Oxygen revolution Flashcards
Great oxidation event (GOE)
- Half of earth’s history had no oxygen in atmosphere
- ~2.3 Ga
- Rise to maybe 1% O2
- Allowed for single-celled eukaryotes to survive
Photosynthesis and O2
Photosynthesis is often associated with the production of O2
- Oxygenic photosynthesis
- CO2 + H2O + hv ⇒ CH2O + O2
- Cyanobacteria (prokaryotes)
Stromatolites
- Layered sedimentary rock
- Domed morphology (cross section)
- Oldest: 3.5 Ga
- Archean stromatolites
- Present-day stromatolites
How they form: when enough sediment inundates microbe mats, the living cells migrate upwards which creates the layered look
Present-day stromatolites
- Today, cyanobacteria can form stromatolitic-like mounds
- Today, these are oxygenic photosynthesizers
O2 in atmosphere 3.5 Ga? Maybe..
- Some cyanobacteria today perform anoxygenic photosynthesis: CO2 + 2H2 + hv ⇒ CH2O + H2O (using methane instead of water)
- Stromatolites can form abiotically
- Stromatolites tell you about O2 in their immediate vicinity
Sugary mats- eaten pretty quickly, not competitive on most places on earth
Banded iron formations (BIFs)
Iron-rich layered rocks
Form/deposited in ocean (marine rock)
- Rock ending up in ocean started by weathering on land (brought up by tectonics)
- Disappear at ~1.9 Ga
- With no O2, Fe flows to ocean in soluble Fe2+ form
- Some process precipitates out Fe3+ in solid form on a massive scale
- With O2, rusting takes place on land; loose sediments buried on land/turned to rock– presence of oxidized iron in terrestrial rocks by 2.3 Ga
Fe2+ Reduced iron: Ferrous, very soluble
Fe3+ Oxidized iron: Ferric, less soluble, precipitates rust (Fe2O3), More common on earth’s surface today
You can’t form these iron rich rocks in ocean in today’s earth (no BIFs)
- Atmosphere that isn’t oxidizing (not much O in it), weathering on land still happens but iron that gets liberated from that weathering process tends to stay in soluble form (Fe2+) so it’s dissolved in water making its way to ocean → when there’s less O2 in atm you get a lot of soluble iron in ocean
- BIFs iron is an oxidized form
- Stromatolite structures perhaps had oxygenic photosynthesizers creating them
O factories in shallow ocean (cyanobacteria mats), locally lots of iron precipitated on these mounds
Carbonate δ13C
- Organic matter burial- geologic “leak”
- Carbonate getting buried, oxygen left behind
- 2.3 Ga
- Decreases CO2 and increases O2
- Linked to cold climates
- Complex multicellular eukaryotic life
Two options for evolution of oxygenic photosynthesis
Option 1
⇒ post-death- organic matter is NOT buried
⇒ oxidative respiration removes O2 and releases CO2
⇒ no net change in atmospheric CO2 or O2
Option 2
⇒ post-death- organic matter IS buried
⇒ O2 produced during photosynthesis stays in atmosphere; CO2 consumed during photosynthesis removed from earth’s surface
⇒ increased O2; decreased CO2
