High organic carbon content and a decrease in radiolarians at the end of the Permian in a newly discovered continuous pelagic section: A coincidence?
High organic carbon content and a decrease in radiolarians at the end of the Permian in a newly discovered continuous pelagic section: A coincidence?
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DOI：
10.1016/j.palaeo.2008.08.016
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
S. Takahashi;S. Yamakita;N. Suzuki;K. Kaiho;M. Ehiro
中科院分区：
文献类型：
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作者：
S. Takahashi;S. Yamakita;N. Suzuki;K. Kaiho;M. Ehiro
最大规模的物种灭绝发生在二叠纪末期。大多数大灭绝的记录不是来自远洋沉积物，而是来自浅海和陆地沉积物。虽然已经发现了几个跨越二叠纪末大灭绝的远洋剖面，但这些剖面包含的指标化石很少，而且由于小断层而经常不连续。在硅质粘土层中发现了Albaillella cf. triangularis （Radiolaria）的指示化石，在上覆的黑色粘土层中发现了Hindeodus parvus（牙形刺）的指示化石，以及Neospathodus cf. cristagalli和Ns的指示化石。日本北部赤森二剖面中硅质粘土层中的牙形石。这些化石表明，该剖面的发育时期为晚二叠世至早三叠世，包括早期的印度河期和奥伦基期。从层状燧石岩到硅质粘土岩、黑色粘土岩再到硅质粘土岩的岩性变化，与日本著名的二叠纪—三叠纪海相层序一致。赤森二段各岩相间无间隙。通过手工抛光样品和薄片的观察，确定了Akkamori剖面2段上二叠统硅质粘土岩层与上二叠统至下三叠统黑色粘土岩层之间的临界岩性连续性。这些古生物学和沉积学证据表明，Akkamori 2剖面是一个连续的远洋剖面，记录了二叠纪末的大灭绝事件。碳质黑色粘土岩层具有较高的总有机碳（TOC）浓度（1.06 ~ 3.31 wt.%），表明海洋缺氧程度至少较深，初级生产力可能稳定。放射虫丰度从26-563个减少到0.27-20个标本/cm2，与二叠世顶部硅质粘土岩与上覆黑色粘土岩层交界处TOC含量从0.01-0.16个增加到1.06-3.31 wt.%一致，表明放射虫灭绝发生在二叠世末期，与海洋缺氧相吻合。虽然TOC含量在奥列基世（史密斯世）早期有所下降，但放射虫丰度并未增加，表明放射虫的恢复延迟了约1.5 m.y。
The greatest mass extinction occurred at the end of the Permian. Most records of the mass extinction are not from pelagic sediments, but from shallow-marine and terrestrial sediments. Although several pelagic sections that span the end-Permian mass extinction have been found, these sections contain few index fossils and are often discontinuous because of small faults. We found the index fossils Albaillella cf. triangularis (Radiolaria) in siliceous claystone beds, Hindeodus parvus (Conodont) in the overlying black claystone beds, and Neospathodus cf. cristagalli and Ns. waageni (Conodont) in the subsequent siliceous claystone beds in Akkamori section-2 in northern Japan. These fossils suggest that this section ranges from the late Permian to the Early Triassic, including the early Induan and Olenekian stages. Furthermore, the lithological changes in the section, i.e., starting from bedded chert through siliceous claystone and black claystone to siliceous claystone, are concordant with those of well-known Permian–Triassic pelagic sequences in Japan. There is no gap between each lithofacie of the Akkamori section-2. Critical lithological continuity between Upper Permian siliceous claystone beds and uppermost Permian to lowermost Triassic black claystone beds of the Akkamori section-2 was recognized by observing hand-polished specimens and thin sections. Such paleontological and sedimentological evidence implies that the Akkamori section-2 is a continuous pelagic section that records the end-Permian mass extinction event. The carbonaceous black claystone beds have high total organic carbon (TOC) concentrations (1.06–3.31 wt.%), suggesting oceanic anoxia at least deep and probably stable primary productivity. A decrease in radiolarian abundance from 26–563 to 0.27–20 specimens/cm2coincided with an increase in TOC content from 0.01–0.16 to 1.06–3.31 wt.% at the boundary of the siliceous claystone and the overlying black claystone beds near the top of the Permian, implying that radiolarian extinction occurred at the end of the Permian coinciding with oceanic anoxia. Although TOC contents decreased in the early Olenekian (Smithian), radiolarian abundance did not increase at that time, indicating that radiolarian recovery was delayed by >1.5 m.y.