|Some of the new data acquired for Tropic Seamount, including|
ship-board multibeam swath bathymetry and geoacoustic
sub-bottom profiler, AUV surveying, ROV sampling and
CTD water information
We are now in the final few days of our exploratory cruise to Tropic Seamount (23.5° N, 20.4° W), about 650 km south of the Canary Islands. We are amassing a significant dataset, having mapped the majority of the area of interest with ship-board multibeam swath bathymetry and geoacoustic sub-bottom profiler. This data is valuable for identifying hard rock areas for more detailed surveying with the autonomous underwater vehicle (AUV) Autosub 6000, and rock sampling using the remotely operated vehicle (ROV) Isis.
We have taken about 35 CTDs to measure conductivity, temperature and depth (pressure), and collect water samples at a range of depths over and around the seamount. We have completed 14 AUV missions to acquire higher resolution swath bathymetry, sidescan, sub-bottom profiles and still images of the seafloor in specific areas of interest. This data has been ground truthed with numerous ROV dive (18 to date), which have used for geological mapping, rock sampling (we have collected more than 250 rock samples, many insitu), core drilling (a total of 46 cores have been acquired) and biological surveying and sampling. This is probably now one of the most thoroughly surveyed seamounts globally, and the new datasets will form a basis for geological and oceanographic research for years to come.
The new rock cores have been acquired using a core drill designed at the National Oceanography Centre specifically for this project and mounted on the front of the ROV. The cores are potentially the most valuable samples for studying the lateral variations in ferromanganese crust composition, texture and thickness as a result of local-scale processes, such as such as micro-topography, currents and sedimentation rates. These cores form the basis for the research BGS is leading on, which will be delivered by a two year post-doc position, held by the newly appointed Pierre Josso. Pierre undertook his PhD with the University of Southampton on the potential for recovery of rare earth elements from metalliferous sediments on Cyprus. He will be joining BGS/NIGL in February to work on these cores and other samples, with the objective of establishing a litho-chemo-stratigraphy across the seamount.
Science operations were suspended at very short notice last week when the RRS James Cook received a ‘mayday’ call from a yacht in the Atlantic Rally for Cruisers transatlantic race, which had departed from Grand Canaria. Just after 14:00 on Wednesday the yacht crew sent a message indicating that their boat was taking in water, their pumps were overwhelmed and they were sinking. As the closest available ship, about one and a half hours away, we were obliged to pull off station and head to the yacht at full speed. With the RRS James Cook using all four engines we accomplished an unheard of speed of 17 knots, reaching the yacht and its crew in about 1 hour. By the time we reached the 36ft yacht ‘Noah’ the crew (three adults and two children) had abandoned the sinking vessel and boarded their small yellow inflatable life raft, which was still close to the yacht. The James Cook pulled up close to the life raft which gradually floated alongside. The crew of the James Cook were fully prepared and well trained, handling the situation very professionally. Lines were thrown to the life raft to provide a link to the deck of the James Cook, some 3-4 metres above sea level and the raft. A ladder was lowered, but it took several attempts, despite the very calm sea conditions, to bring the life raft into a suitable position for the people on board to grab the ladder and clamber onto the James Cook.
|The scene when the RRS James Cook arrived at the abandonded yacht ‘Noah’, with the small life raft containing five people|
floating away from the vessel.The life raft alongside the RRS James Cook as the ship’s crew help the people off the life raft
and up the ladder hanging down the side of the ship.
Despite the good daylight conditions it was a precarious operation to move the five people from the constantly moving life raft onto the deck of the Cook. In about 30 minutes all five of the yacht crew were safely rescued onto the James Cook, where they were warmly welcomed by crew and scientists. We then had to retrieve the AUV, which was still in the water when we received the SOS call. The ship now exceeded its capacity so we steamed back to Tenerife, our departure port, dropping the yacht crew off early on Friday morning. We were literally alongside for about 1 hour (probably the shortest port call in the history of the RRS James Cook) before being led back out by the harbour pilot to travel back to Tropic Seamount. The Captain of the RRS James Cook placed a special request to NERC to use three engines to get us back to Tropic Seamount as soon as possible. This uses considerably more fuel, but as a result the rescue and return journey to Tenerife only cost us about 60 hours.
Rock sampling using the manipulator arm on ROV Isis. Once a rock is collected
it is placed in a compartmentalised and numbered sample box on a retractable tray
which slides out from beneath the ROV. The images also shows push cores with
t-handles for sediment sampling using the manipulator arm. The grey niskin used
for collecting uncontaminated rocks samples for microbial studies is situated
on the bottom lefthand side of the image
Anyway we were back on the science programme by Saturday 26th November. Improved physical and chemical characterisation of ferromanganese crusts is crucial to understanding local-scale processes controlling deposit formation and predicting their occurrence. One of the first missions once back at Tropic Seamount was a 17 hour ROV drilling dive, with the aim of acquiring 12 drill cores (the maximum number the ROV can collect in a single deployment) over a lateral distance of some 100 m of ferromanganese crust pavement. These cores will principally be used by BGS to investigate lateral variations in crust thickness and composition, using a combination of high resolution trace element geochemistry and isotope analysis. The ROV drill requires a flat surface for the ROV to land on, so its use is restricted to the large areas of rock pavement, mainly occurring on the summit of the seamount. However, as water depth is likely to have a strong influence on crust composition and thickness it is important to sample over the full depth range of the seamount, from the summit plateau at about 950 m down to where the flanks meet the abyssal plain in water depths >4000 m. This type of sampling relies on picking up loose rock with one of the manipulator arms on the ROV Isis or preferably snapping off insitu samples from the seabed exposures. The manipulator arms are very powerful and capable of lifting about 200 kg, but it still challenging to break pieces off the highly encrusted rock slabs whilst attempting to keep the ROV static. The crust samples we are obtaining typically have very nice sub millimetre-scale layering, representing millions of years of gradual deposition and reflecting paleooceanographic changes (e.g. water mass provenance, depth of the oxygen minimum zone, biological productivity, current velocity and upwelling patterns) in the north-east Atlantic. To compliment the scanning electron microscope, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotope studies planned at BGS, high-resolution x-ray computer tomography scanning will be done at the University of Leicester. This quick and entirely non-destructive technique allows the reconstruction and visualisation of the internal structure and textures of materials in three dimensions. It is ideally suited to imaging materials such as crusts, which have well-defined, compositionally distinct layers. Combined with compositional data provided by LA-ICP-MS, this technique provides a powerful tool to explore the heterogeneity (or lack of continuity) of litho-chemo-stratigraphy within crusts and nodules at the centimetre scale.
The potential role of microbes in Fe-Mn deposit formation remains unclear. To study this process colleagues at NOC are collecting small pieces of ferromanganese crust to study. In an attempt to transport these from the seabed sampling site to the surface in their ambient environment we have adapted a niskin bottle mounted on the front of the ROV. Once a piece of rock has been placed in the niskin it is closed to seal it from the wall column as the ROV is recovered back to deck. The uncontaminated material will be used to study any microorganisms colonising the crust and which may have role in concentrating E-tech elements in these deposits.
|From L-R: Ferromanganese crust samples collected with the ROV Isis manipulator arm spread out in the laboratory on the|
RRS James Cook, ready for cutting and subsampling; A cut section from a nodular block of ferromanganese crust. Note the
intricate growth layers, which have been deposited on lithic fragments incorporated into the nodule during development
The ROV dives also provide an excellent opportunity to study the marine ecosystem and fauna populating the seamount. Seamounts are generally considered to support high levels of biodiversity and unique biological communities. There is therefore potential for conflict between the location of sessile biology and the highest grade ferromanganese deposits. We are collecting lots of imagery to assess the type and diversity of fauna inhabiting the different seamount environments over varying depth ranges. Images and other habitat data are supported by the collection and photographing of type specimens by the ROV during the geological sampling operations.
|Clockwise from top left: A ‘Dumbo Octopus’ (Grimpteuthis spp.) photographed during a ROV dive. These deep-sea ‘umbrella|
octopuses’ are rare and notable for their fins that resemble elephant ears; A large crab (the red laser dots are 10 cm apart)
walking over ferromanganese nodules sitting in soft sediment on Tropic Seamount; Typical sessile fauna on an area of
ferromanganese crust pavement on Tropic Seamount; Crinoids growing on a sandy sediment covered bank and facing the
current direction (the red laser dots are 10 cm apart).
The remaining few days of the cruise will be spent conducting another ROV-based plume experiment as part of a study examining the potential environmental impacts resulting from exploring for and extracting seabed mineral deposits. We have to recover the three moorings, which we placed across the seamount at the start of the cruise to record hydrographic data over a 30 day period. We are also planning another series of ROV rock sampling dives and further AUV surveys. The RRS James Cook returns to Tenerife on the 8th December where some of the equipment will be offloaded for trials early next year on the RRS Discovery, which is also docked in Santa Cruz.
Co-Chief Scientist JC142