Centimeter-scale diffusion and in situ production effects on greenhouse gas records in the TALDICE ice core

Ice cores provide the only direct archive of past atmospheric greenhouse gases (GHG). However, physical and chemical processes occurring before, during, and after bubble enclosure can alter the atmospheric signal recorded in ice. In particular, gas diffusion within the bubble-clathrate transition zone (BCTZ) has been shown to generate centimeter-scale, non-atmospheric variability, which we refer to as the “Lüthi effect” (Lüthi et al., 2010). Below the BCTZ, diffusive smoothing dampens these non-atmospheric signals but also atmospheric variability. While the Lüthi effect and diffusive smoothing have been documented for CO₂ and the δO₂/N₂ ratio, their impact on N₂O and CH₄ remains poorly constrained. In addition, chemical reactions within the ice can alter atmospheric signals, particularly for N₂O, which has been shown to be produced in situ by nitrate reduction in dust-rich Antarctic ice during glacial periods.

Here we investigate diffusion and in situ production processes potentially affecting CO₂, CH₄, and N₂O by analyzing five samples from the BCTZ of the Talos Dome ice core (TALDICE). GHG concentrations were measured at centimeter-scale resolution using a novel laser sublimation extraction system coupled to a quantum cascade laser absorption spectrometer (Mächler et al., 2023). δO₂/N₂ ratios were analyzed by isotope ratio mass spectrometry following the recapture of the same samples after GHG measurements with the laser spectrometer.

Our results show for the first time that N₂O and CH₄ are also affected by the Lüthi effect in the BCTZ. The strong 1:1 correlation between CO₂ and N₂O variability suggests similar diffusion coefficients for these gases. These findings provide new constraints on N₂O diffusion, relevant for modeling diffusive smoothing in deep and old ice such as the recently drilled Beyond EPICA ice core. Consistent with previous studies, our results indicate that the N₂O record in TALDICE is not significantly affected by the aforementioned in situ production during glacial periods. The atmospheric N₂O signal can therefore be retrieved when measurements are either spatially averaged to smooth the centimeter-scale variability induced by the Lüthi effect, obtained above the BCTZ, or taken well below the BCTZ where diffusive smoothing has attenuated this variability.

References

Lüthi, D., Bereiter, B., Stauffer, B., Winkler, R., Schwander, J., Kindler, P., Leuenberger, M., Kipfstuhl, S., Capron, E., Landais, A., Fischer, H., and Stocker, T. F.: CO₂ and O₂/N₂ variations in and just below the bubble–clathrate transformation zone of Antarctic ice cores, Earth and Planetary Science Letters, 297, 226–233, https://doi.org/10.1016/j.epsl.2010.06.023, 2010.

Mächler, L., Baggenstos, D., Krauss, F., Schmitt, J., Bereiter, B., Walther, R., Reinhard, C., Tuzson, B., Emmenegger, L., and Fischer, H.: Laser-induced sublimation extraction for centimeter-resolution multi-species greenhouse gas analysis on ice cores, Atmos. Meas. Tech., 16, 355–372, https://doi.org/10.5194/amt-16-355-2023, 2023.