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The Importance of Sparse Tephras in Greenland Ice Cores Gill Plunkett,1 Michael Sigl,2 Jonathan R

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The Importance of Sparse Tephras in Greenland Ice Cores Gill Plunkett,1 Michael Sigl,2 Jonathan R RESEARCH ARTICLE Smoking guns and volcanic ash: the importance of sparse tephras in Greenland ice cores Gill Plunkett,1 Michael Sigl,2 Jonathan R. Pilcher,1 Joseph R. McConnell,3 Nathan Chellman,3 J.P. Steffensen4 & Ulf Büntgen5,6,7,8 1Archaeology and Palaeoecology, School of Natural and Built Environment, Queen’s University Belfast, Northern Ireland, UK; 2Climate and Environmental Physics, Physics Institute & Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland; 3Division of Hydrological Sciences, Desert Research Institute, Reno, NV, USA; 4Centre for Ice and Climate, University of Copenhagen, Copenhagen, Denmark; 5Department of Geography, University of Cambridge, Cambridge, UK; 6Swiss Federal Research Institute WSL, Birmensdorf, Switzerland; 7Global Change Research Centre (CzechGlobe), Brno, Czech Republic; 8Department of Geography, Faculty of Science, Masaryk University, Brno, Czech Republic Abstract Keywords Primary ashfall; resuspended volcanic ash; Volcanic ash (fine-grained tephra) within Greenland ice cores can comple- volcanic eruptions; Katla; dust storms; ment the understanding of past volcanism and its environmental and societal tephrochronology impacts. The presence of ash in sparse concentrations in the ice raises ques- tions about whether such material represents primary ashfall in Greenland or Correspondence resuspended (remobilized) material from continental areas. In this article, we Gill Plunkett, Archaeology and Palaeoecology, School of Natural and Built investigate this issue by examining tephra content in quasi-annual samples Environment, Queen’s University Belfast, from two Greenland ice cores during a period of ca. 20 years and considering Belfast BT7 1NN, Northern Ireland, UK. their relationships with sulphur and particulate data from the same cores. We E-mail: [email protected]. focus on the interval 815–835 CE as it encompasses a phase (818–822 CE) of heightened volcanogenic sulphur previously ascribed to an eruption of Katla, Abbreviations Iceland. We find that tephra is a frequent but not continuous feature within CE: Christian Era the ice, unlike similarly sized particulate matter. A solitary ash shard whose GRIP: Greenland Ice Core Project Ky: thousands of years major element geochemistry is consistent with Katla corroborates the attribu- NEEM: North Greenland Eemian Ice Drilling tion of the 822±1 CE sulphur peak to this source, clearly showing that a single research project shard can signify primary ashfall. Other tephras are present in similarly low abundances, but their geochemistries are less certainly attributable to specific sources. Although these tephra shards tend to coincide with elevated sulphur and fine (<10 μm) particulates, they are not associated with increased coarse (>10 μm) particle concentrations that might be expected if the shards had been transported by dust storms. We conclude that the sparse shards derive from primary ashfall, and we argue that low tephra concentrations should not be dismissed as insignificant. To access the supplementary material, please visit the article landing page Introduction sulphur-containing gases that convert to sulphate aero- sols, to deflect incoming solar radiation over the course In recent years, evidence for past volcanic impacts of weeks to years, triggering surface cooling (Robock on climate variation and societal transformation has 2000), although ash particles may offset the cooling in increasingly been posited (Esper et al. 2013; Sigl et al. some instances (Flanner et al. 2014). Volcanic eruptions 2015; Stoffel et al. 2015; Büntgen et al. 2016). The cli- vary greatly in their magnitude, duration and type of mate effectiveness of eruptions stems from the capacity emission, and their latitudinal location is also a signifi- of material injected into the stratosphere, particularly cant factor in the potential extent of their atmospheric Polar Research 2020. © 2020 G. Plunkett et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 1 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Polar Research 2020, 39, 3511, http://dx.doi.org/10.33265/polar.v39.3511 (page number not for citation purpose) Sparse tephras in Greenland ice G. Plunkett et al. impact (Toohey et al. 2019). As volcanic eruptions occur locations and the specific environmental conditions globally on a continual basis, including several that pen- required to resuspend ash from older deposits are amongst etrate stratospheric levels each decade, it remains to be the leading factors that mitigate the incorporation of established which types of events pose the greatest risk to reworked tephra into the ice. Nevertheless, it is conceiv- the climate system. able that strong dust storms could transport resuspended The understanding of past volcanic impacts hinges sediment, including tephra, over long distances to polar substantially on the polar ice core records that present areas. Aeolian resuspension of ash is especially likely in an extended history of volcanic activity through their areas where pedogenesis and stabilization by vegetation chemistry, most notably of eruptions that emitted suf- cover are retarded or absent, namely in very cold or very ficient aerosols to have potentially impacted the Earth’s arid regions. In the Valley of the Ten Thousand Smokes, radiative balance (Hammer et al. 1980; Zielinski et al. Alaska, for instance, tephra beds from the 1912 Novar- 1994; Sigl et al. 2013). For the Late Holocene, ice cores upta eruption remain exposed as climate and topography offer a level of chronological precision that enables their combine to hinder the establishment of pioneering veg- records to be reconciled with historical events, allowing etation (Hildreth & Fierstein 2012). Under certain wind climate perturbations and any ensuing societal impacts conditions, ash from these beds can be resuspended and to be evaluated (Sigl et al. 2015; Büntgen et al. 2016). elevated as much as 3 km into the atmosphere, equiv- Identifying the source eruption is key to deciphering alent to a plume from a Vulcanian-style eruption and the volcanic processes that impact the climate system. sufficient to enable ash to be dispersed over several thou- For example, atmospheric loading of volcanic aerosols is sand kilometres (Hadley et al. 2004). How then do we best reconstructed if the latitude, other eruption param- differentiate primary ashfall from dust deposition events eters (e.g., plume height, eruption style) and season incorporating remobilized tephra? Although reworked are known. However, in many regions, observational glass can sometimes be evidenced by rounded edges, cor- records of eruptions began only in recent centuries, and rosion or altered major element geochemistries (Lowe volcanic histories remain very poorly resolved in many 2011), these indices may depend upon factors such as regions (Siebert et al. 2011). Furthermore, secure attribu- the length of time a tephra has been exposed to weath- tion of volcanic signals to specific eruptions can only be ering prior to remobilization, or transport mechanisms. achieved through the analysis of associated volcanic ash Low analytical totals in geochemical analyses can also (fine-grained tephra) particles whose glass geochemical result from post-deposition hydration of the glass under composition may point to the source eruption, volcano exposed conditions, although other factors contributing or volcanic region. to low totals include thin glass walls and instrumental Transport of volcanic ash to polar regions is con- error (Froggatt 1992; Hunt & Hill 1993). strained by several factors, however, not least injection In terms of the potential source area of reworked vol- height, meteorological conditions at the time of the erup- canic material reaching Greenland, we might reasonably tion over both the source volcano and the deposition site, assume a correlation with the sources of fine particulate and general atmospheric circulation patterns. Given these matter in the ice that regularly reaches polar locations. limitations, it is perhaps not surprising that concentra- Strontium (Sr) and neodymium (Nd) isotope analyses of tions of ash particles large enough (usually >20 μm) to be <5 μm particles in high-altitude Greenland ice cores indi- geochemically typed by using standard tephra-analytical cate a predominantly east Asian origin (Bory et al. 2002; techniques in polar ice are frequently low (Iverson et al. Bory et al. 2003), although others propose that a north 2017). For Late Holocene Greenland ice, Coulter et al. African source may be underestimated in these stud- (2012) examined multi-annual samples with as few as ies (Tanaka & Chiba 2006; Lupker et al. 2010). Winter one tephra shard identifiable by using light microscopy, atmospheric circulation over both the Greenland NEEM while other studies have been limited to analysing par- and Tunu ice-core drilling sites is dominated by the polar ticles <10 μm by using sub-optimal techniques for geo- vortex. In spring to summer, air masses over both sites chemical characterization (Palais et al. 1992; Zielinski et originate mainly from the west, but both sites are also al. 1997; Barbante et al. 2013). Such a very fine-grained influenced by air masses from the North Atlantic, arriving material might not be recognizable as volcanic glass by at
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