The Western Tropical Pacific Ocean has a complicated circulation system and plays a significant role in regulating global marine nitrogen cycles and budgets. Powerful insights can be gained by using nitrate isotope (δ15N-NO3- and δ18O-NO3-) techniques. In this study, nitrate concentrations and δ15N-NO3- and δ18O-NO3- values were obtained along meridional (130°E) and zonal (20°N) transects in the Western Tropical Pacific Ocean. The δ15N-NO3- and δ18O-NO3- values ranged from 2.9 to 10.9 ‰ and from 2.0 to 5.7 ‰, respectively. For the water mass in the thermocline originating in the North Pacific, the simultaneously elevated δ15N-NO3- and δ18O-NO3- values indicated nitrate assimilation. The relatively low δ15N-NO3- values in the intermediate water indicated the remineralization of sinking organic matter produced by N2 fixation at shallow depths, but these signals exhibited a delay in downward propagation in the North Pacific subtropical gyre. For water masses originating in the Southern Hemisphere, the variations in nitrate isotopes might be attributed mainly to physical mixing, including diapycnal mixing between the thermocline and intermediate water and lateral advection from the Equatorial Undercurrent. Furthermore, according to end-member mixing calculations, at least 62 % of the nitrate in the water of the North Subsurface Countercurrent was derived from the Southern Hemisphere, whereas North Pacific Tropical Water, South Pacific Tropical Water and nutrient-rich upwelling accounted for 50 %, 12 % and 38 %, respectively, of the nitrate in the water of the North Equatorial Countercurrent.

中文翻译：

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西热带太平洋硝酸盐的同位素组成：生物地球化学信号及其传输


西热带太平洋具有复杂的环流系统，在调节全球海洋氮循环和预算方面发挥着重要作用。使用硝酸盐同位素（δ15N-NO3- 和 δ18O-NO3-）技术可以获得强大的见解。在这项研究中，沿热带太平洋西部经向 （130°E） 和纬向 （20°N） 样带获得了硝酸盐浓度和 δ15N-NO3- 和 δ18O-NO3- 值。δ15N-NO3- 和 δ18O-NO3- 值分别为 2.9 至 10.9 ‰ 和 2.0 至 5.7 ‰。对于源自北太平洋的温跃层中的水团，同时升高的 δ15N-NO3- 和 δ18O-NO3- 值表明硝酸盐同化。中间水中相对较低的 δ15N-NO3- 值表明 N2 固定产生的下沉有机物在浅层再矿化，但这些信号在北太平洋副热带环流中表现出向下传播的延迟。对于起源于南半球的水团，硝酸盐同位素的变化可能主要归因于物理混合，包括温跃层和中间水之间的双倍混合以及赤道潜流的横向平流。此外，根据端元混合计算，北地下逆流水中至少 62% 的硝酸盐来自南半球，而北太平洋热带水域、南太平洋热带水域和营养丰富的上升流分别占北赤道逆流水中硝酸盐的 50%、12% 和 38%。