Sampling starts on the RRS James Clark Ross: ORCHESTRA (Pt 4)  by Melanie Leng

Melanie Leng is currently part taking in an expedition to the Southern Ocean as part of  ORCHESTRA (Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports), a NERC funded programme with partners at the British Antarctic Survey (lead), the National Oceanography Centre, Plymouth Marine Laboratory, and many more including BGS and several UK universities. This is the fourth blog about her trip where the work begins on the RRS James Clark Ross…

We are currently cruising south from the Falkland Islands, crossing the Drake Passage from the Burwood Bank (an undersea shallow ridge off eastern South America) to Elephant Island (off the tip of the Antarctic Peninsula). We are repeating some established measurements of ocean temperature, salinity, oxygen, and currents that have been made with support from NERC funding since the early 1990’s but we are also adding some new novel measurements, including O, C, N and Si isotopes, nutrients, and micro plastics to help us understand the changes in the ocean. Most of the measurements are made on discrete water samples collected from depth profiles (down to the sea bed which in places is 5km below the sea surface) taken at regular distances along the transect. The water samples are taken using an instrument called a CTD.

The CTD is a carousel of niskin bottles used to collect discrete samples of water at different depths in the ocean as well as make some continuous measurements.

A CTD is an instrument used to measure the Conductivity (used to determine salinity), Temperature, and pressure of seawater (the D stands for “depth,” which is closely related to pressure) of the ocean but also to collect discrete water samples.  The water samples are taken using a rosette or carousel of “Niskin” bottles. Niskin bottles can be opened at both ends. The open bottle is lowered into the ocean on a wire until it reaches a certain depth and then the bottle is closed by a weighted trigger that is sent down the cable from the surface. Our Niskin bottles are set up in a circular rosette of 24 bottles attached around the CTD instrument. This allows us to take samples at different water depths in a way that seals off the sample and allows it to be brought to the surface without mixing with water from different depths. Getting water samples from different depths in the ocean is important to understand how the water chemistry and physical properties changes with depth.

The CTD being recovered from the Southern Ocean.

While the scientist work we have a dedicated crew on board that support us, for the sampling we are indebted to the engineers and deck crew for help with the CTD, as well as those responsible for the successful operation of the ship.

A CTD sampling team.

Cruising track from the south from the the Burwood Bank (an undersea shallow ridge off eastern South America) to Elephant Island (off the tip of the Antarctic Peninsula).

I am tweeting @MelJLeng and @ORCHESTRAPROJ and Facebooking (Orchestra project) during this trip, as well as updating the BGS britgeopeople.blogspot.com and drakepassageblog.wordpress.com when I have time.

Melanie Leng is the Science Director for Geochemistry at the BGS and the BGS lead scientist for ORCHESTRA.