9.3 Sulfur cycle
10 min read•august 21, 2024
The sulfur cycle plays a crucial role in Earth's biogeochemistry, influencing climate, ecosystems, and geological processes. This topic explores the various reservoirs of sulfur, including atmospheric, oceanic, terrestrial, and lithospheric components, and their interactions through biogeochemical processes.
Sulfur isotopes serve as powerful tools for tracing these processes and reconstructing past environmental conditions. The notes cover stable sulfur isotopes, fractionation mechanisms, and analytical techniques used in isotope geochemistry. Applications range from paleoclimate reconstruction to ore deposit exploration and microbial ecology studies.
Sulfur reservoirs
- Sulfur reservoirs play a crucial role in the global sulfur cycle, influencing isotope geochemistry and biogeochemical processes
- Understanding sulfur reservoirs provides insights into the distribution and movement of sulfur through various Earth systems
- Sulfur reservoirs interact dynamically, affecting the overall sulfur budget and isotopic composition in different environments
Atmospheric sulfur
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- Comprises primarily sulfur dioxide (SO2) and hydrogen (H2S) gases
- Atmospheric sulfur concentrations vary globally, ranging from ~0.1 to 1 ppb in remote areas to >10 ppb in polluted regions
- Sulfur aerosols form through oxidation of sulfur gases, impacting climate and air quality
- Residence time of atmospheric sulfur ranges from days to weeks, depending on chemical form and meteorological conditions
Oceanic sulfur
- Largest sulfur reservoir on Earth's surface, containing ~1.3 x 10^21 g of sulfur
- (SO4^2-) dominates oceanic sulfur, with concentrations of ~28 mM in modern seawater
- Dissolved organic sulfur compounds contribute to the oceanic sulfur pool (dimethylsulfide)
- Oceanic sulfur plays a critical role in marine biogeochemistry and climate regulation
Terrestrial sulfur
- Includes sulfur in soils, freshwater systems, and terrestrial biota
- Soil sulfur exists in organic and inorganic forms, with concentrations varying widely (10-1000 mg/kg)
- Freshwater sulfate concentrations range from <1 mg/L in pristine systems to >1000 mg/L in contaminated areas
- Terrestrial plants assimilate sulfur, incorporating it into amino acids and other organic compounds
Lithospheric sulfur
- Largest sulfur reservoir on Earth, containing ~2.9 x 10^22 g of sulfur
- Sulfur-bearing minerals include sulfides (, sphalerite) and sulfates (, anhydrite)
- Igneous rocks contain an average of 300-400 ppm sulfur
- Sedimentary rocks show wide variations in sulfur content, with evaporites containing up to 40% sulfur by weight
Sulfur isotopes
- Sulfur isotopes serve as powerful tools in isotope geochemistry for tracing biogeochemical processes and environmental conditions
- The study of sulfur isotopes provides insights into the sulfur cycle, microbial activity, and paleoenvironmental reconstructions
- Sulfur isotope analysis has applications in various fields, including geology, ecology, and environmental science
Stable sulfur isotopes
- Four stable isotopes of sulfur: ^32S (95.02%), ^33S (0.75%), ^34S (4.21%), and ^36S (0.02%)
- ^32S and ^34S are the most commonly used isotopes in geochemical studies
- Isotope ratios expressed as ^34S/^32S, with variations typically reported in delta notation
- Natural abundance variations of sulfur isotopes range from -50‰ to +100‰ relative to the Vienna Canyon Diablo Troilite (VCDT) standard
Sulfur isotope fractionation
- Occurs during physical, chemical, and biological processes, leading to variations in isotope ratios
- Equilibrium fractionation results from differences in bond strengths between isotopes in different compounds
- Kinetic fractionation arises from differences in reaction rates for different isotopes
- Biological fractionation, particularly during microbial , can produce large isotope effects (up to 70‰)
- Temperature dependence of fractionation factors allows for paleothermometry applications
Delta notation for sulfur
- Expresses the relative difference in isotope ratios between a sample and a standard
- Calculated using the formula: δ^34S = [(^34S/^32S)sample / (^34S/^32S)standard - 1] x 1000
- Results reported in parts per thousand (‰) relative to the VCDT standard
- Positive δ^34S values indicate enrichment in ^34S relative to the standard
- Negative δ^34S values indicate depletion in ^34S relative to the standard
Biogeochemical processes
- Biogeochemical processes drive the cycling of sulfur through Earth's systems, influencing isotope geochemistry and environmental conditions
- Understanding these processes is crucial for interpreting sulfur isotope data and reconstructing past environments
- Microbial activity plays a significant role in sulfur cycling and isotope fractionation
Sulfur assimilation
- Uptake and incorporation of sulfur into organic compounds by organisms
- Plants and microorganisms assimilate sulfate through active transport mechanisms
- Reduction of sulfate to sulfide occurs before incorporation into amino acids (cysteine, methionine)
- Assimilatory sulfate reduction typically produces small isotope fractionations (<5‰)
- Sulfur assimilation influences the distribution of sulfur isotopes in terrestrial and marine ecosystems
Sulfate reduction
- Microbial process that reduces sulfate to sulfide under anaerobic conditions
- Dissimilatory sulfate reduction carried out by sulfate-reducing bacteria and archaea
- Occurs in (marine sediments, wetlands, anaerobic digesters)
- Produces large sulfur isotope fractionations, typically ranging from 20‰ to 70‰
- Fractionation factors influenced by sulfate concentration, electron donor availability, and microbial community composition
Sulfide oxidation
- Conversion of reduced sulfur compounds (H2S, FeS2) to oxidized forms (SO4^2-)
- Can occur through abiotic processes (reaction with O2) or microbial activity (sulfur-oxidizing bacteria)
- Produces smaller isotope fractionations compared to sulfate reduction (<5‰)
- Important in the recycling of sulfur in marine and terrestrial environments
- Contributes to the formation of acid mine drainage in mining-impacted areas
Disproportionation reactions
- Simultaneous oxidation and reduction of sulfur compounds with intermediate oxidation states
- Elemental sulfur (S^0) can disproportionate to sulfate and sulfide
- Carried out by specialized microorganisms (Desulfocapsa, Desulfobulbus)
- Produces distinct isotope fractionation patterns, with ^34S-enriched sulfate and ^34S-depleted sulfide
- Plays a role in sulfur cycling in marine sediments and hydrothermal systems
Sulfur cycle components
- The sulfur cycle encompasses various components that transfer sulfur between different reservoirs
- Understanding these components is essential for interpreting sulfur isotope data in isotope geochemistry
- The sulfur cycle interacts with other biogeochemical cycles, influencing global climate and environmental conditions
Volcanic emissions
- Major natural source of sulfur to the atmosphere, releasing ~10 Tg S/year
- Emit primarily sulfur dioxide (SO2) and hydrogen sulfide (H2S)
- Volcanic sulfur has δ^34S values ranging from -5‰ to +5‰, reflecting mantle-derived sulfur
- Explosive eruptions can inject sulfur into the stratosphere, impacting global climate
- Volcanic emissions contribute to the formation of sulfate aerosols and
Weathering of sulfur minerals
- Release of sulfur from rocks and minerals through physical and chemical processes
- Oxidation of sulfide minerals (pyrite) produces sulfuric acid and sulfate
- Weathering of evaporite deposits releases sulfate to surface and groundwater
- Contributes to the dissolved sulfate load in rivers and oceans
- Weathering rates influenced by climate, topography, and rock type
Marine sulfur cycling
- Complex interplay of biological, chemical, and physical processes in the ocean
- Sulfate reduction in marine sediments produces isotopically light sulfide
- Sulfide can be reoxidized, precipitated as pyrite, or released to the water column
- Organic sulfur compounds (dimethylsulfide) play a role in climate regulation
- Marine sulfur cycling influences the isotopic composition of seawater sulfate over geological time
Atmospheric sulfur deposition
- Transfer of sulfur from the atmosphere to terrestrial and aquatic ecosystems
- Includes wet deposition (rain, snow) and dry deposition (particles, gases)
- Sulfur deposition rates vary globally, influenced by emissions sources and meteorology
- Can lead to soil and water acidification in sensitive ecosystems
- Atmospheric deposition is an important source of sulfur for terrestrial plants in some regions
Anthropogenic impacts
- Human activities have significantly altered the global sulfur cycle, affecting isotope geochemistry and environmental conditions
- Anthropogenic sulfur emissions have increased the flux of sulfur through atmospheric and terrestrial reservoirs
- Understanding these impacts is crucial for interpreting modern sulfur isotope data and assessing environmental change
Industrial sulfur emissions
- Major source of atmospheric sulfur, primarily from fossil fuel combustion
- Global anthropogenic sulfur emissions peaked at ~70 Tg S/year in the 1980s
- Coal combustion accounts for ~50% of anthropogenic sulfur emissions
- Industrial emissions have distinct isotopic signatures, often depleted in ^34S
- Emission control technologies have reduced sulfur emissions in many developed countries
Acid rain formation
- Results from the oxidation of sulfur dioxide to sulfuric acid in the atmosphere
- Lowers the pH of precipitation, typically to values between 4.0 and 5.6
- Impacts terrestrial and aquatic ecosystems, causing soil and water acidification
- Accelerates weathering of buildings and monuments (marble, limestone)
- Acid rain has led to widespread environmental damage in North America and Europe
Ocean acidification
- Increase in seawater acidity due to absorption of atmospheric CO2
- Affects the marine sulfur cycle by altering sulfate reduction rates and organic matter preservation
- May influence the isotopic composition of seawater sulfate over time
- Impacts marine calcifying organisms and ecosystem functioning
- Potential feedback effects on dimethylsulfide production and climate regulation
Sulfur in paleoenvironments
- Sulfur isotopes provide valuable information about past environmental conditions and biogeochemical processes
- Studying sulfur in paleoenvironments helps reconstruct Earth's atmospheric and oceanic evolution
- Paleoenvironmental sulfur studies contribute to our understanding of major events in Earth's history
Sulfur isotopes as proxies
- Used to reconstruct past oceanic and atmospheric conditions
- Provide information on redox states, microbial activity, and sulfur cycling
- Sedimentary pyrite δ^34S values reflect ancient seawater sulfate composition
- Barite (BaSO4) preserves the isotopic composition of seawater sulfate
- Organic sulfur compounds in sediments can record paleoenvironmental information
Archean sulfur cycle
- Characterized by low atmospheric oxygen and oceanic sulfate concentrations
- Evidence for microbial sulfate reduction as early as 3.5 billion years ago
- Mass-independent fractionation (MIF) of sulfur isotopes indicates an anoxic atmosphere
- Archean sulfide δ^34S values show large variations (-20‰ to +20‰)
- Transition to mass-dependent fractionation marks the rise of atmospheric oxygen
Proterozoic sulfur cycle
- Marked by increasing atmospheric oxygen and oceanic sulfate concentrations
- Development of euxinic (sulfidic) conditions in some marine basins
- Larger sulfur isotope fractionations observed due to higher sulfate availability
- Evidence for widespread bacterial sulfate reduction and sulfur disproportionation
- Proterozoic sulfide δ^34S values show extreme variations (-50‰ to +60‰)
Analytical techniques
- Advanced analytical techniques are essential for accurate sulfur isotope measurements in isotope geochemistry
- Continuous improvement in analytical methods has expanded the applications of sulfur isotope analysis
- Proper sample preparation and standardization are crucial for reliable sulfur isotope data
Mass spectrometry for sulfur
- Primary technique for measuring sulfur isotope ratios
- Isotope ratio (IRMS) used for high-precision δ^34S measurements
- Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) allows for analysis of multiple sulfur isotopes
- Secondary ion mass spectrometry (SIMS) enables in situ analysis of sulfur isotopes in minerals
- Continuous flow techniques allow for rapid, automated analysis of large sample sets
Sulfur isotope standards
- Essential for calibrating measurements and ensuring inter-laboratory comparability
- Vienna Canyon Diablo Troilite (VCDT) is the primary reference standard for δ^34S
- International Atomic Energy Agency (IAEA) provides a range of sulfur isotope standards
- Standards include silver sulfide (IAEA-S-1, -2, -3) and barium sulfate (NBS-127)
- Laboratory working standards should be calibrated against international standards
Sample preparation methods
- Vary depending on sample type and analytical technique
- Solid samples often require conversion to SO2 or SF6 for IRMS analysis
- Combustion methods used for organic samples and some inorganic sulfides
- Chemical extraction techniques employed for separating different sulfur species
- Microanalytical techniques (SIMS) require careful sample mounting and polishing
Applications in geochemistry
- Sulfur isotope analysis has diverse applications in geochemistry and related fields
- These applications provide insights into geological processes, environmental conditions, and ecosystem functioning
- Integrating sulfur isotope data with other geochemical proxies enhances interpretations and reconstructions
Ore deposit exploration
- Sulfur isotopes used to trace the origin of sulfur in mineral deposits
- Help distinguish between magmatic, sedimentary, and metamorphic sulfur sources
- Isotopic zonation patterns can indicate fluid flow directions and mineralization processes
- Useful in exploring for volcanogenic massive sulfide (VMS) and sedimentary exhalative (SEDEX) deposits
- Combined with other isotope systems (Pb, Os) to constrain ore-forming processes
Paleoclimate reconstruction
- Sulfur isotopes in marine sediments record changes in oceanic sulfur cycling
- Barite δ^34S values used to reconstruct seawater sulfate isotope composition
- Pyrite δ^34S in sedimentary rocks provides information on past oceanic redox conditions
- Sulfur isotopes in ice cores record atmospheric sulfur deposition and volcanic activity
- Integration with other proxies (δ^18O, δ^13C) improves paleoclimate interpretations
Microbial ecology studies
- Sulfur isotopes trace microbial sulfur metabolism in modern environments
- Used to identify sulfate reduction, sulfide oxidation, and disproportionation processes
- Single-cell techniques allow for linking sulfur isotope signatures to specific microorganisms
- Provide insights into microbial community structure and function in various ecosystems
- Applications in studying extreme environments (hydrothermal vents, hypersaline lakes)
Global sulfur budget
- The global sulfur budget quantifies the fluxes and reservoirs of sulfur in Earth's systems
- Understanding the sulfur budget is crucial for interpreting long-term trends in sulfur isotope geochemistry
- Anthropogenic activities have significantly altered the modern global sulfur budget
Sulfur fluxes
- Quantify the movement of sulfur between different reservoirs
- Major natural fluxes include volcanic emissions, weathering, and marine sulfate reduction
- Anthropogenic fluxes dominated by fossil fuel combustion and industrial processes
- Biogenic fluxes include dimethylsulfide emissions from marine phytoplankton
- Atmospheric deposition represents an important flux to terrestrial and aquatic ecosystems
Residence times
- Measure the average time sulfur spends in a particular reservoir
- Atmospheric sulfur has a short residence time of days to weeks
- Oceanic sulfate has a long residence time of ~10-20 million years
- Lithospheric sulfur has the longest residence time, on the order of billions of years
- Residence times influence the sensitivity of reservoirs to perturbations in the sulfur cycle
Mass balance calculations
- Used to quantify sulfur fluxes and reservoir sizes
- Steady-state assumptions often applied to simplify calculations
- Isotope mass balance equations incorporate isotopic compositions of fluxes and reservoirs
- Box models used to simulate sulfur cycling and predict future changes
- Uncertainties in flux estimates and reservoir sizes can limit the accuracy of mass balance calculations
Key Terms to Review (18)
Acid rain: Acid rain is a form of precipitation that contains high levels of sulfuric and nitric acids, resulting from the emission of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere. These pollutants originate mainly from human activities such as fossil fuel combustion, industrial processes, and vehicle emissions. When these compounds combine with water vapor in the atmosphere, they create acidic precipitation that can have harmful effects on ecosystems, water bodies, and human structures.
Anoxic Environments: Anoxic environments are regions where oxygen is absent or present at extremely low concentrations, inhibiting aerobic processes and influencing the biogeochemical cycles within these ecosystems. These conditions are critical for understanding various ecological and geological processes, particularly in relation to the cycling of elements like sulfur, carbon, and nitrogen. Anoxic environments often occur in settings such as deep ocean floors, wetlands, and sediments, where organic matter accumulates and decomposes anaerobically.
Biogeochemical Cycling: Biogeochemical cycling refers to the movement and transformation of chemical elements and compounds between living organisms and the environment, emphasizing the interconnectedness of biological, geological, and chemical processes. This cycling is crucial for nutrient availability and energy flow within ecosystems, influencing everything from organism growth to climate regulation. Key features include the role of isotopes in tracing these cycles, fractionation effects caused by biological processes, and the unique pathways through which elements like sulfur move through ecosystems.
Dissimilation sulfate reduction: Dissimilation sulfate reduction is a biological process where sulfate is reduced to sulfide by microorganisms in anaerobic environments. This process plays a key role in the sulfur cycle by converting sulfate into forms that can be utilized by other organisms, and it contributes to the cycling of sulfur through various environmental systems, particularly in sedimentary and aquatic ecosystems.
Gypsum: Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, known for its use in various construction materials and as a soil amendment. It plays a significant role in the sulfur cycle, particularly in its formation through evaporation and precipitation in sedimentary environments, contributing to the geochemical processes that recycle sulfur in ecosystems.
Isotope ratio analysis: Isotope ratio analysis is a technique used to measure the relative abundances of different isotopes of the same element in a sample. This method is crucial in understanding various biogeochemical processes, including the sulfur cycle, as it can reveal information about the sources and transformations of sulfur in different environments.
Mass spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, enabling the identification and quantification of different isotopes in a sample. This technique is crucial in isotope geochemistry for analyzing stable and radioactive isotopes, understanding decay processes, and determining isotopic ratios in various materials.
Oxic environments: Oxic environments are areas where oxygen is present in sufficient quantities to support aerobic biological processes. These environments are crucial for various biochemical cycles, including the sulfur cycle, as they facilitate the oxidation of compounds, allowing for the transformation of sulfur species and supporting diverse microbial communities.
Paleoenvironmental reconstruction: Paleoenvironmental reconstruction is the scientific method used to interpret and recreate past environmental conditions based on geological and biological evidence. This process often utilizes isotopic analysis to understand climate changes, ecosystem dynamics, and the geological context in which these environments existed. By examining isotopic compositions and abundances, researchers can infer details about ancient climates, biological activity, and changes over geological time scales.
Pyrite: Pyrite, commonly known as fool's gold, is a sulfide mineral composed of iron and sulfur with the chemical formula FeS\(_2\). It plays an important role in the sulfur cycle due to its ability to influence the geochemical behavior of sulfur in sediments and soils. Pyrite can form in both sedimentary environments and through hydrothermal processes, serving as a source of sulfur for microbial activity during its oxidation.
Sulfate: Sulfate is a chemical compound containing the sulfate ion (SO₄²⁻), which consists of one sulfur atom surrounded by four oxygen atoms. This ion is commonly found in nature and plays a vital role in various biogeochemical cycles, particularly the sulfur cycle, where it is formed through the oxidation of sulfide minerals and the microbial oxidation of organic sulfur compounds.
Sulfate reduction: Sulfate reduction is a biogeochemical process in which sulfate (SO₄²⁻) is reduced to sulfide (S²⁻) by microorganisms, primarily in anaerobic environments. This process plays a critical role in the sulfur cycle, impacting sulfur availability and influencing the geochemistry of various ecosystems.
Sulfide: Sulfides are compounds that consist of sulfur combined with a metal or another nonmetal, typically characterized by the presence of the sulfide ion (S^2−). These compounds play a crucial role in various geochemical processes, particularly within the sulfur cycle, where they can influence the availability of sulfur in the environment and affect biological and geological systems.
Sulfur cycling by microorganisms: Sulfur cycling by microorganisms refers to the biological processes through which various microbial species convert sulfur compounds in the environment, facilitating the movement of sulfur through different oxidation states. This cycling is crucial for nutrient availability and ecological balance, as microorganisms play a key role in processes such as sulfate reduction, sulfur oxidation, and the production of volatile sulfur compounds.
Sulfur dioxide emissions: Sulfur dioxide emissions refer to the release of the gas sulfur dioxide (SO₂) into the atmosphere, primarily from human activities such as burning fossil fuels and industrial processes. This gas plays a critical role in the sulfur cycle, contributing to acid rain formation and affecting air quality and climate change. Understanding these emissions is essential for grasping their environmental impact and the broader implications for ecosystems and human health.
Sulfur oxidation: Sulfur oxidation refers to the biochemical process by which sulfur-containing compounds are converted into sulfate (SO₄²⁻) through microbial or chemical reactions. This process is a crucial part of the sulfur cycle, playing a significant role in the transformation and mobility of sulfur in various environmental settings.
Sulfur-32: Sulfur-32 is a stable isotope of sulfur with 16 protons and 16 neutrons, making it a key player in various geochemical processes. This isotope plays an important role in the sulfur cycle, where it helps to understand the movement of sulfur through different environmental compartments, including the atmosphere, lithosphere, and biosphere. Additionally, its isotopic signatures are used in contaminant source identification, helping to trace pollution back to specific sources based on sulfur's natural variability in different environments.
Sulfur-34: Sulfur-34 is a stable isotope of sulfur, with a relative atomic mass of 34. This isotope plays a significant role in the sulfur cycle and can be used as a tracer for understanding various geological and biological processes. Its unique properties allow scientists to study sulfur sources, transformations, and sinks, making it an essential tool in isotope geochemistry, especially in areas like contaminant source identification.