SULFUR, CHLORINE, AND FLUORINE DEGASSING AND ATMOSPHERIC LOADING BY THE 1783-1784 AD LAKI (SKAFTAR FIRES) ERUPTION IN ICELAND

The 1783-1784 Laki tholeiitic basalt fissure eruption in Iceland was o ne of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing his tory and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorin e in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away fro m the vents. Using the measured sulfur concentrations in glass inclusi ons in phenocrysts and in groundmass grasses of quenched eruption prod ucts, we calculate that the total accumulative atmospheric mass loadin g of sulfur dioxide was 122 Mt over a period of 8 months. This volatil e release is sufficient to have generated similar to 250 Mt of H2SO4 a erosols, an amount which agrees with an independent estimate of the La ki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (similar to 60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrati ons in the samples indicate that the atmospheric loading of hydrochlor ic acid and hydrofluoric acid was similar to 7.0 and 15.0 Mt, respecti vely. Furthermore, similar to 75% of the volatile mass dissolved by th e Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in t he upper magma conduit, and implies that high-discharge basaltic erupt ions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1-2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma , and these emissions were confined to the lowest regions of the tropo sphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki mag ma batch by measurements of sulfur in the volcanic products (the petro logic method) yields a result which is sufficient to account for the m ass of aerosols estimated by other methods.