We learned before that the amount of tritium in water can be used as a stop watch. Today we are going to learn about other tracers used for determining the age of water masses, CFCs.
We are going to follow the team that measures the CFCs from sampling to analysis, but we will begin with a small introduction to CFCs for those who might need a refresher. Skip to 'Why measure CFCs?' if you do not need the introduction.
CFC is an abbreviation for man made compounds called chlorofluorocarbons. These compounds were developed in the 1930's to be used as refrigerants and aerosol propellants. They were widely used in industry because of their useful and unique properties. They condense and evaporate in a small pressure range compare to other gases, which make them great for cooling systems such as refrigerators and air conditioners. They are also non flammable and non-toxic, which make them great as propellants (what sends the hairspray out of the can). So, what happened to them?
The problems with CFC became apparent in the 1980's. It was discovered that CFC rise into the stratosphere (a layer in the atmosphere above 10 km) where they react with and reduce the amount of ozone. Ozone in the stratosphere is good for life on earth as it absorbs some of the harmful ultra violet rays. Less stratosferic ozone means more UV rays which produces cancer. It should be noted that ozone is bad for us to breath, so it is considered a pollutant when found at the surface of the earth, but that is another story. In a great show of environmental concern and action, most industrial countries agreed to end the production of CFC by 1997 on what is known as the Montreal protocol. By 1997 only 10% of the peak production was produced.
There are more than one type of CFC. Generically they are also known as freons, the gas used on refrigerators. Oceanographers measure the following CFC: CFC-11, CFC-12, CFC-113. Each gas was released to the atmosphere at a different known concentration over the years.
Why measure CFCs? Why do we measure in the ocean compounds that have essentially not been produced since 1997?
The ocean and atmosphere exchange gases; when the concentration of a gas is higher on the air than the water, the gas flows to the water, and vice versa (see the March 22 entry for a more detailed explanation).
CFCs in the atmosphere dissolve into the ocean only through its surface, and they essentially do not react with anything in the water. CFC are also only made by humans, there are no natural sources, which makes them great as tracers for water masses. Surface waters eventually sink to greater depths in certain regions of the world taking the CFCs with them. We know what the proportion among the different CFCs was in the atmosphere every year, so we can match the proportion of CFC found in water masses to the atmospheric values and thus date when water masses were at the surface (see April 2 for a more detailed explanation). The actual process is more complicated, since the solubility of each gas in seawater is different, but that is the general idea.
The CFC group also measures another man-made gas that is found in the water for a similar reason. Sulfur hexafluoride (SF6), is a gas used mostly as an insulator in electric equipment. It is a green house gas, but that is not the reason for measuring it in the water. It is another gas that we can use as a stopwatch (see entry of April 2). People measure SF6 because the CFC levels in the atmosphere have been plateauing in recent years, thus making CFCs less useful for dating younger water masses. SF6 is still being produced and used to date. Its atmospheric concentration is on the rise, making it potentially more useful for dating younger water masses.
Here is how we measure the CFC and SF6 on the boat. These samples are the first samples to be taken from rosette bottles to prevent t gas exchange. We take a 500 ml sample in a glass bottle. The bottle is rinsed several times before being filled.
Sarah samples the niskin bottle for measuring CFC and SF6. The glass bottle sits in a plastic cup where the bottle will remain immerse in water from the same niskin bottle until it is analyzed.
The bottle is caped and the cap fixed with a green plastic holder and placed in a plastic cup. The cup is filled with water from the same niskin bottle, completely immersing the sample bottle tin order to achieve a water seal. Here is a box with some of the samples from a rosette.
Box with glass bottles for CFC analysis. The green plastic is a clip that keeps the bottles closed. Each bottle is immersed in water to provide a water seal.
The samples are analyzed right away in a very strange looking machine. Most of the machine is home made by the scientists and technicians. It has taken several years to develop them and you cannot find them in a store. The sample bottle is placed on a tray and connected to a hose.
Eugene places a bottle for the analysis of CFC and SF6 on the machine he has built over the years
The hose removes the right amount of sample and places it on a glass tube with an aerator on the bottom. The SF6 concentration in the ocean is typically much lower that that of CFCS, so we need a larger amount of water for measuring SF6 than for CFCs. The aerator produces very small bubbles of nitrogen forcing the dissolved gases to move from the water to the bubbles. The gases go from the water to the nitrogen bubbles because there is less partial pressure for all gases, save nitrogen, in the bubbles than in the water (see March 22 for a discussion of the effect of partial pressure).
CFC and SF6 extracting column. Pure gas is bubbled into the sample and all dissolved gases, except nitrogen, pass to the nitrogen bubbles.
Very cold temperatures (-87C) generated by the expansion of liquid carbon dioxide trap the purged gases carried by the nitrogen in columns of materials designed to absorb these gases. Heating the columns then releases the gases and sends them to the chromatographer where different gases are separated.
You might have seen chromatography at work if you have dropped a drop of ink on a paper and seen the ink separate in different colors as it spreads on the paper. Each color travels at a different speed on the paper and that allows us to see the separation. A mixture of gases is passed through a glass tube with some special material that looks like sand. Each gas will travel at a different speed through the material based on their polarity, separating the mixture of gases into its components.
Each gas passes separately through an 'Electron Capture Device' that will measure the amount of gas is passing through. The electron capture device works with the same principle as smoke detectors do (I decided to write the more scientific explanation at the end of the entry for those who would like one, since I do not want to loose those who prefer a simpler explanation). At the end we get the amount of CFC and SF6 on a water sample.
Let us take a look at a vertical section of CF-11 for the transect that we are sampling now, with data from 1992. The vertical axis is the depth in meters, the horizontal axis is the distance traveled by the ship and the contours and colors represent the concentration of CFC-11 with dark pink being the highest concentration and green the lowest.
Vertical section of CFC-11 for a cruise in 1992 along one of the sections that we are currently sampling. The vertical axis is the depth in meters, the horizontal axis is the distance traveled by the ship and the contours and colors represent the concentration of CFC-11 with dark pink being the highest concentration and green the lowest.
You can see that the Antarctic Surface Water (AASW) is the water mass with highest concentration of CFC-11, while Lower Circumpolar Deep Water (LCDW) has the lowest. We can say that AASW is the youngest water mass, the LCDW is the oldest and the AABW is in between. Remember that young and old is relative to how long ago the water left the surface of the Ocean. This is consistent with our idea of how each water mass is generated. The Antarctic Surface Water (AASW), a very fresh and cold water mass produced from summer melting of sea ice (see entry for March 9), has the highest concentration because it is in direct contact with the atmosphere. The Antarctic Bottom Water (AABW), cold and salty water produced by the winter freezing of surface waters, was produced longer ago. The LowerCircumpolar Deep Water (LCDW) came from the North Atlantic and was at the surface on the order of 100 of years ago, which is consistent with the very low CFC-11 values (remember that this is not pure North Atlantic Deep Water but was mixed at the Circumpolar Current with other water masses).
It will be interesting to see the same section with the data that we are generating in the cruise and see how the concentrations have changed in 20 years. What do you think we might find at each depth?
Smoke detectors and Electron Capturing Device
Smoke detectors use a radioactive element, Americium, that emits alpha particles and a sensor that detects the particles. Alpha particles are made out of two protons and two neutrons. The smoke alarm will trigger when a gas gets in between the radio active element and the sensor and blocks the alpha particles. Smoke, which is made of gases and particles floating in the air, and water vapor can block the alpha particles.
The electron capturing device uses a radioactive isotope of nickel (Ni-63) that emits ß (beta) particles instead of Alpha particles. ß particles are electrons emitted from the nucleus of an atom when a neutron turns into a proton and an electron. The device has a sensor that detects the amount of electrons flowing. This system works for detecting compounds with halogens. Chlorine and fluorine are two halogens. What makes halogens different from other elements is that they require one electron per atom to be stable. They require it so much that they snatch an electron from wherever they can. That is why pure chlorine is harmful for us; it removes electrons from your atoms.
Halogens in the electron capture device capture the electrons that are the ß particles. The device can detect how many electrons did not make it to the sensor because they were captured by the halogens, and translates that into how much halogenated compounds are in the sample.