10.4 Sediment Biogeochemistry and Diagenesis

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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• Diagenesis 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 ($\text{NH}_4^+$, $\text{H}_2\text{S}$)

• 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. Carbon dioxide

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

• Nutrient cycling implications:

  • Nitrogen cycle: nitrification in oxic zone, denitrification 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 nitrogen

• 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 ($\delta^{13}\text{C}$, $\delta^{15}\text{N}$, $\delta^{18}\text{O}$), 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)