Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation
Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation
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DOI：
10.1073/pnas.1905553116
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发表时间：
2019-11
期刊：
影响因子：
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通讯作者：
B. Naafs;F. Monteiro;A. Pearson;M. B. Higgins;R. Pancost;A. Ridgwell;A. Ridgwell
中科院分区：
文献类型：
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作者：
B. Naafs;F. Monteiro;A. Pearson;M. B. Higgins;R. Pancost;A. Ridgwell;A. Ridgwell
在现代的、氧合良好的海洋中，溶解的生物可利用（固定）氮与磷的比例接近16：1。这种情况--固定氮跟踪磷--通常被认为已经运行了数亿年。在这里，我们使用计算机模拟与代理数据相结合，以证明海洋氮循环的运作非常不同时，在海洋中的溶解氧浓度大大低于目前。不仅硝酸盐被铵取代为固定氮的主要成分，而且总固定库存相对于磷崩溃。这使得海洋中生物泵的强度和状态极易受到破坏，具有潜在的过去和未来影响。当今海洋氮（N）循环受到生物学的强烈调控。海洋表层固定氮和易生物利用氮相对于磷酸盐（P）的可利用性的不足在很大程度上通过固氮生物的活动得到纠正。这种反馈系统，被称为“nitrostat”，被认为是提供了密切的调节固定氮的形态和库存相对于P自元古代。相反，在强烈脱氧的间隔，如白垩纪海洋缺氧事件（OAE）2，一些区域沉积δ 15 N记录暗示存在一种不同的模式的海洋N循环，其中铵在调节输出生产中起着重要作用。然而，这一时期的全球规模动态仍然未知。在这里，使用地球系统模型并以OAE 2为例，我们提供了严重海洋脱氧下全球海洋氮循环的见解。具体来说，我们发现海洋可以表现出氮的物种主导的固定氮库存的根本转变-从硝酸盐（NO3−）到铵（NH 4+）-并且随着这种转变的发生，由于NH 4+氧化，NO3−还原和固氮位点之间的渐进空间解耦，库存可以相对于P部分崩溃。这一发现是相对独立的海洋环流的具体状态，是一致的氮同位素和氧化还原代用数据。在中间脱氧状态下，海洋固定氮存量的大幅减少可能代表了对过去和未来（较暖）海洋具有潜在影响的生物地球化学脆弱性。
Significance The ratio of the dissolved inventories of readily bioavailable (fixed) nitrogen to phosphorus is regulated close to 16:1 in the modern, well-oxygenated ocean. This situation––fixed-nitrogen tracking phosphorus––is generally assumed to have operated for hundreds of millions of years. Here we use computer simulations combined with proxy data to instead demonstrate that the marine nitrogen cycle operates very differently when dissolved oxygen concentrations in the ocean are considerably lower than present. Not only is nitrate replaced by ammonium as the dominant component of fixed nitrogen, but the total fixed inventory collapses relative to phosphorus. This makes the strength and state of the biological pump in the ocean highly susceptible to disruption, with potential past and future implications. The present-day marine nitrogen (N) cycle is strongly regulated by biology. Deficiencies in the availability of fixed and readily bioavailable nitrogen relative to phosphate (P) in the surface ocean are largely corrected by the activity of diazotrophs. This feedback system, termed the “nitrostat,” is thought to have provided close regulation of fixed-N speciation and inventory relative to P since the Proterozoic. In contrast, during intervals of intense deoxygenation such as Cretaceous ocean anoxic event (OAE) 2, a few regional sedimentary δ15N records hint at the existence of a different mode of marine N cycling in which ammonium plays a major role in regulating export production. However, the global-scale dynamics during this time remain unknown. Here, using an Earth System model and taking the example of OAE 2, we provide insights into the global marine nitrogen cycle under severe ocean deoxygenation. Specifically, we find that the ocean can exhibit fundamental transitions in the species of nitrogen dominating the fixed-N inventory––from nitrate (NO3−) to ammonium (NH4+)––and that as this transition occurs, the inventory can partially collapse relative to P due to progressive spatial decoupling between the loci of NH4+ oxidation, NO3− reduction, and nitrogen fixation. This finding is relatively independent of the specific state of ocean circulation and is consistent with nitrogen isotope and redox proxy data. The substantive reduction in the ocean fixed-N inventory at an intermediate state of deoxygenation may represent a biogeochemical vulnerability with potential implications for past and future (warmer) oceans.