Earliest life & isotopes

Question 1

Archaea

Answer 1

Closer related to eukaryotes than bacteria

Question 2

Prokaryotes and diversity

Answer 2

Genetic diversity hinges on prokaryotes

Question 3

Liquid water

Answer 3

Liquid water at 4.4 Ga (billions of years ago)?

Question 4

Earliest life: extremophiles

Answer 4

Based on phylogenetic evidence, hyperthermophiles were likely the earliest form of life

• High temperatures
• No oxygen
• Chemosynthesis- making sugar, chemosynthesizers as basis of food chain
• Not on Earth’s surface, in Earth’s crust
• Photosynthesizers came later- chemosynthesizers came first (bottom of ocean)

Question 5

Life at 3.8 Ga?

Answer 5

Probable that life evolved between 3.8 and 3.5 Ga

Rock evidence at 3.8 Ga:
- Measured stable carbon isotopes
- Analysis is consistent with organic life
- δ13C < -25‰ (per thousand)
- Greenland

Fossil evidence at 3.5 Ga (microfossils?):
- Australia, South Africa

Question 6

Isotopes

Answer 6

• Same element (same number of protons), different mass (different number of neutrons)
• Almost chemically identical- mass differences lead to differences in physical properties and chemical reactivity
• Isotopes can be used as tracers for processes

Question 7

Stable carbon isotopes

Answer 7

C 12- most abundant, higher vibrational frequency (more energy), most plant material has more C12

C 13- present but not abundant

Question 8

Isotopic fractionation

Answer 8

Quantified difference in isotope ratios relative to a reference point
- Driven by equilibrium and non-equilibrium reactions

Question 9

Energy wells

Answer 9

Difference between repulsion curve and attraction curve

• Way to visualize bond difference
• T ∝ ½ mv2
• Light isotopes have a higher energy state than heavy isotope
• Lighter isotopes form weaker bonds than heavy isotopes- lower binding energy
• C12 is lighter than C13 (C12 has a higher energy state, C13 has a stronger bond)
• As you warm a system, difference in binding energy shrinks (easier to break bond)

Question 10

Non-equilibrium reactions

Answer 10

Require energy- e.g., photosynthesis

Lighter isotopes are more energetic, easier for them to go into product, so carbon in sugar form during photosynthesis has more C12
Products tend to concentrate more energetic isotopes

Light isotopes are more energetic and react more easily (they can get into the product more easily)
- CO2 + H2O → CH2O (sugar) + O2
- CO2 has both C12 and C13 in it- more likely that C12 will end up in CH20 (sugar)

Question 11

Equilibrium reactions

Answer 11

Ex. carbonate precipitation

Ca2+(aq) + CO2-3(aq) (carbonate ions) ←→ CaCO3 (calcium carbonate)
- Heavy isotope disproportionately concentrates with the stronger bond in the reaction- in this reaction, stronger bond is CaCO3 (more C13 in it)

Phase changes
- Evaporation strongly selects for lighter isotopes- ex. H2O evaporating from seawater into atmosphere

Question 12

Delta values

Answer 12

Sample value- negative means more C12 relative to standard ratio, positive means more C13 relative to standard

Question 13

Carbon isotope application

Answer 13

Organic matter has low, or “light” isotopic values (more C12, less C13) compared to every other carbon material; more C12 buried than C13