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 (210Pb), cesium-137 (137Cs), and carbon-14 (14C), which are incorporated into sediments
210Pb dating is commonly used for sediments spanning the last 100-150 years, based on the excess 210Pb activity derived from atmospheric fallout and its half-life of 22.3 years
137Cs dating relies on the distinct peak in 137Cs activity associated with the maximum atmospheric nuclear weapons testing in 1963, providing a reliable time marker for sediment chronology
14C 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
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 (δ18O) 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 (δ13C, 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 (δ15N, 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 (δ13C, δ15N, δ18O) 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