We report direct evidence of an inorganic carbon pool that is sensitive to permafrost thaw–siderite. Notably, siderite was absent in the seasonally thawed active layer above permafrost, implying this inorganic carbon reservoir may be lost upon thaw. Assuming siderite weathers quickly once permafrost thaws, we estimate siderite weathering could release carbon equivalent to about 10% of permafrost organic carbon losses over the next half-century. However, studies are needed to understand how widespread siderite is and to quantify its actual weathering rate. This study is submitted as a LESSONS Report because it documents a surprise result that opens up opportunities for new science.