Tephra B contains large particles and has been geochemically correlated to the source volcano Mt. Berlin, which is 670 km from the drill site. Based on SEM observation, the morphology of many of the shards in Tephra B are distinctly magmatic, i.e. more vesiculated and less blocky than those observed in Tephra A and C. Furthermore, the composition is trachytic. Mt. Berlin is capable of producing eruption column heights 25–40 km high, which would allow for distribution of large particles over great distances22. The 92.5 ka22 eruption of Mt. Berlin deposited tephra in marine cores in the Amundsen-Bellingshausen Sea25 and East Antarctic ice cores26 over 3000 km from the eruptive source. The eruption that produced Tephra B would have been on the same order of magnitude.

The two thick coarse-grained tephra layers (A and C) are interpreted to be from local volcanoes, probably within a few hundred km from the drill site, based on the large, blocky particle size and their unique chemical composition27, 28. Neither of these coarse grained, thick tephra are found in the Byrd Core29, which is ~100 km away from the WDC06A core site, suggesting a very directional ash cloud, or one that did not travel far from source, allowing substantial deposition at WAIS and none at Byrd. The presence of these tephra layers in the Byrd Core cannot be absolutely ruled out, but, if present, the layer must have been too fine to be recognized by previous workers.

Several lines of evidence point to Tephra A and C being produced by phreatomagmatic eruptions from subglacial volcanoes nearer to the drill site. Tephra A and C have distinct grain size distributions with large mossy and fluidal particles and finer blocky fragments, observed in SEM and in μCT. The fluidal particles are characteristic of magmatic fragmentation30, whereas the mossy and blocky particles would be more typical of phreatomagmatic eruption style30, 31. In comparison, observations of many other tephra layers found in the WDC06A core are fine grained (<50 μm) and comprised mostly irregular shards 20–30 μm in diameter17. Phreatomagmatic eruptions can produce co-erupted magmatic and phreatomagmatic tephra32, but if that were the case for these tephra layers, there would be large irregular shards throughout the entire deposit. Instead, there are bands of large irregular shards mixed with smaller blocky fragment followed by band of primarily smaller blocky fragments (denoted by black arrows in Fig. 2). This repetitive sequence suggests mixed phreatomagmatic and magmatic phase followed by a more phreatomagmatic phase and finally a mixed phase. This sequence is likely caused by variations in the amount of water contacting the vent33, 34.

Although there are many variables that impact the explosivity of a given volcanic eruption, magmatic composition is a primary controlling factor, with the more evolved and/or H 2 O rich magmas producing higher explosivity eruptions35. Therefore, all other things being equal, the less evolved compositions of tephra layers A and C would suggest less explosive subaerial eruptions. The interaction of glacial meltwater with magma during these eruptions would have created episodic explosivity, but would not have contributed to a high, sustained eruption column36.

A phreatomagmatic genesis of Tephra layer C is further supported by the sulfur (S) concentration in the glass. Sulfur is a volatile component that fractionates into the H 2 O-bearing vapor phase generated during the depressurization that drives volcanic eruptions37. During the near-surface, vigorous degassing that takes place during magmatic eruptions, only small amounts of S stay in the melt, so the glass typically contains <20% of the original, pre-eruptive S content38. During phreatomagmatic eruptions, S is retained in the glass phase because fragmentation and quenching occur at a depth where S does not fully exsolve from the melt. The basanitic Tephra C, which is compositionally similar to Icelandic basalts, contains elevated and variable concentrations of S (1000–2000 ppm), which is consistent with observations made at Icelandic volcanoes that have undergone both magmatic and phreatomagmatic phases during a single eruption (Fig. 4)38. The variable S content of Tephra C, while the other major elements are invariant, suggests that fragmentation occurred at variable depths during the eruption, consistent with the interpretation of a subglacial to subaerial eruption. This tephra may have been generated from an eruption that breached the ice sheet surface, erupted explosively with some mixture of magmatic and phreatomagmatic fragmentation, switched to dominantly phreatomagmatic fragmentation, then had another pulse of mixed fragmentation once the water source was temporarily exhausted. Because Tephra A and B are more evolved and hence have lower S concentrations, S cannot be used to understand changes in fragmentation depth due to lower precision S measurements from the microprobe.

Figure 4 Bivariate plot of S versus TiO 2 /FeO to the assess phreatomagmatic origin of Tephra C. Glass fragments with higher S values are considered to have been erupted prior to degassing as in a phreatomagmatic eruption. Volcanic product best fit regression lines for melt inclusions, tephra, and lava from Icelandic basalts are from Oladottir, et al.38. Error bar for each S measurement shown in lower right. Full size image

Finding an exact analog to use to understand tephra dispersal for the eruptions that produced the two tephra layers found in the WDC06A ice core that we argue may have a begun as subglacial events, is challenging. However, two well-studied Icelandic analogs are presented here. The first is the 2004 subglacial to emergent eruption of the Grimsvotn volcano. Because the Grimsvotn eruption was basaltic, it is likely to have been somewhat less explosive than the eruptions that produced the two subglacial to emergent WDC06A tephra layers. Albedo changes in the ice cap surface, measured by MODIS, were attributed to tephra deposited on the ice sheet from the Grimsvotn eruption39. Based on this information, a tephra load of .01 g/cm2, which would represent a fine dusting of tephra, was calculated to be present 30 km from vent. At the other end of the explosivity spectrum, the 1875 rhyolitic eruption of Askja Volcano produced around 2 km3 of tephra in a series of eruptions with Plinian, subplinian, and phreatoplinian characteristics40. Based on detailed mapping, 5 mm of tephra was present 150 km from the vent, along the maximum dispersal axis for the phreatoplinian part of the event35. Given that Tephra A and C in the WDC06A core are likely to be derived from eruptions intermediate in explosivity between these two end members, it is unlikely that the deposition site sampled by the WDC06A core was no more than around 200 km from the eruptive vents. In comparison, the 2010 eruption of Eyjafjallajökull, deposition thickness of >1 cm were found only within 100 km of the edifice41 .

Three subglacial volcanoes near the WDC06A drill site have been identified by aeromagnetic surveys, and are potential sources for Tephra A and C. These are Mt. Thiel, Mt. Resnik and Mt. Casertz13. Mt. Thiel is the closest subglacial volcano to WDC06A but is 1.5 km below the ice surface. Mt. Resnik is a tall peak, with the summit only 300 m below the ice surface, <100 km away from WDC06A. However, Mt. Resnik has an overall negative magnetic signature suggesting the edifice is older than 760 ka, the last magnetic reversal13. Mt. Casertz is the only subglacial volcano with a current ice depression over the top of the edifice15. However, the volcanic peak is 1.4 km below the surface and ~250 km from WDC06A. Because there are two different tephra with distinct chemical compositions it is likely that more than one subglacial volcano has erupted through the ice sheet. All things considered, the best candidate is Mt. Resnik because of the close proximity and low ice burden. Ice sheet elevation models suggest that WAIS was >200 m lower during the last glacial period42 reducing the ice burden significantly. It is possible that recent volcanic activity at Mt. Resnik during a time of normal polarity may not be large enough to offset the overall reversed polarity of the edifice. Mt. Resnik may also be the source of both tephra but this is difficult to support without geochemical characterization of material from the Mt. Resnik edifice, which cannot be obtained without drilling.