Award: OCE-1936746

At Lamont-Doherty Earth Observatory there is an archive of about 1000 gas samples that were extracted from large volume (250-liter) seawater samples collected in the 1980s and early 1990s throughout the Atlantic Ocean and stored in steel tanks. Each gas sample has a volume of about 4 liters and can be measured for many different gases that are dissolved in the ocean, which includes gases for which there was not a method of measurement in the 1980s. This volume of gas is sufficient for numerous measurements of different gases and subsamples can be taken while the remaining gas continues to be stored in the tanks for other measurements in the future. A large fraction of the deep water in the world ocean forms in the northern North Atlantic Ocean where surface water loses heat to the atmosphere and gains salt from sea ice formation. Both processes increase the density of the water and it sinks and spreads throughout the Atlantic Ocean and into other ocean basins. The concentration of certain gases reveal information on the deep flow paths of this water and the rate the water sinks, which is closely tied to the earth’s heat budget and climate. Measurements made on these stored samples can be compared to measurements made on samples collected today from the same locations, potentially revealing changes that have occurred in the formation and circulation of deep water the Atlantic Ocean over the past 3–4 decades as the global climate has warmed. The objectives of this grant were to determine the integrity of these samples after 3-4 decades of storage in steel tanks and to measure some gases that were could not measured in the 1980s. Subsets of the stored samples were measured for different gases. The pressure in the sample tank was measured when it was initially opened to obtain a subsample and these measurements were compared to the pressure in the tank when the gases were collected. The agreement was excellent, indicating insignificant leakage had occurred. Samples were measured for nitrogen, oxygen and argon by gas chromatography. There was excellent agreement between oxygen measured at sea when the samples were collected and oxygen measured on the stored gas. Comparison of the nitrogen and argon measurements to concentrations expected from their solubility at the temperature and salinity of the water were also in good agreement, although the argon measurements were slightly low. Overall these measurements indicate the samples were not affected by decade long storage in steel tanks. Ar-39 is a radioactive isotope of argon with a half life of 270 years. It enters the ocean from the atmosphere and after surface water sinks, its concentration decreases by radioactive decay. The time since a water parcel was at the surface (mean age) can be estimated from its Ar-39 content. In the 1980s, this measurement could only be made on extremely large samples, but the measurement can now be made on much smaller samples that can be taken from these stored samples. Comparison of Ar-39 measurements on stored samples to measurements on samples collected today will reveal if the mean age of the water, and hence production rate of the water, has changed since the 1980s. Six pairs of samples collected in 1981 and 2019 from the North Atlantic are now awaiting measurement. Isotopic ratios of dissolved oxygen (O-17:O-16, O-18:O-16) can be measured extremely accurately now, compared to the 1980s, and these accurate measurements reveal the fraction of the dissolved oxygen produced by planktonic photosynthesis when the water was at the surface. A few deep water samples were measured showing the presence of photosynthetic oxygen. Measurements of more stored samples and modern samples collected at the same locations should reveal information about primary production in North Atlantic surface waters during the past few hundred years and if changes have occurred during the past few decades. Ratios of Ar:Kr and Ar:Xe and isotopic ratios of these noble gases were also measured on a few deep samples at a precision that was not possible in the 1980s. These data are still being processed, but will likely reveal information on the gas exchange rate in the surface source water regions for deep water since different gases have different exchange rates. Comparison of these measurements with measurements made today in the same locations may reveal changes in the gas exchange rates, which are affected by water temperature and ice cover. These are just a few examples of the potential for measurements on the stored gas samples to yield important information about the Atlantic Ocean and changes that may occur as the climate warms. There may well be other gases we are not yet aware of that can yield important information. This set of stored samples is a valuable resource that is available to the research community. Last Modified: 02/08/2022 Submitted by: William M Smethie Jr.