Oxygen decline in the ocean: Implications for oceanic N2O production and the atmosphere
Atmospheric Science
Final Report Abstract
N2O is an important trace gas in the atmosphere, as it contributes to greenhouse warming and has an atmospheric lifetime that is long enough to be transported to the stratosphere where it undergoes ozone depleting photochemical reactions. The ocean is a significant source of N2O to the atmosphere, accounting for ~35% of natural sources. N2O production in the ocean is regulated by oxygen concentrations, with low oxygen levels leading to high production. Oxygen minimum zones (OMZs) in the ocean have, thus, been identified as major sources of N2O to the atmosphere. Mounting evidence for new and expanding OMZs has serious relevance for oceanic sources of N2O to the atmosphere. Future increases in oceanic N2O production due to reduced oxygen concentrations are likely to have important implications for atmospheric warming and stratospheric ozone depletion. This project used a holistic approach, integrating water column, sea-surface and atmospheric measurements to 1) investigate how changing dissolved oxygen concentrations influence N2O production and production pathways using a suite of isotopic approaches, 2) directly measure sea-to-air N2O fluxes using the eddy covariance technique, and 3) better quantify how various biological, chemical, and physical parameters impact these fluxes. In order to improve current estimates of N2O air-sea fluxes and better predict how these may change under future ocean conditions, it is critical that we develop a more comprehensive understanding of present N2O conditions, define the degree by which N2O production under natural, mixed assemblage conditions may increase as oxygen concentrations decline, and better understand the factors regulating N2O air-sea exchange, all before N2O production in the ocean increases substantially. Two field projects were performed to achieve these objectives: 1) SO243 ASTRA-OMZ research cruise in the eastern tropical South Pacific (ETSP), a large OMZ that is further influenced by the El Niño Southern Oscillation (ENSO), and 2) multi-month research campaign in the seasonally anoxic Saanich Inlet, British Columbia, Canada. In addition, two separate techniques were developed to facilitate reaching these objectives: 1) laser‐based spectroscopy to measure N2O concentrations and isotopic/isotopomeric signatures, and 2) the eddy covariance direct flux measurement technique. The main results from the research cruise illustrate that El Niño significantly reduces N2O surface concentrations and subsequent air-sea exchange. This implies that subsurface waters could serve as a reservoir for N2O, leading to even larger air-sea fluxes in the period following an El Niño event. The interplay between ENSO and increasing OMZ area/decreasing oxygen levels influencing N2O air-sea exchange needs further investigation. The Saanich Inlet study showed that even oxygenated waters can have significant N2O production in the presence of enough ammonium. The newly developed techniques show a lot of promise for tackling related issues in the future. The laser-based method requires smaller sample sizes and is more affordable than traditional methods, while retaining high level performance. The preliminary eddy covariance data show that N2O power spectra shapes and magnitudes follow the trends in air-sea concentration gradient. This is what is expected for flux measurements and indicates that the instrument used has adequate resolution at flux timescales to be used for direct flux measurement techniques. Future work at GEOMAR will focus on mastering the eddy covariance technique for N2O open ocean flux measurements. Finally, repeated measurements coupled with new measurements (e.g., of isotopic signatures) increase the amount of data that can be used to improve our current understanding of N2O cycling and airsea exchange, as well as track any trends that may be occurring.
Publications
- (2016) Observed El Niño conditions in the eastern tropical Pacific in October 2015. Ocean Science, 12 (4)
Stramma, L., Fischer, T., Grundle, D., Krahmann, G., Bange, H. W. & Marandino, C. A.
(See online at https://doi.org/10.5194/os-12-861-2016) - (2016) RV SONNE SO243 Cruise Report / Fahrtbericht Guayaquil, Ecuador: 05. October 2015 Antofagasta, Chile: 22. October 2015 SO243 ASTRA-OMZ: Air Sea Interaction Of Trace Elements In Oxygen Minimum Zones. GEOMAR Helmholtz Centre for Ocean Research Kiel, 81 pp
Marandino, C. A.
(See online at https://doi.org/10.3289/CR_SO243) - (2019) An automated, laser‐based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels. Rapid Communications in Mass Spectrometry, 33 (20). pp. 1553-1564
Ji, Q. & Grundle, D. S.
(See online at https://doi.org/10.1002/rcm.8502) - (2019) Investigating the effect of El Niño on nitrous oxide distribution in the Eastern Tropical South Pacific. Biogeosciences, 16, pp. 2079-2093
Ji, Q., Altabet, M. A., Bange, H. W., Graco, M. I., Ma, X., Arevalo-Martinez, D. L., & Grundle, D. S.
(See online at https://doi.org/10.5194/bg-16-2079-2019) - (2020) Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia. Journal of Geophysical Research: Biogeosciences, 125 (9). Art.Nr. e2020JG005631
Ji, Q., Jameson, B. D., Juniper, S. K. & Grundle, D. S.
(See online at https://doi.org/10.1029/2020JG005631) - (2020) Trends and decadal oscillations of oxygen and nutrients at 50 to 300 m depth in the equatorial and North Pacific, Biogeosciences, 17, 813–831
Stramma, L., Schmidtko, S., Bograd, S. J., Ono, T., Ross, T., Sasano, D., & Whitney, F. A.
(See online at https://doi.org/10.5194/bg-17-813-2020)
