Lightning strikes as a major facilitator of prebiotic phosphorus reduction on early Earth

The study addresses how bioavailable phosphorus required for early biochemistry could have been supplied on the prebiotic Earth. While phosphorus on Earth is typically present as insoluble phosphate in minerals like apatite, reduced phosphorus species such as phosphide (P0) in schreibersite are highly reactive and can generate key prebiotic molecules upon hydration. Schreibersite is abundant in certain meteorites, and meteorite delivery has been widely assumed to be the dominant source of reactive phosphorus on early Earth during a period of declining impact flux. The authors investigate an alternative, terrestrial mechanism: lightning strikes forming fulgurites under high-temperature, highly reducing conditions that can reduce phosphate to phosphide and intermediate species, potentially providing a widespread, quiescent, and continual source of reactive phosphorus in early Earth environments.

Prior work shows that schreibersite corrosion produces activated phosphate and organophosphorus compounds and intermediate species (hypophosphite, phosphite). Schreibersite occurs in meteorites and has been reported in some lightning-formed fulgurites. Models indicate early Earth experienced a monotonic decline in impactors after 4.5 Ga, providing an estimated 10^5–10^7 kg of reduced P annually during the Hadean and early Archean, leading to the assumption that non-meteoritic sources were minor. Experiments and thermodynamic predictions demonstrate electric discharges and high-temperature conditions can reduce phosphate to lower-valence species. Global circulation modeling links higher atmospheric pCO2 to increased lightning. The survival of reduced phosphorus through meteorite impact processes is uncertain due to melting, vaporization, oxidation, and plume interactions with country rock and atmospheric water. Terrestrial settings such as volcanic ponds, lakes, tidal pools, and hot springs have been proposed as favorable locales for concentrating prebiotic compounds.

Empirical component: A decimeter-scale type II fulgurite formed in 2016 in clay-rich soil (Glen Ellyn, Illinois, USA) was sampled. Analyses included Raman spectroscopy (Horiba LabRAM HR Evolution; 532/473 nm lasers), X-ray fluorescence (Rigaku ZSX Primus II) for major elements on parent soil and fulgurite powders (core and rim), X-ray diffraction (Bruker D8) for mineral identification, and SEM-based electron backscatter imaging, EDS mapping (Tescan VEGA3 XM), and EBSD (FEI Quanta 650 with Symmetry detector) to characterize phases and textures. The sample was prepared by sequential polishing with alumina and diamond pastes. Loss on ignition and standard reproducibility were reported for XRF. Analytical results identified amorphous silica glass in the core, crystalline quartz in the rim, silicon carbide (SiC), amorphous graphitic carbon, native Fe, and abundant schreibersite (Fe3P) spherules. Phosphorus balance estimates were made by comparing P2O5 in soil versus fulgurite core and rim relative to an equivalent composition line to infer the minimum fraction of P reduced to phosphide and to estimate schreibersite mass in the recovered ~25 kg fulgurite. Modeling/estimation component: The authors estimated annual reduced phosphorus production by lightning on early Earth using (1) pCO2 evolution through Hadean–early Archean (after Kasting) to infer lightning rates from a global circulation model (Wong et al.), (2) present-day ratios to assume 25–75% of lightning over land and 25% of flashes being cloud-to-ground, (3) an average rock fulgurite mass of 250 g with phosphorus content ranging from 0.0065 wt% P (komatiites) to 0.044 wt% P (flood basalts), (4) fractions of fulgurites experiencing high or mild reduction based on literature (5–10% highly reducing, converting 10–20% of P to phosphide; 25–50% mildly reducing, converting 25–50% of P to phosphite/hypophosphite). They generated order-of-magnitude estimates of annual masses of phosphide and intermediate reduced phosphorus species produced. For comparison, they estimated meteoritic reduced P flux using literature low/high flux scenarios, enstatite chondrite P content (0.169 wt%), 5–25% terrestrial impact frequency, and an assumed 5–50% survival or recondensation of phosphide through impacts.

• The clay-soil fulgurite contains abundant schreibersite (Fe3P) and native iron spherules, with schreibersite spherules ranging from ~10 to hundreds of micrometers in the core and up to tens of micrometers lining vesicles in the rim. SiC and amorphous graphitic carbon are present, indicating highly reducing conditions near the graphite–CO buffer (about 7 log units below IW) and temperatures >1600–2000 K, consistent with lightning heating (>3000 K).
• Bulk XRF shows lower P2O5 in fulgurite than in parent soil (soil 0.22 wt% P2O5; rim 0.11 wt%; core 0.16 wt%), attributed to a nugget effect from phosphorus migrating into schreibersite spherules. Relative to expected matrix composition, at least 55% of P in the rim and 35% in the core was reduced to phosphide. For ~25 kg of recovered fulgurite, an estimated 60–172.5 g of schreibersite formed.
• Graphitic carbon availability is the key limiting factor for extreme reduction; clay-rich soils with organic carbon favor schreibersite formation, while quartz sand and caliche fulgurites, though lacking graphite, still commonly show mild reduction to phosphite and hypophosphite (20–70% of P reduced in some cases).
• Early Earth modeling suggests lightning could have produced 10–1000 kg of phosphide and 100–10,000 kg of phosphite/hypophosphite annually during the Hadean and early Archean. Estimated lightning activity was on the order of 1–5 billion flashes per year, with a significant portion over land.
• Comparison with meteoritic inputs indicates that, given uncertainties in survival of reduced P during impacts (assumed 5–50%), lightning-sourced reduced phosphorus would have surpassed meteoritic sources after ~3.5 Ga as meteoritic flux declined while lightning rates stabilized with atmospheric pCO2.
• Reduced phosphorus from lightning would be concentrated on tropical, basaltic landmasses and could accumulate over time as fulgurites weather, providing a sustained, relatively non-destructive source of reactive P for prebiotic chemistry.

The findings demonstrate that lightning strikes can generate schreibersite and other reduced phosphorus species in terrestrial settings under realistically reducing, high-temperature conditions, offering an alternative or complementary mechanism to meteoritic delivery for supplying reactive phosphorus on early Earth. This pathway produces material directly on land where aqueous environments could dissolve and process phosphides and phosphites into phosphate, aided by UV and small amounts of HS−. Because meteorite flux decreases over time and survival of reduced P through impacts is uncertain, whereas lightning rates are tied to atmospheric conditions and are relatively steady once pCO2 stabilizes, lightning emerges as a significant and continuous terrestrial source, likely becoming dominant after ~3.5 Ga. The mechanism is less disruptive than impacts and spatially focuses resources in tropical volcanic regions, potentially fostering local concentration of prebiotic reagents. The concept extends to other Earth-like planets: given a lightning-rich atmosphere, exposed suitable lithologies, and an active hydrosphere, lightning could continually produce reactive phosphorus independent of meteorite flux.

Lightning strikes can facilitate substantial reduction of phosphorus in surface materials, forming schreibersite and intermediate reduced species within fulgurites under highly reducing, high-temperature conditions. Empirical evidence from a clay fulgurite shows abundant schreibersite formation (60–172.5 g in ~25 kg) with at least 35–55% of P reduced locally, and modeling indicates early Earth lightning could generate 10–1000 kg of phosphide and 100–10,000 kg of phosphite/hypophosphite annually. As meteoritic inputs waned, lightning likely became the predominant source of terrestrial reduced phosphorus after ~3.5 Ga, offering a relatively non-destructive, continuous supply conducive to prebiotic chemistry. This mechanism may operate on other Earth-like worlds with appropriate atmospheric and surface conditions. Potential future research could include systematic surveys of fulgurites across lithologies and climates to quantify global variance in reduced P production, refined models of early Earth lightning distributions and land fractions, experimental constraints on carbon generation from weathering rinds during strikes, and in situ weathering studies to determine release rates of reactive P from fulgurites.

The quantitative estimates are order-of-magnitude and rely on assumptions about early Earth pCO2, lightning rates, the fraction of lightning over land, and cloud-to-ground ratios extrapolated from modern observations. The reduction efficiencies and fulgurite masses/compositions are generalized from limited datasets and one modern clay-rich fulgurite, with strong heterogeneity (nugget effect) affecting bulk measurements. Availability of graphitic carbon is highly variable and critical for forming phosphide, potentially less abundant than in modern soils. Comparisons to meteoritic flux depend on poorly constrained survival or recondensation fractions of phosphide during impacts. Spatial distribution is likely heterogeneous, concentrated in specific tropical basaltic settings, affecting global applicability.