Isotopic evidence for nitrification in the Antarctic winter mixed layer

Abstract

We report wintertime nitrogen and oxygen isotope ratios (δ 15 N and δ 18 O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ 15 N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6-3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5-8‰). The δ 18 O data from the same depth profiles and lateral δ 15 N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ 15 N for nitrite in the winter mixed layer (À40‰ to À20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ 15 N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ 15 N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ 15 N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer.

Enzyme‐level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

“…In the T min , NO 3 À + NO 2 À samples fall slightly above the 1:1 line in δ 18 O versus δ 15 N space, reflecting the decrease in NO 3 À δ 15 N relative to ln(NO 3 À ) within the T min layer (i.e., the kink). This feature has been observed previously in winter data from the Atlantic sector of the AZ and is hypothesized to result from the remineralization of low-δ 15 N N remaining in the mixed layer at the end of the summer [Smart et al, 2015]. ranges from À91 ± 18‰ to À6 ± 22‰, with an average value of À58 ± 52‰ (2σ, n = 55; Figure 5).…”

“…NO 2 À δ 15 N is depth dependent, with an average value above 90 m of À69 ± 33‰ (2σ, n = 41) that is lower than the value below 90 m of À24 ± 38‰ (2σ, n = 14; Figure 5, grey bar). In addition, the surface NO 2 À δ 15 N is significantly lower than that calculated by Smart et al [2015] for the Atlantic sector of the Antarctic Zone; using a similar approach, they estimated NO 2 À δ 15 N to range from À40‰ to À20‰ in the winter mixed layer. We first consider a mechanism suggested previously to explain observations of low-δ 15 N NO 2 À in samples from the wintertime Atlantic AZ.…”

“…[Granger et al, 2004[Granger et al, , 2008[Granger et al, , 2010Karsh et al, 2012Karsh et al, , 2014. [Rafter et al, 2013;Smart et al, 2015]. In the T min , NO 3 À + NO 2 À samples fall slightly above the 1:1 line in δ 18 O versus δ 15 N space, reflecting the decrease in NO 3 À δ 15 N relative to ln(NO 3 À ) within the T min layer (i.e., the kink).…”

“…Smart et al, 2015]. In particular, the Rayleigh model fails to explain the chemistry of the temperature minimum layer, due at least in part to wintertime nitrification of the low-δ 15 N NH 4 + resulting from intensive biological N recycling in the late summer mixed layer [Lourey et al, 2003;Smart et al, 2015].…”

“…This is likely due to the proximity of these stations to the core of the Ross Sea Gyre, where upwelling is expected to be strongest. layer [Sigman et al, 1999;DiFiore et al, 2010;Smart et al, 2015].…”