Exploring the seafloor

RV Kronprins Haakon at the quay in Longyearbyen. Foto: Tobias Himmler.
The new research vessel RV Kronprins Haakon (Fig. 1) set sail on Tuesday 23.10.2018 at 20:00 local time after an unintended extra day at the quay in Longyearbyen, Svalbard.

Days 1 - 3

The new Norwegian research vessel RV Kronprins Haakon set sail on Tuesday 23.10.2018 at 20:00 local time after an unintended extra day at the quay in Longyearbyen, Svalbard.

During the night it headed NNW out of the Adventfjord before steaming SSE along Spitsbergen to its first operational point located SW of Sørkapp within the Storfjord Trough. The scientific party includes 17 international scientists and a team of eight operating the remotely operated vehicle (ROV) ÆGIR 6000 (Fig. 2) from the University of Bergen. Three scientists from NGU involved in the NORCRUST project, Aivo Lepland, Wei-Li Hong, and Tobias Himmler, join this expedition.

Fig. 1. Left: RV Kronprins Haakon at the quay in Longyearbyen. Right: A cosy 3rd-deck cabin shared by two scientists.

Expedition CAGE18-5 is led by Prof. Stefan Bünz from CAGE at the Arctic University of Norway. During the expedition the scientists will mainly use the ROV ÆGIR 6000 to explore the seafloor at three sites with known and suspected methane emissions in the Barents Sea.

The main objective of this expedition is to better understand the dynamics and bio-geochemistry of methane seeps in the Barents Sea. It is planned to take many sediment push cores for subsequent pore water analyses, microbiology studies, and foraminifera sampling. In addition, CTD (conductivity and temperature relative to depth) measurements will be used to study the water column chemistry. Rock samples will be collected from the seafloor using the ROV manipulator arms (Fig. 2). 

Fig. 2. Left: Ongoing preparational work inside the main hangar of RV Kronprins Haakon before launching the ROV ÆGIR 6000. Right: Members of the ROV-team observing the yellow steel frame carrying the ROV below, as it is lowered into the ship’s moon-pool for a successful “wet-test” (i.e. it works just fine).

The extra day was used by the scientists to explore the ship and to set up the sampling equipment in the new and spacious labs. After a safety briefing in the afternoon of 23rd October (Fig. 3), each group presented their sampling strategy for the first sampling site.

Fig. 3. Left: The first officer (sitting on the left) just before the safety briefing. Right: Christiane Schmidt (University of Angers, France) embarking one of the lifeboats on RV Kronprins Haakon.

Whereas the discussion about who is doing what and why the samples are important for post-cruise analyses was very active, all scientists concurred to a sampling plan that meets everyone’s highest prioritized samples. Subsequently the major sampling tools – push cores – were prepared during the transit (Fig. 4). The first sampling site at Storfjord Trough was reached in the afternoon on 24th October. Before the ROV will be deployed later during the night a CTD transect will be completed for water column studies.

Fig. 4. Left: Matteus Lindgreen (left, CAGE, Arctic University of Norway) and Wei-Li Hong (CAGE and NGU) fixing PVC tubes to the steel basket that will be used to retrieve the samples from the seafloor. Right: Strong winds blowing from the East during the transit.

Everyone on board is well and excited for the first ROV dive the coming night, expecting the first overnighter 24/25th October.

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Days 4 - 5

Due to strong winds and high waves, the basket was not deployed so there was no push core sampling during the night 24/25 October. Instead, the time was used for extensive visual seafloor exploration of two mound structures in ca. 380 m water depth, using the ROV.

Fig. 5: Left: Position of RV Kronprins Haakon SSW off Sørkapp. Right: Bathymetry map of the ca. 10 m high mound#3.

Day 4 started with visual seafloor exploration of two neighbouring ~10 m high mounds, previously named mound# 1 and mound#3, respectively, in the Storfjord Trough (Fig. 5). The goal of the visual exploration is to find white microbial mats or rising gas bubbles, both indicating of active methane seepage. Flying with the ROV ca. 1 m above ground and looking at an approximately 2x2 m field of view for tiny bubbles and microbial mats on the vast seafloor is like looking for the needle in the haystack. But that’s what makes seabed exploration for methane seeps an exciting challenge. Few patches of white microbial mats have been observed on the seafloor at mound#1, but no rising gas bubbles. After six hours of survey, it was decided to move to mound#3. Another haystack.

Fig. 6: Gas bubbles rising from the seafloor. In front is a Niskin-bottle for seawater sampling.

The seafloor around the top of mound #3 is covered by extensive white and grey microbial mats. Near the mound top, rising gas bubbles were observed emanating from carbonate crusts: the needle in the haystack (Fig. 6). Carbonates were exposed next to the gas seep, so it was decided to sample seep carbonates. With the help of the skilful ROV pilots, five rock samples have been collected. After 12 h at the seafloor the ROV reappeared inside the moon pool, carrying the carbonates in its drawer. Immediately, sediment attached to the carbonates was sampled for microbiology and micropaleontology analyses before the seep carbonates were stored for further analyses at NGU.

A brief technical inspection of the ROV followed before it was deployed again in the early afternoon. Later in the afternoon the basket was lowered to the seafloor, using the A-frame in the main hanger instead of the aft crane due to the high waves. During the evening, the push core sampling commenced. Based on the previous visual seafloor observation, various sampling sites with and without benthic microbial mats have been selected by the scientists. Unfortunately, a technical problem occurred, and it was decided to stop the sampling. The ROV was heaved up in the late evening of 25th October for a detailed technical check-up during the night. The problem was not trivial and the ROV operations were paused for ca. 24 h. The time was used for additional CTD profiles across the mound and water column sampling.   

In the afternoon of 26th October, the CTD profiles were finished and the ROV was ready for deployment again. Late in the evening the ROV was deployed again to continue the push core sampling. The first push cores were sampled around midnight and the sampling continued until the early morning of 27th October (Fig. 7).     

Fig. 7: Left: The ROV bringing the sediment-filled push cores back to the basket. When all the cores have been filled, the basket will be retrieved on board while the ROV continues exploration at the seafloor. Right: KH crew members retrieving the basket, using the A-frame in the main hangar.

After gusty weather with occasional snow storms during the night, the weather has calmed down in the morning of 27th. Everyone on board is well, working on the sediment samples in the cold rooms and geology labs. The next push cores are already prepared for sampling. The ROV is on its way to the seafloor. After a successful sampling night, day 6 begins with ROV dive #04.

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Days 6 - 9

ROV sampling continued throughout the day of 27th October with numerous push cores and blade cores. The gas sampler was deployed at an active seep collecting the free gas at the seafloor.

Due to another technical problem, it was decided to steam back N to Hornsund fjord in the evening of the 27th to fix the problem there. After this unplanned ~12 h long detour RV Kronprins Haakon has left Hornsund fjord in the morning of the 28th and started the ~12 h long transit to the easternmost sampling location at Storbanken. Because of the time loss resulting from this detour and the delayed departure at Longyearbyen, the cruise leader had to cancel one planned sampling location (Fig. 8). Nevertheless, all scientists are very happy with their samples from mound#3 and used the time to work on the sediment and pore water samples in the labs. 

Fig. 8: Left: Passing Sørkapp in the north, heading towards the east in the morning of 28th October. Right: Cruise leader Prof. Stefan Bünz informing the scientists about the next sampling area Storbanken in the central Barents Sea.

The ship-based echosounder system was used to record a new multibeam track while steaming to Storbanken. RV Kronprins Haakon arrived at Storbanken in the morning of 29th October. The operations started with an ROV multibeam survey along lines connecting previously detected gas seeps. Based on the multibeam data a micro-bathymetry map with a grid spacing of 15 -20 cm was created, allowing for recognition of small-scale sampling target at the seafloor (e.g., distinct rocks, sediment depressions of down to 0.2 m diameter). The resulting map resolution is significantly higher than the ship-based bathymetry. During the survey, the ROV flies at 20 m altitude above the seafloor while the ship follows the ROV track in the “follow-target-mode”.    

Fig. 9: The upper half shows the ROV-acquired microbathymetry map and a 3-D model of the ROV at the seafloor. The lower half shows a 50 cm topographic high (in this case a seep carbonate) right in front of the ROV.

Subsequently to the ~3h long mapping the ROV was heaved up for a quick pitstop on deck. It was equipped with various sampling tools, including four blade cores for sediment sampling, the gas sampler, a hydrophone, and a checkerboard. At about lunch time the ROV was already back in the water for visual seafloor inspection and sampling (Fig. 10).

Fig. 10: The “face” of the ROV just before it is lowered into the opening moon-pool in the main hangar. In the top and middle centre are the ROV’s two main “eyes”, the top and centre cameras. The manipulator arms are on the left and right side, both carrying sampling tools (gas sampler on the left, blade cores on the right).

During the visual inspection several sites with active gas bubbling from white microbial mats (< 30 cm in diameter) and seep carbonate crusts have been observed. The gas sampler was placed on the seafloor above a seep while the ROV continued with sediment sampling. Conveniently, another seep was found on top of a ~50 cm high carbonate crust. The gas escaped from two adjacent orifices, however, at different frequencies and with different bubble sizes: small bubles escaped in a regular “train”-like fashion whereas large bubbles only escaped every 20 to 30 seconds.

For more detailed analyses of the size and shape of the bubbles, a checkerboard was placed behind the rising gas bubbles and the gas bubbles were filmed with the HD camera (Fig. 11). The hydrophone was used to listen to the bubbles’ frequency. Yes, the bubbles make noise but no worries: They are not loud enough to scare the cod.

Fig. 11: Photo of the ROV-pilot control screen. Upper half shows the seep carbonate crust and the manipulator arm placing the checkerboard behind rising bubbles (top camera view); lower half shows close-up of the checkerboard with individual rising gas bubbles (centre camera).

After the gas sampler was successfully recovered from the crust top, four seep carbonate samples have been collected with the ROV manipulator arm. After almost ~6 h at the seafloor, the ROV was heaved up on deck in the late evening of the 29th.

Day 9 ends with an exciting set of samples from a new seep in the central Barents Sea. The ship moved further east during the night. Day 10 started with CTD lines in the night and a ROV multibeam survey in the early morning. During the night the wind got stronger and the waves higher. All on board are well, looking forward to another day with the ROV at the seafloor.

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Days 10-12

During the night the ROV finished the multibeam survey. It was heaved on deck in the morning of the 30th October. The ROV was equipped with various sampling instruments, including the gas sampler, four blade corers, the hydrophone and checkerboard, and a hydraulic chain saw. After a quick functional check of the saw, the ROV was heaved down to the seafloor again and the visual inspection started. The scientists had selected various sites of interest based on the microbathymetry map that has been acquired by the ROV team during the night. The pilots flew the ROV quickly to the sites of interest and the scientists started planning the sampling strategy.

Several small patches of white microbial mats, discrete sites of rising gas bubbles, and seep carbonates were observed on the seafloor. Bingo. Two blade cores were successfully deployed within a microbial mat. The gas sampler collected some gas from the seafloor and the hydrophone was used to record the bubbling noise. Two seep carbonates were sampled from active seeps. The underwater chain saw was used for the first time to cut off a piece of a meter-sized seep carbonate (Fig. 12). In the late afternoon the ROV arrived on deck, carrying lots of exciting samples from an active seep. The scientists retrieved their samples and the ROV team prepared the vehicle for the next dive.

Fig. 12: The pilot’s view of the hydraulic chain saw that was used to cut off a piece of seep carbonate crust. Much appreciated operational skills by the ROV pilots, indeed.

The ship moved NE to the last station: the Storbanken craters (Fig. 13). During the late evening of 30th October, the operation began with a ROV multibeam survey in crater#1. Based on the microbathymetry map of the crater wall a sampling plan set up. During the night numerous rock samples were collected from the crater wall. In contrast to the well lithified seep carbonates, the rather fragile sandstones cropping out at this site were difficult to grab with the manipulator arm. However, the skilled pilots managed to collect 14 rock samples. Just before breakfast in the morning of the 31st October, the ROV came on deck carrying the first rock samples from the crater area.

Fig. 13: Left: Overview of the two working areas in the western
(Strofjord Trough) and central Barents Sea (Strobanken craters).
Right: Bathymetry of the investigated Storbanken craters.

During the morning the ship moved further N above crater#2 and the ROV was heaved down again. The operations followed the same principle: first, acquiring the microbathymetry map, followed by visual inspection and sampling.

During the early morning the next day, the ROV was heaved on board carrying new samples from crater#2. The final ROV operation. EOE – end of expedition. Next port of call is Breivika, Tromsø.

Everyone on board is well and the scientists are very happy with their samples. Thanks to the ship’s crew and the ROV team, expedition CAGE18-5 was successful. After two days of transit (i.e. time for cleaning the labs and preparing the samples for shipment) the scientists will disembark RV Kronprins Haakon in the morning of 02.11.2018, to make space for 50 school kids who will take over the laboratories and cabins for one day.

Maybe some of them will become scientists and continue the long tradition of Norwegian polar exploration (Fig. 14).

Fig. 14: Excerpts of the wall painting on the 5th deck – a reminiscence of the early Norwegian polar explorers.

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