Stable Isotope Analysis

The isotopic abundances of elements such as H, C, N and O were fixed when the earth was formed and, on a global scale have not changed since. Stable isotope analysis is a technique used to measure the relative abundances of these isotopes in different materials, i.e. ²H/¹H, ¹³C/¹²C, ¹⁵N/¹⁴N and ¹⁸O/¹⁶O. This establishes an isotopic profile, or “signature” for those materials.

Subtle variations in the isotopic composition of materials may be introduced during biological, chemical and physical processes. Over geologic time, these processes have varied significantly, on both regional and global scales. The analysis of samples from sequences of sediments or sedimentary rock thus identifies temporal variations at a particular location, providing insight into variations in palaeoenvironment or palaeoclimate. When multiple sequences are analysed, these can be compared in order to establish if isotopic variations are contemporaneous on regional and global scales.

Techniques

At Origin Analytical, we focus on the stable isotopes of carbon and oxygen in sedimentary rocks from marine and lacustrine environments. Our innovative and advanced methodologies mean that we can analyse cuttings samples to the same degree of accuracy and precision as core samples.

Fig.1 Dual-inlet isotope ratio mass spectrometer

Carbonates –  δ¹³C_carb and δ¹⁸O_carb

Analyses of carbonates are performed using a dual inlet isotope ratio mass spectrometer (IRMS) (Fig. 1). Samples containing at least 80% pure calcite are analysed using the ‘common acid bath’ (automated, on-line) method. Mixed-mineralogy carbonates and dolomites are prepared using the ‘sealed vessel’ (offline) method. Isotope compositions are reported as conventional delta values (δ¹³C_VPDB and δ¹⁸O_VPDB) with respect to VPDB. Analytical precision (1σ) is estimated to be better than ±0.1‰ for both isotope ratios based on the replicate measurement of well-characterized, laboratory carbonate quality control materials.

Fig. 2. Continuous-flow elemental analyser and isotope ratio mass spectrometer

Organic-rich sediments – δ¹³C_org

Analyses of organic-rich sediments are performed using a continuous-flow elemental analyser and isotope ratio mass spectrometer (CF-EA-IRMS) (Fig. 2). Samples are prepared by removal of the carbonate carbon by acid washing, prior to analysis. Isotope compositions are reported as conventional delta values (δ¹³C_VPDB) with respect to VPDB. Analytical precision (1σ) is estimated to be better than ±0.2‰ for δ¹³C based on the replicate analysis of pure, well-mixed, organic compounds used as laboratory calibration materials.