Redox condition and nitrogen cycle in the Permian deep mid-ocean: A possible contrast between Panthalassa and Tethys
Redox condition and nitrogen cycle in the Permian deep mid-ocean: A possible contrast between Panthalassa and Tethys
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DOI：
10.1016/j.gloplacha.2018.09.015
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发表时间：
2019-01
影响因子：
3.9
通讯作者：
W. Fujisaki;Y. Sawaki;Y. Matsui;S. Yamamoto;Y. Isozaki;S. Maruyama
中科院分区：
文献类型：
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作者：
W. Fujisaki;Y. Sawaki;Y. Matsui;S. Yamamoto;Y. Isozaki;S. Maruyama
为了限制氧化还原条件和相关的氮循环在中二叠世（瓜达卢普）最晚晚二叠世（乐平）深中泛海拉萨，我们确定了主要的，微量的，稀土元素的丰度沿着与碳和氮同位素比值的页岩夹层深海硅质岩是暴露在Gujo-Hachiman部分在Mino-Tanba带，日本西南部。Guadalupian期和乐平期大部分时间Ce的正异常和Mo、U的低富集因子（MoEF< 1.0; 0.7 < UEF< 2.5），表明中泛海区深部处于氧化环境。另一方面，乐平世中晚期部分页岩的MoEF（2.0 < MoEF< 9.2）和UEF（0.9 < UEF< 2.5）值略高，表明其沉积环境为间歇性的亚氧环境。这些新的发现表明，在中泛海洋深处的氧化还原条件从来没有达到缺氧条件在瓜达卢佩和乐平，直到二叠纪-三叠纪边界（P-TB）的时间间隔。考虑到瓜达卢普世到最晚的Lopingian的氧化还原状态，我们新获得的δ 15 NTN值（−0.12‰到+2.57‰）表明在中泛海拉萨沿着硝酸盐同化过程中的大同位素分馏，处于富硝酸盐的条件下。然而，与富氧和富硝酸盐的深泛海拉萨不同，我们推测缺氧（即，缺氧/缺氧）和生物可利用的氮贫乏的条件下开发的深特提斯之前的瓜达卢佩-Lopingian边界（G-LB）。这些环境压力可能是由全球变冷事件（Kamura事件）以及独特的古地理学驱动的，即，与特提斯洋的极地冰盖没有接触。在全球变冷事件期间，在特提斯深水区积累的缺氧水团的上涌可能引发了G-LB周围浅海区域的海洋缺氧，最终导致G-LB大灭绝。
To constrain the redox conditions and related nitrogen cycles during the Middle Permian (Guadalupian) to latest Late Permian (Lopingian) deep mid-Panthalassa, we determined the abundances of major, trace, and rare earth elements along with the carbon and nitrogen isotope ratios in shales interbedded with deep-sea cherts that are well-exposed at the Gujo-Hachiman section in the Mino-Tanba belt, SW Japan. The positive Ce anomalies together with low Mo and U enrichment factors (MoEF< 1.0; 0.7 < UEF< 2.5) during the Guadalupian and the most of the Lopingian indicate that the deep mid-Panthalassa was under oxic condition. On the other hand, the slightly higher MoEF(2.0 < MoEF< 9.2) and UEF(0.9 < UEF< 2.5) values in some of the middle-late Lopingian shales suggest deposition under intermittent suboxic condition. These new findings indicate that the redox condition in the deep mid-Panthalassa never reached an anoxic condition during the Guadalupian and Lopingian until the Permian-Triassic boundary (P-TB) interval. In view of the redox state from the Guadalupian to latest Lopingian, our newly obtained δ15NTNvalues (−0.12‰ to +2.57‰) indicate nitrate-rich conditions in the mid-Panthalassa along with large isotopic fractionation during nitrate assimilation. However, unlike the oxic and nitrate-rich deep-Panthalassa, we speculate that oxygen-depleted (i.e., anoxic/euxinic) and bioavailable nitrogen-poor conditions developed in the deep Tethys immediately before the Guadalupian-Lopingian boundary (G-LB). These environmental stresses were potentially driven by a global cooling episode (Kamura event) together with the unique paleogeography, i.e., no contact with polar ice caps in the Tethyan Ocean. Upwelling of the anoxic watermass accumulated in the deep Tethys during the global cooling episode likely triggered oceanic anoxia in the shallow-marine regions around the G-LB, which eventually resulted in the G-LB mass extinction.