11:00 〜 11:15
[BCG04-08] 初期太古代炭酸塩岩の地質学的産状と化学組成から解読する海洋中の生命と生命必須元素組成の共進化
★招待講演
キーワード：炭酸塩岩、生命必須元素、イスア表成岩帯、ヌリアック表成岩類、希土類元素、初期太古代
Many geologists and biologists believe that the trace elements utilized in the metalloenzymes of life reflect the availability of trace elements in ancient ocean where those enzymes first appeared (e.g., Fraústo da Silva and Williams, 2001). However, studies to estimate composition of paleoseawater have only been conducted in the last 20 years, so it is necessary to confirm whether the life and bioessential element contents in ocean actually co-evolved.
Geochemistry of banded iron formation (BIF) is commonly utilized to estimate surface environmental conditions such as seawater composition during the Archean. However, an estimation of ocean composition with BIF has some difficulties. For example, trace elements contents in a BIF depends not only these elements contents in ambient seawater but also sedimentary rates, involvement of detrital materials (e.g., Bau, 1993), kinetic effect originated from adsorption rate of element on iron-oxide (e.g., Byrne and Kim, 1990), and other interference elements such as Si (Konhauser et al., 2009). Another problem is that most of modern iron-rich sediments are formed near hydrothermal vents, so they don’t reflect the global seawater composition. Despite the problems with the estimation using the BIF, few studies have been conducted to estimate the concentrations of bio-essential elements in the paleoseawater using carbonate rocks, which also exist as chemical sedimentary rocks. This is because it was reported that the Eoarchean carbonate rocks could be formed secondary to igneous rocks due to an alteration (e.g., Rose et al., 1996).
Here, we estimated the concentrations of V, Co, Ni, and Zn in the Eoarchean seawater using carbonate rocks from Nulliak supracrustal rocks (ca. 3.9 Ga) and Isua supracrustal belt (ca. 3.8 Ga). The Nulliak and Isua carbonate rock used in this study have already been reported to be derived from chemical sedimentary rocks by Yoshida et al. (2021) and (2019; JpGU meeting), respectively.
We analyzed V, Co, Ni, and Zn contents of the whole rocks of the carbonate rocks with High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS).
We estimated bioessential element contents of the Eoarchean seawater using carbonate rocks, which are the least affected by silicification and contamination of detrital material reported by previous studies (Yoshida et al., 2019, 2021). Moreover, we compared these element contents in the Eoarchean carbonate rocks with those of modern carbonate rocks.
The secular change of V, Co, Ni, and Zn contents show that the Eoarchean seawater are enriched in transitional element than modern seawater. The estimation using the BIF reported by previous study is inconsistent with the results of this study because it suggests that V and Zn were more depleted in the Eoarchean ocean than later ocean (Robbins et al., 2013; Aoki et al., 2018). This discrepancy can be attributed to the fact that BIFs in the younger age were strongly influenced by hydrothermal fluids.
Nitrogenase is classified into V-Fe (Vnf), Mo-Fe (Nif), and Fe-Fe (Anf) types, and is thought to have existed as early as the Archean (e.g., Stüeken et al., 2015). Phylogenetic analysis of nitrogenase reported that Nif was the closest to the origin of nitrogenase (e.g, Boyd et al., 2011). This study suggests that nitrogenase and V content in the ocean didn’t co-evolve. Cobalt is indispensable for the production of a vitamin B12 for prokaryote. On the other hand, eukaryotes have an ability to synthesize methionine independently of Co. Therefore, this study suggests that biological Co dependence and Co concentration in the ocean may have co-evolved. Nickel is an essential element for methanogen, which is considered to be one of the candidates for early life. This study indicates that the elemental requirements of methanogen may have co-evolved with ocean composition. These indicate that life and ocean composition co-evolve, but that evolution can also be driven by biological factors.
Geochemistry of banded iron formation (BIF) is commonly utilized to estimate surface environmental conditions such as seawater composition during the Archean. However, an estimation of ocean composition with BIF has some difficulties. For example, trace elements contents in a BIF depends not only these elements contents in ambient seawater but also sedimentary rates, involvement of detrital materials (e.g., Bau, 1993), kinetic effect originated from adsorption rate of element on iron-oxide (e.g., Byrne and Kim, 1990), and other interference elements such as Si (Konhauser et al., 2009). Another problem is that most of modern iron-rich sediments are formed near hydrothermal vents, so they don’t reflect the global seawater composition. Despite the problems with the estimation using the BIF, few studies have been conducted to estimate the concentrations of bio-essential elements in the paleoseawater using carbonate rocks, which also exist as chemical sedimentary rocks. This is because it was reported that the Eoarchean carbonate rocks could be formed secondary to igneous rocks due to an alteration (e.g., Rose et al., 1996).
Here, we estimated the concentrations of V, Co, Ni, and Zn in the Eoarchean seawater using carbonate rocks from Nulliak supracrustal rocks (ca. 3.9 Ga) and Isua supracrustal belt (ca. 3.8 Ga). The Nulliak and Isua carbonate rock used in this study have already been reported to be derived from chemical sedimentary rocks by Yoshida et al. (2021) and (2019; JpGU meeting), respectively.
We analyzed V, Co, Ni, and Zn contents of the whole rocks of the carbonate rocks with High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS).
We estimated bioessential element contents of the Eoarchean seawater using carbonate rocks, which are the least affected by silicification and contamination of detrital material reported by previous studies (Yoshida et al., 2019, 2021). Moreover, we compared these element contents in the Eoarchean carbonate rocks with those of modern carbonate rocks.
The secular change of V, Co, Ni, and Zn contents show that the Eoarchean seawater are enriched in transitional element than modern seawater. The estimation using the BIF reported by previous study is inconsistent with the results of this study because it suggests that V and Zn were more depleted in the Eoarchean ocean than later ocean (Robbins et al., 2013; Aoki et al., 2018). This discrepancy can be attributed to the fact that BIFs in the younger age were strongly influenced by hydrothermal fluids.
Nitrogenase is classified into V-Fe (Vnf), Mo-Fe (Nif), and Fe-Fe (Anf) types, and is thought to have existed as early as the Archean (e.g., Stüeken et al., 2015). Phylogenetic analysis of nitrogenase reported that Nif was the closest to the origin of nitrogenase (e.g, Boyd et al., 2011). This study suggests that nitrogenase and V content in the ocean didn’t co-evolve. Cobalt is indispensable for the production of a vitamin B12 for prokaryote. On the other hand, eukaryotes have an ability to synthesize methionine independently of Co. Therefore, this study suggests that biological Co dependence and Co concentration in the ocean may have co-evolved. Nickel is an essential element for methanogen, which is considered to be one of the candidates for early life. This study indicates that the elemental requirements of methanogen may have co-evolved with ocean composition. These indicate that life and ocean composition co-evolve, but that evolution can also be driven by biological factors.