10.4 Geochemical indicators in sediments

Sediments are natural archives that reveal past environmental conditions in aquatic ecosystems. They contain a wealth of geochemical information, reflecting complex interactions between physical, chemical, and biological processes. Understanding sediment geochemistry is key to reconstructing past climates and assessing human impacts.

Geochemical indicators in sediments include organic and inorganic components, particle size distribution, and chemical properties like pH and nutrient concentrations. Various dating techniques and proxy indicators allow scientists to reconstruct past environments and climate changes. Analyzing these indicators provides valuable insights into ecosystem history and human influences.

Geochemical properties of sediments

• Sediments serve as natural archives of past environmental conditions and provide valuable insights into the history of aquatic ecosystems
• Geochemical properties of sediments reflect the complex interactions between physical, chemical, and biological processes within the water column and at the sediment-water interface
• Understanding sediment geochemistry is crucial for reconstructing past climate, assessing anthropogenic impacts, and predicting future changes in aquatic environments

Sediment composition and structure

Organic vs inorganic components

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• Sediments are composed of a mixture of organic matter derived from living organisms (detritus, fecal pellets, and remains of aquatic plants and animals) and inorganic materials originating from weathering and erosion of rocks and minerals
• The relative proportions of organic and inorganic components in sediments can vary depending on factors such as primary productivity, terrestrial input, and sedimentation rates
• Organic matter content influences the chemical and biological processes within sediments, including nutrient cycling, redox conditions, and microbial activity

Particle size distribution

• Sediment particle size distribution refers to the range and relative abundance of different grain sizes, from clay (<2 μm) to silt (2-63 μm) to sand (63 μm - 2 mm) and gravel (>2 mm)
• Particle size distribution affects the physical properties of sediments, such as porosity, permeability, and surface area available for chemical reactions and microbial colonization
• Variations in particle size distribution can reflect changes in sediment transport processes, depositional environments, and watershed characteristics over time

Porosity and permeability

• Porosity is the fraction of void space within sediments, which can be filled with water or gases
• Permeability refers to the ability of fluids to flow through the interconnected pore spaces within sediments
• Porosity and permeability influence the exchange of solutes and gases between sediments and the overlying water column, as well as the mobility and distribution of contaminants within sediments
• Sediments with high porosity and permeability tend to have greater rates of biogeochemical processes and are more susceptible to diagenetic alterations

Sediment chemistry

pH and redox potential

• Sediment pH and redox potential (Eh) are key parameters that control the speciation, solubility, and mobility of various chemical constituents within sediments
• pH affects the adsorption and desorption of ions on sediment particles, as well as the dissolution and precipitation of minerals
• Redox potential reflects the availability of electron acceptors (oxygen, nitrate, iron, manganese, sulfate) and the dominant microbial metabolic pathways within sediments (aerobic respiration, denitrification, iron reduction, sulfate reduction, methanogenesis)
• Vertical gradients in pH and redox potential develop within sediments due to the sequential utilization of electron acceptors and the production of reduced chemical species

Nutrient concentrations

• Sediments act as a reservoir and source of essential nutrients, such as nitrogen (N), phosphorus (P), and silica (Si), which support primary production in aquatic ecosystems
• Nutrient concentrations in sediments are influenced by the balance between external loading, internal cycling, and burial processes
• Sediments can release nutrients to the overlying water column through diffusion, bioturbation, and resuspension events, contributing to the development of eutrophic conditions and harmful algal blooms
• Nutrient ratios (N:P, Si:N) in sediments can provide insights into the limiting factors for primary production and the potential for ecological shifts in aquatic communities

Trace metal accumulation

• Sediments can accumulate trace metals, such as mercury (Hg), lead (Pb), cadmium (Cd), and arsenic (As), through atmospheric deposition, riverine input, and anthropogenic activities (mining, industrial discharges, urban runoff)
• Trace metals can be adsorbed onto sediment particles, incorporated into mineral phases, or complexed with organic matter
• The bioavailability and toxicity of trace metals in sediments depend on factors such as pH, redox conditions, organic matter content, and the presence of sulfides and iron oxides
• Sedimentary records of trace metal accumulation can be used to reconstruct the history of anthropogenic pollution and assess the ecological risks associated with metal contamination in aquatic ecosystems

Sediment dating techniques

Radiometric dating methods

• Radiometric dating techniques are based on the radioactive decay of naturally occurring isotopes, such as lead-210 ($^{210}Pb$), cesium-137 ($^{137}Cs$), and carbon-14 ($^{14}C$), which are incorporated into sediments
• $^{210}Pb$ dating is commonly used for sediments spanning the last 100-150 years, based on the excess $^{210}Pb$ activity derived from atmospheric fallout and its half-life of 22.3 years
• $^{137}Cs$ dating relies on the distinct peak in $^{137}Cs$ activity associated with the maximum atmospheric nuclear weapons testing in 1963, providing a reliable time marker for sediment chronology
• $^{14}C$ dating is applied to organic matter in sediments and has a much longer time range (up to ~50,000 years), but requires correction for reservoir effects and calibration with other dating methods

Varve counting and chronology

• Varves are annual layers of sediment deposition that form in lakes and marine basins with seasonal variations in sediment input and composition
• Varves can be composed of alternating light (summer) and dark (winter) layers, reflecting changes in biogenic production, terrigenous input, and redox conditions
• Varve counting involves the visual or microscopic identification and enumeration of individual varve couplets, providing a high-resolution, annually resolved chronology for sediment records
• Varve chronologies can be cross-validated with independent dating methods (radiometric dating, tephrochronology) and used to reconstruct past climate variability, lake level fluctuations, and ecosystem dynamics

Biostratigraphic markers

• Biostratigraphic markers are distinct fossil assemblages or species with known ecological preferences and temporal ranges that can be used to date and correlate sediment sequences
• Common biostratigraphic markers in aquatic sediments include diatoms, pollen, chironomids, and ostracods, which are sensitive to environmental conditions and have well-established taxonomic and biogeographic distributions
• Changes in the composition and abundance of biostratigraphic markers within sediment profiles can reflect shifts in climate, hydrology, nutrient status, and other environmental variables over time
• Biostratigraphic dating requires a robust understanding of the ecology and evolution of the indicator species, as well as the development of regional calibration datasets and transfer functions

Paleoenvironmental reconstruction

Proxy indicators in sediments

• Proxy indicators are physical, chemical, or biological variables preserved in sediments that can be used to infer past environmental conditions and processes
• Examples of proxy indicators include stable isotope ratios (oxygen, carbon, nitrogen), elemental concentrations (Ca, Mg, Sr, Ba), organic biomarkers (alkenones, lignin phenols, glycerol dialkyl glycerol tetraethers), and microfossil assemblages (diatoms, pollen, chironomids)
• The interpretation of proxy indicators relies on the understanding of their environmental controls, calibration with modern datasets, and the assessment of potential biases and uncertainties
• Multi-proxy approaches, combining several independent proxy indicators, can provide more robust and comprehensive paleoenvironmental reconstructions

Climate change records

• Sedimentary records can provide valuable archives of past climate variability on local, regional, and global scales
• Climate-sensitive proxy indicators in sediments, such as oxygen isotope ratios ($δ^{18}O$) in biogenic carbonates, can reflect changes in temperature, precipitation, and ice volume over time
• Organic biomarkers, such as alkenones and branched glycerol dialkyl glycerol tetraethers (brGDGTs), can be used to reconstruct sea surface and lake surface temperatures, respectively
• Pollen and diatom assemblages in sediments can reveal shifts in vegetation patterns and lake ecosystem structure in response to climate change
• High-resolution sedimentary records, such as varved sequences and ice cores, can provide insights into abrupt climate events, millennial-scale oscillations, and long-term climate trends

Anthropogenic impact assessment

• Sedimentary records can be used to assess the timing, magnitude, and extent of human impacts on aquatic ecosystems and the environment
• Indicators of anthropogenic influence in sediments include increased trace metal concentrations, shifts in nutrient ratios (N:P), changes in organic matter composition ($δ^{13}C$, C/N ratio), and the appearance of synthetic contaminants (PCBs, PAHs, microplastics)
• Eutrophication histories can be reconstructed using diatom and cyanobacterial pigment concentrations, as well as geochemical proxies for nutrient loading and hypoxia ($δ^{15}N$, Mo/Al ratio)
• Sedimentary records can help to establish baseline conditions, detect the onset and progression of anthropogenic disturbances, and evaluate the effectiveness of management and restoration efforts in aquatic ecosystems

Sediment-water interface processes

Nutrient cycling and fluxes

• The sediment-water interface is a dynamic zone where the exchange of nutrients between sediments and the overlying water column takes place
• Nutrient cycling at the sediment-water interface involves a complex interplay of physical, chemical, and biological processes, including diffusion, advection, adsorption-desorption, and microbial transformations
• Benthic nutrient fluxes can be a significant source of nutrients to the water column, particularly in shallow aquatic systems with high sediment surface area to water volume ratios
• Factors influencing benthic nutrient fluxes include sediment composition, redox conditions, bioturbation, and the presence of benthic microalgae and macrophytes

Benthic-pelagic coupling

• Benthic-pelagic coupling refers to the exchange of energy, matter, and organisms between the sediment and water column compartments of aquatic ecosystems
• Sedimentation of organic matter from the water column provides a food source for benthic communities and fuels microbial processes within sediments
• Benthic organisms, such as suspension feeders and deposit feeders, can influence pelagic food webs and nutrient cycling through their feeding activities and excretion
• Resuspension of sediments and associated nutrients, contaminants, and microorganisms can affect water column processes, such as primary production, microbial dynamics, and contaminant transport

Diagenetic alterations

• Diagenesis encompasses the post-depositional physical, chemical, and biological changes that occur within sediments over time
• Diagenetic processes can modify the original composition and structure of sediments, as well as the preservation and interpretation of paleoenvironmental proxies
• Examples of diagenetic alterations include the degradation of organic matter, recrystallization of minerals, formation of authigenic phases (pyrite, vivianite), and the mobilization and redistribution of elements and compounds within sediments
• Diagenetic overprinting can complicate the use of sedimentary records for paleoenvironmental reconstructions, requiring careful consideration of the potential effects on proxy indicators and the application of geochemical and mineralogical tools to assess the extent of alteration

Geochemical analysis methods

Core sampling and preservation

• Sediment cores are obtained using various coring devices, such as gravity corers, piston corers, and box corers, depending on the water depth, sediment type, and desired core length and diameter
• Proper handling and storage of sediment cores are essential to maintain the integrity of the sediment structure and minimize contamination and oxidation
• Cores are typically sectioned at regular intervals (e.g., 1 cm) and subsampled for different analyses, such as bulk geochemistry, stable isotopes, and microfossils
• Preservation techniques for sediment samples include freezing, drying, and storage in anoxic conditions, depending on the target analytes and the planned analytical methods

Analytical techniques and instrumentation

• A wide range of analytical techniques is employed to characterize the geochemical properties of sediments, including elemental analysis, isotopic analysis, and organic geochemistry
• X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) are commonly used for the determination of major and trace element concentrations in sediments
• Stable isotope ratios ($δ^{13}C$, $δ^{15}N$, $δ^{18}O$) are measured using isotope ratio mass spectrometry (IRMS), following sample preparation and purification steps
• Organic biomarkers and contaminants are analyzed using gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) techniques, often after solvent extraction and compound-specific purification
• Other specialized techniques include X-ray diffraction (XRD) for mineralogical analysis, scanning electron microscopy (SEM) for imaging and elemental mapping, and Fourier-transform infrared spectroscopy (FTIR) for the characterization of organic matter

Data interpretation and limitations

• Interpretation of geochemical data from sediments requires a comprehensive understanding of the environmental context, sediment depositional processes, and potential diagenetic alterations
• Statistical methods, such as principal component analysis (PCA) and cluster analysis, can be used to identify patterns and relationships among geochemical variables and to define sediment geochemical facies
• Geochronological control is crucial for the interpretation of temporal trends and the correlation of sedimentary records across different sites and regions
• Limitations and uncertainties in geochemical data interpretation can arise from factors such as spatial and temporal variability, analytical precision and accuracy, and the potential influence of post-depositional processes
• Integration of geochemical data with other paleoenvironmental proxies, such as biological and physical indicators, can provide a more robust and comprehensive understanding of past environmental conditions and processes in aquatic ecosystems