You guessed right. They are artichoke seeds. This is the first (and I am sure last) year that I missed disposing of the seed heads earlier. Normally I cut them back some time in summer and throw them on a pile to be burnt once the fire season is over. Somehow I missed the boat this year and the seeds started floating in the air. I have cut them back now and they will be properly disposed. I do not want this place to be called 'Artichoke Jungle'.
Oh, if anyone would like some artichoke seeds, I have plenty. You are welcome to them! Artichokes are easy to grow and are a real treat in spring. They do need a lot of space, though. :)