Logbuch SONNE 251
3.10.2016 – 2.11.2016
Yokohama to Yokohama
Extreme events Archived in the GElogical Record of Japan’s Subduction margins (EAGER-Japan)
At the southeastern edge of the Eurasian Plate, Japan hosts two of the most interesting subduction systems, both prone to devastating megathrust earthquakes: The Japan Trench (JT) east of Honshu with subduction erosion/subsidence in the north, and the Nankai Trough (NT) with a huge accretionary prism in the southwest. In 2011 the Japan Trench area was struck by a Magnitude 9 earthquake that caused unusually large slip all the way to the trench, and a series of landslides which are believed to be partly responsible for amplification of the catastrophic tsunami following the event. At Nankai, frequent Magnitude 8+ earthquakes are also documented, and landslides and other sediment remobilization processes related to seismicity are attested.
The overarching goal of R/V Sonne cruise SO251 and subsequent post-cruise research is to investigate fluid- and sediment mobilization processes by mud volcanism, earthquake-triggered seafloor displacement, submarine landslide and related “paleoseismologic event deposits” and to compare inferred earthquake processes and rates along accretionary vs. erosive subduction margins of Japan (Nankai Trough and Japan Trench, respectively).
The cruise is divided into two parts: We will first conduct mapping and coring along the deep Japan Trench and along the upper slope and forearc escarpment from 36°N to 40°N. After a short mid-cruise port-call to partly exchange the science crew and uptake ROV and ROV team, we will then continue to the Kumano Basin in the Nankai Trough working area to pick up instruments in the mud volcano field by ROV followed by further mapping, coring and detailed video surveying of seafloor in the shallow splay fault area.
Post-cruise science will be coordinated among the international science party. With SO251 we will establish a bathymetric and sedimentary inventory of mass-movements, chronology of extreme-event deposits and their paleoseismologic interpretation. It further will investigate geotechnical aspects of earthquake-triggered mass wasting processes to allow for quantitative interpretation and comparison of the established event catalogue. We also will illuminate the relationship between mud volcanic activity (mudflows, seepage, etc.) and local seismicity by using long-term data, time series samples and additional core/data from the cruise. Anticipated conceptual advance in our understanding of sediment dynamic processes related to subduction zone earthquakes are expected to be transformative and may well apply for other active convergent margins worldwide, not just Japan and the circum-Pacific.
Yesterday was our last working day in the Kumano Basin, which we exploited by additional coring and geophysical acquisition. In parallel packing of our equipment had to begin because RV SONNE was already visited by the pilot at 5 a.m. and was tied to the pier before breakfast.
For the entire day, both the main deck and laboratories, got extremely busy with people packing gear while a large Japanese crane was lifting crates and containers off the vessel and delivered a reefer (i.e. chilled container which is shipped at 4°C) for our cores and fluid samples. Both our Japanese colleagues, the Kiel ROV team and MARUM scientists packed their materials to the various boxes and containers very quickly given that the next day is a holiday in Japan. In the evening, everybody was exhausted but happy about the successful cruise after a month at sea and looks forward to be at home soon.
The major part of the research will start in only 5-6 weeks from now, when the containers will arrive back in Bremen and sample analyses starts in the various laboratories in Bremen, Innsbruck and Japan.
On Halloween we set out for the final dive of this cruise, where the only remaining MeBo borehole observatory was to be found and recovered. Given the problems to loosen the hotstab connection of the instrument during an earlier dive, the PHOCA pilots prepared everything for the possibility that the entire MeBo drill rod has to be disconnected. In a laborious operation of unscrewing, grabbing and hooking at the 1.5 m long pole, we released the assembly after four hours bottom time. This success took a large burden from many of us on the vessel and is the highlight of the outstanding performance of the GEOMAR ROV team.
We are now approaching the final stations to work at and look forward to the journey home after four weeks of operations.
Our hydroacoustic surveys during the two previous nights revealed a significant number of flares which were previously unknown. These fluid seepage indicators attest the hydrogeological activity of the entire study area. As a result of the departure of the Japanese technician, we are not in a position to use the JAMSTEC piston corer any longer. We thus improvised and mounted a corer to the heat flow probe. First, the heat flow probe penetrates the seafloor and somewhat later the corer eases its way to the sediment. With this assembly we can now explore whether there are elevated temperatures in the flare areas and also take a core to sample the fluids (gas, but also deep-seated waters).
So far, we used the „new“ instrument in three locations successfully with core recoveries of up to 2 meters. In the meantime, the ROV PHOCA team was busy to set the stage for their final dive on this expedition. Stay tuned.
With the largest of the observatories we were aiming to recover during this cruise safely on deck, a large piezometer probe, we are approaching the weekend. There are, of course, no weekends on research vessels since work continues 24/7.
At the moment, the Japanese weather holds many major surprises for us: heavy rain and sunshine as well as changing winds from various directions, so that we have to exploit very short fair weather windows to deploy the ROV. While we were successfully recovering the SmartPlug piezometer today, we were even more lucky yesterday when two instruments were recovered at night. PHOCA unscrewed the so called MeBoPlug from the MeBo drill string set in 2012, which contained a 4+ years time series of data that will help unravel the relationship between mud volcanic activity and seismicity.
While we are alternately measure heat flow, dive with ROV or acquire hydroacoustic data, shipboard routines by the other crew continue. Last night we safely brought the first MeBo borehole observatory back with ROV PHOCA in splendid weather – today we have stronger winds and deployed the heat flow probe instead.
Because the fresh air and working hours enhance our appetite, Andre Garnitz and his team work hard to have food available at all times. Up to more than 75 bread rolls are baked on RV SONNE each day, and since the cook has many different tasks, he is assisted by a robot which appears to be a R2D2 look-alike. The dishes on SONNE are fabulous – sometimes one even forgets that this is not a restaurant but a working platform.
Yesterday, the ROV control van burnt down. Once a week, there is a safety drill, and luckily, the fire was just the theme of the security measure on RV SONNE. Both science team and regular crew of SONNE participate into this exercise.
At first, everybody gathers at the muster station once the earsplitting alarm invades every corner of the vessel. Each person is then registered and assigned a life boat.
Whilst the scientists are excused afterwards, the crew has to follow through the scenario and, thus, masked crew members in full PPE wander over the deck and along the corridors as a fire brigade.
At that point, we were already back in the lab to continue the heat flow survey over mud volcano MV1. As already anticipated from the short, deformed piston core on this hill, we had trouble penetrating the carbonate crusts with our instrument.
On the way back into our study area we had the opportunity to hold a science meeting during which all the different groups on board can present their results to their colleagues. In short presentations the experts in geophysics, sedimentology, geochemistry, rock mechanics and other fields presented detailed observations, which were then condensed into a more comprehensive understanding of our findings.
One of the most challenging tasks was to tie the relatively short mud volcano cores into the larger picture of the Nankai Trough geological evolution. Opening a less than one meter long core from mud volcano MV1 was particularly exciting, because we suspected hard rocks given that our coring device got partly damaged during penetration of the seafloor. In the split core we discovered numerous hard carbonate rocks of several centimeters thickness, which originate when methane gas precipitates on the seafloor. By doing so, some of the geochemical information is preserved in these "frozen“ pore waters. They hence witness temporal changes in the geodynamic eolution and allow us to reconstruct the mud volcanic activity in the past.
The majority of these clasts appeared to be pieces of chimney structures along which methane migrated upwards before it got oxidised. Unfortunately, we now have to wait for detailed analytical results after the cruise before we know the exact depth of origin of these fluids.
After a tour de force on Sunday where we took a final four piston cores, we are now near Yokohama to meet a pilot boat. Here, we drop off our Japanese coring technician, who unfortunately has to leave early to attend a different expedition.
Coring was difficult yesterday because the sites were located in the Kuroshio current, which reaches velocities in the order of four knots at the surface. As a consequence the piston corer is deviated along the wire and ends up a few hundred meters behind the ship when reaching the seafloor. A precise coring traget can hence only be met when manoeuvring on the bridge is brought to perfection, in particular since there are numerous seafloor cables for tsunami early warning offshore Japan. Operations were further hampered by an upcoming storm with wind speeds of 7 to 8 Beaufort.
Coring was nonetheless successful and the scientific team is looking forward to analysis and interpretation (but also an occasional break enjoying a match of table soccer).
Today is another diving day with PHOCA since the weather is nice and all of us are excited to visit mud volcano MV4. On this submarine hill we deployed two observatories with MARUM seafloor drill MeBo in 2012.
After a short survey PHOCA located one of the instruments, which is connected to the deep borehole via a hotstab and tubing so that pressure and temperature monitoring can provide us with insights to sediment deformation processes prior and during earthquakes. The actual observatory is a short stainless steel cylinder that connects to the deep borehole via a hotstab. Unfortunately, the hotstab connection was too tight to be loosened by ROV. Instead, the skillful handling by the ROV pilots allowed us to unscrew the entire MeBo drill rod underneath the instrument. The assembly was then too long and too heavy to be taken away by PHOCA so that it will require a second dive to safely recover the instrument.
Time is flying and and the international science party, 34 persons from ten nations, uses the equipment onboard RV Sonne most efficiently.
We have meanwhile recovered piston cores from both the Kumano Basin fill and mud volcanoes, the latter of which showed interesting pore waters of assumed deep origin within the accretionary complex. The transformations of clay minerals with increasing pressure and temperature releases fresh water from the mineral lattice and causes lower salinity, but enrichment in other elements.
The exact chemical composition will be analysed post-cruise and will then be indicative of the pressure and temperature regime of the fluid processes. Also, the clasts contained in the extruded mud will allow us to geodynamically reconstruct the evolution of the Nankai subduction system.
As a result of our stranded equipment in various containers on HANJIN vessels, we were forced to send the ROV (remotely operated vehicle) PHOCA to Japan at very short notice. The team around Fritz Abegg prepared the gear quickly and carefully and today PHOCA dove to 2000 m water depth, a new record for the ROV.
We studied a gas hydrate bearing mud volcano, a small underwater hill which evolves from mud, water and gas extruding from depth. With a temperature probe we located the area of maximum activity, which is southwest of the summit and where thermal gradients are a few 100°C per kilometer.
Apparently, fauna is also attracted by the dynamic submarine setting. We saw giant crabs, fish, shrimp and small organisms. An octopus was apparently not so amused about our visit with the bright ROV headlights and wanted to get in a fight.
After 4 days in the port of Yokohama, SONNE is again underway to its next mission in the Kumano Basin in the Nankai Trough forearc. Time in port was busy, most importantly because both the Heat Flow probe as well as the ROV PHOCA by our colleagues from GEOMAR had to be rigged up and tested.
We also had a team from GOOGLE Culture visiting to acquire video footage. The material will serve for a virtual ship tour on RV SONNE, which will soon be available on the internet.
In the evening we arrive in the new research area for Leg 251-2 and ran a heat flow survey across an active mud volcano all night.
Research Vessel Sonne now is back at the Terminal of Yokohama harbor, where our first leg ended on Saturday October 15 around 8 o’clock in the morning. We can look back to a very successful leg during which we achieved all priority objectives of the EAGER-Japan project in the Japan Trench working area:
During our short voyage we mapped the entire along-strike extent of the Japan Trench from 36° to 40.3° N, to now have a complete high-resolution bathymetric map of the trench axis and nearly 2000 km of sub bottom Parasound profiles. We also succeeded to take six 10m long piston cores from more than 7000km water depth, which allow to document and compare the sedimentary processes and extreme-event deposits from the southern to the northern part of the Japan Trench.
Furthermore we could core 3 slope sites of representative location of the landward slope of the subduction system. For each of these slope sites we retrieved two cores, one of which is kept closed and will be analyses by the Marine Geotechnics research group at MARUM to study strength and deformation behavior upon dynamic stresses simulating different earthquake shaking scenarios. With such experiments, we aim at assessing critical earthquake intensities needed to trigger sediment remobilization, towards quantitatively calibrating the geological record of extreme earthquake for past earthquakes.
Additional scientific experiments during our research cruise included the acquisition of a 175km long East-West bathymetric profile perpendicular to the margin at 39.3°N. The track from the incoming oceanic plate across the trench and slope to the shelf followed the exactly same track of a bathymetric survey conducted by JAMSTEC in 2007 (i.e. 4 years before the Tohoku-oki earthquake). By analyzing the differences between the 2007 data set (pre-earthquake) and our newly acquired dataset (post-earthquake) we can test if the seafloor at the location around the northern extend of the Tohoku-oki rupture zone experienced co-seismic displacement during the earthquake. In the recently published literature some rupture and/or tsunami inversion models suggest seafloor displacement in this area, while other models predict no significant movement. With our dataset we will be able to test these models to better constrain the along-strike variation or earthquake rupture along the Japan Trench megathrust.
Last but not least, we successfully retrieved a core from the very same location, where we cored the deep sea trench sediment during previous Sonne cruise SO219-A in 2012. At that time, the geochemists documented striking anomalies in porewater-geochemistry data, which are interpreted to be transient signals induced by remolding and resedimentation triggered by the Tohoku-earthquake. Now, 5 ½ year after the earthquake, we repeat the porewater and solid phase geochemistry analyses at this location to study the transient signal and assess post-depositional processes and rates to learn how the event-deposit becomes archived in the geological record and what distinct chemical signals remain.
Almost half of the science party of the first leg disembarked during the short port-call in Yokohama, including Jess Hillmann, who was contributing the ship’s log entries during the first leg. In exchange, Achim Kopf as chief scientists of the second leg, the Geomar ROV Team, heat flow and seafloor observatories specialists from Bremen, as well as international scientists from partner institutions embarked the ship today. We are getting ready for sailing to the Kumano Basin in the Nankai Trough Subduction zone working area, from where we will report in the next blog.
We are nearing the end of our cruise now, and still have a lot of cores to process so everyone is busy on the ship. Thankfully the sunshine and the calm seas have stayed with us, so we should have ideal conditions for collecting our last two cores tonight and tomorrow morning. After this we will start the long transit back to Yokohama, which will take around 24 hours as we are now in the north of our working area in the Japan Trench. We will no doubt be working away until we are back in port as all the cores must be processed, sampled and packed away carefully for transport to Bremen.
The last stage of processing for the cores is done by the sedimentology team, led by Dr Ken Ikehara and Dr Cecilia McHugh. With the help of their assistants, Tobias Schwestermann and Jana Molenaar, they describe sedimentological features of the cores in detail, providing a valuable record of the core and helping us determine where the sediments in the trench may have come from, when they were deposited, and the mechanisms by which they were transported to the area. This description is done using a variety of techniques on board the ship; first of all, each section of the core is photographed by Dr Kazuko Usami to provide a visual record, then it is described in detail and all the information is documented in a core log. Whilst the core log is compiled, small samples are taken from key sections to be looked at under the microscope as smear slides. The slides are examined by Dr Asuka Yamaguchi, who has been doing a wonderful job of describing these (several hundred!) slides in detail, to tell us about the microscopic components of the sediment. Finally, the magnetic susceptibility of the core is measured by Dr Toshiya Kanamatsu – this tells us whether there are any magnetic minerals present in the sediment, which may be useful in determining where they originated. Also working in the sedimentology group is Dr Witold Szczucinski, who has been collecting samples from some of the cores to try and find DNA from small organisms such as foraminifera in the sediments. This can help provide us with information about where the sediments might have come from, and what processes led to them being deposited where they are now found. Once all of this has been done, the cores are carefully wrapped up and packed away for their long journey back to Bremen where they will be used for further research.
In the last few entries we have discussed the entire core processing workflow on board the ship, but there is one other very important group working on board – the hydroacoustics team. We’ll talk more about their work next time!
After several days of rain and grey skies the sun is finally shining and the sea is lovely and calm. This is a very welcome change as over the last week we have been affected by Typhoon Chaba and several other low pressure systems, resulting in rather rough seas – not ideal conditions for coring. In spite of this we have still managed to collect some great data, so far we have three 10 m long cores collected from the trench, and four 5 m long cores from the slope. We’re planning to get more over the next 24 hours, so we’re keeping our fingers crossed for this good weather to continue!
Once the cores have been sampled by the geochemistry team they are split into two halves. The ‘archive’ half goes to the sedimentology group, whilst the ‘working’ half is passed to the physical property group. On board the ship the physical property group, led by Dr Gauvain Wiemer and Dr Matt Ikari, carry out several different geotechnical analyses with the help of their assistants, Sebastian Trütner, Mareike Höhne, Marie Rex, Jess Hillman and Alex Rösner. These tests allow us to determine the shear strength and the water content of the sediment in the cores. This information is very important in understanding how seismic events affect the strength of sediments on the seafloor, and determining what role different factors play in causing sediment remobilisation and landslides that may result in tsunamis. During the cruise we are collecting cores from the trench (at water depths of 7500-8200 m), and on the slope (at water depths of ~5200 m). Our results so far show a distinct contrast in the physical properties of the sediment at these locations, which we would expect due to the different processes in action at different depths; however, we are also getting some exciting new data that confirms several theories the physical properties team at MARUM have been working on.
The ship was very quiet this morning as most of the scientific crew were sleeping after a busy night of coring. We took advantage of a good weather window and managed to successfully recover a 10 m core from the trench, and two 5 m cores from slightly shallower depths on the slope. The 10 m core from the trench is a repeat of a core from a previous expedition in 2012 (SO219), this is very interesting as it allows us to see what has happened in the sediment over the last 3 years. We will be opening this core today, along with one of the 5 m cores, and they will be processed on board as usual. The second 5 m core will be kept intact until it is safely back in Bremen, where it will be used for further geotechnical analysis at MARUM.
For today’s entry we will focus on the geochemical analysis that is being done onboard by Dr Martin Kölling and his two assistants, Paul Töchterle and Neeske Lübben. The geochemical group are the first people to sample the core once it is on deck, because the parameters they measure are highly time sensitive, so the samples need to be collected as soon as possible. During this cruise they are collecting sediment plugs for measuring methane concentration and pore water for measuring alkalinity. The sediment plugs are acquired whilst the core is being sectioned on the deck, and pore water is extracted from the sealed sections using tiny holes drilled into the liner that allow them to insert small porous tubes. We run some analysis of these samples on board; however, the majority of the geochemistry work will be done back in Bremen.
The geochemical analysis of the porewater and the sediment samples provides us with valuable information about when the sediments were deposited on the seafloor, and whether they have been disturbed by seismic events such as the Tohuku-Oki earthquake in 2011. Events such as these can remobilise large areas of sediment, removing it from some sections, and redepositing it in others. This remobilisation leaves a distinctive geochemical ‘fingerprint’ as it disrupts the normal equilibrium profile that we would expect to see due to the activity of microorganisms in the sediment. We can use this data to try and unravel the story behind the events that are occurring in the trench as it can provide information about where the sediment was transported from, and when the remobilisation event occurred.
Yesterday evening, after patiently waiting for over five hours, there was great excitement on deck as we recovered our first core, 9.76 m of sediment from a depth of 8019 m in the Japan Trench. Collecting cores from such great depth is no mean feat, and we are lucky to have such a great team of scientists and crew on board to make sure the whole process runs smoothly.
The piston core setup is first assembled on deck, this consists of a very heavy (1.1 tons!) weight at the top, connected to a 10 m long metal core barrel, which is lined with a hard, clear plastic tube - the core liner. A piston connected to a cable is then run through the barrel, and the end is capped with the core catcher. Once it is assembled, the piston core is lowered over the side of the ship on a cable. A second, short, 1 m trigger core is attached to the cable by a metal arm above the main piston core. The purpose of this is to act as a ‘trigger’ (hence the name!) for the piston core to fire once it reaches the seafloor. The whole assembly is lowered slowly to the seafloor (in >8000 m of water this takes a few hours!), and once the trigger core hits the seafloor the main piston core is ‘fired’, driving it deep into the sediment. We then wait a few more hours while the recovered core is very slowly winched back up the ship, and brought back onto the deck using a cradle.
Once the core is safely on deck a strict procedure is followed to ensure that it is correctly labelled and all the samples are catalogued. As it is removed from the core barrel the core, contained in the liner, is split into 1 m long sections which are then sealed with plastic caps. By the time all this was finished it was late in the evening, and with hours of further analysis ahead we decided it was best to wait until the morning to split the cores and continue working. We continued with the bathymetric and sub-bottom profiler survey of the trench during the night, until heavy seas and strong winds due to Typhoon Chaba forced us to abandon our track and move further to the south to wait out the storm.
We woke today to wave heights in excess of 4 m, resulting in a rather rocky ride! Nonetheless we were excited to open up the core and start work. Each section of the core is first split into two halves lengthways, one is the working half, which we use for geotechnical measurements and sampling, and the other is the archive half, used for describing the sedimentology. The archive core is kept in pristine condition and will not have any samples taken onboard, this means it can be kept in storage back in Bremen for future research work to be done. We’ll go into more detail regarding the geochemical, geotechnical and sedimentology analysis that is done onboard in the next few blog entries. For now we are busy examining lots of lovely mud!
After a few days of delays in Yokohama we are at last setting sail for the Japan Trench and will soon be in our survey area and on the way to collecting our first cores! Unfortunately, there was a bit of a last minute scramble to source gear from alternate locations as our container of equipment is currently stranded on one of the stricken Hanjin shipping company vessels somewhere near Singapore. Luckily, our wonderful colleagues at JAMSTEC have very kindly loaned us the majority of what we are missing, and the rest was brought over in various scientist’s luggage from Germany and Switzerland!
All of the scientists are now safely on board and we have had plenty of time to prepare our labs and start discussing scientific objectives. The saying goes that “necessity is the mother of invention”, and as our cruise time is being cut short, and some of our gear is missing, we have been trying to prioritise and improvise in order to achieve all of our aims, which has led to the ‘invention’ of several new ways to collect and process core samples. In spite of all this, we are looking forward to collecting valuable new data, hopefully resulting in some exciting new discoveries and ideas. We started today with a ‘mini-conference’ where several scientists on board gave short talks on their previous research in the area, this gave those of us who are less-experienced a valuable introduction, and led to some really interesting discussions of new research concepts.
The only remaining obstacle between us and our survey area now is the imminent arrival of Typhoon Chaba, which is forecast to cross the Japan Trench on the 5th. We will keep an eye on the forecast and sea conditions and plan our movements to stay away from the worst of it, whilst hopefully getting underway with data collection. We may still encounter some rough seas though, so the ship’s doctor, Anke, has been preparing for any sea sick scientists!