130

-

, 9/l

(1990)

FIG. 1. Barium concentrations in surface waters of the San Pedro Basin, approximately 2 km NNE of the Catalina Marine Science Center, Santa Catalina Island, California, over the time period August 18 through September 1, 199 1. These measmements demonstrate that the Ba concentration of the seawater used for the experiments was constant within analytical error.

D. W. Lea and H. J. Spero

2676

samples were subject to an additional cleaning step consisting of 25 rL of 0.001 N 3 times quartz distilled HNOJ to remove any Ba adsorbed during the previous cleaning steps, although this step made no obvious difference in the determinations. Although the cleaning can remove a significant portion of the calcite, previous selective dissolution studies have demonstrated that Ba is distributed homogeneously within foraminiferal calcite (LEA, 1990; LEA and BOYLE, 1991). Upon completion of the cleaning samples were rinsed several times with ultrapure water and then dissolved in 60 pL of 0.08 N Q-HNOa. The vials were centrifuged at 1000 RPM for 5 min and then 10 FL was removed, diluted with 4 mL of 2 g/L KCL solution, and analyzed for Ca by flame atomic absorption spectrophotometry (FAAS) using an air-nitrous oxide-acetylene flame. The use ofthe hotter N20 flame decreases the detection limit of the Ca determination, reducing the volume required for Ca determination. The remaining 50 pL was used for the determination of Ba. 50 WL each of the sample and a 135Ba-emiched spike were diluted with 150 CL of 0.1 N Q-HNO,. The spike concentration is adjusted so that the final ‘35”3*Baratio of the mixture is between 0.5 and 5. (The geometric mean of the 135 to 138 ratio of enriched Ba and natural Ba is I .55 .) The final 250 PL volume was then introduced into a VG PQ2+ inductively coupled plasma mass spectrometer (ICP-MS) as an isolated plug of sample. Scan times of 20 seconds were achieved by using a peristaltic flow rate of 0.5 ml/minute. Using an ICP-MS mode in which data is collected for five points per isotope with a 10 ms dwell time per point over 125 scans, total counts of between 4000 and 100,000 were typical. Signal to noise ranged between 10 and 100. The final Ba concentration is determined by the standard isotope dilution calculation (CHAN et al., 1977; LEA, 1990). Accuracy is controlled by repeatedly determining the ratio of ‘35”)8Baon a mixture of spike and standard, and applying a small correction of about 1 or 2% based on this value to all the samples. Fourteen analyses of a consistency standard with Ba and Ca concentrations similar to the foraminifera samples (CNlh: Ba = 1 nmol/L) had a standard deviation of 4.4% for Ba, 1.4% for Ca, and 4.4% for the Ba/Ca ratio. Barium concentrations of the culturing solutions were determined

by the isotope dilution method on the ICP-MS ( KLINKHAMMER and CHAN, 1990; LEA, 1990). Solutions were spiked with the “‘Baenriched spike, diluted to about 0.1% TDS, ahd then aspirated directly into the ICP-MS. The standard deviations of the seawater Ba determinations are +2% or better. The Ca concentmtion of seawater is virtually conservative (WILSON, 1983). Assuming a surface water value of 10.2 mmol/kg at S = 350/W ( HORIBE et al., 1974), we calculate Ca for the prevailing salinity of 33.7% to be equivalent to 9.8 mmol/kg or 10.1 mmol/L. We also determined Ca concentrations on a few of the culture solutions by FAAS, detecting an average value of 10.0 mmol/L. RESULTS OF THE BARIUM UmAKE

EXPERIMENTS

Figure 1 illustrates the Ba concentration of ambient surface seawater collected from the surface waters of the San Pedro Basin, approximately 2 km NNE of the Catalina Marine Science Center, Santa Catalina Island, California, during the period August 18 through September 1,199O. Over this period the Ba concentration ofsurface seawater was 39.7 ? 0.7 nmol/

L. Since this variation is of the same order as the analytical precision ( f2% ) , the Ba concentration of our starting solutions is considered constant. The Ba concentration of surface waters at this site is about 20% higher than the Ba content of oligotrophic Pacific surface waters (OSTLUND et al., 1987), probably reflecting the input of colder upwelled or advected waters to the San Pedro Basin. One of the largest potential errors that can arise in metal culturing experiments is change in the metal concentration over the course of the experiments (MORSE and BENDER,

1990). Such change might occur by either addition of Ba due to contamination or loss of Ba due to adsorption of Ba onto the container walls. The amount of Ba incorporated in

(number below point indicates # of analyses) 5

BalCa

(foram) = O.lG’Ba/Ca R”2 = 0.94

7

(seawater)

4-

BalCa in foram shells (pmol/mol)

3-

2-

1-

0

10

in seawater (pmollmol) B&a

20

culture

30

solutions

FIG. 2. Ba/Ca of cultured Orbulina universa vs. the Ba/Ca ratios of seawater growth solutions. These analyses were made primarily on single shells, although the smallest shells were pooled for some of the data points (see Table 1). This plot includes all data from medium- and high-light experiments. Seawater Ba ranges from 32 to 80 nmol/kg in surface waters (Ba/Ca = 3.2 to 8.0 rmol/mol) and 45 to 150 nmol/kg in deep waters (Ba/Ca = 4.5 to 15.0 pmmol/ mol ). The line is a linear regression through the data constrained to pass through the origin (see text).

BaZ+in foram shells at 22°C the foraminifera shell is less than 1% of the total Ba present in the culturing solutions. To ascertain if Ba loss or addition was occurring during the experiments, we measured Ba in both the starting solutions and the culturing solutions after the experiments were completed. In all cases the difference was never larger than the measurement error. Thus, Ba concentrations were constant throughout the experiments. Figure 2 illustrates the measured Ba/Ca on shells of Orbulina universa plotted vs. the Ba/Ca content of culturing solutions. These data are primarily from individual shell analyses, although for the smallest shells we pooled two or three shells (Table 1). Figure 2 includes data from all the experiments conducted under medium to high light (300 to 500 /LIE). We found that the response for Ba incorporation is linear within and somewhat above the range of Ba concentrations found in the ocean. The slope of the response curves (which equals the distribution coefficient) can be calculated in two different ways. A simple linear regression yields a slope of 0.15 + 0.01 (95% confidence interval) and an intercept of 0.22 + 0.2 1 (95% CI). A zero intercept is just barely excluded from the 95% confidence interval. However, a more true representation of the distribution coefficient might be obtained

Table 1: Ba/Ca of fotaminifera shellsfrom culturingexperiments

Experiment

hght kvel

Ba/Cs seawater Pdm

Foram

weight w

Bs/C ahe/ Wm

Notes

500E 395 0032 074 T-23 5OOk 3:95 d.030 674 T18 5OQetE 3.95 0.036 0.75 T22 300 PE 3.95 0.033 0.77 T62 3ooPE 3.95 0.042 0.79 T115 3OOPE 3.95 0.030 0.79 T58 5@ 3.95 0.058 0.88 T41.44 5@ 3.95 0.030 1.14 T43.48 3c0* 7 39 0036 103 T70 3OOPE 7139 0:031 1:06 * T66 300 PE 7.39 0.031 1.10 T71 3OOPE 7.39 0.036 1.25 T65.68 500 )rE 7.39 0.070 1.31 * T9495 300 )IE 7.39 0.042 1.34 * T67,69 5CQPE 7.39 0.050 1.39 T98 5OOlJ.E 7.39 0.053 1.65 TlOl 500E 1094 0036 187 Tl 5OOb lOI94 a:043 2:06 T3.5,6 z5 10.94 0.053 2.07 T4 10.94 0.033 2.11 T8 5CQPE 10.94 0.038 2.18 * T2 5w 10.94 0.027 2.16 TlO-T12 l-73 OOE 1794 0027 0:055 2:82 5OOb 17:94 T74.78 500 1E 17.94 0.040 3.38 n7 500 WE 17.94 0.040 4.07 T80 5PE 17.94 0.047 2.94 T26,29,33 5@3 17.94 0.020 3.24 * T35 500E 2490 0041 371 * T85 5&E 24:90 0:058 3:72 T8 1.84 3OOME 24.90 0.043 3.79 TlO7 500 PE 24.90 0.051 3.89 T88 5OOPE 24.90 0.042 3.91 T83 500 WE 24.90 0.053 3.91 T82 300 PE 24.90 0.039 3.92 T103,108 300 NE 24.90 0.034 3.97 T102,lCkl 3OOPE 24.90 0.032 4.14 T105 3OON 24.90 0.031 5.38 ejected T109 Number of experiments indicates number of shells included in the nnalysis Light levels arc in pEinsteins&s To convert Ba/Ca to Ba concentrationin nmol/L, multiply by 10.06 Shell weight includesmassof any remnant dried protoplasm Ba/Ca of shell is determinedon purified shell calcite (see text) *indicates final shell mass analyzed was less than 5 pg

2677

by forcing the line through zero, since foraminifera grown in seawater with no Ba would be expected to have no Ba in the shells. When the linear regression is forced through zero, the slope rises to 0.16 + 0.01 (Fig. 2). The small positive intercept might actually be due to an artifact of the experiments. In most cases we find that the juvenile shell is not present after gametogenesis and was presumably absorbed and incorporated into the adult shell. This would cause shells grown at higher Ba concentrations to record slightly lower Ba/Ca (due to inclusion of low Ba/Ca juvenile material), yielding a slope with a slight positive intercept. A simple mass balance calculation indicates that inclusion of juvenile material amounting to 10% of the adult shell mass can cause a significant positive intercept and negative deviation of the slope. The calculated value of the distribution coefficient (0.14 to 0.17) represents the relative partitioning of Ba into shell calcite; the fact that the calculated value is much less than 1 implies that Ba is discriminated against relative to Ca in the calcification process. The distribution coefficient calculated for Ba is of the same order as that found for Sr incorporation in foraminiferal shell calcite (BENDERet al., 1975; DELANEY et al., 1985; GRAHAM et al., 1982). This might be expected since both Ba and Sr are alkaline earth elements with ionic radii significantly larger than Ca. Empirical D found previously by comparing Ba/Ca for five species of fossil planktonic foraminifera from sediment core tops with estimated surface water Ba compositions was 0.19 f 0.05 (LEA and BOYLE, 199 1); there was no obvious variation of D with environmental parameters such as temperature or salinity, although the calibration was accomplished using almost exclusively tropical and subtropical core tops. The distribution coefficient derived from the culturing experiments (0.14-o. 17 depending on how it is calculated) is in good agreement with the empirically determined value. Figure 3 illustrates a comparison of the mean core-top values found previously with the mean culturing values for the various groups (LEA and BOYLE, 199 1). Note the very small range of Ba/Ca in surface waters of the tropical and temperate oceans vs. the expanded range that can be used in a culturing experiment. One of the reasons for analyzing single shells was to ascertain if any intraspecific variability exists for the incorporation of Ba into shell calcite. Determinations of Ba/Ca on individual shells grown at the same concentration were within analytical error for three of the five concentrations. At this stage we cannot definitively conclude that intraspecific variability in Ba uptake between individuals is negligible. The data derived from the shells grown in the seawater Ba/Ca range do not suggest that variability from individual to individual is of significantly greater magnitude than the errors involved in making the determinations. Our results demonstrate that at the low Ba concentrations present in seawater, Ba occurs in foraminiferal calcite in proportion to seawater concentration. We believe that under these conditions Ba forms an essentially infinitely dilute solid solution in the foraminiferal calcite, reducing the effect of steric incompatibility (A. Navrotsky, Princeton U., pets. commun., 199 1). Therefore, observations of Ba incompatibility and lack of substitution at higher concentrations (PIN-

2618

D. W. Lea and H. J. Spero a

Regression based on culturing experimentr: BaICs (foram) = 0.1 VWCa (seawater)

.

4-

Ba/Ca in foram shells (pmollmol)

l

sedimentary core-top results

0

reds

from culturing experiments

20

10

30

Ba/Ca in seawater (pmollmol) FIG. 3. Comparison of mean E3a/Cameasured on several fossil foraminifera species from core tops in the Pacific Ocean (lowest Ba/Ca), Atlantic Ocean (intermediate Ba/Ca), and Mediterranean Sea (highest Ba/Ca; LEA and BOYLE,199I ) with averaged Ba/Ca of Orbulina universufrom this study. The linear regression is the same as in Fig. 2.

GITORE, 1986) do not apply to coprecipitation of Ba in Orbulina universa at very low seawater concentrations.

The culturing results confirm the observation made previously from study of core-top material that DBa for planktonic foraminifera is about half of that found for benthic foraminifera ( LEA and BOYLE, 1989, 199 1) . This difference

suggests that temperature, pressure, calcification rate, or some

other unknown environmental factor influences the degree of Ba discrimination during precipitation of foraminiferal calcite. Although none of these factors were varied in the experiments described here, a few culturing experiments were carried out under low-light conditions of less than 10 MEm-’

5.

Regreesion based on culturing experiments: Ba/Ca (foram) I o.WBa/Ca (seawater) /

Ba/Ca in foram shells (pmollmol)

werages, 300-500pE 0 lowlight (3-5 PE)

0

-I

I

I

0

30 Ba/Ca i’n” seawater (pmollmol)

cult&

solutions

FIG, 4. Comparison of Ba/Ca of Orbulinauniversashells grown under low-light conditions with Ba/Ca averages of the shells grown at medium and high light. Even though shells grown under low light are inferred to have calcite precipitation rates two to four times lower than those grown at medium to high light, they do not have significantly different Ba/Ca values. The linear regression is the same as in Fig. 2.

Ba*+ in foram shells at 22°C

s-’ (Table 1) . The average weight of the low-light individuals is a factor of two to three times lower than those individuals grown at high light. It has been reported that under low-light conditions dinoflagellate symbiont activity is reduced to 4% of maximum photosynthetic rate and foraminiferal shell mass is also reduced ( SPEROand DENIRO, 1987; SPEROand PARKER, 1985 ). This could indicate that calcite precipitation rates are also lower under these conditions. Figure 4 illustrates Ba/Ca of Orbulina universa shells grown at low light compared to the Ba/Ca averages of the shells grown at medium and high light. Analysis of Ba/Ca in these smaller shells is considerably more difficult because the shells are thinner and more fragile, and therefore they are difficult to clean without intolerable sample loss. However, the values plotted in Fig. 4 suggest that the low-light individuals do not have significantly different Ba/Ca, which qualitatively suggests that biogenic precipitation rates of calcite as they varied in these experiments do not affect the degree of Ba discrimination. CONCLUSIONS

We have sought to verify the incorporation of Ba in the calcite shells of the planktonic foraminifera Orbulina universa by direct culturing experiments at 22°C using a range of Ba equivalent to the range encountered in seawater. We find that under these conditions Ba is incorporated into the shell in proportion to seawater concentration with an effective distribution coefficient between 0.14 and 0.17. This value compares favorably with D = 0.19 f 0.05 found previously by comparing Ba/Ca for five species of fossil planktonic foraminifera from sediment core tops with estimated surface water Ba compositions (LEA and BOYLE, 199 1) . This study confirms the observation made previously that D for planktonic foraminifera is about one half of that for benthic foraminifera (LEA and BOYLE,1989, 199 1) . This difference is presumably due to the influence of temperature, pressure, calcification rate, or some other environmental factor on the uptake of Ba into foraminifera shells. Individuals grown at low-light levels did not exhibit significantly different Ba ratios than those grown at medium to high light. Since individuals grown under low light may calcify more slowly than individuals under high light, these results suggest that calcification rates, at least as they varied in our experiments, do not significantly influence the uptake of Ba into Orbulina universa shells. Acknowledgments-We thank the Catalina Marine Science Center for providing facilities to make this work possible and E. Mochon, S. Haddock, S. Miller, S. Pillings, J. Adkins, and R. Atherton for laboratory and field help. We gratefully acknowledge discussions with E. Boyle, A. Navrotsky, M. Delaney, and J. Erez and thank L. Walter, M. Bender, R. Thunnell, and W. Curry for their reviews. This research was supported by NSF OCE-90 12033 (DWL), LLNL-IGPP #9 I-07 (DWL), NSF OCE-89 I 1827 (HJS), the UCSB Marine Science Institute and the University of California, Santa Barbara. The ICP-MS at UCSB was funded jointly by UC Santa Barbara and the NSF. This is Catalina Marine Science Center contribution # 150. Editorial handling: G. Faure

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