In the summer of 1831, people across the Northern Hemisphere witnessed a surreal transformation in the sky. The Sun appeared blue, at times even violet or green, casting an eerie light across landscapes already gripped by an unseasonable chill. Temperatures plummeted, crops failed, and confusion spread as the skies behaved in ways that defied all expectation.
This atmospheric mystery has long been considered unsolved. For nearly 200 years, scientists suspected that a volcanic eruption had triggered the unusual global cooling, but without a known source, the exact origin remained elusive. Now, researchers have traced the phenomenon to a powerful eruption that had been overlooked for generations — the Zavaritskii caldera in the remote Kuril Islands of Russia.
Ice Cores and a Breakthrough in Geochemical Detective Work
A team of researchers from the University of St Andrews, led by Dr. William Hutchison, employed advanced geochemical analysis to crack the 1831 mystery. Using polar ice cores collected from Greenland and Antarctica, they extracted microscopic volcanic glass shards, known as cryptotephra, and analyzed their chemical makeup. This was then compared to known tephra samples from volcanoes across Asia and the North Pacific.
The results pointed clearly to the Zavaritskii caldera on Simushir Island in the Kuril archipelago, a region between Japan and the Kamchatka Peninsula. When compared to the polar ash, tephra from the “Zav-1” eruption — a gray pumice fall deposit previously mapped on the island — showed a perfect geochemical match.
According to Dr. Hutchison, “The moment we analyzed both ashes together, one from the volcano and one from the ice core, was a true ‘Eureka’ moment. The numbers were identical.” What made this eruption particularly elusive was its geographical isolation and lack of historical documentation.
At the time, Simushir Island was sparsely inhabited, with small groups of Indigenous Ainu, Russian settlers, and Aleut workers occupying its northern coastline. No written records of the eruption survive, and ash fallout was minimal in the only inhabited area, Broughton Bay.
A Powerful Eruption with Climate-Altering Effects
The Zav-1 eruption was a significant event in terms of both magnitude and impact. Scientists estimate that the volcano injected approximately 12 ± 3.5 teragrams (Tg) of sulfur into the stratosphere.
This injection produced sulfate aerosols that reflected sunlight, resulting in widespread cooling across the Northern Hemisphere. Volcanic sulfate layers from Greenland’s ice cores show that this eruption caused cooling of approximately 0.5 to 1°C between 1831 and 1833.
These findings align with the intensity of other historically significant eruptions, including the 1991 eruption of Mount Pinatubo and the 1835 eruption of Cosegüina. The modeled radiative forcing of the Zav-1 event is estimated at −2 ± 1 watts per square meter, comparable to those same eruptions.
Using a model known as EVA_H, the researchers reconstructed the stratospheric aerosol optical depth and radiative forcing for the Zav-1 eruption. Based on this simulation, they calculated that the eruption’s sulfur output and plume height — estimated at 23 ± 12 kilometers above sea level — were sufficient to create a global stratospheric veil.
The resulting reduction in solar radiation would have cooled Earth’s surface and disrupted weather patterns, including rainfall linked to the monsoon systems of Africa and India.
Proceedings of the National Academy of Sciences
Why Previous Theories Fell Short
For years, various candidates were proposed as the source of the 1831 event. Among the most discussed was Ferdinandea, a short-lived submarine volcano that emerged off the coast of Sicily in July 1831. This eruption coincided with “blue sun” sightings in Europe and eastern North America, fueling speculation that it was responsible.
However, the new study provides multiple lines of evidence that rule out Ferdinandea. Geochemically, the ice-core tephra does not match volcanic material from Ferdinandea. Moreover, sulfur isotope analysis reveals that the sulfur from the 1831 ice-core deposits shows no signs of coming from evaporite sources, such as the sedimentary rocks around Sicily.
The study concludes that Ferdinandea’s sulfur emissions, at just 0.3 Tg, were far too low to account for the ice-core sulfate record or for the widespread cooling. Instead, it’s likely that Ferdinandea explains only the short-lived “blue sun” events in August 1831, while the broader cooling effect and atmospheric changes stemmed from Zavaritskii.
The researchers liken this to a similar situation in 44 BCE, when a minor eruption of Mount Etna produced atmospheric phenomena in the Mediterranean while a much larger eruption in Alaska caused long-term global cooling.
Reconstructing the Scale and Timing of Zav-1
Using a combination of field data, radiocarbon dating, and particle analysis, the team reconstructed the scale of the Zav-1 eruption. Tephra fallout deposits across Simushir Island and nearby islands such as Chirpoi and Urup suggest a bulk volume of 3.3 to 4.5 cubic kilometers. When converted to dense rock equivalent, this corresponds to an eruption magnitude of around 5.5 — placing it in the same league as Pinatubo.
Radiocarbon dating of soil and charcoal beneath the Zav-1 tephra layers confirms that the eruption occurred within the last 300 years. One of the strongest pieces of chronological evidence comes from artifacts found in Chirpoi Island’s Peschanaya Bay, where Zav-1 tephra overlays materials from the Russian colonial era, such as muscovite windowpanes and rusted firearms. These archaeological finds, together with tree-ring and ice-core data, point to the summer of 1831 as the eruption date.
Further validation came from sulfur isotope analysis. The Δ33S signals in ice-core samples confirmed stratospheric injection of volcanic aerosols, with a clear shift in sulfur mass-independent fractionation. This type of chemical signature can only occur when sulfur dioxide is oxidized in the presence of ultraviolet radiation, high in the stratosphere — reinforcing the case for a climate-altering eruption.
By integrating all these data — geochemistry, glaciochemistry, radiocarbon dating, aerosol modeling, and stratigraphic mapping — the researchers have established Zavaritskii as the long-missing source of the 1831 climate anomaly.
