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    Liquid and supercritical CO2 as an organic solvent in Hadean seafloor hydrothermal systems: implications for prebiotic chemical evolution

    Takazo Shibuya, Ken Takai

    Origin of life, Liquid and supercritical CO2, CO2 hydrate, Seafloor hydrothermal vent, Prebiotic chemical evolution, Phase separation, Organic solvent, Interface reaction, Metal dissolution

    Schematic of Hadean seafloor hydrothermal systems bearing a liquid/supercritical CO2 (L/SC-CO2) pool. The CO2-rich seawater infiltrates the subseafloor volcanic rocks and reacts with basalts or komatiites, causing carbonation and alteration of basalts or serpentinization of komatiites in the seawater recharge zones. When hydrothermal fluid reaches the reaction zone, it receives abundant degassed CO2 from an underlying magmatic heat source. During fluid upwelling, phase separation of the CO2-rich hydrothermal fluid progressively occurs through depressurization, which likely condenses the CO2 in the vapor phase. If the CO2-rich vapor phase is physically segregated from the main flow channel of the upwelling brine phase, it is continuously mixed with recharged seawater and/or cooler pore fluid, causing further phase separation by the temperature decrease. CO2 is then purified in vapor phase. Eventually, these condensation and purification processes generate many SC-CO2 droplets beneath the seafloor. The SC-CO2 is less dense than seawater and migrates upward. As it cools during the upward migration, its phase converts to L-CO2. These processes form a large L/SC-CO2 pool beneath the seafloor. If the temperature of the bottom seawater is lower than approximately 10 °C, reactions between L-CO2 and the ambient seawater/pore fluid requisitely generate solid CO2 hydrates that cement the pores, voids and cavities of the hydrothermal sediments and permeable volcanics. This self-sealing effect by CO2 hydrates prevents the leakage of L/SC-CO2 through the seafloor. In contrast, when CO2 hydrates are absent in the vicinity of warm/hot hydrothermal vents, L/SC-CO2 leaks from the inner edge of the pool and enters the upwelling hydrothermal fluid just below the seafloor, where it is emitted in the form of bubbles. L-CO2 containing these bubbles transforms to hydrate-coated L-CO2 during upward migration through the water column. The bubbles disappear at depths around 500 m because L-CO2 and CO2 hydrate are unstable at these depths. Eventually, both CO2 gas and the organic molecules dissolved in L/SC-CO2 are released to the ambient seawater. If the hydrothermal vents are black-smoker type, the L-CO2 bubbles will carry heavy metals upward to the shallower part of the water column.

    Prebiotic chemical evolution and the emergence of life in the seafloor hydrothermal systems of Hadean Earth is among the most plausible and popular hypotheses for the origin of earthly life. In contrast, many studies pointed out that this hypothesis intrinsically harbors a critical unsolved problem called the “water paradox”: Abundant water limits dehydration synthesis and instead facilitates hydrolysis of organic molecules during the early stage of chemical evolution. However, many of these criticisms have not referred to the abundant liquid/supercritical CO2 (L/SC-CO2) fluids and pools in modern hydrothermal systems, which not only create dry environments but also behave as hydrophobic solvents at and beneath the seafloor. In this paper, we theorize the generation and preservation of a L/SC-CO2 pool in modern seafloor hydrothermal systems and reinterpret the fossil hydrothermal systems preserved in early Archean seafloor basalts. The theoretical estimation of subseafloor phase separation and phase segregation of CO2-rich hydrothermal fluids suggests the presence of L/SC-CO2 fluids and pools in Hadean seafloor hydrothermal systems. Because they behave as hydrophobic organic solvents, L/SC-CO2 can potentially initiate the dehydration synthesis of organic molecules in seafloor hydrothermal systems. Moreover, at the interface between L/SC-CO2 and H2O-rich fluid (seawater or hydrothermal fluid), amphiphilic molecules might be generated and triggered into self-assembled growth. Based on the abundant occurrence and physicochemical properties of L/SC-CO2 fluids, we propose a new stepwise concept for the origin of life, whereby prebiotic chemical evolution was co-hosted and facilitated by L/SC-CO2 in Hadean water-rich seafloor hydrothermal systems. This “liquid/supercritical CO2 hypothesis” potentially overcomes the water paradox and strengthens the idea that earthly life was hatched in deep-sea hydrothermal systems.