Change of nitrogen chemical tracer N2* between the Okhotsk Sea and the subarctic North Pacific

Abstract

Using the nitrogen chemical tracer N2* (an anomaly of [N2] in terms of the saturation ratio of dissolved nitrogen gas to dissolved argon gas in seawater), we quantitatively investigated the processes that affect the flux of oceanic nitrogen gas at the air-sea interface in the subarctic region of the Sea of Okhotsk (OK) and the North Pacific Ocean (NP): denitrification, air injection, and rapid cooling, in five regions, (i) the inflow region from NP into OK, (ii) the continental shelf region, (iii) the continental slope region, (iv) the Okhotsk Sea pelagic ocean region and (v) the outflow region from OK to NP following the flow of seawater from upstream to downstream. In the inflow region, half of ∼1 μmol kg−1 of N2* amount was due to the denitrification effect, followed by 40% due to the rapid cooling effect, and the remaining about 10% was due to the air injection effect. On the continental shelf, N2* showed a maximum value of ∼8 μmol kg−1, of which about 56% was the rapid cooling effect, followed by the denitrification effect of 44%, which resulted from the formation of the dense shelf water (DSW) and the active biological production on the continental shelf. On the continental slope, the denitrification effect accounted for three-fourths of N2*, and the remaining one-fourth was due to the rapid cooling effect, which is due to the mixing of DSW with the surrounding seawater and the active denitrification at a wide depth on the continental slope. In the pelagic ocean region, the N2* content was the lowest in OK and the fraction of the three effects are almost the same as on the continental slope, which is related to the process of DSW incorporation into the Okhotsk Sea Intermediate Water. In the outflow region from OK to NP, which is downstream of the above regions, the N2* content steeply decreased by 60% compared to the Okhotsk Sea pelagic ocean region due to tidal mixing in the Bussol Strait. The above quantitative analysis of these five regions showed that the Kruzenshtern Strait, the Bussol’ Strait and the continental shelf and slope of OK play an important role in controlling the three effects affecting the flux of ocean nitrogen gas in OK and NP.