Coupled nitrate nitrogen and oxygen isotopes and organic matter remineralization in the Southern and Pacific Oceans

Abstract

[1] The difference between nitrate 15 N and 18 O, or D(15-18), is sensitive to organic matter remineralization and tracks the modification of nitrate as it passes from the deep Pacific Ocean, through the Southern Ocean surface, and into the intermediate-

Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions

“…2a, b). Such values are similar to those observed widely for the deep ocean (see Di-Fiore et al, 2009;Sigman et al, 2000Sigman et al, , 2009bRafter et al, 2013). Although δ 18 O NO 3 values are more scattered, it can be clearly seen that they follow a pattern similar to δ 15 N NO 3 , with values up to 6 ‰ in surface waters, which decrease to < 2 ‰ in the 500 to 1000 m depth interval but tend to increase again in deep and bottom waters and stay close to 2 ‰ (Fig.…”

“…In the present study the effect of NO − 2 was neglected since overall nitrite concentrations were small, representing on average < 0.5 % of the nitrate + nitrite pool (see also DiFiore et al, 2009). However, it has been reported that slightly higher nitrite levels reaching 0.8 % of the nitrite + nitrate pool such as observed here for the surface waters can result in a lowering of the δ 15 N and δ 18 O values by 0.4 and 0.2 ‰ on average (Rafter et al, 2013; their Supplement). We have not corrected our surface water nitrate isotopic values for a possible nitrite effect, as is the case also in work presented by others (see e.g., DiFiore et al, 2009;Rafter et al, 2013), but have considered the impact of this when calculating nitrification (see Sect.…”

“…Several authors have highlighted that knowledge about nitrate stable isotope composition is an essential asset to resolve the complex suite of processes that control the oceanic N cycle (see e.g., Sigman et al, 1999;DiFiore et al, 2006;2009;Rafter et al, 2013). During the early season KEOPS 2 expedition (October-November) to the Kerguelen area, we analysed the N and O stable isotope composition of nitrate over the whole water column to investigate possible imprints of the above-described shallow remineralization + nitrification process, as well as imprints of enhanced primary production on deep ocean nitrate isotopic composition.…”

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

“…[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).…”

Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets

“…were less stressed for iron (Chappell et al, 2012 Hoering and Ford, 1960;Minagawa and Wada, 1986). In contrast, in the Pacific, NO 3 -mixed up from the subsurface is impacted by water column denitrification and can have a NO 3 -δ 15 N >20‰ (e.g., (Brandes et al, 1998;Casciotti et al, 2013;Rafter and Sigman, 2016)), although as upper thermocline waters move westward in the Pacific, the very high NO 3 -δ 15 N signal is diluted and typical values are between 5 and 10‰ (Rafter et al, 2013). The relative importance of each source for 25 supporting export production can be determined using the two end-member mixing model described in Eqn.…”