The sulfur cycle is a natural process that moves sulfur between rocks, water, and living things. It is important in geology because it affects many types of minerals. In living things, sulfur is a necessary element (CHNOPS), meaning it is part of many proteins and important molecules. Sulfur compounds can help microbes breathe by acting as oxidants or reductants. The global sulfur cycle includes changes in sulfur's form as it moves through different stages, which are important for both geological and biological processes. Steps of the sulfur cycle are: These are often termed as follows:
Sulfur oxidation states
Sulfur exists in nature with several different oxidation states, including −2, −1, 0, +2, +2.5, +4, and +6. When two sulfur atoms are in the same type of ion but in different positions, such as in thiosulfate, the oxidation state of each sulfur atom can be calculated using the known charges of other atoms in the compound (hydrogen = +1, oxygen = −2). This calculation may result in a value that seems like an average, such as +2 in thiosulfate, or a value that is not a whole number, like +2.5 in tetrathionate. This happens because the sulfur atoms in the ion have different bonding abilities. The most common sulfur forms involved in the sulfur cycle, listed from the least oxidized to the most oxidized, are as follows:
Sulfur sources and sinks
Sulfur exists in different forms, with oxidation states ranging from +6 in sulfate (SO₄) to −2 in sulfides. Elemental sulfur can either lose or gain electrons depending on its surroundings. On early Earth, which had little oxygen, sulfur was mainly found in minerals like pyrite (FeS₂). Over time, volcanic activity and weathering of Earth’s crust in an oxygen-rich environment increased the amount of mobile sulfur. Today, the main place where sulfur is stored is in ocean water as sulfate (SO₄), which microorganisms use as an electron acceptor in areas without oxygen.
When organisms take in sulfate, they convert it into organic sulfur, a necessary part of proteins. However, the biosphere is not the main place where sulfur is stored. Instead, most sulfur is found in seawater or in sedimentary rocks, such as pyrite-rich shales, evaporite rocks (like anhydrite and baryte), and calcium and magnesium carbonates (such as carbonate-associated sulfate). The amount of sulfate in the oceans is influenced by three main processes:
- Sulfate entering the ocean from rivers
- Sulfate being reduced and sulfide being re-oxidized on continental shelves and slopes
- Sulfate being buried in anhydrite and pyrite within the oceanic crust.
The main natural source of sulfur in the atmosphere is sea spray or windblown dust containing sulfur. These sources do not remain in the atmosphere for long. In recent years, burning coal and other fossil fuels has added large amounts of sulfur dioxide (SO₂) to the air, which harms the environment. In Earth’s history, volcanic activity that caused coal deposits to burn released large amounts of sulfur into the atmosphere, leading to major climate changes and possibly contributing to the Permian–Triassic extinction event.
Dimethylsulfide (DMS), written as (CH₃)₂S, is created when a compound called dimethylsulfoniopropionate (DMSP) breaks down from dead phytoplankton cells in the ocean’s surface layer. DMS is the most important gas produced by living organisms in the sea and is responsible for the "smell of the sea" near coasts. DMS is the largest natural source of sulfur gas, but it stays in the atmosphere for only about one day. Most of it returns to the ocean instead of reaching land. However, DMS plays a key role in the climate system by helping form clouds.
Biologically and thermochemically driven sulfate reduction
Sulfate can be reduced through two processes: bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR). Both processes involve sulfate reacting with organic compounds to produce hydrogen sulfide (H₂S). The main reactants for both processes are organic compounds and dissolved sulfate. The main products or by-products include hydrogen sulfide, carbon dioxide, carbonates, elemental sulfur, and metal sulfides. However, the types of organic compounds involved differ because BSR and TSR occur under different temperature conditions. Organic acids are the main reactants for BSR, while branched or straight-chain alkanes are the main reactants for TSR. In both processes, the inorganic products are hydrogen sulfide (H₂S) and bicarbonate (HCO₃⁻, which becomes CO₂).
BSR and TSR occur in different temperature ranges. BSR typically happens at lower temperatures, from 0°C to 80°C. TSR occurs at much higher temperatures, usually between 100°C and 140°C. The lowest confirmed temperature for TSR is 127°C, and the highest temperatures for TSR are around 160°C to 180°C. At higher temperatures, most sulfate-reducing bacteria cannot survive because their proteins and enzymes break down, so TSR becomes the dominant process. However, in some hot sediment environments near hydrothermal vents, BSR can still occur at temperatures up to 110°C.
BSR and TSR also occur at different depths. BSR takes place in shallow, low-temperature environments such as oil and gas fields, modern marine sedimentary areas like stratified inland seas, continental shelves, organic-rich deltas, and hydrothermal sediments. These environments have high concentrations of dissolved sulfate from seawater. In oil and gas fields, hydrogen sulfide is often produced when sulfate reacts with petroleum hydrocarbons. While microbial processes can contribute to this, most of these reactions are caused by TSR, especially in deep or hot reservoirs. TSR occurs in deep reservoirs where temperatures are much higher. BSR happens quickly in most geological settings, while TSR occurs much more slowly, taking hundreds of thousands of years.
BSR and TSR are important parts of the oceanic sulfur cycle. About 10% of the total hydrogen sulfide produced in the ocean comes from BSR, while 90% comes from TSR. If a deep reservoir has more than a few percent hydrogen sulfide, it is likely that TSR has occurred. This is because thermal cracking of hydrocarbons does not produce more than 3% hydrogen sulfide. The amount of hydrogen sulfide produced depends on factors such as the availability of organic compounds and sulfate, as well as the presence of base and transition metals.
Microbial sulfur oxidation
Sulfide oxidation is carried out by bacteria and archaea in many different environments. In aerobic conditions, autotrophs use sulfide or elemental sulfur to convert carbon dioxide into organic compounds. The process involves creating intermediate sulfur forms, such as elemental sulfur and thiosulfate. When oxygen levels are low, microbes produce elemental sulfur, which collects as sulfur globules inside or outside their cells and is used when sulfur is scarce. To handle low oxygen levels (to find a place for electrons to go), sulfur-oxidizing bacteria like cable bacteria create long chains that connect oxygen-rich and sulfide-rich areas in coastal sediments. Bacteria in sulfide-rich zones oxidize sulfide and send electrons through multiple layers to bacteria in oxygen-rich zones, where oxygen is used to complete the process.
Anaerobic sulfide oxidation is done by phototrophs and chemotrophs. Green sulfur bacteria (GSB) and purple sulfur bacteria (PSB) use sulfide oxidation to power anoxygenic photosynthesis. Some PSB can also oxidize sulfide in the presence of oxygen and grow chemoautotrophically under low light. GSB cannot do this but have developed efficient systems to capture light. PSB live in environments like hot springs, alkaline lakes, and wastewater treatment plants. GSB are found in stratified lakes with high sulfur levels and can grow in hydrothermal vents using infrared light for photosynthesis.
Hydrothermal vents release hydrogen sulfide, which supports chemolithotrophic bacteria that oxidize hydrogen sulfide with oxygen to form elemental sulfur or sulfate. The reactions are:
In modern oceans, Thiomicrospira, Halothiobacillus, and Beggiatoa are main sulfur-oxidizing bacteria that form symbiotic relationships with animals. The host provides materials like carbon dioxide, oxygen, and water to the bacteria, while the bacteria produce organic carbon for the host. The sulfate produced often combines with calcium ions to form gypsum, creating deposits near mid-ocean ridges.
Sulfur-processing microbes often form close relationships with other microbes and animals. PSB and sulfate-reducing bacteria group together in "pink berries" in salt marshes, where sulfur is exchanged directly. Vestimentiferan tube worms near hydrothermal vents lack a digestive system but have trophosomes that house autotrophic, sulfide-oxidizing bacteria. The worms supply the bacteria with sulfide, and the bacteria share fixed carbon with the worms.
δ34S
There are 25 known isotopes of sulfur, but only four are stable and important for studying Earth's chemistry. Of these four, two—light sulfur (S) and heavy sulfur (S)—make up 99.22% of sulfur found on Earth. Most sulfur (95.02%) is in the form of light sulfur, while 4.21% is in heavy sulfur. The ratio of light and heavy sulfur has remained the same in the Solar System since its formation. Scientists believe Earth’s overall sulfur isotope ratio matches the ratio measured in the Canyon Diablo troilite (CDT), a type of meteorite. This ratio is used as the international standard and is set at δ = 0.00. Differences from this standard are measured as δ S, expressed in parts per thousand (‰). Positive δ S values mean more heavy sulfur is present, while negative values indicate more light sulfur in a sample.
When sulfur minerals form through non-living processes, the light and heavy isotopes are not significantly separated. Therefore, sulfur isotope ratios in minerals like gypsum or barite should match the isotope ratio of the water where they formed. However, when sulfate is reduced by living organisms, the light and heavy isotopes are strongly separated because enzymes in organisms react faster with light sulfur. Today, average δ S values in seawater are about +21‰.
Before the 2010s, scientists believed sulfate reduction could split sulfur isotopes by up to 46 parts per thousand. Larger splits in sediment layers were thought to result from sulfur disproportionation, a process that splits sulfur compounds. However, research since the 2010s shows sulfate reduction can split isotopes by up to 66 parts per thousand. Disproportionation has a smaller effect because it depends on limited sulfur compounds produced by sulfate reduction, typically less than 16 parts per thousand.
Throughout Earth’s history, the sulfur cycle and isotope ratios have changed together with life on Earth. As biological processes that reduce sulfate became more common, overall isotope ratios became more negative. However, there are also periods when isotope ratios became more positive. Positive shifts in sulfur isotopes usually indicate more pyrite (a type of sulfur mineral) was formed, rather than sulfide minerals on land being oxidized.
Marine sulfur cycle
The marine sulfur cycle is influenced by sulfate reduction because hydrogen sulfide is used by microbes to create energy or is broken down without life. Dissimilatory sulfate reduction happens when buried organic matter breaks down and when methane is oxidized without oxygen. Both processes create carbon dioxide. In areas where sulfate is low, methanogenesis is common. At the sulfate-methane transition zone (SMTZ), methane rises from below and is used by anaerobic archaea, which break it down using sulfate as an energy source. More sulfate is present at the SMTZ than methane. A 4:1 ratio of sulfate to methane is observed, and the extra sulfate helps break down organic matter. Groups of sulfate-reducing bacteria and methane-consuming archaea work together, sharing electrons directly through special structures.
Hydrogen sulfide created by sulfate reduction can be used by iron minerals to form iron sulfides and pyrite or by microbes to create energy or add sulfur to organic matter. Pyrite forms through two processes: one uses polysulfides, which require sulfur, and the other uses hydrogen sulfide when sulfur is no longer available. Microbes that oxidize sulfur use different oxidants depending on depth. In upper sediment layers, oxygen and nitrate are preferred because they produce more energy. In deeper suboxic zones, iron and manganese take their place. Sulfide oxidation creates sulfur compounds like elemental sulfur, thiosulfate, sulfite, and sulfate. These sulfur compounds are unique to this process and can indicate sulfide oxidation in environmental samples. Differences in sulfur isotope ratios among these compounds and other sulfur types have helped scientists study sulfide oxidation.
The marine sulfur cycle has been studied using sulfur isotope measurements, shown as δS. Today, the oceans hold about 1.3 × 10 kg of sulfur, mostly as sulfate with a δS value of +21‰. Sulfur enters the oceans at a rate of 1.0 × 10 kg per year, with a δS value of ~3‰. Most sulfur comes from rivers, which carry sulfate from the weathering of sulfide minerals (δS = +6‰). Other sources include volcanic activity, metamorphism, and hydrothermal vents (δS = 0‰), which release reduced sulfur like hydrogen sulfide (H₂S) and sulfur (S). Two main ways sulfur leaves the oceans: one is the burial of sulfate as marine evaporites (like gypsum) or in carbonate minerals, which removes 6 × 10 kg per year (δS = +21‰). The other is the burial of pyrite in shelf or deep-sea sediments, removing 4 × 10 kg per year (δS = −20‰). The total sulfur leaving the oceans (1.0 × 10 kg per year) matches the amount entering, showing the modern sulfur cycle is stable. Sulfur stays in the oceans for about 13,000,000 years.
Adding sulfur to organic matter is a major sulfur storage in marine sediments, holding 35–80% of reduced sulfur. These sulfur-containing organic molecules can be broken down to release sulfite and sulfate. This breakdown may help decompose organic matter, deciding whether it is used or buried. Adding sulfur increases the size of organic molecules and adds a new part, which might prevent enzymes from recognizing and breaking them down. The ability of microbes to break down sulfur is shown by the presence of sulfatase genes.
Evolution of the sulfur cycle
The chemical makeup of sulfur in sedimentary rocks gives important information about how sulfur has changed over Earth's history.
The total amount of sulfur on Earth's surface is nearly 10 kg. This amount comes from sulfur released through geological processes over time. Rocks studied for sulfur content are often rich in organic material, meaning their sulfur is likely influenced by biological processes that reduce sulfur. Average seawater sulfur is studied using evaporite deposits from different time periods because these deposits do not favor heavy or light sulfur atoms, so they reflect the ocean's sulfur composition at the time they formed.
Earth formed 4.6 billion years ago with a theoretical sulfur isotope ratio (δ S) of 0. Since there was no life on early Earth, sulfur would not have been split into different forms. All sulfur in the atmosphere came from volcanic eruptions. When oceans formed, sulfur gases dissolved in water, removing them from the atmosphere. During most of the Archean (4.6–2.5 billion years ago), sulfur in the ocean was limited. Some small Archean evaporite deposits suggest that in certain areas, sulfate levels were high enough to form these deposits.
The oldest rocks on Earth, from 3.8–3.6 billion years ago, show no sulfur isotope changes because life had not developed enough to alter sulfur. At 3.5 billion years ago, anoxyogenic photosynthesis began, adding small amounts of sulfate to the ocean. Sulfur isotope ratios remained near 0. Soon after, at 3.4 billion years ago, rocks show early signs of sulfur isotope changes linked to sulfur from volcanic sources, possibly due to bacteria.
At 2.8 billion years ago, the first evidence of oxygen production through photosynthesis appeared. This is important because sulfur cannot be changed into other forms without oxygen. This shows how oxygen and sulfur cycles, along with life, evolved together.
Between 2.7–2.5 billion years ago, the oldest sedimentary rocks with reduced sulfur isotope values appeared, offering clear evidence of sulfate reduction.
At 2.3 billion years ago, sulfate levels in the ocean rose above 1 mM. This increase coincides with the "Great Oxygenation Event," when Earth's surface shifted from low to high oxygen levels. This change likely increased sulfate in the oceans through weathering. For the first time, large sulfur isotope changes linked to bacteria appeared. Although sulfate levels rose, they were still less than 5–15% of today's levels.
At 1.8 billion years ago, Banded Iron Formations (BIFs) were common, made of alternating layers of iron oxides and chert. These rocks form only when water is rich in dissolved iron but lacks oxygen or sulfur, which would otherwise form rust or pyrite. BIFs may have formed when early photosynthetic organisms overproduced oxygen, leading to their own decline and a rise in carbon dioxide. After 1.8 billion years ago, sulfate levels became high enough to increase sulfate reduction faster than iron entered the ocean.
The end of the Paleoproterozoic marks the first large-scale sedimentary deposits linked to increased sulfate in the ocean. Sulfate levels in the Proterozoic were higher than in the Archean but still lower than today's. Sulfate levels also reflect atmospheric oxygen because sulfate forms through weathering in the presence of oxygen. Lower Proterozoic sulfate levels suggest atmospheric oxygen was between Archean and Phanerozoic levels.
750 million years ago, BIFs reappeared, indicating a major change in ocean chemistry. This likely occurred during "snowball Earth" events, when ice covered the globe, blocking oxygen. In the late Neoproterozoic, increased carbon burial raised atmospheric oxygen to over 10% of today's levels. Another major oxygen event later allowed for an oxygen-rich deep ocean and possibly multicellular life.
Over the last 600 million years, seawater sulfur isotope ratios (δ S) have generally ranged between +10‰ and +30‰, similar to today's values. Major changes in δ S occurred during extinction and climate events.
On shorter timescales (10 million years), sulfur cycle changes are easier to study using oxygen isotopes. Oxygen enters the sulfur cycle through sulfate oxidation and is released when sulfate is reduced. Different sulfate sources in the ocean have distinct oxygen isotope values, allowing scientists to trace sulfur processes. Biological sulfate reduction favors lighter oxygen isotopes, similar to how lighter sulfur isotopes are preferred. Studies of oxygen isotopes in ocean sediments from the last 10 million years show that changes in sea level during Pliocene and Pleistocene glacial cycles altered continental shelf areas, disrupting sulfur processing and lowering sulfate levels. This was a major change compared to preglacial times before 2 million years ago.
The Great Oxidation Event and sulfur isotope mass-independent fractionation
The Great Oxygenation Event (GOE) is marked by the end of sulfur isotope mass-independent fractionation (MIF) in ancient rock layers around 2.45 billion years ago. MIF refers to unusual patterns in sulfur isotope measurements (Δ S) that differ from what is expected based on standard rules about how sulfur splits during chemical processes. The GOE caused a major change in Earth's sulfur cycle. Before the GOE, sulfur processes were strongly affected by ultraviolet (UV) light and chemical reactions in the atmosphere, which created these unusual sulfur patterns (Δ S ≠ 0). These patterns only form when oxygen levels in the atmosphere are less than 10 times today's levels. The end of MIF around 2.45 billion years ago shows that oxygen levels in the atmosphere rose above 10 times today's levels after the GOE. After the GOE, oxygen became important in sulfur processes, such as breaking down sulfur compounds through reactions with oxygen. The burial of pyrite (a type of sulfur mineral) in sediments helped increase oxygen levels in Earth's surface environment.
Economic importance
Sulfur plays an important role in the formation of fossil fuels and many metal deposits because it helps in chemical reactions that either add or remove oxygen. Most major mineral deposits on Earth contain significant amounts of sulfur. These deposits include sedimentary exhalative deposits (SEDEX), Carbonate-hosted lead-zinc ore deposits (Mississippi Valley-Type MVT), and porphyry copper deposits. Iron sulfides, galena, and sphalerite can form as by-products when hydrogen sulfide is created, as long as the necessary metals are present or carried to a site where sulfate is reduced. If a system runs out of reactive hydrocarbons, sulfur deposits that can be used economically may form. Sulfur also helps reduce oxygen in natural gas reservoirs, and the fluids that form ores often connect to ancient hydrocarbon seeps or vents.
Sulfur in ores usually comes from deep underground sources, but it can also come from nearby rocks, seawater, or marine evaporites. Whether sulfur is present or not affects how much of precious metals can collect and form deposits. Factors like pH, temperature, and the balance of oxygen and other elements determine if sulfides will form. Most sulfide-rich fluids stay concentrated until they reach conditions with less oxygen, higher pH, or lower temperatures.
Ore-forming fluids are often linked to water rich in metals that has been heated in a sedimentary basin under high temperatures, usually in areas where the Earth's crust is stretched. The oxygen balance in the surrounding rocks influences the oxygen balance of the fluids that carry metals. Deposits can form from both oxygen-rich and oxygen-poor fluids. Since metal-rich fluids usually have little sulfur, sulfur must come from another source during mineral formation. Bacterial reactions in seawater or in water with no oxygen and high hydrogen sulfide levels provide this sulfur. When present, the sulfur in barite often matches the sulfur in seawater, showing that barite forms when hot water containing barium reacts with seawater sulfate.
After fossil fuels or precious metals are found and either burned or processed, sulfur becomes a waste product that must be handled carefully to avoid pollution. Burning fossil fuels has increased sulfur levels in the atmosphere today. Sulfur can be both a harmful pollutant and a valuable resource.
Human impact
Human activities greatly affect the global sulfur cycle. Burning coal, natural gas, and other fossil fuels has increased the amount of sulfur in the atmosphere and oceans while reducing the amount stored in rocks. Without human influence, sulfur would remain in rocks for millions of years until tectonic movements and erosion released it slowly. However, humans are now extracting and burning sulfur-containing fuels at a much faster rate. In highly polluted areas, sulfur levels in the environment have increased by 30 times compared to natural levels.
Although sulfur levels in Earth’s history have changed between periods of oxidation and reduction, the current human impact is likely the largest in Earth’s history. Human actions increase the movement of sulfur into the atmosphere, some of which spreads globally. Humans are extracting coal and petroleum from Earth’s crust at a rate that moves 150 × 10 gS/yr, more than double the rate from 100 years ago. This activity increases the amount of oxidized sulfur (SO₄) in the global cycle, reducing the amount of reduced sulfur stored in Earth’s crust. While human activities do not change the total amount of sulfur in the world, they greatly increase how much sulfur moves through the atmosphere each year.
When sulfur dioxide (SO₂) is released into the air as pollution, it reacts with water to form sulfuric acid. Once sulfuric acid fully dissolves in water, the pH can drop to 4.3 or lower, harming both natural and human-made systems. The EPA defines acid rain as a mix of wet and dry deposits from the atmosphere that contain higher-than-normal amounts of nitric and sulfuric acids. Distilled water, which has no dissolved substances, has a neutral pH of 7. Natural rain has a slightly acidic pH of 5.6 because carbon dioxide in the air reacts with water to form carbonic acid. In Washington, D.C., rain has an average pH of 4.2 to 4.4. pH is measured on a logarithmic scale, so a drop of 1 (from 5.6 to 4.6) makes the acid much stronger. In the United States, about two-thirds of sulfur dioxide and one-fourth of nitrate pollution come from burning fossil fuels for electricity.
Sulfur is an important nutrient for plants and is used in fertilizers. Recently, sulfur shortages have become common in many European countries. Efforts to reduce acid rain have decreased sulfur inputs into the environment. If sulfur fertilizers are not used, the lack of sulfur is likely to grow worse.