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Yes, the Icelandic volcanoes are capable of this level of atmospheric disruption, but there is not yet a smoking hill in Iceland identified for these events:

• runarberg 6y ago
  It is not that long ago, and the land is really well studied. I think I trust the Icelandic volcanologists over the middle age historian regarding the source of the 535 eruption. This size eruption would definitely leave sizeable evidence that wouldn’t be that hard to find given the barren landscape of Iceland. See compare this to the 1783 Laki eruption which caused similar weather events. Unlike 535, the weather events from the Laki eruption were limited to the Northern hemisphere. It is much more likely that this eruption happened close to the equator, where evidence erode much quicker and the ash has an easier time effecting the two hemispheres.