Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS

Abstract

Precise measurements of the isotopic composition of Li in low-concentration natural carbonate samples is demonstrated for quadrupole ICPMS. Cool plasma conditions result in high sample/background ratios for the measured Li signals even for low concentrations. Quantitative chemical separation of Li prior to the measurement on ICPMS is necessary to avoid the matrix-induced isotopic fractionation and the results have to be externally corrected for mass bias and instrumental fractionation. The isotopic composition of lithium can be measured in samples containing only 5–10 ng Li with a within-run precision better than 0.5‰ (2 standard errors of the mean), and a long term reproducibility that is better that 2.1‰ (2 standard deviations). The method has been successfully used to measure Li isotopic composition of 1–10 mg samples of calcium carbonate tests of Pulleniatina obliquiloculata and Globorotalia tumida that were collected from the sediment–seawater interface and have Li concentrations of about 1 ppm. The isotopic composition of Li in studied calcium carbonate tests of P. obliquiloculata is independent of the size of tests and resembles the lithium isotopic composition of modern seawater, suggesting that this species may provide a record of Li isotope variations in past oceans.