Drilling fluid (DF) is one of the main sources of chemical and biological contamination of deep ice cores and lake water samples in the exploration of Subgalcial Antarctic Lake Environments (SALE). In this study, we investigated the contamination of an ice core that represented the first samples of refrozen lake water obtained 1 year after the unsealing of Lake Vostok in 2012. We show that these samples contain inclusions of the DF with a concentration of at least 16.7 mg l⁻¹ (0.0019% or 19 ppmv). This makes it extremely difficult to obtain reliable data on the real chemical composition of the lake water. The focus of our study is the organic components of the DF, which built up in the secondary ice while the water was freezing in the borehole. Of all the possible organic compounds of the DF, only phenol congeners (up to 32.4 mg l⁻¹) and dichlorofluoroethane HCFC-141b (14.4 mg l⁻¹), a DF densifier, were found in the central channel, which is the last part of the core to freeze in the borehole. We conclude that the phenol compounds emerge due to physical processes, namely fractionation, during freezing, rather than any chemical reaction between the DF and the lake water.

Supplementary material: The detailed chemical data are available at https://doi.org/10.6084/m9.figshare.c.3783641