High organic carbon content and a decrease in radiolarians at the end of the Permian in a newly discovered continuous pelagic section: A coincidence?

作者：Takahashi Satoshi^*; Yamakita Satoshi; Suzuki Noritoshi; Kaiho Kunio; Ehiro Masayuki

来源：Palaeogeography, Palaeoclimatology, Palaeoecology, 2009, 271(1-2): 1-12.

DOI：10.1016/j.palaeo.2008.08.016

• Permian-Triassic
• Mass extinction
• Conodont
• Radiolaria
• Organic carbon
• Biostratigraphy
• Oceanic anoxia
• Productivity
• Deep sea
• Japan

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/cm(2) coincided 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.

• 出版日期2009-1-1