Photoinduced Evolutions\nof Permafrost-Derived Carbon\nin Subarctic Thermokarst Pond Surface Waters

In subarctic regions, rising temperature and permafrost\nthaw lead\nto the formation of thermokarst ponds, where organics from eroding\npermafrost accumulate. Despite its environmental significance, limited\nknowledge exists regarding the photosensitivity of permafrost-derived\ncarbon in these ponds. In this study, laboratory experiments were\nconducted to explore the photochemical transformations of organic\nmatter in surface water samples from thermokarst ponds from different\nenvironments in northern Quebec, Canada. One pond near Kuujjuarapik\nis characterized by the presence of a collapsing palsa and is therefore\norganically rich, while the other pond near Umiujaq is adjacent to\na collapsing lithalsa and thus contains fewer organic matters. Photobleaching\noccurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik\nsample exhibited an increase in light absorbance at wavelength related\nto aromatic functionalities, indicating different photochemical aging\nprocesses. Ultrahigh-resolution mass spectrometry analysis reveals\nthat the Kuujjuarapik sample preferentially photoproduced highly unsaturated\nCHO compounds with great aromaticity, while the irradiated Umiujaq\nsample produced a higher proportion of CHON aromatics with reduced\nnitrogen functionalities. Overall, this study illustrates that the\nphotochemical reactivity of thermokarst pond water varies with the\nsource of organic matter. The observed differences in reactivity contribute\nto an improved understanding of the photochemical emission of volatile\norganic compounds discovered earlier. Further insights into the photoinduced\nevolutions in thermokarst ponds may require the classification of\npermafrost-derived carbon therein.