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ES2001 > Early Earth & Dawn Of Complex Life > Flashcards

Early Earth & Dawn Of Complex Life Flashcards

1
Q

When is the Hadean eon?

A

4.5 Ga to 4.0 Ga
Formation of the earth to the oldest preserved rock

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2
Q

What evidence is there for the hadean eon?

A

No rocks are preserved from this time, except for a few mineral grains - zircons

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3
Q

How old is the oceanic rock record?

A

Mostly <200 Ma

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4
Q

Where are the oldest rocks found?

A

Basins

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5
Q

How old is the continental rock record?

A

Mostly <3.5Ga

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6
Q

Why do we not have rocks from the hadean period?

A

Efficient tectonic recycling and asteroid impacts

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7
Q

Give examples of Earths oldest rocks

A

Acasta Gneiss Complex, Canada
Itsaq Gneiss Complex, Greenland

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8
Q

What information can we get from the itsaq gneiss complex in green;and?

A

Stretched pillow basalt suggest an underwater eruption

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9
Q

Why do we think the moon is chemically nearly identical to the earth??

A

There was a moon forming impact that resulted in earth material becoming the moon

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10
Q

When did the moon forming impact occur, and how do we know?

A

Roughly 50 Ma after earth formation
Isotopic dating

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11
Q

What does a planets meteorite impact rate depend on?

A

The planets position in the solar system

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12
Q

Why does the moon have more craters than earth?

A

Earth probably has more, but have been eroded over time

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13
Q

How much more of a gravitational attraction does the earth have to the moon?

A

40x

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14
Q

When is the late heavy bombardment dated?

A

3.8-3.9 Ga

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15
Q

What is a possible cause for the late heavy bombardment stage?

A

The orbits of Uranus and Neptune developed a resonance and switched places which perturbed the planetary discs and comets were flung into the inner solar system

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16
Q

What is the problem with that sampling of the lunar cratering record?

A

Bias - all samples came from the same area
Inaccuracies in early dating methods

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17
Q

What does new evidence about the lunar cratering record suggest?

A

There was a gradual decline in impact rates rather than a later heavy bombardment

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18
Q

What is an indicator of asteroid impact with the lack of craters?

A

Spherule layers with geochemical anomalies caused by lava being flung into the air, cooling, then being crystallised

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19
Q

What is the chemical formula for zircon?

A

ZrSiO4

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20
Q

What is the importance of zircons?

A

They contain a bit of oxygen that proves the existence of water
Suggests there was a surface temperature 0-100 degrees C and presence of H2O

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21
Q

What are the two isotopes of oxygen?

A

O16
O18

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22
Q

What factors affect habitability?

A

Parent Star - lifetime, temp, UV radiation
Orbital parameters - distance, eccentricity
Rate and size of impacts
Planetary mass and composition - iron core formation, magnetic field, crustal differentiation, oceans and continents
Initial inventory of volatiles - atmosphere, biosphere, hydrosphere
Atmospheric gases - greenhouse effect, UV radiation
Tectonics - recycling of volatiles

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23
Q

What are constraints for the habitable zone?

A

Distance from parent star
Atmospheric gases ie ghg
Plate tectonics

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24
Q

In a feedback cycle what does a green box, and arrow, and an arrow with a circle instead of a triangle mean?

A

Square - system component
——> positive coupling (increase=increase, decrease=decrease)
——o negative coupling (increase=decrease, decrease=increase)

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25
Q

Is the carbonate silicate feedback loop stabilising or destabilising?

A

Stabilising

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26
Q

What is the carbonate-silicate feedback loop?

A

Atmospheric CO2 ——> greenhouse effect ——> surface temperature ——> (silicate weathering rate)(carbonate deposition) ——o atmospheric CO2

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27
Q

What is the destabilising water vapour feedback on Venus?

A

——greenhouse effect——>surface temp——>atmospheric H2O ——> greenhouse effect

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28
Q

What is the destabilising ice albedo feedback of mars?

A

——planetary albedo ——o surface temp ——o snow and ice cover ——>planetary albedo

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29
Q

How does earth have water?

A

Icy impacts? - pure luck
Silicate minerals contained absorbed H2O during earths formation?

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30
Q

What is earth buffered against?

A

Large temperature swings

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31
Q

What is the density of the oceanic crust?

A

3.0 g/cm3

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32
Q

What is the density of the continental crust?

A

2.7g/cm3

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33
Q

Where are some of earths earliest crusts?

A

Wawa
Isua
Barberton

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34
Q

How old is the archean-Proterozoic boundary?

A

2.5 Ga

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35
Q

What is the importance of the archean Proterozoic boundary?

A

First evidence of modern style cover sequences

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36
Q

What are the characteristics of archean pilbara craton, west Australia?

A

Granitoids separated by greenstone belts
Thick sedimentary packages

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37
Q

Give examples of a modern style cover sequence.

A

Archean pilbara craton, WA
Shaw granitoid -3 Ga
Stolzberg pluton 3.4-3.2 Ga

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38
Q

What are greenstone belts?

A

Pillow basalt upon pillow basalt
Steeply dipping
TTG

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39
Q

What is TTG?

A

Tonality trondjemite granodiorite

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40
Q

What is the characteristics of komatiite?

A

An archean oddity
Ultramafic extrusive rock
Spinifex texture
Olivine and pyroxene crystals
Indicates very hot mantle

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41
Q

How were archean granite-greenstone terraces formed?

A

Thick mafic crust forces the bottom of the crust to melt.
This crust has been altered and hydrated basalt, producing a felsic melt
Felsic magma is more buoyant than mafic magma so it rises, pushing up older metamorphosed mafic crust, causing a steeply dipping crust

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42
Q

Why are green belts called green belts?

A

Contains green minerals - chlorite, actinolite, epidote

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43
Q

What causes greenstones?

A

Vertical sinking of peripheral basalts and thin sedimentary strata

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44
Q

What are the three stages of geodynamic evolution?

A

Squishy-lid tectonics
Transition
Modern style plate tectonics

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45
Q

What are modern style tectonics?

A

Thick sedimentary packages on buoyant felsic crust - continental crust
Spreading ridges and subducting zones
Thinner, oceanic crust

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46
Q

What are possible causes for the transition from vertical to lateral tectonics?

A
  • mantle cooling, resulting in less voluminous melting events, therefore thinner oceanic crust, which becomes subductable
  • increased production of sediments which acted as a lubricant
  • change in style of mantle convection due to cooling
  • initiation of subduction by late impacts
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47
Q

What could cause mantle cooling?

A

Decreased isotopic decay
Decreased kinetic energy

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48
Q

What is Bob Steins controversial theory?

A

That there wasn’t tectonic activity until the neo-protozoic

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49
Q

What is carbonatization?

A

CO2 reacts with basalt to make carbonate

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50
Q

What is the reaction for carbonatization?

A

CO2 + CaSiO3 = CaCO3 + SiO2 (carbonate+chert)

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51
Q

What are ways of CO2 recycling before horizontal plate tectonics?

A

Carbonatization
Progressive burial and degassing of carbonatized basalt

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52
Q

What colour is hydrothermal barite BaSO4

A

Usually white
Dark colour reflects inclusions of fluids, minor organic matter and pyrite

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53
Q

What is basalt bleached by?

A

Hydrothermal fluids

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54
Q

What is the importance of hydrothermal vents?

A

Super common in the archean
Major influence on seawater chemistry
Dispersed a lot of iron, making the archean ocean Fe rich

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55
Q

What is BIF?

A

Banded iron formation
Alternating bands of chert SiO2 and iron minerals

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56
Q

What are types of iron minerals?

A

Hematite
Magnetite
Siderite
Greenalite

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57
Q

What is earths oldest remaining lake?

A

Lalla rookh fm, 3.0Ga
Pilbara craton, WA

Fault bounded intracontinental basin

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58
Q

What evidence does lalla rookh fm give us?

A

Water
Abundant fluvial sandstone
Partially carbonate cemented
Minor lacustrine mud rocks
Organic matter - microbial life

59
Q

What evidence do we have for archean land masses?

A

Lakes
Sub aerial lava flows
Fluvial ripple marks
Sedimentary cross bedding
Raindrop craters

60
Q

Where can we find raindrop craters?

A

Hadley formation 2.8 Ga

61
Q

What do raindrop craters tell us?

A

2.75 Ga atmospheric pressure was <2x modern

62
Q

What was early life?

A

Microbial

63
Q

What don’t we know about early life?

A

How, when or where life started
How ingredients were assembled
How ingredients were made

64
Q

Why does life favour the 12C isotope?

A

It’s is lighter than other isotopes so requires less energy to react with enzymes

65
Q

What does a distinctly lower 13C/12C ratio compared to carbonate mean?

A

Diagnostic of life

66
Q

What is the average marine biomass?

A

-25%

67
Q

What is the average limestone biomass?

A

0%

68
Q

How is organic biomass preserved?

A

In organic rich sedimentary rocks

69
Q

What’s the oldest possible evidence for life?

A

4.1Ga zircon with possible biogenic carbon preserved
Implies a relatively rapid origin of life soon after earths accretion

70
Q

What example of early life was found in Labrador, Canada - and why was it disputed?

A

3.95Ga sedimentary rocks with biogenic graphite
Martin J Whitehouse claims that the rocks analysed were mesoarchean 3.2-3.4Ga

71
Q

What was the problem with the Greenland stromatolites claim?

A

3.7-3.8Ga
Some of the ‘stromatolites’ are growing in the wrong direction (downwards)
Reinterpreted as metamorphic deformation structures that were originally plane laminated

72
Q

What are stromatolites?

A

Microbial structures

73
Q

How do stromatolites form?

A

Microbial mats create sticky organic film, this layer gets covered by sediment then recolonised
Photosynthesis locally removes CO2 and increases pH
Orientated towards the sun

74
Q

How can nitrogen be used as evidence of life?

A

3.8Ga paleoarchean meta-sedimentary biotite with 50-450ppm total nitrogen
High N2 concentration possibly reflects diagenetic biomass degradation

75
Q

How does ammonium end up being evidence of life?

A

In anoxic environments, lots of NH4 releases into pore waters during diagenetic biomass degradation. Porous mud dries to clay minerals

76
Q

What is the abiotic nitrogen cycle?

A

Sources: lightning N2, volcanism HNO3, impacts N2, hydrothermal reduction NO2/3
Sinks: absorption on clay particles, dissociation to volatile NH3

77
Q

What are assumptions for an abiotic nitrogen cycle in a box diagram?

A

No N2 production - all abiotic sources go to NH4
Use of modern salinity and pH for NH4 adsorption efficiency

78
Q

How does NH4 in the Black Sea?

A

Density stratification of the water column

79
Q

Why is it widely accepted that there was life at 3.5Ga?

A

Multiple lines of evidence - continuous record of life since at least 3.5Ga
Dresser Fm
Strelley Pool Fm, Australia
Moodies Group, 3.2Ga, S Africa

80
Q

What are microborings?

A

Evidence of basalt etching bacteria
Basalt from 3.5Ga, S Africa
Possibly driven by nutrient supply of basaltic glass

81
Q

What are microborings?

A

Evidence of basalt etching bacteria
Basalt from 3.5Ga, S Africa
Possibly driven by nutrient supply of basaltic glass

82
Q

What is evidence for microbial methanogenesis?

A

Gas bubbles preserved in chert contain CH4 with 13C/12C ratios that look biogenic

83
Q

What does the strelley pool formation show?

A

Stromatolite reef at 3.4Ga
Steep angle of cones interpreted as evidence of sticky microbial mats that trapped sediment grains

84
Q

What is the tumbiana formation?

A

Mega stromatolites at 2.7Ga

85
Q

When did stromatolites occur?

A

Mostly Precambrian phenomenon, prior to rise of grazing animals
Often after mass extinctions, or hypersaline lakes

86
Q

What are eukaryotes?

A

Ancestors of plants and animals

87
Q

What isotopes can we use ratios of organic matter and carbonate?

A

Carbon
Sulphur

88
Q

What is the faint young sun paradox?

A

The sun was dimmer in the archean and has become brighter over time, meaning Earth would’ve been cooler

89
Q

How do we explain liquid water with the faint young sun paradox?

A

Lots of greenhouses gases

90
Q

Without greenhouses gases, when would earth have had liquid water?

A

2Ga

91
Q

What mechanism allows us to determine past CO2 pressure?

A

When basaltic rocks turn into soils, pyroxene, olivine and plagioclase are converted into clay minerals. Fe-clay converts to Fe-carbonate at high CO2

92
Q

What are possible reactions for the creation of siderite, and the side products?

A

Greenalite + CO2 = Siderite + water + SiO2
Berthierine + SiO2 + CO2 = siderite +Al2SicO5(OH)4 (kaolinite)

93
Q

What are the inputs and outputs of CO2 in the earth system?

A

Outputs : temperature around the world
Inputs : source and sinks of CO2, land mass distribution, solar radiation

94
Q

How could CH4 survive in earths atmosphere?

A

Archean atmosphere was anoxic

95
Q

What is the geological evidence for the great oxidation event?

A

Photosynthetic life
Banded iron formations
Dentrial pyrite and uraninite
Red beds
Iron leached paleosols
Iron retaining paleosols
Mass independent sulphur isotope fractionation

96
Q

When did oxygenic photosynthesis occur?

A

300Ma prior to GOE

97
Q

Explain the formation of banded iron formations

A

Iron rich deposits from hydrothermal vents upwelled from the deep ocean.
Iron is soluble in anoxic environments and is transported to continental shelves were there is a shift to oxic water and iron precipitates out of solution, depositing on the shelf

98
Q

What happens to hydrothermal Fe today?

A

Oxidised in deep ocean and deposited with red clay

99
Q

What does detritial pyrite and urninite tell us?

A

Rounding implies fluvial transport
A long period of transport would require anoxic atmosphere

100
Q

What does the appearance of red beds after the GOE tell us?

A

FeO3 (hematite) stained siliciclastic sediments in shallow water settings
Implies O2 in atmosphere

101
Q

What are paleosols?

A

Ancient soils

102
Q

What do paleosols show us?

A

Archean soils where leached of iron, making the soil white= anoxic environment

103
Q

What does photochemical sulphur isotope effects tell us about the GOE?

A

Before the GOE there was no ozone shield, so UV radiation caused photochemical reactions in atmospheric sulphur gases, creating 33S isotope
After GOE the ozone was formed, stopping photochemical reactions and sulphur became SO2-4 by oxidation before rainout to the oceans

104
Q

Where does atmospheric sulphur come from?

A

Volcanic input
Weathering

105
Q

What are the dates of oxidation events?

A

Great oxidation event = 2.4Ga
Neoproterozoic oxidation event = 0.6Ga

106
Q

When did the whole planet become oxidised?

A

Soils and shallow marine rocks by 2.4-2.3Ga
Deeper marine rocks remained rich in siderite and pyrite until at least 0.6Ga or the Devonian 0.4Ga

107
Q

If the deep ocean wasn’t oxidised, then why did BIF stop?

A

SO2-4 from atmosphere became more abundant leading to extensive pyrite formation in sediments

108
Q

What depositional environments are most likely to be preserved?

A

Shallow marine

109
Q

What are epicontinental basins?

A

An ocean basin sitting upon an continental crust
I.e Baltic Sea, Hudson Bay

110
Q

What is the issue with epicontinental basins?

A

Are not globally representative

111
Q

What does euxinic mean?

A

Anoxic + H2S rich

112
Q

How does molybdenum act as an indicator of late ocean oxygenation?

A

Dissolved in water but drawn into sediments in euxinic seawater conditions - concentrated in black shales
In oxic conditions, large MoO2-4 reservoirs build up in seawater and any local euxinic setting takes up Mo in high concentrations

113
Q

What brackets the mesoproterozoic?

A

Paleoproterozoic GOE and Neoproterozoic oxygenation of the deep ocean

114
Q

What does the carbon isotope record show about the mesoproterozoic?

A

Relative stable period

115
Q

What does §13Ccarb reflect?

A

Dissolved seawater carbonate and therefore global perturbations in §13Ccarb imply a global shift in the balance between biomass and carbonate burial

116
Q

What are possible causes of the steady state of carbon isotope formation?

A

Unusually high/low productivity
Rapid biomass burial
Re-oxidation of old organic mater

117
Q

What/when was the boring billion?

A

The mesoproterozoic 1.8-0.8Ga
‘Never in the course of earths history did so little happen to so much for so long’ - Buick et al 95

118
Q

When did eukaryotes originate?

A

Proterozoic

119
Q

What are eukaryotes?

A

All complex life including animals

120
Q

What do eukaryotic microfossils in the mesoproterozoic tell us?

A

Live in shallow oxic water

121
Q

When were the first crown group eukaryotes?

A

Red algae ancestors 1.2Ga

122
Q

What are dropstones?

A

Large clasts that fell into much finer grained sediment by melting out of a marine ice sheet

123
Q

What is glacial dimictite?

A

Very poorly sorted sediment that was pushed into the sea by glaciers

124
Q

When were there periods of global glaciation?

A

Marinoan 645-635Ma
Sturtian 720-660Ma

125
Q

What is evidence for glaciation at low latitudes?

A

Thick glacial deposits occurring with carbonate dominated successions ie Namibia

126
Q

What caused snowball earth?

A

Rapid weathering and CO2 drawdown as countries at low latitudes
Franklin Large Igneous Provenance at equator

127
Q

What were the consequences of snowball glaciation?

A

Massive sea level drop
Seawater turned into brine
Return of BIFs

128
Q

What were the conditions of brine in snowball glaciation?

A

Temperature -4.4 degrees C
Salinity 2x present

129
Q

What ended snowball earth?

A

Lack of rainfall and limited silicate weathering means volcanic CO2 emissions build up in the atmosphere increasing greenhouse warming and ice melting

130
Q

What are cap carbonates characteristic of?

A

Post snowball deposits above glacial diamictite

131
Q

What are possible reasons for snowball earth triggering biodiversifcation?

A
  • extinction of mesoproterozoic fauna, recolonisation of habitats after glaciation
  • post snowball weathering, release of nutrients, enhances biological productivity and O2 production, expansion of complex aerobic life
  • chance?
132
Q

What are possible ways life could survive snowball earth?

A

Under cracks in the ice
Near volcanic centres
Around hydrothermal vents
Along shorelines with thinner ice
In cryoconite holes

133
Q

What organisms were present in the ‘garden of ediacara’?

A

Dickinsonia- hopped over seafloor, grazed on microbial mats
Spriggina- anterior/posterior patterning
Rugoconites - tri radial symmetry
Aspidella - evidence of sexual reproduction, larger next to smaller
Parvancorina - possibly resembles a trilobite
Charnia- leave like structures likely standing up 1m

134
Q

Give examples of places were Proterozoic life forms are found/

A

Mistaken point, Newfoundland
Ediacara hills, south Australia

135
Q

What are common characteristics of ediacaran biota?

A

Living on seafloor
Fed on microbial mats
Soft bodied, no shells/bones
Limited evidence of burrowing and swimming

136
Q

What constitutes the oldest evidence of liquid water on earth?

A

Positive oxygen isotope values in zircons

137
Q

True or false:
The formation of the moon predates the oldest evidence of water on earth

A

True

138
Q

What is the approximate global average magnitude of the greenhouse effect on earth?

A

30 degrees Celsius

139
Q

True or false:
Prior to to the onset of modern style plate tectonics, atmospheric CO2 could not be recycled by geological processes

A

False

140
Q

What deposits are linked to hydrothermal activity?

A

Barite
Banded iron formation
Chert veins

141
Q

What is the approximate age of the oldest carbonaceous microfossils in the rock record?

A

3.4 Ga

142
Q

True or false:
Stromatolites have gone extinct and no longer exist today

A

False

143
Q

What is the major source of O2 gas in the atmosphere?

A

Oxygenic photosynthesis

144
Q

What constitutes a possible solution to the faint young sun paradox?

A

Early atmosphere was anoxic, allowing CH4 to accumulate and contribute to climate warming

ES2001 (9 decks)

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