Abstract. The Younger Dryas is considered the archetypal millennial-scale climate change event, and identifying its cause is fundamental for thoroughly understanding climate systematics during deglaciations. However, the mechanisms responsible for its initiation remain elusive, and both of the most researched triggers (a meltwater pulse or a bolide impact) are controversial. Here, we consider the problem from a different perspective and explore a hypothesis that Younger Dryas climate shifts were catalysed by the unusually sulfurrich 12.880 ± 0.040 ka BP eruption of the Laacher See volcano (Germany). We use the most recent chronology for the GISP2 ice core ion dataset from the Greenland ice sheet to identify a large volcanic sulfur spike coincident with both the Laacher See eruption and the onset of Younger Dryasrelated cooling in Greenland (i.e. the most recent abrupt Greenland millennial-scale cooling event, the Greenland Stadial 1, GS-1). Previously published lake sediment and stalagmite records confirm that the eruption's timing was indistinguishable from the onset of cooling across the North Atlantic but that it preceded westerly wind repositioning over central Europe by ∼ 200 years. We suggest that the initial short-lived volcanic sulfate aerosol cooling was amplified by ocean circulation shifts and/or sea ice expansion, gradually cooling the North Atlantic region and incrementally shifting the midlatitude westerlies to the south. The aerosol-related cooling probably only lasted 1-3 years, and the majority of Younger Dryas-related cooling may have been due to the seaice-ocean circulation positive feedback, which was particularly effective during the intermediate ice volume conditions characteristic of ∼ 13 ka BP. We conclude that the large and sulfur-rich Laacher See eruption should be considered a viable trigger for the Younger Dryas. However, future studies should prioritise climate modelling of high-latitude volcanism during deglacial boundary conditions in order to test the hypothesis proposed here.

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

Abstract. The Younger Dryas is the most well-documented millennial-scale cooling event of the Quaternary, but the mechanisms responsible for its initiation remain elusive. Here we use a recently revised chronology for the GISP2 ice core ion dataset to identify a large volcanic sulphur spike coincident with both the sulphur-rich Laacher See volcanic eruption and the onset of Younger Dryas-related cooling (GS-1) in Greenland. Lake sediment and stalagmite records confirm that the 10 eruption's timing was indistinguishable from the onset of cooling across the North Atlantic, but that it preceded westerly wind repositioning over central Europe by ~200 years. We suggest that the initial short-lived volcanic sulphate aerosol cooling was amplified by oceanic circulation shifts or sea ice expansion, gradually cooling the North Atlantic region and incrementally shifting the mid-latitude westerlies to the south. The aerosol-related cooling probably only lasted 2-4 years, and the majority of Younger Dryas-related cooling was instead due to this positive feedback, which was particularly 15 effective during the intermediate ice volume conditions characteristic of ~13 ka BP. We conclude that the large and sulphurrich Laacher See eruption should be considered a viable trigger for the Younger Dryas.