https://doi.org/10.1130/G46004.1 Manuscript received 13 January 2019 Revised manuscript received 27 March 2019 Manuscript accepted 31 March 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 17 April 2019 Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland) Jonathan D. Moles1, Dave McGarvie2, John A. Stevenson3, Sarah C. Sherlock1, Peter M. Abbott4,5,6, Frances E. Jenner1, and Alison M. Halton1 1Faculty of Science, Technology, Engineering and Mathematics, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK 2Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 3British Geological Survey, The Lyell Centre, Research Avenue South, Edinburgh EH14 4AP, UK 4Department of Geography, College of Science, Swansea University, Singleton Park, Swansea SA2 8PP, UK 5Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, 3012 Bern, Switzerland 6School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK ABSTRACT Alternatively, a direct link to the regional paleo- The tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known, but climate archive could be established through the can be investigated through the correlation of eruptive products across multiple depositional identification of tephra from the same eruptions settings. Using geochemistry and geochronology, we correlate a regionally important Pleisto- within ice cores and marine sediments. cene tephra horizon—the rhyolitic component of North Atlantic Ash Zone II (II-RHY-1)—and The distal tephra in this study is II-RHY-1, the Thórsmörk Ignimbrite with rhyolitic tuyas at Torfajökull volcano, Iceland. The eruption the rhyolitic component of North Atlantic Ash breached an ice mass >400 m thick, leading to the widespread dispersal of II-RHY-1 across Zone II, which is dated to the last glacial period the North Atlantic and the Greenland ice sheet. Locally, pyroclastic density currents traveled at 55,380 ± 2367 yr b2k (before A.D. 2000; across the ice surface, depositing the variably welded Thórsmörk Ignimbrite beyond the ice 2σ) (Greenland Ice Core Chronology 2005 margin and ~30 km from source. The widely dispersed products of this eruption represent a [GICC05]; Svensson et al., 2008). II-RHY-1 is valuable isochronous tie line between terrestrial, marine, and ice-core paleoenvironmental an important part of the tephrostratigraphy of records. Using the tephra horizon, estimates of ice thickness and extent derived from the the North Atlantic region due to its widespread eruption deposits can be directly linked to the regional climate archive, which records the distribution and occurrence at a time of abrupt eruption at the onset of Greenland Stadial 15.2. climatic change: the onset of Greenland Sta- dial (GS) 15.2 (Bramlette and Bradley, 1941; INTRODUCTION cinsky and Fink, 2000), and the Hallett Volcanic Zielinski et al., 1997; Austin et al., 2004; Aus- The stratigraphic correlation of volcanic Province, Antarctica (Smellie et al., 2011). Cur- tin and Abbott, 2010). Atmospheric transport products, particularly tephra, is a powerful rent knowledge of the behavior of rhyolitic gla- of the tephra resulted in distal fallout onto the means of studying the past eruptive behavior ciovolcanic eruptions is drawn from proximal Greenland ice sheet and sea ice (Ruddiman and of volcanoes and linking together disparate deposits only (e.g., Stevenson et al., 2011; Owen Glover, 1972; Ram and Gayley, 1991), leading paleo envi ron mental records (Lowe, 2011). The et al., 2013a). Without any established correla- to sea-ice rafting of the tephra as far as 2300 km more depositional settings in which an eruption tions between glaciovolcanic rhyolites and dis- to the south and southwest of Iceland (Ruddiman is identified, the more information can be pooled tal tephras, it is not known whether these erup- and Glover, 1972; Wastegård et al., 2006). The together to understand the eruption and the pre- tions have produced widespread tephra deposits volume of airfall tephra, ice-rafted tephra, and vailing environmental conditions. However, it (Tuffen et al., 2002, 2007; McGarvie, 2009). redeposited tephra in the marine stratigraphy is can be challenging to find correlative volcanic Glaciovolcanic edifices, such as tuyas, are substantial, but poorly constrained (Ruddiman products across multiple realms, especially ter- valuable paleoenvironmental indicators that re- and Glover, 1972; Lackschewitz and Wallrabe- restrial settings that are subjected to periodic cord the presence of ice at the time of their erup- Adams, 1997; Brendryen et al., 2011; Voelker glaciation (Larsen and Eiríksson, 2008). In this tion, and can preserve evidence of the coeval ice and Haflidason, 2015). paper, we use correlation methods to (1) assess thickness and basal thermal regime (Jones, 1968; The II-RHY-1 tephra has been identified in a the tephra dispersal mechanisms of rhyolitic Smellie and Skilling, 1994; Smellie et al., 2011). terrestrial setting as the Thórsmörk Ignimbrite, a glacio volcanic eruptions, and (2) precisely Integration of this information with climate rec- variably welded ignimbrite in southern Iceland inte grate glaciovolcanism-derived paleoenvi- ords has been restricted by the large uncertainties (Sigurdsson, 1982; Lacasse et al., 1996; Tom- ronmental data with the regional climate record. in eruption ages (e.g., 40Ar/39Ar ages, with typical linson et al., 2010; Guillou et al., 2019). It has Rhyolite glaciovolcanism is an abundant uncertainties of thousands of years) relative to been suggested that Tindfjallajökull volcano was feature of the active volcanic zones of Iceland the time scales of climate variability (e.g., the the source of the ignimbrite (Jørgensen, 1980); (McGarvie, 2009) and is also reported in the decadal to centennial scale climate shifts during however, recent observations on the physical Cascades volcanic arc, northwestern USA (Les- the last glacial period; Svensson et al., 2008). volcanology of this deposit by Moles et al. CITATION: Moles, J.D., et al., 2019, Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland): Geology, v. 47, p. 577–580, https:// doi .org /10 .1130 /G46004.1 Geological Society of America | GEOLOGY | Volume 47 | Number 6 | www.gsapubs.org 577 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/6/577/4707802/577.pdf by guest on 29 September 2021 (2018) suggest that this is not the case. Further- Torfajökull volcano Tindallajökull Thórsmörk North Atlanc Ring Fracture Rhyolites Other rhyolites volcano Ignimbrite Ash Zone II more, Grönvold et al. (1995) noted a geochemi- } Kirkjufell Unnamed ridge Rhyolite Ash Pumice Tomlinson II-RHY-1 (locaon map in cal similarity between II-RHY-1 and rhyolites Illihnúkur Bláhnúkur Fiamme Fiamme } et al. (2010) Data Repository, Fig. DR1) Laufafell Hábarmur Dated samples: at Torfajökull volcano, particularly the “Ring Rauðfossaöll North Hábarmur .4 Gvendarhyrna circled markers ( ) B l Fracture Rhyolites”. These suggested sources, atla 21 K ul as well as nearby volcanoes Eyjafjallajökull and 19°W A 1. 3 64°N Katla, are considered here. .0 Eyjaallajök1821–182 ) 81 0. METHODS Torfajökull Torfajökull Ring .6 Potential correlations between samples Fracture Rhyolites CaO (wt% Thórsmörk Ignim. 40 from distal, medial, and proximal settings Other silicic rocks 0. were investigated using both geochemistry .2 and geochronology. II-RHY-1 tephra shards II-RHY-1 and Tindallajökull 00 Thórsmörk Ignimbrite were extracted from four North Atlantic ma- 0. rine sediment cores (Table DR1 and Fig. DR1 2.02.5 3.03.5 4.04.5 5.0 in the GSA Data Repository1). The occurrence FeO (wt%) } and stratigraphic position of II-RHY-1 in the } C 160 cores were determined by Abbott et al. (2018). N Ash and glassy fiamme samples were collected 0 km 5 10 Katla Eyjaallajökull 140 from the Thórsmörk Ignimbrite (Fig. 1A; Table ) DR2). Proximal rhyolite lavas were sampled D ⁴⁰Ar/ ³⁹Ar ages (± 2σ) } 120 i 100 = inverse isochron age at Tindfjallajökull (four samples; Table DR3) } p = plateau age Y (ppm 0 } } and Torfajökull (16 samples; Table DR4). The } p 100 08 selected Torfajökull lavas include those known a) II-RHY-1* p i p p 80 to have erupted during the last glacial period 06 } i } Age (k i } i p } i } (i.e., Ring Fracture Rhyolites, Bláhnúkur, and 04 *GICC05 age ± 2σ “unnamed ridge”; McGarvie, 1984; McGarvie ⁴⁰Ar/ ³⁹Ar ages not determined : 60 (Svennson et al., 2008) 600 800 1000 1200 1400 1600 02 et al., 2006; Clay et al., 2015; Table DR5). Zr (ppm) These deposits contain a significant proportion of fragmental material (e.g., hyaloclastite, ash), Figure 1. A: Location map of Thórsmörk Ignimbrite and nearby volcanoes in southern Iceland. Geological mapping from Jørgensen (1980), Jóhannesson and Sæmundsson (1989), Sæmunds- though samples were sourced from fresh lavas son and Friðleifsson (2001), and Moles et al. (2018). B: Selected major elements plot of tephra to minimize alteration effects. II-RHY-1, Thórsmörk Ignimbrite, and rhyolites from potential source volcanoes. Katla compo- The geochemistry of the samples was de- sition from Lacasse et al. (2007); Eyjafjallajökull (A.D. 1821–1823 eruption) from Larsen et al. termined using electron probe microanalysis (1999). Lavas are plotted as mean and standard