Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)

The stratigraphic correlation of volcanic products, particularly tephra, is a powerful means of studying the past eruptive behavior of volcanoes and linking together disparate paleoenvironmental records (Lowe, 2011). The more depositional settings in which an eruption is identified, the more information can be pooled together to understand the eruption and the prevailing environmental conditions. However, it can be challenging to find correlative volcanic products across multiple realms, especially terrestrial settings that are subjected to periodic glaciation (Larsen and Eiríksson, 2008). In this paper, we use correlation methods to (1) assess the tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions, and (2) precisely integrate glaciovolcanism-derived paleoenvironmental data with the regional climate record.

Rhyolite glaciovolcanism is an abundant feature of the active volcanic zones of Iceland (McGarvie, 2009) and is also reported in the Cascades volcanic arc, northwestern USA (Lescinsky and Fink, 2000), and the Hallett Volcanic Province, Antarctica (Smellie et al., 2011). Current knowledge of the behavior of rhyolitic glaciovolcanic eruptions is drawn from proximal deposits only (e.g., Stevenson et al., 2011; Owen et al., 2013a). Without any established correlations between glaciovolcanic rhyolites and distal tephras, it is not known whether these eruptions have produced widespread tephra deposits (Tuffen et al., 2002, 2007; McGarvie, 2009).

Glaciovolcanic edifices, such as tuyas, are valuable paleoenvironmental indicators that record the presence of ice at the time of their eruption, and can preserve evidence of the coeval ice thickness and basal thermal regime (Jones, 1968; Smellie and Skilling, 1994; Smellie et al., 2011). Integration of this information with climate records has been restricted by the large uncertainties in eruption ages (e.g., ⁴⁰Ar/³⁹Ar ages, with typical uncertainties of thousands of years) relative to the time scales of climate variability (e.g., the decadal to centennial scale climate shifts during the last glacial period; Svensson et al., 2008). Alternatively, a direct link to the regional paleoclimate archive could be established through the identification of tephra from the same eruptions within ice cores and marine sediments.

The distal tephra in this study is II-RHY-1, the rhyolitic component of North Atlantic Ash Zone II, which is dated to the last glacial period at 55,380 ± 2367 yr b2k (before A.D. 2000; 2σ) (Greenland Ice Core Chronology 2005 [GICC05]; Svensson et al., 2008). II-RHY-1 is an important part of the tephrostratigraphy of the North Atlantic region due to its widespread distribution and occurrence at a time of abrupt climatic change: the onset of Greenland Stadial (GS) 15.2 (Bramlette and Bradley, 1941; Zielinski et al., 1997; Austin et al., 2004; Austin and Abbott, 2010). Atmospheric transport of the tephra resulted in distal fallout onto the Greenland ice sheet and sea ice (Ruddiman and Glover, 1972; Ram and Gayley, 1991), leading to sea-ice rafting of the tephra as far as 2300 km to the south and southwest of Iceland (Ruddiman and Glover, 1972; Wastegård et al., 2006). The volume of airfall tephra, ice-rafted tephra, and redeposited tephra in the marine stratigraphy is substantial, but poorly constrained (Ruddiman and Glover, 1972; Lackschewitz and Wallrabe-Adams, 1997; Brendryen et al., 2011; Voelker and Haflidason, 2015).

The II-RHY-1 tephra has been identified in a terrestrial setting as the Thórsmörk Ignimbrite, a variably welded ignimbrite in southern Iceland (Sigurdsson, 1982; Lacasse et al., 1996; Tomlinson et al., 2010; Guillou et al., 2019). It has been suggested that Tindfjallajökull volcano was the source of the ignimbrite (Jørgensen, 1980); however, recent observations on the physical volcanology of this deposit by Moles et al. (2018) suggest that this is not the case. Furthermore, Grönvold et al. (1995) noted a geochemical similarity between II-RHY-1 and rhyolites at Torfajökull volcano, particularly the “Ring Fracture Rhyolites”. These suggested sources, as well as nearby volcanoes Eyjafjallajökull and Katla, are considered here.

Potential correlations between samples from distal, medial, and proximal settings were investigated using both geochemistry and geochronology. II-RHY-1 tephra shards were extracted from four North Atlantic marine sediment cores (Table DR1 and Fig. DR1 in the GSA Data Repository¹). The occurrence and stratigraphic position of II-RHY-1 in the cores were determined by Abbott et al. (2018). Ash and glassy fiamme samples were collected from the Thórsmörk Ignimbrite (Fig. 1A; Table DR2). Proximal rhyolite lavas were sampled at Tindfjallajökull (four samples; Table DR3) and Torfajökull (16 samples; Table DR4). The selected Torfajökull lavas include those known to have erupted during the last glacial period (i.e., Ring Fracture Rhyolites, Bláhnúkur, and “unnamed ridge”; McGarvie, 1984; McGarvie et al., 2006; Clay et al., 2015; Table DR5). These deposits contain a significant proportion of fragmental material (e.g., hyaloclastite, ash), though samples were sourced from fresh lavas to minimize alteration effects.

The geochemistry of the samples was determined using electron probe microanalysis (EPMA; major elements) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS; trace elements). A glassy fiamma from the Thórsmörk Ignimbrite, glass shards from II-RHY-1, and five lava samples from the Torfajökull Ring Fracture Rhyolites were selected for groundmass ⁴⁰Ar/³⁹Ar dating. Full methods are supplied in the Data Repository.

The geochemical data confirm that II-RHY-1 and the Thórsmörk Ignimbrite have highly similar compositions, overlapping on all bivariate plots (Figs. 1B–1C; Figs. DR3–DR5), though both deposits have variable trace element compositions (e.g., the trend to more evolved compositions seen in Fig. 1C). A glassy fiamma from the Thórsmörk Ignimbrite yielded an ⁴⁰Ar/³⁹Ar plateau age of 51.3 ± 4.2 ka (2σ), supporting the observation of Guillou et al. (2019) that the age of the ignimbrite (55.6 ± 4.8 ka [2σ] in their study) is concurrent with the ice core chronology (GICC05) age of II-RHY-1 (Fig. 1D. Fig. DR6). Thus, our new geochemical and geochronological data strengthen the previously recognized correlation between II-RHY-1 and the Thórsmörk Ignimbrite.

Tephras from II-RHY-1 and the Thórsmörk Ignimbrite have compositions that overlap with the Ring Fracture Rhyolites of Torfajökull volcano on all geochemical plots (Figs. 1B–1C; Figs. DR3–DR5), indicating a strong geochemical similarity between these groups. In contrast, known compositions from Tindfjallajökull, Katla, and Eyjafjallajökull volcanoes, and from other Torfajökull rhyolites, are dissimilar to those of these tephras (Fig. 1B; Fig. DR3).

Groundmass ⁴⁰Ar/³⁹Ar inverse isochron ages of the Ring Fracture Rhyolites overlap with the ages of II-RHY-1 and the Thórsmörk Ignimbrite (Fig. 1D; Table DR8; Fig. DR6). Inverse isochrons are the preferred method of age calculation for these samples due to their non-atmospheric initial ⁴⁰Ar/³⁶Ar contents (Table DR8). Dating of groundmass arguably achieves a more representative eruption age than dating of feldspar crystals, which yield older apparent ages for the Ring Fracture Rhyolites (Guillou et al. [2019] feldspar ⁴⁰Ar/³⁶Ar age: 77 ± 6 ka [2σ]; see discussion in the Data Repository, section 7). None of the other Torfajökull rhyolites dated in previous studies (McGarvie et al., 2006; Clay et al., 2015) have similar ages to the tephras. Thus, our new geochemical and geochronological evidence strongly suggests that II-RHY-1, the Thórsmörk Ignimbrite, and the Torfajökull Ring Fracture Rhyolites are the products of the same eruptive event (full results data set in Tables DR9–DR16).

Our new work resolves the long-standing ambiguity regarding the origin of II-RHY-1 and the Thórsmörk Ignimbrite by recognizing Torfajökull, not Tindfjallajökull, as the source volcano. This is supported by the observation that ignimbrite deposits on the flanks of Tindfjallajökull lack proximal facies; in fact, lithic clasts decrease in size and abundance toward the volcano (Moles et al., 2018). Additionally, there was no significant change in local sediment deposition regimes at Tindfjallajökull, as would be expected following a major eruption (Moles et al., 2018).

The identified proximal products of the eruption—the Ring Fracture Rhyolites of Torfajökull volcano—are considered to be the largest rhyolitic eruption deposit in Iceland, with a preserved volume of ∼18 km³ (dense-rock equivalent; McGarvie, 2009). A ring of tuyas was emplaced during the eruption, confined by an ice mass >400 m thick (Tuffen et al., 2002; McGarvie et al., 2006). Explosive activity formed steep-sided tephra piles before the effusive emplacement of intrusions and lava caps (Tuffen et al., 2002, 2008; Owen et al., 2013b).

Our new correlation provides the first documented link between rhyolite tuyas and distal tephra deposits, indicating that a subglacial rhyolitic eruption breached the ice to produce a subaerial eruption plume (Fig. 2). This confirms that widespread and voluminous tephra dispersal can be an important feature, and hazard, of rhyolite glaciovolcanism (as hypothesized by Tuffen et al. [2002] and Stevenson et al. [2011]). Although magma fragmentation during the eruption was initially enhanced by meltwater (Tuffen et al., 2008, 2002), investigations of these and other rhyolite tuyas show that the influence of meltwater rapidly declines with time and explosivity is principally driven by magmatic volatiles (Owen et al., 2013a; Stevenson et al., 2011). The development of volatile-driven subaerial eruption plumes after the ice is breached suggests that rhyolite glaciovolcanism can disperse tephra in the same style as rhyolite volcanism in ice-free settings.

The correlation of rhyolite tuyas with an ignimbrite demonstrates another previously undocumented phenomenon and hazard: pyroclastic density currents (PDCs) can occur during rhyolite tuya-forming eruptions and can travel tens of kilometers across an ice mass (Fig. 2). Welded ignimbrite deposits are present ∼30 km from Torfajökull (Jørgensen, 1980), indicating that the PDCs were still hot after traveling over ice or tephra-covered ice. A significant volume of tephra was likely deposited on the ice surface proximal to the eruption, and is now lost from the terrestrial record (Tuffen et al., 2002; Stevenson et al., 2011). The preserved ignimbrite (estimated volume: 1.5–2 km³ dense-rock equivalent; Thórarinsson, 1969) is interpreted to have been deposited in a largely or wholly ice-free environment (i.e., outside the ice margin), and its outcrop thus defines the minimum extent of this environment at the time of the eruption.

Tephra II-RHY-1 is a widespread isochronous horizon that marks the eruption of the Ring Fracture Rhyolites directly within an array of high-resolution proxy records, such as ice-core δ¹⁸O profiles, which document the climate and the associated environmental conditions (Ruddiman and Glover, 1972; Grönvold et al., 1995; Zielinski et al., 1997; Austin et al., 2004; Abbott et al., 2018). Reconstructions of coeval ice conditions in south Iceland (i.e., ice >400 m thick at Torfajökull; absent in Thórsmörk) can now be directly linked to this vast archive of paleoenvironmental information and can act as boundary conditions for models of the Icelandic ice sheet during the last glacial period. Based on the North Greenland Ice Core Project (NGRIP) ice core record, II-RHY-1 was deposited 40 ± 5 yr after the start of GS 15.2 (Svensson et al., 2008; Rasmussen et al., 2014). The use of tephra to integrate glaciovolcanism-derived data with the regional climate record provides a significant advance in precision compared to absolute dating alone (e.g., Clay et al., 2015), and can be applied to any glaciovolcanic eruption that produced an identifiable distal tephra horizon.

Our data identify the Ring Fracture Rhyolites of Torfajökull volcano, southern Iceland, as the source of the Thórsmörk Ignimbrite and the distal tephra II-RHY-1. This correlation demonstrates that explosive rhyolitic eruptions at subglacial volcanoes can result in widespread tephra dispersal. Additionally, our work shows that pyroclastic density currents can propagate across and beyond an ice mass for ∼30 km to emplace a variably welded ignimbrite. Rhyolitic glaciovolcanic eruptions preserve a record of ice cover at the vent and can also deposit an isochronous tephra horizon in a variety of depositional settings. Tephra from these eruptions can thus be used to precisely date glaciovolcanism-derived paleoenvironmental information relative to the regional climate archive.

This work was supported by the Natural Environment Research Council (grant NE/L002493/1). JAS was funded by a Royal Society of Edinburgh Personal Research Fellowship held at the University of Edinburgh. PMA was financially supported by the European Research Council (TRACE project) under the European Union’s Seventh Framework Programme (FP7/2007–2013)–ERC grant agreement 259253. We thank Mark Chapman, Frederique Eynaud, James Scourse, and Mara Weinelt for providing samples or access to marine cores. For analytical assistance, we thank Sam Hammond, James Malley, and Andy Tindle. Thanks to Siwan Davies for advice and support. We are grateful for the valuable comments provided by Emma Tomlinson, David Pyle, Ben Edwards, John Smellie, and an anonymous reviewer.