Presentation Information
[T12-O-9]Characteristics, habitats, and ages of early eukaryotes from the Mesoproterozoic carbonate succession of India
*Abhishek Kumar Pandey1, Partha Pratim Chakraborty2, Ryoichi Nakada3, Naotaka Tomioka3, Kazumasa Aoki4, Kaushik Das1, Fumito Shiraishi1 (1. Hiroshima University, 2. University of Delhi, 3. JAMSTEC, 4. Okayama University of Science)
【ハイライト講演】真核生物は約21億年前に出現し，約18～8億年前に多様化した．しかしながら，その時期の酸素濃度は，真核生物の生息に必要な量を大きく下回っていたと考えられており，矛盾が生じていた．本講演では，初期の真核生物化石を産するインドの炭酸塩岩が約16億年前のものであり，その当時，シアノバクテリアが放出した酸素を利用して真核生物が生き延びていたことなどが示される．※ハイライト講演とは．．．
The Mesoproterozoic Era is generally characterized by consistently low oxygen concentrations in the atmosphere and seawater. This perception appears at odds with the emergence and diversification of early eukaryotes during the same period. Scientific opinion on this matter is divided: one view holds that early eukaryotes evolved within small, localized, oxygen-rich niches, while another posits that many early eukaryotes survived in anoxic (and sulfidic) environments through symbiotic relationships with bacteria or archaea. To address this discrepancy, this study conducted a paleoenvironmental reconstruction—focusing specifically on redox conditions—of the Mesoproterozoic Chitrakoot Formation on the Indian subcontinent, a strata known for yielding early eukaryotic microfossils.
Integrated field observations revealed that the Chitrakoot Formation comprises three main members: a basal peloidal dolostone, an overlying glauconitic sandstone, and the uppermost Tirohan dolostone. Sedimentary structures indicate a deepening-upward trend, transitioning from nearshore marine waters to a deeper, calmer subtidal environment. Geochemical proxies, such as relatively flat REE patterns characteristic of nearshore depositional environments, further support these sedimentological interpretations. Although no distinct negative Ce anomaly was observed, peloidal dolostone samples exhibited a clear positive Ce anomaly, suggesting the oxidation of Ce(III) to Ce(IV) in nearshore seawater. Collectively, these results suggest that while the Chitrakoot sea was generally anoxic, localized oxygen-rich microenvironments—likely produced by oxygenic prokaryotes—existed, providing niches suitable for the survival and diversification of early eukaryotes.
This study also documents the occurrence of diverse early eukaryotic microfossils (acritarchs) within the silicified peloidal dolostone deposited in the nearshore environment. These specimens bear a strong morphological resemblance to known early eukaryotic taxa such as Dictyosphaera macroreticulata, Shuiyousphaeridium macroreticulatum, and Dictyosphaera delicata. Additionally, microfossils resembling well-preserved coccoidal cyanobacteria were observed within some peloids that had undergone internal silicification, suggesting a close coexistence of early eukaryotes and oxygenic prokaryotes in the Chitrakoot Sea. STXM-based nanoscale analysis revealed that the organic residues of the eukaryotes and the coccoidal cyanobacteria contained distinct organic compositions. Such heterogeneity may reflect differences in constituent organic molecules or metabolic pathways between eukaryotes and prokaryotes.
Furthermore, U–Pb dating of apatite from phosphate rocks in the Chitrakoot Formation indicated a depositional age range of approximately 1629–1551 Ma. These age data are consistent with existing hypotheses proposing that the diversification of early eukaryotes occurred during the transition from the late Paleoproterozoic to the early Mesoproterozoic.
Integrated field observations revealed that the Chitrakoot Formation comprises three main members: a basal peloidal dolostone, an overlying glauconitic sandstone, and the uppermost Tirohan dolostone. Sedimentary structures indicate a deepening-upward trend, transitioning from nearshore marine waters to a deeper, calmer subtidal environment. Geochemical proxies, such as relatively flat REE patterns characteristic of nearshore depositional environments, further support these sedimentological interpretations. Although no distinct negative Ce anomaly was observed, peloidal dolostone samples exhibited a clear positive Ce anomaly, suggesting the oxidation of Ce(III) to Ce(IV) in nearshore seawater. Collectively, these results suggest that while the Chitrakoot sea was generally anoxic, localized oxygen-rich microenvironments—likely produced by oxygenic prokaryotes—existed, providing niches suitable for the survival and diversification of early eukaryotes.
This study also documents the occurrence of diverse early eukaryotic microfossils (acritarchs) within the silicified peloidal dolostone deposited in the nearshore environment. These specimens bear a strong morphological resemblance to known early eukaryotic taxa such as Dictyosphaera macroreticulata, Shuiyousphaeridium macroreticulatum, and Dictyosphaera delicata. Additionally, microfossils resembling well-preserved coccoidal cyanobacteria were observed within some peloids that had undergone internal silicification, suggesting a close coexistence of early eukaryotes and oxygenic prokaryotes in the Chitrakoot Sea. STXM-based nanoscale analysis revealed that the organic residues of the eukaryotes and the coccoidal cyanobacteria contained distinct organic compositions. Such heterogeneity may reflect differences in constituent organic molecules or metabolic pathways between eukaryotes and prokaryotes.
Furthermore, U–Pb dating of apatite from phosphate rocks in the Chitrakoot Formation indicated a depositional age range of approximately 1629–1551 Ma. These age data are consistent with existing hypotheses proposing that the diversification of early eukaryotes occurred during the transition from the late Paleoproterozoic to the early Mesoproterozoic.
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