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
  • $\delta ^{13}{\text{C}}=\left[{{\frac {R_{\text{sample}}}{R_{\text{standard}}}}-1}\right]\times 1000{\text{ permil}}$
  • $R_{\text{sample}}={\frac {^{13}{\text{C}}}{^{12}{\text{C}}}}$
  • $R_{\text{standard}}=R_{\text{PDB}}=0.01124$ 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

formula	CH4	→	CH3OH	→	HCHO	→	HCO2H	→	CO2
molecule	methane		methanol		formaldehyde		formic acid		carbon dioxide
C-O bonds	0		1		2		3		4

• • • 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, C_13-23 in algae, C_23-33 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