In situ measurements of calcium isotopes by ion microprobe in carbonates and application to foraminifera

Abstract

An analytical procedure has been developed for the in situ measurement of calcium isotope composition of carbonates with a spatial resolution of 15-20 μm on a Caméca IMS 1270 ion microprobe. By using two Faraday cup detectors, the 40 Ca and 44 Ca can be measured simultaneously, improving the internal reproducibility. Instrumental mass fractionation (IMF) of calcium isotopes was observed to be independent of primary ion beam intensity and of the Mg content of the carbonate, but can depend on vacuum conditions. Three calcite reference materials were used in this study (ENS 0, MEX and BRET 105E) and their δ 44 Ca values relative to NIST915a were reproducible within a typical 1σ standard deviation of ≈0.15‰.This analytical procedure was applied to planktonic foraminifera, Globorotalia inflata, dated at 2.8 Ma from Shatsky Rise (ODP leg 198). The range of measured δ 44 Ca within a single test is 1.7‰. This intratest variation can be attributed to several processes such as temperature variation, ontogenic effects or differences between primary and secondary calcite (i.e. calcite precipitated by different biomineralization processes).Despite this intratest variation, the averages δ 44 Ca for each foraminifer are similar and are in agreement with published δ 44 Ca values measured for this age. This study shows that in situ δ 44 Ca measurements in tests of foraminifera are an appropriate tool for investigating biomineralization processes.

Controls on calcium isotope fractionation in cultured planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera

“…In situ analyses of d 44/40 Ca in foraminifera revealed >1.5& intra-test variability in species of the globorotaliid family (Rollion- Bard et al, 2007;Kasemann et al, 2008), possibly associated with differences between the biomineralization pathways of ontogenic versus gametogenetic calcite (see Section 1). The deposition of gametogenetic calcite is negligible in G. ruber (Caron et al, 1990) and appears to be small in G. siphonifera (Huber et al, 1997).…”

“…High spatial resolution in situ Ca isotope measurements on planktic foraminifera using ion microprobe revealed intra-test d 44/40 Ca variations of 1.7& in Globorotalia inflata (Rollion-Bard et al, 2007), 1.6& in Globorotalia truncatulinoides and 3.7& in Globorotalia tumida (Kasemann et al, 2008), as compared to the total glacial-interglacial variation in planktic foraminiferal d 44/40 Ca of $1.8& (Kasemann et al, 2008 and references therein). The intra-test variability in G. truncatulinoides and G. tumida was observed to be associated with ontogenic versus gametogenetic layers.…”

Calcium isotopic composition of high-latitude proxy carrier <i>Neogloboquadrina pachyderma</i> (sin.)

“…Ca isotope fractionation by Rayleigh distillation from this biomineralization reservoir could likely explain their foraminiferal Ca isotope data. Nevertheless, recently, high-resolution in situ measurements of the Ca isotopic composition by ion microprobe revealed variations in δ 44/40 Ca of 1.7‰ within two single tests of 2.8 Ma old Globorotalia inflata from Shatsky Rise, ODP leg 198 (Rollion- Bard et al, 2007). They attributed the intra-test variations to several processes, such as ontogenetic effects, differences between primary and secondary calcite, and temperature.…”

“…The study of Griffith et al (2008) marks an important step in this respect. Finally, in order to unravel the complex processes during biomineralization, more high-resolution geochemical methods should be applied, as done by Rollion- Bard et al (2007). Comparative studies on the geochemistry of foraminiferal species dwelling in open ocean conditions vs. species adapted to the specific environments of ecological niches could further result in achieving a mechanistic understanding of how the calcification strategy might be related to the adaptation to a species-specific habitat.…”

In situ boron isotope analysis in marine carbonates and its application for foraminifera and palaeo-pH

“…Such large variations have never been reported using conventional bulk and micro-drill sampling techniques. In addition, preliminary in situ studies on the lithium, carbon, oxygen and calcium isotope composition of biogenic carbonates (Rollion-Bard et al, 2003;Rollion-Bard et al, 2007;Vigier et al, 2007;Kasemann et al, 2008) demonstrate the need and capability of high spatial resolution SIMS to determine the internal isotopic variability and the influence of growth rates, habitat and symbionts on the isotopic composition.…”