It's a trap

If you were fortunate enough to be on the Thompson deck yesterday, or if you were a bird circling the ship, you would have seen our longest, most complicated operation yet. As our final act before leaving station and heading home, we deployed a sediment trap mooring that will stay underwater for the next year.

The mooring is the same one we picked up at the beginning of the cruise (see my post here). It contains two sediment traps at different heights above the bottom, plus meters and meters of chain, rope, floats, and a train wheel to anchor it all to the seafloor. Yes, that's right. A train wheel.

Round 1: train wheel, acoustic release,
and the first sediment trap. Photo by Brie Maillot.

Deploying the mooring took a total of 4 hours, mostly because we had to do it in sections. There was a small group of us designated to help on deck, and others were welcome to watch from an upper level. We had buckets of ice with cold drinks staged at various places on deck, because when you're out for 4 hours in the tropical sun, it's easy to get dehydrated. Some of the onlookers brought their cameras, and one even filmed the whole procedure. At one point, when there was a pause in the deployment process, I turned around and saw one of the onlookers marching out on deck with a bowl of popcorn. It was quite the spectacle indeed.

We started at the bottom of the mooring: the train wheel, the acoustic release, and the bottom sediment trap went overboard first. The distance between the train wheel and the bottom trap was greater than the height of the ship's A-frame, so we had to hoist the trap beside the acoustic release, in effect folding the line between them to make it all fit. Add to the mix several tag lines to keep control of the trap, and you get a complicated web of suspended ropes. Thankfully, none of them got tangled, and the first trap went over smoothly.

Double-checking a sediment trap before putting it over.
Photo by Brie Maillot.

We let out about 100 m of line between the two traps, so that the second one will be suspended a significant distance above the seafloor. We couldn't actually attach the second trap to the mooring until all the line was already deployed, so we had to hold the line in place at the edge of the ship while the second trap was attached. It should have been easy enough, but the chief scientist noticed at the last minute that some of the shackles on the second trap were old enough that he wanted them replaced. We halted the operation temporarily with the first sediment trap already underwater so he could replace the shackles. One of old shackles was already under tension at the edge of the ship by the time he noticed it, so getting it out and replacing it was a delicate operation. We had to hold the mooring line in place with two different ropes to take all the tension off of the shackle in question, then hold our breath while the chief scientist installed the new one at the edge of the ship. It worked out in the end, but there were a few tense minutes there. Note to self: check all shackles before deployment begins.

Float chain gang. Photo by Brie Maillot.

After another couple hundred meters of line, we deployed two strings of floats. The floats will ensure that the sediment traps return to the surface when the time comes, after one year. Transporting the weights was an interesting experience, because they're connected to each other with a heavy chain. In order to carry them, you need about one person per float arranged in a "chain gang." Each person grabs the chain on either side of their float, and we shuffle along in small steps, traveling sideways. I felt slightly like a minion, or a member of a very strange kickline.

Hoisting the floats. Photo by Brie Maillot.

Once we got them to the edge of the deck, the floats were hoisted high into the air and lowered slowly over the side of the ship. We discovered in the process of hoisting them that the float chain was just a hair longer than the height of the ship's A-frame, so the last bit of chain had to be gently pushed over the side by hand. We alternated between hoisting the floats higher and swinging the A-frame out over the water until they were clear of the deck and could be lowered down, the whole time trying to keep control of the suspended floats so they didn't hit the deck or each other. The last thing you want is to damage your floats on deployment, because without them, the mooring will never come back.

The mast sinking away. Photo by Brie Maillot.

The very last item to go overboard was the mast. It was a long pole with weights at one end and floats in the middle to keep it upright, plus a flag at the top to help us see the mooring upon recovery. Once the mast was in the water, we had to turn on the ship's engines and start moving slowly forward so it would stream out behind us and keep the line from getting tangled. Most of the mooring was already underwater at this point, but one line of floats was still suspended from the A-frame. Slowly, the floats were all lowered down, and the mast was allowed to stream behind. We all stepped back from the edge and looked at each other's empty hands. That was it - the last piece. The entire mooring was in the water, and the only thing left to do was set it free. Holding a spool of thin rope in his hand, the chief scientist stepped to the edge of the deck and gave a firm tug on the quick release. One float at a time, the mooring slowly sank away.