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Earth & Life (http://www.geofinds.com), (2006-11-5), 1(2): 15-22.
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Fig. 3. Scanning electronic microphotograph of pyrite framboid (arrow) from Bed 3 of Ziyun section, China. The framboids are generally < 7 μm in diameter and suggest anoxic bottom water. |
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In Ziyun, extinction began at the end of C. yini conodont zone, whereas anoxia did not occur until the H. parvus zone. The same timing of anoxia was interpreted in the low-latitude shallow-marine sections in Sichuan Province, southwestern China (Crasquin-Soleau and Kershaw, 2005). The low-latitude PTB section at Tesero, Dolomites, Italy (Fig. 2B) also began with a basal shallow-marine facies and is subdivided into three beds based on lithology and fossils (Noé, 1987). The basal Bed 1 is a mudstone with a diverse biota of 36 species of calcareous algae, foraminifer, echinoderms, and ostracods. The presence of many stenotropic organisms suggests a pre-extinction biota. Bed 2 consists of grainstone and packstone, with only five species of algae, foraminifers, and bivalves. The absence of stenotropic organisms suggests onset of mass extinction. Dissolution cavities in this and underlying beds (>100 m thick), formed during freshwater phreatic diagenesis, indicate a large sea-level fall beginning at the base of this bed. Mud cracks in the upper part indicate a tidal-flat environment. Bed 3 consists of mudstone in the lower part and grainstone in the upper part, containing only ostracods. The presence of pyrite indicates anoxic conditions, which occurred after the mass extinction at the end of Bed 1. Hindeodus parvus was found in the middle and upper parts of Bed 3 (Perri and Farabegoli, 2003), suggesting a similar timing of anoxia and extinction to that of Ziyun. 2.2 Low-Latitude Deep-Marine Section in Meishan, ChinaThe PTB section in Meishan, Zhejiang Province, southeastern China (Fig. 2C) was located near the plaeo-equator. The uppermost Permian consists of basin-slope facies (Jin et al., 2000; Jiang and Qian, 1986). This section can be subdivided into eight beds. The basal Bed 24e is a wackestone with a diverse biota of fusulinids, brachiopods, ammonoids, and conodonts. The presence of stenotropic fusulinids suggests a pre-extinction biota. Fusulinids are absent at the top of this bed, indicating onset of mass extinction. The top 2 mm is composed of pyrite, indicating anoxic condition (Wignall and Hallam, 1992). Beds 25 to 28 contain sparse eurytropic organisms. Mass extinction and oceanic anoxia began at about the same time at the end of C. yini zone. |
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