The Bottle Cop
Posted by sogasex on March 18, 2008
By Pete Strutton, Oregon State University
Some of the blog entries so far have talked about data collected by satellites and buoys. We’re also performing chemical and biological analyses on water that’s continually pumped through the ship from an intake near the bow. Like the satellite data, this gives us an idea of the distribution of surface properties. To understand the vertical structure of the upper ocean, including the dispersal of the tracers we added, and biological cycling of elements, we use an instrument package referred to as the CTD. The acronym stands for ‘conductivity, temperature, depth’, which are the core measurements, but a more accurate description would probably be something like ‘tethered, profiling analysis system and water sampler’.
The CTD consists of an instrument package mounted in the center of a large metal frame. The instruments make very precise and rapid measurements of ocean temperature, conductivity (for salinity) and pressure (for depth). Salinity and temperature determine the physical structure of the upper ocean because they strongly influence water density. Other instruments can be added to the CTD to measure dissolved oxygen, chlorophyll (for phytoplankton abundance) and particles (for biological carbon concentrations). The data are sent through the wire to a computer on the ship, and vertical profiles of all parameters can be viewed in real time. Around the instrument package are 24 x 12 liter bottles which are open at both the top and bottom on the way down so that water flows freely through them. On the way back up the bottles are closed, or ‘fired’ at depths from which we want to obtain water samples. On this cruise we’ve usually been taking the CTD to 500m and firing most of the bottles in the upper 150m, but the CTD is capable of going to the bottom, which is about 4000m here. Each deployment, putting the CTD over the side, taking it to 500m at about 1 meter per second, and bringing it back to the ship, takes a little less than an hour.
When the CTD comes on deck, the sampling starts. There are at least a dozen different types of samples to be taken from the 24 bottles that we’ve fired on the way up. Some chemical properties, particularly dissolved gasses, are time sensitive, so to get the sampling done rapidly and in the correct order requires some coordination. Some might like to think of the CTD sampling community as a socialist utopia, with each member working towards the common goal of high quality samples, but it’s actually more like a benevolent dictatorship under the direction of the ‘bottle cop’. This is a high pressure job which requires concentration, empathy and a thick skin. On past cruises I’ve been on, particularly long Antarctic cruises, the bottle cop job is usually given to the person with the greatest potential to become bored, such as the doctor. On SO GasEx, the bottle cop duties are clearly beyond the scope of just one person, so I’m doing the night shift while Dave Hebert (URI) does the daytime CTDs.
The CTD package returning to the surface after a trip to 500m. The 12 liter bottles around the outside hide the instrument package in the middle.
Bottle cop, hard at work, while Sarah Purkey samples for oxygen.
The bottle cop’s log sheet – the ordering of samples goes from left (gasses) to right (nutrients). Biology’s such a low priority that it’s on the next page.