High‐Resolution Mg/Ca Measurements of Foraminifer Shells Using Femtosecond LA‐ICP‐MS for Paleoclimate Proxy Development

Determination of Mg/Ca in foraminifer shells as a proxy of seawater temperature is of particular interest in paleoclimate reconstruction. Here we show that femtosecond–200 nm–laser ablation–inductively coupled plasma–mass spectrometry is a suitable technique to precisely and accurately determine Mg/Ca in the micrometer‐sized calcareous chambers of foraminifers. At low fluence (0.3–0.6 J/cm2) the double‐charged 44Ca++ and the single‐charged 25Mg+ ions are measured nearly simultaneously. Integrated single‐shot measurements using a pulse repetition rate of 1 Hz enable precise analyses at a depth resolution of about 50–100 nm/pulse corresponding to an ablated material of 0.3–0.6 ng calcite/pulse for a spot size of 55 μm. High‐resolution analyses can be performed until a depth of 10–20 μm and thus particularly suitable for thin‐shelled foraminifers. Reproducibility (relative standard deviation) is about 5% as approved by homogeneous reference materials. Calibration is performed with the microanalytical synthetic reference material MACS‐3. Magnesium and Ca data of different carbonate and silicate reference materials agree within uncertainties with reference values. The procedure has been successfully applied for detailed analyses of single chambers and shell‐depth profiles of live individuals and empty planktic and benthic foraminifer tests from different ocean basins. Plain Language Summary The ratio of magnesium and calcium (Mg/Ca) of the calcareous shells of singled‐celled marine microorganisms called foraminifers is used for temperature reconstruction of the past ocean and climate. We have been using very short laser pulses of 10−13 s to sample small amounts of calcite and Mg/Ca from very thin layers to analyze temperature at high accuracy and precision. Using low energy densities of 0.3–0.6 J/cm2, Mg/Ca has been determined by mass spectrometry by applying a new single‐shot analysis. Reproducibility of the analyses is about 5% as approved by homogeneous reference materials. The procedure has been successfully applied for detailed analyses of single chambers and shell‐depth profiles of live individuals and empty planktic and benthic foraminifer shells from different ocean basins. The new method allows analyses of fossil shells to determine physiological processes and temperatures of ocean waters dating back several millions of years. Key Points Single‐chamber Mg/Ca measurements of different foraminifers using femtosecond LA‐ICP‐MS Single‐shot analysis to dete