Global Abiotic Sulfur Cycling on Earth-like Terrestrial Planets

This paper presents an open-source dynamical box model to simulate global abiotic sulfur cycling on Earth-like planets, revealing that the absence of life would result in marine sediment sulfate concentrations two orders of magnitude higher and sulfide concentrations four orders of magnitude lower than on present-day Earth.

The Gist

Imagine the Earth as a giant, bustling kitchen where the ingredients for life are constantly being shuffled between the pantry, the fridge, the stove, and the trash can. One of the most important ingredients in this kitchen is Sulfur.

Sulfur is like a versatile chef's knife: it can cut, chop, and change forms easily. It exists in different "flavors" (chemical states): some are "reduced" (like raw, smelly sulfur) and some are "oxidized" (like the clean, white powder in Epsom salts). On our current Earth, life (bacteria, plants, animals) acts as a super-efficient sous-chef, constantly moving these sulfur flavors around, keeping the kitchen balanced.

But what if this kitchen had no chefs? What if the Earth had never developed life? Would the sulfur still be distributed the same way?

This is the question a team of scientists asked in a new study. They built a digital simulation—a "virtual kitchen"—to model how sulfur moves around a planet that looks like Earth but has zero life.

The Virtual Kitchen: How the Model Works

The scientists created a computer program that tracks sulfur as it moves between different "rooms" in the planetary house:

• The Atmosphere: The air above.
• The Oceans: The big water bowl.
• The Crust: The rocky floor (both under the ocean and the continents).
• The Mantle & Core: The deep, hot basement and the very center of the planet.

They simulated this kitchen over 4.5 billion years (the entire history of Earth). They ran two main scenarios:

1. The "Ancient" Kitchen: Before the atmosphere had much oxygen (like early Earth).
2. The "Modern" Kitchen (but empty): After the atmosphere became oxygen-rich, but without any bacteria or plants to help move the sulfur.

The Big Surprise: A Very Different Kitchen

The results were shocking. If you took all the life off Earth, the sulfur would look completely different than it does today.

1. The "Smelly" vs. "Clean" Balance

• On Real Earth: Life loves to eat "reduced" sulfur (the smelly kind) and turn it into "oxidized" sulfur. Because of this, our ocean sediments (the mud at the bottom) are full of reduced sulfur (like pyrite, or "fool's gold").
• On a Lifeless Earth: Without bacteria to eat the smelly sulfur, it doesn't get processed. Instead, the sulfur stays in its "clean" (oxidized) form. The model predicts that the mud at the bottom of a lifeless ocean would be 10,000 times richer in oxidized sulfur and 10,000 times poorer in reduced sulfur than our current oceans.

Analogy: Imagine a laundry room. On Earth, life is the washing machine that scrubs the dirt (reduced sulfur) out of the clothes. On a lifeless Earth, there is no washing machine. The dirt just piles up in a different way, or rather, the "clean" clothes pile up because nothing is there to get them dirty again.

2. The "Great Oxidation" Effect
The scientists also looked at what happens when a planet gets oxygen in its air (like the Great Oxidation Event on Earth, which happened about 2.4 billion years ago).

• With Life: Oxygen helps bacteria work faster, creating a complex cycle.
• Without Life: Even if the air becomes oxygen-rich, the sulfur cycle changes drastically. The oxygen in the air would rust the sulfur rocks on land (a process called "oxidative weathering"), washing huge amounts of clean sulfur into the ocean. Without life to turn it back into smelly sulfur, the ocean would become a giant reservoir of clean, oxidized sulfur.

Why Does This Matter? (The Alien Detective)

Why should we care about a lifeless Earth? Because we are looking for life on other planets (exoplanets).

When astronomers look at distant worlds, they use telescopes to sniff the air for "biosignatures"—chemical clues that say, "Hey, life is here!" Sulfur is a big clue.

• If we see a planet with a lot of smelly sulfur gas (like hydrogen sulfide) in the air, we might think, "Maybe there are bacteria making that!"
• But this paper says: "Wait a minute. If that planet has no oceans or no life, the sulfur might just be stuck there naturally."

The study suggests that if we find a planet with a sulfur cycle that looks too simple or too different from Earth's, it might actually be a sign that life is missing, not that it's present. It helps us avoid "false alarms."

The Takeaway

Think of Earth's sulfur cycle as a complex dance. Life is the lead dancer, guiding the steps and keeping the rhythm. If you remove the lead dancer, the music (the chemistry) doesn't stop, but the dance changes completely. The sulfur ends up in different places, in different amounts.

By understanding how the dance looks without the lead dancer, we can better recognize when we see a real dancer (life) on a stage light-years away. This model is essentially a "control group" for the universe, helping us figure out what a living planet actually looks like compared to a dead one.

Technical Summary

Here is a detailed technical summary of the paper "Global Abiotic Sulfur Cycling on Earth-like Terrestrial Planets."

1. Problem Statement

Sulfur is a critical redox-active element essential for life and a key component of planetary geochemical cycles. However, current understanding of the global sulfur cycle is heavily biased by the presence of life on Earth, particularly microbial processes like sulfate reduction. There is a significant gap in understanding how sulfur cycles on Earth-like terrestrial planets in the absence of life (abiological scenarios).

The authors aim to:

• Quantify the distribution of sulfur reservoirs (atmosphere, oceans, crust, mantle, sediments) on a lifeless Earth-like planet over 4.5 billion years.
• Determine how the absence of biological metabolism (specifically microbial sulfate reduction) alters the redox state and abundance of sulfur species compared to the modern Earth.
• Establish baseline "abiological" models to serve as controls for detecting biosignatures on exoplanets. Specifically, they investigate how a Great Oxidation Event (GOE)-like transition, even if driven by abiotic mechanisms, would alter sulfur cycling.

2. Methodology

The authors developed an open-source, dynamical box model to simulate global sulfur cycling over geological timescales.

• Model Structure: A "box-and-arrow" model tracking two primary redox states:
  • $S_{red}$: Reduced sulfur (dominated by sulfide, $S^{2-}$, and elemental sulfur).
  • $S_{ox}$: Oxidized sulfur (dominated by sulfate, $S^{6+}$).
• Reservoirs: The model tracks mass fluxes between seven distinct reservoirs:
  1. Atmosphere (ATM)
  2. Oceans (Oce)
  3. Oceanic Crust (OC)
  4. Continental Crust (CC)
  5. Marine Sediments (MS)
  6. Upper Mantle (UM)
  7. Lower Mantle (LM)
• Key Processes Modeled:
  • Extraterrestrial (ET) Input: Modeled as an exponentially decaying flux of meteorites/comets.
  • Volcanism: Includes Mid-Ocean Ridge (MOR), Arc, and Hotspot volcanism, with sulfur enrichment factors and exponential decay of volcanic output over time.
  • Subduction: A three-way partition of subducted material: accretion to continental crust, arc volcanism (releasing $S_{ox}$ to the atmosphere), and deep subduction to the mantle (where $S_{ox}$ is reduced to $S_{red}$).
  • Weathering & Erosion: Riverine erosion of continental crust. Crucially, the model distinguishes between Oxidatively Weathered Sulfide (OWS) and non-oxidized sulfide.
  • Atmospheric Rainout: Rapid removal of volcanic $SO_2$ via photochemical reactions (modeled with short timesteps relative to geological steps).
  • Oceanic Processes: Includes solubility limits, barite ($BaSO_4$) precipitation (a sink for low-concentration sulfate), and hydrothermal circulation (removing sulfate to the oceanic crust as anhydrite).
• Simulation Scenarios:
  1. No-OWS Scenario (Pre-GOE mimic): Assumes no oxidative weathering of sulfides; eroded $S_{red}$ reaches the ocean as reduced species.
  2. High-OWS Scenario (Post-GOE mimic): Assumes 99.99% of eroded sulfide is oxidized to sulfate ($S_{ox}$) before reaching the ocean.
  3. Random Seeding: Initial conditions for reservoir masses were randomized to test convergence to steady states.
  4. Parameter Sweeping: Sensitivity analysis on erosion rates, subduction timescales, barite precipitation, and volcanic outgassing efficiency.

3. Key Contributions

• First Abiotic Sulfur Cycle Model: This is the first comprehensive dynamical model specifically designed to simulate the global sulfur cycle on an Earth-like planet without biological intervention, extending previous work on nitrogen and phosphorus cycles.
• Dual-Configuration Approach: The model explicitly compares pre-GOE (reducing) and post-GOE (oxidizing) abiotic environments, isolating the effect of surface oxidation on sulfur speciation without confounding biological factors.
• Quantification of Biological Impact: By comparing the model's abiotic steady states with Present Earth Levels (PEL), the study quantifies the specific magnitude of life's impact on sulfur distribution.
• Exoplanet Biosignature Framework: The results provide a theoretical baseline for interpreting sulfur species in exoplanet atmospheres, helping to distinguish between abiotic false positives and true biosignatures.

4. Key Results

• Convergence to Steady State: Despite wide variations in initial conditions, the model converges to stable sulfur distributions within ~500 million to 2.5 billion years, suggesting that abiotic geochemical processes alone can establish a steady-state sulfur cycle.
• Drastic Differences in Marine Sediments:
  • Abiotic Scenario: In the absence of life, marine sediments are predicted to contain two orders of magnitude more sulfate ($S_{ox}$) and four orders of magnitude less sulfide ($S_{red}$) compared to modern Earth.
  • Cause: This is attributed to the lack of microbial sulfate reduction (MSR), which on Earth converts vast amounts of sedimentary sulfate back into sulfide. Without MSR, oxidized sulfur accumulates in sediments.
• Ocean Stability: The ocean reservoir acts as a transient, saturated system. Its mass stabilizes quickly (<100 Myr) and remains within 10% of present-day values for $S_{ox}$, driven by the balance between erosion inputs and precipitation/sedimentation sinks.
• Atmospheric Deviations: The abiotic atmospheric $S_{ox}$ mass is approximately half of the present-day Earth estimate, likely due to the lack of biogenic sources and different weathering dynamics.
• Sensitivity to Erosion and OWS:
  • The model is highly sensitive to erosion rates and the efficiency of Oxidative Weathering of Sulfide (OWS).
  • The onset of OWS (simulating a GOE-like transition) causes a massive transfer of sulfur from the continental crust to marine sediments in the form of sulfate, depleting crustal $S_{red}$ and enriching sedimentary $S_{ox}$.
• Role of Hydrothermal Circulation: High hydrothermal circulation rates can significantly deplete oceanic sulfate, shifting the system from being precipitation-dominated to hydrothermal-sink-dominated.

5. Significance

• Biosignature Detection: The study demonstrates that the redox state of sulfur in marine sediments is a potent biosignature. A planet with high sedimentary sulfate and low sulfide (relative to abiotic expectations) might indicate a lack of life, whereas a planet with high sulfide suggests active biological reduction.
• Exoplanet Interpretation: High abundances of low-oxidation state sulfur gases (like $H_2S$) in an exoplanet's atmosphere may not necessarily indicate life; they could be transient features on planets lacking liquid water oceans or efficient hydrological cycles (e.g., Venus-like scenarios). Conversely, the presence of liquid water oceans tends to sequester sulfur into the ocean and sediments, lowering atmospheric concentrations.
• Planetary Evolution: The model highlights that the "Great Oxidation Event" fundamentally altered the abiological sulfur cycle by enabling oxidative weathering, independent of whether the oxygen was produced biologically or abiotically.
• Methodological Advance: The open-source nature of the code allows the scientific community to adapt the model for different planetary parameters (e.g., different stellar insolation, atmospheric composition, or tectonic regimes), facilitating broader comparative planetology.

In conclusion, the paper establishes that biology is the primary driver of the reduced sulfur inventory in Earth's marine sediments. Without life, an Earth-like planet would possess a sulfur cycle dominated by oxidized species in sediments, providing a distinct chemical fingerprint for distinguishing living from non-living terrestrial worlds.