Carbonate clumped isotope analysis using isotope dilution

Abstract

The large sample size requirement is a major obstacle while conducting the measurement of isotopic abundances in carbonates for clumped isotope thermometry using MAT 253 isotope ratio mass spectrometer employing McCrea type setup or improved break-seal method at a reaction temperature of 25 °C. A long integration time at high (∼10000 mV) major ion beam intensity permits the highest analytical precision but requires a large sample size for CO2. However, in several instances, carbonate availability is restricted in the natural environment like foraminifera, otoliths, ostracods, etc., in sediment cores, prohibiting the application of the thermometry. Here we introduce a new isotope dilution method for stable and clumped isotope analysis for small size carbonates at a precision of ±0.005‰. In this method, two reference carbonate materials (MAR J1 and OMC) are quantitatively mixed with known proportions or weight fractions. The high-temperature reference carbonate (MAR J1) is treated as a spike end member, whereas the low-temperature carbonate reference is considered as unknown. The spike is identified based on similarity in mineralogy with the sample and well-defined isotopic composition as constrained by its formation temperature. These two carbonates used here are internal laboratory standards and are homogeneous, independently ascertained based on long-term reproducibility of δ13CVPDB and δ18OVPDB values lying within 0.002‰ and 0.003‰ respectively. These two reference carbonate powders have δ13CVPDB and δ18OVPDB values, which differ from each other by ∼6.3‰. The Δ47 values of these carbonates are 0.395‰ and 0.587‰, respectively. In our experiment, we observe a linear relationship governing δ13CVPDB and δ18OVPDB values of a mixture with a known weight fraction; however, Δ47 values follow a non-linear mixing trend upon exceeding the weight fraction ≥0.4 of the low-temperature carbonate (OMC). The linear trend allows the prediction of the Δ47 value of the unknown carbonate powders using a binary mixing model within an acceptable precision of 0.01–0.02‰ necessary for clumped isotope measurement. This new approach provides an alternative method of analyzing small size (∼3 mg) carbonate powder extending the application of the break-seal method for analysis at major ion beam intensity of 10000 mV. Here we demonstrate the application of this method for accurate determination of both stable and clumped isotopic composition of various low-temperature carbonates, including a deep-sea coral, an otolith, and ETH-3 reference carbonate.