Arctic methane seepage: since the end of the last glacial period to the present day – a talk to the public at the University of the Third Age (U3A), Flavel Art Center, Dartmouth

My name is Frances Cooke, I am a PhD student and I live in Tromsø. I work within a project called SEAMSTRESS which is affiliated with the CAGE institute at the Arctic University of Norway. CAGE is a centre of excellence for Arctic Gas Hydrate, Environment and Climate. SEAMSTRESS is funded by the Tromsø Research Foundation and the Research Council of Norway. Scientific outreach is an integral part of my work as a PhD student at the university of Tromsø. I am passionate about making science accessible to the public through public engagements like this talk today, so it is a pleasure to be here to give you an insight into what I do living in the Arctic.

Our main goal in CAGE is to study methane release from gas hydrates beneath the Arctic Ocean to understand the potential impacts on marine environments and global climate systems, while SEAMSTRESS investigates the processes controlling methane release at Arctic continental margins. I will show you in this talk how we go about achieving those goals.

First, where does methane come from? In the ocean (broadly speaking) methane comes from two sources. It is either: 1) biologically formed (biotic) – from the decomposition and subsequent burial of organic material, either in the top 10s of metres (biogenic) or 1000s of metres (thermogenic) below the surface. At greater depth we refer to the breakdown of organic matter as ‘thermogenic’ – as the process is thermally driven (temperatures are higher at greater depth) and sourced from hydrocarbons; or 2) formed by geologic processes (abiotic) through magmatic processes.

Gas is buoyant and always wants to move upwards towards the seafloor any way it can, it eventually reaches the surface of the seafloor, where it will escape into the seawater. That is, if it is not consumed by methane eating bacteria or converted back into gas hydrate – to name a few of the processes.

What is gas hydrate anyway? First we need to understand a bit about the gas hydrate stability zone – where does the gas hydrate exist? We can recover gas hydrate samples by core sampling. If we are lucky the core barrel penetrates into mud containing gas hydrate – and this is within the gas hydrate stability zone. It is generally (in my study area) in the upper 200m beneath the seafloor and around 400m above the seafloor into the water column (however in some parts of the world gas hydrate stability zones can be 500 to 600m – this is dependent on geothermal gradients – i.e. how hot sediments are with depth beneath the surface). In this setting temperatures are between 4 and 18 degrees Celsius. You may find it unusual that methane is frozen between these relatively high temperatures, but it can exist in this state because it is under pressure.

Once gas hydrate is recovered – this is what it looks like. Everyone who has recovered methane from a core sample has a go setting it alight. It is extremely satisfying to watch the ice burning slowly away in your hand.

Huge quantities of methane are stored worldwide as gas hydrate over continents in permafrost and over continental margins, and some deep sea areas in marine settings.

I will now take you on a journey, first starting in the Arctic, in Russia – Siberia, and Arctic Canada – Baffin island; then moving across to the Barents sea where I will talk about the early work CAGE undertook in the Barents sea. Then I will move out of the Arctic into the North Sea – a little closer to home (i.e. U.K) – where I will introduce the Tsunami that happened in the stone ages and its surprising connection to methane seepage. I will finish the talk by introducing a little about my PhD, my study area is located in a deep water setting – 1200m in the Fram Strait – between Svalbard and Greenland.

Time in geology is a bit like distance in astronomy – the numbers are so vast that it is difficult to make sense of them.

You are probably wondering what this is? It is just a stack of paper in the geology museum in Tromsø. It is made up of 22,700 sheets of A4 paper depicting the lifespan of the Earth. The final sheet of paper represents modern human existence.

In this talk we will also be focusing on a time period depicted by this last sheet of paper –the last 20,000 years, which geologically speaking is very recent times!

So what happened in the last 20,000 years? At 20,000 years we were coming to the end of the last ice age. During this time the ice was at its greatest extent, afterwards the ice started to melt and retreat. Then modern humans came into existence during a period called the Holocene period.

The Barents Sea methane release – that I will come to – started 12000 years ago and the North Sea submarine slide that caused a Tsunami event, happened around 8,200 years ago.

But first I will talk a little about what is happening today:

You’ve probably all heard about the permafrost melting in Siberia. Huge amounts of methane are expected to be released from permafrost this century because of warming.

Here is a photo of a village called Churapcha. This used to be an old airport runway but now the land looks more like bubble wrap. It is now a useless swampy field and it could eventually become a lake.

What you don’t see are these wedges of ice that form beneath the ground. They make up the permafrost. Each year the crack gets larger and over time the ice wedges become thicker. The black is an active layer that freezes and melts each year, but what is happening today, is that the white parts are also melting, releasing methane gas and causing the ground to subside in this bubble wrap pattern.

Now we move to Baffin Island – in Arctic Canada:

Here on this aerial photo subsidence has already taken place and the subsided land had since filled with water and the holes have become permafrost thaw ponds. The permafrost areas are vast.

The image to the right looks quite similar, but instead of looking down on the lad we are looking down on the seafloor – in the Barents Sea. Sonar data has been used to collect this information. The colour scale is water depth with blue the deepest and red shallow. There are clearly many craters here, some larger than a kilometer in size.

Now we focus entirely on the Barents Sea:

During the ice age the crater area was covered by a thick 2km ice sheet. When the ice sheets melted away, huge blow out craters formed. There was a sudden release (much like a lid being taken off a pressure cooker). The gas that had been trapped beneath the ice suddenly had somewhere to go, and it is still being release today (see flares on the figure).

What is interesting about this study is that we can use the melting of the Barents Sea ice sheet as an analogue for what may happen if the Greenland and west Antarctic ice sheets melt significantly in the future. We know that there are significant amounts of hydrocarbon gasses generated and stored beneath the ice there.

Now I am going to show you a reconstruction of the last ice age. This animated ice map was created by Henry Patton. He is an ice sheet modeler. The interactive graph in the top left shows the decline in ice volume in blue accompanied by an increase in sea level in green (from 20,000 years ago until 8000 years ago).

The animated map shows the Barents Sea ice disappearing 12,000 years ago, and it also shows the last of the ice disappearing from Norway around 8000 years ago.

This leads us nicely onto the next part – where I will speak about the North Sea Tsunami. It may have wiped out the last remaining part of Doggerland (which was Dogger Island) and the people living there!

Now onwards on our journey out of the Arctic and into the North Sea.

In 2006 I read a special report in the Independent newspaper with the title: ‘Tsunami horror hits Britain.’ This was not long after the Boxing day tsunami so it really made me want to read further and find out what this was all about.

The purpose of the article was to catastrophise what would happen in the year 2060 if nothing was done to prevent climate change, but in reality the article is slightly misleading. The methane bubble was not to blame for the catastrophic seabed slide, it was an effect rather than a cause. The cause of the Storegga slide and tsunami – or the trigger (however it is still debated) was an earthquake.

In the last 2.7 million years there have been cycles of cooling every 100,000 years, during these times ice ages took place.

The first cycle shows a deglacial period when there was a high amount of sediment loading (blue) – the blue sediments are deposited very rapidly and as a result are unstable (they haven’t had time to compact). The sediments are weak and act as a sliding plane. The second part of the cycle shows the ice sheet at its maximum extent, weighing down the continental margin. When the ice melts (stage 3) there is no longer a weight on the continental margin, and the land starts to rebound. This is when the earthquakes happen. So the slide was most likely triggered by a strong earthquake in the area and the unstable sediments on the slope then disappeared with the slide 8200 years ago.

So what about today? It is unlikely that another tsunami will happen at the Storegga slide area. Why? Because a new ice age with infilling of glacial sediments on to of marine clays in the slide scar would be needed to create a new unstable situation.

This illustration nicely summarises what happens once the earthquake has destabilized the sediment.  The debris flow slips on the sliding plane (refer to blue sediments in previous slide) – made up of glacial marine muds. Within this layer are gas hydrates that are released, further triggering disability and sediment slip.

Now to the final part of the journey – we move back to the Arctic, into a deep water setting, 1200m beneath the ocean in the Fram Strait. Here I will  introduce my PhD project.

The title of my PhD project is ‘Near surface (upper 300m) characterization of faults and fluid flow systems (i.e. the plumbing system beneath) at Vestnesa Ridge – east Fram Strait.’ My main research question is: ‘Why is gas seeping on the west of the ridge but not on the east?’ My aim: is to ‘Identify fluid (gas) flow features and structures that show sediment displacement across the ridge.’ My method/data used is: Seismic data (We use seismic data which is a sort of remote sensing that acquires images of subsurface features using technology that sends out a signal and records the signal that has echoed back. If you think of a baby scan, this is a similar technique.) My preliminary conclusion: ‘There is a spatial variation in the rate of sediment deposition across the ridge which may impact the density of small fluid flow escape features.’

How do we know what the plumbing system looks like beneath the seafloor? Here we have a seismic line across the sediment drift (Vestnesa Ridge). Beneath this structure, gas has accumulated and it seepage from one half of the structure (the east) – the ‘active’ area. Craters at the surface (aka pockmarks) give us clues about what is happening beneath the seafloor. Displacements (vertical black lines) in the seismic data beneath the coloured seafloor topology map (aka bathymetry) connect the free gas (arrows point to gas accumulation) to the surface of the seafloor.

I am investigating two different mechanisms controlling the gas seepage in the active and inactive part of the ridge. The ‘inactivity’ in the west refers to a drop in high methane seepage activity in the last few thousand years. A different process controls the release of methane in the west to the continued (last 2.7 Million years) high flux activity in the east. The process is referred to as low flux, and is much less studied. My current project focus is to investigate low flux seepage activity at the west of Vestnesa Ridge.

On the left (final slide) is my actual data within an illustration and to the right a conceptual model. Using seismic data I am documenting sediment deformation events that are confined to specific layers where pressure build up preferentially occurs. Gas may not be able to travel upwards because of different layer properties that act as seals. Gas may either travel horizontally in the trapped layers or form gas ‘turbation’ structures. Gas builds up in local pockets, mounds form and cracks form above the mounds. When the mound eventually collapses a crater is formed at the surface and the gas that is trapped locally escapes.

In the west the craters are preserved in clay sediments that are more likely to open under much less pressure. They are buried over time and we can observe them 100s of metres beneath the seafloor, as well as on the seafloor today. This tells us that the same processes have been occurring for millions of years.

We also know that the region is active with earthquakes because of the proximity to the mid-Atlantic ridge. Earthquakes can also facilitate movement of gas and gas escape in sediments that are extremely sensitive to even the smallest pressure perturbations. We can also factor in sediment deformation on account of post glacial adjustment. As a result of all these processes, sediment deformation is highly varied, and distributions of fractures are extremely intricate. The details are sometimes masked by disturbances/noise in the data, however the disturbances often provide clues to fluid movement.

To conclude: Gas hydrate is a frozen, naturally occurring and highly concentrated form of methane. Significant quantities of methane are stored worldwide as gas hydrate over continents in permafrost and over continental margins, in marine sediments where water depth exceeds 300m. The Arctic is a particularly sensitive region to methane release, widespread disappearance of Arctic near-surface permafrost is projected to occur this century because of warming.

Could decomposition of gas hydrates through increase in ocean temperature trigger abrupt climate change? This is debatable, but unlikely, particularly in deep water settings, given the depth of the gas hydrate stability zones. Bottom water temperature variations affect gas hydrates shallower than 1.6m below the seafloor – this is not deep at all! We should be most concerned about the melting of the permafrost and shallow water settings.

Will there be another North Sea tsunami? I consider this unlikely at the Storegga Slide area, at least within our lifetime. The Storegga type event takes place every ca. 100,000 years and is controlled by glacial cycles. However, we can only reanalyse when the next major event happens. Earthquakes can easily generate tsunamis, but you also need large quantities of sediments for a major event to happen.