New estimates of sulfur degassing and atmospheric mass-loading by the 934 AD Eldgjá eruption, Iceland

The 934 AD Eldgjá basaltic flood lava eruption in southern Iceland is the largest on Earth in the last millennium. The Eldgjá fissures produced 19.6 km³ of transitional basalt in a prolonged eruption that featured at least eight distinct episodes and may have lasted for 3-8 years. The atmospheric SO₂ mass loading by Eldgjá is determined by new measurements of pre-eruption and residual sulfur contents in the products from all phases of the eruption. A pre-eruption sulfur content of ~2150 ppm indicates that the magma carried 232 Mt of SO₂ to the surface, where vent and lava flow degassing released 219 Mt into the atmosphere. This value corresponds to a potential H₂SO₄-aerosol yield of ~450 Mt, increasing previous H₂SO₄-aerosol mass estimates by a factor of 2.6-4.5. Approximately 79% of the original sulfur mass was released at the vents, indicating ~185 Mt SO₂ were discharged into the atmosphere above the Eldgjá fissures and carried aloft by the eruption columns to upper tropospheric and lower stratospheric altitudes (~ 15 km). Consequently, only ~35 Mt SO₂ escaped from the lava into the lower troposphere. These estimates of the SO₂ mass loading from Eldgjá make it the greatest known volcanic pollutant of recent history, exceeding that of 1783 AD Laki and 1815 AD Tambora eruptions by factors of 1.8 and 2.0-2.8, respectively. However, the intensity of climatic effects deduced by the Eldgjá event are not thought to have surpassed that of Laki or Tambora because the eruption was prolonged and subsequently the sulfur emissions were drawn out over several years. The lack of detailed historic records for this period make estimates of the effects of long term but significant release of SO₂ (30-70 Mt/yr) on the atmosphere uncertain.