SRM 915a Research Articles

The Giant Pacific Oyster (Crassostrea gigas) as a modern analog for fossil ostreoids: Isotopic (Ca, O, C) and elemental (Mg/Ca, Sr/Ca, Mn/Ca) proxies

Modern analogs are an essential part of palaeoclimate studies, because they provide the basis for the understanding of geochemical signatures of fossils. Ostreoids are common in many sedimentary sequences and because of their fast growth, high temporal resolution sampling of past seasonal variability is possible. Here, two shell structures of modern Giant Pacific Oysters (Crassostrea gigas), the chalky substance and foliate layers, have been sampled for trace element distributions (Mg, Sr, Mn) and stable isotope variability (C, O, Ca). Oxygen isotopes exhibit a clear seasonal signature. Mean carbon isotope values of different oysters agree within 0.1‰, but ontogenic variability is complicated by shell growth patterns and potential small vital effects. The calcium isotope ratios are found to be constant throughout ontogeny within analytical precision at a value of δ44/40Ca = 0.68 ± 0.16‰ (2 sd) SRM–915a which is consistent with other bivalve species. Calcium isotope ratios in oyster shell material might thus be a possible proxy for palaeo seawater calcium isotope ratios. Element/Ca ratios are significantly higher in the chalky substance than in the foliate layers and especially high Sr/Ca and Mn/Ca ratios are observed for the first growth season of the oysters. Mg/Ca ratios in the chalky substance show a negative correlation with δ18O values, compatible with a temperature dependence, whereas this correlation is absent in the foliate layers. Seasonal changes of Sr/Ca are controlled by metabolic processes, whereas for Mn/Ca an additional environmental control is evident.

Precise analysis of Ca isotope ratios (δ44/40Ca) using an optimized 43Ca–42Ca double-spike MC-TIMS method

We used a Monte Carlo error model to optimize a 43Ca–42Ca double-spike technique for measuring Ca isotope ratios (δ44/40Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). Optimization is non-unique and method-specific because errors for counting statistics and Johnson noise depend on ion beam intensities, integration times, and the total number of duty cycles, while the cumulative effects of measurement-induced Faraday collector damage restrict the double-spike 43Ca/42Ca and spike/sample ratios that can be practically employed. Factors that improve counting statistics and limit Johnson noise also accelerate collector damage and associated drift in measured δ44/40Ca values. Accordingly, better internal precision occurs at the expense of accuracy and external reproducibility. For a 20V 40Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of 43Ca/42Ca ratios should yield internal precisions of ±0.020–0.025‰ (2σSEM) for a runtime of 2.5h/sample, but theoretical constraints for minimizing drift exclude options having relatively high spike/sample ratios. Using a Thermo Fisher MC-TIMS (Triton), we tested 43Ca/42Ca=1 [42Ca/(42Ca+43Ca)=0.50mol/mol] and spike/sample=0.33 [Cadsp/(Cadsp+Casmp)=0.25mol/mol] by repeatedly analyzing OSIL Atlantic seawater (sw), NIST SRM 915a, NIST SRM 915b, and CaF2 over 11 sessions spanning 3 months. The average internal precision for 171 measurements is ±0.024‰, in excellent agreement with the model prediction. We adopted an experimental protocol that eliminates drift for a single measurement session (≤30 runs), thereby increasing accuracy and external reproducibility across consecutive sessions. The average δ44/40Ca values agree well with accepted values: δ44/40Casw-sw=0.000±0.005‰ (n=62), δ44/40Ca915a-sw=−1.865±0.006‰ (n=42), δ44/40Ca915b-sw=−1.134±0.006‰ (n=37), and δ44/40CaCaF2-sw=−1.392±0.008‰ (n=30). The global, long-term external reproducibility for the method is ±0.041‰ (2σSD), which represents a two- to ten-fold improvement over Ca isotope measurements made with existing double-spike MC-TIMS methods.

Calcium isotope measurement by combined HR-MC-ICPMS and TIMS

We report a novel approach for the chemical purification of Ca from silicate rocks by ion-exchange chromatography, and a highly-precise method for the isotopic analysis of Ca—including the smallest isotope 46Ca (0.003%)—by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS), in combination with thermal ionization mass spectrometry (TIMS). Using this approach, we measured the Ca isotope composition of a number of terrestrial rock standards and seawater. Based on these data, we show that the non-mass-dependent abundances of μ43Ca, μ46Ca, and μ48Ca (normalized to 42Ca/44Ca) can be measured with an external reproducibility of 1.8, 45 and 12.5 ppm, respectively, when measured by HR-MC-ICPMS and μ40Ca and μ43Ca to 80 and 23 ppm, respectively, when measured by TIMS (μ notation is the per 106 deviation from the reference material). Comparison with earlier studies demonstrate that it is possible to measure the mass-dependent Ca isotope composition of terrestrial materials using HR-MC-ICPMS with an external reproducibility comparable to that typically obtained with double spike TIMS techniques. The resolution of the mass-independent 43Ca, 46Ca and 48Ca data obtained by HR-MC-ICPMS represents more than a 45-, 120-, and 18-fold improvement, respectively, relative to earlier measurements obtained by TIMS. This improvement allows for a better understanding of the mass fractionation laws responsible for the mass-dependent fractionation of Ca present in natural samples and synthetic standards. For example, the presence of an apparent excess of ∼60 ppm in the μ48Ca composition of the SRM 915a suggests that equilibrium fractionation processes have generated the mass-dependent fractionation of this material. In contrast, the absence of residual anomalies in the mass-independent composition of seawater implies that biogenic and inorganic processes of carbonate formation fractionate Ca kinetically from seawater. Finally, we note that SRM 915b has a mass-dependent and mass-independent Ca isotope composition that is within the resolution of our method identical to that of bulk silicate Earth (BSE). This observation, together with the potential heterogeneity in the 40Ca composition of the SRM 915a inferred from our measurements, suggests that the SRM 915b is a better reference material to study the Ca isotope composition of terrestrial and non-terrestrial materials.

Calcium isotopic ratios and rare earth element abundances in refractory inclusions from the Allende CV3 chondrite

Refractory inclusions in primitive meteorites are the oldest objects formed in the Solar System. They exhibit large mass-dependent isotopic effects in many elements, including Si, Mg and Ca, indicative of their complex origin. We report data for both mass-dependent (δ) and mass-independent (ε) Ca isotopic effects, and rare earth element (REE) abundances in six refractory inclusions from the Allende CV3 meteorite. Our data reveal large mass-dependent Ca isotopic effects in these refractory inclusions, with δ44/40Ca ranging from −5.60 to +0.35 (relative to NIST SRM 915a), which are significantly lower than that of the bulk silicate Earth (δ44/40Ca=1.05±0.04). Importantly, δ44/40Ca is correlated with REE patterns: refractory inclusions with Group II REE patterns have lighter Ca isotope compositions compared to those with Group I REE patterns. Specifically, the studied refractory inclusions form negative 44Ca/40Ca–Tm/Er and 44Ca/40Ca–Nd/Er trends, implying segregation of up to 3% of an ultrarefractory evaporation residue from a chondritic reservoir prior to the formation of refractory inclusions. This residue would have to be characterized by high 44Ca/40Ca and low Tm/Er and Nd/Er. At 2σ levels of ±0.5 and ±1.5 for ε40/44Ca and ε43/44Ca, respectively, the six refractory inclusions studied show no detectable 40Ca or 43Ca anomalies relative to the terrestrial standards. However, five out of the six refractory inclusions do show a 48Ca excess of several ε-units, clearly resolvable from the terrestrial standards.

Calcium isotopic fractionation between clinopyroxene and orthopyroxene from mantle peridotites

We report the first observation of Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene from Kilbourne Hole and San Carlos mantle peridotites. The 44Ca/ 40Ca in orthopyroxenes is 0.36 to 0.75‰ heavier than that in the co-existing clinopyroxenes. Using these isotopic constraints and the relative proportions of major Ca-bearing minerals in the upper mantle, the estimated 44Ca/ 40Ca of the upper mantle is 1.05 ± 0.04‰ heavier relative to NIST SRM 915a. This is slightly higher than our average for basalts (0.97 ± 0.04‰ heavier relative to NIST SRM 915a). Combined with published 44Ca/ 40Ca data on low temperature Ca-bearing minerals (calcite, aragonite and barite), we infer that the inter-mineral fractionation of Ca isotopes at both low- and high temperatures is primarily controlled by the strength of Ca–O bond. Accordingly, the mineral with a shorter Ca–O bond and a smaller Ca coordination number (i.e., stronger Ca–O bond) yields a heavier Ca isotopic ratio (i.e., higher 44Ca/ 40Ca). Since stable isotopes of major elements, such as Ca and Mg, exhibit small fractionations during igneous processes, the estimate of stable isotopic compositions of the bulk differentiated planetary bodies, including the Earth and the Moon, needs to take into account the relative proportions of major rock-forming minerals and their respective isotopic signatures.

Calcium Isotopes (δ44/40Ca) in MPI‐DING Reference Glasses, USGS Rock Powders and Various Rocks: Evidence for Ca Isotope Fractionation in Terrestrial Silicates

We report δ44/40Ca(SRM 915a) values for eight fused MPI‐DING glasses and the respective original powders, six USGS igneous rock reference materials, the U‐Th disequilibria reference material TML, IAEA‐CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM‐GEOMAR and Saskatchewan Isotope Laboratory) using 43Ca/48Ca‐ and 42Ca/43Ca‐double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2‰ at the 95% confidence level. The impact of different preparation methods on the δ44/40Ca(SRM 915a) values was found to be negligible. Except for ML3‐B, the original powders and the respective MPI‐DING glasses showed identical δ44/40Ca(SRM 915a) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of δ44/40Ca(SRM 915a) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS‐1 (1.49 ± 0.06‰) and the peridotite PCC‐1 (1.14 ± 0.07‰) are enriched in 44Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in 44Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.

Reassessing Mg/Ca temperature calibrations of Neogloboquadrina pachyderma (sinistral) using paired δ44/40Ca and Mg/Ca measurements

The Mg/Ca temperature calibration of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma (sinistral) (sinistral indicates left coiling) was refined by a multiproxy approach combining hydrographic temperature and salinity data with Mg/Ca, δ44/40Ca, and δ18O values from Holocene Nordic seas core top samples. Reliable Mg/Ca‐based temperature estimates are limited to foraminiferal tests that calcified in water masses with temperatures above ∼3°C at habitat depth. In these samples, Mg/Ca and δ44/40Ca values are positively correlated (Mg/Ca (mmol/mol) = 0.77 (±0.22) × δ44/40Ca (‰ SRM 915a) + 0.52 (±0.12); n = 20, R2 = 0.76). Both Mg/Ca‐ and δ44/40Ca‐derived temperatures projected onto their corresponding depth intervals reveal that the “apparent” calcification depth of N. pachyderma (sinistral) averaging the specimens' whole life cycle is bound to an isopycnal layer defined by water densities (σt) between 27.7 and 27.8. This implies that N. pachyderma (sinistral) prefers gradually deeper habitats with increasing sea surface temperatures, thus counterbalancing absolute temperature variations. Consequently, the total temperature range recorded in this foraminiferal species is restricted and only partly reflects environmental changes. On the basis of the new Mg/Ca, δ44/40Ca, and δ18O multiproxy data set, we propose a linear Mg/Ca temperature relation for high‐latitude N. pachyderma (sinistral): Mg/Ca (mmol/mol) = 0.13 (±0.037) T (°C) + 0.35 (±0.17); T > 3°C. In core top samples from polar waters with peak summer temperatures below ∼3°C, the temperature response in the Mg/Ca and δ44/40Ca proxy signal is inversed and poorly correlated. Both Mg/Ca‐ and δ44/40Ca‐derived temperature estimates pretend significantly higher calcification temperatures than maximum summer sea surface temperatures of these water masses.

The Calcium Isotope Composition (δ44/40Ca) of NIST SRM 915b and NIST SRM 1486

The calcium isotopic composition of NIST SRM 915b and 1486 provided by the National Institute of Standards and Technology was analysed. The δ44/40Ca values of the two reference materials relative to NIST SRM 915a were: NIST SRM 915b =+0.72 ± 0.04‰ and NIST SRM 1486 =−1.01 ± 0.02‰. NIST SRM 1486 did not require any chemical separation prior to measurement.

Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms

A total of 280 brachiopods of Ordovician to Cretaceous age, complemented by published data from belemnites and planktonic foraminifera, are used to reconstruct the evolution of calcium isotope composition of seawater (δ 44/40Ca SW) over the Phanerozoic. The compiled δ 44/40Ca SW record shows a general increase from ∼1.3‰ (NIST SRM 915a) at the beginning of the Ordovician to ∼2‰ at present. Superimposed on this trend is a major long-term positive excursion from the Early Carboniferous to Early Permian as well as several short-term, mostly negative, oscillations. A numerical model of the global cycles of calcium, carbon, magnesium and strontium was used to estimate whether the recorded δ 44/40Ca SW variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans. The model uses the record of marine 87Sr/ 86Sr ratios as proxy for seafloor spreading rates, a record of oceanic Mg/Ca ratios to estimate rates of dolomite formation, and reconstructed atmospheric CO 2, discharge and erosion rates to estimate continental weathering fluxes. The model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed δ 44/40Ca SW trends, suggesting that the isotope signatures of these fluxes must also have changed. As a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits, i.e. the oscillating ‘calcite–aragonite seas’. The ultimate control of the calcium isotope budget of the Phanerozoic oceans appears to have been tectonic processes, specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic Mg/Ca ratio, which in turn controlled the dominant mineralogy of marine carbonates, hence the δ 44/40Ca SW. As to the causes of the short-term oscillations recorded in the secular δ 44/40Ca SW trend, we tentatively propose that these are related to variable rates of dolomite formation and/or to changing chemical composition of the riverine flux, in particular Ca / HCO 3 - and Ca / SO 4 2 - ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents.

A Reflection on Mg, Cd, Ca, Li and Si Isotopic Measurements and Related Reference Materials

This contribution aims to report the reflections we had with the scientific community during two international workshops on reference materials for stable isotopes in Davos (2002) and Nice (2003). After evaluating the isotopic homogeneity of some existing reference materials, based on either certificates, literature data or specific inter‐laboratory rounds, we confirm these as primary reference materials or propose new ones relative to which stable isotope compositions should be reported. We propose DSM‐3 for Mg, NIST SRM 915a for Ca, L‐SVEC for Li and NBS28 for Si. Cadmium does not yet have a well identified delta zero material, although three commercial mono‐elemental Cd solutions have yielded the same isotopic composition relative to one another. In order to scale the linearity of any mass spectrometer, some secondary reference materials are also proposed: Cambridge‐1 solution for Mg, the “Münster‐Cd” and JEPPIM Cd solutions for Cd and the “Big Batch” silicate for Si. The team from Nancy propose to prepare a mixed spike solution for Li isotopes. Well‐characterised natural samples such as ocean or continental waters, diatoms, sponges, rocks and minerals are needed to validate the entire analytical procedure, particularly to take into account the effect of sample mineralisation and of chemical manipulations for elemental separation prior to analysis.

Proposal for International Agreement on Ca Notation Resulting from Discussions at Workshops on Stable Isotope Measurements Held in Davos (Goldschmidt 2002) and Nice (EGS‐AGU‐EUG 2003)

A proposal is made to standardise the reporting of Ca isotope data to the δ44Ca/40Ca notation (or δ44Ca/42Ca) and to adopt NIST SRM 915a as the reference standard.

Direct measurement of 44Ca/ 40Ca ratios by MC–ICP–MS using the cool plasma technique

Here, we present a new technique for the direct measurement of 44Ca/ 40Ca isotope ratios on a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC–ICP–MS, AXIOM) using the “cool plasma” technique. By reducing the plasma energy to about 400 W, the isobaric effect resulting from 40Ar + can be significantly reduced, enabling the simultaneous and precise measurement of 44Ca and 40Ca beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a 43Ca/ 48Ca double spike, the isotope measurements on MC–ICP–MS can be performed by bracketing standards. We express the calcium isotope variation relative to NIST SRM 915a ( δ 44/40Ca [‰]=[(( 44Ca/ 40Ca) sample/( 44Ca/ 40Ca) NIST SRM 915a)−1]*1000). Isobaric effects of 24Mg 16O + and 23Na 16OH + interfering with 40Ca and 26Mg 16OH 2 + with 44Ca can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of 87Sr 2+ monitored on 43.5 atomic mass units (amu) was found. Repeated measurements of two Johnson Matthey CaCO 3 standards (lot No. 4064 and lot No. 9912) revealed values of about −11.29 (‰ SRM 915a) and 0.57 (‰ SRM 915a). These values are in accordance with previous values published by Russell et al. [Geochim. Cosmochim. Acta 42 (1978) 1075], Heuser et al. [Int. J. Mass Spectrom. 220 (2002) 385], Hippler et al. [Geostand. Newsl. 27 (2003) 267] and Schmitt et al. [Geochim. Cosmochim. Acta 67 (2003) 2607]. Repeated measurement of the NIST SRM 915a CaCO 3 standard showed that the variance of a single δ 44/40Ca measurement is about 0.14‰ RSD being comparable with TIMS. MC–ICP–MS-based δ 44/40Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of δ 44/40Ca and temperature. MC–ICP–MS-based δ 44/40Ca measurements on cultured Orbulina universa showed a slope of about 0.026‰/°C being similar to the TIMS-based δ 44/40Ca measurements showing a slope of about 0.019‰/°C. The large offset of about 5‰ between the two techniques is shown to be caused by a “matrix” effect, indicating that any δ 44/40Ca measurements on MC–ICP–MS are sensitively controlled by the Ca concentration and the acidity of the solution. Our study demonstrates the possibility to measure the whole dispersion of calcium isotopes with MC–ICP–MS, showing that 40Ca can be used for normalization of 44Ca.

Calcium Isotopic Composition of Various Reference Materials and Seawater

A compilation of δ44/40Ca (δ44/40Ca) data sets of different calcium reference materials is presented, based on measurements in three different laboratories (Institute of Geological Sciences, Bern; Centre de Géochimie de la Surface, Strasbourg; GEOMAR, Kiel) to support the establishment of a calcium isotope reference standard. Samples include a series of international and internal Ca reference materials, including NIST SRM 915a, seawater, two calcium carbonates and a CaF2 reference sample. The deviations in δ44/40Ca for selected pairs of reference samples have been defined and are consistent within statistical uncertainties in all three laboratories. Emphasis has been placed on characterising both NIST SRM 915a as an internationally available high purity Ca reference sample and seawater as representative of an important and widely available geological reservoir. The difference between δ44/40Ca of NIST SRM 915a and seawater is defined as ‐1.88 O.O4%o (δ44/42CaNISTSRM915a/Sw= ‐0.94 0.07%o). The conversion of values referenced to NIST SRM 915a to seawater can be described by the simplified equation δ44/40CaSa/Sw=δ44/40CaSa/NIST SRM 915a ‐ 1.88 (δ44/42CaSa/Sw=δ44/42CaSa/NIST SRM 915a ‐ 0.94). We propose the use of NIST SRM 915a as general Ca isotope reference standard, with seawater being defined as the major reservoir with respect to oceanographic studies.