Just speculating: maybe atmospheric dust (pollen, etc.), and/or migrating animals contain it and it gets deposited wherever the dust comes down, or the animal

Just speculating: maybe atmospheric dust (pollen, etc.), and/or migrating animals contain it and it gets deposited wherever the dust comes down, or the animal dies. But in permafrost, it is quickly fixed by humidity and ice, and it isn't returned to the oceans by a constant flow of water?