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.