1 Chapter 10 Major Chemical Cycles

2 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 CO 2 and O 2 ? Where is organic carbon buried in large quantities? How can carbon isotopes reveal the geologic history of carbon burial? How does weathering affect atmospheric CO 2 ?

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4 Major Chemical Cycles Greenhouse gases –Atmospheric gases that trap warming solar radiation near Earth’s surface –Climate change throughout Earth’s history

5 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

6 Feedbacks –Negative feedback Opposes expansion –Positive feedback Accelerates expansion Chemical Reservoirs

7 Carbon Dioxide, Oxygen and Biological Processes Photosynthesis- respiration cycle –Water –CO 2 Photosynthesis –Captures energy –Creates sugars –Oxygen is a by-product Respiration –Releases energy through oxidation of reduced carbon

8 Carbon Dioxide, Oxygen and Biological Processes Plants remove CO 2 for growth and reproduction –Plants: Are consumed Decay Are buried in sediment

9 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 O 2 and CO 2 fluxes

10 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 O 2, release CO 2

11 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

12 Carbon Dioxide, Oxygen and Biological Processes Change in burial can increase atmospheric concentrations –O 2 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

13 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 CO 2 Anoxia was widespread in mid-Cretaceous –Organic-rich mud became black shales

14 Carbon Isotopes Carbon isotopes can trace some aspects of atmospheric chemistry 12 C used by plants in greater proportion than present in the atmosphere Rapid burial impacts atmospheric isotopic ratio –Remove proportionately more 12 C –Atmosphere enriched in 13 C –Oceans follow

15 Carbon Isotopes Isotopes in limestone (CaCO 3 ) Phanerozoic record indicates intervals of great change –Late Carboniferous swamps Excess 13 C in atmosphere and oceans

16 Carbon Isotopes Marine phytoplankton –Preserved in times of anoxia –Store 12 C –Enrich oceans in 13 C

17 Carbon Isotopes Weathering of CaCO 3 releases Ca ++ and HCO3 - –Carried to oceans –Precipitate limestone skeletal material –Carbon is stored for long time period –Released upon subduction

18 Weathering and CO 2 Mountain Building –Weathering and erosion require CO 2 Temperature –Higher temperatures increase rates of chemical reactions Precipitation –Aids in chemical weathering –Continental configuration pattern Vegetation –Weathering in forests higher

19 Phanerozoic Trends in CO 2 Computer model for CO 2 Paleozoic Era –Devonian decrease Widespread forests Increase in weathering –Carboniferous Burial in swamps Aided in Gondwanaland glaciation

20 Mesozoic Era –Reduced mountain building Reduced weathering Increased CO 2 –Evolution of calcareous nannoplankton and foraminifera Pelagic oozes Stored CO 2 as CaCO 2 Phanerozoic Trends in CO 2

21 Cenozoic Era –Increased mountain building Increased weathering Decreased CO 2 –Increased aridity Reduced groundwater flow, further decreased weathering Decreased CO 2 Aided in glaciation Phanerozoic Trends in CO 2

22 Frozen Methane CH 4 –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.)

23 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 12 C dominated Early Jurassic (Toarcian) –Climate warmed –Ocean circulation dropped –Black muds predominated

24 Role of Negative Feedbacks Temperature –High CO 2 High temperatures Increase weathering Decrease CO 2 –Low CO 2 Temperatures decrease Weathering decreases Increase CO 2 Precipitation –Ocean temperatures impact moisture Warm oceans decrease aridity, aid forests

25 Oxygen Isotopes Common isotopes – 16 O and 18 O Organisms incorporate oxygen into shells Ratio depends on –Temperature –Salinity –Ratio of water

26 Oxygen Isotopes Temperature –More 18 O 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

27 Oxygen Isotopes Precipitate skeletons in proportion to water they live in Salinity and glaciers affect seawater ratios –Salinity increases 18 O abundance –Glaciers increase 16 O abundance in ice on land, and 18 O abundance in seawater

28 Oxygen Isotopes Late Pleistocene record of glaciation –Higher 18 O/ 16 O during glacial periods –Lower 18 O/ 16 O during interglacial periods

29 Skeletal Mineralogy Type of CaCO 3 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

30 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

31 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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