By Kelsey Archer Barnhill
Yesterday was the last day of scientific operations in the Cabo Verde region for iMirabilis2. Our last station was a multibeam survey which we finished just after lunch. We are now steaming back to Las Palmas on a four-day transit.
The lander team is happy with their grand total of 16 deployments. The final breakdown was 8 camera landers, 5 respirometer landers, and 3 baited traps. These deployments combined with their ex situ experiment meant the team met all of their cruise goals and are leaving Cabo Verde with a really nice data set. Danielle de Jonge shared with me that it is really cool to have respiration rates for this region as they were not previously known. Over the past few days they team recovered their last baited trap with 10 fish, filtered water from a CTD taken nearby the lander area to look at the quality of incoming flux, and took macrofauna samples from two multicore samples. The images below show some of the highlights from the final deployments (images © Prof. Andrew Sweetman / Lyell Centre-HWU / iMirabilis2)
We had 22 CTD deployments, with the latest taken yesterday off of Brava. With the final CTD, there is now information about the water parameters at all ROV dive sites. With this many CTDs, that means a lot of water filtered! Bea Vinha and Andrea Gori alone filtered 275 litres of water for stable isotopes, fatty acids, and particulate organic carbon.
Ten ROV dives were completed during the cruise. Eight of these were for seamount and continental shelf habitat mapping and sample collection, and two dives were for abyssal biology and testing RoCSI. RoCSI was successfully integrated into ROV Luso for these last two dives, with toughened housing for sample cartridges and a 6000m rated battery pack. The different science objectives for the latest two dives made pilots fly in a new way. In standard ROV dives, they follow a transect and try to film the area in the best way possible, zooming in on interesting fauna. They keep a lookout for spots that are more attractive for finding new things for the scientists to view. They are also prepared to stop and sample when requested. On the latest two dives the main goal was to follow a transect while maintaining position and altitude accuracy to take pictures. It was during the most recent of these 2 abyssal dives that Bruno Ramos obtained a new depth record for ROV Luso of 3510m. When you go deeper there is a concern as to whether or not all your setups are correct. Something can become a problem that was not an issue at previous depths, so the pilots remained extra vigilant throughout the dive. Luckily, everything went according to plan. At the end of the final ROV dive, the mood in the van was great and spirits were high. It is always a significant moment when scientific missions for a cruise come to an end.
On the penultimate dive, the ROV pilots had to fight against a strong current and a swell which grew in strength as the afternoon went on. This made it difficult for the ROV to maintain a constant position and altitude. These conditions combined with some Hypac ship navigation issues meant we could not conduct the transects as initially planned during our three hours on the seafloor. There was also a glitch with the battery pack and RoCSI did not switch on. Back on deck, the adaptations made for the integration seemed successful despite exposure to a high sediment load during the dive. After this dive there were many discussions about how to power RoCSI so we could continue testing. In the end, the amazing ROV team came to the rescue in the early hours of the morning and made a new cable to enable RoCSI to be powered via their depth sensor. Another ROV dive was scheduled later that day and everyone crossed their fingers that the epoxy resin used to protect the electronics had set enough to allow the cable to work at pressure and in water. The second dive went a lot smoother after diverting to a different location and the the ROV was able to maintain position with low altitude variations. The benthic survey lasted four hours and was quite interesting for Erik. While it was not the full quantitative dataset he had hoped for, it is a novel survey in Cabo Verde on the type of feature at 3500m depth. He now has a descriptive dataset to work with, describing what lives in the environment where interesting organisms were seen and algal mats were present which was quite interesting as it indicates a clear transport of nutrients to the site. Unfortunately, RoCSI did not collect any samples on this dive either, and Susan Evans is hard at work investigating to find the exact cause. Despite all this, there are many positives to be taken from these dives as far as RoCSI is concerned: we never set out to test RoCSI on two platforms during the expedition so this was an added bonus and will inform future design and development.
Some other last-minute operations include deploying zooplankton nets during the night off of Fogo, multibeam bathymetry acquisition and a final Autosub mission attempt. Bea Vinha is happy with the 7 plankton net samples we’ve taken which will be used for stable isotope analysis. Across all of our multibeam surveys we acquired over 50 hours of data, often going from late hours of night to early in the morning. For the latest Autosub mission, the team was on call waiting to see if and when the sub would go in. The sub was in great working condition and they were given the green light but unfortunately something happened during the launch and a recovery light came out early and got wrapped around the propeller, meaning the dive had to be aborted.
Now all the teams are packing up, writing up the cruise report, and enjoying their last days at sea. Most teams have been working on the cruise report along the way and are now making their final additions to the document. Some teams are also looking to provide preliminary results in the cruise report. Bea for example is looking through OFOP annotations to see what generalisations across ROV dive sites can be made. Our cruise report will be publicly released six months after the cruise.
Samples are being inventoried, packaged and prepared for transport. Many scientists on board are busy communicating with shipping companies to make sure all boxes will be shipped at the correct temperature to preserve the samples. Some are also arranging for analyses to be done once the samples are back on shore and can be sent off to laboratories. Some samples, like the macrofauna from the multicore and incubations, are being sent all the way to Brazil for our iAtlantic fellow, Daniela Yepes Gaurisas to work with. Most of the packing can be done in transit but the Autosub6000 team will have to wait until we reach port to take down their launch and recovery system. Until then, everyone is catching up on sleep and enjoying the beautiful sunsets on board as we head back to land.