Sampling from the deep

After having left the coast of Panama, we have been steaming into the Panama Basin for the past couple of days. 

Since then, we have performed four CTD casts! CTD stands for Condutivity-Temperature-Depth, and it is an instrument that measures these properties of the water column (with conductivity giving salinity, and depth coming from pressure measurements). As well as this, a carousel of ‘niskin bottles’ is attached to the instrument, which collects seawater samples from different depths. Other measuring instruments can also be added, including an SVP (sound velocity probe) which measures the speed of sound in the water, and devices which measure the oxygen content and turbidity among other things. All in all it’s a pretty fantastic instrument!

The CTD being lowered over the side. The measuring instruments are attached to the metal frame at the bottom and the side, with the carousel of 24 bottles above (credit: Emma)

The CTD is lowered over the side of the ship using the winch system, right down to within meters of the bottom of the ocean. Once we passed the edge of the continental shelf, this was down to ~3000 m deep, so a lot of cable is needed. The continental shelf is where the ocean-covered continental crust changes into proper oceanic crust, which has a lower elevation due to its higher density. This leads to a relatively steep increase in depth over the shelf. Often, there is increased upwelling of nutrient-rich water at continental shelves, which generally means they’ll be more wildlife around. And sure enough, we did see a few pods of dolphins leaping around the ship, though sadly too far away and quick for good photos.

CTD emerging at the surface (credit: Jowan)

CTD being brought back on deck (credit: Jowan)

Once the CTD is back to the surface, we take water samples from the niskin bottles for analysis of oxygen and salinity content, and sometimes helium content too. More about each of these will come in future posts. It was strange thinking that as we were taking the samples from the first bottles, we were being splashed with cold water (~6 °C) from over 2000 m deep in the ocean, a place none of us will ever go (at least for now!). This contrasted to taking the samples from the last bottle, which contained water from close to the surface of the ocean and was at ~27 °C, warmer than the current air temperature and made us want to go for a swim.

Taking water samples from the niskin bottles (credit: Jowan)