Ingredients for microbial life preserved in 3.5 billion-year-old fluid inclusions

Early microbial life likely required small organic molecules both as building blocks for biomass and as substrates for heterotrophic metabolism. Potential sources of such organics on early Earth include redistribution of pre-existing biomass, exogenous delivery via meteorites and dust, and endogenous synthesis from inorganic precursors in atmospheric and hydrothermal settings. Experimental work has shown that hydrothermal reactions (e.g., Fe2O3 to FeS2 driving CO2 reduction; CO reacting with methanethiol on Ni/Fe sulfides) can generate simple organics such as acetic acid relevant to primordial metabolisms and lipid synthesis. The Dresser Formation (ca. 3.5 Ga) preserves barite with abundant primary fluid inclusions within a well-established hydrothermal setting that hosted early microbial communities. While previous work identified the main inorganic constituents (H2O, CO2, H2S, minor CH4) in these inclusions, the presence and diversity of organic molecules remained unknown. This study investigates whether barite-hosted primary fluid inclusions contain indigenous, biologically relevant organic compounds that could have served as substrates for early life.

The Dresser Formation provides a key window into early hydrothermal habitats and contains barite (BaSO4) known for preserving primary fluid inclusions due to its chemical robustness. Prior studies documented hydrothermal origin, stromatolitic associations, and isotopic evidence for microbial activity (e.g., microbial sulfate reduction) in these rocks. Fluid inclusion studies previously identified H2O, CO2, H2S, and minor CH4 as major volatile components, but lacked comprehensive characterization of organic constituents. Experimental geochemistry has demonstrated abiotic synthesis of organics under hydrothermal conditions via Fischer–Tropsch-type processes and sulfide-catalyzed reactions, including production of acetic acid from CO or CO2 in the presence of methanethiol on metal sulfide surfaces. Despite these insights, direct geological evidence for such synthetic products in early Archean rocks had been missing.

• Field sampling: Black, grey, and white barites were collected from the Dresser mine area (Pilbara Craton, Western Australia; 21°09'05.2"S, 119°26'15.3"E). Care was taken to avoid surface contamination; only large, intact pieces (>10 cm) without visible cracks or weathering were retained. Laboratory glassware was combusted at 950 °C for 3 h and solvent-rinsed prior to use.
• Petrography: Hand specimens and thin/thick sections were examined by stereomicroscopy and transmitted/reflected light microscopy to identify primary vs secondary fluid inclusions and textural relations (e.g., trails parallel to barite growth bands).
• Fluid inclusion microthermometry: Doubly polished thick sections (150–200 μm) were analyzed on a Linkam THM 600 heating–freezing stage (calibrated with standards). Recorded transitions included ice/solid melting, phase partitioning (Th of non-aqueous phases), and total homogenization temperatures (Th,total) of the aqueous inclusions.
• Raman spectroscopy: A Horiba LabRAM-HR 800 with 488 nm laser was used to identify volatile and solid phases within inclusions (e.g., CO2, H2S, CH4, N2, COS; solid daughter phases such as sulfur, strontianite, kerogen).
• GC–MS analyses of volatiles and organics from inclusions: • Thermal desorption/decrepitation GC–MS (TD-GC–MS): Barite fragments heated (notably 150 °C and 250 °C) and evolved gases analyzed on a Varian CP-3800 GC coupled to a Varian 1200 MS. This approach releases inclusion contents by cracking/heating inclusions. • Solid phase microextraction GC–MS (SPME-GC–MS): Offline adsorption of volatiles/organics onto SPME fiber at <50 °C prior to GC–MS to minimize thermal artifacts and enable analysis of larger sample masses (grams vs milligrams), improving detection of trace and higher molecular weight compounds. • System blanks and cleanliness checks were performed pre- and post-analyses; results were replicated across multiple TD and SPME runs.
• Bulk geochemistry and isotopes: Total organic carbon (TOC) by RC612 analyzer. Stable carbon and oxygen isotopes measured for bulk materials and online-evolved gases; δ13C reported vs VPDB and δ18O vs VSMOW. δ13C of CO2 released from inclusions measured online for black and grey barites.
• Data interpretation: Combined petrography, microthermometry, Raman, GC–MS, and isotopic data to assess inclusion primacy, composition, potential sources (abiotic vs biotic), and implications for early metabolisms.

• Petrography and inclusion types: Abundant primary fluid inclusions (typically ~10 μm) occur along barite growth bands, with minor secondary trails. Two main inclusion types were identified: (1) aqueous, carbonic-saturated inclusions and (2) non-aqueous, carbon–sulfide inclusions. Both commonly contain solid daughter phases (e.g., native sulfur, strontianite; kerogen and halite observed in some non-aqueous inclusions).
• Volatile compositions (Raman): CO2 and H2S dominate both inclusion types, with minor CH4, N2, and COS. Aqueous inclusions contain 0–24 mol% H2S and occasionally up to 10 mol% N2; non-aqueous inclusions contain 21–36 mol% H2S and small CH4 (<2 mol%).
• Microthermometry of aqueous inclusions: Salinities range from 1–14 wt% NaCl eq (rarely up to 25 wt%). Melting temperatures vary from 0 to −26 °C (peak around −7 °C). CO2/H2S-bearing daughter crystals form upon freezing and melt between ~7 °C (CO2-rich) and ~20 °C (H2S-rich). Total homogenization temperatures (Th,total) range ~100–195 °C, indicating minimum trapping temperatures. Data support primary trapping of immiscible fluids (heterogeneous trapping) with no pressure correction required.
• TD-GC–MS (150 vs 250 °C): Confirmed abundant CO2, H2S, H2O and detected COS, CS2, SO2, and diverse organics including oxygen-bearing (aldehydes, ketones, acetic acid, oxolane) and sulfur-bearing compounds (thiols, thiophene), plus aromatics (e.g., benzene). Compound diversity and yields were markedly higher at 250 °C, consistent with microthermometry showing inclusions remain intact up to ~230 °C. However, SO2 formation at 250 °C indicates thermal artifacts.
• SPME-GC–MS: Revealed numerous oxygen- and/or sulfur-bearing organics (aldehydes, ketones, acetic acid, thiols, polysulfides), and aromatics (benzene, alkylbenzenes, xylenes, styrene), with greater compound diversity—especially higher molecular weight species—than TD. CO2 and H2S were absent due to method-specific adsorption bias.
• Specific biologically relevant compounds: Methanethiol (CH3SH) and acetic acid were detected—stable building blocks of methyl thioacetate (activated acetic acid), a key proposed intermediate in primordial energy metabolism. Additional sulfur species (H2S, COS, CS2) and organosulfur compounds (thiols, thiophene, organic polysulfides) were present, alongside small hydrocarbons and oxygenates.
• Organic carbon and isotopes: Mean TOC of black barite is 0.31 wt% (N=5; SD 0.002). δ13Crock ≈ −27.6 ± 6.0 ‰. Bulk δ13C and δ18O values of −10.0 ± 0.3 ‰ and 34.1 ± 0.6 ‰, respectively (offline). Online δ13C of CO2 from black barites ranges −14.3 to −8.9 ‰ (mean −10.3 ‰; N=11), systematically lighter than grey barites (−8.0 to −4.0 ‰; mean −6.3 ‰; N=17), suggesting a biomass-derived carbon component mixed with magmatic/abiotic CO2 in black barites.
• Data integrity and provenance: Multiple lines of evidence indicate compounds derive from primary fluid inclusions: petrographic primacy, thermal behavior consistent with inclusion decrepitation, detection of highly volatile species, reproducible results across independent techniques and runs, temperature-dependent yields, and blanks showing absence of contaminants. Minor contributions from the rock matrix cannot be entirely excluded.
• Environmental conditions: Inclusion trapping temperatures (~100–195 °C) and variable salinities are consistent with hydrothermal discharge environments and modern analogs (e.g., JADE field 150–200 °C).
• Overall: Early Archean hydrothermal fluids at Dresser contained mixtures of abiotic and biotic compounds, including substrates suitable for ancestral sulfur-based and methanogenic metabolisms and building blocks implicated in primordial carbon fixation pathways.

The study demonstrates that primary fluid inclusions in 3.5 Ga Dresser barites trapped indigenous mixtures of volatiles and organic molecules that could have fueled early microbial life. The presence of H2S, COS, CS2, CH4, methanethiol, acetic acid, and various organosulfur and oxygenated compounds aligns with hypothesized ancestral sulfur cycling and methanogenic metabolisms. Detection of methanethiol and acetic acid supports the availability of building blocks for methyl thioacetate, a proposed key intermediate in primordial energy metabolism and lipid synthesis on metal sulfide catalysts. Isotopically lighter CO2 in black barites relative to grey barites indicates mixing of a biomass-derived carbon component with abiotic sources (e.g., magmatic CO2), consistent with microbial processing (e.g., bacterial or thermochemical sulfate reduction) and redistribution through hydrothermal fluids. The co-occurrence of abiogenic markers (volcanic gases, Fischer–Tropsch-type products, sulfide-catalyzed organosynthesis) and biogenic signals (kerogen δ13C ≈ −28 ‰, sulfur-cycle substrates) suggests complex interactions between abiotic synthesis, biological activity, and fluid mixing in early hydrothermal systems. Collectively, the findings directly address the long-standing question of whether early hydrothermal environments supplied the organic substrates necessary for early life and provide geologic evidence that such ingredients were indeed available and preserved.

This work provides the first detailed molecular inventory of organic compounds preserved within primary fluid inclusions from ca. 3.5 Ga barites, revealing diverse sulfur- and oxygen-bearing organics, including methanethiol and acetic acid—the stable building blocks of methyl thioacetate. The inclusions also contain abundant CO2 and H2S with minor CH4, N2, and COS, and record hydrothermal trapping conditions (~100–195 °C) and variable salinities. Isotopic signatures indicate a mixture of abiotic and biotic carbon sources, consistent with hydrothermal processing and microbial activity. These results substantiate that Archean hydrothermal fluids supplied fertile substrates for early microbial ecosystems and support hypotheses linking hydrothermal chemistry to the emergence of life. Potential future research directions include: targeted searches for methyl thioacetate or direct reaction products in Archean inclusions; compound-specific isotope analyses to apportion abiotic vs biotic sources; exploration of additional Archean localities and mineral hosts; experimental replication of observed compound suites under controlled hydrothermal conditions; and nanoscale mapping of inclusion-organic associations to refine preservation mechanisms.

• Minor contribution of organic compounds from the rock matrix cannot be entirely ruled out despite multiple contamination controls.
• Thermal decrepitation at higher temperatures can produce artifacts (e.g., SO2 formation), complicating interpretation; SPME lacks adsorption of key inorganic volatiles (CO2, H2S), introducing analytical bias.
• Some inclusions show evidence of post-entrapment modifications (e.g., necking down) immediately after trapping, potentially affecting volatile proportions in a subset of inclusions.
• Direct detection of proposed key intermediates (e.g., methyl thioacetate) was not achieved; inference is based on detection of stable building blocks (methanethiol, acetic acid).
• Study focuses on specific barite samples from one locality, which may limit generalizability across Archean settings.