Abstract In the changing Arctic, permafrost thaw is shifting from gradual to abrupt. Although iron‐bound organic carbon (OC‐FeR) critically modulates permafrost carbon‐climate feedbacks, its decadal‐scale variability and response to this regime shift remain poorly constrained. Focusing on the East Siberian Arctic, a hotspot of land‐to‐ocean permafrost carbon transfer, we established critical linkages between marine sedimentary OC‐FeR burial records and terrestrial permafrost‐climate dynamics. Results revealed that coastal OC‐FeR reservoirs exhibited temporally‐distinct responses to permafrost thawing dynamics. During the pre‐2000s, the progressive decline in OC‐FeR coincided with watershed‐scale soil moisture redistribution driven by gradual active‐layer deepening, which preferentially retained OC‐FeR in permafrost, attenuating its land‐to‐ocean fluxes. Since 2000s, although abrupt thaw has rapidly mobilized OC, ∼19% of sedimentary OC remained stabilized as OC‐FeR, thus partially decoupling abrupt thaw from immediate permafrost carbon‐climate feedbacks. Therefore, this mineralogical buffering mechanism can attenuate the net climate impact of accelerating abrupt thaw across the Northern Hemisphere cryosphere.

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