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Planet Earth > Habitable planet > Flashcards

Habitable planet Flashcards

1
Q

what is the order and timescales of the precambrian

A

Hadean 4.6-4 Ga
Archean 4-2.5 Ga
Proterozoic 2.5-0.54 Ga

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2
Q

conditions in precambrian low latitude shallow marine environment

A

bacterial mats
- green + photosynthesising
- silver bubbles - methane from below (previous mat decaying)
- chunks missing = storms

snails destroy bacterial mats, once gastropods develop they decline rapidly

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3
Q

brief timeline of earths history (precambrian 4Ga - 541Ma)

A

Hadean
4.5Ga - Giant impact hypothesis

Archean
≈4Ga - Late heavy bombardment, life quickly after

3.8Ga - oldest marine sediments

3.4Ga - oldest bacteria

Proterozoic
2.1 Ga - first photosynthesis

2.45 - 1.85 Ga - GOE -> consequence of photosynthesis

“boring billion” after GOE

720 - 635 Ma - Snowball Earth
635-541 Ma - first animals immediately after snowball earth

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4
Q

What were the main differences on earth >3 Ga (before 3 billion years ago)

A

Common impacts >K-P event (>150km)

Moon
- 4x closer
- stronger tides

Plate thickness and structure
- thin primordial crust
- no oceanic and continental (until Archean)
- plate tectonics began in Proterozoic

Climatic differences
- weak insolation
- high GHG = high temp = no glaciation
(until GOE decrease methane proportion)

Surface differences
- high radioactivity
- hot metal rich crust
(extreme conditions -> extremophiles)

Atmospheric differences
- no O2 or O3 (ozone) -> high UV

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5
Q

difference between autotroph and heterotroph

A

where gets carbon for biomass

auto = from CO2
hetero = from organic molecules

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6
Q

difference between phototroph and chemotroph

A

source of energy

photo = sunlight
chemo = oxidation of chemicals

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7
Q

difference between lithotroph and organotroph

A

where gets electrons for reducing power

litho = water/H2/rocks
organo = organic molecules

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8
Q

what is likely the sequence of evolution of autotrophic strategies

A

anaerobic chemolithotrophy
(no oxygen, no sun)

anaerobic anoxygenic phototrophy
(no oxygen, sun, no photosynthesis)

oxygenic phototrophy
(photosynthesising)

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9
Q

what are traits common to all cells and why is this useful to know

A
  • limited building blocks (h2o, C, N, P)
  • cell membrane (fatty acid bilayer)
  • chiral molecules (left AA, right nucleic acids) (life selected one)
  • common suite of organic molecules
  • replicate and pass information one generation to the next

useful as ancestors also likely to have had the same
also points to single origin

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10
Q

what is the ‘RNA world’ hypothesis

A

early life had no DNA, but used RNA for same role
- information storage
- replication
- protein synthesis
- enzymatic activity

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11
Q

what are the 3 hypotheses for the origin of life

A
  • Oparin-Haldane hypothesis
  • Panspermia hypothesis
  • Hydrothermal hypothesis
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12
Q

explain the opaline-haldane hypothesis

A

= ‘primordial soup’

energy from lightning could synthesis amino acids easily in early atmosphere, proved by experiments
however, recently with known early composition, very few form + difficult

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13
Q

explain the panspermia hypothesis

A

= delivery of simple organic molecules from space

from - carbonaceous chondrites, meteorites, comets, dust clouds

e.g. uracil (component of RNA) found in meteorite
all 20 AA found in comet
difficult to test due to contamination

ease of synthesis in early solar system?

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14
Q

explain the hydrothermal vent hypothesis

A

= localises synthesis of important simple organic molecules to alkaline vents

  • heat + fluids = favourable conditions for reactions
  • link with extremophiles more common in geological past
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15
Q

how do we estimate dates using molecular clocks

A
  • random mutations that divide a lineage occur at a given rate
  • rate of mutation with percentage genetic differences to another species can be used to estimate a date
  • different mutation rates though time make more complex
  • LUCA estimated in Hadean - remarkable
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16
Q

how do stromatolites form

A
  • grow upwards in columns to reach light
  • domal form
  • bateria growth covered with sediment leads to another bacterial layer to grow above and process repeats
17
Q

what are some issues with identifying stromatolites in the geological past

A

identifying by purely morphological structure can be misleading

e.g.
- Isua, greenland: actually deformation features, identified when cut at 90º
- ‘Taylor-stromatolite’: (convex laminae, concentric circles when cut 90º) -> actually layers of lead based car paint

18
Q

what are 3 strands of evidence used to distinguish biotic from abiotic in the geological record

A
  • viable context (geological setting)
  • biological morphology
  • tiers of biological processing (geochemistry)
19
Q

characteristics of Gunflint Chert, Ontario (1.88 Ga), in the search for early life

A
  • stromatolites (morphology)
  • microfossils in chert bands (setting)
    -> 12 cyanobacteria species
  • spheres under E.M. are cells?
  • C13 values within range of modern organisms (biological processing)
20
Q

characteristics of oldest marine sediments Isua, Greenland (3.8 Ga), in the search for early life

A
  • carbon accumulated around apatite grain -> decayed cell later infilled (morphology)
  • C13 not exact match to average (ish biological processing)
  • marine sediments (setting)
21
Q

characteristics of Apex Chert, Australis (3.46 Ga) in the search for early life

A
  • chains of round structures resembling cyanobacteria (morphology)

issues:
- 1Ga before cyanobacteria
- random distribution
- not clustered
- no pattern with bedding

  • geochem - revealed aluminosilicates
  • context - igneous rocks
22
Q

characteristics of the Strelley Pool formation (3.4 Ga), in the search for early life

A
  • intertidal beach env (setting)
  • hollow chain structures, clustered, show decay, stromatolite (morphology)
  • -ve 13C (biological processing)
23
Q

what are the 3 potential pieces of evidence we found and analysed for evidence of early life

A

3.8 Ga - Isua, Greenland (Earliest marine sediments)
3.4 Ga - Strelley pool, Bus (all 3 factors convincing)
1.8 Ga - Gunflint Chert (gold standard)

24
Q

what evidence should we look for in discovering photosynthesis in the geological past, and what is an estimate for the onset

A
  • body fossils (cyanobacteria)
  • molecular fossils (biomarkers in decay)
  • indirect clues of atmospheric oxidation

estimates: 3.8 - 2.35 Ga

25
Q

what indirect clues of atmospheric oxidation can be used to reveal onset of photosynthesis in the rock record

A

BIF - alternating dark iron oxides and cherts
- common 3.4 - 1.8 Ga
- suggest seasonal ‘rusting’ of oceans, as oxygen rapidly consumed by Fe2+ in solution
Fe2+ + O2 -> Fe3+
- O2 provided by seasonal photosynthesis
- stopped forming when sink saturated -> accumulated in atmosphere

26
Q

how are ‘molecular fossils’ used to reveal onset of photosynthesis

A

specific lipids in cyanobacteria cell walls
-> product of degradation, soluble hydrocarbons, seen 2.1 Ga rocks

issues:
enriched in 13C (+ve) so maybe not organic, contaminated?

27
Q

how is isotope geochemistry used to reveal onset of photosynthesis

A

Fe isotopes from BIF show shallow water = oxygenated, deep water not
- vertical gradient
- due to photosynthesising cyanobacteria at the surface layer

28
Q

cause and impact of GOE

A

cause
- huge injection of photosynthetic oxygen (oceans became saturated?)

consequence
- snowball earth
- increase PO2, decrease PCO2 => glaciation

29
Q

what characteristics are used to recognise eukaryotes in the fossil record

A
  • large size (relative)
  • cell wall
  • cytoskeleton
  • fossil nuclei would be definitive evidence, however difficult to distinguish from bacteria cytoplasm

oldest seen = 1.75 Ga

30
Q

features of the ‘boring billion’ and why did it occur (1.7 - 0.7 Ga)

A
  • no glaciation
  • constant atmospheric O2 levels
  • supercontinent Rodinia
    -> little continental margin = where most bioactivity occurs

after breakup
- photosynthesis increase
-> snowball
-> biodiversity rapid increase

31
Q

when was the Ediacaran, in Ma and in context of other geological periods

A

635 - 541 Ma
latest stage of Proterozoic, before the Cambrian

32
Q

what are ediacara biota, and when are they first seen

A

taxonomic group containing all life forms in the ediacaran
Seen immediately after last glaciation in the ediacaran, up to the base of the cambrian
globally distributed, wide variety of anatomies

33
Q

how did the hypotheses of the affinities of ediacaran biota evolve

A

early hypothesis = octocoral
-> or convergence = same env so diff species evolve similar morphology

now thought = 2 groups both definitely animals, not sure where on tree
-> proved by cholesterol decay products

34
Q

How is the base of the Phanerozoic defined (start of Cambrian)

A

base of Phanerozoic = burrows into substrate

35
Q

what is the Cambrian explosion/what happened

A

= diversification of animal body plans (x3 number), then virtually no more after Cambrian

= substrate revolution
-> deeper burrowing (changed chemistry)
-> animals move up water column

= complex food webs and ecosystems

36
Q

what key innovations occurred in the Cambrian explosion

A

= the through gut
mouth -> anus
peristalsis muscles - also key for burrowing and swimming

= burrowing
- deeper
- introduce oxygen to substrate
- greater complexity with depth

= biomineralisation
- use shell in defence/attack

Planet Earth (3 decks)

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