If you don’t want to get wet, stay at least 4 feet away from dear Rosy during sampling time. As soon as the Rosette is brought into the hangar, eager students, technicians, and scientists flock to the 10-liter Niskin bottles with their sampling tubes and containers to retrieve seawater from specific ocean depths. The first description that comes to mind: Ordered chaos.
Calls of numbers echo off of the hangar walls and trail off into the wind as we steam to the next station. “127 on 5!” announces the chlorofluorocarbon (CFC) team representative, indicating which syringe he will be using to extract the seawater from Niskin 5. “4.8 [degrees C] on 4!” declares the oxygen sampler after measuring the temperature of the seawater contained in Niskin 4. All these numbers may be meaningless to the uninformed bystander casually walking past the hangar during sampling time, but they are crucial to minimizing contamination of the samples, and ensuring that all the necessary information is kept in order.
Who is responsible for maintaining the law of the sampling order? The sample cop. With the sample log and pencil in hand, the sample cop keeps track of who has taken what sample from each bottle so that no one goes out of turn. The order is as follows: CFCs, Helium, Oxygen (O2), Dissolved Inorganic Carbon (DIC), pH, Total Alkalinity, Radiocarbon (13C and 14C), Dissolved Organic Carbon, Tritium, Nutrients, and Salinity. All of these measurements are the basis of the CLIVAR/CO2 repeat hydrography program. In conjunction, they will lead to inferences about climate variability and the carbon cycle.
There is a method to the sampling madness. The order is determined based on the effects that extracting of the sample will have on the chemical property under examination. This is why CFCs must go first. Because these anthropogenically produced compounds have only existed in the atmosphere for approximately 70 years, deep water, which has not been in contact with the atmosphere for more than 1000 years, holds no CFCs. Thus, if the sample of seawater happens to be exposed to open air during extraction, it will attempt to equilibrate, taking up CFCs from the air. This is detrimental to the accuracy of the concentration readings that the science team has so diligently worked to achieve.
This problem is not unique to CFCs, however. In addition to atmospheric contact, many samplers must be weary of microbial growth in their samples. Biological effects on the chemical properties of seawater are immense, and are generally strongest in the upper 1000 meters in situ. Under laboratory conditions, however, these processes can occur rapidly in water taken from all depths. Thus, it is crucial to poison samples with mercuric chloride, or alternative poisons, to eliminate species growth as soon as possible.
All of the measurements taken on this CLIVAR/CO2 cruise have to correct for possible errors, despite the care taken during extraction. Fortunately, there are several measurements which are taken more than one way, and thus, can be compared against each other for accuracy. Among these are salinity, temperature, and O2. The utilization of various instruments and measurement techniques to determine the best results is of the upmost importance to this repeat hydrography program. Hopefully, such diligence will prove fruitful in determining climate variability, and making further predictions about the fate of our world in a global warming scenario.
by Angelica Gilroy and Yongming Sun
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