Nitrous Oxide Cycling and Transport Along the California Current System During the 2021 Summer Upwelling Season

Abstract

The California Current System (CCS) is a globally recognized source of nitrous oxide (N ₂ O), a potent greenhouse gas to the atmosphere; however, little is known about the underlying sources of N ₂ O within the CCS. During the 2021 NOAA West Coast Ocean Acidification cruise, we collected samples for N ₂ O and nitrate (NO ₃ ⁻ ) concentration and isotope ratio measurements–from Dana Point, California (31.78°N) to Haida Gwaii, Canada (52.40°N)–to understand what processes shape the distribution of N ₂ O in the CCS and subsequent emissions. Throughout the primary range of the subsurface California Undercurrent (CUC), with potential density anomalies of 26–27 kg m ⁻³ , we observed elevated ¹⁵ N/ ¹⁴ N in both NO ₃ ⁻ and N ₂ O, corresponding with negative N*. These signals of nitrogen loss and N ₂ O production from denitrification are indicative of northward transport of waters from eastern tropical North Pacific oxygen deficient zone throughout the CUC. Together with contemporaneous data from an underwater glider network, we observed strong upwelling of CUC waters between Pt. Conception and Cape Mendocino in California, corresponding with high surface density and excess N ₂ O concentrations, leading to large N ₂ O fluxes to the atmosphere (as high as 48 μmol m ⁻²  d ⁻¹ ). Thus, the tracers employed in this study link CUC transport to N ₂ O emissions to the atmosphere during the summer upwelling season. Because the CUC is projected to expand and shoal in response to climate change, this source of N ₂ O could be enhanced under these conditions.