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Determination of carbonate-bound barium in foraminifera and corals by isotope dilution plasma-mass spectrometry
CTzemical Geology, 103 ( 1993 ) 73-84 Elsevier Science Publishers B.V., Amsterdam
73
[AL]
Determination of carbonate-bound barium in foraminifera and corals by isotope dilution plasma-mass spectrometry David W. Lea a and Edward A. Boyleb "Department qf Geological Sciences and Marine Science Institute, Universio' of California. Santa Barbara, CA 93106- 9630, USA bDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building E34-258, Cambridge, MA 02139, USA (Received May 22, 1990; revised and accepted August 4, 1992 )
ABSTRACT Lea, D.W. and Boyle, E.A., 1993. Determination of carbonate-bound barium in foraminifera and corals by isotope dilution plasma-mass spectrometry. Chem. Geol., 103: 73-84. The trace-metal content ofbiogenic carbonates reflects seawater concentrations only if one can selectively determine the portion of metal bound into the Ca lattice site. We have developed a procedure to purify benthic foraminifera shells of extraneous sedimentary, organic and diagenetic Ba. The purification procedure involves treatment similar to those developed for the determination of Cd in foraminifera shells but with the addition of an alkaline-DTPA step to dissolve away sedimentary barite associated with the shells. Paired-sample studies and partial dissolution of benthic foraminifera confirm the effectiveness of this procedure. Ba/Ca is determined on the purified shells by measurement of Ba by isotope dilution inductively coupled plasma-mass spectrometry (1D-ICP-MS) and Ca by flame atomic absorption. Long-term precision of the foraminifera Ba/Ca determinations is between + 2% and _+3%. We have also developed a rapid procedure for I D - I C P - M S determination of Ba in aragonite coral samples with routine precisions of < _+2%.
1. Introduction
Determination of trace metals in calcite foraminifera shells and aragonite coral skeletons has developed into one of the most useful means of probing the chemistry of past oceanic water masses (Graham et al., 1982; Delaney et al., 1985; Boyle, 1988, 1990; Lea and Boyle, 1990b; Shen and Sanford, 4990). Carbonatebound trace metals can be used to decipher changes in deep-ocean circulation patterns and oceanic nutrient geochemistry, to recover histories of anthropogenic metal pollution, and to reconstruct temporal and spatial patterns of the E1 Nifio/Southern Oscillation phenomena. The Correspondence to: D.W. Lea, Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630, USA.
0009-2541/93/$06.00
effectiveness of these paleochemical probes rests in large part on accurate and precise determinations of lattice-bound metals; only the fraction of the metal that is bound in the C a C O 3 structure and presumably substitutes directly for Ca in the carbonate lattice can be directly related to paleo-seawater concentrations (Boyle, 1988; Shen and Sanford, 1990). The Ba content of foraminifera shells and coral aragonite can be used to reconstruct oceanic Ba distributions for paleoceanographic studies (Lea and Boyle, 1989, 1990a, b, 1991; Lea et al., 1989). The Ba content of biogenic carbonates is quite low, ranging from ~ 0.5 to ~ 5/tmol of Ba per mol of Ca ( ~ 1 to 7 p p m ) . To reliably measure these low values of carbonate-bound Ba, an effective cleaning protocol to purify biogenic carbonates of all spurious sedimentary, organic or diagenetic
© 1993 Elsevier Science Publishers B.V. All rights reserved.
74
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY 1CP-MS
Ba is mandatory. In addition, since the range of variation in Ba/Ca ratios encountered in a sample set is typically very small, a precise measurement technique is also necessary. This paper details a purification technique developed for the analysis of Ba/Ca in benthic foraminifera shells. A slightly different technique for the purification of planktonic foraminifera shells with analysis by graphite furnace atomic absorption spectrophotometry is presented elsewhere (Lea and Boyle, 1991). This new study also describes a rapid, precise and accurate analytical technique employing isotope dilution plasma-mass spectrometry for determination of Ba in both foraminifera and corals. 2. Purification of benthic foraminifera shells for Ba/Ca analysis Ba can reside in a number of phases in marine sediments: detrital clays and fine-grained CaCO3 ( 10-1000 ppm Ba), organic matter (50-200 ppm Ba), Mn- and Fe-oxides (1001000 ppm Ba), and barite (60% Ba) (Martin and Knauer, 1973; Fischer and Puchelt, 1978; Church, 1979; Collier, 1981; Bishop, 1988; Fisher et al., 1991). Since these Ba-bearing particles become associated with foraminifera shells in the sediments, extraneous phases must be removed from the foraminiferal shell material before analysis, without dissolution or destruction of the shell itself. Fortunately, purification of benthic foraminifera shells presents less of an obstacle than that required for the shells of planktonic foraminifera (Lea and Boyle, 1991 ): ( 1 ) benthic shells contain on average 3-7 times more lattice-bound Ba than planktonic shells, reducing the ratio of spurious sedimentary Ba to lattice-bound Ba (Lea and Boyle, 1989); and (2) benthic foraminifera shells have less surface area than planktonic shells, thus reducing sites on which oxide coatings can precipitate. Initial cleaning follows methods developed by Boyle (1981) for foraminiferal Cd; fora-
minifera shells are gently crushed between gloss plates and transferred to clean, acid-leached 0.5-ml polyethylene centrifuge vials. Samples are then subject to a series of physical and chemical treatments including: repeated ultrasonication in distilled water and methanol to dislodge detrital grains and fine-grained material; oxidation in hot hydrogen peroxide-sodium hydroxide solution to remove organic matter; and reduction in hot hydrazine-ammonium citrate solution to dissolve ferromanganese oxide coatings described in detail in Boyle (1981) and Boyle and Keigwin (1985/1986)1. A unique problem for Ba analysis is the presence of sedimentary barite (BaSO4) (Church, 1979; Dehairs et al., 1980; Bishop, 1988). Barite contamination in planktonic foraminifera was overcome by treating foraminifera samples with hot alkaline diethylene-triaminepentaacetic acid (DTPA), which dissolves the barite by complexing the Ba into solution (Lea and Boyle, 1991 ). Use of the alkaline-DTPA reagent on the smaller samples typical for benthic foraminifera requires extreme care since calcite dissolves quite readily in the cleaning reagent. 50 #1 of ~ 0.1 M DTPA are added to the vials for samples > 0.5 mg, while 50 #1 of ~ 0.05 M DTPA are used for smaller samples. Sample vials are placed in boiling water for 5 min; during the cleaning time vials are ultrasonicated and turned over every minute. Immediately after the 5 min of treatment 3-5 rinses of 30% ammonium hydroxide are applied to rapidly and completely remove the alkaline-DTPA. 3-5 Water rinses are then required to remove the ammonium hydroxide. Comparisons of benthic foraminifera samples cleaned with and without the alkalineDTPA step reveal that the barite dissolution step does make a significant difference for many samples. The first comparison was made on samples of Uvigerina spp. from eastern equatorial Pacific core TRI63-3IB (04°S, 86°W: 3210 m). This core underlies the equatorial high-productivity belt and has a total Ba
75
D . W . L E A A N D E.A. B O Y L E
content of > 0.1% in the sediment (T. Pedersen, UBC, unpublished data, 1982), suggesting that barite is present in significant amounts (Church, 1979). One set of samples was cleaned for Cd assay [Cd data published in Boyle ( 1988 ) ] without any barite dissolution step. A second set of samples (from a re-sampling of the core) was cleaned with the addition of the barite dissolution step described above. Another difference between these two sample sets is that a variety of Uvigerina with particularly small mass and large surface area was included in the first set of analyses but omitted in the second set. A few analyses of this variety of Uvigerina indicates that it contains elevated levels of Ba, Mn and Cd, probably caused by MnCO3 overgrowths (Boyle, 1983 ). Fig. 1 shows the results obtained for this cleaning comparison over the top 160 cm of TR163-31B, corresponding to the last 20 kyr (Lea and Boyle, 1990b). Fig. 2 is an x-y plot of the same data; since the results are from two different samplings of the core, analyses within less than 2-cm depth interval were paired. The data demonstrate that Ba is generally higher in the samples not subject to the alkaline-DTPA barite dissolution step. Results from a comparison of core-top benthic foraminifera cleaned with and without the barite dissolution step are plotted in Fig. 3. Many samples not subject to the barite dissolution step have higher Ba, although for some samples the addition of the barite dissolution step does not make a significant difference. Variability in barite content from core to core presumably causes variation in the degree of difference made by the alkaline-DTPA step. This result validates the general strategy of cleaning all samples with the complete purification procedure, as there is no simple way to predict what degree of cleaning is required. The major drawback to this strategy is that harsher cleaning increases sample size requirement and risk of sample loss. One uncertainty in the cleaning procedure is that alkaline-DTPA solution, because it has an affinity for both Ba and Ca (Ringbom, 1963 ),
TR163.31B: Cleaning comparison Uvigerina spp. Ba/Ca (p.moVmol) 2.5 0
3.0 •
,
3.5 .
4.0
4.5
5.0
" - ~ ! ..........":;--.'-
5.5
20-
40-
60"
Depth (cm)
8O 100-
120-
;o
140-
160
Fig. 1. Uvigerina spp. B a / C a plotted as a function of depth in core TRI63-31B. Two sets of samples are plotted: the first set, cleaned without alkaline-DTPA treatment to dissolve sedimentary barite, is indicated by closed circles. The second set, cleaned with the addition of the alkaline-DTPA treatment, is indicated by open circles.
5.5 []
4.5 ¸
Ba/Ca (#mol/mol) reductive cleaning
[] D ~
m
3.5 ¸
2.5
i
2.5
u
3.5 4.5 Ba/Ca (p.mol/mol) reductive + DTPA cleaning
5.5
Fig. 2. Core TR163-31B Uvigerina spp. B a / C a data plotted as paired data comparing samples cleaned with reductive cleaning only and samples cleaned with both reductire cleaning and the alkaline-DTPA treatment• Samples that fall above the 1:1 line had higher B a / C a when the alkaline-DTPA step was omitted•
76
DETERMINATION
OF CARBONATE-BOUND
3.5
,
3.0'
[]
[]
2.0'
1 :1
line
.5
w C. wuellerstodi + C. kullenbergi
÷
/
1.5
A N D C O R A L S BY I C P - M S
/ +/~
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BaJCa (l~mol/mol) reductive cleaning 2.5'
Ba IN F O R A M I N I F E R A
•
i
i
i
2.0
2.5
3.0
Uvigerina sop.
3.5
Ba/Ca (lamol/mol) reductive + DTPA cleaning Fig. 3. Paired benthic foraminifera] Ba/Ca data from Atlantic Ocean core-tops comparing samples cleaned with and without the alkaline-DTPA treatment. Samples that fall above the 1:1 line had higher Ba/Ca when the alkaline-DTPA step was omitted.
dissolves both barite and calcite. Therefore, lower values found for samples treated with this procedure could result from removal of calcite layers containing higher B a / C a ratios. Evidence drawn from acid leaches and partial acid dissolution of foraminifera samples suggests that this is not the case. However, a cleaning agent that attacks barite exclusive of calcite would be preferable and would also reduce sample size requirement. Further assessment of the cleaning method was accomplished by performing a series of partial dissolution experiments on large samples of purified benthic shells. In addition, to assessing the efficacy of the cleaning protocol, partial dissolution can reveal heterogeneity in the distribution of Ba in the calcite shells. Benthic foraminifera were picked from depth intervals where a single species was relatively abundant; these included C. wuellerstorfi from the Norwegian Sea (V27-60), Uvigerina spp. from the northwestern Atlantic (CHN82I / P C ), and Oridisalis spp. from the equatorial Pacific (TR163-31B). Initial sample weights were 2-4 mg, comprising 50-200 individuals. The samples were first cleaned with the procedure described above. After the final cleaning
step, 100-#1 aliquots of 0.072 N HNO3 were added to the sample followed by ultrasonication for 30-60 s. This was sufficient time for dissolution to take place but a short enough time that pH remained acidic ( < 2.5 ). A 100#1 aliquot was then removed from the sample and transferred to a clean vial. This procedure was repeated 2 to 4 times, depending on the initial size of the sample. The Ba/Ca ratio of each dissolution fraction is plotted in Fig. 4 vs. percent of the total sample dissolved (to that point), calculated from the Ca content of each leachate (analytical methods described on pp. 77-80). The data are relatively uniform, with variability only slightly exceeding analytical reproducibility. The fractions within each dissolution experiment were reproducible to ~ ___5% (1 standard deviation) (SD) compared to analytical reproducibility of _+3% for consistency standards (see pp. 79-80). This difference might indicate some limit to foraminifera as recorders of seawater Ba, or alternatively it might reflect heterogeneity among the shells that were used for the partial dissolution. This _+5% error is a measure of the m i n i m u m variability in Ba/Ca
77
D . W . L E A A N D E.A. B O Y L E
Tr163-31B: 118 Oridorsalis spp.
cm
V27-60:10
3.5-
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.~
2.5
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-
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Fraction
C H N 8 2 . 1 1 P C : 125 Uvigewina sl~.
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•
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Fig. 4. Partial dissolution of three species of benthic foraminifera (experimental procedure described in text). Abscissa error bars indicate the fraction of sample dissolved for that dissolution fraction• Ordinate error bars indicate the estimated I SD error of the Ba/Ca determinations. The second leachate from the T R 1 6 3 - 3 1 B experiment was lost.
of a typical sample pick from a given depth interval in a core. 3. Analysis of benthic foraminiferal Ba/Ca ratios Upon completion of the alkaline-DTPA cleaning step, samples are transferred to acidcleaned 0.5-ml centrifuge vials and then subjected to a series of final acid leaches ( 1-5 × ) in 0.001 N HNO3 to remove any remaining surface contamination (all acids are triple-dis-
tilled in Vycor ® with no measurable Ba blank). These acid leaches also results in more uniform sample sizes since larger samples can be leached several times. Acid is removed via several water rinses, and a pipet is used to remove the last portion of water. At this stage each vial consists of the purified shell material and ~ 5 /d of distilled water. Each sample is dissolved in 100/tl of 0.072 N HNO3, with dissolution encouraged by ultrasonication. Samples are checked visually for complete dissolution, and extra acid is added where needed. A tiny per-
78
DETERMINATION OF CARBONATE-BOUND Ba 1N FORAMINIFERA AND CORALS BY ICP-MS
tion ( 1-2 pl) of each sample is used to check pH on 0-2.5 pH paper. This serves two purposes: ( 1 ) it insures complete dissolution of the samples since calcite that does not dissolve will buffer the pH to high values; and (2) by titrating all samples to a narrow pH range uniform Ca concentrations are ensured. For example, since all samples are dissolved in 72 meq 1-1 acid, a final Ca concentration of 11 m m o l 1- l is achieved if the pH of the samples is titrated to 1.3. [A pH of 1.3 is equivalent to an acid concentration of 50 meq 1-1, and since initial acid concentration was 72 meq 1-1, the difference (22 meq 1-1 of acid) has been consumed by reaction with 11 m m o l 1- 1 or 22 meq 1- 1 of CaCO3. ] After dissolution 100 pl of each sample are transferred to an acid-leached, dry vial. Samples are spiked with 1 or 2 100-/A aliquots of 3 5 B a _ e n r i c h e d spike for determination of total Ba by isotope dilution mass spectrometry (GEOSECS spike S-6; Chan et al., 1977). The ratio of spike to sample chosen depends on the analyst's judgment of the sample size and the expected Ba/Ca ratio. Those samples that require only 100/d of spike solution receive an additional 100/d of 0.072 N HNO3 acid, so that all samples have a final volume of 300 pl. Generally, larger samples (which are titrated to uniform Ca concentration) receive 1 spike aliquot for Atlantic samples ( B a / C a ~ 2-3 pmol mol -~ ) and 2 spike aliquots for Pacific samples (Ba/Ca ~ 4-5/zmol m o l - l ). Small samples always received 1 spike aliquot. Using this method one can routinely achieve 135Ba/138Ba ratios within a factor of 2 of the target ratio, which is 1.55 (Webster, 1960) (Table 1). At this stage one-fourth of the sample (75 /A ) is removed for Ca analysis. These 75 #1 are diluted with 5 ml of La solution (0.4 mg g - l La in a solution of 0.05 N HCI and 0.002 N HNO3 ) to eliminate suppression of the Ca signal due to phosphine in the acetylene. These solutions are then quantified for Ca by comparison to standards on a Perkin-Elmer ® 403 flame atomic absorption spectrophotometer.
TABLE 1 Ba isotopic ratios of t35Ba-enriched spike and natural Ba 135Ba/138Ba Spike S-6-STK (6th GEOSECS Spike, 11/76): 93.6% 135Ba Natural Ba: 6.592% ~3~Ba Target ratio ( = ,,..':0.09194 × 26.29 )
26.29 0.09194 1.55
The remaining 225 ~tl are used for quantification of Ba. A 200-pl flow injection loop is used to inject the sample into a 0.85-ml-mineluant flow (also 0.08 N HNO3). 135Ba/138Ba ratios are determined on a VG ® PlasmaQuad inductively coupled plasma mass spectrometer ( I C P - M S ) . The I C P - M S is run under the standard conditions suggested by VG ® (Gray, 1989), with nebulization accomplished via a Meinhardt* glass nebulizer. Each injection is scanned 360 times from mass 135 to 138 with a 100-/ts dwell time on each of 512 channels (total scan time -- 18 s). Optimal samples ( ~ 10 nmol 1-1 Ba) yield ~ 2 5 0 0 total counts per peak for a relative error (1 SD) of + 2% based on counting statistics. A typical blank yields 20-30 total counts, so a signal to noise ratio of > 100 is usual for most samples, with even the smallest samples achieving signal to noise ratios of better than 50. Samples with very low Ca contents ( < 1 m M Ca in the final 300/~1) are regarded as questionable and are always re-determined if sufficient foraminifera remain. To ensure overall accuracy and reproducibility between runs the 135Ba-enriched spike is repeatedly calibrated to a gravimetric standard. This eliminates the requirement for direct calibration of mass bias inherent in the PlasmaQuad ® system (Klinkhammer and Chan, 1990). This calibration is repeated at least 4 times per run to check for possible machine drift. The 135Ba/138Ba ratio of these spiked gravimetric standards (SGS) are always reproducible to better than +_ 1% over the
D.W. LEA A N D E.A. BOYLE
course of a run. The gravimetric standard used for the calibration was made from reagent grade BaC12-2H20. A second standard was made from SPEX ® ultra pure assayed BaCO3. The Ba concentration of these two standards was intercalibrated by the isotope dilution method; within a +_0.5% analytical error there was no significant difference between the two standards. The Ba concentration of each sample is calculated from the following formula (Chan et al., 1977): 0.0356 [ Ba ]sample-- ~--.~ ~ [ Ba ]spike { spike ~ f R m i x - 2 6 . 2 9 "~ ~sample)volume~0.09194-- Rmix) where Rmi x is the 135Ba/138Ba ratio found for the mixture of sample and spike; 0.0356 is the fraction ~3SBa in the spike (Oak Ridge Nation Laboratories value); 0.717 is the fraction ~38Ba in natural Ba; 26.29 is the ratio of ~35Ba/~38Ba in the spike ( O R N L value ); and 0.09194 is the natural 135Ba/138Ba ratio (Table 1). This expression is evaluated for both the unknown sample and the known standard-spike mixture (SGS). The sample is then adjusted by the deviation of the calculated concentration of the standard from the true value (Webster, 1960). This correction is generally of order 1-2 % and has never been more than 3.5%. Two consistency standards analyzed three times in each run are used as a final check on the precision of the method. These consistency standards were made by adding known amounts of Ba to ion-exchanged Ca (NO 3 ) 2 SOlutions. Ion exchange was used to remove Ba associated with the reagent grade calcite used to make up the Ca(NO3)2 solutions. The concentration and statistical reproducibility of these solutions is given in Table 2. The B a / C a ratio of CN2, the consistency standard with the lower Ba content, is reproducible to ~ 2 3% over 89 analyses. The reproducibility of these consistency
79
standards is one measure of the ultimate precision of the method. However, the analytical precision of real samples might be less favorable. While the Ba content of consistency standards is always known and therefore spiked to an o p t i m u m 135Ba/138Ba ratio, there is uncertainty associated with correctly spiking real samples, although this uncertainty is reduced by maintaining relatively uniform Ca concentrations. To ascertain how much of a problem this effect might be, Ba was determined in a solution spiked with different ratios of spike to standard to yield a range of 135Ba/138Ba ratios (Table 3). Sufficient counts were collected to yield counting statistic of ~ z 0.5%. The range of ~35Ba/'38Ba ratios in this test (0.7-4.8) encompasses the range of ratios obtained over 99% of the time for spiked foraminifera sampies. In this experiment Ba concentrations were reproducible to + 1.5%, with a possible trend towards higher Ba values at higher 135Ba/138Ba ratios. Over 90% of the spiked foraminifera samples analyzed at MIT had 135Ba/138Ba ratios between 0.7 and 3.2; the experimental data in Table 3 indicate that over this range Ba concentrations are reproducible to +1.1%. The spiking error associated with the range of isotope ratios used in this study is clearly small relative to the overall analytical precision. To ascertain if variability in the Ca contents of the injected solutions has a (matrix) effect on isotope ratio determination, Ba was determined in a series of solutions with similar Ba contents but differing Ca contents (Table 4). There was no significant bias on the measured Ba content with differing Ca contents. The average deviation between measured Ba and expected Ba was < _+ 1%. A factor that will clearly degrade how well we determine Ba/Ca in foraminifera samples is reduced count totals for small samples. The mean Ba content of all foraminifera samples analyzed at MIT was ~ 12 nmol 1- l, 20% more Ba than is in the consistency standard that yielded a long-term precision of + 3% (CN2). About 8% of the foraminifera samples had Ba
80
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS
TABLE 2
Statistics from foraminifera consistency standards CN2 and C N 3 Consistency standard C N 2
Mean SD RSD (%)
Number
Consistency standard C N 3
Ba/Ca ( ~ m o l m o l -~ )
Ba ( n m o l l -~ )
Ca ( m m o l l -~ )
Ba/Ca (~molmol -l )
Ba (nmoll
2.36 0.07 3.1 89
28.86 0.75 2.6 96
12.24 0.21 1.7 98
3.12 0.08 2.6 96
58.28 t.30 2.2 104
Ca
1)
(retool 1-1 ) 18.68 0.29 1.6 98
Concentrations were determined on 100-/A aliquots of each consistency standard. SD = standard deviation; R S D = relative standard deviation. TABLE 3 Ba determinations as a function of the 13SBa/138Ba ratio of spiked solutions ID
135Ba/138Ba ratio of spiked solution
Calculated Ba concentration (nmol 1-t )
1 2 3 4 5 6 7 8
0.67 1.21 2.23 2.21 3.21 4.01 4.43 4.77
203 206 206 208 203 209 208 211
Mean
207 +_3
TABLE 4 Ba determination in solutions of varying Ca concentration Ca ( m m o l 1--~ )
Ba a d d e d (/tmol 1-~ )
Ba found (/tmol 1-~ )
Deviation
0 1 2 3 4 5
2.00 2.02 2.04 2.06 2.08 2.10
1.99 2.01 2.01 2.04 2.06 2.08
-0.6 -0.8 - 1.7 -0.6 - 1.0 - 1.1
(%)
Actual Ba concentrations of aspirated solutions after dilution were 2 0 - 2 5 nM.
contents less than one-quarter that of CN2; counting statistics would suggest that these smallest samples have precision a factor of 2 poorer than CN2. Therefore, a rough estimate
of the reproducibility of the least favorable 8% of determinations is a relative error of + 6% ( 1 SD). Most small samples have Ca contents < 3 m M and therefore are replicated if sufficient foraminifera remain. Reproducibility of splits of benthic foraminifera from the same depth in a core are seldom as good as the analytical precision (Lea and Boyle, 1989, 1990a, b). The pooled standard deviation of replicates was + 7-10% for three cores for which sufficient replicates were determined. The decrease in precision associated with real samples is presumably related to bioturbation which mixes together individuals that lived at different times when Ba contents might have been different (Boyle, 1984; Boyle and Rosener, 1990). However, some portion of this variability might be due to imperfect cleaning and/or the imperfect nature of foraminifera as chemical recorders of bottom water Ba. 4. Determination of Ba in coral aragonite The requirements for cleaning and purification of coral aragonite for Ba analysis are not as great as that required for foraminifera shells, since corals are generally not contaminated by sedimentary Ba. Specimens of the coral Pavona clavus from the Gal~ipagos Islands, eastern equatorial Pacific (age 1950-1982) cleaned by the extensive purification procedure developed for Pb and Cd analysis (Shen
D.W. LEA AND E.A. BOYLE
81
and Boyle, 1988) do not have significantly lower Ba than the same samples cleaned with an abbreviated version, of the cleaning procedure (Table 5 ). This "short" cleaning procedure includes ultrasonication in distilled water, methanol and 0.015 N HNO3 followed by several treatments with a boiling solution of 15% HzO2 in 0.2 N NaOH to oxidize residual coral tissue and organic matter. Samples are then leached several times in 0.1 N HNO3. In selected environments and for older, subaerially exposed corals, more extensive cleaning procedures might be warranted. The oxidation step to remove all organic matter in the coral skeleton is especially critical. Coral polyp tissue is enriched in Ba over skeletal material by ~ 10-200 times by weight (Flor and Moore, 1977; Buddemeier et al., 1981 ). Since total organic matter makes up
~ 1% by weight of the skeleton, coral samples not cleaned of organic matter will have organic-Ba contributions equal to 10-200% of skeletal Ba. In a recent study of freshly collected specimens of Acropora palmata, Pingitore et al. (1989) attributed elevated and highly variable skeletal Ba/Ca to a "species effect". However, their results might have been biased by the lack of an oxidative step to remove organic matter from the fresh samples, since samples of "recent-dead" A. palrnata from the same study had significantly lower Ba/Ca with far smaller intraspecific variability. The analytical method employed for determination of Ba/Ca in aragonite skeletons of corals is similar to that employed for foraminifera (see pp. 77-80). Ba is determined by isotope dilution plasma-mass spectrometry and Ca by flame atomic absorption spectro-
TABLE5 Ba/Ca determinations and averages for coral growth bands ofPavona clavus from Punta Pitt, Gal~ipagos Islands, 1950-1959 Coral year-section
950-2 951-1 951-1 951-2" 1952-1 952-2 953-• 953-2* 954-•* 954-2 955-•* 955-2 1956-1" 1956-2" 1957-1 1957-2" 1958-1 1958-I* 1958-2 1959-1"
Date of analysis ( 1988 ) Mar. 16
Mar. 25
4.58 4.26
4.56 4.05
4.28 4.49 4.58 4.62
4.21 4.70 4.51 4.52
Apr. 28
May l
4.69
4.56
4.27 4.44 4.55 4.71
4.35
4.50 4.42
4.65 4.32 4.55
4.83 4.62 4.43 4.64 4.60 4.28
4.71
4.81
4.62
4.67
5.38 4.85 4.63
4.64 4.87 4.28
4.35
4.22 4.28 4.52 4.28
SD
RSD (%)
n
4.57 4.19
0.01 0.10
0.3 2.5
3 4
4.32 4.56 4.55 4.63 4.39 4.49 5.14 4.78 4.70 4.57 4.64 4.72 4.30 4.26
0.10 0.12 0.04 0.08 0.10 0.07 0.23 0.06 0.13 0.13 0.01 0.14 0.04 0.03
2.3 2.7 0.8 1.7 2.2 1.6 4.4 1.3 2.8 2.8 0.1 2.9 0.9 0.6
4 3 3 4 3 2 3 3 3 3 3 3 3 4
4.59 4.36
0.07 0.09
1.4 2.0
3 3
Jun. 13
4.35 4.49
5.12 4.62
4.26
May 22
4.56 4.16
4.93
May 19
Mean
4.27 4.61 4.45
4.34
Key: SD=standard deviation; RSD=relative standard deviation; n = n u m b e r of analyses. Notes: (1) " - I " indicates 1st half of coral growth band; "-2" indicates 2nd half of coral growth band; (2) two entries occur for 1951-I and 1958-1 because two different coral pieces from the same band were analyzed; (3) one value each from 54-1, 55-2, 57-2 and 59-1 was rejected due to a mixing problem for Ca. *Cleaned with the abbreviated purification procedure described in the text.
82
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS 42
5.4
/
-1
5.2
/
5.0
0
/-.*, /
SST Anomaly
/
/l. '4.8 Ba/Ca (l~mol/mol)
(°C) ' 4.6
1 |
• 4.4 2 ' 4.2
3 50
&
4.0
!
Year
(19-) ...... ~ . . . .
a
S S T anomaly
~Ca
Fig. 5. Plot of Ba/Ca determined on sub-divided annual growth bands of the coral Pavona clavus from Punta Pitt (San Christ6bal Island, Galapagos Islands). Error bars indicate 1SD errors of the Ba/Ca measurements (Table 1 ). The Ba/Ca data are compared with sea surface temperature anomalies (E. Rasmusson, Climate Analysis Center, pers. commun., 1986 ). Covariance indicates the association of high Ba levels with cooler sea surface temperatures, as indicates by negative temperature anomalies (Lea et al., 1989). photometry. Because corals are more massive than foraminifera larger samples are generally available for analysis. Therefore, flow injection for Ba analysis is not necessary, although it can be used in cases where sample is limited (i.e. micro-sampling). Coralline Ba is relatively uniform [ see Table 5, Shen and Boyle (1988) and Lea et al. ( 1 9 8 9 ) ] and therefore coral solutions can be spiked quite readily to an o p t i m u m 135Ba/138Ba ratio; for this study the ratio employed was generally between 0.9 and 1.5, which is optim u m for the I C P - M S ( K l i n k h a m m e r and Chan, 1990). Counting times and dilutions were adjusted to yield total counts > 10,000 for each Ba peak. For coral samples, where signal
to noise on the I C P - M S is always greater than 100, the reproducibility of the ratios is very close to that expected from counting statistics. Therefore, 10,000 total counts on each peak generally yields precision on the ratios near + 1% ( 1 R S D ) (relative standard deviation ). Typical Ba concentrations of spiked coral solutions introduced into the I C P - M S are between 15 and 45 nmol 1-1, equivalent for corals to 3-10 mmol 1-1 of Ca or 0.3-1 mg of coral aragonite per ml of solution. Typical volumes of solutions are 0.8-3 ml, with counting times of 1 to 3 min. Reproducibility of coral B a / C a ratios based on replicates of the same coral solutions run on different days is ~ _ 2% ( 1 R S D ) . Data from
83
D.W. LEA AND E.A. BOYLE
a P a v o n a clavus time series ( 1950-1960) from
Punta Pitt in the Gal~ipagos Islands for which replicates were run over 5 separate runs are detailed in Table 5 and plotted with error bars in Fig. 5. Samples were prepared by band-sawing individual growth bands into two equal halves details in Shen and Boyle (1988) ]. Similarity between the coral Ba record and the historical record of sea surface temperature anomalies is due to temporal variability in the upwelling of deeper, colder waters enriched in Ba (Lea et al., 1989). A more detailed coral Ba record was obtained from the same coral over the time period 1965-1978; these data were acquired over a single day's run, and based on consistency standards had an inter-run reproducibility of _+ 1.0% for Ba/Ca (Lea et al., 1989). 5. Conclusions
A procedure has been developed to purify benthic foraminifera shells from deep-sea cores of extraneous Ba not bound by carbonate. Ba/Ca ratios of purified shells are measured by a rapid and precise technique using isotope dilution plasma-mass spectrometry. A modification of this technique has been applied to the determination of Ba/Ca in sub-annual bands of massive reef corals. Acknowledgments
We thank Glen Shen, Ted McConnaughey, Ed Sholkovitz and Lloyd Keigwin for generously providing samples for this work. Comments and suggestions from Lex van Geen, Paula Rosener, Kristin Orians, Debra Colodner, Kelly Kenison Falkner and Gary Klinkhammer aided in the development of this study. We thank John Edmond for access to the MIT PlasmaQuad ® and use of the GEOSECS barium spike. Help and advice from members of the MIT PlasmaQuad ® squad was greatly appreciated. This research was supported by NSF grant OCE8710168 (to E.A.B.) and a Joint Oceanographic Institutions/Ocean
Drilling Program Fellowship to the senior author for 1987/1988. Continuing development of these methods by the senior author at UCSB was supported by OCE-9012033 and the University of California. Appendix - - Recent improvements in Ba analysis of foraminifera Since the time this manuscript was submitted a number of changes have been applied to the analytical methodology described herein. Some of these changes are documented in Lea and Spero (1992). The most substantial changes are for the analysis of benthic foraminifera and are as follows: ( 1 ) To prevent intolerable sample loss during the barite dissolution step, 50/11 of 0.002 MDTPA in 0.1 NNaOH are used for each sample for 10 min in a boiling water bath, ultrasonicating every 2 rain. (2) Samples are subject to only one final 0.001 N nitric acid leach ( 100/A per sample). (3) To eliminate the requirement for flow injection, dissolved foraminifera samples are delivered to the plasma as an isolated sample plug, resulting in easier analysis and improved signal to noise ratios. Careful timing is required so that acquisition times match the residence time ,~f the sample plug in the plasma. (4) Peristaltic flow rates are reduced to as little as 0.5 ml rain- ~, yielding acquisition times for a 0.25-ml "plug" of 20-25 s. (5) An increase in total counts and count rates (and hence improved precision) is achieved by "peak hopping" on the ~3SBaand ~38Bapeaks only, thus eliminating scan time spent in the valleys between peaks and on the masses not required for quantification. The newest generation of plasma mass spectrometers have sensitivities that are about ten times greater than the first-generation machines. (The VG ® PQ2+ Turbo installed at UCSB routinely achieves 20 kHz for a 1 ppb l~Sln solution.) By combining improved ICP-MS sensitivity with the changes described above it is now possible to analyze Ba in single shells of planktonic foraminifera (Lea and Spero, 1992). However, purification of single shells recovered from marine sediments remains a formidable obstacle.
References Bishop, J.K.B., 1988. The barite-opal-organic carbon association in oceanic particulate matter. Nature (London), 332: 341-343. Boyle, E.A., 1981. Cadmium, zinc, copper, and barium in foraminifera tests. Earth Planet. Sci. Lett., 53:11-35. Boyle, E.A., 1983. Manganese carbonate overgrowths on
84
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS
foraminifera tests. Geochim. Cosmochim. Acta, 47: 1815-1819. Boyle, E.A., 1984. Sampling statistic limitations on benthic foraminifera chemical and isotopic data. Mar. Geol., 58: 213-224. Boyle, E.A., 1988. Cadmium: chemical tracer of deepwalet paleoceanography. Paleoceanography, 3:471-489. Boyle, E.A., 1990. Quaternary deepwater paleoceanography. Science, 249: 863-870. 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. Boyle, E.A. and Rosener, P., 1990. Further evidence for a link between Late Pleistocene North Atlantic surface temperatures and North Atlantic Deep-Water production. Palaeogeogr., Palaeoclimatol., Palaeoecol., 89: 113-124. Buddemeier, R.W., Schneider, R.C. and Smith, S.V., 1981. The alkaline earth chemistry of corals. Proc. 4th Int. Coral Reef Symp., Manila, 2:81-85. Chan, L.H., Drummond, D., Edmond, J.M. and Grant, B., 1977. On the barium data from the Atlantic GEOSECS Expedition. Deep-Sea Res., 24:613-649. Church, T.M., 1979. Marine barite. In: R.G. Burns (Editors), Marine Minerals. Mineral. Soc. Am., Rev. Mineral., 6: 175-209. Collier, R.W., 1981. The trace element geochemistry of marine biogenic particulate matter. Ph.D. Thesis, Mass. Inst. Technol.-Woods Hole Oceanogr. Inst., Woods Hole, Mass., 299 pp. Dehairs, F., Chesselet, R. and Jedwab, J., 1980. Discrete suspended particles of barite and the barium cycle in the open ocean. Earth Planet. Sci. Lett., 49: 528-550. Delaney, M.L., B6, A.W.H. and Boyle, E.A., 1985. Li, Sr, Mg, and Na in foraminiferal calcite shells from laboratory culture, sediment traps, and sediment cores. Geochim. Cosmochim. Acta, 49: 1327-1341. Fischer, K. and Puchelt, H., 1978. Barium. In: K.H. Wedepohl (Editor), Handbook of Geochemistry, V.II/4. Springer, Berlin, pp. A I - N 1. Fisher, N.S., Guillard, R.R.L. and Bankston, D.C., 1991. The accumulation of barium by marine phytoplankton grown in culture. J. Mar. Res., 49: 339-354. Flor, T.H. and Moore, W.S., 1977. Radium/calcium and uranium/calcium determinations for Western Atlantic reef corals. Proc. 3rd Int. Coral ReefSymp., Miami, Fla., 2: 555-561. Graham, D.W., Bender, M.L., Williams, D.F. and Keig-
win, L.D., 1982. Strontium-calcium ratios in Cenozoic planktonic foraminifera. Geochim. Cosmochim. Acta, 46: 1281-1292. Gray, A.L., 1989. The origins, realization and performance of ICP-MS systems. In: A.R. Date and A.L. Gray (Editors), Inductively Coupled Plasma Mass Spectrometry. Blackie, Glasgow, pp. 1-42. Klinkhammer, G.P. and L.H. Chan, 1990. Determination of barium in marine waters by isotope dilution inductively coupled plasma mass spectrometry. Anal. Chim. Acta, 232: 323-329. Lea, D.W. and Boyle, E.A., 1989. Barium content of benthic foraminifera controlled by bottom water composition. Nature (London), 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 (London), 347: 269-272. Lea, D.W. and Boyle, E.A., 1990b. Foraminiferal reconstruction of barium distributions in water masses of the glacial oceans. Paleoceanography, 5:719-742. Lea, D.W. and Boyle, E.A., 1991. Barium in planktonic foraminifera. Geochim. Cosmochim. Acta, 55: 33213331. 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., Shen, G.T. and Boyle, E.A., 1989. Coralline barium records temporal variability in equatorial Pacific upwelling. Nature (London), 340:373-376. Martin, J.H. and Knauer, G.A., 1973. The elemental composition of plankton. Geochim. Cosmochim. Acta, 37: 1639-1653. Pingitore, N.E.J., Rangal, Y. and Kwarteng, A., 1989. Barium variation in Acropora palmata and Montastrea annularis. Coral Reefs, 8:31-36. Ringbom, A., 1963. Complexation in Analytical Chemistry. Interscience, New York, N.Y., 361 pp. Shen, G.T. and Boyle, E.A., 1988. Determination of lead, cadmium and other trace metals in anually-banded corals. Chem. Geol., 67: 47-62. Shen, G.T. and Sanford, C.L., 1990. Trace element indicators of climate variability in reef-building corals. In: P.W. Glynn (Editor), Global Ecological Consequences of the 1982-1983 E1 Nifio-Southern Oscillation. Elsevier, Amsterdam, pp. 255-283. Webster, R.K., 1960. Mass spectrometric isotope dilution analysis. In: A.A. Smales and L.R. Wagner (Editors), Methods in Geochemistry. Interscience, New York, N.Y., pp. 202-246.
73
[AL]
Determination of carbonate-bound barium in foraminifera and corals by isotope dilution plasma-mass spectrometry David W. Lea a and Edward A. Boyleb "Department qf Geological Sciences and Marine Science Institute, Universio' of California. Santa Barbara, CA 93106- 9630, USA bDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building E34-258, Cambridge, MA 02139, USA (Received May 22, 1990; revised and accepted August 4, 1992 )
ABSTRACT Lea, D.W. and Boyle, E.A., 1993. Determination of carbonate-bound barium in foraminifera and corals by isotope dilution plasma-mass spectrometry. Chem. Geol., 103: 73-84. The trace-metal content ofbiogenic carbonates reflects seawater concentrations only if one can selectively determine the portion of metal bound into the Ca lattice site. We have developed a procedure to purify benthic foraminifera shells of extraneous sedimentary, organic and diagenetic Ba. The purification procedure involves treatment similar to those developed for the determination of Cd in foraminifera shells but with the addition of an alkaline-DTPA step to dissolve away sedimentary barite associated with the shells. Paired-sample studies and partial dissolution of benthic foraminifera confirm the effectiveness of this procedure. Ba/Ca is determined on the purified shells by measurement of Ba by isotope dilution inductively coupled plasma-mass spectrometry (1D-ICP-MS) and Ca by flame atomic absorption. Long-term precision of the foraminifera Ba/Ca determinations is between + 2% and _+3%. We have also developed a rapid procedure for I D - I C P - M S determination of Ba in aragonite coral samples with routine precisions of < _+2%.
1. Introduction
Determination of trace metals in calcite foraminifera shells and aragonite coral skeletons has developed into one of the most useful means of probing the chemistry of past oceanic water masses (Graham et al., 1982; Delaney et al., 1985; Boyle, 1988, 1990; Lea and Boyle, 1990b; Shen and Sanford, 4990). Carbonatebound trace metals can be used to decipher changes in deep-ocean circulation patterns and oceanic nutrient geochemistry, to recover histories of anthropogenic metal pollution, and to reconstruct temporal and spatial patterns of the E1 Nifio/Southern Oscillation phenomena. The Correspondence to: D.W. Lea, Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630, USA.
0009-2541/93/$06.00
effectiveness of these paleochemical probes rests in large part on accurate and precise determinations of lattice-bound metals; only the fraction of the metal that is bound in the C a C O 3 structure and presumably substitutes directly for Ca in the carbonate lattice can be directly related to paleo-seawater concentrations (Boyle, 1988; Shen and Sanford, 1990). The Ba content of foraminifera shells and coral aragonite can be used to reconstruct oceanic Ba distributions for paleoceanographic studies (Lea and Boyle, 1989, 1990a, b, 1991; Lea et al., 1989). The Ba content of biogenic carbonates is quite low, ranging from ~ 0.5 to ~ 5/tmol of Ba per mol of Ca ( ~ 1 to 7 p p m ) . To reliably measure these low values of carbonate-bound Ba, an effective cleaning protocol to purify biogenic carbonates of all spurious sedimentary, organic or diagenetic
© 1993 Elsevier Science Publishers B.V. All rights reserved.
74
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY 1CP-MS
Ba is mandatory. In addition, since the range of variation in Ba/Ca ratios encountered in a sample set is typically very small, a precise measurement technique is also necessary. This paper details a purification technique developed for the analysis of Ba/Ca in benthic foraminifera shells. A slightly different technique for the purification of planktonic foraminifera shells with analysis by graphite furnace atomic absorption spectrophotometry is presented elsewhere (Lea and Boyle, 1991). This new study also describes a rapid, precise and accurate analytical technique employing isotope dilution plasma-mass spectrometry for determination of Ba in both foraminifera and corals. 2. Purification of benthic foraminifera shells for Ba/Ca analysis Ba can reside in a number of phases in marine sediments: detrital clays and fine-grained CaCO3 ( 10-1000 ppm Ba), organic matter (50-200 ppm Ba), Mn- and Fe-oxides (1001000 ppm Ba), and barite (60% Ba) (Martin and Knauer, 1973; Fischer and Puchelt, 1978; Church, 1979; Collier, 1981; Bishop, 1988; Fisher et al., 1991). Since these Ba-bearing particles become associated with foraminifera shells in the sediments, extraneous phases must be removed from the foraminiferal shell material before analysis, without dissolution or destruction of the shell itself. Fortunately, purification of benthic foraminifera shells presents less of an obstacle than that required for the shells of planktonic foraminifera (Lea and Boyle, 1991 ): ( 1 ) benthic shells contain on average 3-7 times more lattice-bound Ba than planktonic shells, reducing the ratio of spurious sedimentary Ba to lattice-bound Ba (Lea and Boyle, 1989); and (2) benthic foraminifera shells have less surface area than planktonic shells, thus reducing sites on which oxide coatings can precipitate. Initial cleaning follows methods developed by Boyle (1981) for foraminiferal Cd; fora-
minifera shells are gently crushed between gloss plates and transferred to clean, acid-leached 0.5-ml polyethylene centrifuge vials. Samples are then subject to a series of physical and chemical treatments including: repeated ultrasonication in distilled water and methanol to dislodge detrital grains and fine-grained material; oxidation in hot hydrogen peroxide-sodium hydroxide solution to remove organic matter; and reduction in hot hydrazine-ammonium citrate solution to dissolve ferromanganese oxide coatings described in detail in Boyle (1981) and Boyle and Keigwin (1985/1986)1. A unique problem for Ba analysis is the presence of sedimentary barite (BaSO4) (Church, 1979; Dehairs et al., 1980; Bishop, 1988). Barite contamination in planktonic foraminifera was overcome by treating foraminifera samples with hot alkaline diethylene-triaminepentaacetic acid (DTPA), which dissolves the barite by complexing the Ba into solution (Lea and Boyle, 1991 ). Use of the alkaline-DTPA reagent on the smaller samples typical for benthic foraminifera requires extreme care since calcite dissolves quite readily in the cleaning reagent. 50 #1 of ~ 0.1 M DTPA are added to the vials for samples > 0.5 mg, while 50 #1 of ~ 0.05 M DTPA are used for smaller samples. Sample vials are placed in boiling water for 5 min; during the cleaning time vials are ultrasonicated and turned over every minute. Immediately after the 5 min of treatment 3-5 rinses of 30% ammonium hydroxide are applied to rapidly and completely remove the alkaline-DTPA. 3-5 Water rinses are then required to remove the ammonium hydroxide. Comparisons of benthic foraminifera samples cleaned with and without the alkalineDTPA step reveal that the barite dissolution step does make a significant difference for many samples. The first comparison was made on samples of Uvigerina spp. from eastern equatorial Pacific core TRI63-3IB (04°S, 86°W: 3210 m). This core underlies the equatorial high-productivity belt and has a total Ba
75
D . W . L E A A N D E.A. B O Y L E
content of > 0.1% in the sediment (T. Pedersen, UBC, unpublished data, 1982), suggesting that barite is present in significant amounts (Church, 1979). One set of samples was cleaned for Cd assay [Cd data published in Boyle ( 1988 ) ] without any barite dissolution step. A second set of samples (from a re-sampling of the core) was cleaned with the addition of the barite dissolution step described above. Another difference between these two sample sets is that a variety of Uvigerina with particularly small mass and large surface area was included in the first set of analyses but omitted in the second set. A few analyses of this variety of Uvigerina indicates that it contains elevated levels of Ba, Mn and Cd, probably caused by MnCO3 overgrowths (Boyle, 1983 ). Fig. 1 shows the results obtained for this cleaning comparison over the top 160 cm of TR163-31B, corresponding to the last 20 kyr (Lea and Boyle, 1990b). Fig. 2 is an x-y plot of the same data; since the results are from two different samplings of the core, analyses within less than 2-cm depth interval were paired. The data demonstrate that Ba is generally higher in the samples not subject to the alkaline-DTPA barite dissolution step. Results from a comparison of core-top benthic foraminifera cleaned with and without the barite dissolution step are plotted in Fig. 3. Many samples not subject to the barite dissolution step have higher Ba, although for some samples the addition of the barite dissolution step does not make a significant difference. Variability in barite content from core to core presumably causes variation in the degree of difference made by the alkaline-DTPA step. This result validates the general strategy of cleaning all samples with the complete purification procedure, as there is no simple way to predict what degree of cleaning is required. The major drawback to this strategy is that harsher cleaning increases sample size requirement and risk of sample loss. One uncertainty in the cleaning procedure is that alkaline-DTPA solution, because it has an affinity for both Ba and Ca (Ringbom, 1963 ),
TR163.31B: Cleaning comparison Uvigerina spp. Ba/Ca (p.moVmol) 2.5 0
3.0 •
,
3.5 .
4.0
4.5
5.0
" - ~ ! ..........":;--.'-
5.5
20-
40-
60"
Depth (cm)
8O 100-
120-
;o
140-
160
Fig. 1. Uvigerina spp. B a / C a plotted as a function of depth in core TRI63-31B. Two sets of samples are plotted: the first set, cleaned without alkaline-DTPA treatment to dissolve sedimentary barite, is indicated by closed circles. The second set, cleaned with the addition of the alkaline-DTPA treatment, is indicated by open circles.
5.5 []
4.5 ¸
Ba/Ca (#mol/mol) reductive cleaning
[] D ~
m
3.5 ¸
2.5
i
2.5
u
3.5 4.5 Ba/Ca (p.mol/mol) reductive + DTPA cleaning
5.5
Fig. 2. Core TR163-31B Uvigerina spp. B a / C a data plotted as paired data comparing samples cleaned with reductive cleaning only and samples cleaned with both reductire cleaning and the alkaline-DTPA treatment• Samples that fall above the 1:1 line had higher B a / C a when the alkaline-DTPA step was omitted•
76
DETERMINATION
OF CARBONATE-BOUND
3.5
,
3.0'
[]
[]
2.0'
1 :1
line
.5
w C. wuellerstodi + C. kullenbergi
÷
/
1.5
A N D C O R A L S BY I C P - M S
/ +/~
~"
4-
BaJCa (l~mol/mol) reductive cleaning 2.5'
Ba IN F O R A M I N I F E R A
•
i
i
i
2.0
2.5
3.0
Uvigerina sop.
3.5
Ba/Ca (lamol/mol) reductive + DTPA cleaning Fig. 3. Paired benthic foraminifera] Ba/Ca data from Atlantic Ocean core-tops comparing samples cleaned with and without the alkaline-DTPA treatment. Samples that fall above the 1:1 line had higher Ba/Ca when the alkaline-DTPA step was omitted.
dissolves both barite and calcite. Therefore, lower values found for samples treated with this procedure could result from removal of calcite layers containing higher B a / C a ratios. Evidence drawn from acid leaches and partial acid dissolution of foraminifera samples suggests that this is not the case. However, a cleaning agent that attacks barite exclusive of calcite would be preferable and would also reduce sample size requirement. Further assessment of the cleaning method was accomplished by performing a series of partial dissolution experiments on large samples of purified benthic shells. In addition, to assessing the efficacy of the cleaning protocol, partial dissolution can reveal heterogeneity in the distribution of Ba in the calcite shells. Benthic foraminifera were picked from depth intervals where a single species was relatively abundant; these included C. wuellerstorfi from the Norwegian Sea (V27-60), Uvigerina spp. from the northwestern Atlantic (CHN82I / P C ), and Oridisalis spp. from the equatorial Pacific (TR163-31B). Initial sample weights were 2-4 mg, comprising 50-200 individuals. The samples were first cleaned with the procedure described above. After the final cleaning
step, 100-#1 aliquots of 0.072 N HNO3 were added to the sample followed by ultrasonication for 30-60 s. This was sufficient time for dissolution to take place but a short enough time that pH remained acidic ( < 2.5 ). A 100#1 aliquot was then removed from the sample and transferred to a clean vial. This procedure was repeated 2 to 4 times, depending on the initial size of the sample. The Ba/Ca ratio of each dissolution fraction is plotted in Fig. 4 vs. percent of the total sample dissolved (to that point), calculated from the Ca content of each leachate (analytical methods described on pp. 77-80). The data are relatively uniform, with variability only slightly exceeding analytical reproducibility. The fractions within each dissolution experiment were reproducible to ~ ___5% (1 standard deviation) (SD) compared to analytical reproducibility of _+3% for consistency standards (see pp. 79-80). This difference might indicate some limit to foraminifera as recorders of seawater Ba, or alternatively it might reflect heterogeneity among the shells that were used for the partial dissolution. This _+5% error is a measure of the m i n i m u m variability in Ba/Ca
77
D . W . L E A A N D E.A. B O Y L E
Tr163-31B: 118 Oridorsalis spp.
cm
V27-60:10
3.5-
3.5
3.0'
~"
3.0
•...1 2.5,
.~
2.5
rn
2.0
o
cm
C. wuellerstorfi
o
I ~
2.0'
1.5
0.00
-
.
i 0 . 2 0
-
-
i
-
-
0.40 Fraction
i
-
-
|
0 . 6 0
-
1.5 0.0
1.00
0 . 8 0
l
" z
•
,
|
•
i
-
0.4
0.2
Dissolved
Fraction
C H N 8 2 . 1 1 P C : 125 Uvigewina sl~.
.
"
"x;
-
I::I
i
.
.
0.6
I
:
i
0.8
•
.
1.0
dissolved
cm
3.5
1+
~°
3.o
t
E
,
t
i
:
.% x
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1.5 0.0
•
!
•
-
o.2
|
o.4 Fraction
•
!
•
o.6
1
o.e
1.0
Dissolved
Fig. 4. Partial dissolution of three species of benthic foraminifera (experimental procedure described in text). Abscissa error bars indicate the fraction of sample dissolved for that dissolution fraction• Ordinate error bars indicate the estimated I SD error of the Ba/Ca determinations. The second leachate from the T R 1 6 3 - 3 1 B experiment was lost.
of a typical sample pick from a given depth interval in a core. 3. Analysis of benthic foraminiferal Ba/Ca ratios Upon completion of the alkaline-DTPA cleaning step, samples are transferred to acidcleaned 0.5-ml centrifuge vials and then subjected to a series of final acid leaches ( 1-5 × ) in 0.001 N HNO3 to remove any remaining surface contamination (all acids are triple-dis-
tilled in Vycor ® with no measurable Ba blank). These acid leaches also results in more uniform sample sizes since larger samples can be leached several times. Acid is removed via several water rinses, and a pipet is used to remove the last portion of water. At this stage each vial consists of the purified shell material and ~ 5 /d of distilled water. Each sample is dissolved in 100/tl of 0.072 N HNO3, with dissolution encouraged by ultrasonication. Samples are checked visually for complete dissolution, and extra acid is added where needed. A tiny per-
78
DETERMINATION OF CARBONATE-BOUND Ba 1N FORAMINIFERA AND CORALS BY ICP-MS
tion ( 1-2 pl) of each sample is used to check pH on 0-2.5 pH paper. This serves two purposes: ( 1 ) it insures complete dissolution of the samples since calcite that does not dissolve will buffer the pH to high values; and (2) by titrating all samples to a narrow pH range uniform Ca concentrations are ensured. For example, since all samples are dissolved in 72 meq 1-1 acid, a final Ca concentration of 11 m m o l 1- l is achieved if the pH of the samples is titrated to 1.3. [A pH of 1.3 is equivalent to an acid concentration of 50 meq 1-1, and since initial acid concentration was 72 meq 1-1, the difference (22 meq 1-1 of acid) has been consumed by reaction with 11 m m o l 1- 1 or 22 meq 1- 1 of CaCO3. ] After dissolution 100 pl of each sample are transferred to an acid-leached, dry vial. Samples are spiked with 1 or 2 100-/A aliquots of 3 5 B a _ e n r i c h e d spike for determination of total Ba by isotope dilution mass spectrometry (GEOSECS spike S-6; Chan et al., 1977). The ratio of spike to sample chosen depends on the analyst's judgment of the sample size and the expected Ba/Ca ratio. Those samples that require only 100/d of spike solution receive an additional 100/d of 0.072 N HNO3 acid, so that all samples have a final volume of 300 pl. Generally, larger samples (which are titrated to uniform Ca concentration) receive 1 spike aliquot for Atlantic samples ( B a / C a ~ 2-3 pmol mol -~ ) and 2 spike aliquots for Pacific samples (Ba/Ca ~ 4-5/zmol m o l - l ). Small samples always received 1 spike aliquot. Using this method one can routinely achieve 135Ba/138Ba ratios within a factor of 2 of the target ratio, which is 1.55 (Webster, 1960) (Table 1). At this stage one-fourth of the sample (75 /A ) is removed for Ca analysis. These 75 #1 are diluted with 5 ml of La solution (0.4 mg g - l La in a solution of 0.05 N HCI and 0.002 N HNO3 ) to eliminate suppression of the Ca signal due to phosphine in the acetylene. These solutions are then quantified for Ca by comparison to standards on a Perkin-Elmer ® 403 flame atomic absorption spectrophotometer.
TABLE 1 Ba isotopic ratios of t35Ba-enriched spike and natural Ba 135Ba/138Ba Spike S-6-STK (6th GEOSECS Spike, 11/76): 93.6% 135Ba Natural Ba: 6.592% ~3~Ba Target ratio ( = ,,..':0.09194 × 26.29 )
26.29 0.09194 1.55
The remaining 225 ~tl are used for quantification of Ba. A 200-pl flow injection loop is used to inject the sample into a 0.85-ml-mineluant flow (also 0.08 N HNO3). 135Ba/138Ba ratios are determined on a VG ® PlasmaQuad inductively coupled plasma mass spectrometer ( I C P - M S ) . The I C P - M S is run under the standard conditions suggested by VG ® (Gray, 1989), with nebulization accomplished via a Meinhardt* glass nebulizer. Each injection is scanned 360 times from mass 135 to 138 with a 100-/ts dwell time on each of 512 channels (total scan time -- 18 s). Optimal samples ( ~ 10 nmol 1-1 Ba) yield ~ 2 5 0 0 total counts per peak for a relative error (1 SD) of + 2% based on counting statistics. A typical blank yields 20-30 total counts, so a signal to noise ratio of > 100 is usual for most samples, with even the smallest samples achieving signal to noise ratios of better than 50. Samples with very low Ca contents ( < 1 m M Ca in the final 300/~1) are regarded as questionable and are always re-determined if sufficient foraminifera remain. To ensure overall accuracy and reproducibility between runs the 135Ba-enriched spike is repeatedly calibrated to a gravimetric standard. This eliminates the requirement for direct calibration of mass bias inherent in the PlasmaQuad ® system (Klinkhammer and Chan, 1990). This calibration is repeated at least 4 times per run to check for possible machine drift. The 135Ba/138Ba ratio of these spiked gravimetric standards (SGS) are always reproducible to better than +_ 1% over the
D.W. LEA A N D E.A. BOYLE
course of a run. The gravimetric standard used for the calibration was made from reagent grade BaC12-2H20. A second standard was made from SPEX ® ultra pure assayed BaCO3. The Ba concentration of these two standards was intercalibrated by the isotope dilution method; within a +_0.5% analytical error there was no significant difference between the two standards. The Ba concentration of each sample is calculated from the following formula (Chan et al., 1977): 0.0356 [ Ba ]sample-- ~--.~ ~ [ Ba ]spike { spike ~ f R m i x - 2 6 . 2 9 "~ ~sample)volume~0.09194-- Rmix) where Rmi x is the 135Ba/138Ba ratio found for the mixture of sample and spike; 0.0356 is the fraction ~3SBa in the spike (Oak Ridge Nation Laboratories value); 0.717 is the fraction ~38Ba in natural Ba; 26.29 is the ratio of ~35Ba/~38Ba in the spike ( O R N L value ); and 0.09194 is the natural 135Ba/138Ba ratio (Table 1). This expression is evaluated for both the unknown sample and the known standard-spike mixture (SGS). The sample is then adjusted by the deviation of the calculated concentration of the standard from the true value (Webster, 1960). This correction is generally of order 1-2 % and has never been more than 3.5%. Two consistency standards analyzed three times in each run are used as a final check on the precision of the method. These consistency standards were made by adding known amounts of Ba to ion-exchanged Ca (NO 3 ) 2 SOlutions. Ion exchange was used to remove Ba associated with the reagent grade calcite used to make up the Ca(NO3)2 solutions. The concentration and statistical reproducibility of these solutions is given in Table 2. The B a / C a ratio of CN2, the consistency standard with the lower Ba content, is reproducible to ~ 2 3% over 89 analyses. The reproducibility of these consistency
79
standards is one measure of the ultimate precision of the method. However, the analytical precision of real samples might be less favorable. While the Ba content of consistency standards is always known and therefore spiked to an o p t i m u m 135Ba/138Ba ratio, there is uncertainty associated with correctly spiking real samples, although this uncertainty is reduced by maintaining relatively uniform Ca concentrations. To ascertain how much of a problem this effect might be, Ba was determined in a solution spiked with different ratios of spike to standard to yield a range of 135Ba/138Ba ratios (Table 3). Sufficient counts were collected to yield counting statistic of ~ z 0.5%. The range of ~35Ba/'38Ba ratios in this test (0.7-4.8) encompasses the range of ratios obtained over 99% of the time for spiked foraminifera sampies. In this experiment Ba concentrations were reproducible to + 1.5%, with a possible trend towards higher Ba values at higher 135Ba/138Ba ratios. Over 90% of the spiked foraminifera samples analyzed at MIT had 135Ba/138Ba ratios between 0.7 and 3.2; the experimental data in Table 3 indicate that over this range Ba concentrations are reproducible to +1.1%. The spiking error associated with the range of isotope ratios used in this study is clearly small relative to the overall analytical precision. To ascertain if variability in the Ca contents of the injected solutions has a (matrix) effect on isotope ratio determination, Ba was determined in a series of solutions with similar Ba contents but differing Ca contents (Table 4). There was no significant bias on the measured Ba content with differing Ca contents. The average deviation between measured Ba and expected Ba was < _+ 1%. A factor that will clearly degrade how well we determine Ba/Ca in foraminifera samples is reduced count totals for small samples. The mean Ba content of all foraminifera samples analyzed at MIT was ~ 12 nmol 1- l, 20% more Ba than is in the consistency standard that yielded a long-term precision of + 3% (CN2). About 8% of the foraminifera samples had Ba
80
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS
TABLE 2
Statistics from foraminifera consistency standards CN2 and C N 3 Consistency standard C N 2
Mean SD RSD (%)
Number
Consistency standard C N 3
Ba/Ca ( ~ m o l m o l -~ )
Ba ( n m o l l -~ )
Ca ( m m o l l -~ )
Ba/Ca (~molmol -l )
Ba (nmoll
2.36 0.07 3.1 89
28.86 0.75 2.6 96
12.24 0.21 1.7 98
3.12 0.08 2.6 96
58.28 t.30 2.2 104
Ca
1)
(retool 1-1 ) 18.68 0.29 1.6 98
Concentrations were determined on 100-/A aliquots of each consistency standard. SD = standard deviation; R S D = relative standard deviation. TABLE 3 Ba determinations as a function of the 13SBa/138Ba ratio of spiked solutions ID
135Ba/138Ba ratio of spiked solution
Calculated Ba concentration (nmol 1-t )
1 2 3 4 5 6 7 8
0.67 1.21 2.23 2.21 3.21 4.01 4.43 4.77
203 206 206 208 203 209 208 211
Mean
207 +_3
TABLE 4 Ba determination in solutions of varying Ca concentration Ca ( m m o l 1--~ )
Ba a d d e d (/tmol 1-~ )
Ba found (/tmol 1-~ )
Deviation
0 1 2 3 4 5
2.00 2.02 2.04 2.06 2.08 2.10
1.99 2.01 2.01 2.04 2.06 2.08
-0.6 -0.8 - 1.7 -0.6 - 1.0 - 1.1
(%)
Actual Ba concentrations of aspirated solutions after dilution were 2 0 - 2 5 nM.
contents less than one-quarter that of CN2; counting statistics would suggest that these smallest samples have precision a factor of 2 poorer than CN2. Therefore, a rough estimate
of the reproducibility of the least favorable 8% of determinations is a relative error of + 6% ( 1 SD). Most small samples have Ca contents < 3 m M and therefore are replicated if sufficient foraminifera remain. Reproducibility of splits of benthic foraminifera from the same depth in a core are seldom as good as the analytical precision (Lea and Boyle, 1989, 1990a, b). The pooled standard deviation of replicates was + 7-10% for three cores for which sufficient replicates were determined. The decrease in precision associated with real samples is presumably related to bioturbation which mixes together individuals that lived at different times when Ba contents might have been different (Boyle, 1984; Boyle and Rosener, 1990). However, some portion of this variability might be due to imperfect cleaning and/or the imperfect nature of foraminifera as chemical recorders of bottom water Ba. 4. Determination of Ba in coral aragonite The requirements for cleaning and purification of coral aragonite for Ba analysis are not as great as that required for foraminifera shells, since corals are generally not contaminated by sedimentary Ba. Specimens of the coral Pavona clavus from the Gal~ipagos Islands, eastern equatorial Pacific (age 1950-1982) cleaned by the extensive purification procedure developed for Pb and Cd analysis (Shen
D.W. LEA AND E.A. BOYLE
81
and Boyle, 1988) do not have significantly lower Ba than the same samples cleaned with an abbreviated version, of the cleaning procedure (Table 5 ). This "short" cleaning procedure includes ultrasonication in distilled water, methanol and 0.015 N HNO3 followed by several treatments with a boiling solution of 15% HzO2 in 0.2 N NaOH to oxidize residual coral tissue and organic matter. Samples are then leached several times in 0.1 N HNO3. In selected environments and for older, subaerially exposed corals, more extensive cleaning procedures might be warranted. The oxidation step to remove all organic matter in the coral skeleton is especially critical. Coral polyp tissue is enriched in Ba over skeletal material by ~ 10-200 times by weight (Flor and Moore, 1977; Buddemeier et al., 1981 ). Since total organic matter makes up
~ 1% by weight of the skeleton, coral samples not cleaned of organic matter will have organic-Ba contributions equal to 10-200% of skeletal Ba. In a recent study of freshly collected specimens of Acropora palmata, Pingitore et al. (1989) attributed elevated and highly variable skeletal Ba/Ca to a "species effect". However, their results might have been biased by the lack of an oxidative step to remove organic matter from the fresh samples, since samples of "recent-dead" A. palrnata from the same study had significantly lower Ba/Ca with far smaller intraspecific variability. The analytical method employed for determination of Ba/Ca in aragonite skeletons of corals is similar to that employed for foraminifera (see pp. 77-80). Ba is determined by isotope dilution plasma-mass spectrometry and Ca by flame atomic absorption spectro-
TABLE5 Ba/Ca determinations and averages for coral growth bands ofPavona clavus from Punta Pitt, Gal~ipagos Islands, 1950-1959 Coral year-section
950-2 951-1 951-1 951-2" 1952-1 952-2 953-• 953-2* 954-•* 954-2 955-•* 955-2 1956-1" 1956-2" 1957-1 1957-2" 1958-1 1958-I* 1958-2 1959-1"
Date of analysis ( 1988 ) Mar. 16
Mar. 25
4.58 4.26
4.56 4.05
4.28 4.49 4.58 4.62
4.21 4.70 4.51 4.52
Apr. 28
May l
4.69
4.56
4.27 4.44 4.55 4.71
4.35
4.50 4.42
4.65 4.32 4.55
4.83 4.62 4.43 4.64 4.60 4.28
4.71
4.81
4.62
4.67
5.38 4.85 4.63
4.64 4.87 4.28
4.35
4.22 4.28 4.52 4.28
SD
RSD (%)
n
4.57 4.19
0.01 0.10
0.3 2.5
3 4
4.32 4.56 4.55 4.63 4.39 4.49 5.14 4.78 4.70 4.57 4.64 4.72 4.30 4.26
0.10 0.12 0.04 0.08 0.10 0.07 0.23 0.06 0.13 0.13 0.01 0.14 0.04 0.03
2.3 2.7 0.8 1.7 2.2 1.6 4.4 1.3 2.8 2.8 0.1 2.9 0.9 0.6
4 3 3 4 3 2 3 3 3 3 3 3 3 4
4.59 4.36
0.07 0.09
1.4 2.0
3 3
Jun. 13
4.35 4.49
5.12 4.62
4.26
May 22
4.56 4.16
4.93
May 19
Mean
4.27 4.61 4.45
4.34
Key: SD=standard deviation; RSD=relative standard deviation; n = n u m b e r of analyses. Notes: (1) " - I " indicates 1st half of coral growth band; "-2" indicates 2nd half of coral growth band; (2) two entries occur for 1951-I and 1958-1 because two different coral pieces from the same band were analyzed; (3) one value each from 54-1, 55-2, 57-2 and 59-1 was rejected due to a mixing problem for Ca. *Cleaned with the abbreviated purification procedure described in the text.
82
DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS 42
5.4
/
-1
5.2
/
5.0
0
/-.*, /
SST Anomaly
/
/l. '4.8 Ba/Ca (l~mol/mol)
(°C) ' 4.6
1 |
• 4.4 2 ' 4.2
3 50
&
4.0
!
Year
(19-) ...... ~ . . . .
a
S S T anomaly
~Ca
Fig. 5. Plot of Ba/Ca determined on sub-divided annual growth bands of the coral Pavona clavus from Punta Pitt (San Christ6bal Island, Galapagos Islands). Error bars indicate 1SD errors of the Ba/Ca measurements (Table 1 ). The Ba/Ca data are compared with sea surface temperature anomalies (E. Rasmusson, Climate Analysis Center, pers. commun., 1986 ). Covariance indicates the association of high Ba levels with cooler sea surface temperatures, as indicates by negative temperature anomalies (Lea et al., 1989). photometry. Because corals are more massive than foraminifera larger samples are generally available for analysis. Therefore, flow injection for Ba analysis is not necessary, although it can be used in cases where sample is limited (i.e. micro-sampling). Coralline Ba is relatively uniform [ see Table 5, Shen and Boyle (1988) and Lea et al. ( 1 9 8 9 ) ] and therefore coral solutions can be spiked quite readily to an o p t i m u m 135Ba/138Ba ratio; for this study the ratio employed was generally between 0.9 and 1.5, which is optim u m for the I C P - M S ( K l i n k h a m m e r and Chan, 1990). Counting times and dilutions were adjusted to yield total counts > 10,000 for each Ba peak. For coral samples, where signal
to noise on the I C P - M S is always greater than 100, the reproducibility of the ratios is very close to that expected from counting statistics. Therefore, 10,000 total counts on each peak generally yields precision on the ratios near + 1% ( 1 R S D ) (relative standard deviation ). Typical Ba concentrations of spiked coral solutions introduced into the I C P - M S are between 15 and 45 nmol 1-1, equivalent for corals to 3-10 mmol 1-1 of Ca or 0.3-1 mg of coral aragonite per ml of solution. Typical volumes of solutions are 0.8-3 ml, with counting times of 1 to 3 min. Reproducibility of coral B a / C a ratios based on replicates of the same coral solutions run on different days is ~ _ 2% ( 1 R S D ) . Data from
83
D.W. LEA AND E.A. BOYLE
a P a v o n a clavus time series ( 1950-1960) from
Punta Pitt in the Gal~ipagos Islands for which replicates were run over 5 separate runs are detailed in Table 5 and plotted with error bars in Fig. 5. Samples were prepared by band-sawing individual growth bands into two equal halves details in Shen and Boyle (1988) ]. Similarity between the coral Ba record and the historical record of sea surface temperature anomalies is due to temporal variability in the upwelling of deeper, colder waters enriched in Ba (Lea et al., 1989). A more detailed coral Ba record was obtained from the same coral over the time period 1965-1978; these data were acquired over a single day's run, and based on consistency standards had an inter-run reproducibility of _+ 1.0% for Ba/Ca (Lea et al., 1989). 5. Conclusions
A procedure has been developed to purify benthic foraminifera shells from deep-sea cores of extraneous Ba not bound by carbonate. Ba/Ca ratios of purified shells are measured by a rapid and precise technique using isotope dilution plasma-mass spectrometry. A modification of this technique has been applied to the determination of Ba/Ca in sub-annual bands of massive reef corals. Acknowledgments
We thank Glen Shen, Ted McConnaughey, Ed Sholkovitz and Lloyd Keigwin for generously providing samples for this work. Comments and suggestions from Lex van Geen, Paula Rosener, Kristin Orians, Debra Colodner, Kelly Kenison Falkner and Gary Klinkhammer aided in the development of this study. We thank John Edmond for access to the MIT PlasmaQuad ® and use of the GEOSECS barium spike. Help and advice from members of the MIT PlasmaQuad ® squad was greatly appreciated. This research was supported by NSF grant OCE8710168 (to E.A.B.) and a Joint Oceanographic Institutions/Ocean
Drilling Program Fellowship to the senior author for 1987/1988. Continuing development of these methods by the senior author at UCSB was supported by OCE-9012033 and the University of California. Appendix - - Recent improvements in Ba analysis of foraminifera Since the time this manuscript was submitted a number of changes have been applied to the analytical methodology described herein. Some of these changes are documented in Lea and Spero (1992). The most substantial changes are for the analysis of benthic foraminifera and are as follows: ( 1 ) To prevent intolerable sample loss during the barite dissolution step, 50/11 of 0.002 MDTPA in 0.1 NNaOH are used for each sample for 10 min in a boiling water bath, ultrasonicating every 2 rain. (2) Samples are subject to only one final 0.001 N nitric acid leach ( 100/A per sample). (3) To eliminate the requirement for flow injection, dissolved foraminifera samples are delivered to the plasma as an isolated sample plug, resulting in easier analysis and improved signal to noise ratios. Careful timing is required so that acquisition times match the residence time ,~f the sample plug in the plasma. (4) Peristaltic flow rates are reduced to as little as 0.5 ml rain- ~, yielding acquisition times for a 0.25-ml "plug" of 20-25 s. (5) An increase in total counts and count rates (and hence improved precision) is achieved by "peak hopping" on the ~3SBaand ~38Bapeaks only, thus eliminating scan time spent in the valleys between peaks and on the masses not required for quantification. The newest generation of plasma mass spectrometers have sensitivities that are about ten times greater than the first-generation machines. (The VG ® PQ2+ Turbo installed at UCSB routinely achieves 20 kHz for a 1 ppb l~Sln solution.) By combining improved ICP-MS sensitivity with the changes described above it is now possible to analyze Ba in single shells of planktonic foraminifera (Lea and Spero, 1992). However, purification of single shells recovered from marine sediments remains a formidable obstacle.
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DETERMINATION OF CARBONATE-BOUND Ba IN FORAMINIFERA AND CORALS BY ICP-MS
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