15:00 〜 15:15
[BBG01-05] Occurrence and geochemistry of Eoarchean carbonate rocks: Implications for bio-essential trace element contents in Eoarchean seawater
キーワード：生命必須元素、原太古代、イスア表成岩帯、古海水、希土類元素
Earth is a unique planet that has sustained liquid water and life since at least the Eoarchean. Therefore, reconstructing the Eoarchean environment is crucial for understanding the history of life's evolution. The Isua supracrustal belt (ISB; ~3.8 Ga) in southern West Greenland has provided valuable insights into early life evolution since the discovery of the oldest traces of life (e.g., Schidlowski et al., 1979).
Numerous studies have been carried out to determine the content of bio-essential trace elements in ancient seawater, aiming to explore the connection between seawater composition and the evolution of life. For example, Saito et al. (2003) proposed that the Zn content in seawater increased by eight orders of magnitude from the Eoarchean to the modern, based on O2 and H2S contents. In contrast, Robbins et al. (2013) reported secular changes in seawater Zn contents using banded iron formations (BIFs). They argued that Zn contents increased tenfold from the Eoarchean to the present. However, since modern BIFs form exclusively in hydrothermal environments and do not reflect the global ocean composition, comparing the composition of modern BIFs with that of ancient ones presents certain issues. Although carbonate rocks, which precipitate from seawater and have been deposited globally throughout Earth's history, are a potential candidate, carbonate rocks are susceptible to alteration, such as silicification and detrital contamination.
To obtain bio-essential trace element contents in Eoarchean seawater, we utilized ISB carbonate rock and analyzed major and trace elements, such as Ni, Cu, and Zn, using ICP-MS. The ISB carbonate rocks are classified into two types: those interbedded with chert and those interbedded with conglomerate layers. This study focused on the former, which was estimated to have been deposited in the pelagic ocean, based on its geological occurrence and rare earth element content (Yoshida et al., 2019; JpGU).
The concentrations of SiO2 and Al2O3 in the carbonate samples, which serve as indicators of silicification and detrital material incorporation, respectively, showed a positive correlation. Based on this correlation and the composition of ISB clastic rocks (Bolhar et al., 2005), it was estimated that approximately 90% of the SiO2 in the ISB carbonate rocks originates from silicification. Using this SiO2 content, the primary carbonate rock composition prior to silicification was calculated. The Ni and Zn contents in the primary ISB carbonate rocks showed positive correlations with those of Al2O3 and Zr. Since carbonate minerals contain very little Al or Zr, these positive correlations suggest that Ni and Zn were supplied by detrital material and enable the estimation of the composition of the carbonate end-member. As a result, the Ni and Zn concentrations were determined to be 41.1–65.7 μg/g and 19.1–45.3 μg/g, respectively.
The Ni and Zn concentrations in the carbonate end-member of the ISB carbonate rocks are one and two orders of magnitude higher, respectively, compared to those found in modern stromatolites and ooids. The variation in seawater Ni content is largely consistent with previous studies (Konhauser et al., 2009), but that of Zn content contradicts the Robbins et al.'s estimate. This discrepancy can be attributed to the fact that modern BIFs are strongly influenced by hydrothermal fluids.
Ni is essential for methanogens, which are candidates for early life. This study indicates that the elemental requirements of methanogens may have co-evolved with seawater composition. Zn is known to be preferentially utilized by eukaryotes rather than prokaryotes, leading to the assumption that Zn concentrations were lower in the birthplace of prokaryotes. However, this study suggests that Zn concentrations in Eoarchean seawater were higher than those in Phanerozoic seawater. These findings indicate that the elements used in biological metabolism are not necessarily controlled by environmental factors alone.
Numerous studies have been carried out to determine the content of bio-essential trace elements in ancient seawater, aiming to explore the connection between seawater composition and the evolution of life. For example, Saito et al. (2003) proposed that the Zn content in seawater increased by eight orders of magnitude from the Eoarchean to the modern, based on O2 and H2S contents. In contrast, Robbins et al. (2013) reported secular changes in seawater Zn contents using banded iron formations (BIFs). They argued that Zn contents increased tenfold from the Eoarchean to the present. However, since modern BIFs form exclusively in hydrothermal environments and do not reflect the global ocean composition, comparing the composition of modern BIFs with that of ancient ones presents certain issues. Although carbonate rocks, which precipitate from seawater and have been deposited globally throughout Earth's history, are a potential candidate, carbonate rocks are susceptible to alteration, such as silicification and detrital contamination.
To obtain bio-essential trace element contents in Eoarchean seawater, we utilized ISB carbonate rock and analyzed major and trace elements, such as Ni, Cu, and Zn, using ICP-MS. The ISB carbonate rocks are classified into two types: those interbedded with chert and those interbedded with conglomerate layers. This study focused on the former, which was estimated to have been deposited in the pelagic ocean, based on its geological occurrence and rare earth element content (Yoshida et al., 2019; JpGU).
The concentrations of SiO2 and Al2O3 in the carbonate samples, which serve as indicators of silicification and detrital material incorporation, respectively, showed a positive correlation. Based on this correlation and the composition of ISB clastic rocks (Bolhar et al., 2005), it was estimated that approximately 90% of the SiO2 in the ISB carbonate rocks originates from silicification. Using this SiO2 content, the primary carbonate rock composition prior to silicification was calculated. The Ni and Zn contents in the primary ISB carbonate rocks showed positive correlations with those of Al2O3 and Zr. Since carbonate minerals contain very little Al or Zr, these positive correlations suggest that Ni and Zn were supplied by detrital material and enable the estimation of the composition of the carbonate end-member. As a result, the Ni and Zn concentrations were determined to be 41.1–65.7 μg/g and 19.1–45.3 μg/g, respectively.
The Ni and Zn concentrations in the carbonate end-member of the ISB carbonate rocks are one and two orders of magnitude higher, respectively, compared to those found in modern stromatolites and ooids. The variation in seawater Ni content is largely consistent with previous studies (Konhauser et al., 2009), but that of Zn content contradicts the Robbins et al.'s estimate. This discrepancy can be attributed to the fact that modern BIFs are strongly influenced by hydrothermal fluids.
Ni is essential for methanogens, which are candidates for early life. This study indicates that the elemental requirements of methanogens may have co-evolved with seawater composition. Zn is known to be preferentially utilized by eukaryotes rather than prokaryotes, leading to the assumption that Zn concentrations were lower in the birthplace of prokaryotes. However, this study suggests that Zn concentrations in Eoarchean seawater were higher than those in Phanerozoic seawater. These findings indicate that the elements used in biological metabolism are not necessarily controlled by environmental factors alone.