Assessment of Excess Nitrate Development in the Subtropical North Atlantic
Assessment of Excess Nitrate Development in the Subtropical North Atlantic
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DOI：
10.1016/j.marchem.2007.06.005
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发表时间：
2007-08
期刊：
影响因子：
3
通讯作者：
Dennis A. Hansell;D. Olson;F. Dentener;L. Zamora
中科院分区：
文献类型：
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作者：
Dennis A. Hansell;D. Olson;F. Dentener;L. Zamora
北大西洋N2固定的地球化学估计通常作为估计全球海洋固氮作用的基础。然而，尽管被充分研究，在这个盆地的固氮率的估计差异很大。在这里，我们调查的变化，在发表的估计过量氮积累率的主要温跃层的亚热带北大西洋，测试的假设和选择的分析。采用上述方法中的一种，并在此进行了改进，对过量氮的空间梯度和主温跃层的通风率进行了估计，我们确定了总的过量氮积累率为7.8±1.7× 1011 molN yr−1。对过量氮形成的贡献包括高N：P营养物质的大气沉积（以3.0±0.9× 1011 mol N yr− 1的速率增加过量氮，占总量的约38%），高N：P溶解有机物平流进入主温跃层并矿化（以2.2±1.1× 1011 mol N yr− 1的速率增加过量氮，占总量的约28%），以及通过过量氮场的质量平衡计算的N2固定（以2.6±2.2× 1011 mol N yr− 1的速率增加过量氮，占总量的约33%）。假设氮磷比为40，并假设这一过程导致过量氮积累，北大西洋副热带环流的N2固定量估计为0.44 × 1011 mol N yr−1。这一相对较低的固氮速率表明，i）在北大西洋的固氮速率在以前的一些分析中被大大高估，ii）主要的温跃层不是固氮生物固定的N的主要储存库，和/或iii）输出的固氮有机物的N：P比通常假设的要低得多。正是这最后一种可能性，以及我们对支持过量N开发的输出物质的N：P比率的不确定性，大大降低了我们对N2固定的地球化学措施的信心。
Geochemical estimates of N2fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8±1.7×1011mol N yr−1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0±0.9×1011mol N yr−1for ∼38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2±1.1×1011mol N yr−1for ∼28% of the total), and, calculated by mass balance of the excess N field, N2fixation (adding excess N at 2.6±2.2×1011mol N yr−1for ∼33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N2fixation in the North Atlantic subtropical gyre is estimated at ∼4×1011mol N yr−1. This relatively low rate of N2fixation suggests that i) the rate of N2fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N2fixation.