Iron deposition during recovery from Late Devonian oceanic anoxia: Implications of the geochemistry of the Kawame ferromanganese deposit, Nedamo Belt, Northeast Japan
Iron deposition during recovery from Late Devonian oceanic anoxia: Implications of the geochemistry of the Kawame ferromanganese deposit, Nedamo Belt, Northeast Japan
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晚泥盆世海洋缺氧恢复过程中的铁沉积：日本东北部 Nedamo 带 Kawame 铁锰矿床地球化学的意义
DOI：
10.1016/j.gloplacha.2022.103920
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发表时间：
2022
影响因子：
3.9
通讯作者：
Kato Yasuhiro
中科院分区：
文献类型：
--
作者：
Kuwahara Yusuke;Fujinaga Koichiro;Nozaki Tatsuo;Ohta Junichiro;Yano Moei;Yasukawa Kazutaka;Nakamura Kentaro;Kato Yasuhiro
晚泥盆世是地球历史上最重要的时期之一，也是中生代“五大”生物灭绝事件之一。然而，由于俯冲作用，深海记录的缺乏阻碍了对泛海拉萨晚泥盆世远洋环境的阐明。然而，沉积在深海海底，然后增生到大陆边的古热液铁锰沉积物，则作为棕褐色沉积物保存下来，并以增生棱柱体的形式出露。这些沉积物可以提供表征古海洋的关键信息。在日本东北部Nedamo带的Kawame采石场出露了一个上泥盆统（法门期）的棕色存款。本文通过对川梅泥的化学组成和锇同位素比值（187 Os/188 Os）的研究，探讨了晚泥盆世泛海拉萨洋的远洋环境。铁含量与P、V、Os和稀土元素的含量呈正相关，表明热液铁羟基氧化物颗粒以与现今海洋中热液铁锰沉积物相同的方式从环境海水中吸附了这些元素。新获得的晚泥盆世海水187 Os/188 Os初始值为0.49-0.57，低于现今海水187 Os/188 Os的值（~1.06），与古近纪和晚白垩世的值相当。与现今海底热液铁锰沉积物和日本增生杂岩中较年轻的土矿床相比，川目土最显著的特征之一是其铁含量明显较高（高达90重量%）。由于海水187 Os/188 Os值与古近纪和晚白垩世没有明显差异，晚泥盆世异常强烈的热液活动不可能导致Fe沉积的增加。的Kawame编号（归一化后太古代平均澳大利亚页岩）的稀土元素模式显示较弱的负Ce异常比年轻的编号和现今的热液铁锰沉积物，这表明Kawame编号沉积在氧化条件下。考虑到许多红层沉积在~370 Ma，在恢复过程中的海洋缺氧事件（OAE）的晚泥盆世（早期法门），铁的异常富集Kawame数可能反映了增强氧化和沉淀丰富的溶解铁在后OAE从缺氧到好氧条件的过渡。
The Late Devonian, during which one of the “Big Five” Phanerozoic mass extinction events occurred, was one of the most important time intervals in Earth history. Nevertheless, the paucity of deep-sea records due to subduction has hampered elucidation of the pelagic environment during the Late Devonian in Panthalassa. However, ancient hydrothermal ferromanganese sediments, which were deposited on the abyssal seafloor and then accreted onto continental margins, are preserved as umber deposits and exposed in accretionary prisms. These sediments can provide key information to characterize the paleo-ocean. An Upper Devonian (Famennian) umber deposit is exposed in Kawame Quarry, Nedamo Belt, northeastern Japan. We investigated the bulk chemical composition and osmium isotopic ratio (187Os/188Os) of the Kawame umber to discuss the pelagic environment of the Panthalassa Ocean during the Late Devonian. The Kawame umber shows positive correlations between Fe content and the levels of P, V, Os, and rare earth elements, suggesting that hydrothermal Fe-oxyhydroxide particles adsorbed these elements from ambient seawater in the same way as hydrothermal ferromanganese sediments in the present-day ocean. Newly obtained initial187Os/188Os values of Late Devonian seawater are 0.49–0.57, lower than that of present-day seawater (~1.06) and comparable to Paleogene and Late Cretaceous values. One of the most remarkable characteristics of the Kawame umber is its markedly high Fe levels (up to 90 wt%) compared to present-day seafloor hydrothermal ferromanganese sediments and younger umber deposits in Japanese accretionary complexes. Because seawater187Os/188Os values were not notably different from those during the Paleogene and Late Cretaceous, anomalously vigorous hydrothermal activity in the Late Devonian is unlikely to have caused the increased Fe deposition. The rare earth element patterns of the Kawame umber (normalized to post-Archean average Australian shale) show weaker negative Ce anomalies than those of younger umbers and present-day hydrothermal ferromanganese sediments, suggesting that the Kawame umbers were deposited under less oxidizing conditions. Considering that many red beds were deposited at ~370 Ma, during recovery from the oceanic anoxic events (OAEs) of the Late Devonian (early Famennian), the anomalous Fe enrichment in the Kawame umber likely reflects enhanced oxidization and precipitation of abundant dissolved Fe during the post-OAE transition from anoxic to oxic conditions.