No evidence for tephra in Greenland from the historic eruption of Vesuvius in 79 CE: implications for geochronology and paleoclimatology

. Volcanic fallout in polar ice sheets provides important opportunities to date and correlate ice-core records as well as to investigate the environmental impacts of eruptions. Only the geochemical characterization of volcanic ash (tephra) embedded in the ice strata can conﬁrm the source of the eruption, however, and is a requisite if historical eruption ages are to be used as valid chronological checks on annual

The Greenland Ice Core Chronology 2005 (GICC05) provides a synchronized dating for three Greenland ice cores (NGRIP, in which three volcanic marker events were believed to correspond to historic volcanic eruptions (i.e., Vesuvius 79 CE, Hekla 1104 CE and Öraefajökull 1362 CE). These horizons were subsequently assigned an age uncertainty of ±0 years (Vinther et al., 2006), though only for the last-mentioned was the matching supported by a strong geochemical correlation of tephra (Coulter et al., 2012). This ice-core chronology has since been revised, however, following the recognition of distinct solar proton events at 774 and 993 CE that highlighted a temporal offset in the Greenland Ice Core Chronology 70 2005 (GICC05), prompting the construction of revised ice-core chronologies, NS1-2011 (Sigl et al., 2015) and DRI_NGRIP2 . As a result, the date of the sulfate layer formerly assigned to Vesuvius 79 CE is now placed at 88 CE, with a smaller volcanic sulfate concentration peak at 80 CE in NGRIP2 and GISP2 (Fig. 1b). We identify in the northern Greenland NEEM-2011-S1 ice core an unambiguous tephra associated with the 88 CE event. We consider the potential source of the tephra and examine the consequences of attributing ice core volcanic signals to specific events in the absence of 75 supporting evidence.

Methods
Three ice samples bracketing a sulfate peak at 410.56 m in the NEEM-2011-S1 core were cut for tephra analysis between 410.20-410.85 m using a bandsaw in the ice core laboratory at the Desert Research Institute (DRI), Nevada (Fig. 1c). Samples 80 were transferred to precleaned Nalgene bottles and sent to Queen's University Belfast (QUB) for tephra extraction where they were assigned unique identifier codes (QUB-1832: 410.85-410.65 m;QUB-1833: 410.65-410.40 m;QUB-1834: 410.40-410.20 m). The meltwater was centrifuged to concentrate particulates, which were then pipetted onto pre-ground glass slides on a hotplate in a laminar flow cupboard. The samples were covered in Buehler epoxy resin. Tephra shard content was quantified using ×100-400 magnification on a light microscope with the aid of cross polarization. Samples containing tephra 85 were ground using 12 μm alumina powder and polished using 6 μm, 3 μm and 1 μm diamond paste until the surfaces of the shards were exposed for analysis.
To determine the source of the tephra, major element geochemical analysis was performed on a JEOL FEGSEM 6500F at To evaluate possible source eruptions for the recorded tephra, a list of all known eruptions within the period 100 BCE-300 CE with an estimated Volcanic Explosivity Index (VEI) of ≥4 was collated from Siebert et al. (2011), supplemented by data from the Smithsonian's Global Volcanism Program (GVP; http://volcano.si.edu/; Dataset 2 in the Supplement). The wide timespan allows for chronological uncertainty of the events, none of which is historically dated with the exception of Vesuvius 79 CE.
In view of Iceland's vicinity to Greenland, we also considered smaller magnitude eruptions from this region, excluding those 100 volcanic systems that typically produce homogenous basaltic glasses, such as Grímsvötn and Bárdarbunga. Where possible, reference glass geochemical data for the events were collated from published records for comparison with the tephra identified in the NEEM-2011-S1 ice core, and were added to the QUB tephra database. For possible correlatives, we analyzed reference glass samples for geochemical analysis under the same operating conditions as the NEEM-2011-S1 tephra (see Supplementary   Information in the Supplement for sample details). 105 On the basis of three potential source volcanoes suggested by the tephra geochemistry (namely Aniakchak, Chikurachki, and Popocatepétl; see Fig. 1), we modelled ash distribution using the advection-dispersion-sedimentation software Ash3d (Schwaiger et al., 2012) to evaluate the necessary eruption and meteorological parameters needed to transport ash as far as the NEEM coring site (77°27'N 51°3.6'W; 2,000 m above sea level). Ash3d calculates tephra deposition as mass per unit area at 110 sites, even if the fallout is vanishingly small. Hence, even exceedingly small fallout values contributing very low concentrations of tephra can be identified as non-zero values . Model parameters were selected on the basis of each volcanic source under consideration (see Sect. 3.4). As the meteorological conditions at the time of the eruption are not known, we used data drawn from the 2.5 degree NCEP-NCAR Reanalysis II dataset (https://climatedataguide.ucar.edu/climatedata/ncep-reanalysis-r2) that captures 3D time-varying atmospheric structure (including temperature and horizontal and 115 vertical winds). Start dates and times for the events were selected randomly from the period between 1950 to 2010, but limited to just the winter months (November-February), reflecting the likely season of the eruption based on sulfate and tephra deposition in the ice core (Sigl et al., 2015;McConnell et al., 2018). For each of the three sources, 350 simulations were run.
Additionally, to distinguish meteorological differences from differences in the eruption source parameters, we randomly selected 1,000 start times from 1950 to 2010, and simulated events using identical source parameters at the three locations of 120 interest.

The NEEM-2011-S1 tephra
Cryptotephra was identified in samples QUB-1832 (n = 18) and QUB-1833 (n = 5), demonstrating that ash deposition preceded the maximum of sulfuric aerosol depositions at this location in Greenland. Shards were pale brown, ranged between 24 and 85 125 µm in longest axis, and displayed a varied morphology, including blocky, bubble-wall and platy shards, some with elongated or occasionally spherical vesicles (Supplementary Information in the Supplement). Several shards contained minerals, and one https://doi.org/10.5194/cp-2021-63 Preprint. Discussion started: 16 June 2021 c Author(s) 2021. CC BY 4.0 License.
was microlite-rich. Major element analyses of the glass were obtained for six and three shards in these samples, respectively, and trace element data for five and three of the shards. The data reveal a homogenous, andesitic glass that evidently derives from a single source and the sample is hereafter named QUB-1832/3. Major and trace element compositions suggest a source 130 along an active arc (cf. Tomlinson et al., 2015;Fig. 2).
The GVP database lists five VEI 6, three VEI 5 and 31 VEI 4 eruptions dating to between 100 BCE and 300 CE. The VEI 6 eruptions include that of Churchill, Okmok and Ksudach, whose chronologies have all been recently revised and no longer encompass the first century CE (Ponomareva et al., 2017;McConnell et al., 2020;Reuther et al., 2020). The magnitude of 135 Ambrym, Vanuatupreviously thought to have entailed a cataclysmic collapse of the calderahas also been questioned and was likely a less pronounced, gradual event (Németh et al., 2009). Of the three VEI 5 eruptions, only Masaya is located on an active arc. We have been unable to find major element glass geochemistry for many of the VEI 4 eruptions, but we draw upon trace element data (for glass or whole rock), tectonic setting and information from other eruptives to evaluate their correspondence with QUB-1832/3 (Dataset 2 in the Supplement). 140

Major element geochemistry
The QUB-1832/3 glass geochemistry is clearly distinct from Vesuvius glass and from the data presented for the GRIP ash particles by Barbante et al. (2013). On the basis of major element compositions, we can also firmly reject correlations with the VEI 6 eruptions from Ksudach, Okmok, Churchill (WRAn tephra), Apoyeque (Chiltepe tephra) and Taupo, and the VEI 5 eruptions from Furnas and Masaya (Fig. 3a, b). We lack a full suite of data for Ambrym, but glasses from this eruption do not 145 include an andesitic component (Robin et al., 1993). We find no geochemical matches with any of the VEI 4 eruptions for which we have data. We also exclude Iceland as a source, as the geochemistry matches neither the known eruptions of this time period nor any other Icelandic tephras for which we have data. Similarly, we discount Jan Mayen from consideration as only basaltic tephras have been reported from this source (Gjerløw et al., 2016), and no eruption is known in the timeframe under consideration. The geochemistry does not correlate with any known Kamchatkan source (Kyle et al., 2011;Ponomareva 150 et al., 2017;V. Ponomareva, pers. comm.). Extending our search beyond the age range of the QUB-1832/3 tephra, we find geochemical correlations with two shards (Shards #5 and #7) within the mixed population of QUB-1859 from NEEM-2011-S1, dated to 536 CE (NS1-2011 chronology; Sigl et al., 2015;Fig. 4). A comparison with published data reveals a close similarity to the andesitic component of Aniakchak 155 (Kaufman et al., 2012;Davies et al., 2016) and to material from Mount Spurr (Riehle, 1985;Béget et al., 1994;Child et al., 1998). QUB-1832/3 has a slightly lower TiO2 content than the Aniakchak tephra, however. Although no large Aniakchak eruptions are known from the timeframe of interest, the GVP reports an eruption with no assigned VEI at 255±200 CE. Point data are not available for the Mount Spurr tephras but the average FeOtotal content also tends to be lower than the NEEM-2011-S1 tephra (Fig. 5). No activity at this volcano has been reported in the timeframe of the 88 CE event.
The Chikurachki eruptions are not listed in the GVP database, but large Popocatépetl eruptions are dated to 200±300 BCE (VEI 5: Lorenzo Pumice) and 822-823 CE (VEI 4: Pink Pumice), the latter date based on an assumed linkage with a sulfate 165 peak in the Greenland ice core GISP2 (Siebe et al., 1996(Siebe et al., , 2017 at this time (817 CE on the NS-2011 timescale). We reanalyzed OSC1-5 glass and reference samples from Popocatépetl, including material from the Lorenzo and Pink Pumices, and from reference ash samples provided by the Smithsonian Institution, on the JEOL FEGSEM 6500F (Fig. 4). Our results show that QUB-1832/3 differentiates from OSC1-5 in FeOtotal and CaO content, rendering Chikurachki an unlikely source of the NEEM-2011-S1 tephra. Analyses from the Pink and Lorenzo Pumices from Popocatépetl demonstrate homogenous dacitic to 170 rhyodacitic glass compositions, that do not overlap with the andesitic datapoints of QUB-1832/3. The Smithsonian samplesfrom an unspecified event or eventsshow greater compositional variability along a trend from rhyolitic to basaltic trachyandesitic. The majority of analyses obtained from these samples have a rhyolitic to dacitic composition, seemingly on a compositional trend towards the QUB-1832/3 population except in FeOtotal concentrations. Some rhyolitic shards of QUB-1859 (Shards #1, #10, #11 and #2: Sigl et al., 2015) attributed to the 536 CE fallout closely match the high-silica Popocatépetl 175 analyses, but divergence in TiO2 (Shards #1, #10, #11) or K2O (Shard #2) is evident.

Trace element geochemistry
The trace element signature of QUB-1832/3 is clearly distinct from candidate eruptions in Latin America ( Fig. 5a-c). Lesser Antilles sources have a Rb-U and Ta>Na signature that distinguishes them from the cryptotephra. Although less evolved than the Japanese tephras reported by Albert et al. (2019), the trace element composition of the QUB-1832/3 glass is inconsistent 180 with Japanese sources (Fig. 5d).
Our best matches using major element geochemistry are with two andesitic shards in QUB-1859, various Holocene andesitic eruptives of Aniakchak and two recent eruptives from Mount Spurr; our analysis of Popocatépetl is inconclusive. Trace element data were collected from the pertinent shards in QUB-1859 and confirm that these shards derive from the same source as QUB-185 1832/3 (Fig. 6). Attempts to analyze other shards within QUB-1859, specifically those lying close to the rhyolitic end of Popocatépetl compositional field, were unsuccessful due to the entrapment of minerals.
We compared our results with published trace element data for Aniakchak glass (Kaufman et al., 2012) and with newly obtained data from the four Smithsonian reference samples from Popocatépetl. Our analyses show that the ice-core tephras are 190 lower in trace element concentrations than Aniakchak but share consistent high field strength element (HFSE) ratios, and a link with this source or source region cannot be excluded ( Fig. 6b-d). In contrast, clearly distinctive HFSE ratios (e.g., Th: Nb; https://doi.org/10.5194/cp-2021-63 Preprint. Discussion started: 16 June 2021 c Author(s) 2021. CC BY 4.0 License. Fig. 6d) differentiate the tephra from Popocatépetl and allow this source to be eliminated. We were unable to locate trace element data for Mount Spurr.

Modelling ash dispersal 195
Our geochemical analysis of the QUB-1832/3 tephra demonstrates that it derives from an eruption other than a known event of VEI 4 or higher listed in the GVP database. The geochemical composition also enables us to eliminate from consideration well-characterized volcanoes in Iceland, Kamchatka and, on the basis of trace element composition, the Lesser Antilles and Japan. Other likely source regions include Alaska, the Kuriles arc and Mexico. We therefore explore the eruption parameters needed for tephra to be transported from these sources to Greenland, by modelling ash dispersal from the three candidates 200 (Aniakchak, Chikurachki and Popocatépetl) highlighted by major element geochemical similarities.
Despite a lack of clear correspondence in the major and trace element composition, we cannot firmly reject Aniakchak or a neighboring system as a possible source of the QUB-1832/3 tephra. Aniakchak is not known to have erupted in the timeframe of interest, implying that any such eruption is unlikely to have been very large, or deposits would have been preserved locally. 205 Since the caldera-forming eruption (CFE) at ~1640 BCE (GICC05 chronology) whose tephra has been securely identified in Greenland ice (Coulter et al., 2012;Plunkett et al., 2017), at least two VEI 4 eruptions have occurred. To simulate ash dispersal, we therefore used parameters for a VEI 4 event erupting 0.5 km 3 dense rock equivalent (DRE) volume of tephra with a 12 km plume height over a duration of 15 hours. In ~10% of the simulations, fine tephra (~30 μm) reached the NEEM ice core drilling site, but not larger particles. The model suggests that a VEI 4 eruption could, under suitable meteorological conditions, disperse 210 ash to Greenland, but probably not a smaller event (Fig. S1 in the Supplement).
Our major element data suggest that Chikurachki is an unlikely source of QUB-1832/3, but large (VEI 4)  prior to the settlement of the region in the 18th century but at least four other VEI 4 eruptions from Chikurachki are known to have occurred during the mid-to late Holocene (Siebert et al., 2011). We therefore examined the potential ash dispersal of a comparable event (0.25 km 2 DRE, 11 km-high plume, eruption duration 7 hours, with grain size distribution as described by Belousova and Belousov, 2001) to transport tephra as far as northern Greenland. In none of the Ash3d simulations did tephra 220 reach Greenland (Fig. S2 in the Supplement).
Thirdly, we examine the possible dispersal of Popocatépetl ash. Recent eruptions have here been of a modest nature (VEI 3 or less), injecting small volumes of ash and sulfur into the troposphere. Larger eruptions dating to 200±300 BCE (VEI 5) and ~750 CE (VEI 4) are inferred from tephra fall deposits (known respectively as the Lorenzo and Pink Pumices; Siebe and Macías, 2004). For the Ash3d simulations, we used the parameters outlined for an older VEI 6 event, the ~3700 BCE Ochre These simulations used eruption source parameters (plume height, eruptive volume, duration, grain-size distribution) that were drawn from past events for Chikurachki and Popocatépetl. To address the question of how big or how small would an eruption need to be or could be for ash from each volcano to fall out in Greenland, we randomly selected 1,000 events and applied the 235 same eruption source parameters to each of the three volcanoes, with eruptions commencing at randomly selected times from a uniform distribution between 1950 and 2010. Eruptive volume was selected from a log-normal distribution between 0.01 and 2.0 km 3 (dense rock equivalent). Plume heights were calculated using a best-fit relationship between eruptive volume and plume height with a random adjustment (Gaussian with μ = 0 km and σ = 2.9 km). Mass eruption rate was calculated from a best-fit relationship with plume height and duration using the mass eruption rate and eruptive volume (see Mastin et al., 2020). 240 For all of these 1,000 cases, the same grain-size distribution (GSD) was used (15% 0.250 mm, 20% 0.125 mm, 40% 0.063 mm, and 25% 0.031 mm). Using a GSD with this greater fine component and considering plume heights in a broader range (7-35 km), hypothetical eruptions at each of the three volcanoes resulted in ash deposited in Greenland. The output of all simulations can be found in Plunkett et al. (2021).

245
Generally, for both Aniakchak and Chikurachki, the greater the plume height of the hypothetical eruption, the more likely ash will fall out in Greenland (Fig. 8). With Popocatépetl, the trend is not as pronounced. In all cases, no 31 μm ash was deposited in Greenland for plume heights less than 9 km for Aniakchak or 11 km for Chikurachki and Popocatépetl. In less than 1% of cases for both Aniakchak and Popocatépetl were 63 μm particles deposited and exceptionally large plume heights were required to achieve this (26 and 18 km, respectively). In none of the 1,000 simulations of Chikurachki did 63 μm ash reach Greenland. 250 For these hypothetical cases, 44% of the simulations of Aniakchak resulted in fine ash deposited in Greenland, as opposed to 25% and 13% from Chikurachki and Popocatépetl, respectively. There is a weak sensitivity to season, with a slight increase in the odds of ash reaching Greenland for each of these three volcanoes during wintertime (51%, 26%, and 20% for Aniakchak, Chikurachki and Popocatépetl; Fig. 8d). Again, these likelihoods are only with consideration of the source location with respect 255 to the meteorology. When considering the geologic history, Chikurachki becomes much less likely due to its historically lower plume heights and coarser grains size.

Deconstructing the ice-core "Vesuvius" marker horizon
We identify in the NEEM-2011-S1 ice core a significant concentration of large cryptotephra shards in association with the 260 sulfate layer previously attributed to the Vesuvius 79 CE eruption. Our geochemical characterization of the tephra demonstrates conclusively that it is not derived from Vesuvius. Coupled with the revised NS1-2011 ice core chronology that now places this event in winter 87/88 CE (Sigl et a., 2015), our findings call into question the robustness of the geochemical data from glass shards in the GRIP ice core that were reported by Barbante et al. (2013). In our simulations, ash was deposited almost entirely within days to weeks following the event, although the model is not sufficiently tuned to consider extremely fine particles. We 265 consider it unlikely that Vesuvius ash was being deposited in Greenland up to nine years after the eruption, but the GRIP tephra raises uncertainty about the validity of diminutive (<5 μm) ash particles in polar ice cores. Under the revised NS1-2011 and DRI_NGRIP2 chronologies (Sigl et al., 2015;McConnell et al., 2018), a small volcanic sulfate peak in NGRIP2 and GISP2 at 80 CE is observed which may derive from the VEI 5 eruption of Vesuvius in 79 CE, or equally an unknown eruption of similar age (Fig. 1b). Drilling of the NEEM-2011-S1 core had been stopped at an age of 86 CE so we could not sample the 270 corresponding depth range for tephra. We suggest that additional screening for potential tephra should be extended by the icecore community to the corresponding core sections of archived ice cores (e.g., NGRIP, NEEM, Dye-3, and others).
Although we have been unable to determine the source of our 88 CE tephra, we rule out on the basis of glass geochemistry the likelihood that it was from Iceland, Greenland's closest volcanic source, or from any known event of VEI 4 or higher presently 275 dated to within ~200 years of the event, emphasizing that eruption inventories are incomplete and often imprecisely dated (Brown et al., 2014, Rougier et al., 2016. Our ash models suggest that tephra from smaller magnitude eruptions in Mexico or the north Pacific region is unlikely to reach Greenland, and that plumes from the northwestern Pacific must attain elevations of 9 km or more to have potential to disperse ash as far as the north Atlantic. Stevenson et al. (2015) similarly found that ash from 10 km-high plumes in Iceland could rapidly travel 850 km, but our findings extend the range of moderately high ash 280 clouds to considerably greater distances. For ash particles as large as those in QUB-1832/3 and QUB-1859 to disperse such distances, however much higher (>18 km) plumes are likely needed. The glass geochemistry of the 88 CE tephra most closely resembles products from the eastern Aleutian range, although no candidate eruption can be identified despite the likelihood that it was a large magnitude event. This serves to emphasize the incompleteness of the volcanic record, a salutary reminder that large, unknown eruptions may be candidates for prominent volcanic events in Greenland ice cores even when there are 285 close temporal associations with documented eruptions.

Geochronological implications
The misattribution of the 88 CE signal can be traced back to early ice core research in which Vesuvius was posited as a possible source because the ice layer counts were within 10 years of the historical eruption (Zielinski, 1995). Subsequent adherence to 290 this unconfirmed tie-point within the Greenland ice-core chronology impeded the recognition of coeval historical and proxy signals of past environmental change despite calls for a reconsideration of the ice-core dating on the basis of similar intervals between events in the ice core and tree-ring records (Baillie, 1996(Baillie, , 2008Baillie and McAneney, 2015). Nonetheless, the marker event played a pivotal role in the constraining the Common Era section of the GICC05 chronology (Vinther et al., 2006), which is widely considered as the reference chronology for Northern Hemisphere paleoclimate (Rasmussen et al., 2014). 295 All major deep Greenland ice cores are aligned on GICC05 (Rasmussen et al., 2013;Seierstad et al., 2014;Vinther et al., 2006Vinther et al., , 2008, including the most recently drilled EGRIP ice-core (Mojtabavi et al., 2020), similar to the way in which Antarctica ice cores are increasingly aligned to the WD2014 chronology (Baggenstos et al., 2018;Buizert et al., 2018;Cole-Dai et al., 2021;Winski et al., 2019).

300
The development of the independent NS1-2011 chronology, unconstrained by this assumed tie-point (and another previously assumed tie-point at 1105 CE ascribed to the Hekla 1104 CE eruption), has since enabled a more precise dating, confirmed by tephra linkages, of past volcanic eruptions (e.g., Ilopango 431 ± 2 CE - Smith et al., 2020;Katla 822 CE -Plunkett et al., 2020;Churchill 853 CE -Jensen et al., 2014;Eldgjá 939-40 CE -Oppenheimer et al., 2018;Changbaishan 946 CE -Sun et al., 2014;Veiðivötn 1477CE -Abbott et al., 2021. The chronology has also enabled a more accurate appraisal of volcanic 305 effects on climate and society, including on Northern Hemisphere summer temperatures Helama et al.;Zhu et al., 2020), Nile summer flood variability and socioeconomic effects in ancient Egypt (Manning et al., 2017), and Chinese dynastic collapses (Gao et al., submitted). It is largely consistent with an independently annual-layer counted chronology from the NGRIP2 ice core  used to constrain the date and climate effects of the caldera-forming Okmok II eruption in winter 44/43 BCE . 310 Over the last decade, numerous continental-to global-scale temperature reconstructions have been made based on multi-proxy records that include ice cores (McKay and Kaufman, 2014;PAGES 2k Consortium, 2013, 2019Neukom et al., 2019;Kaufman et al., 2020;Konecky et al., 2020) and data-assimilation employing these databases (Steiger et al., 2018;Tardif et al., 2019). These reconstructions are increasingly used as a benchmark in assessments of anthropogenic global warming (Steig 315 et al., 2013;Abram et al., 2016;Neukom et al., 2019b) or to explore the role of natural forcing on past climate (Tejedor et al., 2021). In these reconstructions, the relative contribution of ice-core records to the overall proxies is generally increasing with increasing age. In the most recent PAGES2k reconstruction (PAGES 2k Consortium, 2017), 48 ice cores (8%) are included between 1800-1850 CE, and 22 (24%) between 500-550 CE. Of the latter 22 records, 10 have been aligned to the GICC05 chronology using volcanic synchronization and only two have been aligned to the new WD2014 chronology .
Because of the reduced fraction of proxies with annual dating precision (i.e., tree-rings) in the first millennium CE and in combination with coarse resolution, dating uncertainties and biases in other proxies, multi-proxy reconstructions consequently depict smaller amplitudes of natural climate variability in the first millennium CE than in the second millennium CE under comparable external climate forcings . Accurate alignment of the available annual resolution datatreerings and polar ice coresis critical to redress this issue, and hinges on the integration of data based on a robust ice-core 325 chronology. Given our firm refutation of the Vesuvius 79 CE tie-point in the GICC05 chronology, we urge that corresponding databases (e.g., PAGES 2k, PAGES Iso2k, Last Millennium Reanalysis, Paleo Hydrodynamics Data Assimilation) be updated to capture accurately the timing and full amplitude of paleoclimatic variability.

Sulfate deposition for 88 CE and implications for climate forcing
Unsupported linkages between volcanic signals and their source have consequences also for the understanding of volcanic 330 impacts. For pre-satellite era eruptions, determination of atmospheric aerosol loadingthe leading mechanism by which volcanoes impact climatehinges upon models of aerosol dispersal and deposition based on suspected source and ice-core sulfur concentrations (Gao et al., 2007). Injection height may also determine the duration of aerosol persistence in the stratosphere, and consequently, its climate effectiveness (Toohey et al., 2019;Marshall et al., 2019;Aubry et al., 2020). Some prior knowledge of the source and eruption parameters is therefore needed to refine the modelling and interpretation of climate 335 response.
Here, our ash dispersal models enable us to constrain the minimum injection height needed for eruptions in three volcanic regions to extend tephra to Greenland. Our results demonstrate that stratospheric injection was very likely required to disperse large ash particles to northern Greenland from either Aniakchak in the high-latitudes or Popocatépetl in the tropics. These two 340 scenarios have implications for the determination of stratospheric sulphate loading, with a considerably larger sulfur emission expected from the tropical source to account for the aerosol deposition observed in Greenland. Consequently, were this event to be attributed to Popocatépetl rather than high-(e.g., Aniakchak) or mid-latitude (e.g., Vesuvius) sources, we would anticipate a larger global mean forcing, similar to the Huaynaputina eruption of 1600 CE. Instead, an extratropical location implies an event of smaller stratospheric sulfur injection and therefore lesser global climate significance, roughly equivalent to some 345 estimates of the sulfur injection from the 1912 Katmai eruption (Stothers, 1996). Annual resolution climate records for this period are rare, but an analysis of limited tree ring records from Northern Eurasia suggests a small cooling signal associated with the eruption (Sigl et al., 2015).
Although inconclusive, the tephra evidence points to the eastern Aleutians as the most likely origin of the 88 CE eruption. 350 Whatever the source, the same volcano contributed to the volcanic signal in Greenland ice cores in 536 CE, following which there was a major climate perturbation with societal consequences (Sigl et al., 2015;Büntgen et al., 2016). Other tephras in the NEEM-2011-S1 ice core at this time illustrate that the 536 CE sulfur signal is potentially a composite of three simultaneous https://doi.org/10.5194/cp-2021-63 Preprint. Discussion started: 16 June 2021 c Author(s) 2021. CC BY 4.0 License. eruptions of uncertain origin (Sigl et al., 2015), confounding the estimation of atmospheric aerosol loading. Like QUB-1832/3, the large (>50 μm) shard sizes in the 536 CE ice implicate stratospheric injection. The Okmok II eruption of 43 BCE has also 355 been tied to climate impacts , clearly demonstrating the significant forcing potential of high-latitude eruptions.
The 43 BCE, 88 CE and 536 CE events seem to share a similar volcanic source region and magnitude (VEI 5 or greater), but their aerosol representation in Greenland ice cores is spatially divergent (Table 1). Ice-core records of cumulative volcanic 360 sulfate deposition in Greenland for the 88 CE event are available from NEEM-2011-S1 (Sigl et al., 2015), NGRIP2 (McConnell et al., 2018), GISP2 (Zielinski, 1995), GRIP and Dye-3 (Clausen et al., 1997). These records demonstrate a strong spatial gradient in volcanic sulfate deposition following this eruption: high sulfate mass deposition is observed in northern Greenland (NEEM: 83 kg km -2 y), but only moderate deposition is recorded in central and southern Greenland (NGRIP2: 32 kg km -2 y; GRIP: 26 kg km -2 y; GISP2 29 kg km -2 y; Dye 3: 31 kg km -2 y). The mean deposition (40 kg km -2 y) over the five ice-core sites 365 is much lower than the deposition following the 536 CE (75 kg km -2 y) or Okmok II (117 km -2 y) events. Moreover, the duration of volcanic sulfate deposition is markedly shorter (0.9 years) for the 88 CE eruption compared to that of the 536 CE (1.7 years) and Okmok II (2.3 years) events, suggesting more rapid atmospheric aerosol removal. The relative importance of other variablesduration of eruption, contributing effects of other eruptions, prior state of the climate systemcannot presently be determined. 370

Conclusions
The NS1-2011 revised ice-core chronology that differentiates the conspicuous, 1st century CE volcanic signal in Greenland ice from the historical age of Vesuvius is now supported by tephra evidence that demonstrates the sulfate was from an eruption other than Vesuvius. Although we have been unable to pinpoint the precise source eruption, the geochemical signature of its glass shards most strongly points to the Aleutian arc as the source region. Shard characteristics, combined with ash dispersal 375 models, implicate a large magnitude eruption with stratospheric injection. Constraining the location and minimum eruption height of the event promises to improve the estimation of stratospheric aerosol loading and radiative forcing when combined with understanding of the dependence of aerosol lifetime on these eruption parameters (Marshall et al., 2019;Toohey et al, 2019). We note, however, that unlike notable events such as the 43 BCE Okmok II and 536 CE unidentified eruptions, the 88 CE eruption is not presently associated with any known major climatic perturbations. 380 Increasingly, the recognition of solar proton events in tree-ring and ice core archives is demonstrating offsets in the ice core chronology, and it is imperative that ice-core chronologies are now updated in recognition of the known dating bias in GICC05.
Multi-proxy compilations (PAGES2k, LMR, PHYDA and others) in which ice-core derived  18 O and D records from