Comparison of δ C analyses of individual foraminifer (Orbulina universa) shells by secondary ion mass spectrometry and gas source mass spectrometry.

Rationale: The use of secondary ion mass spectrometry (SIMS) to perform micrometer-scale in situ carbon isotope (δ C) analyses of shells of marine microfossils called planktic foraminifers holds promise to explore calcification and ecological processes. The potential of this technique, however, cannot be realized without comparison to traditional whole-shell δ C values measured by gas source mass spectrometry (GSMS).

Methods: Paired SIMS and GSMS δ C values measured from final chamber fragments of the same shell of the planktic foraminifer Orbulina universa are compared. The SIMS-GSMS δ C differences (Δ C ) were determined via paired analysis of hydrogen peroxide-cleaned fragments of modern cultured specimens and of fossil specimens from deep-sea sediments that were either untreated, sonicated, and cleaned with hydrogen peroxide or vacuum roasted. After treatment, fragments were analyzed by a CAMECA IMS 1280 SIMS instrument and either a ThermoScientific MAT-253 or a Fisons Optima isotope ratio mass spectrometer (GSMS).

Results: Paired analyses of cleaned fragments of cultured specimens (n = 7) yield no SIMS-GSMS δ C difference. However, paired analyses of untreated (n = 18) and cleaned (n = 12) fragments of fossil shells yield average Δ C values of 0.8‰ and 0.6‰ (±0.2‰, 2 SE), respectively, while vacuum roasting of fossil shell fragments (n = 11) removes the SIMS-GSMS δ C difference.

Conclusions: The noted Δ C values are most likely due to matrix effects causing sample-standard mismatch for SIMS analyses but may also be a combination of other factors such as SIMS measurement of chemically bound water. The volume of material analyzed via SIMS is ~10 times smaller than that analyzed by GSMS; hence, the extent to which these Δ C values represent differences in analyte or instrument factors remains unclear.