Sulfate concentration in ice core from Central Greenland, accurately dated by counting oxygen isotope seasonal variations. The year 1815 AD was due to the <a href="https://en.wikipedia.org/wiki/Mount_Tambora" class="extiw" title="w:Mount Tambora">Mt. Tambora</a> eruption, while prior to that is an unknown eruption.
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<cite style="font-style:normal">Dai, J.; Mosley-Thompson, E.; Thompson, L. G. (1991). "<a rel="nofollow" class="external text" href="http://www.agu.org/pubs/crossref/1991/91JD01634.shtml">Ice core evidence for an explosive tropical volcanic eruption six years preceding Tambora</a>". <i>Journal of Geophysical Research (Atmospheres)</i> <b>96</b> (D9): 17361–17366.</cite>