  The latest caldera-forming eruption of Okmok volcano, Alaska, had a global
atmospheric impact with tephra deposits found in distant Arctic ice cores and a
sulfate signal found in Antarctic ice cores. The associated large-scale 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 this climactic phase with the two-phase flow model MFIX-TFM under
axisymmetric conditions with several combinations of injection mass flux,
emission duration, and topography. Results suggest that a steady mass flux of
$8.6-28\times 10^9$ kg/s is consistent with field observations. Stratospheric
injections occur in pulses issued from 1) the central plume initially rising
above the caldera center, 2) successive co-ignimbrite clouds caused by the
encounter of the pyroclastic density currents with topography, and 3) the
buoyant lift-off of dilute parts of the currents at the end of the eruption.
Overall, 2.5 to 25% of the emitted volcanic gas reaches the stratosphere if the
mass flux at the vent is steady. A fluctuating emission rate or an efficient
final lift-off due to seawater interaction were unlikely to have increased this
loading. Combined with petrological estimates of the degassed S, our results
suggest that the eruption emitted 46.5-60.4 Tg S into the troposphere and
injected 1.6-15.5 Tg S into the stratosphere, which controlled the atmospheric
forcing and the subsequent climate response.
