11:30 〜 11:45
[BCG06-10] Constraints on bio-essential trace elements in Paleoproterozoic seawater: Implications for Paleoproterozoic biological consumptions
キーワード：炭酸塩置換態硫酸、LA-ICP-MS、古海水、原生代
The Paleoproterozoic is a significant period in Earth’s history, characterized by the Great Oxidation Event and the emergence of eukaryotes. The Francevillian Group in the Gabonese Republic has garnered recent attention due to the discovery of macroscopic structures that may represent some of the earliest potential eukaryotic fossils. Consequently, these strata offer valuable insights into the evolution of life.
Several studies have been conducted to reconstruct the bio-essential trace elements content in paleoseawater with the aim of investigating the relationship between seawater composition and the evolution of life. For example, Saito et al. (2003) proposed that the Zn content in seawater increased by ten orders of magnitude from the Paleoproterozoic to the modern, based on the redox state and H2S content. Robbins et al. (2013) and Scott et al. (2013) reported secular changes in seawater Zn content using banded iron formations (BIFs) and black shales, respectively. The former argued that Zn contents increased tenfold from the Paleoproterozoic to the present, whereas the latter claimed that Zn contents have remained constant since the Proterozoic. However, modern BIFs form only in hydrothermal settings and, therefore, do not reflect the global ocean composition. Similarly, black shales are accumulations of detrital material, and many sources dominate their compositions. Thus, ongoing debates exist about estimating seawater contents using sedimentary rocks. Carbonate rocks, which precipitate from seawater and have been deposited globally throughout Earth’s history, are a potential candidate. However, carbonate rocks are susceptible to post-depositional diagenesis and contamination of sulfide. To obtain bio-essential trace element contents in primary Paleoproterozoic carbonate rocks, we estimated the effects of diagenesis and contamination.
We utilized dolostone samples of the FB Formation of the Francevillian Group deposited in the Lastoursville basin and analyzed major and trace elements, including S, Cu, and Zn, using ICP-MS/MS coupled with a laser ablation sampling technique. The analysis points (100×100 µm2) were selected from the most primary parts based on petrological observation. The depositional environments of these samples were classified into two types based on their Fe and Mn, as well as rare earth element contents: those deposited below the Mn chemocline and those deposited near the Mn chemocline (Yoshida et al., 2024). The ratios of Cu/Ca and Zn/Ca in both types of dolostones are positively correlated with the S/Ca value.
The positive correlations between the S/Ca value and Cu/Ca and Zn/Ca values suggest the presence of fine-grained sulfides in the ablated areas, indicating the mixing of carbonates with sulfide end-members. Estimated concentrations of Cu and Zn in the carbonate end-member ranged from 0.82–2.70 and 3.97–5.40 µg/g, respectively, for samples deposited below the Mn chemocline. In contrast, those in samples deposited near the Mn chemocline were estimated to be nearly zero.
The carbonate end-member Zn composition of samples deposited below the chemocline was 2–5 times higher than that of modern stromatolites and ooids. This result contradicts the compilation of BIFs. This discrepancy is likely because modern BIFs form exclusively near hydrothermal vents, reflecting the high Zn concentrations in hydrothermal fluids.
Given that the earliest candidate fossils of eukaryotes have been discovered from black shales interbedded between the two types of dolostone, this study revealed a sharp depletion in Cu and Zn concentrations across the occurrence of the fossils. Considering that the seawater in the Lastoursville basin was relatively oxic (Ossa Ossa et al., 2018; Yoshida et al., 2024), allowing for sufficient dissolution of Cu and Zn, this depletion may reflect the consumption of these elements by organisms, possibly eukaryotes.
Several studies have been conducted to reconstruct the bio-essential trace elements content in paleoseawater with the aim of investigating the relationship between seawater composition and the evolution of life. For example, Saito et al. (2003) proposed that the Zn content in seawater increased by ten orders of magnitude from the Paleoproterozoic to the modern, based on the redox state and H2S content. Robbins et al. (2013) and Scott et al. (2013) reported secular changes in seawater Zn content using banded iron formations (BIFs) and black shales, respectively. The former argued that Zn contents increased tenfold from the Paleoproterozoic to the present, whereas the latter claimed that Zn contents have remained constant since the Proterozoic. However, modern BIFs form only in hydrothermal settings and, therefore, do not reflect the global ocean composition. Similarly, black shales are accumulations of detrital material, and many sources dominate their compositions. Thus, ongoing debates exist about estimating seawater contents using sedimentary rocks. Carbonate rocks, which precipitate from seawater and have been deposited globally throughout Earth’s history, are a potential candidate. However, carbonate rocks are susceptible to post-depositional diagenesis and contamination of sulfide. To obtain bio-essential trace element contents in primary Paleoproterozoic carbonate rocks, we estimated the effects of diagenesis and contamination.
We utilized dolostone samples of the FB Formation of the Francevillian Group deposited in the Lastoursville basin and analyzed major and trace elements, including S, Cu, and Zn, using ICP-MS/MS coupled with a laser ablation sampling technique. The analysis points (100×100 µm2) were selected from the most primary parts based on petrological observation. The depositional environments of these samples were classified into two types based on their Fe and Mn, as well as rare earth element contents: those deposited below the Mn chemocline and those deposited near the Mn chemocline (Yoshida et al., 2024). The ratios of Cu/Ca and Zn/Ca in both types of dolostones are positively correlated with the S/Ca value.
The positive correlations between the S/Ca value and Cu/Ca and Zn/Ca values suggest the presence of fine-grained sulfides in the ablated areas, indicating the mixing of carbonates with sulfide end-members. Estimated concentrations of Cu and Zn in the carbonate end-member ranged from 0.82–2.70 and 3.97–5.40 µg/g, respectively, for samples deposited below the Mn chemocline. In contrast, those in samples deposited near the Mn chemocline were estimated to be nearly zero.
The carbonate end-member Zn composition of samples deposited below the chemocline was 2–5 times higher than that of modern stromatolites and ooids. This result contradicts the compilation of BIFs. This discrepancy is likely because modern BIFs form exclusively near hydrothermal vents, reflecting the high Zn concentrations in hydrothermal fluids.
Given that the earliest candidate fossils of eukaryotes have been discovered from black shales interbedded between the two types of dolostone, this study revealed a sharp depletion in Cu and Zn concentrations across the occurrence of the fossils. Considering that the seawater in the Lastoursville basin was relatively oxic (Ossa Ossa et al., 2018; Yoshida et al., 2024), allowing for sufficient dissolution of Cu and Zn, this depletion may reflect the consumption of these elements by organisms, possibly eukaryotes.