[B-CG06] 地球史解読：冥王代から現代まで

[BCG06-08] Bio-essential element contents in the Paleoproterozoic seawater: Insights from in situ analysis of carbonate rocks in the Francevillian Group, Gabonese Republic

*吉田 聡^1,2、浅沼 尚³、佐藤 友彦⁴、澤木 佑介²、平田 岳史⁵ (1.東北大学　東北アジア研究センター、2.東京大学大学院　総合文化研究科、3.京都大学　人間・環境学研究科、4.岡山理科大学　基盤教育センター、5.東京大学大学院　理学系研究科地殻化学実験施設)

キーワード：炭酸塩岩、LA-ICP-MS、古海水、原生代、酸化還元度

The Paleoproterozoic is one of the most significant periods in Earth's history because the interval is characterized particularly by the emergence of eukaryotes. Recently, the Paleoproterozoic Francevillian Group in the Gabonese Republic has attracted increasing attention because distinctive macroscopic structures, interpreted as potentially the earliest eukaryotic fossils, have been discovered there. Therefore, the chemical sediments of the Group can provide information about the evolution of life.Several studies have been conducted to reconstruct the bio-essential element contents of paleo-seawater. For example, given the redox state and H₂S content, Anbar (2008) predicted that the Co content in the Paleoproterozoic seawater was about two orders of magnitude higher than the modern equivalent. Based on the secular variation in Co contents in black shales, banded iron formations (BIFs) in various depositional settings, and pyrites, Swanner et al. (2014) argued that there was a relatively Co-rich ocean from 2.8 Ga to 1.8 Ga, whereas thereafter, the decline in hydrothermal activity led to a decrease in Co content. However, because black shale is an accumulation of not only chemical precipitates but also detritus and organic matter, interpretation of the obtained data is not straightforward. Similarly, interpreting the trace element contents in pyrite is complex due to the variety of formation environments and processes involved. Carbonate rocks, chemical sedimentary rocks are proposed as another potential candidate for tracking trace element contents of seawater. Carbonate rocks directly precipitate from seawater, and trace elements are incorporated through solid solutions. On the other hand, carbonate rocks are susceptible to post-depositional processes including dissolution of Fe-Mn oxide and silicification, and dolostones in the Francevillian Group also suffered from these processes (Yoshida et al., 2024). To constrain bio-essential trace element contents in the Paleoproterozoic seawater, we tried to extract trace element contents of primarily precipitated domains in the dolostones.We utilized the dolostones of the FB Formation of the Francevillian Group deposited in the Lastoursville basin and analyzed major and trace elements, including Co, Ni, Cu, and Zn using an ICP-MS/MS coupled with a laser ablation sampling technique. During this process, we checked the line profiles of all analyses and excluded data where detrital silicate minerals were ablated.The results demonstrated that Ni, Cu, and Zn contents are positively correlated with Si content rather than Fe content. Thus, these trace elements can be explained as mixing between the carbonate domain and the silicified part, and we calculated the elemental contents of the carbonate domain. The Ni, Cu, and Zn contents of primary carbonate rock are calculated as 4.87 ± 0.92, 1.07 ± 1.07, and 7.64 ± 2.03 μg/g (1s), respectively.
Co content is positively correlated with Fe content rather than Si content, which indicates that the influence of silicification on Co content was negligible. Given that the positive correlation between Fe and Co contents is seen in modern Fe-Mn nodules, the correlation observed in the dolostone can be attributed to mixing between the primary dolomite and the diagenetic Fe-rich dolomite originated from carbonation of Fe-Mn oxides. By comparing the slope of the obtained correlation (Co ppm = 4.64×Fe wt.%) with that derived from modern Fe-Mn nodules (Co ppm = 10.5×Fe wt.%), it is estimated that the Co content in the Paleoproterozoic seawater was about half of its modern level.Among the results, the estimated Ni, Cu, and Zn contents are lower than those reported from Eoarchean primary carbonate rocks. The estimated Co contents contradict the previous studies. This indicates that the marine environment is not solely governed by redox conditions and that the contents of BIFs and pyrites are strongly influenced by the contents of each depositional setting.