Monday, April 22, 2013

Sampling the Depths: How oceanographers study the ocean

by Evan Howard, Woods Hole Oceanographic Institution

We are in the South Atlantic, but some of the water underneath us came from near Greenland, and some from Antarctica. That water has traveled a long way, and the central goal of this cruise is to see what happens to dissolved organic carbon carried by the water on its journey away from the surface. How do we know where the water came from, and how do we sample water from great depths?  

The vertical ocean and the CTD 
At any given location, all ocean water is not the same with depth and time—parcels of water move around and under each other. As water from different sources meet, the heavier water slides beneath lighter water. Water parcels move most easily through similarly light or heavy water, and only mix with difficulty into other parcels—it is hard for light water to sink into heavy water. This movement can carry along with it chemicals and organisms, or prevent them from moving vertically to mix with other water parcels. For example, we are interested in the amount of nutrients available to phytoplankton. If there are only shallow and deep parcels of water, we may want to consider nutrient concentrations from each water mass separately—phytoplankton grow near the surface and cannot easily access deep nutrients, even if there is plenty below.

Monica Torres Beltran and Erin Eggleston recover the
CTD rosette. (Winn Johnson, WHOI)
The Conductivity, Temperature, Depth instrument (CTD) is one of the most widely used oceanographic tools. We use the CTD to determine at what depths different water parcels are found. Salinity tells us how fresh or salty the water is. Your tongue is very good at detecting even tiny amounts of salt, but the CTD can’t taste salt. However, when salt is dissolved in water it conducts electricity more easily—this is the conductivity in the name of the CTD, and what the instrument can actually measure in place of salinity. Temperature also varies between water parcels. At the surface of the ocean, the sun heats up the water, so there is often a layer of warmer water at the surface. But warm water can travel deeper too if it is in a heavy water parcel. Heavier water parcels tend to not change salinity or temperature much from where they first began to sink away from the surface. For this reason, temperature and salinity measured on the CTD can tell us where deep water originally came from.

Often an oxygen sensor is also attached to the CTD. Oxygen is mixed into the water from the air, particularly in the cold regions where deep water forms, because cold water holds more gas than warm water—have you ever noticed how your soda goes "flat" as it warms up and the bubbles escape? It can’t hold as much gas when it is warmer, just like the ocean. So the amount of oxygen in the water can help identify water parcels that have traveled from far away, just like temperature and salinity—this is how we know that some of the water beneath us came from the far ends of the North and South Atlantic (see the video below for more).

Bringing water back to the surface with Niskin bottles 
 When we look at CTD and oxygen measurements they pinpoint at what depths important or exciting features are found in the water parcels. If we want to measure other physical, chemical, or biological properties in that water, we need to bring some back to the surface. The most common way to do this is using a Niskin bottle, a special sampling container that closes around a sample at a target depth. Such bottles were originally designed by Norwegian oceanographer and Nobel Peace Prize winner Fridtjof Nansen for exploring the Arctic and North Atlantic Oceans. The bottles were later improved by an inventor named Niskin (hence their name).

The Niskin bottles are on  a circular rack called a rosette that is secured to a metal cable. The CTD and other sensors are also attached to the rosette. A crane lifts the rosette using the cable and lowers it into the ocean. The Niskin bottles are held open at both ends until they reach the depth we want. Aboard ship, we send an electronic command down the cable—this triggers a latch that releases the end caps to snap shut on one or more bottles. When all the Niskin bottles on the rosette are full, we return it to the surface so that we can collect the water from each depth. This water is analyzed to develop a more detailed picture of the vertical ocean beneath us.