About 74,000 years ago Earth's climate abruptly transitioned to particularly severe cold and dry conditions, which lasted for several millennia. An incomplete eruption record may be why volcanic eruptions were dismissed as the trigger.The Youngest Toba Tuff eruption of Toba volcano, Indonesia, is the largest known eruption of the Quaternary with a magnitude of 8.8. Events exceeding a magnitude of 8.0 are those which demonstrate a total erupted mass in excess of 10 15 kg (~1000 km 3 dense-rock-equivalent), and represent the largest expressions of explosive volcanism on Earth. The eruption date of 73.88 ± 0.32 thousand years ago 1 is within error of the transition into a cold period, recorded in Greenland ice cores as Greenland Stadial 20 with an onset 74.1 ± 0.06 thousand years ago 2 . This coincidence in timing has led to speculation that there may exist a causal link between the Youngest Toba Tuff eruption and Greenland Stadial 20, and that the eruption's exceptional magnitude alongside a release of 1700-3500 Mt of sulfur dioxide (SO 2 ) 3 was sufficient to have precipitated a cold period in the Northern Hemisphere that lasted for about 2000 years. We know that the 1991 AD Pinatubo eruption released about 18 Mt SO 2 and affected climate globally: the Pinatubo eruption caused a global temperature anomaly of 0.4 °C, and a Northern Hemisphere anomaly of 0.5 °C4 . The Toba eruption was two orders of magnitude larger than Pinatubo and released between 94 and 194 times as much sulfur into the atmosphere. Additionally, modern aerosol-climate models suggest that direct radiative cooling of 3.5 to 9 °C for multiple years would ensue following an eruption of this size, with significant implications for atmospheric circulation and ocean dynamics 5,6 .The coincidence in the timing between the Youngest Toba Tuff eruption and the onset of Greenland Stadial 20 is tantalizing, with the eruption date within dating uncertainty of the initiation of the cooling. Additionally, positive feedback mechanisms associated with sea ice and oceanic circulation that extend sub-decadal scale aerosol effects to thousands of years have been proposed 7 . However, uncertainty regarding the timing of the Youngest Toba Tuff eruption relative to Greenland Stadial 20, as well as its effective sulfur load 8 , contribute to the substantial debate concerning the role Toba played in driving global climate change and the onset of Greenland Stadial 20.Recent work may hold the key to better understanding the cause of Greenland Stadial 20. Cisneros de León et al. 9 have used zircon (U-Th)/He dating to show that the Los Chocoyos eruption of Atitlán volcano, Guatemala, occurred 75 ± 2 thousand years ago: about 9000 years after the previously accepted eruption time of about 84 thousand years ago. The Los Chocoyos eruption was not only sulfur-rich, but also the third largest eruption of the Quaternary with an estimated magnitude of 8.1 10 . The new eruption date possibly makes the Toba and Atitlán events the first known doublet of super-eruptions occurring withi...

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<p>Sulfate aerosols are a primary driver of climate impacts during and following volcanic eruptions and form from erupted SO<sub>2 </sub>gas. However, the amount of SO<sub>2</sub> that is delivered to the stratosphere is not clearly related to the amount dissolved in the magma (the ‘sulfur excess problem’). Therefore, magma properties and eruption magnitude are not necessarily predictive of climate impacts from eruptions, which is exacerbated by the as-yet unknown importance of the insulated, hot transport pathway. During a magnitude 6 explosive volcanic eruption there is up to 100 seconds of transport between the magma fragmentation depth – where volcanic ash is formed and the mixture accelerates – and the Earth’s surface. Here, we present a numerical implementation of a theoretical framework which predicts the rapid reactions between gases and volcanic ash in this transport interval, which include: (1) iron oxidation state changes; (2) SO<sub>2</sub> uptake via calcium sulfate surface crystallization; (3) HCl uptake via NaCl surface crystallization; and (4) incipient nanolite crystallization that may be related to (1). In all cases, these processes are rate-limited by a suite of diffusive exchanges between the ash bulk and surface, for which our model solves. To demonstrate the upscaled importance of these processes, we couple our models to volcanic plume simulations (using a 1991 Pinatubo baseline simulation), and output the bulk SO<sub>2</sub> that can be captured by ash. We find that depending on the source parameters of the eruption, anywhere between 30 and 100 wt.% of the total erupted SO<sub>2</sub> can be removed from the plume gas and captured by ash. This effectively changes the sink of SO<sub>2</sub> from the stratosphere to the hydrosphere, as CaSO<sub>4</sub> crystals are soluble and ultimately wash into the environment following ash deposition. We propose that these hot sulfur scrubbing processes may be crucial in mediating SO<sub>2</sub> delivery to the atmosphere, and therefore may explain much of the complexities associated with correlating eruption magnitude with climate impacts in the recent past or back into the Last Glacial period.</p>

The Laacher See Tephra (LST) is a key Late Pleistocene chronostratigraphic unit across Europe, and an accurate date for the deposit is critical for understanding Late Glacial sedimentary sequences. Reinig et al.1 recently used radiocarbon measurements of subfossil trees trapped within the LaacherSee eruption's (LSE) pyroclastic deposits to date the eruption to 13,006 +/-9 BP, ~130 years older than the previously accepted varve counting (12,880 ± 40 BP2) and 40Ar/39Ar (12,900 ± 560 BP3) age determinations. However, Reinig et al. did not correct for the incorporation of radiocarbon 'dead' magmatic CO2 into the growing trees, and here we highlight the possibility that the date is in fact ~130 years too old. The implications of incorporating a high precision yet inaccurate LST age into the

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