Revisiting the question of the magnitude and intensity of the ~73 ka Toba eruption

The Younger Toba Tuff eruption, approximately 73 ka ago, is the largest known for the Quaternary and its climate, environmental and human consequences are keenly debated (Oppenheimer, 2011). However, to date, the estimates of its magnitude, intensity and duration are based on crude calculations. More robust estimation of these parameters with uncertainty bounds would represent a significant step in improving our understanding of the event. We report here on preliminary steps towards refined estimates of the YTT eruption parameters based on the recent development of a computational model that accounts for the complex dynamics of co-ignimbrite ash clouds (Herzog et al., 2010), and constraints from grain size data for distal deposits of YTT ash fallout. Recent studies (Herzog and Graf, 2010) showed that the height of co-ignimbrite plumes is largely insensitive to the areal forcing above a certain size limit due to the formation of multiple updrafts. Depending on the eruption intensity internal plume dynamics can dominate the horizontal transport in the umbrella region over large areas. Therefore the column height could act as self-limiting factor in the distribution of co-ignimbrite ash, evidencing the major role played by other factors, such as internal plume dynamics. Transport by atmospheric mean winds will only become important at a certain distance away from the source. Here, we parameterize the Active Tracer High Resolution Atmospheric Model (ATHAM, Herzog et al., 2003) for YTT co-ignimbrite cloud simulations, so as to reproduce particle size distributions known for terrestrial and marine YTT deposits. The particle size distributions, characterized through laser diffraction, have been collected from 350 km, 3,000 and 3,700 km from the vent. We report on the model setup, new constrains for the Toba ash distribution and on initial sensitivity studies that identify key sources of error propagation.