04 Jan 2023
04 Jan 2023
Status: this preprint is open for discussion.

A mechanism of post-depositional processes affecting chlorine and its isotope in the upper snowpack of High Antarctic Plateau

Xavier Giraud1, Mélanie Baroni1, and Rita Traversi2 Xavier Giraud et al.
  • 1Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, 13100, France
  • 2Dept. of Chemistry Ugo Schiff, University of Florence, Florence, I-50019, Italy

Abstract. The main purpose of this work is to propose a mechanism of post-depositional processes affecting chlorine and its chlorine-36 cosmogenic nuclide in the upper snowpack of the High Antarctic Plateau. We suggest that the observed decrease of total chlorine content in the upper meters of the snowpack is due to a progressive release of the HCl content from ice. We also propose a consistent framework, combining diffusion in bulk ice and snow microstructure. The observation of the low chlorine content in ice at depth leads to the robust hypothesis that the chemical equilibrium of chlorine between the ice and the snowpack interstitial air (SIA) is close to zero. HCl is thought to diffuse in ice, and to be progressively released in the SIA, and exported to the Antarctic atmosphere by the wind-ventilation. The time required to expel all the mobile species of chlorine (i.e., HCl) from snow depends on the diffusion coefficient of chlorine in ice combined with the snow grain size and its evolution with depth. This work is synthesised in a model combining the microstructure evolution of the upper meters of a snowpack (changes in mean snow grain size) and the diffusion of chlorine in ice applied to single spherical grains. The variability observed in chloride concentration profiles with depth, at a same site but different sampling time or different snow pits, or among different sites of the High Antarctic Plateau, is mostly due to the variations in initial concentrations in HCl and sodium chloride (NaCl) species and the snow grain size evolution. This model offers a common framework for understanding the fate of chlorine in Antarctica, from coastal to inland locations, including low accumulation sites on the plateau, far from the ocean. Applications of this post-depositional model to chlorine and to 36Cl allows to picture a recycling mechanism of chlorine at the scale of Antarctica. In particular, the 36Cl concentration in the surface snow of the Vostok site illustrates this recycling mechanism and the persistent contamination of inland Antarctica by anthropogenic 36Cl originating from the marine nuclear tests of the 1950s to the 1970s.

Xavier Giraud et al.

Status: open (until 01 Mar 2023)

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Xavier Giraud et al.


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Short summary
The snowpack in High Antarctic Plateau is an interface medium between the atmosphere and the firn, where past climate conditions are recorded. Originating mainly from oceanic sources, chlorine is deposited along with snow. We propose a mechanism implying the diffusion of HCl at the scale of snow grains, longing a few decades for its release to the Antarctic atmosphere. Based on this scenario, the fate of the anthropogenic 36Cl originating from the nuclear tests can be forcasted.