11.5: Measuring the Ocean’s Layers

Since the time of the Challenger expedition, oceanographers used various types of water sampling bottles for obtaining seawater samples at depth. The Pettersson–Nansen bottle, co-invented by Norwegian oceanographers Otto Pettersson (1848–1941) and Fridtjof Nansen (1861–1930), became commonplace on oceanographic vessels by the early 20th century (e.g., Mill 1900). Typically, a single bottle or a string of bottles is lowered in the open position until the bottles reach the desired depth, whereupon they’re closed, trapping a volume of seawater inside. When the bottles are brought back on board, samples of water are taken for immediate analysis or preserved for future analysis.

While perfectly fine for many types of chemical analyses, this method introduces problems for measurements of temperature. As the sample is raised, its temperature may change as it exchanges heat with the surrounding seawater. This gives an inaccurate reading of the sample’s actual temperature at depth. The invention of reversing thermometers solved the problem of temperature measurements at depth. These thermometers could be attached to sampling bottles and turned upside down at depth using a weighted messsenger sent down the line. This to cut off the supply of mercury and prevented any further changes in the reading (e.g., Deacon 1971).

So, too, oceanographers desired a faster means for determining salinity. The salinometer—an electronic instrument for measuring salinity—reduced the need for laborious chemical measurements. Combined, reversing thermometers and salinometers permitted oceanographers to take more samples in more places in the world ocean (e.g., Salinometry 2023).

Improvements in computing technology and reductions in the size of computers by the early 1970s paved the way for development of a new kind of instrument, the CTD—the conductivity–temperature–depth instrument. Based on an instrument developed by American engineer Neil Brown and colleagues in the early 1960s, the CTD is considered the workhorse of oceanographic research (e.g., Salinometry 2023). There is perhaps no better instrument for obtaining a near-instantaneous measurement of the properties of the water column. If CTDs had public appeal, you can imagine an ad campaign similar to one for a popular credit card: “The CTD—Don’t leave your home port without it.” (See The Drum Team 2016 for the history of the ad from which this popular phrase is borrowed.)

It didn’t take long for oceanographers to trick out their CTDs. One of the first things they did was attach the instrument package (the CTD) to a frame on which water sampling bottles could be placed. Modern CTDs come equipped with a rosette, a series of water sampling bottles. Most often these are Niskin bottles, invented by American businessman Shale Niskin (1926–1988; Niskin 1962). Arranged in a circle of 12 to 24 bottles and triggered electronically, the Niskin bottles provide a means of taking water samples on the fly. An oceanographer can look at the vertical profile of temperature and salinity and choose a depth to take a water sample simply by raising or lowering the CTD rosette to that depth. Other kinds of electronic sensors can be attached for measurements of submarine light intensity (i.e., radiometers), chlorophyll concentration (i.e., fluorometers), pH (pH probes), dissolved oxygen (oxygen probes), and more. When an oceanographer raises a CTD to go over the side and watches it swinging back and forth as the ship rocks in the waves, they pray that it will come back safely. One cable snap can send a $250,000 fully outfitted CTD rosette to the bottom of the sea—and an oceanographer crying all the way home.