The Mid-Atlantic Ridge

At the northern end of our travels we are looking at the Bear Island Fan. This is one of the largest packages of fan sediments in the world. It extends from the continental shelf just south of Bear Island westwards to the Mid-Atlantic Ridge (MAR). We wanted to see if any of the debris flows that make up the fan reach the centre of the MAR. This gave us an opportunity to survey along this dramatic structure to see if there was a suitable site to sample. First as we approached the ridge we could see on the 3.5kHz pinger records blocks of rock protruding through the well layered sediments. The multibeam showed that these blocks were parallel to the MAR and increasing in height and extent as we moved towards to the actual ridge.

Then we reached the ridge itself. As we steamed along its axis with Pelagia’s  starboard side above the European Plate and its port side above the American Plate the multibeam display was a confirmation of all those sketch diagrams on plate tectonics explaining the formation of oceanic crust at spreading centres, with each line of multibeam data added to the swath showed ridges parallel to the axis or evidence of volcanic activity. The ridge exhibits sudden large changes in bathymetry that makes following the seabed on the 3.5kHz pinger involve frequent changes of the display offset. It showed the rocky seafloor but we could see no thick sequences of sediments so we moved away to investigate the debris flows at the edge of the Bear Island Fan only a short distance to the east where we recovered a 10m long core with a very good record of the Bear Island Fan. 

Image from the Swath multibeam onboard the RV Pelagia of the Mid-Atlantic Ridge.

David Long

Trænadjupet Slide

So we've heard about the Afen Slide and seen some of the great cores from the Trænadjupet Slide, as well the wide array of sediment types that exist within them. Here's a little background on why the Trænadjupet Slide is important and what the cores might tell us.

Extent of the Trænadjupet Slide (Courtesy of SpringerLink)

We know from recent submarine landslides (Grand Banks, 1929), and from the geological record (Storegga, ~6000 BC), that submarine landslides have the potential to cause tsunami evens similar to the 2004 Christmas tsunami in the Indian Ocean. Despite this tsunami-causing potential, the Trænadjupet Slide appears to have left little or no evidence of a tsunami on nearby coastlines as one might expect. Tsunamis often deposit marine sand much higher up on land than normal coastal processes and thus we have a record of the event stored for us to study. There are some potential deposits being explored, but there is still some debate about the significance of these deposits. Much of the uncertainty arises due to a lack of sufficient age-control on when the Trænadjupet Slide took place. Uncertainty also surrounds how fast the collapse happened and how fast it moved.  Fast moving landslides will tend to have more of an impact than those that move slowly or those that fail in multiple smaller stages. 

Schematic of how underwater landslides can cause tsunami waves (Courtesy of IranPetroTech)

So, where do the cores come in? Cores will be used to radiocarbon date marine background sediment which has accumulated on top of the landslide debris, and help constrain how old it is. Cores also allow us to look at the internal morphology of the slide and how the debris is distributed both spatially and in time. If the slide took place in numerous small stages rather than as one large failure, it may not have had enough energy to create a tsunami. The cores and radiocarbon dating will help us to better understand why this slide might not have generated a tsunami which will assist in assessing the tsunami risk to countries with nearby continental margins. 

Tsunami deposits within terrestrial soil on The Shetland Islands

Josh Allin

Progress in the Far North

After a successful coring campaign to date, and the continuing excellent weather, we are now moving towards our most northerly site at 74° north. We have now collected over 300m of piston core from a range of sites in the Nordic Seas, and are now focused on collecting samples that will help us to understand the timing of deposition of sediments on the Bear Island Fan. This feature is one of the largest depositional formations on earth, and we are hoping to understand the timing of sediment delivery to the slope, and hopefully this will inform our research on slope stability.

The weather has made a turn for the worse, we currently have 5-6 meter swells and the temperature feels truly "Arctic" today, so much so that we experimented with the survival suits and the padded thermal boiler suits whilst working on deck this morning!

The night team in their warm gear

Enjoying the spray at the end of our shift

Glorious Mud.

We have been coring in the Traenadjupet region, and have seen some fabulous examples of a range of processes recorded in the mud. We are hopeful for some great results from these cores, though this will take some time to work up, but in the meantime, here are a few examples of some of the wonderfully colourful and exciting mud cores we have taken recently:

This layer has been described as the Pistachio Green horizon after its distinctive colour. This was the first core we found it in, though it has appeared in several others. The exact chemistry and mineralogy of the layer will be part of the post cruise work, but this is an unusual colour, and will unfortunately fade over time.

This very unusual deposit looks remarkabaly like a butterfly, but is in fact a series of mud boulders that were caught up in a submarine landslide. The mud would have been partially consolidated at the time, and the layers ripped up and rolled into boulders, which we have then cored through.

This picture is a close up of one of these contorted mud boulders, and shows that the sequences of colours is symmetrical across the pale brown horizon. When interpreted as having symmetry, andthe fact that these layers terminate against the side of the core, we can be sure that they are boulders of mud and not an artefact of the coring process, which can sometimes cause disturbance to the layers, as seen below:

The darker mud visible in the centre of the core is running from top to bottom, and there are very few processes that can produce this type of feature. It could be bioturbation, the traces left behind by burrowing fauna that are infilled by sediment, but in our current location in very deep water, this is too large a burrow to be possible. This is unfortunately a product of suction at the base of the core, which can cause mud from the base to be injected upwards into stratigraphically higher layers. This limits the usefulness of the core, but is normally confined to the lower sections.

The unusual black block in the lower core is a boulder of peat that was caught in the flow. It is surrounded by clasts of mud set in a sandy matrix, typical of the type of deposit left by a large submarine landslide. The peat boulder is spongy and soft, and will be useful for us to determine where the flow originated as it should contain macrofossils and pollen.

This wonderful striped section is from a deep basin core, and each of the layers represents a very distal deposit from the submarine landslide. Cores like this one have the deposits separated by a thin section of hemipelagite: "normal" marine sedimentation whcih contains forams and potentially other material that will allow us to date each event. These long basin records are one of the key objectives of the project, areas that capture a long record will allow us to assess how frequently landslides happen, whilst the cores taken on top of the landslides are helping us to understand what makes some landslides tsunamigenic or not.

This final picture shows some very pretty laminations, couplets of sediment that will be part of our research into sedimentation patterns on the margin. They appear only in certain locations and within small sections of core, and are potentially the result of seasonal/cyclic changes in sediment source or the energy of the current.

Millie

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

 

Progress so far…

We have had a really successful coring campaign so far, after three days of fairly intensive activity we have collected over 20 cores, from a variety of sites. One of our first objectives was to core both inside and outside a set of large cracks close to the Storegga Slide, these features are over 100m wide, and have not yet been reliably dated. The cores we retrieved we quite spectacular, with abundant carbonised material and several distinct horizons that will allow us to correlate the cores, and refine the date at which these cracks formed. 

James and Matthieu assessing a core during the night shift.

As we moved north over the Voring Plateau, we crossed a large pockmark field, which is the subject of current research at NOC. We managed to collect two 12 m cores here, one from within a Pockmark and one from the adjacent sea bed. The pockmark core still contained small amounts of methane clathrate when it was cut on deck, nice to see as we had expected in to have dissociated before reaching the surface, however, they have a particularly unpleasant to smell!

We are currently in the Traendajupet region, looking at the nature of the landslide here, and hoping to collect a variety of data to help us understand how the landslide moved, when it happened and how much material was involved.

Life on board has settled into a steady rhythm of shifts, to maximise our time at sea we are split into two teams, the night shift run s from midnight to noon, and day shift for the opposing 12 hours. We have three meals a day, though the night shift tends to sleep through dinner, and the same for the day shift at breakfast. The chef of board is fantastic, and we have been spoilt with food, our main meal is always at lunch time, normally a soup, main and desert, and he leaves the night shift something tasty in the fridge for when we get hungry in the very early hours of the morning, the lasagne was definitely a high point!

We have a daily science meeting at midnight on the shift change, to catch everyone up on the plans for day, where we are surveying and how many cores to take in each location. We have several intense days of work between now and Wednesday, and though we have been fortunate with the weather so far, this is likely to change on Sunday night as a storm heads our way, however we are not expecting anything too severe.

The plan for the next week is to continue to the northern edge of our study area, hopefully with as much good luck as we have had with the cores to date, before heading back towards Iceland, then back into Bergen for the 31^st. 

Big things in small shells

Contrary to popular belief, euphemisms, idioms and trite sayings often have little relevance to real life. Foraminifera on the other hand, lend credibility to the notion that big things do indeed come in small packages.

Foraminifera are small single-celled marine organisms; more specifically known as Protists. They exist at various depths within the oceans but are most commonly found in shallow waters above 50 m. Many feed on small marine plants and other detritus, but some are carnivorous and feed on other smaller species. They have lived in the oceans for millions of years and are found throughout every ocean on Earth. This fact makes them remarkably useful in studying ancient oceans.

Many species construct their shells out of calcium carbonate (CaCO₃), while others construct it using fragments of other shells, or even grains of sand. The CaCO₃ which makes up the shell contains a record of a large number of environmental variables which can help us understand the past ocean. Carbon and Oxygen both have 2 stable isotopes (¹²C, ¹³C, ¹⁶O and ¹⁸O respectively). The record of past changes in these isotopes locked in these Foraminifera shells can be used to infer changes in water mass distribution, changes in ice volume on land and nutrient distribution in the oceans. 

Aside from basic stable isotopes, Foraminifera also contain record radioactive isotopes. One of these isotopes called radiocarbon (¹⁴C), is particularly important to paleoceanographic studies. When Nitrogen 14 (¹⁴N) in the upper atmosphere is bombarded by incoming solar radiation, it gains a proton and becomes ¹⁴C. This radiocarbon diffuses into the oceans and is taken up in small amounts into the shells of the Forams. By measuring how much of this radiocarbon is left in an ancient shell we can know its age and the age of the sediment which surrounds it. This helps us tie down the ages of important ocean events like landslides and ice-sheet break-up.

Several additional proxies exist including Uranium decay series elements. These are used to detect the source of water masses by using their chemical fingerprint to trace the source of their terrestrial suspension material. Even the numbers of Forams can help us. Certain species thrive in cold water while others prefer the warm waters of the sub-tropics. Using the % of these different species in a sediment sample, we can compare this past assemblage with modern ones and infer sea surface temperature changes that help us understand de-glacial events

To sum up, these guys may only be the remains of tiny dead critters, but they provide us with a immensely powerful tool in our quest to understand the oceans and their role in the changing climate.

Josh Allin