Geochemical Cycling

TERMINOLOGY

• RESERVOIRS -BODIES OF CHEMICAL ENTITIES THAT OCCUPY PARTICULAR SPACES
  • Examples - Atmosphere, Ocean, Biosphere, Crust
• FLUXES - MOVEMENT OF CHEMICAL MATERIALS FROM ONE RESERVOIR TO ANOTHER
  • Source - Originating reservoir
  • Sink - Receiving reservoir
• FEEDBACK - RESPONSE OF A RESERVOIR TO A CHANGE IN FLUX

WATER CYCLE

• RESERVOIRS BY SIZE -	OCEAN	97.25000%
  	GLACIERS	2.05000%
  	GROUNDWATER + SOIL	0.68500%
  	LAKES + RIVERS	0.01010%
  	ATMOSPHERE	0.00100%
  	LAND PLANTS	0.00004%

• 1ST ORDER CYCLE
  • Water evaporates from the ocean reservoir into the atmosphere reservoir
  • The atmosphere reservoir transports water vapor over land
  • Precipitation falling on land is incorporated into the glacier, groundwater + soil, lake + river, and land plant reservoirs
  • Water is returned to the ocean mostly by rivers and groundwater flow
• 2ND ORDER CYCLES
  • Water transpires from land plants and evaporates from lakes and rivers into the atmosphere reservoir
  • Precipitation will fall on the ocean as well as land
• INTERACTION WITH ROCK CYCLE
  • Water chemically reacts with igneous and metamorphic rocks to form sediments
  • Water erodes and transports sediments

PHOTOSYNTHESIS-RESPIRATION CYCLE

• PHOTOSYNTHESIS-RESPIRATION CYCLE IN PLANTS

  • Photosynthesis

    Sunlight ->

    CO2 + H2O + inorganic N & P + energy <-> CH2O+ other organic compounds with N & P + O2

    <- Metabolic processes

    Respiration

  • Plants use some sugars & other organic compounds to build tissue
  • So, plants extract CO₂ + H₂O + inorganic N & P from atmosphere, groundwater & crust reservoirs and put them into the biosphere reservoir
    • CO₂ & N FROM ATMOSPHERE (N₂ converted to usable form by certain microorganisms)
    • H₂O FROM GROUNDWATER
    • P WEATHERING OF PHOSPHORUS-BEARING MINERALS IN CRUST
  • O₂ into atmosphere reservoir
• FATE OF ORGANIC TISSUE
  • Plant tissue eaten by animals, OR decomposed by bacteria or fungi
    • RECYCLES CO₂ + H₂O + INORGANIC N & P BACK INTO ATMOSPHERE, GROUNDWATER & CRUST RESERVOIRS AND EXTRACTS O₂ FROM ATMOSPHERE
  • Burial in sediments, usually in anoxic (no O₂ present) environments (like swamps)
    • NET EFFECT IS EXTRACTION OF CO₂ + H₂O + INORGANIC N & P FROM ATMOSPHERE, GROUNDWATER & CRUST RESERVOIRS AND LONG TERM STORAGE IN CRUST (See figure 10-5)
    • PUTS O₂ INTO ATMOSPHERE (See figure 10-6)
      • Some organic tissue turns into petroleum, natural gas and coal
  • Large changes in the rate of burial has large effect on the CO₂ & O₂ concentrations of the atmosphere reservoir
    • HIGH RATES OF BURIAL (like during the Carboniferous) = CO₂ DECREASES AND O₂ INCREASES IN ATMOSPHERE
    • LOW RATES OF BURIAL = CO₂ INCREASES AND O₂ DECREASES IN ATMOSPHERE

USE OF ISOTOPES

• CARBON ISOTOPES (¹²C AND ¹³C)
  • d¹³C relates the ratio of ¹³C to ¹²C in a sample to that in a standard
  • Plants use relatively more ¹²C than ¹³C during photosynthesis, so the ratio of ¹³C to ¹²C in organic tissue is less than the ratio of ¹³C to ¹²C in the atmosphere
  • When rates of burial increase, the crust reservoir becomes enriched in ¹²C and the atmosphere and ocean resevoirs become enriched in ¹³C
  • Shallow-marine limestones record the carbon isotope composition of the ocean (& the atmosphere because of rapid mixing between the two)
    • WHEN d¹³C OF SHALLOW-MARINE LIMESTONES INCREASES, RATES OF ORGANIC CARBON BURIAL INCREASED AND WHEN d¹³C OF SHALLOW-MARINE LIMESTONES DECREASES, RATES OF ORGANIC CARBON BURIAL DECREASED
    • A PLOT OF d¹³C FOR PHANEROZOIC EON LIMESTONES SHOWS A PRONOUNCED PEAK DURING THE CARBONIFEROUS WHEN LARGE VOLUMES OF CARBON ACCUMULATED IN COAL SWAMPS (See figure 10-9A)
    • d¹³C FOR PHANEROZOIC EON LIMESTONES ALSO SERVES AS AN ESTIMATE OF THE O₂ CONCENTRATION OF THE ATMOSPHERE BECAUSE WHEN BURIAL RATES OF ORGANIC CARBON INCREASE, ATMOSPHERIC O₂ INCREASES AND VICE VERSA (See figure 10-9B)
• ATMOSPHERIC CO₂
  • Atmospheric CO₂ concentrations are controlled not only by the size of the buried organic carbon reservoir but also by weathering reactions
  • Atmospheric CO₂ combines with water to form carbonic acid (H₂CO₃) which dissolves limestone and silicate minerals to form dissolved Ca⁺⁺ and bicarbonate ions (HCO₃^-)
    • CACO₃ IS PRECIPITATED TO FORM LIMESTONE AS CARBONATE PLATFORMS ON CONTINENTAL SHELVES OR AS DEEP-SEA PELAGIC DEPOSITS AFTER RIVERS HAVE TRANSPORTED DISSOLVED CA⁺⁺ AND BICARBONATE TO THE OCEAN (See figure 10-11)
    • METAMORPHISM AND MELTING OF LIMESTONE AT SUBDUCTION ZONES RELEASES CO₂ BACK TO THE ATMPOSPHERE
  • Factors controlling weathering processes
    • MOUNTAIN BUILDING ACCELERATES WEATHERING
    • WEATHERING IS FASTER IN WARM, TROPICAL CLIMATES THAN COLD CLIMATES
    • WEATHERING IS FASTER IN MOIST CLIMATES THAN IN DRY CLIMATES
      • Global distribution of moist and dry climates is controlled by the distribution and size of continents
    • WEATHERING IS FASTER IN VEGETATED AREAS THAN IN NON-VEGETATED AREAS
• PHANEROZOIC TRENDS IN ATMOSPHERIC CO₂ (See figure 10-13)
  • Geochemical computer models used to estimate trends using information on amount of organic carbon buried, amount of vegetation and other factors derived from carbon isotope ratios and other data sources
  • CO₂ content of the atmposphere is very high in the Early Paleozoic, moderately high in the Mesozoic, and low in the Late Paleozoic and Cenozoic
    • STEEP DECLINE BEGINNING IN DEVONIAN RESULTS FROM 1) INCREASED WEATHERING BECAUSE OF THE EVOLUTION OF LAND PLANTS CAPABLE OF LIVING IN UPLAND AREAS AND 2) BURIAL OF ORGANIC CARBON IN CARBONIFEROUS COAL SWAMPS
      • Reduction in atmospheric CO₂ probably led to global cooling and the advent of widespread glaciation in the southern hemisphere
    • VERY HIGH CO₂ IN THE EARLY PALEOZOIC PROBABLY DID NOT LEAD TO EXCESSIVE TEMPERATURES DUE TO A DIMMER SUN
      • Global distribution of moist and dry climates is controlled by the distribution and size of continents
    • MODERATE CO₂ DURING THE MESOZOIC DUE TO 1) DECREASED MOUNTAIN BUILDING AND 2) MORE LIMESTONE DEPOSITED AS PELAGIC SEDIMENT, WHICH IS RECYCLED MORE QUICKLY THAN CONTINENTAL SHELF PLATFORM CARBONATES
  • Negative Feedbacks: How CO₂ Levels are Held in Check
    • INCREASING CO₂ RESULTS IN INCREASED TEMPERATURE - LEADING TO INCREASED CHEMICAL WEATHERING AND PRECIPITATION, WHICH BOTH LEAD TO INCREASED RATE OF CO₂ EXTRACTION FROM THE ATMOSPHERE DECREASING CO RESULTS IN DECREASED TEMPERATURE - LEADING TO DECREASED CHEMICAL WEATHERING AND PRECIPITATION, WHICH BOTH LEAD TO DECREASED RATE OF CO₂ EXTRACTION FROM THE ATMOSPHERE
• OXYGEN ISOTOPES (¹⁶O AND O)
  • d¹⁸O relates the ratio of ¹⁸O to ¹⁶O in a sample to that in a standard
  • The d¹⁸O composition of shells and skeletons made of CaCO₃ differs from the environment and depends on temperature during formation - relatively more ¹⁸O than ¹⁶O during low temperatures
  • d¹⁸O varies with glacial ice volume and salinity - increased evaporation extracts ¹⁶O and leaves behind ¹⁸O
    • DURING THE QUATERNARY, d¹⁸O OF MARINE CARBONATE REPRESENTS MOSTLY GLACIAL ICE VOLUME CHANGES RATHER THAN TEMPERATURE
• OCEAN CHEMISTRY AND SKELETAL MINERALOGY
  • Whether calcium carbonate forms calcite or aragonite and high-magnesium calcite depends on the Mg⁺⁺/Ca⁺⁺ ratio of seawater (calcite forms when the Mg⁺⁺/Ca⁺⁺ ratio is low, aragonite and high-magnesium calcite when the Mg⁺⁺/Ca⁺⁺ ratio is high)
    • THE MG⁺⁺/CA⁺⁺ RATIO DEPENDS ON THE SEAFLOOR SPREADING RATE AT THE MID-OCEAN RIDGE - HIGHER SPREADING RATES = GREATER ALTERATION OF OCEAN CRUST BY HYDROTHERMAL WEATHERING AND A LOWER MG⁺⁺/CA⁺⁺ RATIO
  • The kinds of organisms forming reefs and pelagic sediment depends on whether calcite or aragonite and high-magnesium calcite is stable (See figure 10-18)

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