The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling
The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling
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DOI：
10.1029/2022jc019413
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发表时间：
2023-03-01
影响因子：
3.6
通讯作者：
Fawcett, Sarah E.
中科院分区：
文献类型：
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作者：
Marshall, Tanya A.;Sigman, Daniel M.;Fawcett, Sarah E.
大阿古拉斯海流区是气候系统的重要组成部分，但其对碳和营养循环的影响知之甚少。在这里，我们使用硝酸盐同位素（δ N-15，δ O-18，δ（15-18）= δ N-15-δ O-18）来追踪区域水团循环，并调查在Agulhas Current和邻近的再循环沃茨的氮循环。深海和中层沃茨记录了发生在远处的过程，包括南大洋的部分硝酸盐同化和阿拉伯海的脱氮作用。在温跃层和表面，热带来源的沃茨的地球化学性质与相邻的亚热带来源的沃茨不同，证实了目前的核心抑制横向混合。（亚）热带温跃层硝酸盐δ N-15低于（4.9-5.8千分之一）亚温跃层来源，亚季风模式水（6.9千分之一）;我们将这种差异归因于当地的N-2固定。使用一箱模型来模拟新固定的硝酸盐通量，我们估计局部N-2固定率为7-25 Tg N.a（-1），上限可能偏高。在混合层，硝酸盐三角洲N-15和三角洲O-18一致上升，表明浮游植物硝酸盐同化占主导地位的表面沃茨，硝化作用仅限于较深的沃茨。由于硝酸盐同化和硝化作用在垂直方向上是不耦合的，因此硝酸盐同化加N-2固定的速率可以用来近似计算碳输出。温跃层和混合层硝酸盐三角洲（15-18）是低的，由于N-2固定和耦合的部分硝酸盐同化和硝化。同样，Agulhas环中的低三角洲（15-18）硝酸盐表明低三角洲N-15氮泄漏到南大西洋，这应该记录在沉入海底的有机物中，为过去Agulhas泄漏提供了潜在的示踪剂。
The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (delta N-15, delta O-18, delta(15-18) = delta N-15-delta O-18) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate delta N-15 is lower (4.9-5.8 parts per thousand) than the sub-thermocline source, Subantarctic Mode Water (6.9 parts per thousand); we attribute this difference to local N-2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N-2 fixation rate of 7-25 Tg N.a(-1), with the upper limit likely biased high. In the mixed layer, nitrate delta N-15 and delta O-18 rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N-2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate delta(15-18) is low, due to both N-2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-delta(15-18) nitrate in Agulhas rings indicates leakage of low-delta N-15 nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.