Award: OCE-1923915

This project supported the development and first applications of a new technique for high-precision dissolved gas measurements in the ocean, shedding new light on how the physical exchange of gases between the atmosphere and ocean occurs. As excess carbon dioxide (CO2) continues to be emitted to the atmosphere, much of that CO2 will end up in the ocean. The rate at which the ocean takes up CO2 from the atmosphere depends on both physics and chemistry, and there are major outstanding questions about how gases (like CO2) move across the sea surface, from the atmosphere to the ocean. Like CO2, oxygen (O2) is a dissolved gas of great importance to global biological cycles that is undergoing change in our ocean, with most models predicting a global decline in global marine dissolved oxygen concentrations. Improving our ability to trace the physical processes that govern the efficiency of atmosphere-ocean gas exchange is therefore important to better predict the future of carbon, oxygen, and other important gases in the global ocean. To specifically address these outstanding questions of air-sea gas exchange, this project focused on high-precision measurements of noble gas isotopes, which are naturally occurring gases that are chemically and biologically unreactive. As a result, only physical processes can change the noble gas isotope composition of seawater, making these gases useful tracers of physical air-sea exchange. This project involved co-development of a new analytical technique for at-sea collection of large seawater samples in robust leak-tight vessels in two labs - one at Woods Hole Oceanographic Institution (WHOI) and another at Scripps Institution of Oceanography (SIO) - helping to establish the core method of an early career principal investigators new lab at WHOI and contributing to the dissertation of a doctoral student at SIO. The collaborative nature of this project enabled interlaboratory comparisons to be evaluate the analytical method, resulting in two methodological peer-reviewed publications. Additionally, to interpret real-world oceanographic measurements, new high-precision determinations of the background (air-equilibrated) noble gas isotope composition of water were required. These new measurements were made at WHOI, and contributed both to establishing the basis for interpretation of oceanographic measurements and to a physical chemistry-focused publication, led by the early-career PI, analyzing these solubility isotope effects in the context of molecular dynamics simulations and quantum mechanics theory. Although the sea-going aspects of the project were delayed due to the COVID-19 pandemic, the first oceanographic application of the new method was carried out in the North Atlantic Ocean in 2022, to investigate air-sea disequilibrium with respect to noble gas isotopes in the deep ocean, near Bermuda. This study, published in 2023, documented a pronounced deficit in the heavy noble gas isotopes (e.g., a lower ratio of argon-40/argon-36 than air-equilibrated seawater) that was used as a quantitative constraint for an inverse model, to optimize a parameterization for air-sea gas exchange due to diffusion across the air-sea interface and bubbles, as well as complete dissolution and injection of small bubbles. The implication of the paper is that bubbles play a more important role in gas exchange than previously recognized, especially for gases like oxygen and nitrogen, in high latitude locations where a disproportionately large fraction of the global ocean is formed. During the pandemic, the project also supported the implementation of noble gas isotopes into a general circulation model as physical tracers, for comparison with observations. The data generated from this project have all been made freely accessible online (e.g., via BCO-DMO for the measurements and model simulations in the North Atlantic study). The project supported ongoing K-12 outreach efforts focused on marine chemistry, including air-sea gas exchange demonstrations, to public school students in southeastern MA. Additionally, the project supported the intial development of a complementary technique for in situ gas analysis, to provide higher temporal resolution of important air-sea exchange components. Finally, despite the COVID-19 cruise delays, the project supported the collection of new samples, using the new technique, from three cruises (in the equatorial Pacific, Southern Ocean, and Labrador Sea), which will be analyzed at WHOI after the official end date of the project. Last Modified: 05/29/2024 Submitted by: AlanMSeltzer