Nitrate Supply Routes and Impact of Internal Cycling in the North Atlantic Ocean Inferred From Nitrate Isotopic Composition

Abstract

The biological carbon pump plays a crucial role in the regulation of Earth's climate and the distribution of biogeochemical properties in the ocean, by exporting to the deep ocean CO 2 that is fixed into biomass during photosynthesis in the sunlit surface layer, through the sinking of particles and their subsequent remineralization. Phytoplankton requires macro-nutrients for the synthesis of organic matter. Nitrogen (N) is considered as one of the most important nutrients since it limits productivity in many oceanic regions (Moore et al., 2013). The North Atlantic Ocean, which hosts one of the most productive spring phytoplankton blooms of the world's ocean (Longhurst, 2007), is estimated to be a significant contributor to the global oceanic export production (Falkowski et al., 1998;Sanders et al., 2014).The North Atlantic Ocean is characterized by the classical double gyre system of the subtropical and subpolar gyres (Figure 1). Both gyres harbor contrasting physical and biogeochemical features (Sanders et al., 2014) and are separated by a strong transition area, the North Atlantic Current (NAC). In addition, the Atlantic Meridional Overturning Circulation is a key component of the Earth's climate system, with the NAC carrying a northward flow of warm and salty waters balanced by a southward flow of colder deep waters (North Atlantic Deep Water) (Buckley & Marshall, 2016). The North Atlantic subtropical gyre is considered to be a year-round stratified oligotrophic N-limited area (Moore et al., 2008(Moore et al., , 2013, where N 2 fixation performed by diazotrophs plays an important role in providing fixed N (or bioavailable N) to the surface waters (Capone et al., 2005(Capone et al., , 2008. In contrast, a strong seasonality is observed in the North Atlantic subpolar gyre. The relief of winter light limitation induces the onset of a spring phytoplankton bloom supported by

The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

“…NADW is formed by deep convection in the subarctic North Atlantic where it has a mean nitrate concentration of 17.5 μM and δ 15 N of 4.8‰ (Marconi et al., 2015; Sigman et al., 2000). Its relatively low nitrate δ 15 N derives from N 2 fixation in the (sub)tropical North Atlantic (Deman et al., 2021; Knapp et al., 2008; Marconi et al., 2015; Marconi et al., 2017). By the time NADW reaches the southwest Indian Ocean, its mean nitrate concentration has increased to 27.4 ± 0.8 μM and its δ 15 N to 5.0 ± 0.0‰, similar to observations from the Cape Basin (26.0 μM and 5.1‰, respectively; (Campbell, 2016; Marconi et al., 2017)).…”

“…The nitrate generated from the remineralization and nitrification of this newly fixed organic matter is similarly low in δ 15 N while its δ 18 O, which is set by δ 18 O H2O (equal to ∼0‰) plus an isotopic offset, is ≥1.1‰ (Fawcett et al., 2015; Knapp et al., 2008; Lehmann et al., 2018; Marconi et al., 2019; Rafter et al., 2013; Sigman et al., 2005, 2009). The second process is partial nitrate assimilation occurring in the same water parcel as nitrification (Deman et al., 2021; Fawcett et al., 2015; Smart et al., 2015; Wankel et al., 2007). These co‐occurring processes generate nitrate that is high in δ 18 O relative to that initially removed by phytoplankton but leave nitrate δ 15 N unchanged (since in net, N is neither gained nor lost from the water parcel; Fawcett et al., 2015; Rafter et al., 2013; Sigman et al., 2005, 2009).…”

“…These co-occurring processes generate nitrate that is high in δ 18 O relative to that initially removed by phytoplankton but leave nitrate δ 15 N unchanged (since in net, N is neither gained nor lost from the water parcel; Fawcett et al, 2015;Rafter et al, 2013;Sigman et al, 2005Sigman et al, , 2009. The signal of coupled nitrate assimilation and nitrification can be generated in situ (i.e., nitrate assimilation and nitrification co-occurring at the base of the euphotic zone; Fawcett et al, 2015) or as a result of partial nitrate assimilation in surface waters that then subduct and flow laterally along subsurface isopycnals where nitrification occurs (Deman et al, 2021;Fawcett et al, 2018). While both N 2 fixation and co-occurring nitrate assimilation and nitrification decrease the Δ(15-18) of nitrate, N 2 fixation also lowers its δ 15 N (Figure 6a vs.…”

The Impact of Incomplete Nutrient Consumption in the Southern Ocean on Global Mean Ocean Nitrate δ15N

“…Outside these areas, relatively constant depth‐weighted δ 15 N values are reported with small isotopic gradients (<0.2‰) across ocean basins (Figure 2d; Table S1 in Supporting Information S1) (Fripiat, Marconi, et al., 2021; Fripiat, Martínez‐García, et al., 2021). In the North Atlantic, low depth‐weighted δ 15 N values are observed (Figure 2d), which have been attributed to an excess of N 2 fixation relative to denitrification and a lack of water column denitrification (Deman et al., 2021; Fawcett et al., 2015; Knapp et al., 2005, 2008; Marconi et al., 2015; Marconi, Sigman, et al., 2017). In the OMZs, water column denitrification raises the residual nitrate δ 15 N above the mean pycnocline value (Casciotti et al., 2013; Liu & Kaplan, 1989; Martin & Casciotti, 2017; Sigman et al., 2005).…”

Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean

“…Despite the rise in nitrate δ 15 N and δ 18 O in the ASCA16 surface layer, Δ(15–18) was lower than in the thermocline (mean of 1.5‰). This low nitrate Δ(15–18) can be explained by co‐occurring partial nitrate assimilation and nitrification (near the base of the mixed layer) (Marshall et al., 2023), which in net approximately conserves the δ 15 N of nitrate while raising its δ 18 O (Deman et al., 2021; Fawcett et al., 2015). The isopycnal delineating the top of the thermocline also represents the immediate nitrate supply to surface waters (Figure 2j).…”