Tempo rubato for Tempo-Mini and BARS

Tempo rubato for Tempo-Mini and BARS

Perseverance paid off in the final hours of this NC Fall Cruise: 12 hours before having to transit home, the waves subsided just enough to attempt one last very important dive. On the ambitious agenda (14-page dive plan) for this Main Endeavour vent Field (MEF) dive were: deployment of Tempo-Mini and a Benthic And Resistivity Sensor (BARS) instrument plus the sampling of fluids and gas. To complicate matters, these operations required synchronized international collaboration. The deployment of Tempo-Mini required online presence of our French colleagues from IFREMER in Brest, and BARS needed the cyber presence of our Seattle colleague Marv Lilley who just happened to be in Zurich, Switzerland that night.

Why did it get so late?

If you haven't followed our previous blogs involving weather (Weather, ROCLS, Food), here’s a quick review of how it came to such a tight call for our last dive. Consider the Endeavour itinerary after we finished our Barkley Canyon work with Wally and Barkley Benthic Pod 3 (see blog post):

Tuesday, Sep 20

We steamed post haste back to Endeavour because the weather pattern indicated a chance for deploying a cable drum to the Mothra site before arrival of an impending storm. We went directly to Endeavour without stopping for the planned bottom pressure recorder (BPR) deployment at ODP 1027.

Wednesday, Sep 21

A whole day with 8+ m swell; no dive or deployment, but we used the time to finish building our MEF extension cable.

Thursday/Friday, Sep 22-23

With inclement conditions at Endeavour, we returned to ODP 1027 and deployed autonomous BPRs via the ship's winch (ROPOS wasn’t required for this, so we could deploy in higher waves). After that, we steamed straight back to Endeavour.

Saturday, Sep 24

In the wee hours we were able to sneak in a real dive to deploy ROCLS (see blog post) with ROPOS near the Endeavour node. But due to strengthening winds (35+ kts) and some issues with a ROPOS manipulator, we were forced to abandon the dive, leaving ROCLS on the seafloor (see ROCLS blog post). With unpredictable weather, we had no choice but to hold out and hope.

Sunday, Sep 25

Luck came in the morning, when a change in wind direction resulted in such a confused sea-state that the total wave height was actually low enough to dive! This was such a surprise that it was difficult to find the shore support required for this dive. But everything ended in success, the cable was connected, MEF got power, and our previously installed instruments all sprung back to life! But alas, when coming up from the dive, the weather worsened once again.

Monday, Sep 26

Hoping for better weather...

Tuesday, Sep 27

Hoping for better weather...

Wednesday, Sep 28, morning

Yes, better weather! Now the swell has to subside quickly...

Wednesday, Sep 28, 18:00

OK, let's do it!

So all in all we'd spent 7 days on site (plus two days at ODP1027 in between), and finally we were getting our third dive in, but only just!

Diving against clock and swell

Excitement took over quickly, but also a foreboding sense of time pressure, as our return to port could not be delayed. And weather conditions could, of course, worsen again. The deck crew loaded Tempo-Mini and BARS onto the ROPOS tool basket, ROPOS latched on, and down they went. Watching blue water and monitoring the pressure gauges while descending, the dive log read: "Strong surface swell is visible in the video as a reversal of direction."

Two hours later, ROPOS reached the seafloor. A place to put down the tool basket was found near the instrument platform, and BARS installation was the first order of business. At this time it was 5 a.m. in Europe; a short night for our scientific colleagues in France and Switzerland. But everyone involved was suddenly wide awake because something was unusual: Far more black smokers than were seen there before came into view! Marv Lilley Skyped from Zurich to say that this area is "pretty active" – a comment that made it into the dive log.

Deploying BARS

The vent for BARS was certainly active enough, but we also checked with a poker that the vent was deep enough, too. Then, gas samples were taken using so-called gas tights. After everything was deemed good, ROPOS zoomed back and forth, first fetching BARS from the tool basket, placing the BARS canister, then connecting its cable at the IP. While flying back to the vent, BARS was powered on and data began streaming immediately.

BARS, which stands for Benthic And Resistivity Sensor, measures fluid resistivity (yielding chloride concentration), temperature and redox potential. Its sensor is located directly inside an active vent.

In order to avoid shock heating of the probe by inserting it directly into the hot vent, the wand was carefully preheated for several minutes while moving it gradually closer to the plume and down into the vent opening. When the temperature stabilized at 150°C just above the vent, ROPOS inserted it fully into the vent where it measured 325°C. Some like it hot! Marv Lilley could then proceed with his remote testing and set-up.

Deploying Tempo-Mini

Since larger patches of level seafloor right next to an active vent are sparse, the chosen placement for Tempo-Mini happened to be right next to the RAS (Random Access water Sampler). Tempo-Mini's installation procedure included a 3D site survey. If the RAS anchorage was in the way, we intended to move and then re-deploy RAS; not a trivial operation. But, with a time running low, this step was quickly skipped.

Instead, ROPOS placed a checkerboard calibration target against the vent wall, and flew back and forth at various depths and distances, while we photographed the site from all angles. These pictures can later be used for a 3D reconstruction.

By this time, 5 hours had lapsed and we were making good progress. The next step was to install Tempo-Mini 's base platform. The base platform has 4 adjustable legs so it provides a horizontal base even on a slanted uneven seafoor. Adjusting the base plate was no small feat, when you consider it was done by a two-armed remotely operated vehicle flying 2.3 km below stormy seas, working on a cramped, yet fragile hot vent flange structure! 90 minutes and a shift change later, the base platform was adjusted and ready for Tempo-Mini.

Meanwhile, in France, morning coffee had arrived at the IFREMER conference room where a dozen scientists gathered to watch the drama live via streaming video. Much to their delight, they witnessed ROPOS skillfully install Tempo-Mini on the base plate with very little room to operate. The French were certainly impressed when Skyping "Wow!"

The cable was inspected and adjusted, then Tempo-Mini was switched on from shore. Breathless, IFREMER logged on to test the camera, and it began streaming clear images of the hot vent tubeworm community!

Tempo-Mini, by the way, is a benthic observation platform that includes an HD camera with lights and temperature, oxygen and ion sensors. These several metres long sensor strings were unfolded from Tempo-Mini and placed on the benthic field. Plenty of twirling and untwisting was necessary to disentangle the mess.

In the end, all sensors worked properly, a happy conclusion that drew joyful applause from our French collaborators. After years of planning, building, testing and testing, Tempo-Mini had finally become a reality!

The not-so-bitter end

By now, 10 hours had elapsed and time was running out, so we quickly fired two Niskin bottles above the vent to sample its plume water, turned and followed the MEF cable to pick up the ROCLS grating, zoomed back to the tool basked, threw the grating in and began our last the ascent of this cruise

No doubt, our perseverance riding out the waves was paid off. It was absolutely worth it to get this final dive done!

PS: In case you wondered: tempo rubato ~ time robbing, a musical term.