Sampling for Science

By Hannah Dawson, Seth Travis and Dave Webb

We’ve left the Antarctic Circle and the icebergs behind and have been heading north along our transect for 13 days now. If you’re curious about the ocean properties we’re sampling and why then this is the post for you!

Recovering the CTD after a cast on one of the nicest days the Southern Ocean has to offer.  Photo courtesy of Cara Nissen

The photo above shows us retrieving the rosette package, which contains a sequence of bottles that can be closed independently to collect water samples at depth. In additional to the water samples, the rosette has a number of other instruments to collect additional information. One of the instruments installed on the rosette package is called a CTD which measures the conductivity, temperature, salinity and depth of the ocean.

Photo courtesy of Cara Nissen

The following figures show the potential temperature, salinity and oxygen levels that we have measured along this current occupation of I08S.

Once the rosette package is back on deck, it’s taken into the hangar for sampling. On this cruise we have scientists sampling the following water properties: chlorofluorocarbons (CFC’s), oxygen, dissolved inorganic carbon (DIC), pH, alkalinity, chlorophyll, coloured dissolved organic matter (CDOM), radioisotopes of carbon (14C and 13C), dissolved organic carbon (DOC), δ18O, δ15N, salinity and nutrients. By sampling these properties, we can identify different features of the ocean and further our understanding of the processes operating. Without taking the time to describe all of the great science being done, we wanted to describe a few of the samples in more detail, and how they help us understand the ocean.

Sampling at the rosette. (Photos courtesy of Seth Travis)

Chlorofluorocarbons (CFC’s) are known to cause ozone depletion in the atmosphere but are useful tracers of water in the ocean. CFC’s are man-made compounds that were used for a long time as refrigerant gases. The CFCs can be thought of a dye that is added to the ocean surface waters. When ocean waters are in contact with the atmosphere, there is some transfer of CFC’s from the atmosphere to the ocean. These waters can then be subducted to depth, and move throughout the ocean. By knowing the concentration of CFC’s in the water at different locations, and the concentration of CFC’s in the atmosphere at different times, we are able to infer how long it has been since a water at a particular location has been in contact with the atmosphere, which is often called the “age” of the water. By following water masses from oldest to youngest, and CFC concentrations can act as a map of the paths water takes, similar to pouring dye into a stream and watching it be carried away by the currents. Now that CFCs concentrations are starting to decline in the atmosphere we are also measuring Sulfur Hexafluoride (SF6). This man-made compound also acts as a dye just like the CFCs and it continues to increase in concentration in the atmosphere.

CFC Gas Chromatography Machine (Photo courtesy of Seth Travis)

Radiocarbon (14C and 13C), can also be a useful tool for tracking various oceanic parameters. 14C occurs naturally in the atmosphere, and can be transferred into the ocean waters at the ocean surface. On this cruise, Sarah is trying to use radiocarbon to determine the age of dissolved organic carbon (DOC) pool (or supply), in the South Indian Ocean. Generally, DOC has an age of between 2000-6000 years, but there are few published profiles of 14C. Most of these profiles are in the Atlantic, Pacific, and Southern Oceans, with none taken in the Indian Ocean. The profiles taken on this cruise would help improve our knowledge of global 14C pools, and how these pools of DOC circulate between different oceans.

On the cruise, there are a number of different nutrients being measured. These include nitrate, nitrite, phosphate, silicate, and ammonia. From a biological perspective, the levels of these nutrients can give indications of available primary productivity (a measure of the conversion of inorganic carbon to organic carbon), and can help to understand the food chain in the area. These nutrient measurements are also a key component in understanding organic and inorganic carbon cycling. Beyond the biological implications, physical oceanographers are able to use the nutrients as tracers of water masses. By examining data from the previous times that this transect of the ocean has been sampled, we are able to see decadal changes in the levels of these nutrients, and how they vary with other parameters, such as temperature and salinity.

Thanks to Jim Happell, Sarah Bercovici, and Susan Becker for taking the time to explain the work they are undertaking on this cruise.