Ion microprobe measurement of strontium isotopes in calcium carbonate with application to salmon otoliths

Geochimica et Cosmochimica Acta, Vol. 69, No. 5, pp. 1225–1239, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 ⫹ .00

doi:10.1016/j.gca.2004.05.051

Ion microprobe measurement of strontium isotopes in calcium carbonate with application to salmon otoliths PETER K. WEBER,1,* CHARLES R. BACON,2 IAN D. HUTCHEON,3 B. LYNN INGRAM4 and JOSEPH L. WOODEN5 1

3

Department of Geography, 507 McCone Hall, University of California, Berkeley, CA 94720, USA 2 U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, CA 94025, USA Chemical Biology and Nuclear Science Division, Lawrence Livermore National Laboratory, P.O. Box 808, L-231, Livermore, CA 94551, USA 4 Department of Earth and Planetary Science, 307 McCone Hall, University of California, Berkeley, CA 94720, USA 5 U.S. Geological Survey, Stanford–USGS Ion Microprobe Laboratory, Green Earth Science Research Building, Stanford, CA 94305 USA (Received April 1, 2003; accepted in revised form May 26, 2004)

Abstract—The ion microprobe has the capability to generate high resolution, high precision isotopic measurements, but analysis of the isotopic composition of strontium, as measured by the 87Sr/86Sr ratio, has been hindered by isobaric interferences. Here we report the first high precision measurements of 87Sr/86Sr by ion microprobe in calcium carbonate samples with moderate Sr concentrations. We use the high mass resolving power (7000 to 9000 M.R.P.) of the SHRIMP-RG ion microprobe in combination with its high transmission to reduce the number of interfering species while maintaining sufficiently high count rates for precise isotopic measurements. The isobaric interferences are characterized by peak modeling and repeated analyses of standards. We demonstrate that by sample-standard bracketing, 87Sr/86Sr ratios can be measured in inorganic and biogenic carbonates with Sr concentrations between 400 and 1500 ppm with ⬃2‰ external precision (2␴) for a single analysis, and subpermil external precision with repeated analyses. Explicit correction for isobaric interferences (peak-stripping) is found to be less accurate and precise than sample-standard bracketing. Spatial resolution is ⬃25 ␮m laterally and 2 ␮m deep for a single analysis, consuming on the order of 2 ng of material. The method is tested on otoliths from salmon to demonstrate its accuracy and utility. In these growth-banded aragonitic structures, one-week temporal resolution can be achieved. The analytical method should be applicable to other calcium carbonate samples with similar Sr concentrations. Copyright © 2005 Elsevier Ltd during the sputtering process (Table 1). Calcium dimers are particularly important interferences in these samples because Ca is a major element and its dimers can only be fully resolved from the Sr species of interest with very high mass resolving power (M/⌬M ⬎ 9000). These interferences significantly perturb Sr isotope ratio measurements for samples with less than 5000 ppm Sr. In previous studies, two approaches have been used to compensate for these isobaric interferences: explicit corrections (peak stripping) and high mass resolving power. Exley (1983) used explicit corrections at low mass resolving power (M/⌬M ⫽ 200) on a 400-ppm Sr calcite. He obtained an 87Sr/86Sr ratio in agreement with published data within analytical precision (2␴ ⫽ 2‰ for a total of 480 cycles) but neglected to correct for Ca dimers at 86 and 87 amu, calling into question the validity of this approach (Scatena-Wachel, 1986; Kennedy et al., 1990). Scatena-Wachel (1986) analyzed the same 400-ppm Sr sample with the same ion microprobe using the same analytical conditions but making the additional Ca-dimer corrections. She found that the measurement could not be made accurately at low mass resolving power because of the isobaric interferences. Kennedy et al. (1990) made both low and high mass resolving power measurements on the same 400-ppm Sr sample and confirmed that the measurement could not be made accurately at low mass resolving power. At high mass resolving power (M/⌬M ⫽ 4800), the measurement was accurate, but precision was low (2␴ ⫽ 12‰) because of low count rates. Here, we use the Stanford-USGS SHRIMP-RG (Sensitive High Resolution Ion Microprobe with Reverse Geometry), a high-transmission, high-mass resolution ion microprobe, to achieve higher precision and accuracy. The double focusing mass spectrometer is

1. INTRODUCTION

Secondary ion mass spectrometry (SIMS) performed by ion microprobe provides the highest spatial resolution of existing techniques for high precision isotopic measurements. The conventional ion microprobe performs in situ isotopic measurements in areas as small as a few microns across. Precision better than one part in one thousand can be achieved, depending on the isotopic measurement of interest and sample composition (e.g., Paterson et al., 1997; McKeegan et al., 1998; Riciputi et al., 1998; Fritzsimons et al., 2000; Ireland, 2004). Results of in situ isotopic analyses can be directly related to complex microstructure of heterogeneous or growth-zoned samples, yielding significantly more information than bulk analyses. The isotopic composition of strontium in a sample, as measured by the 87Sr/86Sr ratio, is a powerful tracer for a wide range of scientific applications (see Faure, 1986). SIMS is sensitive for Sr, but attempts to measure 87Sr/86Sr ratios by ion microprobe have had mixed results. Accurate and precise measurements of 87Sr/ 86 Sr ratio have been achieved in samples with high Sr concentration (⬎5000 ppm Sr by weight) (Exley, 1983; Scatena-Wachel, 1986; Kennedy et al., 1990). Many of the samples of interest, however, are calcium carbonate and have lower Sr concentrations. Calcium carbonate presents difficulties for SIMS because it generates numerous isobaric interferences in the 86 to 88 amu range

* Author to whom correspondence should be addressed (weber21@ llnl.gov). Present address: Chemical Biology and Nuclear Science Division, Lawrence Livermore National Laboratory, P.O. Box 808, L-231, Livermore, CA 94551, USA. 1225

1226

P. K. Weber et al. Table 1. Selected isobaric interferences for Sr isotope measurements at atomic masses 86, 87 and 88 with calculated mass resolving power (M.R.P. ⫽ M/⌬M) relative to the respective Sr species. See Analytical Methods. Species

M.R.P. at 86 amu

42

44

a

17,800 12,200 10,400 4,300 3,600 2,900 2,600

Ca Ca Ca46Caa Ca2a 54 Fe16O2 44 Ca26Mg16O 39 24 K Mg23Na 40 Ca23Na2 40 43

at 87 amu 87

a

300,000 16,200 11,800 10,600 4,600 3,500 3,100 3,000

Rb Ca44Caa 48 Ca39Ka 86 1 a Sr H 55 Mn16O2 39 24 K Mg2 48 Ca23Na16O 40 Ca24Mg23Na 43

at 88 amu 44

a

16,400 13,100 9,200 7,900 4,600 3,300 3,200 2,900 2,700 2,700

Ca2 Ca46Caa 40 Ca48Caa 87 1 a Sr H 56 Fe16O2 48 Ca24Mg16O 40 Ca24Mg2 39 26 K Mg23Na 40 Ca25Mg23Na 42 Ca23Na2 42

a

Unresolved species in this study.

designed with the electrostatic analyzer downstream from the magnet (reverse geometry) to increase mass resolving power with high transmission (Compston, 1996; Ireland and Wooden, 1998). The SHRIMP-RG is potentially well suited for 87Sr/86Sr analyses because of its high mass resolving power (6000 to 10,000 M.R.P. at 10% peak height with full transmission) and high transmission (for 88Sr, ⬃400 cps · ppm⫺1 with a primary beam current of 10 nA O2⫺). At greater than 6000 M.R.P., many of the potentially important isobaric interferences are fully resolved (Table 1). To develop the analytical method, we characterize the Ca dimer interferences in low-Sr synthetic CaCO3 samples, analyze a set of 87 Sr/86Sr CaCO3 standards, and compare two methods of data correction. Then the preferred method of data correction is applied to a small set of modern salmon otoliths (aragonitic “earstones”) to demonstrate the accuracy and utility of the analytical method. The analytical procedure is designed to achieve precision better than 1‰. 2. SAMPLES

2.1. Synthetic Calcite and Aragonite Synthetic calcite and aragonite reference materials were crystallized at high pressure and temperature in a piston-cylinder

apparatus at the United States Geological Survey (USGS) in Menlo Park, California. Starting powders were sealed in Au, Ag, or Pt capsules (typically 5 mm diameter) in 2.54 cm diameter furnace assemblies and generally run for 24 h., followed by rapid quenching by turning off power to the furnace assembly. Successful runs produced 40 –100 ␮m diameter crystals. Run conditions ranged from 1.0 to 2.65 GPa at 700 to 1100 °C. The following samples were made: 1) Two high-purity, low-Sr (⬃2 ppm) calcites synthesized from NIST SRM 915a and Specpure CaCO3 for characterization of Ca-dimer production. 2) Calcite and aragonite synthesized from modern Tridachna shell (EN-1) powder (87Sr/86Sr ratio ⫽ 0.709167 ⫾ 0.000016 (⫾2 standard error, or 2␴) (Sugarman et al., 1997). The Sr concentration of the EN-1 powder (1325 ppm) was determined by isotope dilution using thermal ionization mass spectrometry (TIMS) at the Center for Isotope Geochemistry at the University of California, Berkeley (UCB). 3) A series of calcites made from Specpure CaCO3 and doped with small amounts of other carbonates, including reagent grade SrCO3, in various concentrations. The Sr concentration and 87Sr/86Sr ratio in the resultant products were determined by solution ICP-MS (100 to 1500 ppm Sr) and TIMS, respectively, at USGS. 2.2. Natural Inorganic Calcites A set of ⬃60 natural calcites was selected from the UCB Earth and Planetary Science mineral collection. These calcites were analyzed by electron microprobe and ion microprobe at Lawrence Livermore National Laboratory (LLNL) to assess homogeneity and Sr concentration. Sr concentration was also determined independently by ICP-MS at LLNL. Based on homogeneity and Sr concentration (125 to 1400 ppm), five calcites were selected for 87Sr/86Sr analysis by TIMS at the Center for Isotope Geochemistry. 2.3. Otoliths Otoliths are acellular aragonitic (CaCO3) accretionary structures in the inner ear of bony fish that are used extensively for chemical investigations in fisheries research (see Campana, 1999), as well as for paleoenvironmental reconstruction (e.g., Patterson et al., 1993). They are not vascularized and, unlike bone, are not subject to mineral reworking (Ichii and Mugiya, 1983; Campana and Neilson, 1985; Campana, 1999). Otolith CaCO3 accretes in a protein-rich matrix in concentric layers around one or more centers of calcification (primordia). Prominent bands (checks) in the otolith demarcate when the fish hatches out of the egg and when it begins feeding, and visible daily increments accrete after feeding begins (Neilson and Geen, 1982; Campana and Neilson, 1985). Three otoliths form in the labyrinth of each inner ear. The largest of the three, the sagittal otolith, is commonly used for chemical investigations. They are elliptical and laterally compressed with complex internal structure, and in juvenile chinook salmon (Oncorhynchus tshawytscha), they range in length from 0.5 mm at first feeding, to ⬃3 mm at 6 months, and ⬃1 cm in adults (Fig. 1). Daily increment thickness is on the order of 2 to 10 ␮m in juvenile

Ion microprobe measurement of strontium isotopes

1227

Fig. 1. Obliquely illuminated photomicrographs showing the relative size, orientation and morphology of the juvenile portion of an adult chinook salmon. (a) A sagittal view of a juvenile sagittal otolith superimposed on an adult sagittal otolith. The juvenile portion accretes while the fish is in freshwater, before migrating to the ocean. (b) A transverse section of an adult otolith with (c) a detail of the juvenile portion. This view shows the orientation of the juvenile portion within this adult otolith. The otolith does not grow uniformly along the sagittal plane (here, horizontal), creating the curvature visible on the left and right of image b. This curvature is irregular and differs among individuals, making the exact internal morphology and the orientation of the juvenile portion unpredictable. The optimal section to achieve maximum temporal resolution is sagittal.

chinook salmon in the direction of greatest linear extension, revealed in a sagittal section (Fig. 2). Otoliths from chinook salmon with known histories were analyzed to test the method. The fish were all from the Sacramento-San Joaquin river system, California, USA (⬃38°N, 122°W). All otoliths were cleaned in deionized water to remove tissue, mounted in Araldite or Eraldite epoxy, sectioned sagittally to expose banding, and gold coated to allow charge dissipation (Fig. 2). Two of the otoliths (FRH4 & FRH6) came from juvenile chinook salmon from the Feather River Hatchery, Oroville, California, that were transported to the California Department of Fish and Game Fish Health Laboratory, Rancho Cordova, California in April 2000, where they were raised on hatchery feed for three months. The mean water 87Sr/86Sr ratio for that period is 0.70618 ⫾ 0.00016 (⫾2 standard deviations, or 2SD) at the Feather River Hatchery, and 0.70997 ⫾ 0.00061 at the Fish Health Laboratory (Weber, 2002). Four otoliths (MRH8, MRH10, MKH12 & MKH14) came from juvenile chinook salmon that were raised and coded-wire tagged at the Merced River (MRH; 87Sr/86Sr ⫽ 0.70852 ⫾ 0.00085) and Mokelumne River (MKH; 87Sr/86Sr ⫽ 0.70700 ⫾ 0.00026) hatcheries. The fish were released into the wild in the spring of 1997 and recaptured in the Sacramento-San Joaquin river delta (0.70660 ⫾ 0.00087) ⬃20 days later. For comparison with the data for MRH8 and MRH10, we have multiple whole otolith 87Sr/86Sr measurements for the Merced River Hatchery (0.70866 ⫾ 0.00042, n ⫽ 11) from previously published work, including analyses of the other sagittal otolith from each of the sampled fish (Ingram and Weber, 1999). We have similar data for the Mokelumne River Hatchery for comparison with MKH12 and MKH14, but the data are more variable, apparently because the data set includes fish that were transferred from the Feather River Hatchery to supplement the Mokelumne River Hatchery’s stocks (Weber, 2002). One otolith (BCA10) came from an adult chinook salmon that was coded-wire tagged as a juvenile in its natal river, Butte Creek (0.70475 ⫾ 0.00022), and recovered in that river after returning from the ocean and spawning. This fish is known to be naturally-spawned because there is no hatchery or other stocking in this river.

Otolith 87Sr/86Sr ratio is expected to be the same as the Sr/86Sr ratio of the water in which the fish is living at the time the otolith increment is deposited, except when the fish are given non-local food, as in the case of the marine-based feeds used in these hatcheries. In the case of hatchery rearing, expected otolith 87Sr/86Sr is based on replicate analyses of whole otoliths from hatchery-raised chinook salmon from the Sacramento-San Joaquin river system (Ingram and Weber, 1999) or on the relationship of otolith 87Sr/86Sr to water 87Sr/86Sr for hatchery-raised chinook salmon established by Weber (2002): 87

Sr ⁄ 86Srotolith ⫽ 0.213(⫾0.015)

87

⫹ 0.700(⫾0.021) · 87Sr ⁄ 86Srwater . (1) For hatchery-raised chinook salmon, the variability in Srotolith is ⬃0.0004 (2SD).

87

Sr/

86

3. ANALYTICAL METHODS 87

86

Sr/ Sr measurements were made with the Stanford-USGS SHRIMP-RG in six analytical sessions over two years. The general approach was to characterize the interfering species and 87 Sr/86Sr measurement precision and accuracy with repeated

Fig. 2. Transmitted-light photomicrograph of the early juvenile region of the sagittal section of an adult chinook salmon otolith (BCA10). The hatch (inner) and first-feeding (outer) check marks are indicated. The section was first analyzed for Sr isotopes and then later for a second isotopic marker. The otolith banding is visible through the gold coat, and the analysis craters show as bright spots.

1228

P. K. Weber et al.

analyses of calcium carbonate samples over a range of Sr concentrations. High-purity, low-Sr synthetic calcites were analyzed to assess production and instrumental mass-dependent fractionation of Ca dimers. Natural and synthetic calcites with known 87Sr/86Sr ratios and Sr concentrations were used as standards and were analyzed to evaluate the effect of Sr concentration on the measured 87Sr/86Sr ratio. Calcite and aragonite crystallized from EN-1 powder were analyzed in tandem to determine if crystal structure affected instrumental mass-dependent fractionation and Ca-dimer production. Finally, otolith 87 Sr/86Sr was measured and compared to expected ratios to test the application of this SIMS method and the calcite standards to biogenic aragonite samples. Table 1 presents potentially important isobaric interferences in the 86 to 88 amu range. The selected elements can be significant components of inorganic or biogenic carbonates. The table includes singly charged clusters of three or fewer species with terrestrial abundances greater than 10% for non-Ca clusters and greater than 0.1% for clusters with Ca and non-Ca species. All Ca dimers and Sr hydrides are included. The Ca-dimer and Rb hydrides are not included because they are negligible. Of the species in Table 1, this study addresses the Ca dimers, 48Ca39K, 87Rb and the Sr hydrides because they are the only species that the SHRIMP-RG cannot fully resolve. The Ca dimers are the major focus of this study because they were the most abundant of the unresolved isobaric interferences. Sr hydride production was ⬃0.1% of the Sr monomer, making the 87Sr1H species insignificant relative to 88Sr. 48 Ca39K and 87Rb production was negligible in the standards and significant (⬎0.1% of Sr) in the otoliths. The Sr hydrides and 48Ca39K were effectively resolved (See Results). The SHRIMP-RG primary column was fitted with a duoplasmatron and tuned to deliver a primary beam of 7 to 15 nA of 16O2⫺. Kohler illumination was used to produce oval (⬃25 ⫻ 35 ␮m) flat-bottom analysis craters in the samples. The beam was rastered for 1 to 2 min. before analysis to clean the area around the analysis spot and to allow count rates to stabilize. Positive secondary ions were accelerated to 10 kV and focused into the mass spectrometer, which was tuned for 7000 to 9000 M.R.P., depending on the specific analysis session. Each peak top had a minimum width of three magnet steps. Secondary ion intensities were measured by magnetic peak jumping and pulse counting on a single electron multiplier (deadtime 20 to 25ns, depending on date). In a typical run, the following species were measured: 40Ca2, 40Ca42Ca, 85Rb, 86Sr, 87Sr, 88Sr and 44Ca48Ca (2, 5, 5, 10, 10, 2 and 5 s per cycle, respectively). 40Ca2, 40 Ca42Ca, 40Ca43Ca, 40Ca44Ca, 42Ca44Ca, 40Ca48Ca and 44 Ca48Ca were measured in high-purity synthetic calcites at 8500 M.R.P. The measurement of 42Ca44Ca included ⬃100% of 40Ca46Ca (⌬Mspecies ⫽ 0.0022 amu) because we centered on the combined peak, and the measurement of 40Ca48Ca included ⬃37% of 44Ca2 (⌬Mspecies ⫽ 0.0042 amu) based on the ideal peak shape model described below. 42Ca43Ca was not measured because of its low abundance, and 43Ca44Ca could not be measured because of interference from 87Sr. Each analysis (or run) typically consisted of the maximum number of cycles through the mass table allowed by the software, which was 10 cycles and lasted ⬃15 min. Count rates were 7000 to 30,000 per second for 87Sr, depending on analysis conditions and Sr concentrations in the sample. For a single analysis, the Poisson

counting statistics limit of precision (2␴cs; see Eqn. 12–14) is 2.7 to 1.3‰, respectively. Replicate analyses were made at the same spot or adjacent spots to achieve measurement precision goals (2␴ ⬍ 1‰). Analyses at a spot were stopped when count rates began to decline (hole depth ⬃3 ␮m; 10 –30 cycles). Measurement precision tended to degrade if the primary beam illumination became inhomogeneous. During the otolith analyses, standards were analyzed periodically to assure 87Sr/86Sr measurement accuracy. 3.1. Data Processing: Ca-dimer Corrections Two approaches to data processing are compared: (1) implicit correction for interfering Ca-dimer species using a working curve defined by the standards (sample-standard bracketing); and (2) explicit correction for all isobaric interferences (peak stripping). In both cases, the data are first corrected for the counting system deadtime and time-interpolated to produce time-independent ratios (Ireland, 1995). 3.1.1. Implicit Ca-dimer correction Under the implicit Ca-dimer correction, three adjustments are made to the data. First, the count rate at 87Sr is adjusted for the 87Rb interference, then the 87Sr/86Sr ratio is corrected for mass-dependent fractionation, and finally, the measured 87 Sr/86Sr ratio is corrected for the Ca-dimer interferences based on a working curve. The count rate at 87Sr is adjusted for 87Rb based on the count rate at 85Rb using the linear law for mass-dependent fractionation. C M Ri,j ⫽ Ri,j (1 ⫹ ␣⌬mi,j),

(2)

87 where RC Rb/ i,j is the corrected ratio of isotopes i and j (for M 85 C Rb, R87 , ⬅ 0.3856), R is the measured ratio, ⌬m i,j i,j ⫽ mi Rb 85Rb – mj, and ␣ is the fractionation factor. N M ␣ ⫽ (Ru,v ⁄ Ru,v ⫺ 1) ⁄ ⌬mu,v .

(3)

N u,v 87 86

R

is the natural ratio for reference isotopes u and v. The Sr/ Sr ratio is also corrected for mass-dependent fractionation using the linear law. In both cases, ␣ is determined from N the measured 88Sr/86Sr ratio (R88 , ⬅ 8.3752; ␣ ⬇ 0.005 per Sr 86Sr amu). This value of ␣ may be too high for Rb because its ionization probability is higher than Sr’s, but the potential error in the corrected 87Sr/86Sr is negligible (⬍0.1‰). The count rate at 85Rb can be adjusted for 42Ca43Ca (15,000 M.R.P. relative to 85 Rb), but this correction is implicit in the working curve correction and in samples with ⬎500 ppm Sr, the intensity of 42 Ca43Ca is negligible (⬍0.01% of 87Sr). Combining these first steps, the working equation is:

冋

(87Sr ⁄ 86Sr)c ⫽ (87Sr ⁄ 86Sr) ⫺ (85Rb ⁄ 86Sr)R87C Rb,85Rb

冢

⫻ 1⫹

R88MSr,86Sr R88N Sr,86Sr

R88N Sr,86Sr R88MSr,86Sr

⫺1

⌬m88Sr,86Sr

冣

,

册 (4)

where the raw ratios are calculated from count rates calculated from total counts divided by counting time.

Ion microprobe measurement of strontium isotopes

1229

The (87Sr/86Sr)c ratio is next corrected for the Ca-dimer interferences based on a working curve defined by replicate analyses of the standards for the analytical period. For the working curve, ⌬87Sr/86Sr is plotted against 88Sr/40Ca2. Here, ⌬87Sr/86Sr is the permil (‰) deviation of the SIMS 87Sr/86Sr ratio from the TIMS 87Sr/86Sr ratio: ⌬87Sr ⁄ 86Sr ⫽ [(87Sr ⁄ 86SrSIMS ⫺ 87Sr ⁄ 86SrTIMS) ⁄ (87Sr ⁄ 86SrEN⫺1)] · 1000. (5) All data are referenced to the TIMS 87Sr/86Sr ratio for EN-1. The 88Sr/40Ca2 ratio is a direct measure of the relative importance of the interfering Ca-dimer species for a given analysis. These data are not converted to Sr concentration because 88Sr/ 40 Ca2 is related to, but is not linearly dependent on, the sample Sr concentration. Analyses of the calcite and aragonite standards and otoliths with the Lawrence Livermore National Laboratory Cameca ims-3f showed that the production of the 40Ca2 dimer relative to the 42Ca monomer differs by as much as 18% among samples, demonstrating that secondary factors affect the production of Ca dimers relative to Ca and Sr monomers (see Electronic Annex EA1). To correct data for unknowns, ⌬87Sr/ 86 Sr is derived from the working curve and subtracted from the measured ratios. 3.1.2. Explicit Ca-dimer correction Under the explicit Ca-dimer correction approach, measurements of 86Sr, 87Sr and 88Sr are adjusted individually for the unresolved Ca-dimer species before making both the 87Rb correction and the instrumental mass-dependent fractionation correction to 87Sr/86Sr, described above. Measurements of 40 Ca2 and 40Ca42Ca are used to calculate the abundance of unresolved dimer species at 86, 87 and 88 amu. These calculations include the measured instrumental mass-dependent fractionation for the Ca dimers. The expected relative abundance of each dimer species is calculated based on the linear combination equation:

冉兺 冊 48

2

[iCa] ⫽ [40Ca]2 ⫹ [42Ca]2 ⫹ 2[40Ca][42Ca] ⫹ . . . , (6)

i⫽40

where [iCa] is the natural terrestrial abundance of the respective isotope, and the individual terms on the right hand side of the equation are the expected relative abundances for the respective combinations of species. Two models were used to estimate the contribution of the unresolved interfering dimer species to the measurements of 86 Sr, 87Sr and 88Sr. The first model uses an empirically derived peak to estimate the contribution of each species at the center of the Sr peak of interest (Fig. 3). The 40Ca2 peak from a low-Sr calcite was parameterized for the model because this peak is free of significant interferences. Peak shape measurements were made at 5000 and 7000 mass resolving power. The contribution of each interference to the corresponding Sr measurement is estimated by scaling the model peak to the mass and calculated abundance of the interference and determining the intensity of the peak at ⌬Mspecies from the peak center on the low mass side. ⌬Mspecies is the difference in mass between the interfering species and the Sr species of interest.

Fig. 3. Comparison of empirical and idealized trapezoidal peakshape models at 7000 M.R.P. Here, the 87Sr (larger) and 43Ca44Ca peaks are modeled based on their approximate relative intensity in a 1000 ppm Sr calcite sample. Note that the scale for relative intensity is logarithmic. The idealized trapezoidal model (dashed line) approximates the empirical peak shape (solid line) to within 10% of the maximum count rate.

The second model is an idealized trapezoidal peak-shape model for which the ratio of peak-top width to peak-base width is set at 1:3 (Fig. 3). The fractional contribution of the interfering species, I, is calculated based on this idealized geometry using the following equations: ● For ⌬Mspecies ⬎ 0.54 ⌬Mpeak, I ⫽ 0 (no contribution; 7) ● For ⌬Mspecies ⫽ 0.54 ⌬Mpeak, to 0.18 ⌬Mpeak, I ⫽ (0.54 ⌬Mpeak ⫺ ⌬Mspecies)/(0.36 ⌬Mpeak) (partial contribution; 8) ● For ⌬Mspecies ⬍ 0.18 ⌬Mpeak, I ⫽ 1 (100% contribution; 9) ⌬Mpeak is the peak width at 10% of the maximum peak height, calculated from M.R.P.machine ⫽ M/⌬Mpeak. The contribution of each interference to the corresponding Sr measurement is I times the calculated abundance of the interference. Model results for M.R.P.machine of 5000, 7000, and 9000 are compared. 3.2. Data Processing: Precision and Accuracy The data for the standards are presented in terms of the difference between the measured and the true ratio (⌬87Sr/ 86Sr) in permil notation, as defined in Eqn. 5. Internal precision for a single analysis is based on first-order, second-moment error propagation:

冪冉 m

␴

冉 冊⫽ 87Sr

86Sr

c

兺 j⫽1

⫻

冉

⭸ 共87Sr ⁄ 86Sr兲 ⭸x j

⭸ 共87Sr ⁄ 86Sr兲 ⭸x j

冊 冊冉 共 2

␴x j

␴x j

⫹2

⭸

m

m

兺兺

j⫽1 k⫽j⫹1

Sr ⁄ 86Sr兲

r x jxk

冊

87

⭸xk

␴ xk , (10) 87

where m is the number of species used to make the Sr/86Sr estimate, and rxjxk is the correlation between all combinations of these species. Practically, this precision can be calculated based on the uncertainty in the measured 87Sr/86Sr, 88Sr/86Sr and 85Rb/86Sr ratios using the formula:

1230

␴

P. K. Weber et al.

冪

␴

冉 冊⬇ 87Sr

86Sr

c

冉 冊 ⫹ 共 ⫺ ␴冉 冊 R 87Sr 2

85Rb

86Sr

85Rb

⫹ 2r

兲 ⫹ 共 ␴冉 冊 C兲 ␴冉 冊 ␴冉 冊 C, 2

N 87Rb,85Rb

87Sr 88Sr

87Sr

88Sr

86Sr 86Sr

86Sr

86Sr

,

Table 2. Ca-dimer abundance relative to 40Ca2, as calculated based on natural abundance, and as measured in high-purity, low-Sr synthetic calcites.

2

88Sr

86Sr

(11) Ca dimer

where C ⫽

⫺共87Sr⁄ 86Sr兲EN⫺1

⫽ ⫺0.0423. The correlation facR88N Sr,86Sr⌬m88Sr,86Sr tor, r , , is positive under normal conditions (⬃0.7), and 86Sr 86Sr therefore the correlation term reduces the total uncertainty relative to the variability in the raw 87Sr/86Sr ratio, in keeping with the reduced scatter in the data with the mass-dependent fractionation correction. The error due to the Rb correction is negligible for Rb corrections less than 1% of 87Sr. The external precision for a single analysis is based on the standard deviation of replicate analyses of the standards (Fritzsimons et al., 2000). The external precision for replicate analyses of unknowns is calculated by summing in quadrature the total measurement uncertainty for the replicate analyses, the uncertainty in ⌬87Sr/86Sr for the working curve, and the uncertainty in the TIMS 87Sr/86Sr ratio for the standards. To assess accuracy, the standard data are compared to TIMS measurements and the otolith data are compared to TIMS measurements and expected ratios. To assess precision, internal measurement precision 共␴共87Sr ⁄ 86Sr兲c兲 is compared to the Poisson counting statistics limit (␴cs). The counting statistics limit is calculated by applying Eqn. 10 to Eqn. 4 in terms of counts of individual species, yielding: 87Sr 88Sr

␴cs ⬇

冪

Calculated abundance (⫻ 10⫺3)a

Measured abundance (⫻ 10⫺3)

2␴ (⫻ 10⫺3)

at 82 amu 40

Ca42Ca

13.35

40

Ca43Ca

2.79

12.80

0.03

at 83 amu 2.626

0.008

at 84 amu 40

Ca44Ca 42 Ca2

43.04 0.044

40.30

0.05

at 85 amu 42

43

Ca Ca

42

44

0.0186 at 86 amu

Ca Ca Ca46Ca 43 Ca2 40

0.287 0.083 0.002

0.333b

0.003

3.47b

0.01

0.072

0.002

at 87 amu 43

44

Ca Ca

40

48

0.060 at 88 amu

冉兹

cts87Sr ⁄ t87Sr

冋

cts86Sr ⁄ t86Sr

冉

兹cts86Sr ⫺

冊

Ca Ca Ca2 42 Ca46Ca 44

2

⫹

(87Sr ⁄ 86Sr) cts86Sr

⫹

⫹

(87Sr ⁄ 86Sr)R88N Sr,86Sr cts88Sr⌬m88Sr,86Srt86Sr ⁄ t88Sr

冉

冊册

⫺ 兹cts88Sr(87Sr ⁄ 86Sr) cts88Sr ⌬m88Sr,86Sr

冊

3.86 0.463 0.0006 at 92 amu

2

2

,

44 46

48

Ca Ca Ca2 a b

0.083 ⬍0.0001

Determined from Eqn. 6. Measurements had significant contribution from adjacent Ca dimer.

(12)

where cts is the total number of counts of each species, 兹cts is the counting statistics uncertainty in that total, and t is the counting time for each respective species. The uncertainty due to 85Rb is omitted because it is negligible. Reference and natural ratios are used for measured ratios with negligible change in calculated uncertainty (i.e., 共87Sr ⁄ 86Sr兲 ⬇ 共87Sr ⁄ 86Sr兲EN⫺1 & R88N Sr,86Sr ⁄ R88MSr,86Sr ⬇ 1). Eqn. 12 can be simplified using these assumptions to:

␴cs ⬇ 共87Sr ⁄ 86Sr兲EN⫺1

冑

1.28 cts87Sr

.

(14)

Uncertainty is presented as 2 standard errors (2␴), except for the uncertainties in site means for water and otolith 87Sr/86Sr ratios, which are presented as 2 standard deviations (2SD), as noted. 4. RESULTS

␴cs ⬇ 共 Sr ⁄ Sr兲EN⫺1 87

冑

1 cts87Sr

86

⫹

1 cts86Sr

冉

1 ⫺ ⌬m88Sr,86Sr ⌬m88Sr,86Sr

冊

4.1. Ca-dimer Production and Fractionation 2

⫹

1 cts88Sr ⌬m88Sr,86Sr2

.

(13)

For general use, Eqn. 13 can be reduced to a function of cts87Sr by substituting cts87Sr for cts86Sr and cts88Sr based on counting times in this study (10, 10 and 2 s, respectively) and the reference and natural ratios, yielding:

40 Ca and 44Ca are the most abundant calcium isotopes (97 and 2%, respectively) and the primary constituents of the most abundant Ca-dimer interferences in the Sr range (Table 2). The fractionation of the measured Ca-dimer data relative to 40Ca2 (mass 80) is plotted against atomic mass to assess the results (Fig. 4). In this plot, the data for 44Ca48Ca (mass 92) lie significantly above the trend for the other data, suggesting that

Ion microprobe measurement of strontium isotopes

1231

the idealized model is sufficiently accurate for this application. At 5000 M.R.P., however, the models diverge because of differences in the way they account for the reduction in mass resolution. For the empirical model, the mass spectrometer exit slit was opened until 5000 M.R.P. was achieved, thereby increasing the peak-top width while maintaining steep side slopes. The idealized model maintains the peak-top to peakbase width ratio at a constant ratio of 1:3, regardless of mass resolving power. The idealized model is used to compare correction scenarios in the next section. 4.3.

Fig. 4. Mass-dependent fractionation of Ca dimers in low-Sr calcites. The Ca-dimer data at each mass are represented as the permil deviation from natural abundance relative to 40Ca2 for 10 replicate measurements. The measurements of 44Ca48Ca at 92 amu appear to include an unidentified interference and therefore are excluded from the regression. The mean mass-dependent fractionation is the slope of the regression line, ⫺17.2 ⫾ 0.5‰ per amu (2␴).

the measurement of 44Ca48Ca included an unidentified interference. Excluding 44Ca48Ca, the mean mass-dependent fractionation for the Ca dimers is ⫺17.2 ⫾ 0.5‰ per amu. The Ca dimer data at mass 86 and/or 88, which are based on estimates of Ca-dimer contributions to the measurements, can be excluded without changing the estimate of fractionation a statistically significant amount (⫺17.6 to ⫺17.1‰ per amu). In samples with moderate Sr concentrations, the abundances of the Ca dimers at 86, 87 and 88 amu are low, but significant, relative to the abundance of the respective Sr species (Table 3). As a percentage of the Sr species of interest, the unresolved Ca dimers at mass 88 are the most significant (1.0% at 1000 ppm Sr), followed by those at mass 86 (0.7%), and then those at mass 87 (0.2%). However, 40Ca48Ca at mass 88 is effectively resolved at ⬎7000 M.R.P., reducing the relative importance of the Ca-dimer interferences on 88Sr to 0.1% for 1000 ppm Sr samples. Therefore, for these measurements, the interferences to 86Sr are the most significant.

87

Sr/86Sr in Calcite Standards

The calcite standard data for a single analytical session are presented in Figure 5, and Table 5 compares implicit and explicit Ca-dimer corrections for these data. The trends in ⌬87Sr/86Sr vs. 88 Sr/40Ca2 are consistent with our modeling results. Without explicit corrections for the Ca-dimer interferences, ⌬87Sr/86Sr becomes increasingly negative with decreasing 88Sr/40Ca2 (Fig. 5a) because the combined 44Ca42Ca⫹40Ca46Ca interference on 86Sr is larger than the 44Ca43Ca interference on 87Sr (Table 4). Explicitly correcting the data based on the idealized peak model improves the accuracy of the 87Sr/86Sr data, with the 7000 M.R.P. scenario resulting in a better correction to the data than 9000 or 5000 M.R.P. (Fig. 5b, c and d), consistent with the actual mass resolving power of the SHRIMP-RG during this analytical session (⬃7000 M.R.P.). For the corrections based on 9000 M.R.P., ⌬87Sr/86Sr values diverge negatively with decreasing 88Sr/40Ca2, and for corrections based on 5000 M.R.P., ⌬87Sr/86Sr values diverge positively. No modeled mass resolving power, however, yields accurate, explicitly-corrected 87Sr/86Sr ratios (i.e., ⌬87Sr/86Sr within error of zero) for the full range of 88Sr/40Ca2 ratios in our standards. The best-fit correction with the idealized peak-shape model is achieved with ⬃6500 M.R.P., but it does not yield accurate 87Sr/86Sr ratios for the full range of Sr concentrations. At this mass resolving power, the 87Sr/86Sr data are undercorrected (⌬87Sr/86Sr ⬍ 0) for 88Sr/40Ca2 ratios greater than 0.3 (⬎1000 ppm Sr) and over-corrected (⌬87Sr/86Sr ⬎ 0) for Table 3. Estimated abundance of the partially resolved Ca-dimer species relative to the Sr peaks for calcites with 1000 and 500 ppm Sr.

4.2. The Interference Models The contributions of the unresolved isobaric interferences to the respective Sr isotopes are strongly affected by mass resolving power. At 7000 to 9000 M.R.P., 87Sr1H and 48Ca39K are unresolved in the sense that they cannot be separated from the Sr peaks, but based on our interference models, they are effectively resolved because they contribute negligible counts to the measurement of 87Sr at the peak center (Table 4). The Ca dimers can be considered partially resolved because they do not contribute fully to the 86Sr, 87Sr and 88Sr measurements. At 7000 M.R.P., the Ca dimers contribute less than 40% of their maximum intensity to the corresponding Sr measurements, and at 9000 M.R.P., they contribute less than 10%. Of the isobaric interferences listed in Table 1, only 87Rb is fully unresolved at the mass resolving power of the SHRIMP-RG. The empirical and the idealized models for the dimer interferences are in close agreement at 7000 M.R.P., suggesting that

Relative abundancea Ca dimer

M.R.P.

1000 ppm Sr

500 ppm Sr

at 86 amu 42 40 43

44

Ca Ca Ca46Ca Ca2

17,800 12,300 10,400

0.56% 0.16% ⬍0.01%

1.13% 0.32% 0.01%

at 87 amu 43

44

Ca Ca

40

48

16,200

0.17%

0.34%

at 88 amu Ca Ca Ca2 42 Ca46Ca 44

a

9,260 16,500 13,200

0.92% 0.11% ⬍0.01%

1.75% 0.21% ⬍0.01%

See Table 4 for estimated contribution to Sr measurements.

1232

P. K. Weber et al. Table 4. Modeled contribution from partially resolved isobaric interferences to the measured Sr isotope peaks, presented as percent of maximum peak height for the interfering species. Idealized model Species

9000 M.R.P.

7000 M.R.P.

Empiric model 5000 M.R.P.

7000 M.R.P.

5000 M.R.P.

32% 0% 0%

95% 61% 19%

25% 0% 0% 100%

92% 58% 24% 100%

0% 29% 0%

2% 92% 72%

at 86 amu 42

Ca44Ca 40 Ca46Ca 43 Ca2

8% 0% 0%

40% 0% 0%

71% 36% 15% at 87 amu

43

44

Ca Ca Ca39K 86 1 Sr H 87 Rb 48

0% 0% 0% 100%

29% 0% 0% 100%

64% 33% 18% 100% at 88 amu

40 44 42

48

Ca Ca Ca2 Ca46Ca

0% 0% 0%

0% 31% 2%

0% 65% 44%

Fig. 5. Comparison of correction scenarios for unresolved Ca dimers: (a) no Ca-dimer correction, (b) 9000 M.R.P., (c) 7000 M.R.P. and (d) 5000 M.R.P. Natural (x) and synthetic calcite (o) data are included. These analyses were made in a single session. Of these Ca-dimer corrections scenarios, 7000 M.R.P. provides the least divergence from the correct 87 Sr/86Sr ratio (⌬87Sr/86Sr 3 0) for the full range of Sr concentrations, consistent with these measurements being made at ⬃7000 M.R.P. Conversion of 88Sr/40Ca2 to ppm Sr is ⬃3000 ppm Sr per 88Sr/40Ca2.

Ion microprobe measurement of strontium isotopes Table 5. Summary table of Standard

87

Sr/86SrTIMS

87

1233

Sr/86Sr data for natural and synthetic calcite standards for the October 2001 session. 88

ppm Sr

Sr/40Ca2

⌬87Sr/86Srca (‰)

2␴b (‰)

n

⌬87Sr/86Srccc (‰)

⫺1.79 ⫺1.83 ⫺2.00 ⫺2.87 ⫺6.26

0.52 0.42 0.51 1.58 1.13

12 23 18 6 12

⫺1.21 ⫺1.15 ⫺0.30 ⫺0.69 ⫺2.62

⫺1.95 ⫺3.46 ⫺4.48 ⫺14.59

0.67 0.76 0.78 1.65

10 7 4 6

⫺1.32 ⫺2.68 ⫺2.16 ⫺5.93

Natural calcities Ca49 Ca9 Ca70 Ca50 Ca48

0.71168 0.70560 0.70812 0.70870 0.70895

1400 1161 424 333 125

0.443 0.377 0.149 0.116 0.068 Synthetic calcites

1507 1556 1500 1508

0.70998 0.71040 0.70994 0.71002

1435 1192 403 107

0.403 0.339 0.110 0.028

a

Corrected for 87Rb and mass-dependent fractionation only. Precision for a single analysis is ⬃2‰ (2␴) for samples with ⬎400 ppm Sr. c Explicitly corrected for Ca-dimer interferences based on the empirical 7000 M.R.P. interference model. b

88

Sr/40Ca2 ratios less than 0.15 (⬍500 ppm Sr). It is also notable that the uncorrected ⌬87Sr/86Sr values at higher 88Sr/ 40 Ca2 ratios appear to approach a non-zero asymptote (Fig. 5a). The standard data for other analytical sessions also did not approach an asymptote of zero (Fig. 6). 4.4. Comparison of Synthetic Calcite and Aragonite Analyses Many applications require analysis of Sr isotope ratios in aragonite, whereas the available standards are calcite. To evaluate the effect of crystal structure on instrumental mass-dependent fractionation and Ca-dimer production, calcite and aragonite were synthesized from EN-1 powder. Both materials were analyzed in the same SHRIMP-RG session by alternating between them (Table 6). The results for the calcite and aragonite samples are indistinguishable within analytical uncertainties, although the precision of the aragonite measurements is relatively poor. Calcite crystals 40 to 100 ␮m across can be routinely grown in the piston-cylinder apparatus. We were less successful in growing relatively coarse aragonite and our EN-1 aragonite is a somewhat porous aggregate of equant crystals, several of which are included within an analysis spot. The non-uniformity of the aragonite sample surface may be the explanation for the greater variability in the data for EN-1 aragonite compared to calcite. Nevertheless, this experiment demonstrates that any difference in instrumental mass-dependent fractionation of Sr isotopes and Ca dimers between calcite and aragonite is small.

Table 6. Comparison of uncorrected ⌬87Sr/86Sr for synthetic calcitic and aragonitic crystals of EN-1. Mineralogy Calcite Aragonite

⌬87Sr/86Sr (‰)

2␴ (‰)

n

⫺7.7 ⫺7.8

1.5 4.2

8 6

4.5. Otolith

87

Sr/86Sr

The otolith data were corrected using the implicit Ca-dimer correction because of the inaccuracy of the explicit corrections (Table 7). A working curve spanning the Sr concentration for the unknowns was established for each analytical session by replicate analyses of calcite reference standards run under the same analytical conditions (Fig. 6). Otolith Sr concentrations ranged from 400 to 1100 ppm (88Sr/40Ca2 from 0.165 to 0.375). The measurement precision of the 87Sr/86Sr ratio differed among analyses primarily based on total 86Sr and 87Sr counts. Total Sr counts differed among sessions based on primary beam intensity and counting time. For example, FRH4 and FRH6 were analyzed with a 15 nA primary beam and each analysis consisted of 10 cycles, yielding ⬃1.8 ⫻ 106 counts of 87 Sr per analysis (2␴cs ⫽ 1.7‰). By comparison, MRH8, MRH10, MKH12, and MKH14 were analyzed with a 7 nA primary beam, the counting time was 6 s for all species, and each analysis consisted of 6 cycles, yielding ⬃0.7 ⫻ 106 counts of 87Sr per analysis (2␴cs ⫽ 2.6‰). Sr concentration in the sampled volume also was a factor in determining total counts. Another factor influencing measurement precision was ion microprobe performance. Suboptimal performance during some sessions reduced measurement precision (e.g., Fig. 6c). For the analyses presented here, internal measurement precision is one to two times the counting statistics limit (␴(87Sr/86Sr)c ⫽ 1 to 2␴cs). For replicate analyses within defined zones, external measurement precision is as good as 0.6‰ (2␴). The measured 87 Sr/86Sr ratios are within error of the expected 87Sr/86Sr ratios in all cases (See section 5.1. for discussion). Daily banding in these otoliths is ⬃5 ␮m per increment. With the ⬃25 ⫻ 35 ␮m primary beam oriented with the longer axis parallel to the banding, temporal resolution of less than one week was achieved for individual analyses. This resolution was maintained for replicate analyses when the primary beam current was low, allowing repeated analysis at the same spot before count rates and measurement precision began to decline. For measurements consisting of multiple analyses along a

1234

P. K. Weber et al.

rates are ⬃0.1 and 0.15%, respectively, of the 87Sr count rate for a 1000 ppm Sr otolith. Our interference models indicate that at ⬃7000 M.R.P., 48Ca39K is adequately resolved and does not contribute significantly to the 87Sr measurement (Table 4). At 5000 M.R.P., 30 to 60% of the full intensity of the 48Ca39K interference contributes to the 87Sr measurement. In contrast, 87 Rb is completely unresolved, and its abundance must be calculated for each analysis using 85Rb and subtracted from the mass 87 signal, as described in section 3.1.1. 5. DISCUSSION

The data presented here demonstrate for the first time that Sr/86Sr can be measured accurately with subpermil external precision in calcites and otoliths using a high-transmission, highmass resolving power ion microprobe. Data for unknown samples are corrected using a working curve (implicit Ca-dimer correction) defined by reference standards spanning the range of Sr concentrations in the unknowns and run under the same operating conditions as the unknowns. These results represent a significant improvement in accuracy and precision over previous SIMS measurements of moderate-Sr calcium carbonate samples (Exley, 1983; Scatena-Wachel, 1986; Kennedy et al., 1990). Ca-dimer interferences are significant (⬎0.1%) at the Sr concentrations in this study (⬍1500 ppm Sr), and the measured 87 Sr/86Sr ratios must be corrected for these interferences to achieve accurate results. Relative to the implicit Ca-dimer correction, explicit Ca-dimer corrections yield less precise and less accurate data. Because the dimer peaks are partially resolved at high mass resolving power, the magnitude of the interferences must be calculated based on peak shape and instrumental massdependent fractionation of the Ca dimers. The associated uncertainties have the effect of reducing measurement precision. Furthermore, the corrected 87Sr/86Sr ratios are not accurate (⌬87Sr/ 86 Sr ⫽ 0) for the full range of 88Sr/40Ca2 ratios for any modeled mass resolving power because of unaccounted interferences or other effects (Fig. 5 and Table 5). Additional explicit corrections could potentially be incorporated to produce more accurate data, but measurement precision would be further compromised relative to using the implicit correction of a working curve. It is worth noting that the implicit Ca-dimer correction is based on the relative production of the Ca-dimer and Sr species as measured by 88Sr/40Ca2, not the Sr concentration in the sample. We take this approach because (1) 88Sr/40Ca2 can readily be measured during the Sr isotope measurements to account for variations in Sr concentrations in samples, and (2) 88 Sr/40Ca2 reflects the production of Ca dimers directly. Using the 88Sr/40Ca2 ratio for the Ca-dimer correction also opens up the possibility of applying this sample-standard bracketing approach to materials that are not uniform with respect to Ca, provided matrix effects are not significant. The accuracy of the implicit Ca-dimer correction in the working-curve approach relies on the interferences from the Ca dimers for a given 88Sr/40Ca2 ratio being constant from spot to spot relative to the uncertainty of the 87Sr/86Sr measurement. This assumption appears to be valid. Interference variability does not appear to be significant because measurement uncertainty does not become increasingly large relative to counting statistics at lower Sr concentrations. At 150 ppm Sr, the lowestconcentration natural reference sample analyzed (Ca48), inter87

Fig. 6. Working curves for otolith 87Sr/86Sr measurements. (a) Otoliths MRH8, MRH10, MKH12 and MKH14 were analyzed during the September 1999 session, which had low total counts compared to later sessions. ⌬87Sr/86Sr ⫽ ⫺3.89 ⫾ 0.67‰, n ⫽ 32. For a single analysis, 2␴ ⫽ 3.8‰, as compared to 2␴cs ⬇ 2.6‰. (b) Otoliths FRH4 and FRH6 were analyzed during the October 2000 session. ⌬87Sr/86Sr ⫽ ⫺4.96 –13.66 (88Sr/40Ca2), n ⫽ 65, 2␴ ⫽ 0.32‰ at the mean 88Sr/40Ca2 ratio. For a single analysis, 2␴ ⬇ 2‰, as compared to 2␴cs ⬇ 1.7‰. (c) Otolith BCA10 was analyzed during the April 2002 session, which had suboptimal performance. ⌬87Sr/86Sr ⫽ ⫺4.26 ⫾ 0.44‰, n ⫽ 87. For a single analysis, 2␴ ⫽ 4.2‰, as compared to 2␴cs ⬇ 2.1‰.

group of growth increments, temporal resolution was as good as one week. Based on the average count rates for 40Ca39K (mass 79) and 85 Rb (mass 85) in the otoliths, the 48Ca39K and 87Rb count

Ion microprobe measurement of strontium isotopes Table 7. Measured otolith

87

Sr/86SrSIMS vs. expected Whole otolith 87 Sr/ 86SrTIMS

Otolith & zone

87

Sr/86Sr ratios, with whole otolith

87

1235

Sr/86SrTIMS of complement if applicable. See text. 2␴ (external)

nc

0.7086 0.7069 0.7096 0.7071 0.7092

0.00083 0.00051 0.0010 0.00044 0.00060

10 6 7 10 4

0.7087 0.7087

0.7091 0.7086 0.7092 0.7068c

0.00090 0.00083 0.0012 0.00095

4 4 4 4

0.7087 0.7087

0.7087 0.7087

0.0011 0.0015

4 3

0.7073f

0.0023

2

0.7083 0.7062f

0.00091 0.00068

4 5

0.7072 0.7071 0.7056 0.7060

0.0012 0.0016 0.00084 0.0013

5 5 4 4

Expected Sr/ 86Sra

87

87

Measured Sr/ 86SrSIMSb

Hatchery Juveniles d

FRH4-ph&pf FRH4-inner FRH4-outer FRH6-inner FRH6-outer MRH8 MRH8-phd MRH8-inner MRH8-outer MRH8-rime MRH10 MRH10-inner MRH10-outer MKH12 MKH12-innerf MKH14 MKH14-phd MKH14-innerf

0.7071 0.7097 0.7071 0.7097 0.70822

0.70866 0.7077 0.70726

Wild Adult (juvenile portion) BCA BCA BCA BCA

d

10-ph 10-pfd 10-inner 10-mid

0.7048

2SD ⫽ 0.0004. Determined from implicit Ca-dimer correction and a working curve. c Number of analyses within otolith zone. d Pre-hatch (ph) and post-hatch-pre-feeding (pf) zones expected to reflect a mixture of marine (87Sr/86Sr ⫽ 0.7092) and riverine Sr. e Water 87Sr/86Sr ratio at location of recapture is 0.7066. f Two possible expected 87Sr/86Sr ratios: 0.7063 and 0.7076. a

b

nal precision for a single analysis is 3.9‰ (2␴), which is approximately at the counting statistics limit (Table 5). The internal precision for the 400-ppm Sr natural reference sample (Ca70) is also approximately at the counting statistics limit. Furthermore, in the range of Sr concentrations in the otoliths (400 to 1100 ppm Sr), changes in the dimer interferences would have to be on the order of 25 to 50% to make significant changes (⬎1‰) in the Sr isotope measurements. At higher Sr concentrations (1000 to 1500 ppm Sr), measurement precision per analysis does not improve with increasing Sr count rates (2␴ of ⬃2‰), suggesting that factors other than the Ca-dimer interferences and Sr counts limit measurement precision at higher Sr concentrations. 5.1. Otolith

87

Sr/86Sr

Among other applications, this high spatial resolution method of measuring 87Sr/86Sr will be useful for extracting life history information from fish otoliths. Otolith 87Sr/86Sr ratio reflects river water 87Sr/86Sr ratio, which can differ significantly among watersheds because of differences in lithology (Kennedy et al., 1997; Ingram and Weber, 1999; Kennedy et al., 2000; Weber, 2002; Bacon et al., 2004). Spatially resolved analyses are necessary because bulk otolith 87Sr/86Sr can represent an average of disparate sources of Sr, even in juvenile salmon. The prefeeding otolith is expected to reflect maternal chemistry (Kalish, 1990), and

thereafter, water and to a lesser extent, food chemistry (Farrell and Campana, 1996; Weber, 2002). After hatching and emerging from the streambed gravel, juvenile salmon can be flushed from their natal locations within days, and migration among rearing habitats can be on similar time scales (Kjelson et al., 1982; Healy, 1991). The Sr isotopic composition of otoliths will be zoned based on changes in Sr sources over the life of the fish being recorded in the growth increments of the otolith. Therefore, to construct a meaningful chronology of a fish’s migration history using otolith 87Sr/ 86 Sr in this case, the analyses must sample the minimum number of daily increments possible. The otoliths examined in this study are from chinook salmon taken from the Sacramento-San Joaquin river system. Our SIMS analyses of these otoliths reveal the Sr isotope zonation within individual otoliths (Table 7). For the purposes of this study, we focus on assessing the accuracy of the technique by comparing our results to the expected 87Sr/86Sr ratios. Secondarily, we consider other information drawn from these measurements, including differences in 87Sr/86Sr ratios among otoliths in the prefeeding portion. Our confidence in the expected 87Sr/86Sr ratio differs among otoliths and zones, based on what we know about the history of the fish. We have direct knowledge of the history of the fish from which otoliths FRH4 and FRH6 were taken because these fish were transferred from the Feather River Hatchery to a

1236

P. K. Weber et al.

holding location with a significantly different mean 87Sr/86Sr signature for this study. The measured 87Sr/86Sr ratios in the inner otolith zones are consistent with the fish starting their lives at the Feather River Hatchery, and the measured 87Sr/86Sr ratios in the outer zones are consistent with the fish being transferred to the holding location. These measurements also consist of the most replicates and include the highest precision otolith measurement in this data set (2␴ ⫽ 0.00044 or 0.6‰). Therefore, the agreement between the measured and expected 87 Sr/86Sr for the inner and outer zones of these otoliths is the strongest evidence in this study of the accuracy of this technique. Our knowledge of the history of the other fish in this study comes from coded-wire tags recovered from these fish. The codes on the wire tags identify where the fish were tagged and the dates and locations of release and recapture. The tagging information can be directly translated into holding location before release for fish from hatcheries that raise all of their own fish, which is the case for most hatcheries in the SacramentoSan Joaquin river system, including the Feather River Hatchery and the Merced River Hatchery. Therefore, we have confidence in the expected 87Sr/86Sr ratios for the inner and outer zones of otoliths MRH8 and MRH10, which should have been deposited during early and later growth at the Merced River Hatchery before release. The measured and the expected 87Sr/86Sr ratios are within error, providing further evidence of the accuracy of this technique for otoliths. In the case of otoliths MKH12 and MKH14, however, there are two locations at which the fish could have spent their early growth periods. The fish from which these otoliths came were tagged by the Mokelumne River Hatchery, which received fish from the Feather River Hatchery in the same year that the fish with otoliths MKH12 and MKH14 were raised. The Feather River Hatchery fish were raised, tagged and released with Mokelumne River Hatchery fish, and the two sets of fish cannot be distinguished based on the tag (Weber, 2002). Therefore, we expect the inner portion of these otoliths to reflect the fish starting life at one of these two hatcheries. In the case of MKH12, the measurement of the inner portion of the otolith agrees with the expected 87Sr/86Sr ratio for the Mokelumne River Hatchery, but with only two replicates and poor precision, no conclusion can be reached regarding sample origin. By contrast, the 87Sr/86Sr measurement of the inner zone of MKH14 has good precision (0.7062 ⫾ 0.00068), and therefore, based on this measurement, it is very likely that this fish started its life at the Feather River Hatchery (0.7063) before being transferred to the Mokelumne River Hatchery (0.7076). It is worth noting that the Feather River Hatchery had an anomalously low 87Sr/86Sr signature in the winter of 1997, the year these fish were raised (0.70633 ⫾ 0.00012 (⫾2SD), n ⫽ 3 vs. 0.7071 in other years), apparently due to extreme winter flooding changing the mix of Sr sources in the river (Weber, 2002). The likely transfer of this fish is also suggested by the lower than expected whole otolith TIMS 87Sr/86Sr ratio for the other sagittal otolith from the sampled fish (0.70726 ⫾ 0.00002), which is only marginally consistent with the fish being raised solely at the releasing hatchery. The spatially resolved SIMS analysis reveals the chemical zonation within the otolith and the likely cause of the lower than expected whole otolith 87Sr/86Sr ratio. This result is more of a demonstration of the utility of this high-spatial resolution method than a demonstration of its accuracy.

BCA10 is the only otolith from a wild fish in this study. We have good confidence that it spent its entire pretagging life in Butte Creek, but we cannot be certain of where in the river it was, and therefore, what the exact 87Sr/86Sr ratio should be. At the tagging site, the 87Sr/86Sr ratio is potentially somewhat higher than in the region where the fish spawn, for which we have data (0.7048). This uncertainty in expected 87Sr/86Sr is matched by low measurement precision during this particular session. Therefore, although the measured 87Sr/86Sr ratio for the inner portion of BCA10 (0.7056 ⫾ 0.00084) is marginally higher than expected, it is within error of our data for Butte Creek water 87Sr/86Sr ratio. While location information is typically poor for the time period after release into the wild, we can reasonably constrain the expected 87Sr/86Sr ratio for otolith MRH8 after release. The fish was raised in the Merced River Hatchery, released into the lower Tuolumne River on April 23, 1997 and recaptured ⬃150 km downstream in the Sacramento-San Joaquin river delta on May 11, 1997. The SIMS analyses of MRH8 sampled ⬃5 bands deposited between 5 and 11 days after release. The mean of these analyses (0.7068 ⫾ 0.00095) is distinctly lower than the 87Sr/86Sr signatures of the Merced River Hatchery (0.7087) and the upper Tuolumne River (0.7079), and is consistent with our limited data for the 87Sr/86Sr ratio of the release location (0.70650, n ⫽ 1) and the downstream river passage (0.70707, n ⫽ 1), as well as the delta (0.70660 ⫾ 0.00087). The 87Sr/86Sr measurements of the central portions of otoliths provide insight into the Sr budget of prefeeding fish. The origin of the Sr in the otolith primordia and the immediately surrounding material out to the hatch line is of interest because the concentration of Sr in this region (reported as the Sr/Ca ratio) is commonly used to distinguish the progeny of anadromous and resident fish (Kalish, 1990; Rieman et al., 1994; Volk et al., 2000; Zimmerman and Reeves, 2000; Howland et al., 2001; Zimmerman and Reeves, 2002). The prehatch portion of the otolith (Fig. 2; in chinook salmon, ⬃150 ␮m thick ⫻ 200 ␮m wide ⫻ 250 ␮m long) is formed while the fish is in the egg, and its Sr concentration should be strongly influenced by maternal chemistry, which in anadromous fish is expected to reflect the higher Sr/Ca ratio in the ocean, as compared to Sr/Ca ratios in many freshwater environments. The region just outside of the prehatch zone forms after the fish hatches out of the egg but is still dependent on the egg yolk for nutrition (to ⬃300 ⫻ 400 ⫻ 500 ␮m). We refer to this region as the posthatch—prefeeding zone. While maternal Sr in anadromous fish is expected to be largely marine (87Sr/86Sr ⫽ 0.7092), some variation in egg Sr is expected depending on how much time the adult female fish spends in fresh water before spawning (Volk et al., 2000; Bacon et al., 2004). Sr may also be derived from the river water via diffusion across the egg membrane in prehatch fish and directly through the gills in posthatch, prefeeding fish. The proportions of marine and riverine Sr in the prefeeding otolith can be estimated by comparing the otolith 87Sr/86Sr ratios to the marine and local river water 87Sr/86Sr ratios. The prehatch and posthatch—prefeeding 87Sr/86Sr measurements are consistent with differences in Sr sources based on maternal spawning instinct. Otoliths FRH4 and MKH14 are from ocean-type salmon, which spawn soon after entering fresh water. Their prefeeding 87Sr/86Sr ratios (0.7086 ⫾ 0.00083 & 0.7083 ⫾ 0.00091) are distinctly higher than the 87Sr/86Sr ratios for the adjacent postfeeding zones (0.7069 & 0.7062) and the river water

Ion microprobe measurement of strontium isotopes

(0.7062), and they are low relative to the marine 87Sr/86Sr ratio (0.7092), although within two standard errors. Otolith BCA10 is from a stream-type salmon, which spend more time in fresh water before spawning. Its prefeeding 87Sr/86Sr ratios (0.7072 ⫾ 0.0012 & 0.7071 ⫾ 0.0016) are distinct from the marine 87Sr/86Sr ratio and the site 87Sr/86Sr ratio (0.7048), and are high relative to the 87 Sr/86Sr ratios in the zone just outside the core (0.7056), although within two standard errors. The measured 87Sr/86Sr ratio for the prehatch zone of MRH8 (0.7091 ⫾ 0.0009) is indistinguishable from the Merced River Hatchery water 87Sr/86Sr (0.7085) and marine 87Sr/86Sr. Otolith MRH8 is from an ocean-type salmon. Taken together, these data support the hypothesis that the longer the maternal parent spends in freshwater before spawning, the more riverine Sr will be incorporated into the prefeeding otolith. Similar results were found for chinook salmon from Washington and Idaho, USA (Bacon et al., 2004). 5.2. Comparison with Other Methods Microsampling for a conventional 87Sr/86Sr analysis and laser ablation (LA) ICP-MS are the two alternatives to SIMS for extracting spatially-resolved 87Sr/86Sr data from a solid sample. To our knowledge, all of the published work on CaCO3 samples with moderate Sr concentrations (500 to 1500 ppm Sr) has been on otoliths. Kennedy et al. (2002) used a computer controlled micromill to generate temporally-resolved Sr isotope records for Atlantic salmon (Salmo salar), extracting 10 to 25 ␮g samples to obtain ⬃20 ng Sr for TIMS analysis (2␴ ⬇ 0.02‰). To maximize temporal resolution, they minimized the sampling width (40 –50 ␮m) by extending the sampling volume along the otolith banding. Sampling length was shown as 1 to 2 mm, and sampling depth was not reported. LA-ICP-MS has also been tested on otoliths (Thorrold and Shuttleworth, 2000; Outridge et al., 2002), but the issues of standardization, interferences and measurement precision for this method have not been addressed for moderate-Sr samples. This work did not use multicollection, which would improve precision. These initial LA-ICP-MS studies suggest that ⬃1 ␮g of otolith would be necessary to achieve subpermil precision. The SIMS method demonstrated in this study has higher spatial resolution and far less depth penetration than these other methods. For example, 10 replicate SIMS analyses could be made in five spots along 200 microns of a set of growth increments. Such an analysis would consume on the order of 20 ng of material, which is ⬃1000 times less material than needed for a TIMS analysis. Lateral resolution as good as 30 ␮m can be achieved with careful placement of the analysis spot, and sampling depth is less than a few microns. With such low sample consumption, sampling location can be verified after the analyses. By comparison, to collect sufficient material for a TIMS analysis from a microsampling track 50 ␮m deep and 50 ␮m wide would require a sampling length at least 1.5 mm long, which is the full circumference of the growth increments deposited in the first weeks after the fish emerge from the gravel and start to feed. Theoretically, if a chinook salmon otolith is sectioned precisely relative to the juvenile banding structure, the sampling depth for the exposed increments can be on the order of 50 ␮m all the way around the first-feeding region without sampling adjacent growth zones at depth (Fig. 1b and 1c). In practice, however, the exact internal morphology can only be discerned by destructive sectioning, and crossover into adjacent zones below the sample surface would be

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difficult to monitor with techniques that penetrate tens of microns. The shallow sampling depth of the ion microprobe largely eliminates the potential for crossing into adjacent increments at depth. Overall, the low sample consumption of the ion microprobe results in higher resolution and more precise sampling. The trade-off is in measurement precision. The measurement precision of the TIMS technique is ⬃0.02‰ per analysis (2␴), compared to ⬃2‰ for a single SIMS analysis and subpermil precision with replicate SIMS analyses. The spatial resolution of this SIMS technique will be useful for applications for which this lower level of measurement precision is sufficient. In the case of wild juvenile chinook salmon in the SacramentoSan Joaquin river system, the measurement precision of SIMS is sufficient, and its spatial resolution is necessary. The measurement precision of SIMS is sufficient because most of the major spawning sites in the Sacramento-San Joaquin river system can be identified based differences in river water 87Sr/86Sr ratio greater than 1‰ (Ingram and Weber, 1999; Weber, 2002). Sites with smaller differences in 87Sr/86Sr ratio (⬍0.5‰) cannot be readily differentiated by 87Sr/86Sr ratio alone because of natural variability in these rivers. The spatial resolution of SIMS is necessary because juvenile chinook salmon can be flushed from their natal stream directly after emerging from the gravel and beginning to feed, making high temporal resolution essential to correctly identifying natal location (Kjelson et al., 1982; Healy, 1991). The lateral resolution achieved in this study (⬃30 ␮m), which represents approximately one-week temporal resolution in the otoliths of these salmon, could resolve such early life movements. The spatial resolution of the SIMS method is also valuable because the small sample volume consumed allows for additional analyses for other isotopic or elemental markers directly adjacent to the 87Sr/86Sr analyses (Fig. 2). Additional markers can potentially differentiate sites with similar 87Sr/86Sr ratios. For example, in the Sacramento-San Joaquin river system, the offset in otolith 87Sr/86Sr relative to water 87Sr/86Sr caused by the marine-based feeds used in the hatcheries results in similar otolith 87Sr/86Sr signatures for some hatchery and river sites (e.g., Feather River Hatchery, 0.7071, and Mokelumne River, 0.7070). Because the marine-based feeds have elevated 34S/32S relative to the freshwater diets in the wild, SIMS analysis of otolith sulfur isotopic composition can be used to distinguish hatchery-raised and naturally spawned salmon to allow the correct assignment of origin (Weber et al., 2002). 6. CONCLUSION 87

Measurement of Sr/86Sr by ion microprobe in CaCO3 is a difficult problem because of the isobaric interferences. Our results show that a high-mass resolving power, high-transmission ion microprobe can accurately make 87Sr/86Sr measurements with precision approaching the limit of counting statistics, using sample-standard bracketing and a working curve. For Sr concentrations greater than 400 ppm, the 87 Sr/86Sr measurement can be made to a precision of ⬃2‰ per spot (2␴), with subpermil precision with replicate analyses. Measurement of 87Sr/86Sr in aragonite does not differ significantly from calcite 87Sr/86Sr measurement using this method. The most important differences between the calcite standards and the otoliths are the interfering species. Otoliths contain higher concentrations of Na, K, Mg and Rb than

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the calcite standards (⬃1000, 1000, 10 and 1 ppm, respectively, in otoliths). These elements produce interferences (Table 1) that must be resolved or corrected because the abundance of these species is likely to change significantly from spot to spot due to sample inhomogeneity. At the mass resolving powers used for this study, all of the interferences based on these elements are adequately resolved, except for 87 Rb, for which a correction must be made based on 85Rb. The development of this in situ method establishes the highest spatial resolution technique with subpermil precision available for measuring 87Sr/86Sr ratio in calcium carbonate materials with moderate Sr concentrations. The high spatial resolution of this SIMS technique allows otolith analysis on a temporal scale relevant to juvenile fish migration. Temporal resolution less than one week can be achieved with the standard analysis spot size (⬃25 ⫻ 35 ␮m), and higher resolution is possible with changes in tuning of the analysis beam. This level of resolution also allows for additional types of analyses directly adjacent to the 87Sr/86Sr analyses, enabling migration chronologies constrained by multiple geochemical markers. As a result, this method will enable the collection of data on migration that cannot currently be obtained for individual fish, allowing researchers to better understand complex behavior. Acknowledgments—Funding for this project was provided by the California Department of Water Resources and the Institute for Geophysics and Planetary Physics. The U.S. Geological Survey provided time on the SHRIMP-RG at the Stanford/USGS Micro-Analytical Center. Work at Lawrence Livermore National Laboratory was performed under the auspices of U.S. Department of Energy. Harold Persing advised us in setting up and tuning the SHRIMP RG, and Persing and Cynthia Schwartz aided in characterizing instrument performance and reducing data. Ben Hankins synthesized carbonates and John Fitzpatrick performed the related elemental and isotopic analyses. Peggy Genaro provided natural calcite samples, and Tom Owens, Helen Tolliver and Kathleen Johnson performed all related Sr separations and TIMS analyses. Elizabeth Pickett provided much valued sample preparation, and Pickett and Marilyn Vogel helped with ion microprobe analysis. Ross Williams wrote the program used for calculating isobaric interferences. Special thanks go to William Cox and Tresa Veek for providing the facilities for the fish transfer study. Fish samples were provided by the California Department of Water Resources, California Department of Fish and Game, and the U.S. Fish and Wildlife Service. We thank Christie Beeman, Eteine Deloule, Stephen Galer, Rachel Johnson, Jim Kirchner, Brian Marshall and Renee Takesue for comments on the manuscript. Associate editor: S. Galer REFERENCES Bacon C. R., Weber P. K., Larsen K. A., Reisenbichler R., and Fitzpatrick J. A., and Wooden J. L. (2004) Ion microprobe Sr isotope and Sr/Ca analyses of otoliths reveal migration and rearing histories of chinook salmon (Oncorhynchus tshawytscha). Can. J. Fish. Aquat. Sci. 61 (12), (in press). Campana S. E. (1999) Chemistry and composition of fish otoliths: pathways, mechanisms and applications. Mar. Ecol. Prog. Ser. 188, 263–297. Campana S. E. and Neilson J. D. (1985) Microstructure of fish otoliths. Can. J. Fish. Aquat. Sci. 42, 1014 –1032. Compston W. (1996) SHRIMP: origins, impact and continuing evolution. J. R. Soc. West. Aust. 79, 109 –117. Exley R. A. (1983) Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates. Earth Planet. Sci. Let. 65, 303–310.

Farrell J. and Campana S. E. (1996) Regulation of calcium and strontium deposition on the otoliths of juvenile tilapia, Oreochromis niloticus. Comp. Biochem. Physiol. A 115, 103–109. Faure G. (1986) Principles of Isotope Geology. John Wiley & Sons. Fritzsimons I. C. W., Harte B., and Clark R. M. (2000) SIMS stable isotope measurements: counting statistics and analytical precision. Min. Mag. 64, 59 – 83. Healy M. C. (1991) Life history of chinook salmon (Oncorhynchus tshawytscha). In Pacific Salmon Life Histories (ed. C. Croot and L. Margolis), pp. 311–393. University of Br. Columbia Press. Howland K. L., Tonn W. M., Babaluk J. A., and Tallman R. F. (2001) Identification of freshwater and anadromous inconnu in the Mackenzie River system by analysis of otolith strontium. Trans. Amer. Fish. Soc. 130, 725–741. Ichii T., and Mugiya Y. (1983) Comparative aspects of calcium dynamics in calcified tissues in the goldfish Carassius auratus. Bull. J. Soc. Sci. Fish. 49, 1039 –1044. Ingram B. L., and Weber P. K. (1999) Salmon origin in California’s Sacramento-San Joaquin river system as determined by otolith strontium isotopic composition. Geol. 27, 851– 854. Ireland T. R. (1995) Ion microprobe mass spectrometry: techniques and applications in cosmochemistry, geochemistry and geochronology. In Adv. in Analytical Geochemistry, Vol. 2 (ed. M. Hyman and M. W. Rowe), pp. 1–118. JAI Press Inc. Ireland T. R. (2004) SIMS measurement of stable isotopes. In Handbook of Stable Isotope Analytical Techniques (ed. P. A. de Groot), Vol. I. Elsevier Science. Ireland T. R., and Wooden J. L. (1998) A SHRIMP-RG at Stanford University. EOS 79, F953. Kalish J. M. (1990) Use of otolith microchemistry to distinguish the progeny of sympatric anadromous and non-anadromous salmonids. Fish. Bull. 88, 657– 666. Kennedy A. K., Hutcheon I. D., Wyllie P. J., and Wasserburg G. J. (1990) Ion microprobe analysis of 87Sr/86Sr in carbonates. Geol. Soc. Australia Absts. 27, 5419. Kennedy B. P., Blum J. D., Folt C. L., and Nislow K. H. (2000) Using natural strontium isotopic signatures as fish markers: Methodology and application. Can. J. Fish. Aquat. Sci. 57, 2280 –2292. Kennedy B. P., Folt C. L., Blum J. D., and Chamberlain C. P. (1997) Natural isotope markers in salmon. Nature 387, 766 –767. Kennedy B. P., Klaue A., Blum J. D., Folt C. L., and Nislow K. H. (2002) Reconstructing the lives of fish using Sr isotopes in otoliths. Can. J. Fish. Aquat. Sci. 59, 925–929. Kjelson M. A., Raquel P. F., and Fisher F. W. (1982) Life history of Fall-run chinook salmon Oncorhynchus tshawytscha, in the Sacramento-San Joaquin estuary, California. In Estuarine Comparisons (ed. V. S. Kennedy), pp. 393– 411. Academic Press. McKeegan K. D., Leshin L. A., Russell S. S., and MacPherson G. J. (1998) Oxygen isotopic abundances in calcium-aluminum-rich inclusions from ordinary chondrites: implications for nebular heterogeneity. Science 280, 414 – 418. Neilson J. D., and Geen G. H. (1982) Otolith of chinook salmon (Oncorhynchus tshawytscha): daily growth increments and factors influencing their production. Can. J. Fish. Aquat. Sci. 39, 1340 – 1347. Outridge P. M., Chenery S. R., Babaluk J. A., and Reist J. D. (2002) Analysis of geological Sr isotope markers in fish otoliths with subannual resolution using laser ablation-multicollector-ICP-mass spectrometry. Env. Geol. 42, 891– 899. Paterson B. A., Riciputi L. R., and McSween H. Y. Jr. (1997) A comparison of sulfur isotope ratio measurement using two microprobe techniques and application to analysis of troilite in ordinary chondrites. Geochim. Cosmochim. Acta 61, 601– 609. Patterson W., Smith G. R., and Lohmann K. C. (1993) Continental paleothermometry and seasonality using the isotopic composition of aragonitic otoliths of freshwater fishes. Geophys. Mon. 78, 191–202. Riciputi L. R., Paterson B. A., and Ripperdan R. L. (1998) Measurement of light stable isotope ratios by SIMS: Matrix effects for oxygen, carbon and sulfur isotopes in minerals. Int. J. Mass. Spectrom. 178, 81–112.

Ion microprobe measurement of strontium isotopes Rieman B. E., Myers D. L., and Nielsen R. L. (1994) Use of otolith microchemistry to discriminate Oncorhynchus nerka of resident and anadromous origin. Can. J. Fish. Aquat. Sci. 51, 133–141. Scatena-Wachel D. E. (1986) Ion microprobe measurements of radiogenic nuclides: cosmochemical and geochemical tracers. Ph.D. Dissertation, University of Chicago. Sugarman P. J., McCartan L., Miller K. G., Feigenson M. D., Pekar S., Kistler R. W. and Robinson A. G. (1997) Strontium-isotopic correlation of Oligocene to Miocene sequences, New Jersey and Florida. In Proceedings of the Ocean Drilling Program, Scientific Results (ed. K. G. Miller and S. W. Snyder), Vol. 150X, pp. 147–159. Thorrold S. R., and Shuttleworth S. (2000) In situ analysis of trace elements and isotope ratios in fish otoliths using laser ablation sector field inductively coupled plasma mass spectrometry. Can. J. Fish. Aquat. Sci. 57, 1232–1242. Volk E. C., Blakeley A., Schroder S. L., and Kuehner S. M. (2000) Otolith chemistry reflects migratory characteristics of Pacific salmonids: Using otolith core chemistry to distinguish maternal associations with sea and freshwaters. Fish. Res. 46, 251–266. Weber P. K. (2002) Geochemical Markers in the Otoliths of Chinook Salmon in the Sacramento-San Joaquin River System, California. Ph.D. Dissertation, University of California, Berkeley.

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Weber P. K., Hutcheon I. D., McKeegan K. D., and Ingram B. L. (2002) Otolith sulfur isotope method to reconstruct salmon (Oncorhynchus tshawytscha) life history. Can. J. Fish. Aquat. Sci. 59, 587–591. Zimmerman C. E., and Reeves G. H. (2000) Population structure of sympatric anadromous and nonandromous Oncorhynchus mykiss: evidence from spawning surveys and otolith microchemistry. Can. J. Fish. Aquat. Sci. 57, 2152–2162. Zimmerman C. E., and Reeves G. H. (2002) Identification of steelhead and resident rainbow trout progeny in the Deschutes River, Oregon, revealed with otolith microchemistry. Trans. Amer. Fish. Soc. 131, 986 –993.

APPENDIX

SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.gca.2004.05.051.