early traces of life, evolution and oxygenation of earth

Question 1

how old were the first footsteps of man

Answer 1

3.6Ma first obligate bipedal humans

Question 2

how can you look at biomineralisation to reconstruct the past?

Answer 2

the hard parts preserved, and it enables reconstruction of earths history. Made of carbon calcium phosphate. This only started 400 Ma in the oceans.

Question 3

what was discovered about fossils in Australia?

Answer 3

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.

Question 4

what evidence is there fore the first oxygenic photosynthesiser?

Answer 4

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),

Question 5

what are stromalites?

Answer 5

layered accretionary structures of biological/chemical origin.

Question 6

how old do stromalites go back to?

Answer 6

3.5 Ga, but the chemical origin argued for these that you can get the layering by weathering.

Question 7

how do the stromalites form?

Answer 7

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.

Question 8

how do stromalites fossilise?

Answer 8

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.

Question 9

what evidence do we have for the atmosphere?

Answer 9

evidence of oxygen
sedimentary rocks that contain minerals
identify paleosols (ancient rocks)
use isotopic variations of gas phase elements

Question 10

how do we trace ancient chemical and biogeochemical cycles?

Answer 10

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.

Question 11

what do sulphur isotopes do in rich atmosphere?

Answer 11

In modern O2 rich atmosphere, most sulphur gases are rapidly oxidised to sulphuric acid, and rained out and accumulate in oceans as dissolved sulphate.

Question 12

what do sulphur isotopes do in very low oxygen atmospheres?

Answer 12

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.

Question 13

what are pyrite grains?

Answer 13

Pyrite grains are rounded due to prolonged sediment transport. Pyrite turns to rust. No chemical alteration, good evidence atmosphere was oxygen free.

Question 14

why are pyrite and uraite not in equilibrium ?

Answer 14

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.

Question 15

how do you identify paleosols that record interactions with the atmosphere?

Answer 15

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.

Question 16

what evidence is there for the great oxygenation event?

Answer 16

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.

Question 17

what does BIF stand for?

Answer 17

banned iron formation

Question 18

why do you need high volumes of dossilved SiO2 and dissolved iron?

Answer 18

Hydrothermal fluids can leach Si and Fe from crust and form hydrothermal plume in seawater.
Fe2+ therefore waters must be anoxic

Question 19

why must photosynthesis encounter O2?

Answer 19

must encounter O2, as precipitates out as Fe3+ , process must happen over a vast area to generate extensive BIF’s.

Question 20

what occurs in glacial environments?

Answer 20

• 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.

Question 21

what are the other factors of photosynthesis?

Answer 21

• 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.

Question 22

what is the chemcial evolution of the atmosphere and ocean Archean-Proterozoic?

Answer 22

• 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

Question 23

what were the climation feedbacks in the snowball earth cycle?

Answer 23

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

Question 24

what happened to life under snowball earth?

Answer 24

• 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

Question 25

what was Rodina and what happened to the rifting?

Answer 25

Supercontinent Rodinia formed 1000 Ma. Rodinia breaks up around 825 Ma. Rifting produced smaller land masses in quatorial latitudes, increased weatherability of land surface, making it wetter, moving to warmer. This resulted in drawn down of CO2 and cooling

Question 26

what does rifting lead to?

Answer 26

Rifting also led to the eruption of vast basaltic lava provinces- higher weatherable volcanic rocks, enhanced weathering and CO2 drawn down.

Question 27

what were prgoressive oxygenation of ocean?

Answer 27

• Post glacial oxygenation led the way for the development of animal life
  
  • Also charging redox then supplied metal cofactors that helped stimulated radiation of the phytoplankton.

Question 28

what was the evolution and diversification of metazoan life in late proterozoic?

Answer 28

• The Ediacaran faunas
  
  • Significance- earliest identified complex metazoan life.
  • Time of slime, bacterial and cyanobacterial mats covered the shallow oceans. Some sat on the mats and ingested them, some mined into or under the mats, some were fronds sticking up into the water column.
  • Grazers and filter feeders helped clear the water column, so taht light could penetrate further and sustain more phytoplankton.

Question 29

what did the proterozoic life lead to?

Answer 29

the cambrian explosion

Question 30

what happened in the cambrian explosion?

Answer 30

• All the known groups started to evolve.
  
  • Why? Due to a race product, as there were lots of carbon ions in the sea.
  • Onset of biomineralization, calcification initially as “detoxification”, then selected as defence against (increasing) levels of predation.

Question 31

what did the cambrian explosion coincide with?

Answer 31

• Coincide with increase to near-modern levels of atmospheric O2, likely due to rapid expansion of eukaryotic phytoplankton.

Question 32

why is oxygen important?

Answer 32

• Reduction of O2 provides largest free energy release per electron.
  
  • Aerobic metabolism provides an order of magnitude more energy for a given intake of food than anaerobic metabolism
  • Oxygen pressures comparable to PAL are required to support the circulatory systems of complex metazoans.
  • The timescale to reach critical O2 threshold for multicellular life 3.9 Ga, is half of its main sequence lifetime.
  • Large life has co-evolved with the history of atmospheric O2.