Fine scale variability of greenhouse gases in polar and subpolar ocean waters : insights from high resolution sampling and numerical models

We present high resolution surface water measurements and discrete water column profiles of nitrous oxide (N₂O) and methane (CH₄) saturations in two distinct oceanographic regions, the Canadian Arctic Archipelago, and the Northeast Subarctic Pacific. Utilizing a high frequency, automated measurement system, we were able to reveal fine-scale variability in surface water N₂O and CH₄ saturations not previously captured using discrete sampling in earlier studies. We combine these high frequency observations with depth profile measurements and numerical model output to better understand how vertical mixing influences gas concentrations on small scales. In doing so, we found that elevated gas supersaturation in both study regions was associated with strong vertical mixing, as indicated by increased vertical eddy diffusivity (κ𝚣) values. In the Eastern Canadian Arctic, we observed CH₄ saturations 300% larger than previously reported, based on existing measurements mostly conducted in summertime. We suggest that these summer measurements may have failed to capture elevated surface water concentrations associated with weakened stratification and increased mixing during early autumn. In the Northeast Subarctic Pacific, the range of N₂O and CH₄ saturations we measured was consistent with prior observations, although our high-resolution measurements provided greater detail on the spatial and temporal variability of these gases. In particular, we were able to identify new regions of elevated N₂O and CH₄, associated with regions of strong tidal mixing around the west coast of Vancouver Island and Salish Sea. Time-series observations conducted along the west coast of Vancouver Island revealed strong temporal variability in N₂O and CH₄ saturations associated with a combination of wind-driven and tidal mixing. Our work highlights the utility of high frequency measurements, combined with numerical model output, to provide insight into the oceanographic processes affecting surface water N₂O and CH₄ distributions in dynamic marine systems.