Stable Isotopic Analyses of Small-Mass Carbonate Samples: Results from Isotope Dilution and Pressure Correction Experiments

Abstract

Studies of subdecadal climate variability have used individual foraminifera analysis (IFA) to investigate high frequency paleoceanographic signals. Although typical carbonate stable isotope studies use masses larger than ~30μg, IFA requires high sensitivity instrumentation that is able to detect and accurately measure individual tests of foraminifera that range from 5 to upwards of 30 μg. There is currently no commonly utilized, standardized methodology that outlines best practices for IFA. In this work, we systematically investigate analytical precision and optimal sample processing for IFA using two lines of investigation on a Kiel IV carbonate device coupled to a MAT 253 Plus isotope ratio mass spectrometer housed at the University of Arizona’s Paleoceanography Laboratory. This setup currently has detection limits that permit analysis of samples as low as 8 μg under a high sensitivity (“IFA-mode”) setting and larger samples under a lower sensitivity (“bulk mode”) setting. Our first approach is to perform pressure correction experiments on the IFA mode in which we modify Kiel parameters including reference refill time to optimize the methodology pertaining to the high sensitivity setting. In the second experiment, we employ an isotope dilution approach to test the ability of our instrumentation to accurately measure small-mass samples under “bulk mode” operation. Here, we use standards with accepted stable isotope values as either “unknowns” or “spikes” in this experiment. The unknown standard mimics a small-mass carbonate sample whereas the spike is used to bring the entire sample up to mass-detection limits. We find that the isotope dilution method can be an effective approach in analyzing oxygen and carbon stable isotopes but is limited by the accuracy of mass measurements. The pressure correction approach suggests that more negative carbonate samples will require a pressure correction only in the smaller range of CO2 yields (<400 μbar) while more positive carbonates must be corrected at larger yields (>600 μbar).