Deep time: Lectures 1-8 Flashcards
Dates and key events for Hadean, Archean, Proterozoic, Phanerozoic, Cenezoic era and Quaternary period
Hadean (4.6-3.8 Ga) - origin of Earth, anoxic atmosphere
Archean (3.8-2.5 Ga) - origin of life in form of single-celled bacterial organisms, lowered CO2
Proterozoic (2.5-0.54 Ga) - The Great Oxidation, Snowball Earths, origin of Eukaryotes, boring billion, and animals by 0.58 Ga
Phaneozoic (0.54-0 Ga) - plants, mass extinctions
Cenezoic (65-0 Ma) - declining and levelling off CO2
Quaternary (Last 2.6 billion years) - Pleistocene and Holocene (11.5 Ka)
Describe the early atmosphere (Kasting, 1993)
High amounts of CO2, N2, CH4, H2O
O2 rose naturally and later, whilst greenhouse gases lowered to counter brightening Sun
Describe the Big Bang and formation of the elements
- Occurred 13.7 Ga, taking 9.1 billion years for star formation, and eventually Earth 4.5 Ga
- Red-shift and CMBR suggest a continually expanding universre
- Initial cooling after big bang formed neutrons - decay and collide to form H atoms; nuclear synthesis (stops, unstable masses 5/8)
- Nuclear fusion in large stars produces heavier elements up to Fe
- Elements heavier than Fe from neutron capture after large stars explode into super novas then cool into neutron stars; elements dispersed
Describe the formation of the solar system i.e. rocky planet formation
- Coagulation - dust collisions (10^4 yrs)
- Runaway accretion - growth by collisions to 1000km planetesimals
- Oligarchs - growth by accretion
- Embryo planet merging - embryos collide, Earth-sized, v. slow timescale (10^8)
Describe the formation of Earth. Refer to volatiles, the core and the formation of the moon.
Earth & core:
- Non-volatile elements formed by stars assumed to form core of rocky planets e.g. Si and Fe
- Gravitational energy heated Earth’s inteior as accretion occurred
- Melting caused dense Fe droplets to form the core
- Fe and Ni abundant/dominant in core, but Si is not despite not volatile; because Fe and Si are immiscible
- Si convected upwards in partial melt of mantle = core differentiation
Moon:
- Anomalously large - from collision with ‘Theia’
- Moon has less dense, small core
- Was once closer to Earth, debris ring coalesced into moon - Earth’s spin momentum transferred
How did the oceans and atmosphere form? What evidence is there for liquid water?
Oceans & atmosphere:
- Out-gassing - some volatiles stored in mantle, out-gassed to surface
- Extraterrestrial impacts from water-rich bodies; carbonaceous chondrites
Liquid water:
- Zircons (4.4 Ga); dated and found within igneous rock
- Existence of zircons suggests active hydrological cycle for erosional/weathering and tectonic processes
- Also, enriched in 18^O, suggests water was present (similar to present day isotopes??)
Oldest rocks dated at 4.28 and 4.04 Ga (Quebec and Canada)
When was the first life evidenced; where and how (x5)?
All occurring within 3.8-3.2 Ga…
- Banded Iron Formations (3.8 Ga) in oldest sedimentary rocks - evidence for existence of some O
- Turbidites (3.8 Ga) - oldest reduced carbon, C rich; graphite with 12-C, could have been microorganisms
- Stomatolites (3.5 Ga) - odd bunps/dome structures indicating microbial communities
- 13-C depleted carbon in oldest fossil cells (3.5 Ga) - although contradicted by Brasier et al., 2002
- Dividing cells (3.26 Ga, Swaziland); caught in the act =THE WIDELY ACCEPTED POINT OF ORIGIN
Alternatives - Rare Earth / Transpermia / Deep Hot Biosphere
What is continental drift? What evidence is there for it, sea-floor spreading, and fault lines/boundaries? What is the rate of divergence/convergence?
Alfred Wegner (1880-1930) - continents on plates move over time; oceanic (more dense) and continental (less dense); theory of Pangea
Rate of convergence/divergence; 100mm/yr
Pangea evidence:
Biological (fossils matching up), Geological (shelves, cratons) and Climatological (glacial till at tropics)
Boundary evidence:
- 1960s mapped ocean bathymetry - marine mountain ranges etc.
- Global distribution of seismicity; fault lines
Sea-floor spreading evidence:
- Paleomagnetism (aligning and matching ion orientation according to geomagnetic reversal)
- Sediment depth (deeper further away, dated older too)
Types of plate boundaries? What are cratons?
Divergent/Constructive - move apart; crust created = rift valleys, mid-ocean ridges e.g. East African
Convergent/Destructive - plates collide; denser crust subduct beneath other, melting, volatiles and pressure, magma rises = trenches and volanoes (oceanic-continental) or island arcs (ocean-ocean) or buckling and mountains (continent-continent) e.g. Andes, Himalayas
Passive - innactive boundaries
Cratons = continental shield
- Oldest rock found on surface since most mountain ranges conain “young” rock because of erosion
- E.g. Canada, Greenland, Australia up to 3 Ga
What are supercontinent cycles? How many have occurred since formation of Earth and how many years dows a cycle take to complete?
Movement of supercontinents; broken up, move, then join again in THE WILSON CYCLE
Estimated 8-10 cycles in Earth’s history; complete in 500x10^6 years
3 currently proposed supercontients:
- Nuna (1.3 Ga)
- Rodinia (750 Ma)
- Pangaea (225 Ma)
What is the equation for Silicate Weathering including Carbonate deposition?
CaSiO3 + CO2 –> CaCO3 + SiO2
What is the FYSP, HZ, and H-R Diagram? Whilst your at it, describe star lifecycles…
Faint Young Sun Puzzle - when Earth formed 4.6 Ga Sun was 30% dimmer, so Earth should have been frozen. BUT there is evidence for liquid water at this time (zircons) - so how was the Earth kept warm despite the lack of solar irradiation
Habitable Zone - zone in which liquid water can exist on planet based on distance from star and how bright it is; temps of 0-70C; not fixed, HZ will move further as star brightens
Herzprung-Russell Diagram - categorises stars based on luminosity (brightness) and temp (colour); shows evolution of stars
Bigger than Sun = main sequence –> produce Fe -> super red giant –> supernova –> neutron star/black hole
Same/smaller than Sun = red giant –> white dwarf
How is the solution to the puzzle proposed? What is the role of silicate weathering and then what role does life play?
GHG:
- Greenhouse gas effect must have been greater
- High levels of certain GH gases; many suggest CO2 or ammonia
Carbon cycle/weathering:
- Largest C store = buried oxidized carbonate
- Smallest C store = surface
- Input (volcanoes & heterotrophs) balanced by organic carbon burial (dead animals) and silicate weathering
- CO2 dissolves in rainwater, produces carbonic acid, reacts with Ca or Mg in rocks, ions washed into ocean, carbonate used by organisms (shells)
- FEEDBACK; temp increases from brightening Sun, Si weathering speeds up, more C buried, less CO2 = less heat trapping…
Climate regulation:
- Plant amplification of Si weathering (e.g. fungus breaking rocks to reach minerals)
- Also increase phosphorus cycle which increases burial of also (since it increases ocean productivity)
- Plants caused CO2 to decline and level out
How many snowball events have been thought to have occurred; give their dates.
3 within the proterozoic and neoproterozoic eon
- Makganyene, 2.22 Ga
- Sturnian, 710 Ma
- Marinoan, 640 Ma
How does a Snowball Earth occur - what triggered it (x2)? How would it ‘get out’ of one? How did complex life ancestors survive?
Causes/triggers:
- Runaway feedback; cooling causes ice sheet expansion, if reaches 30 degree latitude tipping point, albedo so high that ice sheets ‘snap shut’ - what drove a reduction in CO2 then?
GEOLOGICAL = Rodinia supercontinent breaking up, continents dispersed at tropics = rapid weathering, increased C burial AND Large Igenous Province = increased basalt weathering
BIOLOGICAL = colonisation by fungi, cyanobacteria, algae and lichens (1430-600 Ma) = x10 faster weathering = increased C burial
Escape?
- Accumulation of CO2 in atmosphere since output inhibited (dry, arid, no rain, no Si weathering)
- Increased greenhouse effect; volcanic areas melting ice, degassing of ocean floor, cracks in ice etc.
- Dark ocean reduces albedo, runaway feedback enables global melt
Survival?
Warm refuges by hot springs
