The triggers

68.5^oN 8^oE

Notes: Sun has now not set for three days, although the weather has been overcast for the last two days. Graveyard shift currently leads the daytime shift by 17 to 11 cores, not that we are counting. Alessandro is pretty poor at darts. There is a storm coming!!!!

Core Puns:

He who dares.....Cores

The Good, the Bad and the Corer

Cora! Cora! Cora!

The Italian Core

Return of the Corer

Cool Coring

The Beauty and the Corer

101 Cores

Core Story

Harry Potter and the Half Sand Core

The Core

Reservoir Cores

One of the purposes of the 2014 Pelagia Cruise is to collect information relating to the triggering of submarine landslides. Many possible triggers for submarine landslides have been identified. These include earthquakes, rapid sedimentation and gas hydrate dissociation. Working out the specific triggering factor behind individual slides is, however, extremely difficult. Which factor has preconditioned the slope to failure? Which factor has triggered the actual slope failure? Today I was covered in 4000 year old mud by certain members of the night shift (Camilla Watts), what the actual reason for this is unclear [Editorial note: it was well deserved]. Was it the fact that I was inanely putting the word core or corer in film and book titles for two hours? Was it because I thought Harry Potter deserved to be kicked in the shin or because I have never watched or am not likely to watch Game of Thrones? Or was it because I have been talking constantly about ball sports for the last year? Or was it that she randomly got bored and decided it was a good idea?

Although a submarine landslide will not be triggered by a poor pun this example indicates the complexity of the question that we are attempting to answer. Many of the submarine landslides around the Norwegian Basin are closely associated with the numerous trough mouth fans which line the continental slope. These features are produced by ice sheets. Glacial ice is a much more effective erosive agent of sediment and bedrock than rivers and is therefore able to deliver extremely large volumes of sediment to the continental margin very quickly. Rapid sediment loading from ice streams (areas of extremely fast flowing ice) is thought to lead to high pore pressures and instabilities which could lead to failure. Similarly dissociation of gas hydrates, an ice like crystalline structure, into their gaseous constituent parts can also generate high pore pressures which could lead to failure of the sediment. Dissociation of gas hydrates can be caused by a number of factors. These include pressure changes related to changes in sea level and temperature changes caused by ocean warming and cooling. Whilst these factors could trigger a submarine landslide, they could equally precondition the slope to fail.

Where rapid deposition of sediment gas hydrate dissociation has generated a slope preconditioned to fail a further trigger may be needed in order to actually achieve failure. In many cases this trigger is an earthquake. Shaking of the sediment can lead to a loss of structure and subsequent failure. Large magnitude earthquakes are currently relatively rare around the margins of the Norwegian basin. However, as we moved out of the last glacial period earthquake magnitudes in this region increased in response to glacial unloading of the crust as the ice sheets retreated. These earthquakes may therefore have represented a common trigger for many events. Despite the increase in magnitude of earthquakes associated with crustal rebound, not all earthquakes will cause slope failure to occur. Some might even lead to a strengthening of the sediment and reduced likelihood of slope failure.

I hope from this post that it is clear that isolating an individual trigger for large submarine landslides is extremely difficult and presents one of the main questions to be addressed as part of the landslide tsunami project. Only with precise dating of the landslides will we increase the possibility of linking landslides to individual triggering mechanisms, although this may in fact be impossible. Who knows?

Ed Pope

 

A-Level Science and Geography Post

As part of some outreach in June, the cruise team took part in a visit day from a Somerset sixth form college, Richard Huish, who came to the National Oceanography Centre for a series of talks about our on-going research. As part of their visit day, they had a lecture from Dr. James Hunt on the history of landslides from the Canaries, and a visit to the core store (BOSCORF) to see several cores and get an overview of how we interpret landslide deposits. This post is aimed specifically at students during their A-Levels, and hopes to explain the science we are working on within the context of the A-Level syllabus. If you are taking your A-Levels at the moment, please take part in our Q and A on the “Chat to the Team” post; we would love to hear from you!

Part of the A-Level geography syllabus covers tsunamis as a hazard in addition to climatic hazards that affect the UK. Though most case studies focus on the recent earthquake induced events in Indonesia and Japan, though it is worth remembering, that over the Holocene (the most recent geological time period spanning 12000 years ago to present), several landslides have occurred on European continental margins that had the potential to generate tsunamis that would affect the UK. The Arctic Landslide Tsunami Project is playing a key role in working out how much of a hazard submarine landslides pose, and when or if they are likely to occur.  

Shot of dawn from the Pelagia while winching back the CTD (Conductivity, Temperature Depth: measures the characteristics of the water column)

One of the biggest research questions of the Arctic Landslide Tsunami Project, is to assess the link between when these landslides occur, and the climate at the time. This is largely driven by one of the biggest coincidences in timing between two events that occurred approximately 8200 years ago: the Storegga Landslide, and the 8.2 ka BP cooling event.

The 8.2 event was the last of the major climatic shifts to occur, though there have been several others (the Younger Dryas event is a case study within the Climate module, during which time half of the deglacial warming occurred in year (almost 10-12°), at 8.2 ka, a 5.4-11.7° C drop in temperature over Northern Europe occurred in less than 10 years). The 8.2 event is interesting to climatologists as it occurred during a period of relative warmth and stability. During glacial periods, there are numerous records of rapid and large climate shifts known as Dansgaard-Oeschger cycles, which follow a pattern of slow cooling and rapid warming, but few rapid climate shifts are known from the warmer interglacial periods.

Why this matters, is that we are currently in a warm period, not too different to the conditions just before the 8.2 event, and we need to understand not only what triggered the event, but also, the other hazards that were potentially generated by it. The widely accepted theory for the cause of the 8.2 is that an ice dam that had been holding back a large volume of very cold fresh water, generated by the melting of Laurentide Ice sheet (covering North America during the last glacial) suddenly broke, and released this water into the North Atlantic. The North Atlantic is one of the most important components of the global climate system, as the formation of deep water in the Nordic Seas and to the south of Greenland helps drive the northward flow of warm water held within the Gulf Stream that keeps the UK nice and warm.

The second event, the Storegga Landslide, is the largest known and dated submarine landslide in the North Atlantic, and has been placed at 8.15 ka BP. Though this is a hard date to refine, it falls exactly within the coldest period of time recorded in the Greenland Ice records (8.16 ka BP). The landslide generated a tsunami that was 10 m high when it reached Scotland and the Shetland Islands (comparable in height to the two recent tsunamis), and tsunami deposits have been found along the Norwegian coast and as far afield as Greenland. The landslide itself moved enough sediment to cover all of Scotland in a 100 m thick layer, and an event of this size today would cause significant damage to UK industry and infrastructure, and represent a significant risk to the large oil and gas operations in the North Sea (The headwall of the Storegga Slide is very close to one of the largest complexes: the Ormen Lange field, which was subject to a comprehensive assessment of stability and landslide frequency in 2005 before operations began).

Landslide events are recorded as turbidites, distinctly different layers of silt or fine sand in an otherwise muddy (hemipelagite) background, by looking at the nature of the material in the turbidites, its size, chemical composition, how well sorted it is and the structures it shows, we can tell where the landslide came from, how old it is and whether or not it happened in one big slide (likely to generate a tsunami) or in several smaller slides from the same region (less tsunamigenic potential, but still likely to cause a hazard).

The key question for my PhD, is looking at the timings of these two events, in order to determine if there is a relationship between them. Did the cooling cause the landslide, or did the landslide contribute to the cooling? Are landslides caused by rapid changes in the oceans? If so, are we more likely to see one happen with contemporary global warming?

These questions can only be answered by heading to the deepest parts of the Nordic Seas, the Lofoten basin, and to the parts of the ocean floor that sit directly beneath the deep water currents. This current is generated by the sinking of water in the Nordic Seas, where it splits and part heads north along the Voring Plateau margin towards the Barents sea, and part heads south over the Iceland Scotland Ridge, a shallow sill of 800 m water depth where we are hoping to collect a core that records the strength of this current, and any landslide events that occurred over the Holocene.

If you have a question, A-Level student or not, please feel free to join in the live chat next week, and keep an eye on our other social media streams:

Twitter: #ArcticSlides

Pinterest: http://www.pinterest.com/millie0692/arctic-landslide-tsunami-project-pe391-cruise/

Millie

Mud, Silt and Sand

So, when I said to a couple of people that I was heading off on a research cruise to the Norwegian Sea, I got responses of 'Oh how lovely, the fjords are supposed to be beautiful'.  I am sure that the fjords are exceedingly picturesque, but the likelihood of seeing them is a prospect that might only happen from a helicopter, and hey, let's not go there!  We, the team of ten scientists from the National Oceanography Centre, Southampton, Swansea University, BGS Nottingham and BGS Edinburgh are going to do something even more exciting... We are going to come face to face with mud from the sea floor that is thousands of years old, and which flowed down the continental slope (the rise between the deep abyssal plain and the shallow shelves, the ancient relict of palaeo ice margins) as mighty bulldozers in the deep – large submarine debris flows and turbidites.  But why is this exciting?, I hear you ask. Well, let me tell you why.

Mud, silt and sand (aka. sediment) are deposited in layers on the sea-floor.  They form from admixtures of organic and inorganic matter (soft and hard parts of living organisms), and sediments that either settle through the water column that could have been derived from rivers, or remobilised by deep ocean currents.  These layers of mud build up over time and often contain fossilised remains of past environments.  Protists (animal-like, single celled organisms) called foraminifera have a hard calcitic (the same substance that limestone is made from) test (shell: see below) and can be found today pretty much in every ocean basin.  These live either at the surface or and the bottom of the ocean.  Their tests take on the geochemistry of the ambient sea water as they grow and different species, which are distinguished by their different shapes are fussy about what temperature of water they live in.  Information on the ocean environment can also be obtained from the physical characteristics of the sediments themselves, which may give clues to the ice-sheets that abutted the ocean basins.  Therefore if we core through this layer-cake of sediment, we can obtain a record of environmental change in the ocean through time (see next blog post for more details). 

Scanning Electron Micrograph of a Planktonic Foraminifera (Photo J. Stanford)

Occasionally, these layers of sediment can become very thick due to high rates of sediment being delivered to that particular area. One such site was just west of Norway, not now, but around 18 – 14 thousand years ago.  During this time, the ice-sheet that covered Norway during the peak of the last ice-age was melting rapidly as the air temperatures in the Northern Hemisphere started to warm.  Ice contains large amounts of sediment, which have been worn away from the bedrock that the glacier once flowed over.  Today in the Arctic, ice melt tends to happen during the relatively short summer season, and in the past, mixtures of meltwater and sediment would have been injected into the Norwegian Sea as highly sediment laden plumes.  The sediment may have entered at the surface, or may have been injected as periodic, highly dense flows.  These high density flows which form from a steady pulse of sediment laden waters are called hyper- (super) pyncal (density) flows.  Sediment from the last deglaciation (18-14 ka BP or so) accumulated in piles, which today is seen as a <25 m thick sediment drape over much of the Vøring Plateau.  These sediments became unstable over time due to one, or a combination of three key factors (1)  due to gravity (just like when you are digging a hole in the garden and eventually the sides become so steep the mud collapses back in), (2) the shear weight of the sediments causing loading of the thin rigid crust that covers the Earth, triggering earthquakes and/or (3) organic matter, trapped within these sediments started to decompose over time within these thick deposits, releasing gas that escaped through the sediment pile.  Eventually, a crack would have propagated from deep within the sediment pile, all the way to the sea-floor, spawning what is known as a submarine 'gravity flow'.  Depending on whether the sediment flowed downslope as one block, or whether it disintegrated into a much more fluid flow, defines whether these flows as a debris flow or turbidity flow, respectively (see http://www.youtube.com/watch?v=krVkYvJI-PI for a visual demonstration of what a turbidity flow looks like and future blog post).

As these sediments fail and rush down the submarine continental slope, they displace the sea-water around them, giving rise to the possibility of powerful and destructive tsunamis (a recent example of this can be seen in Lituya Bay).  One such failure on the Norwegian margin, is known as the Storegga Slide, which mobilised around 900 km3 of sediment and occurred around 8 thousand years ago. It is thought that there are tell-tale traces of this large tsunami that resulted from this failure as far afield as the Shetland Islands.  However, the exact timing and nature of this event is still unsure, despite decades of research. Other slides include Andøya and Traenadjupet (~4 thousand years ago), and Nyk (~16 thousand years ago), precise ages are also still unsure for these.

There is a need to know what caused these large failures in order to mitigate against future catastrophic events, since large accumulations of these sediments still exist on the Norwegian margin today, as a relict legacy of the past cold climate that persisted between throughout the last ice age.  What makes Storegga even more interesting is that fact that its failure roughly coincided with an extreme cold snap in Northern Hemisphere temperatures around 8.2 thousand years ago and therefore, we need to untangle whether abrupt climatic change has a role in destabilising the sediments.  Given that global temperatures have on average risen by 0.72 degrees Celsius since 1951 (IPCC, 2013), and that this change is not uniformly distributed, with enhanced warming in the polar regions (a process known as polar amplification), increased urgency surrounds this need to discover the mechanism behind these potentially catastrophic events.  A series of other, much smaller, but still large failure scars can be seen on the Norwegian sea floor and previously recovered cores of sediment have dated these events as having occurred at intervals during the current warm period, the Holocene that followed the last ice age.  So, we are heading to these sites to try to discover (a) how and why these large failures occurred, (b) when these failures occurred and (c) what were their impacts.  That is why we are excited about going to the Norwegian Sea for some very hard graft, but some really rewarding returns!

Jenny.