Substantial halogenated organic chemicals stored in permafrost soils on the Tibetan Plateau

Permafrost in the polar regions potentially stores large amounts of toxic chemicals, including organic compounds bound with halogens. The release of such halogenated organic chemicals (HOC) from thawing permafrost represents a potential global concern with climate change. However, the exact inventory of HOC remains uncertain because conventional analytical techniques largely overlook nonextractable residues. Here we present an inventory of HOC in permafrost soils sampled from the Tibetan Plateau using stepwise chemical treatment following conventional solvent extraction to release and analyse the nonextractable residues. We identify more than 270 types of HOC, with total mean concentration of 310,000 ng g −1 , of which 180,000 ng g −1 are naturally sourced based on their molecular structures. We also find unexpectedly high fractions of the nonextractable residues, contributing more than 99% of the total HOC, much higher than those reported for other soils and sediments. Up to 85% of the nonextractable residues are physically entrapped in soils rather than chemically bound, such that they could readily be remobilized if soil properties change. We suggest that this substantial stock of HOC in Tibetan Plateau permafrost poses potentially important future risks to local ecosystems in a warming climate. Chemical analyses show permafrost soils on the Tibetan Plateau contain large amounts of halogenated organic chemicals that could be remobilized in a changing climate.