early traces of life, evolution and oxygenation of earth Flashcards
how old were the first footsteps of man
3.6Ma first obligate bipedal humans
how can you look at biomineralisation to reconstruct the past?
the hard parts preserved, and it enables reconstruction of earths history. Made of carbon calcium phosphate. This only started 400 Ma in the oceans.
what was discovered about fossils in Australia?
landmark discovery in early 1990s, the Apex chart. 3.5 billion years old (ga). Marble bar. Deposit contained flint-like cherts. Originally deposition in a shallow water setting. Microfossils of the early Archean apex chart- new evidence of the antiquity of life. The archaeologist found cell like structures, thin section studies which resembled microbes. Belonged to grounds not seen on fossil records until 800 Ma.
what evidence is there fore the first oxygenic photosynthesiser?
Cyanobacteria and chloroplasts use the same photosystems.
Finding the oldest unequivocal fossil evidence for life- cyanobacteria.
Oldest accepted cyanobacterial fossils are 2,000 Ma (billions),
what are stromalites?
layered accretionary structures of biological/chemical origin.
how old do stromalites go back to?
3.5 Ga, but the chemical origin argued for these that you can get the layering by weathering.
how do the stromalites form?
You get multifilament’s that grow, water comes past and gets trapped in the filaments, and you have the grown the bacteria, and this repeats- winter or summer rains.
how do stromalites fossilise?
They fossilise by starting to bind the material which stabilises the sediment. Alternating layers of grains, binding to the filaments. Within the alkal grown, you get mucus, which helps them move. Mineralisation occurs in this, due to the carbonate.
what evidence do we have for the atmosphere?
evidence of oxygen
sedimentary rocks that contain minerals
identify paleosols (ancient rocks)
use isotopic variations of gas phase elements
how do we trace ancient chemical and biogeochemical cycles?
Use stable isotopes.
Use of carbon stable isotopes for tracking biogeochem cycles.
Mostisotpoc fractionation is mass-dependent, eg, carbon
All forms of carbon-fixation preferentially takes up light isotopes.
Light isotopes 12C in all biologically fixed organic matter
Photosynthesis is 12C enriched in take up (diffusion) and carboxylation.
what do sulphur isotopes do in rich atmosphere?
In modern O2 rich atmosphere, most sulphur gases are rapidly oxidised to sulphuric acid, and rained out and accumulate in oceans as dissolved sulphate.
what do sulphur isotopes do in very low oxygen atmospheres?
sulphur can exist in a range of oxidation states from -2 to +6
In archaen atmosphere, mass-independent fractionation due to gas phase photochemical reactions.
what are pyrite grains?
Pyrite grains are rounded due to prolonged sediment transport. Pyrite turns to rust. No chemical alteration, good evidence atmosphere was oxygen free.
why are pyrite and uraite not in equilibrium ?
Sedimentary rocks contain minerals that are in equilibrium with the earths atmosphere. But pyrite and uraninite not found after 2.3 Ga. Uraninite only stable during anoxic conditions.
how do you identify paleosols that record interactions with the atmosphere?
Post 2 Ga red terrestrial sediments “red beds” common- but none in Archean.
Form in fluvial or alluvial environments with Fe3+ haematite red cement.
Thus require O2 in atmosphere to form.
what evidence is there for the great oxygenation event?
Look for marine sediments- banned iron formations (BIF)
Sedimentary deposits of alternating iron-rich haematite, and iron-poor, siliceous layer
Banding on 3 scales. Worlds main supply of iron core, at an estimated 10^14 tonnes
There are no bands after 1.8 Ga.
what does BIF stand for?
banned iron formation
why do you need high volumes of dossilved SiO2 and dissolved iron?
Hydrothermal fluids can leach Si and Fe from crust and form hydrothermal plume in seawater.
Fe2+ therefore waters must be anoxic
why must photosynthesis encounter O2?
must encounter O2, as precipitates out as Fe3+ , process must happen over a vast area to generate extensive BIF’s.
what occurs in glacial environments?
• Due to the movement of life. Ice carries and dumps every grain size. Creates glacial till, poorly sorted gravel, silt clay and sand.
• Formation of ice rafted detritus IRD
• Evidence for the snowball earth comes from glacial deposits.
• Ice-rafted drop-stones boulders and pebbles in diamictites.
There is also evidence of the snowball earth globally. Due to BIFS around the world.
what are the other factors of photosynthesis?
- Proposal taht plate tectonics collisions creating supercontinents led to increases in O2 levels, due to rapid erosion and runoff would introduce nutrients and micronutrients Fe to the oceans increasing photosynthesis. • Rapid erosion would also lead to rapid carbon burial so lessening the respirative consumption of oxygen.
- Carbon dioxide + water+ CH2O + oxygen.
- Supercontinent formation led to step O2 increase. Peaks in detrital zircons evidence of increased granite magma formation, shows increase and decrease of number.
what is the chemcial evolution of the atmosphere and ocean Archean-Proterozoic?
- Climatic feedbacks in the snowball earth cycle as more radiation is reflected, will cool more.
- Ice-albedo effect.
- Positive feedback loops. Climate cooling, increased snow and ice, less solar radiation, greater cooling.
- Long term carbon cycle
what were the climation feedbacks in the snowball earth cycle?
Initial cooling, causing ice formation, polar icecaps grow, volcanic aerosols cause cooling, increased albedo reflects solar radiation enhances cooling, ocean cycle stops, weathering stops, strong greenhouse effect and melts snowball earth, resulting in the hothouse earth. Ocean cycle starts, weathering starts, Co2 drawn down reducing greenhouse effects
what happened to life under snowball earth?
- Phototrphs survive through cracks or in “lushy” tropics.
- Creates evolutionary pressures- fittest survive
- Potential, when ice melts, the exploit massive input of nutrients to oceans and thrive and diversify into new niches
- Increases oxygenation
- Oxygenation on deglaciation
