Coexistence of nitrous oxide undersaturation and oversaturation in the surface and subsurface of the western Arctic Ocean

Abstract

The nitrous oxide (N2O) distributions in a shelf‐slope‐basin section from the Chukchi Sea shelf to the Chukchi Abyssal Plain (CAP) in the western Arctic Ocean were observed for the first time during the fourth Chinese National Arctic Research Expedition (4th CHINARE) in late August 2010. The N2O concentrations were 11.4–16.9 nmol L−1, corresponding to saturation values of 92–118% in the surface water. In the surface, biological factor was negligible to N2O production, and physical process dominated the surface N2O distribution. The N2O undersaturation may be the result of dilution by sea ice meltwater. The oversaturation may be caused by the diffusing of N2O‐rich shelf bottom water. Below the surface, N2O concentrations ranged from 11.4 to 21.4 nmol L−1. On the Chukchi Sea shelf, N2O concentrations increased from the surface to the near‐bottom water, indicating sediment N2O produced by nitrification or denitrification emission may be a significant source. The subsurface maxima in the upper halocline layer (UHL) may receive partial contribution from nitrification production but was primarily caused by the spreading of shelf water.

Methane and nitrous oxide distributions across the North American Arctic Ocean during summer, 2015

“…Our results, and those of previous studies [ Hirota et al ., ; Zhang et al ., ] strongly suggest that the sediments of the Bering and Chukchi shelves are a primary site of N 2 O production in the North American Arctic Ocean. By comparison, net N 2 O production is likely far more limited in the shallow water columns of these continental shelf systems due to light inhibition of nitrifying bacteria [ Ward , ], and aerobic oxygen concentrations (200 µmol L −1 ) that inhibit denitrification.…”

The Air-Sea Nitrous Oxide Flux along Cruise Tracks to the Arctic Ocean and Southern Ocean

“…The SA values range between −10% and 40% along the cruise tracks. Previous studies have also reported N 2 O undersaturation in surface waters [14,17,29]. The exchange rate of N 2 O is slower than that of heat, which can result in surface water N 2 O oversaturation or undersaturation, and the melting of sea ice in polar regions can dilute the surface water, resulting in surface water N 2 O undersaturation.…”

“…There are reports of N 2 O undersaturation in the Arctic region. Wu et al [34] and Zhang et al [17] both showed undersaturation in the Canadian Basin, and Kitidis, Upstill-Goddard and Anderson [15] also found that the lowest N 2 O undersaturation of approximately 82% along a cruise track occurred in the Northwest Passage of the Arctic Ocean, which probably resulted from ice melt water dilution. In the present study, N 2 O undersaturation is also observed at high latitudes, and the lowest salinity in the surface water is present between 70 • N and 80 • N ( Figure 6).…”

“…The highest SA value of the cruise track is observed in the Bering Strait and on the Chukchi Sea continental shelf, where the water depth is approximately 60 m. Different studies have described the oversaturation of N2O in this region. The highest N2O saturation observed by Hirota, Ijiri, Komatsu, Ohkubo, Nakagawa and Tsunogai [16] was approximately 157% in continental shelf waters of the Bering Sea and Chukchi Sea; Zhang et al [17] showed an oversaturation of 106-118% on the Chukchi Sea shelf south of 69° N; and Wu et al [34] observed N2O oversaturation on the continental shelf between 65° N and 73° N during the 5th CHINARE, suggesting that the Bering Sea to Chukchi Sea shelf is a hotspot for N2O production. Farther north, surface water undersaturation can be observed.…”

“…These discrepancies suggest that further work is needed; in particular, field observations must be conducted in the Southern Ocean. Another less studied region is the Arctic Ocean, which is mainly reported to act as a source along its shelf region [15,16] and as a sink in the open surface waters [17]. With the retreat of sea ice and the exposure of seawater in the Arctic Ocean, the source and sink characteristics of this ocean also need to be addressed.…”