Interannual variability in methane and nitrous oxide concentrations and sea-air fluxes across the North American Arctic Ocean (2015–2019)

Abstract

Between 2015–2018, we collected ~2000 measurements of methane (CH4) and nitrousoxide (N2O) concentrations in the North American Arctic Ocean during summer and early fall from water column profiles. We also obtained 25 measurements of CH4 and N2O concentrations in rivers along the Northwest Passage and Ellesmere Island in mid-summer 2017–2019. Our results show that N2O is generated in the highly productive Bering and Chukchi Seas and transported northeastward, producing a persistent subsurface N2O peak in the Beaufort Sea. The Chukchi and Beaufort Sea sediments are a significant source of CH4 to the water column. These sedimentary sources and associated water column consumption display significant spatial gradients and interannual variability. CH4 isotope data demonstrate the importance of CH4 oxidation across the study region. We find that rivers are not a significant source of CH4 or N2O to the Arctic Ocean at the time of year sampled. The estimated annual sea-air flux across the study region (2.3 million km2) had a median (interquartile range) of 0.009 (0.002, 0.023) Tg CH4 y-1 and −0.003 (−0.013, 0.010) Tg N y-1. These results suggest that the North American Arctic Ocean currently plays a negligible role in global CH4 and N2O budgets. Our expansive dataset, with observations at many repeat stations, provides a synopsis of present-day Arctic CH4 and N2O distributions and their range of variability, as well as a benchmark against which future climate-dependent changes can be evaluated.

Enhanced Transport of Dissolved Methane From the Chukchi Sea to the Central Arctic

“…In addition, repeat measurements taken from the Chukchi shelf and coastal Beaufort Sea have suggested the presence of elevated CH 4 concentrations in the subsurface, and the authors attributed it to sedimentary sources (seafloor CH 4 seeps, permafrost, and CH 4 hydrates) (Manning et al., 2022), which also supports our view that the CH 4 pool was enhanced in the shelf/slope areas due to the sedimentary release of CH 4 .…”

“…Similar to the retreat of sea ice, the transport of shelf‐derived materials (Kipp et al., 2018) together with CH 4 enhancement during the 1990s–2010s have important implications for the effect of climate warming when using the increased material in the central Arctic as a new climate‐change indicator in the warmer, increasingly ice‐free Arctic. Although our results as well as those of others (Manning et al., 2022) suggest that the Pacific sector of the Arctic Ocean is currently a minor source for the atmosphere, the increasing ΔCH 4 in the UHC and the reduction in sea ice indicate that a potential CH 4 emission is undergoing. Therefore, it is imperative to understand how these climate‐change‐driven processes function, how the surface ocean and low atmosphere affect the ocean interior, and how new techniques constrain the CH 4 flux from hotspots (Steinbach et al., 2021; Thornton et al., 2020).…”

“…Sedimentary release in shallow marine waters was considered to be an important source of dissolved CH 4 in, for example, the Chukchi Sea (Lapham et al., 2017), the Beaufort Sea (Manning et al., 2022) and the East Siberian Arctic shelf (Shakhova, Semiletov, Leifer, et al., 2010; Shakhova, Semiletov, Salyuk, et al., 2010). Due to the effect of ebullition from ocean hotspots, significant CH 4 in the East Siberian Arctic shelf was vented to the atmosphere, yielding the same magnitude as existing estimates of total CH 4 emissions from the entire world ocean (Shakhova, Semiletov, Salyuk, et al., 2010; Shakhova et al., 2014).…”

Interannual Variability in Methane and Nitrous Oxide Concentrations and Sea‐Air Fluxes Across the North American Arctic Ocean (2015–2019)

“…These off‐transect stations (14 out of 129 stations, indicated by white triangles in Figure 1) were only sampled in one of the four years and cannot be used to investigate interannual variability in gas distributions. In accordance with the principles of findable, accessible, interoperable, and reusable data (Wilkinson et al., 2016), all station locations are shown in Figure 1 and all data have been archived in the open access repository PANGAEA (Manning et al., 2022).…”

“…All CH 4 , N 2 O, temperature, salinity, pressure, O 2 , and δ 13 C‐CH 4 data from oceanic and river samples, as well as sea ice cover, sea level pressure, and derived sea‐air flux estimates, are archived at PANGAEA (Manning et al., 2022) at https://doi.org/10.1594/PANGAEA.941201. Software for calculating sea‐air fluxes is available on GitHub (Manning & Nicholson, 2022) at https://doi.org/10.5281/zenodo.6126685.…”