Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean
Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean
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DOI：
10.1130/g33930.1
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发表时间：
2013-06-01
期刊：
影响因子：
5.8
通讯作者：
Fralick, Philip W.
中科院分区：
文献类型：
--
作者：
Godfrey, Linda V.;Poulton, Simon W.;Fralick, Philip W.
氮循环已在古元古代晚期阿尼米奇盆地（北美），跨越地球的最后一个时期的全球铁沉淀结束，并在高生产力地区的主要过渡到富氧条件的深度横断面进行了评估。从附近的海岸，那里的生产力最高的沉积物，有d（15）N组成高达类似的千分之三高于在更远的网站。这表明，当NH 4+从缺氧的海洋内部垂直向上混合进入好氧透光层时，它要么被生物同化，要么被氧化。随后通过反硝化或厌氧氨氧化（厌氧氨氧化）增强N-2的产生，导致观察到的靠近海岸的d（15）N增加。任何生物有效氮的不足都可以被固氮生物所克服，但是来自该过程的低d（15）N的N输入并不能压倒来自反硝化的d（15）N的增加。由于没有证据表明，在过渡到富氧条件下，痕量金属限制（特别是钼）的N固定所产生的严重N应力条件下，N的损失要么非常小（可能是因为低O-2水平限制NH 4+氧化），或保留N的替代途径是重要的。事实上，钼似乎仍然生物可利用的N固定，无论是表明，程度或严重程度的硫化物水柱条件是不足以在全球范围内定量隔离钼，或河流直接提供钼表面沃茨的内架。N-2固定对d（15）N的影响增加到大陆架的更远部分，与广泛大陆边现代上升流区的模型一致。
Nitrogen cycling has been evaluated across a depth transect in the late Paleoproterozoic Animikie Basin (North America), spanning the end of Earth's final period of global iron precipitation, and a major transition to euxinic conditions in areas of high productivity. Sediments from near shore, where productivity was highest, have d(15)N compositions up to similar to 3 parts per thousand higher than at more distal sites. This suggests that as NH4+ mixed vertically upward into the oxic photic zone from the anoxic ocean interior, it was either assimilated by organisms or oxidized. Subsequent enhanced production of N-2 by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in d(15)N close to shore. Any deficit in biologically available N was overcome by N-2-fixing organisms, but the input of N with low d(15)N from this process did not overwhelm the increase in d(15)N from denitrification. Because there is no evidence for conditions of severe N stress arising from trace metal limitation (particularly Mo) of N fixation during the transition to euxinic conditions, losses of N were either very small (potentially because low O-2 levels limited NH4+ oxidation), or alternative pathways that retained N were important. The fact that Mo appears to have remained bioavailable for N fixation, either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester Mo on a global scale, or that rivers directly delivered Mo to surface waters on the inner shelf. The effects of N-2 fixation on d(15)N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.