Monday, 21 October 2013

Past blast: The lost volcano that set off an ice age

A volcano erupted so powerfully in the 13th century that it triggered global cooling, yet no one knew where it was. Have we cracked the puzzle at last?
THE clincher was a 13th-century text, written in Old Javanese on palm-leaf parchment. When Franck Lavigne discovered it in a Dutch library, his instincts told him that it was what he had been looking for. Now, as he read the translation, his suspicions were confirmed. The text described a cataclysmic eruption on the Indonesian island of Lombok – a blast so powerful that it obliterated the long-forgotten volcano Mount Samalas and destroyed a kingdom now all but lost to history.
Lavigne had already been to Lombok in his quest to solve one of volcanology's biggest mysteries. There he found charcoal fragments and layers of volcanic debris surrounding a large crater lake. Now the geographer from the Panthéon-Sorbonne University, Paris, had written evidence to support his hunch. A caldera, the text indicated, was all that was left of Mount Samalas. It dispelled any lingering doubt: Lavigne and his colleagues had discovered the location of the mystery eruption of 1257, possibly the biggest volcanic event since the Stone Age.
Their findings, just published in PNAS (DOI: 10.1073/pnas.1307520110), not only mark the culmination of three years of hard detective work, but also have far-reaching implications. The eruption of 1257 may have had a role in one of the most intriguing events in recent climate history, when three centuries of relative warmth and stability gave way to the Little Ice Age, a period marked by famines, epidemics and social turbulence. More generally, concerns over global warming have made it imperative to better understand Earth's climate, and the potential impacts of very large volcanic eruptions are an important part of that puzzle.
The first hint of a major volcanic episode in the mid-13th century only emerged in the early 1980s, when Danish researchers unveiled a new technique for measuring changes in acidity in ice cores from ancient glaciers. That allowed them to obtain a record of years when sulphur dioxide from eruptions rained down from the stratosphere as droplets of sulphuric acid. Using cores taken from Greenland and spanning the past 10,000 years, the team identified the signatures of major events such as the Indonesian eruptions of Tambora and Krakatoa in 1815 and 1883 respectively. They also discovered many previously unknown blasts. One of the strongest signals corresponded to the winter of 1258-59, indicating a major eruption one or two years before that. But there was no historical record of such an event.
Initially, the team thought that an eruption not far from Greenland must have produced the mystery spike, which would explain its large size. But subsequent ice-core studies found the same strong signal elsewhere, including Antarctica. This implied that the eruption was in the tropics, from where its gases spread over both poles.
Clearly, the explosion was far bigger than originally thought. Just how big emerged in 2008, when researchers combined the results from 54 ice-core studies to try to work out just how much sulphuric acid had been injected into the atmosphere by major eruptions of the past 1500 years. The signal for 1258-59 stood out as easily the biggest spike over the entire period. It was more than eight times the size of Krakatoa's and more than twice the size of Tambora's, an eruption that killed an estimated 71,000 people and triggered widespread climate disturbances, including Europe's "year without a summer". You have to go back more than 7,000 years to find anything bigger.
The monster blast must have had a global impact, and there are signs it did. Good evidence indicates that during the late 1250s much of the world experienced unusual weather typical of a volcanically perturbed atmosphere. European accounts of the time described excessive rain that ruined crops and triggered famines, which probably contributed to outbreaks of disease. Matthew Paris, a monk in St Albans, north of London, chronicled the conditions: "No one, indeed, could remember ever having before beheld such misery and such a famine." By his estimate, it led to 15,000 deaths in London alone in 1258, while another eyewitness put the figure at around 20,000. The magnitude of the calamity came into vivid focus last year when researchers from the Museum of London Archaeology released details of their excavation of more than 10,500 skeletons from mass graves at St Mary Spital, a former medieval monastery in London. Radiocarbon dating of the bones indicated that the majority belonged to people who had perished in the wake of crop failures from late 1257 to 1260.

Unusual conditions

Conditions were as bad across much of Asia. Bill Atwell, a retired professor of history who has studied the impacts of volcanism on Asian economies, says the period was clearly atypical even in an era known for turbulence. "Whether you're looking at England or Japan or China or Korea, there are real problems beginning in the mid-1250s which continue into the 1260s," he says.
The widespread nature of these unusual conditions around this time is confirmed by tree-ring research. One recent study of Vietnamese trees shows that the monsoon season of 1258 was by far the wettest in South-East Asia in the past 1000 years. And tissue damage in growth rings provides evidence that temperatures in Mongolia in the summer of 1258 dipped below freezing. "It's very unusual," says Rosanne D'Arrigo at Columbia University, New York, who was involved in the Mongolian study. "In some trees, they're the only frost rings we see."
As the circumstantial evidence has grown, so too has the mystery surrounding the eruption itself. If it was such an enormous blast, why was there no physical record – no obvious crater? And why were there no historical accounts of the event or its catastrophic after-effects? Several candidate volcanoes have been proposed, but the evidence has been unconvincing. Indeed, some researchers have even wondered if the spike in volcanic gases was simply the result of two or more smaller eruptions that occurred around the same time.
Enter Lavigne. In 2010 he was writing a paper on the environmental impacts of large volcanic eruptions when he learned of unknown blasts revealed in the ice-core records. Particularly eye-catching was the giant spike of 1258-59. "I found it unbelievable that nobody knew where it had occurred," he says. Assuming that volcanologists were too busy studying known eruptions to look for unknown ones, he set out to crack the case.
Since the evidence suggested that the blast had tropical origins, Lavigne's focus soon fell on Indonesia, where many large eruption sites remain undated and under-studied. To narrow his search further, he looked for pumice mines, often located where significant and geologically recent eruptions have taken place. It was only after unsuccessful missions to potential sites in Sumatra and Java that he made his way to Segara Anak, a crater lake just west of Mount Rinjani in northern Lombok (see map).
The first hint that Lavigne and his team were on to something came from 22 charcoal fragments found around the caldera. Radiocarbon dating revealed that every one of them predated 1257, indicating an absence of trees thereafter. Next, a detailed study of the surrounding landscape, including analysis of the layers in 130 cliffs and outcrops, showed that the caldera had been formed by a massive eruption involving several stages that probably spanned a three-day period. The picture that emerged was of explosive plumes pumping ash and volcanic gases between 34 and 52 kilometres into the sky. At some point the eruption plume collapsed, propelling heavy clouds of scorching rubble sideways and burying most of Lombok's northern half under around 35 metres of debris. A layer of ash up to 50 centimetres thick settled over the entire island, while a thinner layer covered all of Bali and parts of Java to the west. When it was all over, Mount Samalas – estimated to have been 4200 metres high – was gone, replaced by a hole 6 by 8.5 kilometres wide and 800 metres deep.
This story of devastation was confirmed in the Javanese text. It tells of an explosion that caused avalanches on the western flank of nearby Mount Rinjani – where a massive scar is visible in satellite images today – and that blanketed the vicinity in debris. "All houses were destroyed and swept away, floating on the sea, and many people died," the text reads.
Since then, Lavigne's team has found evidence that directly links Mount Samalas with the 1258-59 ice-core spike: glass from Samalas pumice turns out to have a crystal structure similar to that of shards extracted from the 1258-59 ice-core layer. The team also has unpublished research indicating that the eruption was rich in sulphur – a key finding that suggests it was capable of generating the huge signal seen in the ice cores. "I'm 100 per cent sure this is the one," says Lavigne. Going on ash dispersal evidence and seasonal wind patterns, he places the eruption as happening between May and October of 1257.
"It looks like they have a very good case," says Olivier Bachmann, a volcanologist at the Swiss Federal Institute of Technology in Zurich. There is no doubting the potential significance of the team's conclusions: finding the 1257 eruption "would be a huge development", says climatologist Michael Mann at Pennsylvania State University in University Park.
The question now is how much damage did Samalas cause? If Tambora is anything to go by, such a blast would have killed most of Lombok's plant and animal life. There is no clue as to how many people perished, but the void in the island's history during this period may be telling. Lavigne's Javanese manuscript indicates that there were survivors, including the king and his family. But the kingdom and its capital Pamatan were erased from history.
The effects were felt far beyond Indonesia. "From what we've found, it's quite clear that the global climate impacts of Samalas were bigger than Tambora," says Lavigne. "They were more widespread. Almost everywhere in the world, there was cooling."
But even if the eruption did cause the foul weather that ruined crops and led to several years of famine across Europe and Asia, could it have triggered a long-term change in the climate? That is now being hotly debated.
Much of Europe experienced distinct climatic phases over the past millennium. The weather was relatively warm between around 900 and 1250 – the Medieval Climate Anomaly – when agriculture expanded into northern Scandinavia and winemaking flourished in England. This was followed by centuries with more frequent bouts of nasty weather, including wetter summers and colder winters. Rivers such as the Thames in London froze over, and glaciers grew until they engulfed farms and villages in the Alps. Exactly when this Little Ice Age began and how widely it was felt has never been completely clear, but a large body of evidence points to the second half of the 13th century, and to its influence extending far beyond Europe.
The period from 1270 to 1400 is also notable for social turmoil worldwide. In Europe, deteriorating climate has been fingered in the onset of famines and epidemics, including the Great Famine of 1315 and the Black Death of the 1340s, which practically halved the population of central Europe. The 1250 to 1400 timeframe also saw the decline of the Norse settlements in Greenland, the disintegration of the Mongol empire, and the collapse of three major South-East Asian states: Pagan in Burma, Angkor in Cambodia, and Dai Viet in Vietnam.
The discovery of an overlap between periods of diminished sunspot activity and some of the coldest decades of the Little Ice Age had led some researchers to blame the medieval climate shift on a dip in the amount of solar radiation reaching Earth's surface. However, it is not clear whether this could account for all the temperature changes that occurred. Some studies suggest that the Little Ice Age was just a blip within the climate's natural range of variability. But other evidence points to a different culprit: volcanic activity, especially large tropical eruptions.
This idea has recently been given a boost. Last year, a team led by Gifford Miller at the University of Colorado in Boulder reported that surges in the formation of Arctic sea ice during the Little Ice Age coincided with periods of active volcanism in the late 13th century – which saw at least three other prominent blasts – and the mid-15th century. This led Miller to wonder whether post-eruption cooling can trigger a persistent change in climate. Sure enough, when he and his colleagues modelled the effects of an expansion of ice into the North Atlantic, they found it could weaken warm currents from the south and so trigger runaway cooling.
"The work suggests the Little Ice Age was caused not by solar forcing but by volcanic activity," says Alan Robock at Rutgers University in New Brunswick, New Jersey. More than three decades ago, he provided some of the earliest evidence that volcanism contributed to cooling in the Little Ice Age. The case is still not closed, but if the idea does become widely accepted, it will have important implications for our understanding of Earth's climate. Knowing the causes of earlier climate shifts is crucial to the quest to forecast what will happen next.
If the eruption of Samalas did help trigger the Little Ice Age, there can be little doubt that volcanism can have sudden, far-reaching consequences both for the climate and for civilisation. With the mystery of 1257 seemingly solved, we are one giant step closer to understanding the role of volcanic activity in dramatic climate change.

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