[BCG09-P01] 87Sr/86Sr chemostratigraphy of Ediacaran Doushantuo Formation in the Yangtze Craton, South China
キーワード：放射性起源ストロンチウム同位体比、南中国、エディアカラ紀
The Ediacaran period records one of the most dramatic biological episodes in Earth’s history. To track environmental changes occurring in the Ediacaran, multi-geochemical proxies have been reported by a number of studies. Ediacaran sedimentary rocks in South China figure prominently in such studies, because they are fossiliferous and accumulated at various depositional settings from shallow platform to basin facies. Recent extensive geochemical works for the Doushantuo Formation in South China demonstrate that δ13C values of inorganic carbon were variable throughout the Ediacaran period. On the other hand, a drastic change in weathering influx from continents is thought to have major influences on the change in seawater composition and on biological activity. Its flux can be estimated from the 87Sr/86Sr ratios of carbonate rocks. However, the existing 87Sr/86Sr values are limited to shallow marine deposits, which leaves ambiguity in a variation of 87Sr/86Sr ratios in outer ocean.
We conducted drilling at Siduping, Tianping, and Weng’an sections in South China to obtain the Ediacaran complete sedimentary sequences deposited at slope and shoal facies. We newly report stratigraphic profiles of the 87Sr/86Sr ratios at the three sections. 87Sr/86Sr chemostratigraphy demonstrated some diachronous natures of δ13C within the Doushantuo Formation. The enhanced continental weathering during Gaskiers glaciation likely promoted bacterial sulfate reduction and aerobic respiration of organic matter. These resulted in low δ13C values of dissolved inorganic carbon and accumulations of phosphate and dissolved CO2 species in seawater, and eventually induced the deposition of phosphorites at the shelf margin. High 87Sr/86Sr ratios during the largest negative δ13C anomaly in the Ediacaran can be also recognized in the continental slope sediment. This fact supports that globally high continental weathering rate led to massive remineralization of organic matter and a consequent significant negative δ13Ccarb excursion.
We conducted drilling at Siduping, Tianping, and Weng’an sections in South China to obtain the Ediacaran complete sedimentary sequences deposited at slope and shoal facies. We newly report stratigraphic profiles of the 87Sr/86Sr ratios at the three sections. 87Sr/86Sr chemostratigraphy demonstrated some diachronous natures of δ13C within the Doushantuo Formation. The enhanced continental weathering during Gaskiers glaciation likely promoted bacterial sulfate reduction and aerobic respiration of organic matter. These resulted in low δ13C values of dissolved inorganic carbon and accumulations of phosphate and dissolved CO2 species in seawater, and eventually induced the deposition of phosphorites at the shelf margin. High 87Sr/86Sr ratios during the largest negative δ13C anomaly in the Ediacaran can be also recognized in the continental slope sediment. This fact supports that globally high continental weathering rate led to massive remineralization of organic matter and a consequent significant negative δ13Ccarb excursion.