🌋Geochemistry Unit 7 – Sedimentary geochemistry

Key Concepts and Definitions

• Sedimentary geochemistry studies the chemical composition, processes, and interactions within sedimentary rocks and environments
• Sedimentary rocks form through the deposition and lithification of sediments derived from weathering, erosion, and biological processes
• Weathering breaks down rocks and minerals through physical, chemical, and biological processes (mechanical weathering, dissolution, oxidation)
• Erosion transports weathered materials from their source to depositional environments (rivers, glaciers, wind)
• Diagenesis encompasses the physical, chemical, and biological changes sediments undergo after deposition but before lithification
  • Includes compaction, cementation, and recrystallization
• Lithification converts loose sediments into solid sedimentary rocks through compaction and cementation
• Geochemical cycles describe the movement and exchange of elements between Earth's reservoirs (atmosphere, hydrosphere, lithosphere, biosphere)
• Provenance refers to the origin and source area of sedimentary materials

Sedimentary Rock Formation

• Sedimentary rocks form through a multi-step process involving weathering, erosion, transportation, deposition, and lithification
• Weathering and erosion break down pre-existing rocks and minerals, producing sediments
• Transportation moves sediments from their source area to depositional environments via water, wind, or ice
• Deposition occurs when sediments settle out of the transporting medium and accumulate in layers
  • Depositional environments include rivers, deltas, lakes, oceans, and deserts
• Diagenesis modifies the deposited sediments through compaction, cementation, and recrystallization
• Lithification transforms the diagenetically altered sediments into solid sedimentary rocks
  • Compaction reduces pore space and increases density
  • Cementation binds sediment grains together with minerals precipitated from pore fluids (calcite, quartz, hematite)
• The resulting sedimentary rocks record information about the source area, transport processes, depositional environment, and post-depositional changes

Mineral Composition and Classification

• Sedimentary rocks are classified based on their mineral composition, texture, and mode of formation
• Clastic sedimentary rocks consist of rock and mineral fragments (clasts) derived from pre-existing rocks (sandstone, conglomerate, breccia)
  • Classified by grain size (gravel, sand, silt, clay) and composition (quartz, feldspar, lithic fragments)
• Chemical sedimentary rocks form through the precipitation of minerals from aqueous solutions (limestone, dolostone, evaporites)
  • Reflect the chemistry of the depositional environment and pore fluids
• Biochemical sedimentary rocks form through the accumulation and lithification of organic materials (coal, chert, phosphorites)
  • Indicate the presence and productivity of organisms in the depositional environment
• The mineral composition of sedimentary rocks provides insights into the source area, weathering conditions, and depositional environment
• Common minerals in sedimentary rocks include quartz, feldspar, clay minerals, calcite, dolomite, and evaporite minerals (halite, gypsum)

Chemical Weathering Processes

• Chemical weathering involves the breakdown of rocks and minerals through chemical reactions with water, air, and organic acids
• Dissolution is the process by which minerals dissolve in water, releasing their constituent ions (calcite, halite)
• Hydrolysis breaks down silicate minerals by replacing cations with hydrogen ions from water (feldspar to clay minerals)
• Oxidation occurs when minerals react with oxygen, often changing their chemical composition and properties (pyrite to hematite)
• Carbonation is the reaction between minerals and carbonic acid, formed when atmospheric CO2 dissolves in water (limestone dissolution)
• Chemical weathering rates depend on factors such as temperature, precipitation, rock composition, and vegetation cover
  • Higher temperatures and precipitation accelerate chemical weathering
  • Rocks with unstable minerals (olivine, pyroxene) weather more rapidly than those with stable minerals (quartz, muscovite)
• Chemical weathering plays a crucial role in the geochemical cycling of elements and the formation of secondary minerals (clays, oxides)

Diagenesis and Lithification

• Diagenesis refers to the physical, chemical, and biological changes sediments undergo after deposition but before lithification
• Compaction reduces pore space and increases the density of sediments due to the weight of overlying layers
  • Leads to the expulsion of pore fluids and the reorientation of grains
• Cementation binds sediment grains together through the precipitation of minerals from pore fluids (calcite, quartz, hematite)
  • Depends on the availability of dissolved ions and the chemistry of the pore fluids
• Recrystallization involves the transformation of existing minerals into new minerals with the same chemical composition (aragonite to calcite)
• Diagenetic processes can alter the original composition, texture, and porosity of sediments
  • May result in the formation of concretions, nodules, and cement layers
• Lithification converts the diagenetically altered sediments into solid sedimentary rocks through compaction and cementation
• The extent and nature of diagenesis depend on factors such as burial depth, pore fluid chemistry, and geothermal gradient
• Diagenetic changes can influence the reservoir properties of sedimentary rocks (porosity, permeability) and the preservation of fossils and sedimentary structures

Geochemical Cycles in Sedimentary Environments

• Geochemical cycles describe the movement and exchange of elements between Earth's reservoirs (atmosphere, hydrosphere, lithosphere, biosphere)
• The carbon cycle involves the transfer of carbon between the atmosphere, oceans, land, and sedimentary rocks
  • Weathering of silicate rocks consumes atmospheric CO2, while weathering of carbonate rocks releases CO2
  • Organic carbon burial in sediments removes CO2 from the atmosphere on geological timescales
• The sulfur cycle encompasses the transformations of sulfur compounds in sedimentary environments
  • Bacterial sulfate reduction in anoxic sediments produces hydrogen sulfide (H2S) and pyrite (FeS2)
  • Evaporite deposits (gypsum, anhydrite) sequester sulfur in sedimentary basins
• The phosphorus cycle plays a vital role in marine productivity and the formation of phosphorite deposits
  • Weathering of phosphate-bearing rocks releases phosphorus to the oceans, stimulating primary productivity
  • Phosphorus burial in sediments removes it from the water column, limiting marine productivity
• Redox conditions in sedimentary environments influence the speciation and mobility of elements (iron, manganese, uranium)
  • Oxic conditions favor the precipitation of iron and manganese oxides, while anoxic conditions promote the formation of sulfides
• Geochemical cycles in sedimentary environments are closely linked to global climate, tectonics, and biological processes

Analytical Techniques in Sedimentary Geochemistry

• X-ray fluorescence (XRF) measures the elemental composition of sedimentary rocks and minerals
  • Determines major and trace element abundances based on the emission of characteristic X-rays
• X-ray diffraction (XRD) identifies the mineralogical composition of sedimentary rocks and sediments
  • Analyzes the diffraction patterns produced by the interaction of X-rays with crystalline materials
• Inductively coupled plasma mass spectrometry (ICP-MS) quantifies trace element and isotope ratios in sedimentary materials
  • Ionizes the sample in a high-temperature plasma and separates the ions based on their mass-to-charge ratio
• Stable isotope analysis (δ13C, δ18O, δ34S) provides insights into the environmental conditions and processes during sediment deposition
  • Measures the relative abundances of stable isotopes in minerals and organic matter
  • Reflects changes in temperature, salinity, and biological productivity
• Scanning electron microscopy (SEM) allows high-resolution imaging and chemical analysis of sedimentary components
  • Produces detailed images of grain morphology, cement textures, and diagenetic features
• Cathodoluminescence (CL) reveals the growth history and alteration of carbonate and silicate minerals
  • Emits light when bombarded with an electron beam, highlighting different generations of cement and recrystallization
• Radiometric dating techniques (U-Pb, Rb-Sr, K-Ar) determine the absolute ages of sedimentary rocks and minerals
  • Measures the decay of radioactive isotopes and their accumulation in minerals over time

Environmental and Climate Applications

• Sedimentary geochemistry provides valuable insights into past environmental and climatic conditions
• Stable isotope ratios (δ18O, δ13C) in carbonate rocks and fossils record changes in ocean temperature, salinity, and global ice volume
  • Higher δ18O values indicate cooler temperatures and/or increased ice volume, while lower values suggest warmer conditions and/or decreased ice volume
  • Variations in δ13C reflect changes in the global carbon cycle, such as the burial and oxidation of organic matter
• Clay mineral assemblages in sedimentary rocks can indicate weathering conditions and climate in the source area
  • Kaolinite forms under warm, humid conditions with intense chemical weathering, while illite and chlorite are more common in cool, dry climates
• Evaporite deposits (halite, gypsum) record periods of aridity and restricted water circulation in sedimentary basins
  • Thick evaporite sequences suggest prolonged dry conditions and high evaporation rates
• Redox-sensitive trace elements (molybdenum, vanadium, uranium) in sediments provide information about ocean oxygenation and productivity
  • Enrichment of these elements in organic-rich sediments indicates anoxic conditions and enhanced preservation of organic matter
• Sedimentary geochemistry contributes to the reconstruction of past atmospheric CO2 levels and their impact on global climate
  • Weathering rates of silicate rocks, as inferred from geochemical proxies, regulate long-term atmospheric CO2 and global temperature
• The study of sedimentary geochemistry improves our understanding of Earth's climate history and its response to various forcing factors (orbital variations, volcanic activity, anthropogenic influences)