09:45 〜 10:00
[BCG05-04] Combination of in-situ sulfur isotope and trace element analysis of pyrites: Quantitative estimate of chemistry and redox condition of seawater in the Ediacaran
キーワード：エディアカラ紀、局所イオウ同位体分析、環境と生物の共進化、古海洋組成と酸化還元推定
The earth has experienced coevolution between the environment and life, and became a unique planet, where oxic ocean and atmosphere are present and many highly diversified organisms of prokaryotes, eukaryotes and even metazoans live. The Ediacaran and early Cambrian period is a key period because surface environment drastically changed and metazoans first appeared. Especially, chemical composition and redox condition of seawater dramatically changed from the ferruginous and euxinic ocean, which is depleted in bioessential elements such as Mo, to oxic ocean. Thus, it is important to estimate the surface environment and decode the coevolution through the period because they possibly influenced on the biological evolution. However, it is difficult to estimate the paleoenvironment such as chemistry and redox condition of seawater; therefore, it is necessary to establish a new proxy to estimate them.
Previous studies used iron chemical species, redox sensitive element contents, REE patterns, and isotope compositions of Fe, Mo, and Cr, in order to estimate the redox condition of seawater through the Ediacaran, and suggested that the seawater remained ferruginous and euxinic with intermittent oxidation events even after the Marinoan Snowball Earth. However, the quantitative estimate of the redox state remains ambiguous. Previous studies showed that euxinic areas were present in addition to a layered structure of oxic shallow and ferruginous deep water in the Ediacaran. However, the extent of euxinic areas was obscure: the euxinic areas were limited to edges of continental shelf, widely distributed in deep water, or sporadically over the world. Therefore, this study consists of two purposes: Establishment of a new method to more quantitatively estimate of the redox condition of paleoseawater and its application to core samples of the Siduping section in the Yangtze block, corresponding to upper slope of the continental shelf connected with the open ocean, in order to estimate the extent of euxinia in the early Ediacaran.
It is well known that Mo content is the most sensitive to redox condition, especially the extent of euxinia. However, Mo contents of black shale are too varied by over 2-3 orders of magnitude to quantitatively estimate Mo content and extent of euxinia of seawater in the Ediacaran. We assumed that the large variation was caused by setting and timing of pyrite formation: water column vs. pore water and immature vs. mature pore water, respectively. Sulphur isotope ratios of sulfide (δ34Spy) depend on a δ34S value of ambient water and microbial isotope fractionation between sulfate and sulfide. Although seawater δ34S values changed through geologic time, the values kept constant in the short term because seawater was enriched in sulfate even in the Ediacaran. On the other hand, the δ34S values of pore water drastically change as sulfide is precipitated. Therefore, the δ34Spy values constrain the setting and timing of pyrite formation; thus pyrite with low δ34S values was formed in water column or immature pore water so that it preserves geochemical information of seawater.
We conducted in-situ measurement of sulfur isotopes and Mo contents of pyrites in black shale (~630 Ma) just above Cap Carbonate of the Siduping section using SIMS at Kochi Institute, JAMSTEC and LA-ICP-MS at Geochemical Research Center of University of Tokyo. The δ34S values range from -22.93 to 51.93 ‰ and Mo contents from 0.1 to 2211 ppm. Framboidal pyrites have the minimum δ34S values with a nadir of ca.-23 ‰, possibly equilibrated with the deep seawater. A negative correlation between their δ34S values and Mo contents suggests that the pyrite was formed in closed pore water and the δ34S values and Mo contents were controlled by Rayleigh fractionation. Assuming that the pyrite equilibrated with seawater had -20 ‰, it is estimated that the pyrite equilibrated with seawater had ca. 13000 ppm in Mo content from the regression curve. The estimated content is equivalent to the maximum Mo content of pyrite in a modern euxinic environment (Cariaco Basin), suggesting that the extent of euxinia was much smaller even in the Ediacaran than the expected.
Previous studies used iron chemical species, redox sensitive element contents, REE patterns, and isotope compositions of Fe, Mo, and Cr, in order to estimate the redox condition of seawater through the Ediacaran, and suggested that the seawater remained ferruginous and euxinic with intermittent oxidation events even after the Marinoan Snowball Earth. However, the quantitative estimate of the redox state remains ambiguous. Previous studies showed that euxinic areas were present in addition to a layered structure of oxic shallow and ferruginous deep water in the Ediacaran. However, the extent of euxinic areas was obscure: the euxinic areas were limited to edges of continental shelf, widely distributed in deep water, or sporadically over the world. Therefore, this study consists of two purposes: Establishment of a new method to more quantitatively estimate of the redox condition of paleoseawater and its application to core samples of the Siduping section in the Yangtze block, corresponding to upper slope of the continental shelf connected with the open ocean, in order to estimate the extent of euxinia in the early Ediacaran.
It is well known that Mo content is the most sensitive to redox condition, especially the extent of euxinia. However, Mo contents of black shale are too varied by over 2-3 orders of magnitude to quantitatively estimate Mo content and extent of euxinia of seawater in the Ediacaran. We assumed that the large variation was caused by setting and timing of pyrite formation: water column vs. pore water and immature vs. mature pore water, respectively. Sulphur isotope ratios of sulfide (δ34Spy) depend on a δ34S value of ambient water and microbial isotope fractionation between sulfate and sulfide. Although seawater δ34S values changed through geologic time, the values kept constant in the short term because seawater was enriched in sulfate even in the Ediacaran. On the other hand, the δ34S values of pore water drastically change as sulfide is precipitated. Therefore, the δ34Spy values constrain the setting and timing of pyrite formation; thus pyrite with low δ34S values was formed in water column or immature pore water so that it preserves geochemical information of seawater.
We conducted in-situ measurement of sulfur isotopes and Mo contents of pyrites in black shale (~630 Ma) just above Cap Carbonate of the Siduping section using SIMS at Kochi Institute, JAMSTEC and LA-ICP-MS at Geochemical Research Center of University of Tokyo. The δ34S values range from -22.93 to 51.93 ‰ and Mo contents from 0.1 to 2211 ppm. Framboidal pyrites have the minimum δ34S values with a nadir of ca.-23 ‰, possibly equilibrated with the deep seawater. A negative correlation between their δ34S values and Mo contents suggests that the pyrite was formed in closed pore water and the δ34S values and Mo contents were controlled by Rayleigh fractionation. Assuming that the pyrite equilibrated with seawater had -20 ‰, it is estimated that the pyrite equilibrated with seawater had ca. 13000 ppm in Mo content from the regression curve. The estimated content is equivalent to the maximum Mo content of pyrite in a modern euxinic environment (Cariaco Basin), suggesting that the extent of euxinia was much smaller even in the Ediacaran than the expected.