14:30 〜 14:45
[MIS20-04] Long-term evolution of hydrothermal fluid chemistry on Earth
キーワード：熱水系、熱水実験、原生代
Hydrothermal systems on seafloors have played many roles in biogeochemical cycles on Earth, as sinks of dissolved species in the oceans and sources of reductants that can support chemoautotrophic life. Black-smoker-type hydrothermal fields are commonly found on modern Earth and are characterized as acidic pH fluids and abundant sulfides (e.g., Seyfried et al., 1991). On the other hand, Archean hydrothermal fields would have possessed alkaline pH fluids owing to high CO2 in the atmosphere-ocean systems (e.g., Shibuya et al, 2010; 2013). This suggests that transitions in water chemistry of hydrothermal fluids would have been occurred in response to the long-term evolution of the seawater composition over Earth’s history. One major transition may be the Great Oxidation Event (GOE), by which the concentrations of dissolved sulfate in seawater has dramatically increased due to oxidative chemical weathering of sulfides (e.g., Fakhraee et al, 2019). Another general trend of long-term seawater evolution would be gradual decreases in the concentration of dissolved carbonates due to the carbon cycles and silicate-carbonate feedback in response to the increase in the solar luminosity (e.g., Krissansen-Totton and Catling, 2020). With this regard, dissolved carbonate and sulfate-rich seawater may have existed in the early Proterozoic. However, there is no experimental and numerical study that investigates the effects of the carbonate and sulfate-rich seawater on water chemistry of hydrothermal systems.
Here we examine the long-term evolution of hydrothermal fluid chemistry in response to changes in dissolved carbonate and sulfate in seawater by conducting both hydrothermal experiments and thermodynamic equilibrium calculations. In the experiments, we used synthetic basaltic rocks and an initial solution containing high levels of dissolved carbonate and sulfate simulating early Proterozoic oceans after GOE. Our experimental results show that in-situ pH of hydrothermal fluids at 300 deg.C was alkaline (pH~7) due to the high concentrations of dissolved carbonate in fluids and precipitation of calcite, which is similar to the previous results for high dissolved carbonate and no sulfate simulating Archean oceans (Shibuya et al., 2010, 2013). In the alkaline fluids, most of dissolved sulfate precipitates as Ca sulfate because sulfate reduction is kinetically inhibited due to the alkaline condition, suggesting that hydrothermal systems in the early Proterozoic would have been a sink of dissolved sulfate in the oceans. Our calculation results show that with a decrease in the concentration of dissolved carbonate in seawater, hydrothermal fluid pH at 300 deg. C shifts from ~7 (alkaline) to ~5 (acidic) upon disappearance of calcite. This transition occurs at around 50 mM of total dissolved carbonate in seawater. Considering the previous models (e.g., Krissansen-Totton et al., 2018), this transition would have happened in the middle of the Proterozoic era. Before this transition in the early Proterozoic era, H2 and sulfate-rich hydrothermal fluids can be generated owing to inhabitation of sulfate reduction at alkaline pH, where sulfate reducing bacteria can be supported. After this transition, abiotic sulfate reduction can proceed in hydrothermal systems due to acidic pH, which can be seen modern Black-smoker-type hydrothermal systems. Our results suggest that fluid chemistry and autotrophic ecosystem in hydrothermal systems would have evolved over Earth’s history in response to the long-term evolution of redox states of the atmosphere and oceans.
Here we examine the long-term evolution of hydrothermal fluid chemistry in response to changes in dissolved carbonate and sulfate in seawater by conducting both hydrothermal experiments and thermodynamic equilibrium calculations. In the experiments, we used synthetic basaltic rocks and an initial solution containing high levels of dissolved carbonate and sulfate simulating early Proterozoic oceans after GOE. Our experimental results show that in-situ pH of hydrothermal fluids at 300 deg.C was alkaline (pH~7) due to the high concentrations of dissolved carbonate in fluids and precipitation of calcite, which is similar to the previous results for high dissolved carbonate and no sulfate simulating Archean oceans (Shibuya et al., 2010, 2013). In the alkaline fluids, most of dissolved sulfate precipitates as Ca sulfate because sulfate reduction is kinetically inhibited due to the alkaline condition, suggesting that hydrothermal systems in the early Proterozoic would have been a sink of dissolved sulfate in the oceans. Our calculation results show that with a decrease in the concentration of dissolved carbonate in seawater, hydrothermal fluid pH at 300 deg. C shifts from ~7 (alkaline) to ~5 (acidic) upon disappearance of calcite. This transition occurs at around 50 mM of total dissolved carbonate in seawater. Considering the previous models (e.g., Krissansen-Totton et al., 2018), this transition would have happened in the middle of the Proterozoic era. Before this transition in the early Proterozoic era, H2 and sulfate-rich hydrothermal fluids can be generated owing to inhabitation of sulfate reduction at alkaline pH, where sulfate reducing bacteria can be supported. After this transition, abiotic sulfate reduction can proceed in hydrothermal systems due to acidic pH, which can be seen modern Black-smoker-type hydrothermal systems. Our results suggest that fluid chemistry and autotrophic ecosystem in hydrothermal systems would have evolved over Earth’s history in response to the long-term evolution of redox states of the atmosphere and oceans.