EAGER: Measurement of Sulfur Hexafluoride and Argon-39 on Archived Samples from the Atlantic Ocean Collected in the 1980s

During the 1980s about 1200 dissolved gas samples were collected from 250-liter water samples at locations throughout the Atlantic Ocean. These samples have been archived and can be analyzed for a variety of gases that could provide information on deep water formation and circulation in the 1980s. In particular, they could be measured for 1) the natural radioactive isotope, argon-39, which is chemically inert, has a well-known input to the ocean, and half-life of 269 years, and 2) sulfur hexafluoride (SF6), a man-made gas that is chemically inert and has been entering the ocean for the past 6 decades with a well-known, nearly linear increasing input through time. The argon-39 half-life is ideal for investigation of the mean circulation of the deep Atlantic Ocean and SF6 traces the circulation of recently formed deep water. The current technology to measure SF6 in the ocean and argon-39 on small water samples was not available in the 1980s. This work will determine if the stored samples collected in the 1980s are uncontaminated by leakage during storage and if the samples can be accurately measured for SF6. If this is the case, it will be possible to compare measurements made on these samples to measurements on samples collected from the same locations on cruises carried out today and in the future. This will provide information on changes in the deep Atlantic circulation that may be occurring as the Earth's climate warms. This project will employ two undergraduate students, providing them laboratory and research experience as they learn how to measure gases with gas chromatography.

The objectives of this EAGER (Early Concept Grant for Exploratory Research) project are to check the integrity of the stored gas samples collected in the 1980s, to determine if these samples are suitable for SF6 measurements, to develop a procedure for transferring aliquots of these samples to other labs, and to assemble all of the information on these samples into a single file that can be distributed to other investigators and posted on oceanographic websites. The samples were collected in two types of containers, 2.8-liter high pressure steel tanks for the Transient Tracers in the Ocean/North Atlantic Study (TTO/NAS) and Tropical Atlantic Study (TTO/TAS) and 4-liter low pressure steel tanks for the South Atlantic Ventilation Experiment (SAVE). The investigators will test the integrity of these samples by comparing the tank pressures to the pressure at the time of collection and by measuring the nitrogen to argon ratio, which will show if there is any atmospheric contamination, on a subset of about 100 samples. They will determine if the samples are suitable for SF6 measurements by measuring SF6 on this subset using the method used routinely to perform these measurements at sea, and comparing these measurements to CFC-11 and CFC-12 measured at sea on these cruises. The sample subset will include vertical profiles from surface to bottom at about 8 stations and surface samples from about 20 stations. SF6 concentrations in the surface samples will be compared to the expected concentrations assuming equilibrium with the atmospheric concentration at the time of sample collection. If successful, these measurements will ensure the integrity of samples collected in the 1980s and open the door for using these samples to investigate deep water circulation in the Atlantic Ocean during the 1980s and changes in this circulation since the 1980s. There may also be interest in making other tracer and chemical measurements that were not possible in the 1980s.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.