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Recently, a record of large, mostly unknown volcanic eruptions occurring during the younger half of the last glacial period (12–60 ka) has been compiled from ice-core records. In both Greenland and Antarctica these eruptions led to significant deposition of sulfate aerosols, which were likely transported in the stratosphere, thereby inducing a climate response. Here we report the first attempt to identify the climatic impact of volcanic eruptions in the last glacial period from ice cores. Average negative anomalies in high-resolution Greenland and Antarctic oxygen isotope records suggest a multi-annual volcanic cooling. Due to internal climate variability, glaciological noise, and uncertainties in the eruption age, the high-frequency noise level often exceeds the cooling induced by individual eruptions. Thus, cooling estimates for individual eruptions cannot be determined reliably. The average isotopic anomaly at the time of deposition also remains uncertain, since the signal degrades over time as a result of layer thinning and diffusion, which act to lower the resolution of both the oxygen isotope and sulfur records. Regardless of these quantitative uncertainties, there is a clear relationship of the magnitude of isotopic anomaly and sulfur deposition. Further, the isotopic signal during the cold stadial periods is larger in Greenland and smaller in Antarctica than during the milder interstadial periods for eruptions of equal sulfur deposition magnitude. In contrast, the largest reductions in snow accumulation associated with the eruptions occur during the interstadial periods. This may be the result of a state-dependent climate sensitivity, but we cannot rule out the possibility that changes in the sensitivity of the isotope thermometer or in the radiative forcing of eruptions of a given sulfur ejection may play a role as well. [ABSTRACT FROM AUTHOR]

Large volcanic eruptions occurring in the last glacial period can be detected by their accompanying sulfuric acid deposition in continuous ice cores. Here we employ continuous sulfate and sulfur records from three Greenland and three Antarctic ice cores to estimate the emission strength, the frequency and the climatic forcing of large volcanic eruptions that occurred during the second half of the last glacial period and the early Holocene, 60–9 kyr before 2000 CE (b2k). Over most of the investigated interval the ice cores are synchronized, making it possible to distinguish large eruptions with a global sulfate distribution from eruptions detectable in one hemisphere only. Due to limited data resolution and large variability in the sulfate background signal, particularly in the Greenland glacial climate, we only list Greenland sulfate depositions larger than 20 kgkm-2 and Antarctic sulfate depositions larger than 10 kgkm-2. With those restrictions, we identify 1113 volcanic eruptions in Greenland and 737 eruptions in Antarctica within the 51 kyr period – for which the sulfate deposition of 85 eruptions is found at both poles (bipolar eruptions). Based on the ratio of Greenland and Antarctic sulfate deposition, we estimate the latitudinal band of the bipolar eruptions and assess their approximate climatic forcing based on established methods. A total of 25 of the identified bipolar eruptions are larger than any volcanic eruption occurring in the last 2500 years, and 69 eruptions are estimated to have larger sulfur emission strengths than the Tambora, Indonesia, eruption (1815 CE). Throughout the investigated period, the frequency of volcanic eruptions is rather constant and comparable to that of recent times. During the deglacial period (16–9 kab2k), however, there is a notable increase in the frequency of volcanic events recorded in Greenland and an obvious increase in the fraction of very large eruptions. For Antarctica, the deglacial period cannot be distinguished from other periods. This confirms the suggestion that the isostatic unloading of the Northern Hemisphere (NH) ice sheets may be related to the enhanced NH volcanic activity. Our ice-core-based volcanic sulfate records provide the atmospheric sulfate burden and estimates of climate forcing for further research on climate impact and understanding the mechanism of the Earth system. [ABSTRACT FROM AUTHOR]

The Younger Toba Tuff eruption ∼74 ka ago in Indonesia, is among the largest known supereruptions in the Quaternary and its potential impact on the climate system and human evolution remains controversially debated. The eruption is dated radiometrically to 73.88 ± 0.32 ka (1σ, Storey et al., 2012) and it occurred at the abrupt cooling transition from Greenland Interstadial 20 to Greenland Stadial 20. The precise stratigraphic position of volcanic fallout detected in ice cores from both polar ice sheets has previously been narrowed down to four potential candidates. Here, we compile all available Greenland and Antarctic sulfate records, together with electrical conductivity records and recently obtained sulfur isotope records to identify, quantify and characterize these Toba candidates in terms of their likely latitudinal position of eruption, sulfur emission strength and radiative forcing. We identify that the youngest event of the four candidates is composed of two separate eruptions, both likely located in the extra-tropical Northern Hemisphere. We deem the two older events unlikely candidates for the Toba eruption because of their limited sulfur emission strengths. The second youngest event has the largest sulfur output of the Toba candidates, and it is also larger than any other volcanic event identified in ice core records over the last 60 kyr. Comparable amounts of sulfate deposits in Greenland and Antarctica strongly suggest a tropical source. We thus propose the second youngest event (74,156 years before 2000 CE) to be most likely associated with the Toba eruption. The estimated stratospheric sulfate loading of the proposed Toba eruption is 535 ± 96 Tg, which is 3 times that of Samalas 1258 CE, 6 times that of Tambora (1815) CE and 20 times that of Pinatubo (1991) CE. We derive the continuous time-series of volcanic sulfate deposition, sulfur emission strength and radiative forcing over the 74.8–73.8 ka time window, suitable for conducting experiments with climate models that either require prescribed forcing field or interactively reproduce aerosol processes. We estimate the cumulative volcanic sulfur emission strength and the radiative forcing of the two younger events and they are found to be much stronger than those at the onset of the Younger Dryas and those preceding the Little Ice Age. Stacked Greenland water isotope records show an accelerated transition trend and abrupt shift after the proposed Toba eruption and suggest that the Greenland moisture source moved southward shortly after the Toba eruption. The Toba eruption may thus have an amplifying effect on the cooling transition leading to Greenland Stadial 20. • Identified precise stratigraphic position in ice cores and quantified sulfur emission strength for the Toba eruption. • The stratospheric sulfate loading of the Toba eruption is 535 ± 96 Tg, which is 20 times that of Pinatubo (1991) CE. • Derived the continuous time-series of volcanic sulfur emission strength and radiative forcing over the 74.8–73.8 ka. [ABSTRACT FROM AUTHOR]