Dating water

The concept of water age was completely foreign to me before starting graduate school. How can the ocean have anything but one age, i.e. really, really old? I soon realized that oceanographers definition of age refers to the time elapsed since a water parcel was last in contact with the atmosphere. As an aside, the year in which the water left the surface is known as the vintage, as in “I have a fine bottle of Southern Ocean seawater vinted in 1982 for your enjoyment.” From a physical standpoint, we can use age, amongst other things, to figure out how quickly water in the ocean is transported around and which areas are flushed more rapidly with water that comes from the surface. Age is also important from a biogeochemical point of view. For example, if the water’s age is more than 300 years old, it came from a time when anthropogenic carbon dioxide wasn’t significant.

How do we figure out the age of a water parcel? This is where chlorofluorocarbons (CFCs) (among other tracers) come in. CFCs were used as propellants and refrigerants starting about 1920s and increased exponentially until the early 1990s when their use was banned because they caused damage to the ozone in the upper atmosphere. Once in the ocean, they are essentially inert, so we can match a water sample’s CFC concentration to the year when the atmosphere had the same concentration (as illustrated in the cartoon below, courtesy of Michelle Weirathmueller).

So what does CFC data look like? Generally speaking, it’s high near the surface and decreases the deeper you go reflecting the fact that the shallower water were exposed more recently to an atmosphere with higher CFC concentrations (check out my not-so-amazingly accurate hand contour of our data thus far). The current 2013 P02 CLIVAR / CO₂ cruise is repeating a section first taken in 1994, repeated in 2004. By comparing these different data together, we can get a sense of how the oceanic circulation has changed over the past few decades and perhaps provide a physical explanation for some of the changes the other measurement groups on this cruise have observed.

Carefully hand-contoured plot of CFC-11 measurements. The vertical axis is depth, the horizontal axis station number and longitude. Yellow shows the highest concentration.

by Andrew Shao