Precise dating of the 686 ± 2 CE Newberry Pumice eruption and insights into 7th century volcanism from cryptotephra and sulfur isotopes in Greenland ice

Greenland ice core sulfate records reveal several large-magnitude volcanic events during the 7th Century of the Common Era (CE). The largest eruptions, in 626 and 682 CE, coincide with negative tree ring growth anomalies and documented evidence of climate cooling and societal crises. However, their volcanic sources remain unidentified, leaving major uncertainties about eruption latitude, sulfur injection height and climate forcing potential. Here, we analyse sulfur isotopes of deposited sulfate aerosol and cryptotephra geochemistry in Greenland ice core Tunu2013 to better constrain eruptive sources for the 626 and 682 CE eruptions, and additionally, a sulfate peak at 698 CE and a tephra deposit at 686. We present the first identification of the Newberry Pumice tephra in Greenland ice (686 ± 2 CE), extending the known spatial distribution of this North American tephrostratigraphic marker. Sulfur isotope data indicates this Newberry eruption injected sulfur primarily into the troposphere, consistent with previous studies. Isotopic evidence confirms that the 626 and 682 CE eruptions had stratospheric plume heights and were from extratropical Northern Hemisphere and tropical sources, respectively. The 698 CE sulfate peak is revised from an assumed tropical eruption, to a tropospheric, extratropical Northern Hemisphere eruption. Although cryptotephra geochemistry does not conclusively match known volcanic events, shards coincident with the 626 CE peak suggest an unidentified North Pacific arc source. Recurrent rhyolitic tephra throughout our sample set may reflect pulses of Southern Mono Craters activity, multiple unrelated volcanic events, or secondary remobilisation and deposition. These results demonstrate the value of integrating sulfur isotope and tephra analyses to refine reconstructions of volcanic–climate linkages.