Fig. 1: Conceptual model of sea ice sublimation and the isotopic two-source mixing model leading to ¹⁸O enrichment of the lower snowpack.

a The upper image shows an annotated microCT sample that highlights the processes of mass redistribution from the substrate into the snow from a previous lab experiment, where the underlying ice lens is now representing a sea-ice substrate. This microCT sample was first published by Mareike Wiese²⁴https://www.research-collection.ethz.ch/entities/publication/133e13d0-e64f-413a-9676-6b93893f66cf. We have obtained permission from the rights-holder for its use in this publication, and it is now licensed under a CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/. The original blue background and original annotations have been removed, and all annotations on this figure are modifications. Ice thickness equivalent (ITE, mm) is the thickness of sublimation of the sea-ice surface over 21 days. The pie charts show the approximate ratio of ¹⁶O (red) to ¹⁸O (blue) of solid ice (block color) and vapor (dashed). The red arrows represent vapor fluxes. The lower figure shows a cross-section of sea ice, where h_s = snow height, h_i = ice thickness, not drawn to scale. b One point represents one winter snow sample. The solid and dotted lines are the average profile with one standard deviation ( ± 1σ). 0 cm = snow-ice interface. Samples  > 30 cm are in ridged areas. The pie charts show the snow-ice interface ¹⁸O enrichment.