Early Earth & Dawn Of Complex Life Flashcards
(144 cards)
1
Q
When is the Hadean eon?
A
4.5 Ga to 4.0 Ga
Formation of the earth to the oldest preserved rock
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
3
Q
How old is the oceanic rock record?
A
Mostly <200 Ma
4
Q
Where are the oldest rocks found?
A
Basins
5
Q
How old is the continental rock record?
A
Mostly <3.5Ga
6
Q
Why do we not have rocks from the hadean period?
A
Efficient tectonic recycling and asteroid impacts
7
Q
Give examples of Earths oldest rocks
A
Acasta Gneiss Complex, Canada
Itsaq Gneiss Complex, Greenland
8
Q
What information can we get from the itsaq gneiss complex in green;and?
A
Stretched pillow basalt suggest an underwater eruption
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
10
Q
When did the moon forming impact occur, and how do we know?
A
Roughly 50 Ma after earth formation
Isotopic dating
11
Q
What does a planets meteorite impact rate depend on?
A
The planets position in the solar system
12
Q
Why does the moon have more craters than earth?
A
Earth probably has more, but have been eroded over time
13
Q
How much more of a gravitational attraction does the earth have to the moon?
A
40x
14
Q
When is the late heavy bombardment dated?
A
3.8-3.9 Ga
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
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
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
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
19
Q
What is the chemical formula for zircon?
A
ZrSiO4
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
21
Q
What are the two isotopes of oxygen?
A
O16
O18
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
23
Q
What are constraints for the habitable zone?
A
Distance from parent star
Atmospheric gases ie ghg
Plate tectonics
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)
25
Is the carbonate silicate feedback loop stabilising or destabilising?
Stabilising
26
What is the carbonate-silicate feedback loop?
Atmospheric CO2 ——> greenhouse effect ——> surface temperature ——> (silicate weathering rate)(carbonate deposition) ——o atmospheric CO2
27
What is the destabilising water vapour feedback on Venus?
——greenhouse effect——>surface temp——>atmospheric H2O ——> greenhouse effect
28
What is the destabilising ice albedo feedback of mars?
——planetary albedo ——o surface temp ——o snow and ice cover ——>planetary albedo
29
How does earth have water?
Icy impacts? - pure luck
Silicate minerals contained absorbed H2O during earths formation?
30
What is earth buffered against?
Large temperature swings
31
What is the density of the oceanic crust?
3.0 g/cm3
32
What is the density of the continental crust?
2.7g/cm3
33
Where are some of earths earliest crusts?
Wawa
Isua
Barberton
34
How old is the archean-Proterozoic boundary?
2.5 Ga
35
What is the importance of the archean Proterozoic boundary?
First evidence of modern style cover sequences
36
What are the characteristics of archean pilbara craton, west Australia?
Granitoids separated by greenstone belts
Thick sedimentary packages
37
Give examples of a modern style cover sequence.
Archean pilbara craton, WA
Shaw granitoid -3 Ga
Stolzberg pluton 3.4-3.2 Ga
38
What are greenstone belts?
Pillow basalt upon pillow basalt
Steeply dipping
TTG
39
What is TTG?
Tonality trondjemite granodiorite
40
What is the characteristics of komatiite?
An archean oddity
Ultramafic extrusive rock
Spinifex texture
Olivine and pyroxene crystals
Indicates very hot mantle
41
How were archean granite-greenstone terraces formed?
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
42
Why are green belts called green belts?
Contains green minerals - chlorite, actinolite, epidote
43
What causes greenstones?
Vertical sinking of peripheral basalts and thin sedimentary strata
44
What are the three stages of geodynamic evolution?
Squishy-lid tectonics
Transition
Modern style plate tectonics
45
What are modern style tectonics?
Thick sedimentary packages on buoyant felsic crust - continental crust
Spreading ridges and subducting zones
Thinner, oceanic crust
46
What are possible causes for the transition from vertical to lateral tectonics?
- 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
47
What could cause mantle cooling?
Decreased isotopic decay
Decreased kinetic energy
48
What is Bob Steins controversial theory?
That there wasn’t tectonic activity until the neo-protozoic
49
What is carbonatization?
CO2 reacts with basalt to make carbonate
50
What is the reaction for carbonatization?
CO2 + CaSiO3 = CaCO3 + SiO2 (carbonate+chert)
51
What are ways of CO2 recycling before horizontal plate tectonics?
Carbonatization
Progressive burial and degassing of carbonatized basalt
52
What colour is hydrothermal barite BaSO4
Usually white
Dark colour reflects inclusions of fluids, minor organic matter and pyrite
53
What is basalt bleached by?
Hydrothermal fluids
54
What is the importance of hydrothermal vents?
Super common in the archean
Major influence on seawater chemistry
Dispersed a lot of iron, making the archean ocean Fe rich
55
What is BIF?
Banded iron formation
Alternating bands of chert SiO2 and iron minerals
56
What are types of iron minerals?
Hematite
Magnetite
Siderite
Greenalite
57
What is earths oldest remaining lake?
Lalla rookh fm, 3.0Ga
Pilbara craton, WA
Fault bounded intracontinental basin
58
What evidence does lalla rookh fm give us?
Water
Abundant fluvial sandstone
Partially carbonate cemented
Minor lacustrine mud rocks
Organic matter - microbial life
59
What evidence do we have for archean land masses?
Lakes
Sub aerial lava flows
Fluvial ripple marks
Sedimentary cross bedding
Raindrop craters
60
Where can we find raindrop craters?
Hadley formation 2.8 Ga
61
What do raindrop craters tell us?
2.75 Ga atmospheric pressure was <2x modern
62
What was early life?
Microbial
63
What don’t we know about early life?
How, when or where life started
How ingredients were assembled
How ingredients were made
64
Why does life favour the 12C isotope?
It’s is lighter than other isotopes so requires less energy to react with enzymes
65
What does a distinctly lower 13C/12C ratio compared to carbonate mean?
Diagnostic of life
66
What is the average marine biomass?
-25%
67
What is the average limestone biomass?
0%
68
How is organic biomass preserved?
In organic rich sedimentary rocks
69
What’s the oldest possible evidence for life?
4.1Ga zircon with possible biogenic carbon preserved
Implies a relatively rapid origin of life soon after earths accretion
70
What example of early life was found in Labrador, Canada - and why was it disputed?
3.95Ga sedimentary rocks with biogenic graphite
Martin J Whitehouse claims that the rocks analysed were mesoarchean 3.2-3.4Ga
71
What was the problem with the Greenland stromatolites claim?
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
What are stromatolites?
Microbial structures
73
How do stromatolites form?
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
How can nitrogen be used as evidence of life?
3.8Ga paleoarchean meta-sedimentary biotite with 50-450ppm total nitrogen
High N2 concentration possibly reflects diagenetic biomass degradation
75
How does ammonium end up being evidence of life?
In anoxic environments, lots of NH4 releases into pore waters during diagenetic biomass degradation. Porous mud dries to clay minerals
76
What is the abiotic nitrogen cycle?
Sources: lightning N2, volcanism HNO3, impacts N2, hydrothermal reduction NO2/3
Sinks: absorption on clay particles, dissociation to volatile NH3
77
What are assumptions for an abiotic nitrogen cycle in a box diagram?
No N2 production - all abiotic sources go to NH4
Use of modern salinity and pH for NH4 adsorption efficiency
78
How does NH4 in the Black Sea?
Density stratification of the water column
79
Why is it widely accepted that there was life at 3.5Ga?
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
What are microborings?
Evidence of basalt etching bacteria
Basalt from 3.5Ga, S Africa
Possibly driven by nutrient supply of basaltic glass
81
What are microborings?
Evidence of basalt etching bacteria
Basalt from 3.5Ga, S Africa
Possibly driven by nutrient supply of basaltic glass
82
What is evidence for microbial methanogenesis?
Gas bubbles preserved in chert contain CH4 with 13C/12C ratios that look biogenic
83
What does the strelley pool formation show?
Stromatolite reef at 3.4Ga
Steep angle of cones interpreted as evidence of sticky microbial mats that trapped sediment grains
84
What is the tumbiana formation?
Mega stromatolites at 2.7Ga
85
When did stromatolites occur?
Mostly Precambrian phenomenon, prior to rise of grazing animals
Often after mass extinctions, or hypersaline lakes
86
What are eukaryotes?
Ancestors of plants and animals
87
What isotopes can we use ratios of organic matter and carbonate?
Carbon
Sulphur
88
What is the faint young sun paradox?
The sun was dimmer in the archean and has become brighter over time, meaning Earth would’ve been cooler
89
How do we explain liquid water with the faint young sun paradox?
Lots of greenhouses gases
90
Without greenhouses gases, when would earth have had liquid water?
2Ga
91
What mechanism allows us to determine past CO2 pressure?
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
What are possible reactions for the creation of siderite, and the side products?
Greenalite + CO2 = Siderite + water + SiO2
Berthierine + SiO2 + CO2 = siderite +Al2SicO5(OH)4 (kaolinite)
93
What are the inputs and outputs of CO2 in the earth system?
Outputs : temperature around the world
Inputs : source and sinks of CO2, land mass distribution, solar radiation
94
How could CH4 survive in earths atmosphere?
Archean atmosphere was anoxic
95
What is the geological evidence for the great oxidation event?
Photosynthetic life
Banded iron formations
Dentrial pyrite and uraninite
Red beds
Iron leached paleosols
Iron retaining paleosols
Mass independent sulphur isotope fractionation
96
When did oxygenic photosynthesis occur?
300Ma prior to GOE
97
Explain the formation of banded iron formations
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
What happens to hydrothermal Fe today?
Oxidised in deep ocean and deposited with red clay
99
What does detritial pyrite and urninite tell us?
Rounding implies fluvial transport
A long period of transport would require anoxic atmosphere
100
What does the appearance of red beds after the GOE tell us?
FeO3 (hematite) stained siliciclastic sediments in shallow water settings
Implies O2 in atmosphere
101
What are paleosols?
Ancient soils
102
What do paleosols show us?
Archean soils where leached of iron, making the soil white= anoxic environment
103
What does photochemical sulphur isotope effects tell us about the GOE?
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
Where does atmospheric sulphur come from?
Volcanic input
Weathering
105
What are the dates of oxidation events?
Great oxidation event = 2.4Ga
Neoproterozoic oxidation event = 0.6Ga
106
When did the whole planet become oxidised?
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
If the deep ocean wasn’t oxidised, then why did BIF stop?
SO2-4 from atmosphere became more abundant leading to extensive pyrite formation in sediments
108
What depositional environments are most likely to be preserved?
Shallow marine
109
What are epicontinental basins?
An ocean basin sitting upon an continental crust
I.e Baltic Sea, Hudson Bay
110
What is the issue with epicontinental basins?
Are not globally representative
111
What does euxinic mean?
Anoxic + H2S rich
112
How does molybdenum act as an indicator of late ocean oxygenation?
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
What brackets the mesoproterozoic?
Paleoproterozoic GOE and Neoproterozoic oxygenation of the deep ocean
114
What does the carbon isotope record show about the mesoproterozoic?
Relative stable period
115
What does §13Ccarb reflect?
Dissolved seawater carbonate and therefore global perturbations in §13Ccarb imply a global shift in the balance between biomass and carbonate burial
116
What are possible causes of the steady state of carbon isotope formation?
Unusually high/low productivity
Rapid biomass burial
Re-oxidation of old organic mater
117
What/when was the boring billion?
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
When did eukaryotes originate?
Proterozoic
119
What are eukaryotes?
All complex life including animals
120
What do eukaryotic microfossils in the mesoproterozoic tell us?
Live in shallow oxic water
121
When were the first crown group eukaryotes?
Red algae ancestors 1.2Ga
122
What are dropstones?
Large clasts that fell into much finer grained sediment by melting out of a marine ice sheet
123
What is glacial dimictite?
Very poorly sorted sediment that was pushed into the sea by glaciers
124
When were there periods of global glaciation?
Marinoan 645-635Ma
Sturtian 720-660Ma
125
What is evidence for glaciation at low latitudes?
Thick glacial deposits occurring with carbonate dominated successions ie Namibia
126
What caused snowball earth?
Rapid weathering and CO2 drawdown as countries at low latitudes
Franklin Large Igneous Provenance at equator
127
What were the consequences of snowball glaciation?
Massive sea level drop
Seawater turned into brine
Return of BIFs
128
What were the conditions of brine in snowball glaciation?
Temperature -4.4 degrees C
Salinity 2x present
129
What ended snowball earth?
Lack of rainfall and limited silicate weathering means volcanic CO2 emissions build up in the atmosphere increasing greenhouse warming and ice melting
130
What are cap carbonates characteristic of?
Post snowball deposits above glacial diamictite
131
What are possible reasons for snowball earth triggering biodiversifcation?
- 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
What are possible ways life could survive snowball earth?
Under cracks in the ice
Near volcanic centres
Around hydrothermal vents
Along shorelines with thinner ice
In cryoconite holes
133
What organisms were present in the ‘garden of ediacara’?
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
Give examples of places were Proterozoic life forms are found/
Mistaken point, Newfoundland
Ediacara hills, south Australia
135
What are common characteristics of ediacaran biota?
Living on seafloor
Fed on microbial mats
Soft bodied, no shells/bones
Limited evidence of burrowing and swimming
136
What constitutes the oldest evidence of liquid water on earth?
Positive oxygen isotope values in zircons
137
True or false:
The formation of the moon predates the oldest evidence of water on earth
True
138
What is the approximate global average magnitude of the greenhouse effect on earth?
30 degrees Celsius
139
True or false:
Prior to to the onset of modern style plate tectonics, atmospheric CO2 could not be recycled by geological processes
False
140
What deposits are linked to hydrothermal activity?
Barite
Banded iron formation
Chert veins
141
What is the approximate age of the oldest carbonaceous microfossils in the rock record?
3.4 Ga
142
True or false:
Stromatolites have gone extinct and no longer exist today
False
143
What is the major source of O2 gas in the atmosphere?
Oxygenic photosynthesis
144
What constitutes a possible solution to the faint young sun paradox?
Early atmosphere was anoxic, allowing CH4 to accumulate and contribute to climate warming
