User:Benjah-bmm27/degree/2/RPE

Geochemistry, RPE
"Origin and Fate of Organic Matter in the Geosphere"
 * Biogeochemistry
 * Organic geochemistry
 * Oxygen evolution and history of life on Earth:
 * Earth began with a reducing atmosphere, composed mostly of reduced gases
 * 3700-2000 Ma - banded iron formations (alternating Fe3O4/Fe2O3 and silicate (chert) layers) due to early marine photosynthesis: O2 causes Fe2+ → Fe3+
 * 2000 Ma - Great Oxidation Event - O2 started to evolve from cyanobacteria in the oceans into the atmosphere
 * 2000 Ma-present - Continental Red Beds - widespread oxidation at Earth's surface
 * 1500 Ma - Gypsum formation: atmospheric SO2 + O2 → SO42−, then + Ca2+ → CaSO4
 * Isotope geochemistry
 * Isotope ratio mass spectrometry
 * $$\delta ^{13} {\text{C}} = \left[ {\frac - 1} \right] \times 1000{\text{ permil}}$$
 * $$R_ = \frac$$
 * $$R_ = R_  = 0.01124$$ from Peedee belemnite
 * Carbon cycle
 * Biosphere:
 * Photosynthesis: CO2 + H2O → CH2O + O2
 * Respiration: CH2O + O2 → CO2 + H2O
 * Exchange of CO2 with oceans
 * Keeling Curve


 * Methane CH4:
 * Atmospheric methane
 * Sources:
 * Natural: wetlands (habitat for methanogenic bacteria), termites, oceans, methane hydrates
 * Methane catastrophes, e.g. Permian–Triassic extinction event 250 Ma
 * Anthropogenic: energy, ruminants, rice paddies, landfills, biomass burning (biofuel), waste
 * Arctic methane release, methane clathrates
 * Sinks: tropospheric destruction, loss to stratosphere, soils (consumed by methanotrophs)
 * Low affinity (high capacity) methanotrophs - live in methanogenic environments, reoxidise ~90% of CH4 to CO2 before it escapes, adapted to [CH4] up to 40 ppm
 * High affinity (low capacity) methanotrophs - live in well aerated soils (upland, forests, grasslands, farmland), adapted to [CH4] up to 12 ppm
 * Investigate high affinity methanotrophs by following their characteristic PFLAs. Analyse 13CH4-fed methanotroph colonies for 13C-rich PFLAs to find out which PFLAs are characteristic.
 * Methanotrophs cannot do their job if their habitats are destroyed - woodland → agricultural land means [N] ↑ so methanotrophs inhibited
 * Need to use land correctly to protect methanotrophs and allow them to help prevent [CH4] ↑
 * Biochemicals:
 * Carbohydrates for structure and energy storage, e.g. cellulose, polymer of ~10000 glucose monomers
 * Lignins for structure of cell walls in higher plants, high m.w. polyphenolic heteropolymers of coumaryl alcohol, coniferyl alcohol, sinapyl alcohol
 * Proteins for structure, catalysis (enzymes), storage - biopolymers of amino acids, 1-500 kDa
 * Lipids - all biochemicals that are insoluble in water but soluble in organic solvents, <1000 Da, often fully saturated chains and rings - e.g. alkanes, wax esters, triglycerides, sterols:
 * straight-chain lipids - n-alkanes (odd numbers predominate, C13-23 in algae, C23-33 in higher plants), n-fatty acids and n-fatty alcohols (even numbers predominate, C16 and C18 everywhere, while C20 and C30 found in higher plant leaf waxes
 * branched-chain fatty acids (phospholipids) characteristic of bacteria, esp. C15, C17, plus corresponding alcohols and alkanes; acyclic isoprenoids (e.g. phytol) based on C5 isoprene
 * sterols and hopanols are diagnostic of particular organisms - hormones and membrane rigidifiers, e.g. C27 cholesterol in animals, C28 campesterol and C29 sitosterol in higher plants, C28 ergosterol in fungi, C30 dinosterol in algae and bacteriohopanetetrol in bacteria (which cannot make sterols)
 * C3 and C4 plants:
 * C3 carbon fixation: C3 plants adapted to moderate climates, use Calvin cycle to fix CO2 as 3-phosphoglycerate, δ13C = −28 to −34 ‰ in alkanes (bulk tissue still −26 ‰)
 * C4 carbon fixation: C4 plants adapted to hot or dry climates, use Hatch-Slack pathway to fix CO2 as oxaloacetate, δ13C = −20 to −24 ‰ in alkanes (bulk tissue still −26 ‰)
 * Bengal fan - an alluvial fan: may be contaminated, but very good temporal resolution. Palaesol and ocean sediment carbon isotope records match and suggest a shift from C3 to C4, starting 9 Ma, peaking 4 Ma, and now returning towards C3 values.
 * Paleosols: in situ, so less contamination, but poor temporal resolution because mixing occurs
 * Biomarkers of terrestrial vegetation, e.g. C27, C29, C31, C33 n-alkanes


 * News reports:
 * Key role of forests 'may be lost', BBC News Online, 18 April 2009