CTD

Weather: The winds picked up overnight and the sea has lost its glassy, lake-like surface, but the seas are still considered relatively calm, for the Axial Seamount area. Thin clouds have blown in and the skies are overcast.

Science Update:

Today’s Objectives:
1) Recover ROV Jason dive J2-731 – check!
2) Deploy CTD (conductivity-temperature-depth) to bottom and back to surface – check!
3) Launch ROV Jason dive J2-732 – check!  

Marine organisms and oxygen

The CTD is a package of instruments that measure Conductivity (to calculate water salinity), Temperature, and Depth. As described in the blog, “Axial Volcano Buoys,” the CTD also has a remotely operated “rosette” of bottles that collect water samples on command in the water column. In the first days of the cruise we deployed the CTD to collect water away from the volcano not associated with the hydrothermal plumes, which serves as background data. Today, the CTD Rosette collected samples over Castle Vent in the International District Hydrothermal Vent Field. These expeditions to Axial Seamount routinely collect CTDs over the volcano’s main hydrothermal vent fields in order to see how the height and intensity of the hydrothermal plumes vary with time. Hydrothermal plumes are detected by the CTD as temperature and turbidity anomalies because they are warm and cloudy. CTDs can also show other interesting patterns about the ocean’s properties vary with depth.

Marine Tech Patrick A’Hearn explains that in addition to collecting water samples, the CTD also measures oxygen concentration (in % concentration) as it is lowered and raised on a wire. The graph above is a screen shot of the data during the CTD deployment that shows the oxygen concentration (blue curve) on the x-axis and depth on the y-axis. The yellow arrow points to 500 m depth, below which oxygen concentration remains very low until nearly at the seafloor at 1500 meters. This shows that oxygen in the ocean mainly comes from mixing with the atmosphere at the surface.

Also on board the ship is a multibeam sonar system that emits sound waves into the water and converts the return travel time of those waves into distance to the ocean floor, which is shown on the graph above by the white line (the display is a cross-section from the surface of the ocean at the top to the seafloor at the bottom – the ship is at the apex of the colored triangle, which shows the strength of the sound echoes). The dark blue semi-circle shows the water column not affected by sonar echoes. The darkest blue is uninterrupted water, but towards the surface the sonar display shows a lighter blue color, indicating the water has lots of objects reflecting the sonar sound energy. Note that those reflectors essentially disappear at a single depth of 500 m, as indicated by the yellow arrow.

Why do the shallow reflectors stop at 500 m and what are those reflectors? We can use the oxygen data to interpret that the reflectors are organisms that live in the water column but that at 500 m depth where the dissolved oxygen concentration decreases substantially, the organisms are not present. It is a well known that plankton and other organisms live in the shallow part of the ocean, and many of them have tiny swim bladders (air sacs to help them move up and down in the ocean) that reflect sound.

At the end of the CTD measurements, the CTD is lifted back on board, but yesterday it was held just at the surface while the deck crew organized its return to the ship (above left). Notice the still, clear water in which it sits- this is an example of the calm seas mentioned in the weather reports that we have enjoyed here for the last few days. The image above right shows the CTD winched into the air before being brought onboard. The grey cylinders are the “rosette” of water sample bottles.

 The images below show a few of the organisms we’ve seen during dives here at Axial Seamount during the cruise, both in the upper water column and closer to the ocean floor. The left image is a jellyfish captured with the “Pilot-cam” on ROV Jason at 31.4 m depth; the center image is a Rojo Grande jellyfish at 1458 m; and the image on the right is a squid that attached momentarily to the “Pilot-cam” just before we captured this image of it with the overhead “Brow-Cam.”