Evolutions of Non‐oxygenated Compounds in Sunlit Subarctic Thermokarst Pond Surface Waters

Abstract With global climate change, ground subsidence caused by permafrost thawing leads to the formation of thermokarst ponds, where organics from eroding permafrost accumulate in part as dissolved organic matter (DOM). Surface waters were collected from two subarctic thermokarst ponds with distinct characteristics: one palsa‐affected pond, which is organic‐rich, and another lithalsa‐affected pond, which is organic‐poor. Here, the photo‐evolutions of non‐oxygenated compounds (referred to as CH*) in the waters are investigated to elucidate the environmental fate of permafrost‐derived DOM. Photochemistry alters the CH* community, and the differences among various CH* subgroups, as well as between non‐oxygenated and oxygenated groups, highlight the need for a detailed classification of DOM constituents in future studies. A comparison of the thermokarst ponds revealed that the photochemical evolution of CH* is more closely linked to the specificity of DOM than to its quantity. Furthermore, the formation pathways of liquid and gaseous products were examined, emphasizing the distinct roles of liquid‐phase photochemistry and air‐water interface photochemistry. In summary, this study contributes to elucidating the extended environmental fate of permafrost‐derived DOM and underscores its critical role in shaping both liquid and gaseous environmental media in thermokarst landscapes.