In this study we report full‐depth water column profiles for nitrogen and oxygen isotopic composition (δ¹⁵N and δ¹⁸O) of nitrate (NO₃⁻) during the GEOTRACES GA01 cruise (2014). This transect intersects the double gyre system of the subtropical and subpolar regions of the North Atlantic separated by a strong transition zone, the North Atlantic Current. The distribution of NO₃⁻ δ¹⁵N and δ¹⁸O shows that assimilation by phytoplankton is the main process controlling the NO₃⁻ isotopic composition in the upper 150 m, with values increasing in a NO₃⁻ δ¹⁸O versus δ¹⁵N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO₃⁻ consumption and residual NO₃⁻ δ¹⁵N supports the view that NO₃⁻ is supplied via Ekman upwelling and deep winter convection, and progressively consumed during the Ekman transport of surface water southward. The co‐occurrence of partial NO₃⁻ assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO₃⁻ δ¹⁸O in comparison to deep oceanic values. This signal propagates through isopycnal exchanges to greater depths at lower latitudes. With recirculation in the subtropical gyre, cycles of quantitative consumption‐nitrification progressively decrease subsurface NO₃⁻ δ¹⁸O toward the δ¹⁸O of regenerated NO₃⁻. The low NO₃⁻ δ¹⁵N observed south of the Subarctic Front is mostly explained by N₂ fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO₃⁻ δ¹⁵N signal observed between 600 and 1500 m depth close to the Iberian margin.

中文翻译：

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从硝酸盐同位素组成推断北大西洋硝酸盐的供应途径及其内部循环的影响

在这项研究中，我们报告为氮和氧的同位素组成全深度水柱型材（δ ¹⁵ N和δ ¹⁸ O）的硝酸盐（NO ₃ ^- ）的GEOTRACES GA01巡航（2014）中。该横断面与北大西洋的亚热带和亚极地区的双回旋系统相交，该双回旋系统被一个强过渡带（北大西洋洋流）分隔开。NO的分布₃ ^- δ ¹⁵ N和δ ¹⁸ ö表明，通过同化浮游植物是控制NO的主要过程₃ ^-在上部150米同位素组成，其值在增加NO ₃ ^- δ ¹⁸ø与δ ¹⁵连同一个朝向表面的斜率的线N的空间。在极地环流，NO度之间的单个关系₃ ^-消费和残余NO ₃ ^- δ ¹⁵ Ñ支持NO视图₃ ^-是表面的埃克曼运输期间经由埃克曼上升流和深冬季对流供给，并逐渐消耗水向南。局部NO的共现₃ ^-同化和硝化作用的副极地环流提升地下深混合层NO ₃ ^- δ ¹⁸与深海值相比是O。该信号通过等渗交换传播到较低纬度的更大深度。在亚热带环流再循环，定量消耗硝化的周期逐渐减小地下NO ₃ ^- δ ¹⁸ O侧向的δ ¹⁸再生的NO的O- ₃ ^- 。低NO ₃ ^- δ ¹⁵ Ñ南观察到的亚北极阵线大多由N个说明₂固定，虽然从地中海流出一个贡献是必需的来解释下NO ₃ ^- δ ¹⁵在接近伊比利亚边界的600至1500 m深度之间观察到N信号。