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Polar ice cores and historical records evidence a large-magnitude volcanic eruption in 1831 CE. This event was estimated to have injected ~13 Tg of sulfur (S) into the stratosphere which produced various atmospheric optical phenomena and led to Northern Hemisphere climate cooling of ~1 °C. The source of this volcanic event remains enigmatic, though one hypothesis has linked it to a modest phreatomagmatic eruption of Ferdinandea in the Strait of Sicily, which may have emitted additional S through magma–crust interactions with evaporite rocks. Here, we undertake a high-resolution multiproxy geochemical analysis of ice-core archives spanning the 1831 CE volcanic event. S isotopes confirm a major Northern Hemisphere stratospheric eruption but, importantly, rule out significant contributions from external evaporite S. In multiple ice cores, we identify cryptotephra layers of low K andesite-dacite glass shards occurring in summer 1831 CE and immediately prior to the stratospheric S fallout. This tephra matches the chemistry of the youngest Plinian eruption of Zavaritskii, a remote nested caldera on Simushir Island (Kurils). Radiocarbon ages confirm a recent (<300 y) eruption of Zavaritskii, and erupted volume estimates are consistent with a magnitude 5 to 6 event. The reconstructed radiative forcing of Zavaritskii (−2 ± 1 W m⁻²) is comparable to the 1991 CE Pinatubo eruption and can readily account for the climate cooling in 1831–1833 CE. These data provide compelling evidence that Zavaritskii was the source of the 1831 CE mystery eruption and solve a confounding case of multiple closely spaced observed and unobserved volcanic eruptions. 

Abstract Existing global volcanic radiative aerosol forcing estimates portray the period 700 to 1000 as volcanically quiescent, void of major volcanic eruptions. However, this disagrees with proximal Icelandic geological records and regional Greenland ice-core records of sulfate. Here, we use cryptotephra analyses, high-resolution sulfur isotope analyses, and glaciochemical volcanic tracers on an array of Greenland ice cores to characterise volcanic activity and climatically important sulfuric aerosols across the period 700 to 1000. We identify a prolonged episode of volcanic sulfur dioxide emissions (751–940) dominated by Icelandic volcanism, that we term the Icelandic Active Period. This period commences with the Hrafnkatla episode (751–763), which coincided with strong winter cooling anomalies across Europe. This study reveals an important contribution of prolonged volcanic sulfate emissions to the pre-industrial atmospheric aerosol burden, currently not considered in existing forcing estimates, and highlights the need for further research to disentangle their associated climate feedbacks. 

Abstract Tephra is a unique volcanic product with an unparalleled role in understanding past eruptions, long-term behavior of volcanoes, and the effects of volcanism on climate and the environment. Tephra deposits also provide spatially widespread, high-resolution time-stratigraphic markers across a range of sedimentary settings and thus are used in numerous disciplines (e.g., volcanology, climate science, archaeology). Nonetheless, the study of tephra deposits is challenged by a lack of standardization that inhibits data integration across geographic regions and disciplines. We present comprehensive recommendations for tephra data gathering and reporting that were developed by the tephra science community to guide future investigators and to ensure that sufficient data are gathered for interoperability. Recommendations include standardized field and laboratory data collection, reporting and correlation guidance. These are organized as tabulated lists of key metadata with their definition and purpose. They are system independent and usable for template, tool, and database development. This standardized framework promotes consistent documentation and archiving, fosters interdisciplinary communication, and improves effectiveness of data sharing among diverse communities of researchers. 

Abstract. Volcanic eruptions are a key source of climatic variability, andreconstructing their past impact can improve our understanding of theoperation of the climate system and increase the accuracy of future climateprojections. Two annually resolved and independently dated palaeoarchives –tree rings and polar ice cores – can be used in tandem to assess thetiming, strength and climatic impact of volcanic eruptions over the past∼ 2500 years. The quantification of post-volcanic climateresponses, however, has at times been hampered by differences betweensimulated and observed temperature responses that raised questions regardingthe robustness of the chronologies of both archives. While manychronological mismatches have been resolved, the precise timing and climaticimpact of two major sulfate-emitting volcanic eruptions during the 1450s CE, including the largest atmospheric sulfate-loading event in the last 700 years, have not been constrained. Here we explore this issue through acombination of tephrochronological evidence and high-resolution ice-corechemistry measurements from a Greenland ice core, the TUNU2013 record. We identify tephra from the historically dated 1477 CE eruption of theIcelandic Veiðivötn–Bárðarbunga volcanic system in directassociation with a notable sulfate peak in TUNU2013 attributed to thisevent, confirming that this peak can be used as a reliable and precisetime marker. Using seasonal cycles in several chemical elements and 1477 CEas a fixed chronological point shows that ages of 1453 CE and 1458 CE can beattributed, with high precision, to the start of two other notablesulfate peaks. This confirms the accuracy of a recent Greenland ice-corechronology over the middle to late 15th century and corroborates thefindings of recent volcanic reconstructions from Greenland and Antarctica.Overall, this implies that large-scale Northern Hemisphere climatic coolingaffecting tree-ring growth in 1453 CE was caused by a Northern Hemispherevolcanic eruption in 1452 or early 1453 CE, and then a Southern Hemisphereeruption, previously assumed to have triggered the cooling, occurred laterin 1457 or 1458 CE. The direct attribution of the 1477 CE sulfate peak to the eruption ofVeiðivötn, one of the most explosive from Iceland in the last 1200 years, also provides the opportunity to assess the eruption's climaticimpact. A tree-ring-based reconstruction of Northern Hemisphere summertemperatures shows a cooling in the aftermath of the eruption of −0.35 ∘C relative to a 1961–1990 CE reference period and−0.1 ∘C relative to the 30-year period around the event, as well as arelatively weak and spatially incoherent climatic response in comparison tothe less explosive but longer-lasting Icelandic Eldgjá 939 CE and Laki1783 CE eruptions. In addition, the Veiðivötn 1477 CE eruptionoccurred around the inception of the Little Ice Age and could be used as achronostratigraphic marker to constrain the phasing and spatial variabilityof climate changes over this transition if it can be traced in moreregional palaeoclimatic archives.