Timing and duration of the eruption

Figure 2 shows the sub-annual resolution chlorine (Cl), calcium (Ca) and non-sea salt sulphur (nssS) stratigraphy for the NEEM-2011-S1 core. The Millennium Eruption sulphur anomaly, confirmed by the presence of identifiable ash (Sun et al. 2014), is modest. Far more striking is a pronounced nssS signal that begins 7.5 years earlier, i.e. in 939 CE (Fig. 2). This can be firmly attributed to Eldgjá because the prominent horizon is also present in the GISP2 ice core, from which glass shards have been extracted and matched geochemically to Eldgjá compositions (Zielinski et al. 1995).

Fig. 2 Chlorine (Cl), calcium (Ca), and non-sea salt sulphur (nssS) concentrations in the NEEM-2011-S1 ice core. The timescale is pinned to the late 946 CE date for the Millennium Eruption (Oppenheimer et al. 2017) and is equivalent to the NS1-2011 timescale (Sigl et al. 2015). The time lag between initial increases in nssS linked to Eldgjá and the Millennium Eruption is 7.5 years. The modest nssS spike attributed to the Millennium Eruption is co-located with glass shards that have been chemically matched to type materials from the Millennium Eruption (Sun et al. 2014) Full size image

Seasonality is pronounced for Ca (typically peaking in late winter/spring; Kang et al. 2015) and Cl (peaking in midwinter and associated with long-range transport of marine aerosol from the sea surface and/or from sea ice by North Atlantic storms; Davidson et al. 1989; Rhodes et al. 2017). A smaller summer peak in Cl occurs around July (Davidson et al. 1989), attributed to sea spray from Greenland coastal areas. These seasonal signals pinpoint the timing and prolongation of the Eldgjá eruption. The volcanic sulphur signal picks up strongly as the 938/939 CE winter Cl and Ca peaks wane but before the summer 939 CE Cl peak. We cannot be certain that the initial deposition indicated in the record derives from the first phase of the eruption, but if it does, it points to an eruption onset in spring 939 CE, assuming transport of aerosol between source and sink of a few weeks at most. Sulphur concentration reaches a peak before the 939/940 CE winter Cl spike, then falls and then rises again coincident with the winter 939/940 CE Cl peak. Sulphur abundance declines in early 940 CE but remains elevated through much of the year. There is a subsidiary sulphur spike before the winter 940/941 CE Cl peak, suggestive of the eruption’s persistence to around autumn 940 CE. While it is possible that the eruption continued after this time without further deposition to northern Greenland, it appears to have been significantly shorter (with episodic activity spanning around 1.5 years) than has previously been suggested. For instance, Zielinski et al. (1995) considered a 3–6-year duration based on sulphate measurements for the GISP2 ice core, while Thordarson et al. (2001) entertained episodic activity spanning up to 8 years. This matter is pertinent to the timeline and intensity of sulphur emission to the atmosphere and thus for understanding atmospheric and climatic change resulting from the eruption.

One striking feature of the glaciochemical signal is the high magnitude of the Cl spike in the 939/940 CE winter. Inspection of the Na record (not shown here) indicates that the Cl abundance exceeds that which can be accounted for by sea salt deposition, pointing to a contribution from volcanic fallout. This is confirmed by the presence of trace metals such as thallium and bismuth (not shown here). This winter Cl spike is accompanied by a peak in S whereas the initial S deposition (spring-to-autumn 939 CE) is not coupled with a Cl anomaly. Zielinski et al. (1995) noted the same sequence in their analysis of the GIPS2 ice core (recovered at a site in central Greenland, more than 600 km from the NEEM drill site): sulphate preceding chloride deposition. The tephra they extracted from the GISP2 core were co-located within the high chloride section.

These observations and comparisons are puzzling. The variable combinations of sulphur, chlorine and tephra in the Greenland ice could reflect source conditions and/or plume chemistry and transport. The Eldgjá products are dominated by lava flows but include an estimated 1.3 km3 (dense-rock equivalent) of tephra associated with at least 16 explosive phases of the eruption, sourced at various points along the fissure system (Moreland 2017). These explosive phases included sub-glacial phreatomagmatic eruptions earlier in the eruption history, which gave way to magmatic eruptions as the fissure system extended well beyond the ice cap (Moreland 2017). This progression may well be reflected in the nature and content of tephra and volatile species carried in the volcanic cloud and in the scavenging of volatiles by hydrometeors and ash particles. However, we also have to consider that we lack information on the chemistry (including photochemistry) and transport of the plume during the course of the eruption. In particular, we do not know the transit times, trajectories or heights of the plumes that deposited materials to the surface at the ice core sites.

The high amplitudes of the winter 940/941 and winter 941/942 Cl peaks might be explained by increased sea ice (a significant source of Na and Cl to the interior of Greenland in winter; Rhodes et al. 2017). If correct, this postulated growth in sea ice might be attributable to climate forcing of the Eldgjá eruption and might represent a pattern common to other boreal volcanic eruptions.

Climatic variation during/after the eruption

We next examine the climate response of the eruption using the two tree-ring-based reconstructions (NH1 and NH2; see Sect. 2; Stoffel et al. 2015). These represent Northern Hemisphere (NH; > 40° N) summer (JJA) land temperatures (Fig. 3a) for the past 1500 years. Cooling associated with Eldgjá amounts to ~ 0.7 °C for NH1 and ~ 1.5 °C for NH2, the 42nd and second largest summer cooling signals, respectively, in the 1500-year-long reconstructions. For comparison, Fig. 3b also shows the reconstructions spanning the period of the 1783–1784 Laki eruption. Summer cooling linked to Laki is similar for both reconstructions, ~ 1.2 °C, the sixth and eighth largest summer cooling in the NH1 and NH2 datasets, respectively. All these anomalies lie below the fifth percentile of coolest years for both reconstructions. JJA-gridded temperature reconstructions are shown for 939, 940 and 941 CE (Fig. 3c–e) and for the years around the 1783–1784 Laki eruption (Fig. 1 in Supplementary Content). In 940 CE, cooling is most pronounced in Central Europe, Scandinavia, the Canadian Rockies, Alaska and Central Asia and reaches − 2 °C (with respect to 1961–1990). Notably, for both Icelandic eruptions, the hemispheric cooling is seen in just one summer (although the anomaly persists in parts of Canada and Central Asia in 941 CE; Fig. 3e). This contrasts with the 2–5-year summer cooling signals associated with large, sulphur-rich explosive eruptions of equatorial volcanoes (Stoffel et al. 2015).

Fig. 3 Northern hemisphere summer (JJA) temperature anomalies reconstructed from the two tree-ring chronologies (NH1 and NH2; Stoffel et al. 2015) spanning the eruptions of a Eldgjá and b Laki. Shaded areas denote uncertainties (2.5 and 97.5 percentiles) related to the NH1 tree-ring reconstruction (Sect. 2). Spatial extent of the JJA temperature anomalies are for c 939 CE, d 940 CE and e 941 CE Full size image

Annals and chronicles attesting to atmospheric optical phenomena, climatic anomalies and food crises

Our secure date for the eruption focuses attention on an entry in the Chronicon Scotorum, an Irish chronicle for 939 CE (McCarthy and Breen 1997): ‘the Sun was of the colour of blood from the beginning of one day to the middle of the following day’ (Supplementary Content). The entry is egregious—nowhere else in the entire chronicle is a blood-red Sun recorded. We find a similar report in the Annales Casinates, compiled in an abbey southeast of Rome (Supplementary Content). An entry plausibly dated to mid-July 939 CE reads ‘We looked at the sun, it did not have any strength, neither light nor heat. But we saw the sky and the colour [or ‘appearance’] of it changed, as though viscous. And others said that they saw the sun as though half’ (Supplementary Content). Such descriptions are echoed in observations made eight centuries later as volcanic haze sourced from the 1783–1784 Laki eruption spread across the Mediterranean (Grattan and Brayshay 1995; Thordarson et al. 1996).

The near-contemporary chronicle, Res Gestae Saxonicae, written at the abbey of Corvey (east of Paderborn in Germany), similarly records blood-red sunlight and also tells that the winter of 939/940 CE was exceptionally severe (Supplementary Content). Irish annals also attest to a severe winter of 940/941 CE, with rivers and lakes freezing over (Supplementary Content). Chinese sources as well indicate uncommonly severe winters in 939/940 CE, 940/941 CE and 941/942 CE. Remarkably, snow is reported in Inner Mongolia in June 939 CE. Unusual snowfall reoccurred the following winter (939/940 CE) when the Later Jin emperor Shi Jingtang lamented 50 continuous days of snow, which ‘harmed the people’ (Supplementary Content). The next winter was likewise difficult. In early February 941 CE, ice was observed in Qingzhou (modern Shandong province) to extend 100 li (50 km) out from the coast, into the Bohai Sea. The winter of 941/942 CE was as well ‘extremely cold’. In late December 941 CE, the Grand Canal in Jiangdu (modern Jiangsu province) froze over (Supplementary Content). Again, these observations mirror records of the severe winter of 1783/1784 during the Laki eruption (Witham and Oppenheimer 2004), though the relationship with volcanic forcing remains unclear (D’Arrigo et al. 2011; Schmidt et al. 2012; Cole-Dai et al. 2014).

A further point of comparison between these two great Icelandic eruptions is suppressed flow of the Nile—records from the Roda nilometer in Cairo indicate anomalously low discharge in 939 CE (Popper 1951). This phenomenon has been interpreted as a dynamical response of the atmosphere to volcanic aerosol with weakened summer monsoonal precipitation in the source areas of the Nile (Oman et al. 2006; Manning et al. 2017).

These reports of climatic anomalies coincide with food shortages from 940 CE recorded in a variety of narrative sources (Supplementary Content; Newfield 2015), implicating the Eldgjá eruption in substantial reductions in arable output in some regions of Western and Eastern Eurasia for which evidence is extant. For instance, the Ta’rīkh Jazīrat Şiqilliya or Cambridge Chronicle, a tenth- or eleventh-century list of events in Sicily, recounts a dire subsistence crisis in AH 328 (939–940 CE) that led to desertion and abandonment of fortresses and countryside (Amari 1880). In his Antapodosis, written circa 950 CE, Liudprand of Cremona mentions a food shortage that ‘devastated Italy’ in 941 CE (Supplementary Content). Famine also persisted into 941 CE at least in parts of the Benelux countries (Supplementary Content) and, according to later sources, Germany (Zusätze Röchell’s zu Frühern Chronisten 1856, p.187–8). Parts of France, Switzerland and the Benelux countries were afflicted still in 942 CE when Flodoard of Rheims wrote in his Annales of a ‘great food shortage through all Francia and Burgundia’ (Supplementary Content). Dearth and subsistence crises also visited the Baghdad area from 940 to 941 CE (Ibn al-jawzi 1992; Bar Hebraeus 2003) and the Maghreb (Skylitzes 2010).

As indicated, hardship accompanied the uncommonly cool temperatures experienced in eastern China as well. In mid-May 941 CE, there are reports of both widespread starvation and public relief efforts in Henan and Shandong provinces, which could suggest an uneven occurrence of harvest failure (regions without public granaries suffered more), and in winter 941/942 CE of starving soldiers (Supplementary Content). Extensive flooding reported in eastern China in the spring of 941 CE, possibly fuelled by heavy winter snowfall, and inundations in the autumn of the same year undoubtedly aggravated food shortages. Flooding recurred in the late spring of 942 CE, which may indicate that winter 941/942 CE was unusually long (Supplementary Content).

Although historical sources do not record pronounced aridity in Europe, the Old World Drought Atlas identifies exceptionally dry conditions north of the Alps in the summers of 942–944 CE (Cook et al. 2015). Severe spring–summer drought conditions are observed widely in eastern China in 942–943 CE as well. At the same time, there were locust infestations both in Central Europe in 941 CE (Zusätze Röchell’s zu Frühern Chronisten 1856, p.187) and in the Yellow River basin in 942–943 CE (Supplementary Content). In 943 CE, famine inflated foodstuff prices and families migrating for want of food are referenced alongside reports of tens of thousands of deaths in eastern China. Hundreds of thousands of deaths were reported the following year (Supplementary Content). The European evidence is terser and less speculative, but subsistence crises there likely contributed greatly to human mortality and to a vast cattle plague, exacerbating the agricultural consequences of summer cooling (Newfield 2015). It appears that multiple environmental anomalies, potentially synergistic, coalesced to challenge the subsistence strategies of contemporaries in many and disparate regions. In Europe, the Middle East and China, the conditions for famine and mortality mounted over a number of years after the Eldgjá eruption.

Vǫluspá and the Eldgjá eruption

Returning to the eruption itself, we focus on one of Iceland’s most celebrated medieval poems, the apocalyptic Vǫluspá (‘the prophecy of the seeress’). It is included in the Codex Regius (Fig. 1c), written circa 1270 CE, but recapitulating much earlier poetry and Scandinavian and Germanic folklore (Orchard 2011). We suggest it contains literary references to the Eldgjá eruption and that these assume wider significance in the context of Iceland’s conversion to Christianity. The poem describes how the revered pagan god Odin raises a prophetess from the dead. She foretells the end of the pagan pantheon and the coming of a new (and singular) god in a series of portents, one being the rearing of a monstrous wolf that will swallow the Sun. The descriptions, found notably in stanzas 41 and 57, are strongly suggestive of an eruption of the magnitude, duration and likely impacts of the Eldgjá eruption:

41: Fylliz fjǫrvi

feigra manna,

rýðr ragna sjǫt

rauðum dreyra;

svǫrt verða sólskin

of sumur eftir,

veðr ǫll válynd.

Vituð ér enn—eða hvat? [The wolf] is filled with the life-blood

of doomed men,

reddens the powers’ dwellings

with ruddy gore;

the sun-beams turn black

the following summers,

weather all woeful:

do you know yet, or what? 57: Sól tér sortna,

sígr fold í mar,

hverfa af himni

heiðar stjǫrnur.

Geisar eimi

ok aldrnara,

leikr hár hiti

við himin sjálfan. The sun starts to turn black,

land sinks into sea;

the bright stars

scatter from the sky.

Steam spurts up

with what nourishes life,

flame flies high

against heaven itself.

The poetic circumlocution ‘what nourishes life’ from stanza 57 is generally taken to refer to fire. The two stanzas taken together, describing an explosive and fiery event reaching up to the sky in which the air above first turns blood-red, while the land seems to recede, and then the Sun and other heavenly bodies are obscured by darkness over several years, suggest a close literary interpretation of what might have been observed at first hand when Eldgjá erupted (see also Supplementary Content). Referring to stanza 41, the Icelandic mediaevalist, Sigurður Nordal, who edited Vǫluspá, noted that ‘I do not think I have ever understood this description until I saw volcanic ash falling in a clear sky from the eruption of Katla in 1918. The sun was shining then, but it was dark, as was all its shine. Impressive as a total eclipse of the sun is (as popular beliefs and folktales witness), this was yet much more grim and terrible’ (Nordal 1923). Thorarinsson (1981) also recognised the volcanic allusions in the poem but suggested that these were references to an eruption of Katla in 1000 CE.

Further lines in the poem suggest volcanic manifestations. In stanza 38, ‘venom-drops flowed in through the roof-holes’, which may recall acid rain associated with volcanic plumes. This finds an echo in Pastor Jón Steingrímsson’s account of the Laki eruption: ‘more poison fell from the sky than words can describe: ash, volcanic hairs, … rain full of sulphur and saltpetre…’ (Steingrímsson and Kunz 1998). In stanza 52, there is a reference to the fire giant ‘Surt comes from the South with what damages branches’. In verse 60, the gods are said to come together at their home to commemorate these ‘mighty events’. The fact that the devastation of the mythical fire-giant Surt is said to ‘come from the South’ maps on to the geographical location of Eldgjá, which is even now designated as situated in Suðurland (‘the southern part of the country’).

If our attribution of these parts of Vǫluspá to manifestations and climatic consequences of the Eldgjá eruption is correct, our secure date for the eruption implies that Vǫluspá was composed after 939/940 CE.

The Christianisation of Iceland

In calling attention to experiences and memories of the Eldgjá eruption as signs that the old pagan ways were doomed, Vǫluspá suggests that the eruption acted as a catalyst for the profound cultural change brought about by conversion to Christianity. While this was a gradual process over the latter half of the tenth century, and there existed a Christian population of Celtic roots even since the settlement of Iceland, the official date for conversion (kristnitaka) is 999/1000 CE. This followed a heated national parliament at Þingvellir (where annual gatherings had been held since 930 CE, before the Eldgjá eruption). In Kristni saga (‘the story of the conversion’), written in the thirteenth century, one of the key events is the sudden arrival of a man reporting a volcanic eruption. He claims that lava is threatening the land of one of the chieftains, Þóroddr of Ölfus, who was evidently on the Christian side. When the pagans noted that this was a sign of the gods’ displeasure, another powerful chieftain, pointing to the dramatic landscape of Þingvellir itself, asks ‘What were the gods enraged by when the lava we are standing on here and now was burning?’ (Grønlie 2006). Later traditions evidently associated the conversion with volcanic events, just as Vǫluspá does, in that case likely the Eldgjá eruption itself.