The Sulfur Cycle: A Simple Guide

We explain the 4 steps of the Sulfur Cycle as well as its impact on the environment. In particular we look at the pollution effects of acid rain (sulfur dioxide + water) - and show how chemical weathering of rocks by sulfuric acid rain releases carbon dioxide into the air, thus adding to global warming.
Coal Fired Power Station
The largest source of sulphur in the atmosphere is the burning of fossil fuels like coal. Photo: © Arnold Paul (CC 3.0)

The sulfur (sulphur) cycle, one of the major biogeochemical cycles, is a continuous circular process by which sulfur atoms move through the main departments of Planet Earth, being continually used and re-used in countless chemical reactions, that help to sustain life. Other chemical pathways include: the carbon cycle, the nitrogen cycle, the phosphorus cycle, the oxygen cycle and the water cycle.

Sulfur (S) is a tasteless, odorless and yellow-colored non-metallic element. It is referred to 15 times in the Bible, in which it was best known for destroying Sodom and Gomorrah (Genesis 19:24). It was mined near Mount Etna in Sicily and used for bleaching cloth and preserving wine. Most of the sulfur in the world is stored underground in soils, rocks and minerals, including sulfate salts buried in sediments deep under the ocean floor. 1

Why is Sulfur Important?

Sulfur is essential to all living things. It is a key constituent of certain amino acids, such as methionine and cysteine, which play an important role in the structure of proteins. It is also needed in some co-enzymes. It is also a minor constituent of body fluids, and skeletal minerals.

In plants and animals, sulfur is present in all polypeptides, proteins, and enzymes containing the amino acids methionine and cysteine. Hydrogen sulphide (H2S) replaces water in the photosynthesis performed by some bacteria.

Sulfur is the tenth most abundant element in the universe and the seventh or eighth most abundant element in the human body.

The 4 Steps of the Sulfur Cycle

In the sulfur cycle, sulfur moves through Earth’s main departments – the atmosphere (air), the hydrosphere (rivers, lakes, oceans), the pedosphere (soil and surface rock), and the lithosphere (deeper rocks). Along the way, it changes its form as necessary from a gas (e.g. sulfur dioxide, hydrogen sulfide) to a liquid (e.g. sulfuric acid) to a solid (various sulfates). Since the Earth and its atmosphere form a closed environment, the actual amount of sulfur in the system remains constant, although its form and location is always changing.

Sulfur Cycle: Step 1. In the Atmosphere

Atmospheric sulfur (sulphur) is mostly sulfur dioxide (SO2). This compound reacts easily with other chemicals to form other toxic sulfur compounds, such as sulfurous acid, sulfuric acid, and sulfate particles. Thus, it reacts with water (H2O) in rainfall to form a weak sulfuric acid (H2SO4), which falls as “acid rain”. One-third of all sulfur that reaches the atmosphere – including 90 percent of sulfur dioxide – stems from human activities. 2

In addition to SO2, there is a large group of less important gaseous sulfur oxides (SOx) including sulfur trioxide (SO3) which are found in the atmosphere in smaller quantities.

Another important sulfur gas is hydrogen sulfide (H2S), which is famous for smelling like rotten eggs. Hydrogen sulfide is flammable and highly toxic – more toxic than cyanide. Together with sulfur dioxide (SO2) and several other organo-sulfur gases, hydrogen sulfide as we shall see plays a key role in shaping the earth’s climate 3

Sulfur (sulphur) gases enter the atmosphere in three ways.

Fossil Fuel Combustion

The largest source of SO2 in the atmosphere is the burning of fossil fuels, such as coal, by electricity generating power plants. 4 Other sulfur-polluting industrial facilities include plants involved in the smelting of mineral ores that contain sulfur, such as aluminum, copper, zinc, lead, and iron.

Petroleum is another major source, as sulfur dioxide is also a significant presence in car exhaust emissions. Wood-burning is another source, notably in those developing countries where the use of fuelwood is widespread, as are ships that burn fuel with a high sulfur content.

Volcanic Eruptions

Volcanoes spew out huge amounts of gas when they erupt. Mt. Pinatubo, for example, is estimated to have released more than 250 million tons of gas into the upper atmosphere during the course of a single day. 5 Most of this volcanic gas is water vapor, which is harmless. However, significant amounts of sulfur compounds, such as sulfur dioxide and hydrogen sulfide are also emitted.

But volcanic eruptions may not be the biggest danger. A recent study that analyzed emissions data gathered by NASA’s Aura satellite, found that volcanoes collectively spew out 20-25 million tons of sulfur dioxide (SO2) annually. 6 Volcanologist Simon Carn, the lead author of the study, explained: “Volcanoes are continuously releasing quite large amounts of gas, and may do so for decades or even centuries.” “On average, volcanoes release most of their gas when they are not erupting.”

In addition, gases can often escape continuously into the air from fumaroles, geothermal vents and hydrothermal systems.

Decomposition of Organic Soil Matter

In addition to volcanic activity, another important source of hydrogen sulfide is decaying organic matter or minerals in the soil, in mangrove swamps, wetlands, tidal flats and similar environments in which anaerobic microorganisms thrive. As the marine microbes break down organic or inorganic forms of sulfur anaerobically, hydrogen sulfide is released to the air, where it becomes oxidized to produce sulfur dioxide, which returns to earth in rainfall. Here it is taken up again by plants to make amino acids, and duly travels through the food web and back to decomposition. Atmospheric hydrogen sulfide also originates from livestock production.

Human emissions of sulfur gases, which now exceed natural sulfur gas emissions, is estimated at 70–100 million tons per year. 7 Most of this sulfur is emitted as SO2, though approximately 3 million tons per year are released as H2S 8

Sulfur Cycle: Step 2. In the Biosphere

Sulfur (sulphur) enters the biosphere in three main ways: by direct fallout from the atmosphere, by precipitation, and by rock weathering. In all three cases, the sulfur eventually finds its way either into the soil, or into the ocean.

Fallout from Atmosphere

Sulfur particles (also called “aerosols“) are found both in the stratosphere and in the troposphere. Stratospheric sulfur particles are usually propelled into the high-altitude stratosphere by volcanic eruptions. Here, they produce a cooling effect, both by reflecting sunlight back into space. This cooling can persist for several years before the particles fall out. Eventually, when they do descend into the troposphere, they boost cloud formation by causing the number of cloud droplets to increase, but the droplet size to decrease. This makes the clouds denser, thus raising their albedo so they reflect more sunlight. 9

Sulfur pollution in the troposphere stems mostly from sulfate particles resulting from the incomplete combustion of coal and oil, and because of smoke from slash-and-burn methods of deforestation and wildfires exacerbated by climate change, such as the recent Australian bushfires and the extensive Arctic fires that engulfed large areas of Siberia, Alaska and Canada.

Precipitation

As we have seen, the most widespread form of atmospheric sulfur gas is sulfur dioxide (SO2). This comes from electricity power plants, factories, animal agriculture, and motor vehicle tailpipe emissions. As rain falls through the atmosphere, the SO2 is dissolved in the rainwater and forms weak droplets of sulfuric acid (H2SO4) between 0.1 to 1.0 microns (a millionth of a meter) in diameter. This form of precipitation is commonly known as “acid rain”.

Chemical Weathering

Another pathway for sulfur (sulphur) to find its way into the pedosphere is through chemical weathering. Acid rain helps to break down sulfur-containing rock surfaces. This process releases sulfur into the air where it is converted into sulfate (SO4). It then moves into the soil, or the water system.

Sulfur Cycle: Step 3. Plant and Animal Uptake

Plants and Soil

Once the sulfur (sulfate) arrives in the terrestrial biosphere, it is taken up by plants and microorganisms. If these life forms are consumed by animals, the sulfate inside them moves up through the food chain. If they, or the animals that consume them, die, their remains are broken down by various fungi and bacteria, and either remains in the soil or else is released back into the air as hydrogen sulfide (H2S) gas. (See also: Why is Soil So Important to the Planet?)

Ocean

Sulfur enters the ocean via the freshwater system of groundwater, streams, rivers and lakes runoff from land, or from underwater geothermal vents. In addition, some sulfur enters the ocean through fallout from the atmosphere. Some marine ecosystems rely on primary producers like chemoautotrophs (e.g. extremophiles, bacteria or archaea), who use sulfur as a biological energy source. This sulfur then supports marine ecosystems in the form of sulfates. 10

Some sulfur re-enters the atmosphere from the ocean. Sea spray, for instance, releases large amounts of sulfur from the ocean into the atmosphere.

Sulfur Cycle: Step 4. Lithification and Outgassing

Oceanic sulfur, like terrestrial sulfur, circulates through the food chain and other microbial chains. Sulfur that is not recycled falls into the depths, and combines with iron to form ferrous sulfide (FeS), which is responsible for the black colour of ocean sediments. Over geological time, these sediments are lithified and then moved to land by a process of geological uplift, after which their sulfur is outgassed into the atmosphere, or chemically weathered back into the biosphere.

Effects of Sulfur on the Environment

The effects of acid rain (sulfur dioxide + water) – one of the widespread environmental effects of fossil fuels – are well documented. It damages forests, lowers the pH of lakes, rivers and soils, killing insect and aquatic life-forms, causing corrosion of steel structures (e.g. bridges), and weathering of stone buildings and statues, as well as health impacts on humans.

Sulfur dioxide also reacts with ground level ozone to boost levels of photochemical smog – a nasty form of urban air pollution that mainly affects the northern hemisphere during the summer.

Effects of Sulfur on Climate Change

Sulfur dioxide (SO2) is well known in paleoclimatology for being strongly associated with episodes of climate change, including both warming and cooling. Large amounts of SO2 spewed out by volcanoes appear to massively boost the oxidizing capacity of the atmosphere, resulting in very rapid warming. Yet all major historic volcanic eruptions have generated sulfuric acid aerosols in the lower stratosphere that cooled the earth’s surface by around 0.5°C (0.9°F) for about three years. 11

Sulfur, in the form of sulfuric acid, has a major impact on chemical weathering of rocks, boosting global warming in the process. In mined areas, for example, sulfuric acid rain weathers carbonates from rock surfaces (like excavated mountaintops), thus releasing carbon dioxide into the atmosphere. Which means that, long after mining has stopped, anywhere between 20 and 90 percent of the carbon absorbed by plants on the surface will be nullified by the release of rock carbon dioxide into the atmosphere.

The reclaimed mountaintop removal coal mines of central Appalachia, for example, have exceptionally high weathering rates, with sulfuric acid weathering of carbonate rock accounting for more than 531,000 tons of CO2 emissions each year. 12

Effect of Hydrogen Sulfide on Plant Growth

The net effect of hydrogen sulfide on climate change is less clear. According to one study, the impact of atmospheric hydrogen sulfide (H2S) on plants and vegetation is strangely contradictory. On the one hand, H2S may negatively affect plant growth and survival. On the other hand, plants seem to be able to use the gas as a source of growth. H2S levels found in polluted regions can significantly contribute to plant growth. 13

To understand more about the timeline of our planet, its atmosphere and sulfur cycle, see: History of Earth in One Year (Cosmic Calendar).

References:

  1. Sulfur.” Royal Society of Chemistry. []
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  3. The future of airborne sulfur-containing particles in the absence of fossil fuel sulfur dioxide emissions.” Véronique Perraud, et al; PNAS November, 2015 112 (44) 13514-13519 []
  4. “What is SO2 and how does it get in the air?” []
  5. Volcanic gases can be harmful to health, vegetation and infrastructure.” []
  6. A decade of global volcanic SO2 emissions measured from space.” Carn, S., Fioletov, V., McLinden, C. et al. Sci Rep 7, 44095 (2017). []
  7. A global catalogue of large SO2 sources and emissions derived from the Ozone Monitoring Instrument.” Vitali E. Fioletov, et al; Atmos. Chem. Phys., 16, 11497–11519, 2016. []
  8. The last decade of global anthropogenic sulfur dioxide: 2000–2011 emissions.” Z Klimont, et al; Jan 2013. Environmental Research Letters, Volume 8, Number 1. []
  9. Impact of geoengineering schemes on the global hydrological cycle“. Bala, G.; Duffy, B.; Taylor, E. (June 2008). PNAS. 105 (22): 7664–7669. []
  10. LumenLearning.” []
  11. Sulfur dioxide initiates global climate change in four ways.” Peter L. Ward. ScienceDirect. Thin Solid Films. Volume 517, Issue 11, 2 April 2009, Pages 3188-3203. []
  12. Pyrite oxidation drives exceptionally high weathering rates and geologic CO2 release in mountaintop-mined landscapes.” Matthew R. V. Ross, et al; Global Biogeochemical Cycles, 2018; []
  13. Atmospheric H2S: Impact on Plant Functioning.” Ties Ausma, Luit J. De Kok. Front. Plant Sci., June 2019. []
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