̽��ֱ�� of Cambridge - glacier

Shrinking Arctic glaciers are unearthing a new source of methane

cmm201 — Thu, 06 Jul 2023 14:23:15 +0000

As the Arctic warms, shrinking glaciers are exposing bubbling groundwater springs which could provide an underestimated source of the potent greenhouse gas methane, finds new research published today in Nature Geoscience.

UK and US join forces to understand how quickly a massive Antarctic glacier could collapse

sc604 — Mon, 30 Apr 2018 08:45:00 +0000

̽��ֱ��collapse of the Thwaites Glacier in West Antarctica could significantly affect global sea levels. It already drains an area roughly the size of Britain or the US state of Florida, accounting for around four percent of global sea-level rise —an amount that has doubled since the mid-1990s.

As part of a new £20 million research collaboration, the UK Natural Environment Research Council and the US National Science Foundation will deploy scientists to gather the data needed to understand whether the glacier’s collapse could begin in the next few decades or centuries.

NERC and NSF have jointly funded eight large-scale projects that will bring together leading polar scientists in one of the most inhospitable regions of the planet. ̽��ֱ��programme, called the International Thwaites Glacier Collaboration (ITGC), is the largest joint project undertaken by the two nations in Antarctica for more than 70 years - since the conclusion of a mapping project on the Antarctic Peninsula in the late 1940s.

In addition to the £20 million-worth ($25 million) of awards to the research teams, the logistics of mounting a scientific campaign in one of the most remote places in Antarctica could cost as much again in logistical support. ̽��ֱ��nearest permanently occupied research station to the Thwaites Glacier is more than 1600km away, so getting the scientists to where they need to be will take a massive joint effort from both nations. While researchers on the ice will rely on aircraft support from UK and U.S. research stations, oceanographers and geophysicists will approach the glacier from the sea in UK and U.S. research icebreakers.

Dr Poul Christoffersen from the ̽��ֱ�� of Cambridge’s Scott Polar Research Institute is co-leading one of the eight projects with Professor Slawek Tulaczyk from the ̽��ֱ�� of California, Santa Cruz. Their project, Thwaites Interdisciplinary Margin Evolution (TIME) also includes researchers from the ̽��ֱ�� of Leeds, Stanford ̽��ֱ��, the ̽��ֱ�� of Texas and the ̽��ֱ�� of Oklahoma. ̽��ֱ��team will investigate how the margins of the drainage basin will evolve and influence ice flow over the coming decades.

“These margins have so far never been studied directly, due to the logistical challenges of working in such a remote region of Antarctica,” said Christoffersen. “ ̽��ֱ��margins, which separate the fast-flowing glacier from the surrounding slow-moving ice, are often thought of as being stationary, but they might not be. ̽��ֱ��hypothesis that drives our science is that they can move and thereby exert powerful control on the future evolution of ice flow in the whole drainage basin.”

“This international collaboration will lead to a step change in our understanding of ice sheet stability,” said Cambridge’s Dr Marion Bougamont, who will use observational data records gathered in the field to improve computer models needed to predict sea level rise. “ ̽��ֱ��glacier’s response will depend on where the margins are and how they evolve.”

Today’s collaboration involves around 100 scientists from world-leading research institutes in both countries alongside researchers from South Korea, Germany, Sweden, New Zealand and Finland, who will contribute to the various projects. These projects aim to deliver answers to some of the big questions for scientists trying to predict global sea-level rise.

Antarctica’s glaciers contribute to sea-level rise when more ice is lost to the ocean than is replaced by snow. To fully understand the causes of changes in ice flow requires research on the ice itself, the nearby ocean, and the Antarctic climate in the region. ̽��ֱ��programme will deploy the most up-to-date instruments and techniques available, from drills that can make access holes 1,500 meters into the ice with jets of hot water to autonomous submarines like the Autosub Long Range affectionately known around the world as Boaty McBoatface.

“Rising sea levels are a globally important issue which cannot be tackled by one country alone,” said UK Science Minister, Sam Gyimah. “ ̽��ֱ��Thwaites Glacier already contributes to rising sea levels and understanding its likely collapse in the coming century is vitally important. Science, research and innovation are at the heart of our Industrial Strategy and this UK-U.S. research programme will be the biggest field campaign of its type ever mounted by these countries. I’m delighted that our world-leading scientists will help to lead this work.”

̽��ֱ��science programme and logistics on the five-year programme begins in October 2018 and continues to 2021. ̽��ֱ��funding is for eight research projects and a co-ordination grant to maximise success.

Adapted from a NERC/NSF press release.

A Cambridge researcher will lead one of eight projects in a new joint UK-US research programme that is one of the most detailed and extensive examinations of a massive Antarctic glacier ever undertaken.

These margins have so far never been studied directly, due to the logistical challenges of working in such a remote region of Antarctica.

Poul Christoffersen

US National Science Foundation/US Antarctic Program

Reconnaissance flight over the Thwaites glacier

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Frozen in time: glacial archaeology on the roof of Norway

lmb97 — Wed, 24 Jan 2018 09:49:48 +0000

Climate change is one of the most important issues facing people today and year on year the melting of glacial ice patches in Scandinavia, the Alps and North America reveals and then destroys vital archaeological records of past human activity.

Enter the glacial archaeologists – specialists who rescue now-threatened artefacts and study the relationship between variability in climate and the intensity of human use of alpine landscapes.

Focusing on Jotunheimen and the surrounding mountain areas of Oppland, which include Norway’s highest mountains (to 2649m), an international team of researchers have conducted a systematic survey at the edges of the contracting ice, recovering artefacts of wood, textile, hide and other organic materials that are otherwise rarely preserved.

To date, more than 2000 artefacts have been recovered. Some of the finds date as far back as 4000 BC and include arrows, Iron Age and Bronze Age clothing items and remains of skis and packhorses.

By statistical analysis of radiocarbon dates on these incredibly unusual finds, patterns began to emerge showing that they do not spread out evenly over time. Some periods have many finds while others have none. What could have caused this chronological patterning – human activity and/or past climate change?

These questions are the focus of a new study published today in Royal Society Open Science.

“One such pattern which really surprised us was the possible increase in activity in the period known as the Late Antique Little Ice Age (c. 536 – 660 AD)," says Dr James H. Barrett, an environmental archaeologist at Cambridge's McDonald Institute for Archaeological Research and senior study author.

"This was a time of cooling; harvests may have failed and populations may have dropped. Remarkably, though, the finds from the ice may have continued through this period, perhaps suggesting that the importance of mountain hunting (mainly for reindeer) increased to supplement failing agricultural harvests in times of low temperatures. Alternatively, any decline in high-elevation activity during the Late Antique Little Ice Age was so brief that we cannot observe it from the available evidence.

“We then see particularly high numbers of finds dating to the 8th – 10th centuries AD, probably reflecting increased population, mobility (including the use of mountain passes) and trade – just before and during the Viking Age, when outward expansion was also characteristic of Scandinavia.

"One driver of this increase may have been the expanding ecological frontier of the towns that were emerging around Europe at this time," says Barrett. "Town-dwellers needed mountain products such as antlers for artefact manufacture and probably also furs. Other drivers were the changing needs and aspirations of the mountain hunters themselves."

There is then a decrease in the number of finds dating to the medieval period (from the 11th century onwards). Lars Pilø, co-director of the Glacier Archaeology Program at Oppland County Council and lead author on the study further explains, “There is a sharp decline in finds dating from the 11th century onwards. At this time, bow-and-arrow hunting for reindeer was replaced with mass-harvesting techniques including funnel-shaped and pitfall trapping systems. This type of intensive hunting probably reduced the number of wild reindeer.”

Professor in medieval archaeology Brit Solli, of the Museum of Cultural History in Oslo, who led the study of the recovered artefacts, comments: “Once the plague arrived in the mid-14th century, trade and markets in the north also suffered. With fewer markets and fewer reindeer the activity in the high mountains decreased substantially. This downturn could also have been influenced by declining climatic conditions during the Little Ice Age.”

̽��ֱ��ongoing research of the Glacier Archaeology Program in Oppland can be followed on the Secrets of the Ice blogpost: http://secretsoftheice.com/

Artefacts revealed by melting ice patches in the high mountains of Oppland shed new light on ancient high-altitude hunting.

Town-dwellers needed mountain products such as antlers for artefact manufacture and probably also furs

James Barrett

Johan Wildhagen, Palookaville

Glacial archaeologists systematically survey the mountainous areas of Oppland, Norway

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Increase in volcanic eruptions at the end of the ice age caused by melting ice caps and glacial erosion

sc604 — Tue, 02 Feb 2016 06:00:00 +0000

̽��ֱ��combination of erosion and melting ice caps led to a massive increase in volcanic activity at the end of the last ice age, according to new research. As the climate warmed, the ice caps melted, decreasing the pressure on the Earth’s mantle, leading to an increase in both magma production and volcanic eruptions. ̽��ֱ��researchers, led by the ̽��ֱ�� of Cambridge, have found that erosion also played a major role in the process, and may have contributed to an increase in atmospheric carbon dioxide levels.

“It’s been established that melting ice caps and volcanic activity are linked – but what we’ve found is that erosion also plays a key role in the cycle,” said Dr Pietro Sternai of Cambridge’s Department of Earth Sciences, the paper’s lead author, who is also a member of Caltech’s Division of Geological and Planetary Science. “Previous attempts to model the huge increase in atmospheric CO₂ at the end of the last ice age failed to account for the role of erosion, meaning that CO₂ levels may have been seriously underestimated.”

Using numerical simulations, which modelled various different features such as ice caps and glacial erosion rates, Sternai and his colleagues from the ̽��ֱ�� of Geneva and ETH Zurich found that erosion is just as important as melting ice in driving the increase in magma production and subsequent volcanic activity. ̽��ֱ��results are published in the journal Geophysical Research Letters.

Although the researchers caution not to draw too strong a link between anthropogenic (human-caused) climate change and increased volcanic activity as the timescales are very different, since we now live in a period where the ice caps are being melted by climate change, they say that the same mechanism will likely work at shorter timescales as well.

Over the past million years, the Earth has gone back and forth between ice ages, or glacial periods, and interglacial periods, with each period lasting for roughly 100,000 years. During the interglacial periods, such as the one we live in today, volcanic activity is much higher, as the lack of pressure provided by the ice caps means that volcanoes are freer to erupt. But in the transition from an ice age to an interglacial period, the rates of erosion also increase, especially in mountain ranges where volcanoes tend to cluster.

Glaciers are considered to be the most erosive force on Earth, and as they melt, the ground beneath is eroded by as much as ten centimetres per year, further decreasing the pressure on the volcano and increasing the likelihood of an eruption. A decrease in pressure enhances the production of magma at depth, since rocks held at lower pressure tend to melt at lower temperatures.

When volcanoes erupt, they release more carbon dioxide into the atmosphere, creating a cycle that speeds up the warming process. Previous models that attempted to explain the increase in atmospheric CO₂ during the end of the last ice age accounted for the role of deglaciation in increasing volcanic activity, but did not account for erosion, meaning that CO₂ levels may have been significantly underestimated.

A typical ice age lasting 100,000 years can be characterised into periods of advancing and retreating ice – the ice grows for 80,000 years, but it only takes 20,000 years for that ice to melt.

“There are several factors that contribute to climate warming and cooling trends, and many of them are related to the Earth’s orbital parameters,” said Sternai. “But we know that much faster warming that cooling can’t be caused solely by changes in the Earth’s orbit – it must be, at least to some extent, related to something within the Earth system itself. Erosion, by contributing to unload the Earth’s surface and enhance volcanic CO₂ emissions, may be the missing factor required to explain such persistent climate asymmetry.”

Reference:
Pietro Sternai et al. ‘Deglaciation and glacial erosion: a joint control on magma productivity by continental unloading.’ Geophysical Research Letters (2016). DOI: 10.1002/2015GL067285

Inset image: 3D model simulation of a glaciation on the Villarrica Volcano (Chile). Credit: Pietro Sternai

Researchers have found that glacial erosion and melting ice caps both played a key role in driving the observed global increase in volcanic activity at the end of the last ice age.

It’s been established that melting ice caps and volcanic activity are linked – but what we’ve found is that erosion also plays a key role in the cycle.

Pietro Sternai

Matthew.landry at English Wikipedia

Arenal Volcano in November 2006

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1.5 million years of climate history revealed after scientists solve mystery of the deep

bjb42 — Fri, 10 Aug 2012 02:51:00 +0000

Scientists have announced a major breakthrough in understanding the Earth’s climate machine by reconstructing highly accurate records of changes in ice volume and deep-ocean temperatures over the last 1.5 million years.

̽��ֱ��study, which is reported in the journal Science, offers new insights into a decades-long debate about how the shifts in the Earth’s orbit relative to the sun have taken the Earth into and out of an ice-age climate.

Being able to reconstruct ancient climate change is a critical part of understanding why the climate behaves the way it does. It also helps us to predict how the planet might respond to man-made changes, such as the injection of large quantities of carbon dioxide into the atmosphere, in the future.

Unfortunately, scientists trying to construct an accurate picture of how such changes caused past climatic shifts have been thwarted by the fact that the most readily available marine geological record of ice-ages – changes in the ratio of oxygen isotopes (Oxygen 18 to Oxygen 16) preserved in tiny calcareous deep sea fossils called foraminifera – is compromised.

This is because the isotope record shows the combined effects of both deep sea temperature changes, and changes in the amount of ice volume. Separating these has in the past proven difficult or impossible, so researchers have been unable to tell whether changes in the Earth’s orbit were affecting the temperature of the ocean more than the amount of ice at the Poles, or vice-versa.

̽��ֱ��new study, which was carried out by researchers in the ̽��ֱ�� of Cambridge Department of Earth Sciences, appears to have resolved this problem by introducing a new set of temperature-sensitive data. This allowed them to identify changes in ocean temperatures alone, subtract that from the original isotopic data set, and then build what they describe as an unprecedented picture of climatic change over the last 1.5 million years – a record of changes in both oceanic temperature and global ice volume.

Included in this is a much fuller representation of what happened during the “Mid-Pleistocene Transition” (MPT) - a major change in the Earth’s climate system which took place sometime between 1.25 million and 600 thousand years ago. Before the MPT, the alternation between glacial periods of extreme cold, and warmer interglacials, happened at intervals of approximately 41,000 years. After the MPT, the major cycles became much longer, regularly taking 100,000 years. ̽��ֱ��second pattern of climate cycles is the one we are in now. Interestingly, this change occurred with little or no orbital forcing.

“Previously, we didn’t really know what happened during this transition, or on either side of it,” Professor Harry Elderfield, who led the research team, said. “Before you separate the ice volume and temperature signals, you don’t know whether you’re seeing a climate record in which ice volume changed dramatically, the oceans warmed or cooled substantially, or both.”

“Now, for the first time, we have been able to separate these two components, which means that we stand a much better chance of understanding the mechanisms involved. One of the reasons why that is important, is because we are making changes to the factors that influence the climate now. ̽��ֱ��only way we can work out what the likely effects of that will be in detail is by finding analogues in the geological past, but that depends on having an accurate picture of the past behaviour of the climate system.”

Researchers have developed more than 30 different models for how these features of the climate might have changed in the past, in the course of a debate which has endured for more than 60 years since pioneering work by Nobel Laureate Harold Urey in 1946. ̽��ֱ��new study helps resolve these problems by introducing a new dataset to the picture - the ratio of magnesium (Mg) to calcium (Ca) in foraminifera. Because it is easier for magnesium to be incorporated at higher temperatures, larger quantities of magnesium in the tiny marine fossils imply that the deep sea temperature was higher at that point in geological time.

̽��ֱ��Mg/Ca dataset was taken from the fossil record contained in cores drilled on the Chatham Rise, an area of ocean east of New Zealand. It allowed the Cambridge team to map ocean temperature change over time. Once this had been done, they were able to subtract that information from the oxygen isotopic record. “ ̽��ֱ��calculation tells us the difference between what water temperature was doing and what the ice sheets were doing across a 1.5 million year period,” Professor Elderfield explained.

̽��ֱ��resulting picture shows that ice volume has changed much more dramatically than ocean temperatures in response to changes in orbital geometry. Glacial periods during the 100,000-year cycles have been characterised by a very slow build-up of ice which took thousands of years, the result of ice volume responding to orbital change far more slowly than the ocean temperatures reacted. Ocean temperature change, however, reached a lower limit, probably because the freezing point of sea water put a restriction on how cold the deep ocean could get.

In addition, the record shows that the transition from 41,000-year cycles to 100,000-year cycles, the characteristic changeover of the MPT, was not as gradual as previously thought. In fact, the build-up of larger ice sheets, associated with longer glacials, appears to have begun quite suddenly, around 900,000 years ago. ̽��ֱ��pattern of the Earth’s response to orbital forcing changed dramatically during this “900,000 year event”, as the paper puts it.

̽��ֱ��research team now plan to apply their method to the study of deep-sea temperatures elsewhere to investigate how orbital changes affected the climate in different parts of the world.

“Any uncertainty about the Earth’s climate system fuels the sense that we don’t really know how the climate is behaving, either in response to natural effects or those which are man-made,” Professor Elderfield added. “If we can understand how earlier changes were initiated and what the impacts were, we stand a much better chance of being able to predict and prepare for changes in the future.”

Study successfully reconstructed temperature from the deep sea to reveal how global ice volume has varied over the glacial-interglacial cycles of the past 1.5 million years.

̽��ֱ��only way we can work out what the likely effects of the changes we are making to the climate will be is by finding analogues in the geological past. That depends on having an accurate picture of the past behaviour of the climate system.

Harry Elderfield

Julian Dowdeswell.

Tabular iceberg. ̽��ֱ��production of tabular icebergs is a major mechanism of mass loss from the Antarctic Ice Sheet.

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Ice Age, interrupted

bjb42 — Mon, 09 Jan 2012 08:33:12 +0000

In terms of the ebb and flow of the Earth’s climate over the course of its history, the next Ice Age is starting to look overdue. Periods between recent Ice Ages, or ‘interglacials’, average out to be around 11 thousand years, and it’s currently been 11, 600 since the last multi-millennial winter. Although it is almost impossible to predict exactly when the next Ice Age will occur (if it will at all), it is clear that a global freeze is not on the horizon; the amount of CO₂ emitted by human activity and the enhanced greenhouse effect that results all but preclude it. But what if we weren’t around and CO2 was lower?

In a paper published in Nature Geoscience this week, new research proposes that the next Ice Age would have been kick-started sometime in the next thousand years, just round the corner in the context of the Earth’s lifespan, if CO₂ was sufficiently low.

By looking at the onset of abrupt flip-flops in the temperature contrast between Greenland and Antarctica (extreme climate behaviour that would have only been possible if vast and expanding ice sheets were disrupting ocean circulation), the researchers believe they have been able to identify the fingerprint of an Ice Age activation, or the ‘glacial inception’.

By applying this fingerprinting method to an interglacial period with nearly identical solar radiation, or ‘insolation’, to our own - some 780 thousand years ago - the researchers have been able to determine that glacial inception would indeed be expected to occur sometime soon.

“ ̽��ֱ��mystery of the Ice Ages, which represent the dominant mode of climate change over the past few million years, is that while we can identify the various ingredients that have contributed to them, it’s the arrangement of these ingredients, and how they march to the beat of subtle changes in seasonality, that we lack an understanding of,” says Dr Luke Skinner from the Department of Earth Sciences, who helped to conduct the research with Professor David Hodell and their colleague Professor Chronis Tzedakis from ̽��ֱ�� College London.

Insolation, the seasonal and latitudinal distribution of solar radiation energy, changes over tens of thousands of years due to the variations in the Earth’s orbit around the sun. It has long been apparent that insolation changes have acted as a pace-maker for the Ice Ages. But, like a metronome paces music, it sets the beat of climate change but not its every movement. ̽��ֱ��changing concentrations of greenhouse gases, CO₂ in particular, are evidently what determine when a shift in insolation will trigger climate change.

“From 8,000 years ago, as human civilization flourished, CO₂ reversed its initial downward trend and drifted upwards, accelerating sharply with the industrial revolution,” says Skinner. “Although the contribution of human activities to the pre-industrial drift in CO₂ remains debated, our work suggests that natural insolation will not be cancelling the impacts of man-made global warming.”

Research shows that a new Ice Age could well have been upon us in the next millennium were it not for increases in CO2 due to humans, despite the advantageous trend in solar radiation of our current age.

Our work suggests that natural insolation will not be cancelling the impacts of man-made global warming.

Dr Luke Skinner

Trey Ratcliff from Flickr

Deep into the Patagonia Glacier

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'Extreme Sleepover #11’ - moulins and meltwater on the Greenland ice sheet

bjb42 — Sun, 01 Jan 2012 09:00:57 +0000

“What? No coffee?” We had been dropped off by helicopter onto the Greenland ice sheet where we were to live and work for three weeks. ̽��ֱ��sky was clear and the sun was out, but the air temperature hovered around 0^oC and a cutting katabatic wind blew from the top of the ice sheet in the east, through our ‘windproof’ jackets, and down the coast to our west. We had speedily put up our personal tents and three mess tents (the ‘kitchen’, the ‘workshop’ and the ‘office’). By late afternoon, we had set up the stove; dug through the snow to get water; the kettle was boiling; and now all we needed was the coffee. But someone had forgotten to pack it. “Three weeks without coffee?” Our heads throbbed, our hands shook and our moods sank into our felt-lined boots.

That early-June morning, we had flown to the ice sheet from the small coastal town of Ilulissat, Disko Bay, west central Greenland. Ilulissat, meaning ‘icebergs’ in the native language, has around 4,000 people and is remarkable for its brightly coloured wooden buildings. To the south is Ilulissat Fjord, which has at its head one of Greenland’s biggest and fastest moving outlet glaciers, Jakobshavn Isbræ. As it shudders forwards, thousands of icebergs calve off the front each year, some the size of aircraft carriers. For the last decade, local people and visiting scientists have witnessed one of the world’s most noticeable effects of climate change, as the floating front of the glacier has retreated by around 10 km. ̽��ֱ��glacier has also sped up; ten years ago it flowed at 7 km a year, but now that figure is closer to 15 km.

Our goal was to measure three key things on the ice sheet surface: melting, water flow through snow and along channels etched into the ice, and the filling (and hopefully drainage) of lakes. We radiated out from our camp each day, sometimes for 2–3 km, trussed up in harnesses jangling with ice screws and carabiners, roped together like beads on a string in case anyone disappeared into a crevasse or a hole under the snow. We advanced slowly as we probed for these hidden dangers, but also in an attempt to avoid large patches of slush that developed within the melting snow. We had 24 hours of daylight, of course, and quickly learnt to tell the time from the direction of the sun gyrating around us, higher to the south around midday and lower towards the north, reminding us to crawl into our sleeping bags at the end of each day.

For several days we had watched a nearby lake grow into what was now a thick sky blue ribbon, tapering at the edges, spread across the bright white of the ice sheet – about 800m across. But, as we watched, it suddenly began to shrink: the thick ribbon became a thin band, then a tiny thread, and then it was gone. During the final dying moments of the lake, ice blocks the size of truck containers swirled on the water like pieces of soap around an emptying plug hole. A volume equivalent to 600 Olympic-sized swimming pools (or 15 Royal Albert Halls) had drained in just 2.5 hours. On the floor of the former lake was a new fracture 600 m long that had been produced by the weight of the water. Six large holes or ‘moulins’ had formed along the fracture, the largest of which was around 10 m wide and still had water thundering into it when we reached it.

After nearly three weeks on the ice sheet we were reluctantly ready to leave. It took a few days to bring our instruments back to camp, collapse our tents and pack our things into the boxes they had arrived in. ̽��ֱ��helicopter retrieved us and whisked us back to Ilulissat which felt warm, peculiarly dry underfoot, and was fused with colours and smells that we had been deprived of for three weeks. And at last there was coffee.

Dr Ian Willis and Alison Banwell

Ian is a Senior Lecturer and Alison is a PhD student at the Scott Polar Research Institute, and both are members of St Catharine’s College. Ian has over 20 years of extreme sleepover experience on top and in front of several of the World’s glaciers. In addition to his Greenland work, he currently has projects in Svalbard, Iceland and New Zealand. Alison has recently been awarded a Dow Sustainability Innovation Student Challenge Prize that will allow her to extend her Greenland work to the study of Himalayan glaciers. Ian and Alison’s work was supported by the Natural Environment Research Council, ̽��ֱ�� of Cambridge Travel Fund, BB Roberts Fund, Scandinavian Studies Fund and St Catharine's College.

A longer version of this article was originally published in the St Catharine’s College Society Magazine 2011.

In the eleventh of a series of reports contributed by Cambridge researchers, glaciologists Dr Ian Willis and Alison Banwell watch as a lake disappears before their eyes.

During the final dying moments of the lake, ice blocks the size of truck containers swirled on the water like pieces of soap around an emptying plug hole.

Dr Ian Willis

Ian Willis

Peering into a moulin

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Britain's island heritage: half a million years of history

bjb42 — Sun, 01 Nov 2009 00:00:00 +0000

Deep below the Bay of Biscay, where the English Channel meets the Atlantic Ocean, layers of sediment hold precious information about how Britain came to be separated from mainland Europe. Until recently, the clues had remained hidden, off limits owing to the impracticalities and cost of obtaining long-piston core samples and high-resolution acoustic data in this area. However, thanks to an Anglo-French collaboration between Professor Phil Gibbard, who leads the Quaternary Palaeoenvironments Group in the Department of Geography, and PhD student Sam Toucanne and his colleagues from the ̽��ֱ�� of Bordeaux and the French Research Institute for Exploitation of the Sea (IFREMER), the seabed has now yielded its secrets. In doing so, it provides the final instalment in a story that has been unfolding for two decades, since Professor Gibbard first began his detailed palaeogeographic and paleoenvironmental reconstructions of the southern North Sea.

When Britain actually was in Europe

But the story really starts 500,000 years ago, five ice ages before present times, when Britain was connected to Europe through a land mass that stretched from ̽��ֱ��Netherlands to the Dover Strait. Between Britain and France was a wide shallow area into which southerly rivers like the Somme and the Seine drained, while the Thames and the Rhine drained northwards into the North Sea.

Indications as to what happened to the landmass over the ensuing half a million years have been found in sedimentary investigations of the geological history of the southern North Sea region and neighbouring areas by Professor Gibbard. These provided a read-out of past environments, including climatic changes, and of how and where the rivers flowed during the past. Together with bathymetric maps (sonar readings of the floor of the English Channel, showing the topography of deep valleys running the length of the waterway), it has been possible to build up a detailed picture of the conditions of the region: where rivers drained, glaciers formed and landmasses changed.

Most recently, the scientists have been able to build up a continuous record of the Earth’s climate variability for the past 1,200,000 years, showing a correspondence between temperature changes caused by the waxing and waning of the glaciers and the peaks and lows in sedimentary deposits. ̽��ֱ��data show that three of the five ice ages – 450,000 years, 160,000 years and 30,000 years ago – seem to have had the most dramatic impacts on the sedimentary deposits. Could one or more of these ice ages have precipitated the events that resulted in Britain becoming separated from Europe?

Megafloods and super-rivers

̽��ֱ��first breakthrough came when Professor Gibbard’s investigations of the southern North Sea region and review of the drainage systems led him to suggest the existence some 450,000 years ago of a massive lake. ̽��ֱ��meeting of a glacier flowing down across Britain from the north and a glacier advancing from Denmark strongly modified the flow directions of central European rivers, effectively blocking their flow into the Atlantic Ocean. ̽��ֱ��result was the build-up of a large, freshwater, glacial lake, about the size of East Anglia, just north of what is now the Dover Strait. ̽��ֱ��lake was dammed by an ice sheet, landmass and, crucially, the upfolded spit of land that linked Britain and France at the Dover Strait.

̽��ֱ��first morphological evidence came to light in 2007 from the bathymetric maps of the English Channel. Dr Sanjeev Gupta at Imperial College London observed that the existence of long grooves of erosion and deep valleys running longitudinally along the bedrock floor fitted with an extraordinary conclusion. It seems that the freshwater lake filled until, like an overflowing bath, it breached the Dover Strait. A catastrophic discharge of water surged at least once and probably twice down the basin between Britain and France, overwhelming the rivers and streams below it, and spreading out across the basin as a megaflood. This massive southwards discharge of meltwaters merged with the river-water from the Seine, Somme and others, to form the ‘Fleuve Manche’ (Channel River) palaeoriver, one of the largest river systems on the European continent.

Further ice ages followed. Each time, as the glaciers receded, sea levels rose and the Channel would continue to be carved out; as glaciers returned, sea levels dropped and the landmass would once more connect Britain to the Continent. A second megaflood seems to have happened 160,000 years ago, only this time the gap at the Dover Strait was enlarged enough that it would never reform: Britain was now an island.

A full reconstruction

̽��ֱ��recently published Anglo-French study has provided the final piece of the puzzle by looking, for the first time, at what flowed out of the Channel into the Bay of Biscay during these crucial periods of Quaternary history. Corroborating the story that had emerged from the upstream sedimentary analyses and the bathymetric data, this new research demonstrates peaks and troughs in character and volume of the sedimentary material relating to the interglacial and glacial periods. Significantly, at the times identified for the megaflood, the scientists found that the volume and character of the sedimentary material vastly increased, consistent with massive surges in ice-rafted debris and sediment having swept down the Fleuve Manche super-river and out into the Bay of Biscay. With this new research, there is now a complete record that reconstructs these dramatic and far-reaching events that were to give Britain its island status.

Like any other large-scale geographical upheaval, the implications of the megaflood are profound. ̽��ֱ��large volume of freshwater released into the Atlantic would have caused changes in ocean circulation, with knock-on effects on the climate in an area stretching from the Bay of Biscay to North Africa. Flora and fauna (including early humans) would also have been affected: during interglacial periods such as the one we are currently experiencing, the high sea level would cut Britain off from mainland Europe, forming a barrier to the migration of humans, animals and plants alike. But in glacial periods, the water level would fall and, although there would never again be a continuous landmass linking Britain with Europe, the Channel River would provide a major migration routeway. In fact it was probably possible to wade across from France to Britain as few as 9,000 years ago, and this may yet be possible tens of thousands of years from now, when we enter the next ice age.

For more information, please contact Professor Phil Gibbard (plg1@cam.ac.uk) at the Quaternary Palaeoenvironments Group (www.qpg.geog.cam.ac.uk/) in the Department of Geography or Sam Toucanne (Samuel.Toucanne@ifremer.fr). This research was published in Quaternary Science Reviews (2009) 28, 1238–1256 and Nature(2007) 448, 259–260.

̽��ֱ��latest instalment of a 20-year study to understand how Britain became an island completes a tale of megafloods and super-rivers.

With this new research, there is now a complete record that reconstructs these dramatic and far-reaching events that were to give Britain its island status.

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