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摘要
The Capitanian minimum in the Permian represents one of the most significant features in the Phanerozoic seawater Sr-87/Sr-86 history. In order to establish the detailed Sr chemostratigraphy around the Guadalupian minimum, Sr-87/Sr-86 ratios were measured for the Capitanian (upper Middle Permian) paleo-atoll limestones at Akasaka in Japan. The limestone was primarily deposited on a paleo-seamount in the low-latitude mid-Panthalassa, and was secondarily accreted to Japan (South China block) margin in the Jurassic. As being free from local continental influences, the Akasaka limestone recorded well-mixed seawater isotope composition of the Permian low-latitude mid-superocean. We detected extremely low Sr-87/Sr-86 ratios (ca. 0.7068-0.7069) in the 70 m-thick Capitanian interval, immediately below the Guadalupian-Lopingian (Middle-Late Permian) boundary (G-LB), of the Akasaka limestone. This Sr isotopic profile at Akasaka suggests that the global seawater was least affected by radiogenic continental flux throughout the Capitanian. As these values correspond to the lowest in the Paleozoic, this interval with low Sr-87/Sr-86 ratios, lasted for at least some milllion years, represents the Capitanian minimum, which marks the significant turning point from the Late Paleozoic decrease to Early Mesozoic increase in seawater Sr-87/Sr-86 ratio. The geological lines of evidence indicate that the Capitanian minimum was caused likely by the mid-Permian cooling that may have driven extensive ice-cover over continental crusts to suppress continental flux enriched in radiogenic Sr into the superocean. The rapid increase in Sr-87/Sr-86 values after the minimum can be explained either by the deglaciation or by the Pangean rifting.
- 出版日期2013-7
