Saturday, October 31, 2009

Ghosts in the machine?

All clear: Here’s Andy checking for stow-aways in the tool sled prior to Nereus launch. Could that have been the cause of the excess baggage? Or was it just a Halloween haunting?

Please, please, please, let me get what I want (with apologies to Morrisey and the Smiths). Here goes Nereus, back to the seafloor late Sunday afternoon.
Position at 16h00 EST – End of CTD 48
Lat: 18° 33’N Long: 081° 44’W
Water depth: ~5000m

We had a bit of an adventure on Halloween, but not a specially fun one. The launch of Nereus went extremely well and the pay out of our depressor to 2500m went well too. We worried, briefly, whether the release of the fiber-optic tether that would be used for Nereus to get the rest of the way to the seafloor would work OK - we quickly dubbed this “separartion anxiety” - but that too went flawlessly as well.

And so we continued, over 4 hours, all the way to ~5000m, where we dropped our descent weights and aimed to put Nereus into hover-mode. But instead, we kept descending slowly. Even when Chris had the thrusters trying to drive the vehicle up as hard as they could. All the way to the seabed.

We’re still not sure quite why, but the vehicle had arrived at the seabed heavier than expected – not by much, but by enough to have us rooted just where we sat. National Geographic is out with us for a few days at present, filming, but this was NOT quite the kind of drama we had wanted to share with them.

Of course, our engineers are not easily left stumped for long and after 15 minutes or so we already had a first plan of action lined up and put in motion. We jettisoned some of our “ascent weights” (the ones we normally would only drop at the end of the dive, so Nereus would float back to the surface) and that allowed us to start to head south and explore the seafloor from our launch point on thick soft sediment toward our proposed work area (and hydrothermal vents?) about 600m distant and 150-200m shallower.

As we headed south, and began to climb quite steeply uphill, we came across fields of large boulder-sized blocks of basaltic lava and, further uphill again, intact tongues of basaltic lava that had been frozen in place as they had poured over and flowed down the rock face toward the direction Nereus was arriving from.

This already gained us useful information – we now know the hill that we are heading toward must have previously been a site of active and extensive volcanic eruptions. You might think that obvious if we’re working on a mid-ocean ridge (i.e. an underwater volcanic chain), but there was actually something like a 50:50 chance, since the Mid Cayman Rise is a very slow-spreading ridge, that at any given site the local geology would be dominated by long-lived and deep rooted faulting extending all the way down from the seafloor toward the underlying mantle. Sadly, however, that is also where the tale of our first ROV dive to the Mid-Cayman Rise ends.

After some rapid and earnest conversations among the Nereus team, Andy was able to advise us that the consensus was this: the only way to be SURE that we could get Nereus back on deck safely and ready to dive another day was to conserve power (i.e. leave the seafloor immediately) AND to also shed more weight to make sure we came all the way back and didn’t run out of buoyancy while Nereus was still submerged – i.e. never to be seen again. So, unsurprisingly, that’s what we did.

Making sure we had excellent navigational fixes, we lifted one of Jeff Seewald’s titanium vent-fluid samplers carefully off the basket and placed it on the seafloor. The moment it was released from the manipulator arm, Nereus rose up from the seabed. We released all the remaining ascent weights and the vehicle began its long flight upward and home. Happily, the rest of the recovery went smoothly and just around sunset Nereus came back up to the ocean surface where it was promptly recovered and lifted back aboard ship.

Overnight and into this morning we slotted in two more deployments of the CTD rosette, each of which measured the strongest sets of hydrothermal anomaly data we have seen all cruise confirming (surely!) that we really DO have our target narrowed down now. And tonight we are back in the water with Nereus and heading back down. Once again, we’re telling ourselves: this really COULD be the one.

No comments:

Post a Comment