Glossary

10.4 Sediment Biogeochemistry and Diagenesis

3 min read•july 25, 2024

Sediment biogeochemistry is a complex dance of physical, chemical, and biological processes. From organic matter breakdown to mineral transformations, these interactions shape the underwater world, influencing nutrient cycles and ecosystem health.

Redox reactions in sediments create distinct zones, each with its own microbial community. This layered structure affects nutrient cycling, contaminant behavior, and even serves as a record of past environmental conditions, offering insights into Earth's history.

Sediment Biogeochemistry Fundamentals

Processes of sediment diagenesis

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  • encompasses physical, chemical, and biological changes in sediments after deposition occurring at low temperatures and pressures (< 200℃)

  • Organic matter degradation involves microbial decomposition of organic material releasing nutrients and dissolved organic compounds while producing reduced substances (NH4+\text{NH}_4^+, H2S\text{H}_2\text{S})

  • Mineral precipitation and dissolution form authigenic minerals (pyrite) and dissolve primary minerals (calcite) altering sediment composition and porosity

  • Redox reactions oxidize reduced compounds and reduce oxidized species influencing elemental cycling (Fe, Mn, S)

  • Pore water chemistry changes create concentration gradients driving diffusion and advection of dissolved species (nutrients, metals)

Redox zonation in marine sediments

  • Vertical zonation of electron acceptors follows a sequence based on energy yield:

    1. Oxygen
    2. Nitrate
    3. Manganese oxides
    4. Iron oxides
    5. Sulfate
    6. dioxide

  • Redox cascade results in sequential use of electron acceptors creating depth-dependent distribution of microbial communities (aerobes, denitrifiers, sulfate reducers)

  • Nutrient cycling implications:

    • : nitrification in oxic zone, and anammox in suboxic zone
    • Phosphorus cycle: adsorption to iron oxides in oxic zone, desorption in anoxic zone
    • Iron and manganese cycling: reduction in anoxic zone, oxidation at redox boundaries

  • Benthic ecology effects shape distribution of benthic organisms (burrowing worms, clams) influence bioturbation and bioirrigation impacting microbial community structure

Sediments as nutrient sources and sinks

  • Nutrient sources regenerate nutrients from organic matter decomposition and release dissolved species through diffusion and resuspension (phosphate, ammonium)

  • Nutrient sinks bury organic matter, adsorb phosphate to iron oxides, and promote denitrification in anoxic sediments removing bioavailable

  • Contaminant dynamics involve adsorption and sequestration of heavy metals (lead, copper), accumulation of persistent organic pollutants (PCBs, PAHs), and methylation of mercury in anoxic sediments

  • Factors influencing source/sink behavior include sediment composition and grain size (clay vs sand), organic matter content, redox conditions (oxic vs anoxic), and hydrodynamic regime (calm vs turbulent)

Sediment records for environmental reconstruction

  • Sedimentary archives include lake sediments (varves), marine sediments (deep-sea cores), and coastal sediments (salt marshes)

  • Geochemical proxies utilize stable isotopes (δ13C\delta^{13}\text{C}, δ15N\delta^{15}\text{N}, δ18O\delta^{18}\text{O}), trace element ratios (Mg/Ca, Sr/Ca), and redox-sensitive elements (Mo, U) to infer past conditions

  • Biomarkers encompass lipid biomarkers for source identification (terrestrial vs marine), pigments as indicators of primary productivity (chlorophyll), and molecular fossils for reconstructing past organisms (dinoflagellates)

  • Applications in paleoenvironmental reconstruction include climate change (temperature, precipitation), ocean circulation patterns (thermohaline circulation), productivity changes (upwelling intensity), and anoxic events (oceanic anoxic events)

  • Limitations and challenges involve diagenetic alterations (selective preservation), temporal resolution (bioturbation mixing), and spatial variability (local vs regional signals)

Key Terms to Review (18)

Acidification: Acidification refers to the process by which the pH level of a substance decreases, becoming more acidic over time. This phenomenon is critical in understanding how changes in biogeochemical cycles, driven by natural and anthropogenic factors, can influence aquatic and terrestrial ecosystems. Increased acidity can lead to significant impacts on biodiversity, nutrient availability, and chemical interactions within sediments and water bodies.
Carbon: Carbon is a versatile chemical element, essential to life on Earth, that plays a crucial role in the biogeochemical cycles, particularly in sediment biogeochemistry and diagenesis. It exists in various forms, such as organic and inorganic carbon, influencing processes like nutrient cycling, energy flow, and the formation of sedimentary rocks. Understanding carbon's behavior in sediments is vital for grasping how ecosystems function and how they respond to environmental changes.
Carbon cycle: The carbon cycle is the series of processes through which carbon atoms circulate in the Earth's systems, including the atmosphere, biosphere, hydrosphere, and geosphere. This cycle plays a crucial role in regulating Earth’s climate, supporting life, and maintaining ecological balance by involving various reservoirs and fluxes of carbon across different spheres.
Cementation: Cementation is the process by which sediment grains are bound together by minerals precipitating from groundwater, forming solid rock. This process is crucial in the transformation of loose sediments into sedimentary rock and plays a significant role in diagenesis, influencing the physical and chemical properties of the sediment.
Compaction: Compaction is the process by which sediment particles are pressed together under pressure, reducing the space between them and increasing the sediment's density. This process is crucial in the formation of sedimentary rocks and plays a significant role in how sediments behave in various environments, particularly during diagenesis, where sediments undergo physical and chemical changes after deposition.
Denitrification: Denitrification is a microbial process that converts nitrates and nitrites into nitrogen gas (N₂) or, to a lesser extent, nitrous oxide (N₂O), thus removing nitrogen from the soil and returning it to the atmosphere. This process plays a crucial role in the nitrogen cycle by reducing excess nitrogen in ecosystems, which can help mitigate issues like nutrient pollution and promote the balance of biogeochemical cycles.
Diagenesis: Diagenesis refers to the physical and chemical processes that transform sediments into sedimentary rock through various stages of compaction, cementation, and alteration. This process occurs after sediment deposition and before metamorphism, involving changes in mineral composition, texture, and structure, which play a crucial role in the cycling of nutrients and carbon within sediments.
Eutrophication: Eutrophication is the process by which water bodies become enriched with nutrients, often leading to excessive growth of algae and other aquatic plants. This phenomenon can disrupt ecosystems, contribute to oxygen depletion, and harm aquatic life, making it a critical concern in the study of biogeochemistry.
Hans Jenny: Hans Jenny was a prominent Swiss soil scientist known for his work on soil formation and classification, particularly his theories on the processes of weathering and the role of biogeochemical cycles in soil development. His contributions greatly influenced our understanding of how soils evolve, especially in relation to phosphorus release, organic matter composition, and sediment biogeochemistry.
Isotope tracing: Isotope tracing is a method used to track the movement and transformation of substances in natural systems by utilizing isotopes, which are variants of elements with different numbers of neutrons. This technique allows scientists to understand processes such as nutrient cycling, organic matter dynamics, and sediment diagenesis. By analyzing the ratios of stable isotopes, researchers can gain insights into sources, pathways, and rates of changes in ecological and geological contexts.
Methanogenesis: Methanogenesis is the biological process by which microorganisms, specifically methanogens, convert organic matter into methane (CH₄) in anaerobic conditions. This process plays a crucial role in the global carbon cycle and is especially significant in environments where oxygen is limited, like wetlands and sediments. Methanogenesis contributes to greenhouse gas emissions but also serves as a critical step in the degradation of organic materials.
Mineralization: Mineralization is the process by which organic matter is broken down into inorganic substances, making nutrients available for uptake by plants and microorganisms. This process plays a vital role in nutrient cycling, affecting the availability of essential elements like nitrogen and phosphorus, and influencing soil health and fertility.
Nitrogen: Nitrogen is a colorless, odorless gas that makes up about 78% of the Earth's atmosphere and is an essential element for all living organisms, primarily because it is a key component of amino acids, proteins, and nucleic acids. In biogeochemical cycles, nitrogen undergoes various transformations, including fixation, mineralization, nitrification, and denitrification, which play vital roles in nutrient availability and ecosystem functioning.
Nitrogen cycle: The nitrogen cycle is the biogeochemical process through which nitrogen is converted between its various chemical forms, enabling it to be used by living organisms. This cycle involves several key processes including nitrogen fixation, nitrification, denitrification, and ammonification, connecting various Earth's spheres and influencing ecosystem dynamics.
Organic-rich sediments: Organic-rich sediments are sedimentary deposits that contain a significant amount of organic material, primarily derived from the remains of living organisms. These sediments play a crucial role in biogeochemical cycles, as they contribute to the storage and release of nutrients and carbon within ecosystems, influencing both sediment biogeochemistry and diagenesis.
Robert G. Bates: Robert G. Bates is a prominent geochemist known for his influential work in sediment biogeochemistry and diagenesis, particularly regarding the interactions between biological processes and sedimentary environments. His research has greatly contributed to understanding how organic matter decomposes and how nutrients cycle through sediments, which is crucial for ecosystem health and functioning.
Sandy sediments: Sandy sediments are granular materials composed primarily of sand-sized particles that have been transported and deposited by natural processes such as wind, water, or ice. These sediments play a crucial role in sediment biogeochemistry and diagenesis as they provide habitats for various microorganisms, influence nutrient cycling, and affect the physical and chemical properties of aquatic and terrestrial environments.
Sediment core analysis: Sediment core analysis is a scientific technique used to study the physical, chemical, and biological properties of sediment layers accumulated over time. This method helps researchers understand past environmental conditions, biogeochemical processes, and the diagenesis of sediments as they transform after deposition.
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