Abstract. The ∼74 ka Toba eruption was one of the largest volcanic events of the Quaternary. There is much interest in determining the impact of such a large event, particularly on the climate and hominid populations at the time. Although the Toba eruption has been identified in both land and marine archives as the Youngest Toba Tuff, its precise place in the ice core record is ambiguous. Several volcanic sulfate signals have been identified in both Antarctic and Greenland ice cores and span the Toba eruption 40Ar/39Ar age uncertainty. Here, we measure sulfur isotope compositions in Antarctic ice samples from the Dome C (EDC) and Dronning Maud Land (EDML) ice cores at high temporal resolution across 11 of these potential Toba sulfate peaks to identify candidates with sulfur mass-independent fractionation (S-MIF), indicative of an eruption whose plume reached altitudes at or above the stratospheric ozone layer. Using this method, we identify several candidate sulfate peaks that contain stratospheric sulfur. We further narrow down potential candidates based on the isotope signatures by identifying sulfate peaks that are due to a volcanic event at tropical latitudes. In one of these sulfate peaks at 73.67 ka, we find the largest ever reported magnitude of S-MIF in volcanic sulfate in polar ice, with a Δ33S value of −4.75 ‰. As there is a positive correlation between the magnitude of the S-MIF signal recorded in ice cores and eruptive plume height, this could be a likely candidate for the Toba super-eruption, with a plume top height in excess of 45 km. These results support the 73.7±0.3 ka (1σ) 40Ar/39Ar age estimate for the eruption, with ice core ages of our candidates with the largest magnitude S-MIF at 73.67 and 73.74 ka. Finally, since these candidate eruptions occurred on the transition into Greenland Stadial 20, the relative timing suggests that Toba was not the trigger for the large Northern Hemisphere cooling at this time although we cannot rule out an amplifying effect.

Abstract. The ~74 ka Toba eruption was one of the largest volcanic events of the Quaternary. There is much interest in determining the impact of such a huge event, particularly on the climate and hominid populations at the time. Although the Toba eruption has been identified in both land and marine archives as the Youngest Toba Tuff, its precise place in the ice core record is ambiguous. Multiple volcanic sulfate signals have been identified in both Antarctic and Greenland ice cores within the uncertainty of age estimates as possible events for the Toba eruption. We measure sulfur isotope compositions in Antarctic ice samples at high temporal resolution across 11 of these potential Toba sulfate peaks in two cores to identify candidates with sulfur mass-independent fractionation (S-MIF), indicative of an eruption whose plume reached altitudes at or above the ozone layer in the stratosphere. Using this method, we identify several candidate sulfate peaks that contain stratospheric sulfur. We further narrow down potential candidates based on the isotope signatures by identifying sulfate peaks that are due to a volcanic event at tropical latitudes. In one of these sulfate peaks at 73.67 ka, we find the largest ever reported magnitude of S-MIF in volcanic sulfate in polar ice, with a Δ33S value of −4.75 ‰. As there is a positive correlation between the magnitude of the S-MIF signal recorded in ice cores and eruptive plume height, this could be a likely candidate for the Toba supereruption, with a plume height in excess of 45 km. These results support the 73.7 ± 0.3 ka (1σ) ka Ar/Ar age estimate for the eruption, with ice core ages of our candidates with the largest magnitude S-MIF at 73.67 and 73.74 ka. Finally, since these candidate eruptions occurred on the transition into Greenland Stadial 20, the relative timing suggests that Toba was not the trigger for the large Northern Hemisphere cooling at this time although we cannot rule out an amplifying effect.

1. A method to model the minimum volume and mass to inundate an at-risk city from a volcano source site We use a Bayes Belief Network (BBN) related to each model to characterize uncertainties on the MinVol estimate, including in the calculational scheme correlation coefficients between the input variables and the calculated MinVol [Ababei (2016); UNINET BBN: https://lighttwist-software.com/ accessed 25 Nov 2019]. The large rectangular panels in each BNN illustrate the shape of the uncertainty distribution for the minimum volume needed to reach the at-risk city, based on the uncertainties in the relevant input variables, with the mean value and standard deviation shown in the lower frame of the panel. Larger correlation values (marked on linking arc arrows) highlight the more sensitive variables. Negative correlation means that an increase of that input decreases the MinVol required to reach the at-risk city. Positive correlations produce the opposite effect. The correlation coefficients are based on the joint DM distribution from elicitation and on the 130 km maximum runout distance. Further research is merited to develop better constraints on the uncertainty distributions of the most influential input parameters. Acknowledgements: This work was supported by Aspinall & Associates. We thank Charles Connor, Laura Connor, Britt Hill and the other experts participating in the elicitation sessions. .

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Abstract. Volcanic eruptions are the dominant cause of natural variability in climate forcing on timescales up to multidecadal. Large volcanic eruptions lead to global-scale climate effects and influence the carbon cycle on long timescales. However, estimating the frequency of eruptions is challenging. Here we assess the frequency at which eruptions with particular deposition fluxes are observed in the EPICA Dome C ice core over the last 200 kyr. Using S isotope analysis we confirm that most of the largest peaks recorded at Dome C are from stratospheric eruptions. The cumulative frequency through 200 kyr is close to linear, suggesting an approximately constant rate of eruptions. There is no evidence for an increase in the rate of events recorded in Antarctica at either of the last two deglaciations. Millennial variability is at the level expected from recording small numbers of eruptions, while multimillennial variability may be partly due to changes in transport efficiency through the Brewer–Dobson circulation. Our record of events with sulfate deposition rates > 20 and >50 mg m−2 contains 678 and 75 eruptions, respectively, over the last 200 kyr. Calibration with data on historic eruptions and analysis of a global Quaternary dataset of terrestrial eruptions indicates that sulfate peaks with deposition rates > 20 and >50 mg m−2 correspond to explosive eruptions of magnitude ≥ 6.5 and ≥7, respectively. The largest recorded eruption deposited just over 300 mg m−2.