Glossary

10.4 Geochemical indicators in sediments

9 min read•august 20, 2024

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 . 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

Top images from around the web for Organic vs inorganic components

  • Frontiers | Impacts of Global Change on Ocean Dissolved Organic Carbon (DOC) Cycling View original

    Is this image relevant?

  • Biogeochemical Cycles | Microbiology View original

    Is this image relevant?

  • Frontiers | Impacts of Global Change on Ocean Dissolved Organic Carbon (DOC) Cycling View original

    Is this image relevant?

1 of 3

Top images from around the web for Organic vs inorganic components

  • Frontiers | Impacts of Global Change on Ocean Dissolved Organic Carbon (DOC) Cycling View original

    Is this image relevant?

  • Biogeochemical Cycles | Microbiology View original

    Is this image relevant?

  • Frontiers | Impacts of Global Change on Ocean Dissolved Organic Carbon (DOC) Cycling View original

    Is this image relevant?

1 of 3
  • 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 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 , 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 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

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

  • 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 (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 (δ18Oδ^{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 (δ13Cδ^{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 (δ15Nδ^{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 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
  • (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 (δ13Cδ^{13}C, δ15Nδ^{15}N, δ18Oδ^{18}O) are measured using isotope ratio mass spectrometry (IRMS), following sample preparation and purification steps
  • Organic biomarkers and contaminants are analyzed using -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

Key Terms to Review (26)

Biodiversity assessment: Biodiversity assessment is the systematic evaluation of the variety and variability of life forms within a specific habitat or ecosystem. This process often involves analyzing species richness, abundance, and distribution to understand the health and stability of ecological systems. By examining biodiversity, researchers can identify changes in ecosystems, determine the impacts of human activities, and inform conservation efforts.
Biogeochemical cycling: Biogeochemical cycling refers to the movement and transformation of essential elements and compounds through biological, geological, and chemical processes within ecosystems. This cycling is crucial for maintaining ecosystem health as it ensures the availability of nutrients like carbon, nitrogen, phosphorus, and sulfur, supporting life forms across various habitats. By cycling these elements, ecosystems can maintain their productivity and balance over time.
Biostratigraphic markers: Biostratigraphic markers are fossilized remains or trace fossils used by scientists to correlate and date rock layers. These markers help in establishing the relative ages of sedimentary sequences and understanding the geological history of an area. By identifying specific fossil assemblages, researchers can make inferences about past environments and the evolution of life during different geological periods.
Clay sediment: Clay sediment refers to fine-grained particles that are less than 0.002 mm in diameter and are primarily composed of clay minerals. These sediments form through the weathering of rocks and can accumulate in various aquatic environments, serving as an important indicator of geological processes and environmental conditions over time.
David Schindler: David Schindler is a prominent limnologist known for his extensive research on freshwater ecosystems and the impacts of human activities on lakes and rivers. His work has significantly contributed to understanding how nutrient loading, particularly phosphorus, affects freshwater health and biodiversity, particularly in the context of pollution and eutrophication.
Decomposition: Decomposition is the biological process through which organic matter breaks down into simpler forms, primarily facilitated by microorganisms, fungi, and detritivores. This process is crucial in recycling nutrients back into ecosystems, making them available for plant growth and influencing various ecological dynamics, including carbon cycling and oxygen levels in water bodies.
Deltaic environments: Deltaic environments are landforms that occur at the mouth of a river, where it meets a body of water such as an ocean or lake, leading to the deposition of sediment. These regions are characterized by unique physical and biological processes due to the interaction between riverine and marine or lacustrine systems, resulting in rich habitats for various species and influencing sediment geochemistry.
Diagenetic alterations: Diagenetic alterations refer to the physical, chemical, and biological changes that sediments undergo after deposition and during burial, transforming them into sedimentary rock. These changes can significantly impact the composition and characteristics of sediments, influencing factors like porosity, permeability, and mineralogy, which are crucial for understanding geochemical indicators in sediments.
Erosion: Erosion is the process of wearing away and removing soil and rock material from one location and transporting it to another. This natural process can be influenced by various factors such as water flow, wind, ice, and human activities. Erosion plays a significant role in shaping landscapes, affecting sediment transport and deposition, and influencing the formation of coastal lakes, as well as impacting the analysis of sediments through coring techniques and geochemical indicators.
Fluxes: Fluxes refer to the rates at which substances move or transfer from one reservoir to another within an ecosystem or environment. This concept is crucial in understanding the dynamics of nutrient cycling, energy flow, and the movement of chemicals in aquatic systems, where sediments can act as both sources and sinks for various elements.
Gas chromatography: Gas chromatography is a technique used to separate and analyze compounds that can vaporize without decomposition. It works by passing a mixture through a column filled with a stationary phase, allowing the different components to be separated based on their interaction with the stationary phase and their volatility. This method is crucial for analyzing geochemical indicators in sediments, helping identify and quantify organic compounds present in sediment samples.
Heavy metal concentrations: Heavy metal concentrations refer to the levels of metallic elements that can be toxic or harmful at elevated levels in the environment, often found in sediment samples. These metals, such as lead, mercury, cadmium, and arsenic, can accumulate in aquatic ecosystems, impacting both aquatic life and human health. The study of these concentrations in sediments serves as a critical geochemical indicator of environmental pollution and ecosystem health.
Lacustrine systems: Lacustrine systems refer to ecosystems associated with freshwater lakes and their surroundings. These systems encompass a variety of physical, chemical, and biological interactions within the lake and its watershed, influencing everything from sediment deposition to nutrient cycling. Understanding lacustrine systems is crucial for interpreting the geochemical indicators found in sediments, which can provide insights into past environmental conditions and changes over time.
Margaret A. Palmer: Margaret A. Palmer is a prominent limnologist known for her extensive research on freshwater ecosystems and her contributions to the understanding of the interactions between biotic and abiotic factors in aquatic environments. Her work has emphasized the importance of geochemical indicators in sediments, particularly in assessing the ecological health of water bodies and the impacts of anthropogenic activities.
Nutrient concentrations: Nutrient concentrations refer to the amounts of essential elements and compounds found within a specific environment, often measured in water or sediment. These concentrations are critical for understanding the productivity of aquatic ecosystems, as they directly influence the growth of organisms and the overall health of the environment. Additionally, variations in nutrient concentrations can indicate changes in environmental conditions and human impacts on water bodies.
Nutrient Cycling: Nutrient cycling refers to the movement and exchange of organic and inorganic matter back into the production of living matter. It is a fundamental ecological process that ensures the availability of essential nutrients for organisms, maintaining ecosystem productivity and health.
PH levels: pH levels refer to the measurement of acidity or alkalinity of a solution on a scale of 0 to 14, with lower values indicating higher acidity, and higher values indicating higher alkalinity. The pH of aquatic systems is crucial as it affects the availability of nutrients and overall health of ecosystems. Additionally, pH levels can influence the growth of organisms, chemical reactions, and the solubility of minerals in water.
Radiometric dating methods: Radiometric dating methods are scientific techniques used to determine the age of materials, such as rocks or sediments, by measuring the abundance of specific radioactive isotopes present in them. This method relies on the principle of radioactive decay, where unstable isotopes transform into stable ones at a predictable rate. Understanding these processes helps in reconstructing geological timelines and studying environmental changes over time.
Sediment transport: Sediment transport refers to the movement of solid particles, such as sand, silt, and clay, from one location to another, primarily by water, wind, or ice. This process plays a crucial role in shaping landscapes and ecosystems, influencing river morphology and sediment deposition patterns, which can greatly affect aquatic habitats and geochemical processes within sediments.
Silty sediment: Silty sediment consists of fine particles that are smaller than sand but larger than clay, typically measuring between 0.002 mm and 0.06 mm in diameter. This type of sediment plays a crucial role in aquatic environments, affecting water quality, nutrient cycling, and habitat availability for various organisms.
Stable isotope ratios: Stable isotope ratios refer to the relative abundance of stable isotopes of an element in a sample, which can provide valuable information about environmental conditions and biological processes. These ratios can indicate changes in sources of organic matter, nutrient cycling, and past climate conditions by analyzing sediment samples. By examining these ratios in sediments, researchers can infer historical ecological dynamics and biogeochemical processes over time.
Total organic carbon: Total organic carbon (TOC) is a measure of the amount of carbon found in organic compounds within a sample, typically used to assess the amount of organic material present in sediments or aquatic systems. It serves as a crucial indicator of the biological productivity and health of aquatic environments, as it reflects the quantity of decaying plant and animal matter. High TOC levels can indicate rich ecosystems while low levels may suggest poor productivity or environmental stress.
Trace metal accumulation: Trace metal accumulation refers to the gradual buildup of trace metals, such as lead, mercury, and cadmium, in sediments over time. This process is significant because it can indicate anthropogenic influences, pollution sources, and changes in environmental conditions. The study of trace metal accumulation helps in understanding the historical impacts of human activities on aquatic systems and the overall health of these ecosystems.
Trophic Status: Trophic status refers to the biological productivity of a water body, indicating its nutrient levels and the resulting ability to support aquatic life. It is commonly categorized into oligotrophic, mesotrophic, and eutrophic states, which are influenced by various factors, including nutrient concentrations, light penetration, and organic matter decomposition. Understanding trophic status is essential for assessing water quality and ecological health, as it can affect species diversity and distribution within aquatic ecosystems.
Varve counting: Varve counting is the method of analyzing sedimentary layers, specifically annual layers, known as varves, which are deposited in lakes and other water bodies. This technique helps in determining the chronological age of sediments and can reveal past environmental conditions, as each varve typically represents one year of sediment deposition, showing seasonal variations.
X-ray fluorescence: X-ray fluorescence is a non-destructive analytical technique used to determine the elemental composition of materials by measuring the characteristic X-rays emitted from a sample when it is irradiated with X-rays. This method is particularly valuable in sediment analysis as it allows for rapid and accurate detection of trace elements, helping to interpret geochemical indicators found in sediments.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide