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Chapter 10

Major Chemical Cycles

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Guiding Questions• What are the chemical reservoirs in the Earth system? • What is the difference between photosynthesis and respiration?• What happens to sugars that plants produce?• How does burial of dead plant tissue affect atmospheric CO2

and O2?• Where is organic carbon buried in large quantities?• How can carbon isotopes reveal the geologic history of carbon

burial? • How does weathering affect atmospheric CO2?

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Major Chemical Cycles• Greenhouse gases

– Atmospheric gases that trap warming solar radiation near Earth’s surface

– Climate change throughout Earth’s history

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Chemical Reservoirs• Bodies of chemical

entities that occupy particular spaces– Atmosphere– Oceans– Portion of crust– Biomass

• Flux– Expansion and contraction

of reservoirs with changes in rates at which elements or compounds flow through them

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• Feedbacks– Negative feedback

• Opposes expansion

– Positive feedback• Accelerates

expansion

Chemical Reservoirs

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Carbon Dioxide, Oxygen and Biological Processes

• Photosynthesis- respiration cycle– Water– CO2

• Photosynthesis– Captures energy– Creates sugars– Oxygen is a by-product

• Respiration– Releases energy through

oxidation of reduced carbon

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Carbon Dioxide, Oxygen and Biological Processes

• Plants remove CO2

for growth and reproduction– Plants:

• Are consumed• Decay• Are buried in

sediment

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Carbon Dioxide, Oxygen and Biological Processes

• Respiration– Gases are exchanged with

the environment– Animals respire to gain

energy from sugars from plants

• Plant and animals are in balance– To increase animals, must

increase plants – Double biomass, double O2

and CO2 fluxes

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Carbon Dioxide, Oxygen and Biological Processes

• Decomposers– Break down dead

organic debris not consumed by animals

– Bacteria, Fungi• Use respiration to

break down tissues

– Extract O2, release CO2

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Carbon Dioxide, Oxygen and Biological Processes

• Burial of plant debris affects atmospheric chemistry

• Removal of plants from system– Burial

• Reservoir of reduced C

• Erosion usually balances it

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Carbon Dioxide, Oxygen and Biological Processes

• Change in burial can increase atmospheric concentrations– O2 increases when carbon is

buried• Decomposers cannot act on

reduced carbon• Oxygen remains in atmosphere• In marine systems, aquatic

planktonic algae fulfills roles of plants

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Carbon Dioxide, Oxygen and Biological Processes

• Carbon is introduced to oceans through rivers

• Marine plankton provide additional carbon

• Anoxia aids in burial of carbon– Virtual absence of CO2

• Anoxia was widespread in mid-Cretaceous– Organic-rich mud became

black shales

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Carbon Isotopes• Carbon isotopes can trace some

aspects of atmospheric chemistry

• 12C used by plants in greater proportion than present in the atmosphere

• Rapid burial impacts atmospheric isotopic ratio– Remove proportionately more 12C– Atmosphere enriched in 13C– Oceans follow

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Carbon Isotopes

• Isotopes in limestone (CaCO3)

• Phanerozoic record indicates intervals of great change– Late Carboniferous

swamps• Excess 13C in

atmosphere and oceans

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Carbon Isotopes

• Marine phytoplankton– Preserved in times

of anoxia– Store 12C– Enrich oceans in

13C

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Carbon Isotopes

• Weathering of CaCO3 releases Ca++ and HCO3-

– Carried to oceans– Precipitate limestone

skeletal material– Carbon is stored for

long time period– Released upon

subduction

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Weathering and CO2• Mountain Building

– Weathering and erosion require CO2

• Temperature– Higher temperatures increase rates

of chemical reactions

• Precipitation– Aids in chemical weathering– Continental configuration pattern

• Vegetation– Weathering in forests higher

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Phanerozoic Trends in CO2

• Computer model for CO2

• Paleozoic Era– Devonian decrease

• Widespread forests• Increase in weathering

– Carboniferous• Burial in swamps• Aided in Gondwanaland

glaciation

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• Mesozoic Era– Reduced mountain

building• Reduced weathering• Increased CO2

– Evolution of calcareous nannoplankton and foraminifera

• Pelagic oozes• Stored CO2 as CaCO2

Phanerozoic Trends in CO2

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• Cenozoic Era– Increased mountain

building• Increased weathering

• Decreased CO2

– Increased aridity• Reduced groundwater flow,

further decreased weathering

• Decreased CO2

• Aided in glaciation

Phanerozoic Trends in CO2

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Frozen Methane• CH4

– Most produced by Archean prokaryotes

• Herbivore flatulence– Significant warming

• Stored frozen on sea floor and deep under tundra– Low temperature, high

pressure formation– Also found on continental

slope (400–1000 m w.d.)

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Frozen Methane• Release of frozen methane releases

carbon– Water at depth warms– Rapid release of greenhouse gases

(methane)– Positive feedback

• Continue to warm• Signal is 12C dominated

• Early Jurassic (Toarcian)– Climate warmed– Ocean circulation dropped– Black muds predominated

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Role of Negative Feedbacks• Temperature

– High CO2

• High temperatures• Increase weathering• Decrease CO2

– Low CO2

• Temperatures decrease• Weathering decreases• Increase CO2

• Precipitation– Ocean temperatures impact moisture

• Warm oceans decrease aridity, aid forests

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Oxygen Isotopes• Common isotopes

– 16O and 18O

• Organisms incorporate oxygen into shells

• Ratio depends on– Temperature– Salinity– Ratio of water

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Oxygen Isotopes• Temperature

– More 18O at lower temperatures

– Value can change with diagenesis and recrystallization

• Rudists– Cretaceous reef builders– Indicates seasonal

temperature range from 22–32°C

• Warmer than today

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Oxygen Isotopes• Precipitate skeletons in

proportion to water they live in

• Salinity and glaciers affect seawater ratios– Salinity increases 18O

abundance– Glaciers increase 16O

abundance in ice on land, and 18O abundance in seawater

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Oxygen Isotopes• Late Pleistocene record of glaciation

– Higher 18O/ 16O during glacial periods– Lower 18O/ 16O during interglacial periods

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Skeletal Mineralogy• Type of CaCO3 to precipitate

depends on abundance of Ca++ and Mg ++

• Mg, Ca swap in calcite– High-magnesium calcite– Mg too small to fit into

aragonite lattice

• High Mg ++/Ca ++ precipitates aragonite and high-magnesium calcite

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• Mid-ocean ridge ion exchange system– Extract Mg++ from seawater,

release Ca ++ to it– Lower Mg ++/Ca ++ when

ridges are abundant

• Correlates with sea-level change– High MOR volume, high

sea level

Skeletal Mineralogy

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Skeletal Mineralogy• Upper Cretaceous

Series chalks– Prolific calcareous

nannofossils– Accumulated rapidly

• 1 mm/year

– Driven by very low ratio of Mg++ to Ca ++

• Easy precipitation of calcite

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