JC142 cruise blog #3 – Geological sampling by Pierre Josso (MarineE-tech post-doc)

The half-way though milestone (known as hump-day) of the cruise has just passed! We now have a much better idea of what lurks beneath our ship, typically at water depths in the range 1000-3000m. Nearly complete surveying of the summit of the seamount by the AUV has produced excellent and detailed imagery of the sunken volcano to guide our latest ROV dives. This imagery has helped us target sediment free areas for sampling. As previously indicated, the ROV can take multiple tools depending on the objectives of the dive and one has been specifically designed for this expedition: a new rock drill. With a key objective being to study variation in the composition of the crusts in relation to depth and other morphological features of the seamount, it is of great importance to be sure that the piece of rock you are collecting on the seabed actually comes from where you think it does. This may be a problem on the flanks of the volcano where slopes of 15 to 45° are likely to result in debris moving down slope from outcrops at shallower depths. The top of the seamount is relatively flat and sometimes exposures only offers scarce loose rocks for us to pick up with the manipulator arms of the ROV. This is where the drill comes in to its own.

The rock drill is assembled on the front of the ROV for its next mission, behind is crate containing core catchers for extracting the core from the drill hole, and boxes for storing samples during the dive

When the geologist in the ROV shack sees an interesting pavement of crusts (only a geologist will say that apparently!), he gives the signal for drilling. With a barrel of 30 cm, the diamond drill typically manages to produce a complete core in about one hour. Probably not the most exciting part of dives but one of the most important and delicate operations as the drill needs to go down as straight as possible for us to recover the best sample. It took a few tries to develop the best drilling strategy…indeed a few core catchers got stuck in the seabed or the core couldn’t be recovered, but the ROV team did a great job adapting to the challenging drilling conditions and our technique (this is the first attempt at seabed drilling for them). The drilling is now as good as it can probably get. It is notable that, as far as we are aware, this is the first time ever that seamount Fe-Mn crust deposits have been sampled with an ROV-mounted drill. The drilling part of these missions (which can last for up to 24 hours) provides a short break from the almost continuous seafloor observation/mapping that the scientists undertake. The drill seems to be a particular attraction to the local crabs and shrimps, curious about this bright intruder in their dark environment.

One of the many platforms found on the top of the seamount encrusted by iron and manganese, a perfect flat platform for ROV drilling.

The drill is deployed and running with a constant weight applied by the two iron blocks lifted onto the rig once the drill bit is on the seafloor. Brown dust indicates we are drilling through crust!

As the drill goes deeper, the plume changes colour as we attack the sedimentary platform under the crusts.

In this lateral view of the drill, our sampling attracts the local residents curious about this noisy intruder. 

The drill has completed a hole in the seabed, time to grab the core, still in its hole, using the core catcher shown! The laser beams are 10cm apart.

One of the many samples we have recovered during our dives (laser dots are 10 cm apart).

During the cruise we plan to acquire a dense grid of cores to study changes in crust composition at the meter, hundreds of meter and tens of kilometer scale, with control over the location of each sample. These will be complimented by grab samples of rocks. An untold competition runs between the two science shifts to see who will bring back the biggest slab, and the race is tight as samples in the 20 to 25 kg range have already been recovered! These big pieces of rocks provide the opportunity for us to study the lateral and vertical variations in these deposits at a small scale. In total, 70 kg of rocks per dive can be acquired, if we do not take the drill or other heavy equipment. It will always be a trade-off between equipment requirements e.g. taking the drill and sample capacity on this vehicle.   

Once the ROV dive is finished, we usually have a good hour to get ready for handling the samples as the ROV is recovered from the seabed. In order to quickly process and preserve the samples we have to work as an efficient team transferring them to the lab where they are photographed, cut into slices and described before being bagged and stored for further analysis back at base. 

Back on the deck, samples are cut carefully…

…. photographed and described.

One of the cores extracted from its barrel showing crust above the phosphorite substrate.