It is time to go back to a subject I left unfinished almost a month ago. How do we know the time it takes water masses to move from their place of origin to where we find them?
Here is a review of what we have covered about water masses with the date in which I wrote about that topic, in case you have not read those posts.
Water masses are large volumes of water that have similar water properties (temperature, salinity, dissolved gases, etc.) [March 9].
Water acquires its properties mostly at the surface of the ocean where it exchanges energy and gases with the atmosphere, receives energy from the sun, and where freezing, precipitation, evaporation and melting occur [March 9].
The exchange of gases between the ocean and the atmosphere depends on the relative values of partial pressure of the gas in the water and in the air [March 22].
Surface waters sink when they become more dense than the surroundings [March 6].
The density of water depends on temperature, salinity and pressure. Some surface waters in the polar regions become very dense in winter as other water freezes and releases a very cold brine [March 6].
We can trace the origin of a water mass by looking into its properties. We can therefore estimate how far the water mass has travelled [March 9].
In order to predict where a water mass will be in the future we need to know the velocity at which it is moving. We all think we know what velocity means: how fast an object is moving, but most people forget to think of the direction of the motion as part of velocity. In physics, this is the difference between a scalar and a vector. Speed is the scalar quantity, as it does not involve direction; velocity is speed with direction. If I want to predict where a car will be in 20 seconds it matters how fast it is going, but it also matters in which direction it is going. Anther way of looking at this is given by the difference between traveling at 50 miles per hour towards a big truck or 50 miles per hour away from the big truck. The result is very different, even though the car travels at the same speed (I do not encourage you to try this experiment).
By looking at water properties we have the distance and direction in which the water has moved (the direction and distance form the vector called displacement). We only need to know the time it took the water to cover that displacement to calculate its velocity. But, how can we time the motion of a water mass?
Oceanographers realized that water masses come with their own stopwatches. The basic principle is to identify an atmospheric gas that has the following characteristics:
- Its concentration in the atmosphere has changed over time. 2. We know its concentration in the atmosphere at different times. 3. The gas concentration remains constant in the water, or we have a way of knowing how its concentration changes over time.
Compounds that satisfy this criterion are called tracers because they help us trace the water mass back in time (not to be confused by the term 'trace' as in very small concentration, the way it is used in 'trace metals').
One example is tritium, which is an isotope of the element hydrogen with two neutrons (isotopes are atoms of the same element that have different number of neutrons). Humans released large quantities of tritium to the atmosphere during the atomic weapons tests back in the 50's. We know the concentration of tritium in the atmosphere for different years. The oceans took some of the tritium at the surface, which then got trapped as the water sank. Tritium is radioactive with a half life of 12.5 years. It releases ß particles (one neutron of the tritium turns into a proton + electron. The electron flies away and the proton remains) so tritium turns into helium.
We measure the amount of tritium and helium in a sample to figure out the initial tritium concentration in the water. Scientists need to know what was the initial concentration of tritium and match it to when the atmosphere had that concentration. This way they can estimate how long ago was the water at the surface. For all this they need to measure the mount of tritium that is in the water today as well as the helium as its decay product. Ingenious, is it not?
Anthony Dachille is in charge of the helium and tritium measurements. If I were to do a movie portraying a science lab with strange looking equipment and cool procedures aboard I would probably chose his. While everybody else is using glass bottles to sample from the rosette, Anthony uses fancy metallic cylinders with valves and everything for collecting helium. He still uses glass bottles for the tritium samples.
Tubes for sampling water from the rosette to measure helium
I cannot say much about what happens to the tritium samples, since some samples will be analyzed back at Lamont Doherty Earth Observatory and others at Woods Hole Oceanographic Institution.
Helium samples, on the other hand, need more processing here on the ship before being sent back to those institutions. Anthony extracts the helium gas from the water sample and stores it in glass ampules. The principle for extraction method is simple increase helium's partial pressure in the water as much as possible and decrease its partial pressure above the water so the helium flows into the air (see March 22 for an explanation on partial pressure). Let us follow the hole process. Anthony first connects the sampling tubes to other stainless steel structure with more valves. The large cylinder on this structure is where the sample will be heated later on.
Sampling tubes fro helium attached to part of the extraction equipment. The sample of water will go on the large metallic cylinder where it will be heated with an electric heater.
Anthony connects this larger structure to the extracting machine. The helium will end inside the glass bulbs that you see in the next picture.
Machine for extracting helium from the water. The glass bulbs will collect the helium gas.
He places plastic cups with water an ice on the machine to give the glass bulbs an ice bath.
Plastic cups hold the water and ice for the ice bath for the glass bulbs that will collect the helium.
Anthony opens the valves and starts the pumps to create a vacuum on the tubes around the sample cylinder. He then closes the valves to the pumps and opens the valves of the cylinder. The water drains to the larger cylinder below the sampling tube where it is in a vacuum.
Having the sample in a vacuum (no air above it) means that the partial pressure of any gas is zero above the water. Anthony starts the electric heaters that are under the water sample to increase the partial pressure of the helium. The partial pressure of helium is now high on the water sample and zero above it, so the helium flows out of the water. Some of the water in the sample evaporates because it is hot and in a vacuum. The water vapor flows upwards and carries the heavier helium with it to the glass capsule in the ice bath.
The helium gets trapped inside the glass bulb. There is a narrow section of glass above the glass bulb where we can see the water vapor carrying the helium.
Upper part of the glass bulb for collecting helium from water samples. The section is narrow so it can be broken with a torch.
Anthony uses a torch to break the narrow section of glass and seals the upper part of the bulb turning it into an ampule. The helium and a small amount of water are enclosed in the glass ampule.
Anthony breaking the helium container and sealing it with a torch. He now knows how to seal the ampoule so it is easier to break with the spectrometer.
The ampules will be placed on a spectrometer back on land that will pierce the top of the ampule and measure the amount of helium in the sample. Anthony said that the tritium is left on its bottle to decay into helium fro about six months and will later be extracted in a similar way back home. Knowing how much tritium decayed in six months, and knowing that half of the sample decays in 12.5 years, a simple exponential equation tells them how much tritium was in the bottle at the beginning.This is a good method for estimating the age of a water mass (years since it was at the surface) that is about 60 years old when nuclear testing was being carried out. There are other methods for estimating the age of older waters.