Estimates of sulfur and chlorine yield to the atmosphere from volcanic eruptions and potential climatic effects

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Abstract
We report analytical methods based on electron microprobe analysis of volcanic tephra which allow estimation of minimum masses of sulfur and chlorine released to the atmosphere by volcanic eruptions. Differences between sulfur and chlorine contents of glassy melt inclusions (trapped by tephra phenocrysts) and matrix glasses, respectively, scaled to estimated eruption volumes, provide estimates of minimum sulfur and chlorine yield to the atmosphere. Declines in mean northern hemisphere surface temperatures associated with several Recent eruptions (Laki 1783, Tambora 1815, Krakatau 1883, Agung 1963, Mount St. Helens 1980) are positively correlated with our estimates of the rnimimum mass of sulfur released to the atmosphere, reinforcing the idea that sulfate aerosols have a greater climatic impact than volcanic “dust.” Injection of volcanic emissions into the stratosphere increases the optical thickness of the atmosphere which may result in cooling of the earth's surface. Our results indicate that decreases in mean surface temperatures associated with the eruptions listed above appear to vary linearly with the cube root of the estimated minimum mass of sulfur released by the eruptions. This apparent geometric relationship suggests that increases in optical thickness may be completely related to increases in the mass of particulates in the stratospheric aerosol layer‐Correlations between observed mean surface temperature decreases and other eruption‐related parameters (estimated chlorine yield, estimated total sulfur and chlorine yield, estimated tephra volume dense rock equivalent) are poor compared to the correlation with our estimates of sulfur yield. Application of our methods to the 1982 eruption of El Chichon volcano results in underestimation of the minimum mass of sulfur released to the atmosphere, judging from independent estimates based on airborne sampling and satellite observations of the eruption plume. The presence of anhydrite phenocrysts in El Chichon tephra suggests that decomposition of anhydrite may have contributed to the sulfur yield of this eruption. Potential contributions of volatile species to the atmosphere from eruptions of basaltic magma are generally about an order of magnitude higher in sulfur than eruptions of equal volumes of silicic magma, in spite of the fact that silicic magmas inject more fine ash and total volatiles (H2O, CO2) into the upper atmosphere. In view of the significant climatic impact of the Laki 1783 fissure eruption of basaltic magma, we suggest that flood or fissure basalt eruptions should be taken into account along with explosive silicic volcanism in paleoclimatic reconstructions.