Numerical simulations of the latest caldera-forming eruption of Okmok volcano, Alaska

Abstract

The 2050 14C yBP caldera-forming eruption of Okmok volcano, Alaska, had a global atmospheric impact. The associated global climate cooling was driven by the amount of sulfur injected into the stratosphere during the climactic phase of the eruption. This phase was dominated by pyroclastic density currents, which have complex emplacement dynamics precluding direct estimates of the sulfur stratospheric load. We simulated the dynamics of the climactic phase with the two-phase flow model MFIX-TFM under axisymmetric conditions with several combinations of mass eruption rate, jet water content, vent size, particle size and density, topography, and emission duration. Results suggest that a steady mass eruption rate of 1.2-3.9e11 kg/s is consistent with field observations. Minimal stratospheric injections occur during emission as most of the volcanic gas is injected into the stratosphere by the buoyant liftoff of dilute parts of the currents at the end of the eruption. Overall, 58-64 wt percent of the total amount of gas emitted reaches the stratosphere. Combined with petrological estimates of the degassed S, our results suggest that the eruption injected 11 to 20 Tg S into the stratosphere, consistent with the subsequent climate response and Greenland ice sheet deposition. Our results also show that the combination of the source Richardson number and the mass eruption rate is able to characterize the buoyant-collapse transition at Okmok. We extended this result to 141 runs from 10 published numerical studies of eruptive jets and found that this regime diagram is able to capture the first-order layout of the buoyant-collapse transition in all studies except one. An existing multivariate criterion yields the best predictions of this regime transition.