Shibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60
https://doi.org/10.1186/s40645-022-00510-6

Progress in Earth andPlanetary Science

Open Access

REVIEW

Liquid and supercritical ­CO2 as an organic
solvent in Hadean seafloor hydrothermal
systems: implications for prebiotic chemical
evolution
Takazo Shibuya*   and Ken Takai

Abstract
Prebiotic chemical evolution and the emergence of life in the seafloor hydrothermal systems of Hadean Earth isamong the most plausible and popular hypotheses for the origin of earthly life. In contrast, many studies pointed outthat 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 C­ O2 (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 poolin 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 C
­ O2-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 H­ 2O-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 thewater paradox and strengthens the idea that earthly life was hatched in deep-sea hydrothermal systems.
Keywords: Origin of life, Liquid and supercritical ­CO2, CO2 hydrate, Seafloor hydrothermal vent, Prebiotic chemical
evolution, Phase separation, Organic solvent, Interface reaction, Metal dissolution
1 Introduction
An increasing interest in research on the origin of life
(OOL) has emphasized two substantial aspects of OOL
research: the process of prebiotic chemical evolution*Correspondence: takazos@jamstec.go.jp
Super‑cutting‑edge Grand and Advanced Research (SUGAR) Program,
Institute for Extra‑cutting‑edge Science and Technology Avant‑garde
Research (X‑star), Japan Agency for Marine-Earth Science and Technology(JAMSTEC), 2‑15 Natsushima‑cho, Yokosuka 237‑0061, Japanand the birthplace of life. The former aspect began withthe pioneering Miller–Urey experiments in a given primordial atmosphere (Miller 1953; Miller and Urey
1959) and was later sophisticated with genetics-centric
theories such as RNA/Protein world hypotheses (Rich
1962; Gilbert 1986; Eddy 2001; Ikehara 2005; Robertson
and Joyce 2012; Harish and Caetano-Anolles 2012) andmetabolism-centric theories such as the autotrophic origin of life hypothesis (Wächtershäuser 1988, 1990, 1994;
Huber and Wächtershäuser 1997, 2003; Russell and Hall© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Shibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60

Page 2 of 15

1997; Martin et al. 2008; Russell et al. 2010). The latteraspect began with the primordial organic-soup hypothesis in oceans (Oparin 1924, 1957; Haldane 1929) andhas been updated with several plausible candidates andmodels such as the seafloor hydrothermal vent hypothesis (Russell et al. 1988, 2021; Russell and Hall 1997; Takai
et al. 2006; Russell et al. 2014; Shibuya et al. 2015, 2016;
Yamamoto et al. 2017; Barge et al. 2019; Kitadai et al.
2018, 2019, 2021; White et al. 2020), on-land hot spring(warm little pond) hypothesis (Powner et al. 2009; Mulkidjanian et al. 2012; Maruyama et al. 2019; Damer andDeamer 2020), and others (e.g., panspermia hypotheses)(Arrhenius 1908; Hoyle and Wickramasinghe 2000). Bothaspects can be overlapped to create unified scenariosand contexts of OOL, whereas each hypothesis has long
been debated because it certainly embraces both advantages and disadvantages. In fact, seafloor hydrothermal
vents and on-land hot springs are now considered as themost plausible birthplaces of life, but the hypotheses havebeen hotly debated recently (Damer and Deamer 2020;
Marshall 2020; Russell 2021; Korenaga 2021; Rosas andKorenaga 2021). Furthermore, they have been extensively
investigated in their analog environments on the present
Earth (e.g., Takai et al. 2004; Suzuki et al. 2013; Suda et al.
2014) and Saturn’s moon (Hsu et al. 2015; Sekine et al.
2015; Glein et al. 2015; Waite et al. 2017).
The hot spring hypothesis is supported by the availability of various energy sources (hydrothermal chemistry, sunlight, and cosmic ray irradiation), the supply offreshwater and atmospheric feedstock materials, and thecondensation and polymerization of organics throughpossible wet–dry and freeze–thaw cycles on the groundsurface (Powner et al. 2009; Mulkidjanian et al. 2012;
Damer and Deamer 2020; Rosas and Korenaga 2021). In
contrast, the ground surface of the Hadean should havebeen exposed to long-term cosmic ray irradiation, meteorite showers, and ultraviolet (UV) irradiation in theabsence of an ozone shield. These natural processes havethe potential to induce polymerization and molecular
evolution of organics under appropriate conditions (e.g.,
McKay and Borucki 1997; Damer and Deamer 2020), butsustaining the prebiotic chemical evolution and the generation and continuity of primordial life would be difficultin such harsh environments. Furthermore, the spatiotemporal abundance and stability of hot spring environmentson the exposed land might be stochastically weak points
for the sustainable prebiotic chemical evolution and thegeneration and continuity of life as compared to those ofdeep-sea hydrothermal systems.
Since the discovery of hydrothermal systems on themodern ocean floor (Corliss et al. 1979), the seafloorhydrothermal vent hypothesis has been widely accepted
as the birthplace of life and the hatchery habitat of thelast universal common ancestor (LUCA) because many
thermophilic, lithotrophic, and potentially ancient lineages of microbes sustained only by geo-fuels have been
identified in hydrothermal fluids (Corliss et al. 1981;
Takai et al. 2004; Stetter 2006; Nakamura and Takai2014). Recent reverse-evolution studies using an enormous dataset of prokaryotic genomes have reinforced theconcept that early seafloor hydrothermal vents hosted
the emergence of LUCA and the post-LUCA propagation and evolution (Weiss et al. 2016; 2018). However,whether the origins of life and LUCA were spatiotemporally related is still uncertain. Rather, the seafloor hydrothermal vent hypothesis for OOL has been established interms of prebiotic chemical evolution, such as the generation of auto-catalytic and autotrophic proto-metabolisms in Hadean hydrothermal vents (Wächtershäuser
1988, 1990, 1994; Huber and Wächtershäuser 1997, 2003;
Russell and Hall 1997; Martin et al. 2008; Russell et al.
2010; Yamamoto et al. 2017; Barge et al. 2019; Kitadai
et al. 2018, 2019, 2021; White et al. 2020). In addition,
limited but certain amounts of geological and geochemical evidences have constrained the environmental conditions that would support the seafloor hydrothermal venthypothesis. For instance, seafloor hydrothermal vents canprovide a sustainable and stable chemical energy supply for microbial metabolisms (McCollom and Shock1997; Amend et al. 2011; Nakamura and Takai 2015)
and are insulated from lethal extraterrestrial inputs (e.g.,
UV and cosmic rays and meteorite impacts) (Morbidelli
et al. 2001; Takai 2019). Moreover, seafloor hydrothermal activities have existed since the initiation of plate
tectonics in the Hadean (e.g., Komiya et al. 1999), andmicrofossils were found within the 3.5 Ga subseafloorhydrothermal deposits (e.g., Ueno et al. 2001). Nevertheless, all seafloor hydrothermal systems are present inseawater that generally inhibits dehydration synthesis
rather facilitates hydrolysis of organics. This so-called
water paradox or water problem is the most challenging inherent dilemma of the seafloor hydrothermal venthypothesis.
In fact, various prebiotic organics have been synthesized in laboratory experiments under water-rich conditions that simulate seafloor hydrothermal systems (e.g.,
Yanagawa and Kojima 1985; Huber and Wächtershäuser
1997; Imai et al. 1999; Simoneit 2004; Kawamura et al.
2005a, b; Lemke et al. 2009; Sakata et al. 2010; Burcar
et al. 2015; Barge et al. 2019; Kitadai et al. 2019, 2021).
Early experiments on hydrothermal organic synthesis focused on the polymerization of amino acids fromamino acid monomers, while recent geo-electrochemical
experiments have synthesized various building-blockorganics (amino acids, organic acids, and thio-esters)from inorganic compounds (Kitadai et al. 2018, 2019,Shibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60

Page 3 of 152021). However, the amounts and sizes of the organic
polymers generated in simulated hydrothermal solutions
were demonstrably insufficient, possibly emphasizing the“water paradox” of seafloor hydrothermal vents.
Besides these two major hypotheses, there are a viewthat prebiotic chemical evolution might be facilitatedby supercritical C­ O2 (SC-CO2) fluids (e.g., Fujioka et al.
2009; Schreiber et al. 2012). The critical point of C
­ O2
(Tc = 31.1 °C; Pc = 73.8 bar) is located among the mild
temperature and pressure conditions, which frequently
occurs in those of geologic environments. Condensed­CO2 behaves as a nonpolar hydrophobic solvent in liquidand supercritical states (e.g., its solubility parameter is
14.6 ­MPa1/2 at 40 °C and 20 MPa; Marcus 2018), and canbe industrially used for polymer synthesis because of its
high miscibility with many organic materials (e.g., Hyatt
1984; Cooper 2000; Shi et al. 2015). Thus, as potential
environments facilitating prebiotic chemical evolution,
Fujioka et al. (2009) assumed on-land soda fountains
whereas Schreiber et al., (2012) proposed deep strikeslip fault zones in exposed proto-continents. However,partial pressure of C
­ O2 in atmosphere and ocean wasprobably reduced by water–rock reactions during formation process of primordial ocean to a level that L/SC-CO2
is unstable (Ueda and Shibuya 2021). Moreover, theexposed land of continental crusts is not guaranteed toexist in the Hadean (Korenaga 2021). In addition, it might
be difficult to examine these models in analogue sites onmodern Earth. Nevertheless, these prior SC-CO2 models
certainly provide hints for solving the “water paradox” ofthe seafloor hydrothermal vent hypothesis because ongoing generation of liquid/supercritical C­ O2 (L/SC-CO2)
fluids is observed in several modern seafloor hydrothermal systems.
In this regard, this paper reviews previous studies ofmodern hydrothermal systems that host L/SC-CO2 fluids
and pools at and beneath the seafloor and theorizes thegeneration and preservation mechanisms. Archean geologic records that can be the evidence for L/SC-CO2 fluids in seafloor hydrothermal systems are also reviewed.
These discussions shed light on the possible key processesof prebiotic chemical evolution driven by L/SC-CO2 fluids and pools in Hadean seafloor hydrothermal systems.

2 Liquid ­CO2 in modern seafloor hydrothermal
systems
L/SC-CO2 fluids and pools have now been recognizedin some seafloor hydrothermal fields of the Okinawa
Trough (Sakai et al. 1990a, b; Ishibashi et al. 1995; Inagaki et al. 2006; Konno et al. 2006; Kawagucci et al.
2011) and the Mariana Arc (Lupton et al. 2006, 2008)(Table 1). The hydrothermal vents in these fields generally emit C­ O2-rich fluids with or without lesser amountsof L/SC-CO2 bubbles/droplets, which can be recognizedboth visually (e.g., Sakai et al. 1990b; Lupton et al. 2006)
and acoustically (e.g., Nakamura et al. 2015). The C­ O2
concentrations in hydrothermal fluids vary widely but
are typically at least one order of magnitude higher thanthose in typical hydrothermal fluids of mid-ocean ridge
systems (10–30 mM) (Table 1). However, these analyzedhydrothermal fluids are apparently mixtures of brine
and gas-rich vapor phases because it is difficult to sample the brine and vapor phases, separately. Therefore, thereported ­CO2 concentrations in hydrothermal fluids indicate the maximum and minimum values of the brine andvapor phases, respectively.
According to stable carbon isotope ratios of C
­ O2 inthese hydrothermal fluids, C­ O2 likely originated from a
combination of mantle carbon and thermogenic ­CO2from sedimentary organics in the Okinawa Trough systems (Kawagucci et al. 2011). In the Champagne field(NW Eifuku) of the Mariana Arc, relatively heavy carbon
isotopes point to decomposition of marine carbonates inthe subducted slab, suggesting that a local melting process at subduction zone also potentially contributes togeneration of a C­ O2-rich magma. (Lupton et al. 2006).
In any case, the high C­ O2 concentrations in fluids indicate that a considerable amount of C
­ O2 is injected into
the reaction zone from surrounding heated organicsrich sediment layers and/or the underlying heat source(e.g., the magma chamber). As previously suggested,
the upwelling of substantially C­ O2-rich hydrothermal

Table 1 Modern seafloor hydrothermal systems with a L/SC-CO2 pool beneath the seafloor
Site

Venting depth below
sea surface (m)Temperature of vent CO2 concentration in
fluid (°C)
hydrothermal fluid (mM)

JADE (Okinawa Trough)

1300–1550< 220, 320

26–198

Yonaguni Knoll IV (Okinawa Trough)

1370–1385

160–328

18.1–329

Iheya North (Okinawa Trough)

1000

< 311

2.7–227

Champagne, NW Eifuku (Mariana Trough)1600

11–10872.3–2711

δ13CCO2 (VPDB)− 5.0 to − 4.7− 8.0 to − 7.2− 10.1 to − 6.1− 1.8 to − 1.2

JADE: Sakai et al. (1990a, b) and Ishibashi et al. (1995), Yonaguni Knoll IV: Suzuki et al. (2001) and Konno et al. (2006), Iheya North: Kawagucci et al. (2011), Champagne,
NW Eifuku: Lupton et al. (2006)

(2022) 9:60

3.1 Phase separation by fluid upwelling as a CO2
condensation process

Phase separation is a frequent phenomenon in seafloorhydrothermal systems on the present Earth. It has been
observed in the Rainbow (Charlou et al. 2002), East
Pacific Rise (9°50’ N) (especially immediately after magmatic events; Lilley et al. 2003), and Edmond (Gallant
and Von Damm 2006) hydrothermal fields on the midocean ridges, and even in the world’s deepest hydrothermal system of the Piccard field, the Mid-Cayman Rise
(McDermott et al. 2018). In addition, the processes ofphase separation during subseafloor hydrothermal circulation are also well simulated by theoretical consideration and numerical modeling (Fox 1990; Scholl et al.
1993; Coumou et al. 2009). Subseafloor phase separationis generally recognized as a clear difference in salinity of
the hydrothermal fluid against the seawater value. Higher
and lower Cl concentrations in hydrothermal fluids thanin seawater result from phase separation and segregationof brine and vapor phases during fluid upwelling (Butterfield et al. 2003). Phase separation occurs when thevapor pressure of water or partial pressure of the gases
dissolved in the hydrothermal fluid exceeds the headpressure. Hence, a pressure decrease in the upwellinghydrothermal fluid causes phase separation, depending on the pressure, temperature, and fluid composition. If a hydrothermal fluid is significantly enriched in­CO2, a high partial pressure of ­CO2 likely causes phaseseparation during upwelling. Furthermore, the H­ 2O–
CO2 system has a wide immiscibility field that changes
with pressure and temperature (Fig. 1; Takenouchi and
Kennedy 1964; Blencoe et al. 2001; Kaszuba et al. 2006).
When a pressure and/or temperature change drives­CO2-rich hydrothermal fluid into the immiscibility fieldon the P–T diagram, the fluid is physically separated into
­CO2-poor liquid and ­CO2-rich vapor phases. Actually,
NaCl in fluids also influences conditions of phase separation in natural systems (Butterfield et al. 2003), but there
is no available phase diagram of ­H2O-NaCl-CO2 systemwith seawater salinity at present. Therefore, we tentatively use the phase diagram of NaCl-free system for thefollowing discussions. It is noted that the immiscibility

5°C

Critical cu
rv

30

0°

500

32

C

C

Arc

35

5°

Mid-ocean ridge
Back arc

27e

150 °C

200 °C
250 °C

150 °C
200 °C

3 Generation and formation mechanisms of liquid/
supercritical ­CO2 fluids and pools in seafloorhydrothermal systems

1000

250 °C

fluid inevitably drives phase separation through depressurization (e.g., Konno et al. 2006). Such processes likely
generate a ­CO2-rich vapor phase during hydrothermal
circulation, which should nucleate and grow L/SC-CO2
fluids and pools at and below the seafloor. The following
section discusses the mechanisms of phase separation in­CO2-rich hydrothermal systems.

Page 4 of 15

Pressure (bars)

Shibuya and Takai ﻿Progress in Earth and Planetary Science

0°C

Seafloor pressure of hydrothermal systems with L/SC-CO2

0

0

50

100

CO2 concentration (mol%)Fig. 1 Isotherm diagram of fluid compositions in the ­CO2–H2O
system (modified from Takenouchi and Kennedy 1964), on which theseafloor pressure of hydrothermal systems hosting a L/SC-CO2 poolprojected. The ranges of seafloor pressures of hydrothermal systems
at mid-ocean ridges, back arcs and arcs are summarized from theInterRidge Vents Database (https://​vents-​data.​inter​ridge.​org/)fields probably expand slightly if seawater salinity is
assumed for the fluids (e.g., Kaszuba et al. 2006; Dubacq
et al. 2013).
Such processes are expected in present hydrothermal
systems hosting abundant L/SC-CO2 fluids and pools.
For example, a hydrothermal fluid containing 2 mol%
­CO2 (~ 1.1 mol/kg) is assumed to be generated at 300 °C
in a reaction zone located 2 km beneath a 1-km-deepseafloor for simplicity. In the assumption, we lack geological and geochemical data supporting the depth of reaction zone and the ­CO2 concentration of fluid. However,
the assumed seafloor depth and temperature of reaction
zone are broadly comparable to those of the Iheya Northhydrothermal field in the Okinawa Trough, where diveobservations with human-occupied and remotely operative vehicles have detected many L/SC-CO2 droplets andbubbles exhaled along with high-temperature hydrothermal fluid flows (Kawagucci et al. 2011) (Fig. 2a andTable 1). As the hydrothermal fluid upwells, its hydraulic head pressure decreases and the fluid encountersthe 300 °C immiscibility field at around 170 bar (700 m
below the seafloor). At this point, phase separation
begins with a very small amount of vapor phase containing 35 mol% ­CO2 (V0 in Fig. 2a). Although the density
differs between the vapor phase and the brine phase (B0;chemically almost identical to the original hydrothermal
fluid), the vapor phase should occur as small droplets inthe main brine phase due to its very small amount. As the
upwelling proceeds, the brine phase continuously releases
vapor phases and reduces its C­ O2 concentration along
the outer boundaries of the immiscibility field (Fig. 2a).
Simultaneously, the volume of vapor phase graduallyShibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60

(a) Phase separation by fluid upwelling°C
300

Hydrothermal fluidat reaction zone;
CO2 = 2 %, 300 °C

0°

C

300
35

Pressure (bars)

110 °C

200 °C

250 °C

260 °C

270 °C
275 °C

°C

°C

325

300

400

150 °C

500

200

B0
V0

B1

Seafloor pressure

100

0

V1

B1’

V1’

10

0

20

30

40

50

60

70

80

90

100

CO2 concentration (mol%)(b) Phase separtaion by fluid mixing
°C
300
0°

C

300
35

Pressure (bars)

110 °C

200 °C

250 °C

260 °C°C°C

270 °C
275 °C

300

325

400

150 °C

500

200
B2

100

M2 (250 °C)
B3

V1

V2

Seafloor pressureV3

M3 (150 °C)

0

0

10

20

30

40

50

60

70

80

90

100

CO2 concentration (mol%)Fig. 2 Evolution of fluid compositions associated with phaseseparation caused by a depressurization due to fluid upwelling andb temperature decrease due to fluid mixing (see text for details).
Isotherms are enlarged from those in Fig. 1 (Takenouchi and
Kennedy 1964). B, V, and M stand for brine, vapor, and mixed phases,
respectively

increases and its ­CO2 concentration also decreases alongwith the outer boundary of immiscibility field. Whenthe bubbles of vapor phase reach a certain volume, thevapor phase (V1) might physically segregate from themain brine phase (B1) due to the density difference andmight become concentrated during flow through thechannels and mixing with low-temperature seawater (asdiscussed below). When the hydrothermal brine phasereaches the seafloor (100 bar) (B1’), its ­CO2 concentration should be reduced to 0.3–0.4 mol% (170–220 mmol/kg) through the continuous release and segregation ofthe ­CO2-rich vapor phase. This value approximates the
maximum ­CO2 concentration of the hydrothermal fluids
in the Iheya North hydrothermal field (Table 1). Meanwhile, the C­ O2 concentration in the vapor phase reduces
to approximately 11 mol% at 100 bar (near the seafloor)(V1’), but its amount relative to the brine phase increases
with decreasing pressure if physical segregation is limited. In summary, depressurization from 170 to 100 bar

Page 5 of 15(from 700 to 0 m below the seafloor) causes continuousor progressive phase separation that condenses ­CO2 into
the vapor phase during fluid upwelling.
Above, the case close to Iheya North field was discussed in detail. We now briefly discuss the effect of varying the initial conditions. When the ­CO2 concentration of
the hydrothermal fluid generated in the reaction zone ishigher than the above-assumed value, the amount of generated vapor phase relative to the brine phase increases
and the C­ O2 concentration in the vapor phase can reach46 mol% (the right edge of the 300 °C immiscibility fieldin Fig. 2a). In contrast, if the temperature of the hydrothermal fluid in the reaction zone reaches 350 °C, the­CO2 concentration in the generated vapor phase is lower
than at lower temperatures because the immiscibility
field is narrowed. Therefore, high-temperature hydrothermal fluids (> 350 °C) might be less likely to generate
L/SC-CO2 because the low ­CO2 concentration in thevapor phase has less potential to generate the density difference that potentially drives the physical segregation
between the vapor and brine phases. Nevertheless, thehigher original ­CO2 concentration in fluid leads to thegreater amount of generated vapor phase, which might
assist physical segregation of the vapor phase.
Under the above ­CO2 concentration and temperature
condition ­(CO2 = 2 mol%, 300 °C), the phases cannot
separate below a water depth of 1700 m (170 bar) because
the fluid composition is outside the immiscibility field.
This indicates that upwelling of ­CO2-rich hydrothermal
fluid does not necessarily cause phase separation if theseafloor is much deeper than 1700 m. For example, if the­CO2 concentration and temperature are approximately
2 mol% and 350 °C, respectively, the upwelling hydrothermal fluid reaches the 350 °C immiscibility field at ahydrostatic pressure corresponding to a depth of 2200–
2300 m. Therefore, in typical mid-ocean ridge hydrothermal systems (e.g., 2500 m water depth at the seafloor),
phase separation of hydrothermal fluids is not triggeredby high C­ O2 concentrations. This reasoning explains thelack of L/SC-CO2 fluids and pools reported in/near midocean ridge hydrothermal systems.
3.2 
Phase separation by fluid mixing as a process of CO2
purification

If the temperature decreases due to thermal conduction, an upwelling C­ O2-rich vapor phase segregatedfrom the main flow channel would elevate its ­CO2 concentration by releasing an H
­ 2O-rich phase. This mechanism is driven by the expanding immiscibility field in the­H2O–CO2 system as the temperature decreases (Fig. 1).
This mechanism might work in some regions of hydrothermal systems hosting L/SC-CO2 fluids and pools. In
general, however, upwelling hydrothermal fluids actively

(2022) 9:60

Page 6 of 15

mix with cool seawater infiltrating down into the subseafloor environments, which effectively decreases thetemperature of hydrothermal fluids (e.g., Butterfield et al.
2004). Although the mixing process usually dilutes thechemical species specifically dissolved in hydrothermal
fluids (e.g., H­ 2aq, ­H2Saq, and ­Fe2+), the mixing-driven temperature reduction can enrich the ­CO2 only in ­CO2-richvapor phases. To briefly explain this mechanism, we
assume simple initial conditions discussed above. First, a­CO2-rich vapor phase (V1; C­ O2 = 20 mol%) at 300 °C is
segregated from the main flow channel at 200 m below
the seafloor (120 bar) during the fluid upwelling processwith isothermal depressurization (Fig. 2a and b). Next,
the vapor phase (V1) mixes with seawater near the seafloor (100 bar). For simplicity, the temperature and ­CO2
concentration of the seawater are assumed as 0 °C and
0 mol%, respectively, because their actual values are
negligible relative to those of the vapor phase. In thiscase, when the mixing ratio of seawater-to-vapor phase
is 0.2, a mixed phase (M2) with a ­CO2 concentration of
16.7 mol% is formed at 250 °C (Fig. 2b). This ­CO2 concentration falls within the immiscibility field at 250 °C,
which is unstable and causes phase separation generating a ­CO2-poor brine phase (B2) and a C­ O2-rich vapor
phase (V2; ­CO2 = 42 mol%) (Fig. 2b). Similarly, whenthe mixing ratio of seawater-to-vapor phase is 1.0, themixed phase (M3) has a ­CO2 concentration of 10 mol%and a temperature of 150 °C. This mixed phase also falls
within the immiscibility field at 150 °C and separates intoa ­CO2-poor brine phase (B3) and a C­ O2-rich vapor phase(V3; ­CO2 = 88 mol%). In natural systems, such mixing processes continuously occur in many places of thesubseafloor hydrothermal circulation. Hence, the mixing ratio of the seawater/vapor phase generally increases
over distance and time from the main flow channel or
the reaction zone; accordingly, the width of the immiscibility field expands with decreasing temperature in the­H2O–CO2 system. This implies that once the ­CO2-richvapor phase has separated from the high-temperaturehydrothermal fluids of the main flow channel, the C­ O2enrichment can be accelerated by continuous mixing
processes because the expansion of the immiscibility
field due to the temperature decrease exerts a greater
effect than ­CO2 dilution through mixing with seawater.
Therefore, at the seafloor and in relatively shallow subseafloor regions dominated by the mixing process, nearly
purified L/SC-CO2 fluid is finally generated from thehighly ­CO2-enriched vapor phase. If there are appropriate geological structures such as alternation of porous
and impermeable layers in the sediments and umbrella(flange)-shaped metal sulfide deposits, the L/SC-CO2 fluids can be isolated and stored around the seafloor of thehydrothermal system (as discussed below). When spaces

become filled with L/SC-CO2 fluids, L/SC-CO2 pools
would begin to form.
3.3 
Geological processes that preserve liquid/supercritical
CO2 around the seafloor

Because the density of L/SC-CO2 at the temperatures ofmodern deep seawater is lower than that of seawater atpressures lower than around 300 bar (e.g., Qanbari et al.,2011), the L/SC-CO2 fluid should slowly rise throughthe fractures, voids, and cavities of subseafloor environments in hydrothermal systems shallower than 3000 m.
On geological timescales, L/SC-CO2 fluid might slowly
permeate even the overlying impermeable sediments
and eventually enter the seawater. To ensure the growthand long-term preservation of L/SC-CO2 pools at andbeneath the seafloor, other physical and chemical processes are required. At 100 bar and temperatures lower
than approximately 10 °C, liquid C­ O2 (L-CO2) reacts
with water to form clathrate hydrate (Fig. 3). As a solid,­CO2 hydrate can agglomerate to fill the pore spaces of
not only volcanics but also sediments. These agglomerates significantly reduce the permeability of sediments
and block the leakage of L-CO2 into the overlying seawater (Evrenos et al. 1971; Gauteplassa et al. 2018). Suchself-sealing ability (hydrate formation) of L-CO2 can promote the widespread occurrence and preservation of L/
SC-CO2 pools at and beneath the seafloor (Gauteplass
et al. 2020). Indeed, C­ O2 hydrate sealing the underlying
L-CO2 has been visually confirmed in the Yonaguni Knoll

200

Champagne, NW Eifuku

150

JADE
Yonaguni Knoll IV

Pressure (bar)Shibuya and Takai ﻿Progress in Earth and Planetary ScienceCO2 hydrate+liquid CO2100

H2O + liquid CO2

Iheya North

CEP
Jan Mayen50

H2O + CO2 gas

Q

CO2-rich hydrothermal fields in Mariana andTonga-Kermadec arcs (except Champagne, NW Eifuku)

CO2 hydrate + CO2 gas

0
0

10

20

30

Temperature (°C)

Fig. 3 A relationship between seafloor pressures of ­CO2-richhydrothermal fields and phase relations in the ­CO2–H2O system(modified from Spycher et al. 2003). ­CO2-rich hydrothermal systems
seem to host a L/SC-CO2 pool when seafloor pressures exceed thecritical pressure of ­CO2-rich fluid. In contrast, Jan Mayen field in the
Arctic Mid-Ocean Ridge (Pedersen et al. 2010; Stensland et al. 2019)and ­CO2-rich hydrothermal fields in the Mariana (except Champagne
field) and Tonga-Kermadec arcs (Lupton et al. 2008) discharge­CO2-rich fluids as gas bubbles (with/without C­ O2 hydrate). Q, aquadruple point; CEP, a critical end point for the ­CO2–H2O system

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IV field and other fields of Okinawa Trough (e.g., Inagaki
et al. 2006) and hydrate-sealed L-CO2 droplets in deep
seawater have also been verified near many hydrothermal
fields (Lupton et al. 2006, 2008). In addition, although­CO2 hydrate can coexist with L-CO2 at pressures above
35–45 bar at low temperatures, pressures higher than c.
74 bar are required for stabilizing SC-CO2 that eventually becomes L-CO2 when it cools beneath the seafloor(Fig. 3). Therefore, the formation of L/SC-CO2 pools atand beneath the seafloor requires water depths of several hundred meters or more depending on temperature. This feature is consistent with the venting depths(> 1000 m) of present hydrothermal fields bearing L-CO2
pools (Table 1) and the emission of gas-phase C­ O2 bubbles (and ­CO2 hydrate) in ­CO2-rich hydrothermal fields
at shallow seafloor depths (< 700 m) (Lupton et al. 2008;
Pedersen et al. 2010; Stensland et al. 2019). In contrast,
L-CO2 becomes heavier than seawater at pressureshigher than approximately 300 bar (Qanbari et al. 2011).
Therefore, in hydrothermal systems on seafloors deeper
than 3000 m, the L-CO2 does not ascend even when L/
SC-CO2 is generated in the deeper part by the above-discussed processes. In such cases, L/SC-CO2 pools cannot
form around the seafloor.

et al. 2010). This range of values corresponds to the maximum ­CO2 concentrations of hydrothermal fluids in the
Okinawa Trough systems that host abundant L/SC-CO2
fluids and pools (Table 1). Therefore, the early Archean
seafloor hydrothermal systems could potentially generate
L/SC-CO2 fluids during hydrothermal circulation.
Other geological evidence is consistent with subseafloor phase separation. In the ca. 3.5 Ga North Pole area,
Pilbara Craton, Western Australia, there is a bedded
chert/barite (Dresser Formation, Warrawoona Group)
and underlying thick basaltic sequence, in which ancient
seafloor hydrothermal alteration is well preserved (Kitajima et al. 2001a; Terabayashi et al. 2003; Van Kranendonk 2006). In the basaltic sequence, the temperature ofalteration increases stratigraphically downward, reachingapproximately 350 °C at 1000 m below the ancient seafloor. At this depth, two types of mineral assemblages arefound at nearly identical alteration temperatures: calcitebearing (calcite–rutile–quartz) and calcite-free (epidote–
actinolite–titanite) assemblages. The former and latterassemblages thermodynamically point to their coexistence with ­CO2-rich and ­CO2-poor ­(H2O-dominated)
fluids, respectively. As these two fluids should have very
different ­CO2 concentrations, they might have been
generated by subseafloor phase separation in the 3.5 Ga
seafloor hydrothermal system (Kitajima et al. 2001b).
This implies that L/SC-CO2 fluids could be generated
in Archean seafloor hydrothermal systems similarly tomodern L/SC-CO2-bearing hydrothermal systems.
More direct evidence of subseafloor phase separation in the Archean greenstones is found in hydrothermal quartz deposits in the 3.5 Ga North Pole area. In
this area, flow channels of hydrothermal fluids are wellpreserved as silica dikes and barite/quartz veins in thebasaltic sequence (Ueno et al. 2006; Van Kranendonk
2006). Such hydrothermal deposits preserve ancient circulating fluids as many fluid inclusions in the quartz/barite crystals (Kitajima et al. 2001b; Ueno et al. 2006). The
frequency distribution of homogenization temperaturesof the fluid inclusions shows a single peak at approximately 150 °C in the deeper part of the basaltic sequence(> 1000 m below the ancient seafloor), whereas samples from the shallower part (< 1000 m deep) show wide
ranges of homogenization temperatures (87–367 °C).
Judging from this upward change in homogenization
temperatures, phase separation likely occurred around1000 m below the ancient seafloor (Kitajima et al. 2001b).
Under a microscope, most of the fluid inclusions consistof two phases ­(H2O-rich liquid and vapor) at room temperature, but fluid inclusions containing L-CO2 as a third
phase are also seen (Kitajima et al. 2001b). This finding
indicates extreme ­CO2 enrichment in the L-CO2-bearing
fluid inclusions, which probably corresponds to a vapor

4 Geological evidence of phase separation in earlyArchean seafloor hydrothermal systems
Fossils of ancient subseafloor hydrothermal systems arepreserved in the Archean/Proterozoic greenstone belts(e.g., Kitajima et al. 2001a; Van Kranendonk 2006; Hofmann and Harris 2008; Rouchon and Orberger 2008).
The Archean basaltic greenstones show a series of evidence of hydrothermal carbonation, suggesting thatancient seawater was extremely enriched in C
­ O2 (Nakamura and Kato 2004; Shibuya et al. 2013b). Furthermore,
the carbonate content in the altered seafloor basalts
decreased from the early Archean to the Paleoproterozoic (Nakamura and Kato 2004; Shibuya et al. 2012,2013a, 2017), indicating that the ­CO2 concentrationin seawater decreased over this period (Shibuya et al.
2017). This finding is consistent with many other geologic
records, such as paleosols and evaporates (Hessler et al.
2004; Lowe and Tice 2004; Kanzaki and Murakami 2015),
and with climate models dealing with the faint young Sun
paradox (Kasting 1993; Sleep and Zahnle 2001).
Since the early Archean seawater was indeed extremely
enriched in ­CO2, the hydrothermal fluids in most of theearly Archean seafloor hydrothermal systems may also
have been enriched in ­CO2 (e.g., Kitajima et al. 2001a).
For example, the mineral assemblages of the seafloor
basalts altered under high-temperature conditions(300–400 °C) indicate that the C­ O2 concentration in
the hydrothermal fluid exceeded 200 mmol/kg (Shibuya

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phase generated by phase separation. Furthermore, thestable carbon isotopes of the ­CO2-rich fluid inclusions(ca. −7.4 to 0 ‰ vs. Vienna Pee Dee Belemnite) are generally explained by a mixture of mantle-derived ­CO2 andseawater (Ueno et al. 2006). Therefore, even in the earlyArchean, the hydrothermal fluids enriched with seawater- and mantle-derived ­CO2 probably underwent phaseseparation in the subseafloor hydrothermal systems,with consequent generation of C
­ O2-rich vapor phases. In
addition, the temperature of phase separation recordedin the fluid inclusions (ca. 150 °C) implies that phase separation occurred even in off-axis hydrothermal circulation systems (e.g., Kitajima et al. 2001b).
As discussed above, multiple lines of evidence in
the Archean geological records show that ­CO2-rich
vapor phases were formed by phase separation in earlyArchean seafloor hydrothermal systems. If the abovedescribed condensation and purification processes wereeffective, nearly pure L/SC-CO2 fluids are expected inthese hydrothermal systems as observed in the modern
equivalents, although this expectation must be validated
in further analyses of the geological records. In particular, the Archean bottom seawater must have been sufficiently cool to form ­CO2 hydrates beneath the seafloor(see above). The oxygen isotopes of phosphates in theArchean sediments indicate that the surface seawater
was indeed cool during that period (26–35 °C) and did
not significantly differ from the modern equivalent (e.g.,
Blake et al. 2010). Therefore, the temperature of theArchean bottom seawater was probably below ~ 10 °C,
suggesting that hydrothermal systems hosting abundant
L/SC-CO2 pools could exist in the early Archean ocean.

degassed ­CO2 into reaction zones in komatiite-hostedhydrothermal systems. Therefore, generation of L/
SC-CO2 fluids caused by phase separation is expected
to be more frequent in Hadean seafloor hydrothermal
systems than in modern systems (Fig. 4). Furthermore,during the weathering of impact ejecta, the Earth’s surface temperature might have been temporarily lower inthe Hadean than in the Archean (Kadoya et al. 2020). At
this time, the deep seawater should have been sufficiently
cool to initiate ­CO2 hydrate formation at the interface
between L-CO2 and seawater at and beneath the seafloor,
leading to the widespread formation of L/SC-CO2 pools.
Even if the temperature of deep seawater exceeded thatof the stability field of C
­ O2 hydrates (~ 10 °C), sealing
might have been provided by hydrothermal sediments(e.g., bedded chert and/or banded iron formation) chemically precipitated from hydrothermal plume (by mixing
between hydrothermal fluid and seawater) instead of C
­ O2
hydrate (Fig. 4). Such sediments should be less permeable
than those in modern seafloor hydrothermal environments because the hydrothermal fluid probably mixed
with silica-saturated seawater, thus cementing the porespaces and fractures as observed in Archean greenstone
belts (e.g., Kitajima et al. 2001a; de Vries and Touret 2007;
Shibuya et al. 2010). The sealing effects of C
­ O2 hydrate
and/or chemical sediments may have prepared the conditions under which the seafloor hydrothermal systems
could host abundant L/SC-CO2 pools in the Hadean era
when prebiotic chemical evolution and life’s emergence
took place. Accordingly, we propose that L/SC-CO2 contributed to prebiotic chemical evolution in early seafloorhydrothermal systems. We call this hypothesis the “liquid/supercritical ­CO2 hypothesis.”
In addition, the L/SC-CO2 fluids associated with
Hadean hydrothermal circulations likely played an
important role in heavy metal cycles through the Hadean
ocean. As heavy metals are highly soluble in L/SC-CO2
under conditions with chelating agents (e.g., Smartet al. 1997; Erkey 2000; Lin et al. 2014), the L/SC-CO2
pools generated in high-temperature hydrothermal systems (e.g., black-smoker-type systems) might have dissolved the heavy metals if natural chelating agents were
present/generated. Furthermore, hydrate-coated ­CO2
bubbles released from hydrothermal systems have also
the potential to transport polymetallic nanoparticles
upward and release them at shallower depths throughtheir buoyancy and dissociation at depths around 500 m(e.g., Stensland et al. 2019) (Fig. 4). Therefore, the emission of L/SC-CO2 fluids into seawater probably provided
the chemical pump for the vertical distribution of heavy
metals in the Hadean Ocean, in addition to active plume
expansion derived from high-temperature hydrothermal
vents (Fig. 4). This process would have contributed to the

5 Implications for prebiotic chemical evolution
in Hadean seafloor hydrothermal systems
5.1 Liquid/supercritical CO2 in Hadean seafloorhydrothermal systems

As discussed above, L/SC-CO2 fluids and pools could
be generated in the subseafloor hydrothermal systemsof the early Archean. In the Hadean, the atmosphericand oceanic ­CO2 levels were probably as high as thoseof the early Archean although the ­CO2 levels had temporarily decreased through weathering of impact ejecta(Sleep and Zahnle 2001; Kadoya et al. 2020). Regarding
solid Earth, the upper part of oceanic crust was highly
carbonated through reactions with seawater (Nakamura
and Kato 2004; Shibuya et al. 2012), and its subduction
might cause decomposition of carbonates and resulting­CO2-rich melt production at subduction zones. Moreover, komatiite magma could achieve buoyancy to erupt
on the seafloor, likely because its enrichment of ­CO2
decreased melt density (e.g., Anderson 1995; Shimizu
et al. 2009), which would potentially inject abundantShibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60

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Fig. 4 Schematic of Hadean seafloor hydrothermal systems bearing a L/SC-CO2 pool. The C­ O2-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 rechargezones. When hydrothermal fluid reaches the reaction zone, it receives abundant degassed C­ O2 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 C­ O2
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. C­ O2 is then purified invapor phase. Eventually, these condensation and purification processes generate many SC-CO2 droplets beneath the seafloor. The SC-CO2 is lessdense 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 theambient seawater/pore fluid requisitely generate solid ­CO2 hydrates that cement the pores, voids and cavities of the hydrothermal sediments andpermeable 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

widespread formation of metal-rich chemical sedimentsin the Hadean and Archean oceans.
5.2 Possible roles of liquid/supercritical CO2 in prebiotic
chemical evolution

As L/SC-CO2 fluids have similar chemical properties tohydrocarbon solvents (with a few notable differences)(e.g., Hyatt 1984), the L/SC-CO2 pools in the Hadean
seafloor hydrothermal systems would have provided
novel reaction fields for prebiotic chemical evolution.
At present, the reaction processes of prebiotic chemical

evolution is uncertain because we lack experimental dataof the reaction properties of L/SC-CO2 fluids and theirinteractions with seawater and hydrothermal fluid. In
addition, in situ measurements of gaseous and condensed­CO2 in natural seafloor hydrothermal systems are stillchallenging (e.g., Pasteris et al. 2004). In the following
paragraphs, however, we briefly discuss three representative features of L/SC-CO2 fluids: (1) provision of a dry
environment, (2) behavior as an organic solvent, and (3)provision of an interface between L/SC-CO2 and water.

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The behavior as a dry environment is among the mostimportant chemical characteristics of L/SC-CO2 fluids
and pools. The solubility of water in L-CO2 is 0.003–
0.004 mol% at 25 °C and > 64 bar (Spycher et al. 2003),
which is only approximately one order of magnitude
higher than the content of saturated water vapor in air
and is sufficiently low to facilitate dehydration synthesis.
Therefore, the presence of L/SC-CO2 pools in Hadean
seafloor hydrothermal systems might resolve the “waterparadox” that has stymied previous seafloor hydrothermal vent hypotheses—the filling of seafloor hydrothermal systems with abundant water is considered to limit
organic synthesis and instead facilitate the hydrolysis oforganic molecules. The dry environments of L/SC-CO2
pools in seafloor hydrothermal systems would have been
spatiotemporally abundant, stable, and safe, even in the
Hadean era when the Earth was continuously bombarded
with harmful extraterrestrial inputs such as solar UV
radiation, cosmic rays, and meteorites (e.g., Morbidelli
et al. 2001; Nishizawa et al. 2020).
Various organic molecules such as n-alkanes arehighly miscible with L/SC-CO2, which reflects the
behavior of L/SC-CO2 as an organic solvent (Hyatt
1984; Swaid et al. 1985). In addition, most of the solutes
in SC-CO2 can also dissolve in L-CO2, while the difference in solubilities of a given solute between L-CO2
and SC-CO2 is a matter of degree (Hyatt 1984). Therefore, if organic molecules dissolvable in L/SC-CO2 weresynthesized somewhere in the hydrothermal circulation process (e.g., in the reaction and mixing zones),
they could be finally transported and trapped in the L/
SC-CO2 fluids and pools. This suggests that L/SC-CO2
pools can ultimately condense and store hydrophobicorganics in hydrothermal systems. If L/SC-CO2 pools
establish dry conditions and promote the condensationof certain organics synthesized during hydrothermal
circulation, they might be the efficient sites of dehydration synthesis and polymerization of L/SC-CO2-solubleorganics in addition to other possible environmentswith L/SC-CO2 (e.g., soda fountains and deep-reaching
tectonic fault zones; Fujioka et al. 2009; Schreiber et al.
2012; Mayer et al. 2015; 2018). Furthermore, L/SC-CO2
dissolves various types of organic molecules such as
methanol and ascetic acid (Hyatt 1984), which not only
induce various synthetic reactions but also elevate thesolubilities and reaction rates of other organic molecules via the entrainer effect in certain cases (Jouyban
et al. 2005; Machmudah et al. 2006). Indeed, simple
organic carbon species such as formate and C­ 2+ alkanes
are abiotically synthesized during serpentinizationin modern seafloor hydrothermal systems associatedwith ultramafic rocks (Lang et al. 2010, 2012; McDermott et al. 2015). According to geo-electrochemical

experiments, the synthesis of simple building-blockorganics (amino acids, organic acids, and thio-esters)
can be catalyzed by elemental metals/metal sulfide
assemblages putatively included in early hydrothermal
mineral deposits (Kitadai et al. 2019; 2020; 2021). Thus,
in Hadean seafloor hydrothermal systems associatedwith ultramafic rocks (e.g., komatiite), and possibly inthose hosted by other rocks, the L/SC-CO2 pools might
operate as factories that enrich and process certain
organic molecules from the hydrothermal fluids andminerals and even from seawater.
The physicochemical properties of L/SC-CO2 fluids
and pools create different phases of fluids in water andform abundant interfaces between the water and hydrophobic fluids. Across these interfaces, hydrophilic molecules derived from H­ 2O-rich fluid (e.g., heated seawaterand hydrothermal fluid at > ~ 10 °C) would react withhydrophobic molecules dissolved in L/SC-CO2, possibly forming amphiphilic molecules that can arrange
along the interface and might self-assemble into membrane-like structures (e.g., Mayer et al. 2017). It can be
speculated that physicochemical interactions among L/
SC-CO2 and H­ 2O-rich fluids triggered by varying environmental conditions (e.g., temperature and pressure)lead to the formation of vesicles and droplets and theirinteractions (e.g., coalescence and splitting). According
to previous seafloor hydrothermal vent hypotheses, celllike mineral compartments formed within hydrothermal
mineral deposits at the time of proto-metabolism emergence (e.g., Russell and Martin 2004). Of course, suchself-assembled amphiphilic molecules created by interface reactions between water and L/SC-CO2 may become
immobilized in the mineral compartments of hydrothermal mineral deposits, where they cooperate with minerals as prebiotic cellular membranes.
In addition to the above-described features of L/
SC-CO2 fluids and pools, the high dissolution capacities of L/SC-CO2 for heavy metals might have contributed to the formation of metal-bearing biomolecules(e.g., enzymes and co-factors) because SC-CO2 can beindustrially used for heavy metal extraction (e.g., Linet al. 2014). Moreover, polynucleotides such as RNA andDNA, which are generally hydrolyzed in water, may havebeen stabilized and preserved in L/SC-CO2 fluids andpools because dry small interfering RNA (siRNA) and dry
plasmid DNA (pDNA) can be pharmaceutically preparedusing SC-CO2 (e.g., Okamoto et al. 2005; Okuda et al.
2013). These chemical characteristics of L/SC-CO2 fluids
and pools might increase the reactivity and stability of
typical biomolecules. One can therefore hypothesize thatthe abundant L/SC-CO2 fluids and pools in Hadean seafloor hydrothermal systems served as novel and important reaction fields for prebiotic chemical evolution.

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In the proposed hypothesis, not all processes of prebiotic chemical evolution are associated with L/SC-CO2
fluids and pools in the Hadean seafloor hydrothermal
systems. Instead, the L/SC-CO2 fluids and pools justfacilitate significant steps of prebiotic chemical evolution
such as condensation, storage, and synthesis of molecular building blocks, which were previously considered tobe difficult in water-filled seafloor hydrothermal environments. Abundant L/SC-CO2 fluids and pools arewidespread at and beneath the modern hydrothermally
active seafloors. They are associated with high- and lowtemperature vent fluids and are found in the cavities andspaces of hydrothermal mineral deposits and also in sediments. In the Hadean, therefore, various simple organic
compounds might be abiotically produced under various physicochemical conditions, in different zones of thewhole hydrothermal circulation (e.g., reaction zone andmixing zone) and in different fluids and solids (e.g., hightemperature hydrothermal fluids, seawater, and various
minerals). In this case, it would be possible that the many
local environments in the seafloor hydrothermal system co-worked as a series of factories to drive prebiotic
chemical evolution owing to the whole hydrothermal circulation. This study highlights that many of the Hadean
seafloor hydrothermal systems could provide abundant
L/SC-CO2 fluids and pools by the whole hydrothermal
circulation processes. Our new hypothesis, named the“liquid/supercritical ­CO2 hypothesis,” is consistent withprevious hypotheses related to seafloor hydrothermal
systems, some of which refer to low-temperature alkaline vents (e.g., Russell and Hall 1997; Russell et al. 2014;
Shibuya et al. 2016) or high-temperature vents (Takai
et al. 2006; Shibuya et al. 2010, 2015). Our hypothesis
synergistically strengthens the grand “seafloor hydrothermal vent hypothesis.”geological records, widespread L/SC-CO2 pools in seafloor hydrothermal systems probably formed throughout Earth history and were likely more dominant in the
Hadean and early Archean eras than in the modern era.
Based on theoretical considerations and observations,
we here propose that the abundant L/SC-CO2 fluids
and pools in the Hadean seafloor hydrothermal systems
served as a novel and important reaction field for prebiotic chemical evolution, particularly of condensation,
storage, and maturation of hydrophobic and amphiphilic molecules. Our “liquid/supercritical C­ O2 hypothesis” potentially overcomes the intrinsic “water paradox”
of the “seafloor hydrothermal vent hypothesis.”
The solvent and reaction properties of L/SC-CO2
interacting with seawater and hydrothermal fluids aremostly unknown in natural seafloor hydrothermal environments and remain unclear even in laboratory studies. Therefore, the basic properties of L/SC-CO2 should
first be investigated under different physical and chemical conditions (e.g., temperature and pressure and with/without seawater and other gases) in the laboratory. The
dissolubility, preservability, and reactivity of organic
compounds such as amino acids, organic acids, nucleosides and nucleotides, proteins, nucleic acids, and lipids
should also be studied in L/SC-CO2 systems. Second, weneed to characterize the types of inorganic and organic
compounds that dissolve in the natural L/SC-CO2 fluids
and pools of modern seafloor hydrothermal systems. This
investigation will require elaboration of seafloor sampling
and preservation tools. The analyzed data of natural L/
SC-CO2 fluids and pools in modern seafloor hydrothermal systems, which contain both abiotically and biologically produced compounds, will help to reveal the in situ
behaviors and properties of L/SC-CO2 fluids and poolsin the whole hydrothermal circulation processes. Theselaboratory and field experiments and analyses will provide key insights for predicting the types of compounds
that dissolve and condense in L/SC-CO2 fluids and thetypes of reactions occurring in the L/SC-CO2 fluids andpools of modern and Hadean seafloor hydrothermal systems. The physicochemical conditions of the L/SC-CO2
fluids and pools in Hadean seafloor hydrothermal systems can be predicted by analyzing the Archean geologic
records (e.g., alteration of volcanic rocks and fluid inclusions). Particularly, the natural entrainers and chelating
agents of L/SC-CO2 are possibly constrained by compositions of fluid inclusions containing L-CO2 in Archeanhydrothermal deposits. Through these field observations,laboratory experiments, and theoretical approaches, ournew “liquid/supercritical ­CO2 hypothesis” will be further
combined with the “seafloor hydrothermal vent hypothesis” for a comprehensive understanding of the OOL.

6 Conclusions and future perspectives
L/SC-CO2 fluids and pools can provide dry environments serving as organic solvents in water-rich seafloor hydrothermal systems. Based on preliminary
experimental and analytic data in industrial and pharmaceutical laboratories, L/SC-CO2 is considered topossess unique physicochemical properties enabling
the potential dissolution and condensation of heavy
metal elements and organics, and possible dehydration
synthesis of certain molecules (e.g., Hyatt 1984; Swaid
et al. 1985). Widespread L/SC-CO2 fluids and pools inmodern seafloor hydrothermal systems are theorized
to form by phase separation and phase segregation processes of ­CO2-enriched hydrothermal fluid in the wholehydrothermal circulation at and beneath the seafloor.
Considering the geological time scale of the Earth andre-interpreting many lines of evidence in the known

Shibuya and Takai ﻿Progress in Earth and Planetary Science(2022) 9:60

Abbreviations
L-CO2: Liquid ­CO2; SC-CO2: Supercritical ­CO2; L/SC-CO2: Liquid and supercritical­CO2; OOL: Origin of life.
Acknowledgements
We are grateful to two anonymous reviewers for constructive comments andK. Kobayashi for editorial handling and suggestions.
Author contributions
TS and KT proposed the topic and conceived and designed the study. All theauthors read and approved the final manuscript.
Funding
This work was partially supported by JSPS/MEXT KAKENHI Grant Numbers
JP25707038, JP15K13583, JP17H06455, and JP20H00209.
Availability of data and materials
Not applicable.

Declarations
Competing interests
The authors declare that they have no competing interest.
Received: 1 April 2022 Accepted: 27 September 2022

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