Isotopic evidence for nitrification in the Antarctic winter mixed layer
Isotopic evidence for nitrification in the Antarctic winter mixed layer
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南极冬季混合层硝化作用的同位素证据
DOI：
10.1002/2014gb005013
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发表时间：
2015
影响因子：
5.2
通讯作者：
D. Sigman
中科院分区：
文献类型：
--
作者：
S. Smart;S. Fawcett;S. Thomalla;M. A. Weigand;C. Reason;D. Sigman
本文报道了非洲南部南大洋冬季海水硝酸盐氮氧同位素比值（δ 15 N和δ 18 O）。2012年7月收集的深度剖面和正在进行的表面样本从亚热带延伸到南极冬季海冰边缘之外。我们在这里集中在南极地区（50.3°S以南），在那里应用瑞利模型深度剖面δ 15 N数据产生的硝酸盐同化的同位素效应（同位素歧视程度）的估计（1.6-3.3‰），显着低于通常观察到的夏季南极（5-8‰）。然而，来自相同深度剖面的δ 18 O数据和混合层内δ 15 N的横向变化，暗示O和N同位素效应更类似于夏季数据所建议的那些。这些发现指向活性硝化（即，在南极冬季混合层内，有机物再生为硝酸盐）。从样品中去除亚硝酸盐显示了冬季混合层中亚硝酸盐的低δ 15 N（−40‰至−20‰），与硝化作用一致，但不能消除硝酸盐的异常低δ 15 N的观察结果。冬季数据及其所揭示的硝化作用解释了以前在夏季深度剖面的温度最低层（残留冬季混合层）中观测到的硝酸盐δ 15 N异常低的现象。与此同时，冬季数据要求秋季混合层中有机氮和铵结合的δ 15 N较低，这可用于冬季硝化作用，表明强烈的N再循环是南极夏末的普遍条件。
We report wintertime nitrogen and oxygen isotope ratios (δ15N and δ18O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ15N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6–3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5–8‰). The δ18O data from the same depth profiles and lateral δ15N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ15N for nitrite in the winter mixed layer (−40‰ to −20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ15N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ15N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ15N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer.