Exact timing, sulfur spread and global climate footprint of the caldera-forming Mt. Mazama eruption, the largest volcanic eruption of the Holocene.

Large volcanic eruptions are key time markers in paleoclimatology because they inject large quantities of volcanic fallout (such as sulfuric acids and tephra) into the atmosphere which is then widely distributed and deposited in environmental archives such as ice cores, lakes and peat bogs. They also produce strong climate effects, imprinted in climate archives such as tree-rings. The caldera-forming eruption of Mount Mazama (Crater Lake, Oregon, USA) some 7700 years ago ranks among the largest eruptions of the Holocene but little is known about its exact timing and global-scale climate impacts. Here we use new high-resolution ice-core analyses of volatiles (S, Cl), particle-size distribution, crypto-tephra and sulfur isotopes (33S, 34S), from ice cores in Greenland and Antarctica, to constrain the date, stratospheric sulfur injection, global aerosol distribution and climate forcing of this eruption. We further demonstrate that the climatic effects left distinctive fingerprints in ultra-long tree-ring chronologies from North America and Europe allowing the date of this eruption to be pinned to a specific year, thereby aligning climate proxy records in North America, Greenland and Europe on a common timeline. Using an ensemble of fully-coupled Earth System Model simulations we identify some key regions experiencing large anomalies in temperature and hydro-climate following the Mt. Mazama eruption. These extreme conditions were not only relevant for hunter-gatherer communities and early agricultural societies emerging in Eurasia, that experienced these compounding effects, but they also help us in identifying a global existential risk arising from comparable eruptions in the future.