42° 53.684’ N, 59° 48.143’ W
Winds: 21 knots
The CTD is back on Atlantis. What is it, anyway? While the letters “CTD” are shorthand for Conductivity, Temperature, Depth, “the CTD” commonly refers to a collection of instruments and water bottles (Niskin bottles) bundled onto a steel frame (see photo: WHOI's Joe Murray collects water from the CTD for Radium analyses, 14 Sep 2010). Collectively, this is the primary tool that I use to collect water samples anywhere between the surface and the bottom of the sea. I’m often asked if I dive to collect my samples. While I would love to experience the deep sea untethered, the CTD is a much easier way to grab a few liters of water. Here’s one reason why. Liquid water is a very dense fluid (1 gram per milliliter). Think of a gallon of milk. That’s about 8 pounds. Now think of a stack of 10,000 milk jugs resting one on top of the other on top of your head. That’s about how much pressure we would feel at the depths we are sampling. I would rather use the CTD.
Basically, we lower the CTD over the side of the ship while monitoring the instruments on a computer. This gives us a continuous readout of the water temperature, pressure, salinity (conductivity), oxygen concentrations, etc… throughout the water column beneath us. Among other things, this information reveals where life is most abundant, where particles are raining down, and where the water comes from. After the CTD reaches its maximum depth, we hoist it back to the surface, stopping frequently to fill our Niskin bottles with water samples. We can do all of this from the comforts of our ship.
Recovering the CTD takes a good team and clear communication. It is heavy. And it happens to swing from a crane above the perpetually wet deck of a rolling ship. So you gear up with steel-toe boots, a life vest, and a hard hat. You look over the rail at the wire that disappears beneath the waves. The winch operator keeps winding it in. Faintly, it comes into view—a pale blue leviathan rising from the deep. It breaches the surface with a roar and sprays foam through the waves. Our job at that moment is to grab the swinging mass with 20-foot long hooks, safely assist it over the rail and onto the deck, and then bolt it down for safety.
All of this takes time, of course. Our first CTD cast at Station-6 lasted 5 hours from deployment to recovery. It will be weeks before we can begin analyzing these water samples back at WHOI. It will be months before we will see results. In the meantime, we have 4 more CTD casts and 1 sediment core to complete before moving to deeper waters…