16:45 〜 17:00
[BPT26-03] 原生代海洋ユーキシニアのダイナミクスと生物圏への影響
キーワード：原生代, 海洋酸化還元状態, 海洋物質循環モデル, ユーキシニア
Proterozoic is characterized by substantial changes in the redox state of atmosphere and oceans at both ends of the eon and stasis between them. Accumulating geological/geochemical records demonstrate that the first major oxygenation of the Earth's surface at 2.45-2.22 billion years ago (Ga) (known as "Great Oxidation Event") may have been followed by a subsequent drop of atmospheric oxygen level at the end of the Lomagundi-Jatuli event (LJE) (~2.08-2.05 Ga). Such waxing and waning of the oxygenation state of Earth's surface would have caused substantial changes in oceanic chemical composition and would surely have impacted the biosphere. In this context, the evidence for strongly sulfidic (euxinic) oceanic environments in the LJE aftermath in Gabon and Karelia are notable because their low molybdenum isotopic values (less than 0.95‰ and 0.85‰, respectively) imply widespread euxinia at that time. The spatiotemporal variation of euxinia should have played a crucial role in Proterozoic biological and geochemical evolution not only because of its toxicity to eukaryotes but also because of its fundamental role on bioessential trace metal availability in the ocean interior. However, the nature of oceanic biogeochemical dynamics and its impact on the biosphere in the Proterozoic remains unclear.
To investigate the dynamics of oceanic biogeochemical response during the Paleoproterozoic eon, we improved upon the CANOPS model, in which coupled C-N-O-P-S marine biogeochemical cycles and a series of redox reactions were adequately taken into account. We newly developed an open system modeling approach for marine sulfur cycling in which the oceanic sulfur balance is explicitly evaluated.
In this study we show that a plausible cause and effect chain of events that is consistent with the geological records of the LJE aftermath can be obtained when a dramatic decrease in atmospheric oxygen level (from > 50%PAL to < 1%PAL; PAL=present atmospheric level) is assumed: the deoxygenation of an atmosphere-ocean system stimulates the microbial sulfate reduction, resulting in a transitional (~20 Myr) expansion of euxinia until the ocean reaches a new steady state. A mass balance calculation of Mo also demonstrates that LJE aftermath is marked by an expansion of euxinia to ca. 9-40% of the whole seafloor. Under such conditions Mo levels would decrease to as low as ca. 2.0-6.5 nM where nitrogen fixation by Fe-Mo nitrogenase is very sensitive to Mo concentration. We therefore suppose that waxing and waning of the atmospheric oxygenation state in the Paleoproterozoic could have affected evolution and diversification of the aerobic bacteria and/or eukaryotes through dynamic oceanic euxinia.
To investigate the dynamics of oceanic biogeochemical response during the Paleoproterozoic eon, we improved upon the CANOPS model, in which coupled C-N-O-P-S marine biogeochemical cycles and a series of redox reactions were adequately taken into account. We newly developed an open system modeling approach for marine sulfur cycling in which the oceanic sulfur balance is explicitly evaluated.
In this study we show that a plausible cause and effect chain of events that is consistent with the geological records of the LJE aftermath can be obtained when a dramatic decrease in atmospheric oxygen level (from > 50%PAL to < 1%PAL; PAL=present atmospheric level) is assumed: the deoxygenation of an atmosphere-ocean system stimulates the microbial sulfate reduction, resulting in a transitional (~20 Myr) expansion of euxinia until the ocean reaches a new steady state. A mass balance calculation of Mo also demonstrates that LJE aftermath is marked by an expansion of euxinia to ca. 9-40% of the whole seafloor. Under such conditions Mo levels would decrease to as low as ca. 2.0-6.5 nM where nitrogen fixation by Fe-Mo nitrogenase is very sensitive to Mo concentration. We therefore suppose that waxing and waning of the atmospheric oxygenation state in the Paleoproterozoic could have affected evolution and diversification of the aerobic bacteria and/or eukaryotes through dynamic oceanic euxinia.