A stable isotope toolbox for water and inorganic carbon cycle studies
Stable isotopes of hydrogen, carbon and oxygen are used to investigate numerous physical and chemical processes in the water and inorganic carbon cycles. Measuring and comparing natural isotopic variations requires reliable primary reference materials and consistent data treatment. However, these reference materials have changed over time, while advances in technology have led to better constrained isotopic compositions of the reference materials. In this Technical Review, we provide an overview of the historical evolution of such materials, and explain their relationships across time and isotopic scales. Recommendations are provided for the measurement and reporting of isotopic compositions against the consensual VPDB and VSMOW scales in light of the newest carbonate and water reference materials distributed by the International Atomic Energy Agency. Stable isotope fractionation factors and their temperature dependence in processes specific to the water cycle (2H, 18O, 17O) and the CO2–water–carbonate system (13C, 18O) are described, including for carbonate clumped isotope thermometry.
Propagation of errors is also addressed for a consistent reporting of real uncertainties of isotopic measurements and calculations. Lastly, current gaps in knowledge on the behaviour of stable isotopes in the water cycle and the CO2–water–carbonate system are highlighted for future studies.
Isotopes in the water and carbon cycles. a- The main isotope fractionation processes within the water cycle. b- Major elements governing the 13C content of the Earth’s surface carbon. δ13C mean values (versus VPDB) are indicated in each box.
Référence :
Claude Hillaire-Marcel, Sang-Tae Kim, Amaëlle Landais, Prosenjit Ghosh, Sergey Assonov, Christophe Lécuyer, Marc Blanchard, Harro A. J. Meijer and Hans Christian Steen- Larsen, 2021. A stable isotope toolbox for water and inorganic carbon cycle studies. Nature Reviews Earth & Environment Vol. 2, pp. 699–719. DOI: 10.1038/s43017-021-00209-0
Water Isotopic Signature of Surface Snow Metamorphism in Antarctica
Water isotope ratios of ice cores are a key source of information on past temperatures. Through fractionation within the hydrological cycle, temperature is imprinted in the water isotopic composition of snowfalls. However, this signal of climatic interest is modified after deposition when snow remains at the surface exposed to the atmosphere. Comparing time series of surface snow isotopic composition at Dome C with satellite observations of surface snow metamorphism, we found that long summer periods without precipitation favor surface snow metamorphism altering the surface snow isotopic composition. Using excess parameters (combining D,17O, and 18O fractions) allow the identification of this alteration caused by sublimation and condensation of surface hoar. The combined measurement of all three isotopic compositions could help identifying ice core sections influenced by snow metamorphism in sites with very low snow accumulation.
Fig. 1. Time series of surface snow isotopic compositions at Dome C: 17O-excess (purple), d-excess (lighter blue: observations, darker blue: precipitation modelling approach (Casado et al., 2018)), δ18O (lighter green: observations, darker green: precipitation modelling approach (Casado et al., 2018)), grain index (black) (Picard et al., 2012), temperature from ERA-interim reanalysis (red), and precipitation amounts from ERA-interim reanalysis (grey bars). For the isotopic compositions the shaded area indicates uncertainty range. The vertical shading indicates the periods when intense metamorphism is detected using the grain index, a red bar indicates a period during which metamorphism is associated with sublimation while a blue bar indicates a period associated with condensation.
Reference: Mathieu Casado, Amaelle Landais, Ghislain Picard, Laurent Arnaud, Giuliano Dreossi, Barbara Stenni, Frederic Prié, 2021. Water Isotopic Signature of Surface Snow Metamorphism in Antarctica. Geophysical Research Letters 48 (17). doi: 10.1029/2021GL093382