Precambrian : The Hadean and Archean Life Flashcards
What time period was the Hadean?
4.6-4 billion years ago
What are the years encompassing the Precdambrian?
4.6 billion years ago to 541 million years ago
*takes up most of the geologic time scale → we don’t have as much evidence for the older times
What marks the upper boundary of the Hadean?
The appearance of the Acasta Gneiss (in canada)
*oldest rock
List the Hadean geologic and biologic events
- Oldest meteorites
- earth bombarded with meteorites and comets
- Origin of earth’s moon
- detrital zircons (Australia)
- Acasta Gneiss (Canada)
How did the accretion of Earth start?
- swept up debris in the vicinity of Earth’s orbit
- differentiated into core and mantle (Homogenous Accretion Theory)
What is the theory for how the moon came into being? when?
- about 4.53 Ga, a Mars-sized planetoid collided with Earth
- this planet and a part of Earth’s mantle were disintegrated
- the collision debris formed a ring around the Earth
- the debris coalesces and forms the moon
*evidence = the moon has a composition similar to Earth’s mantle
When did the continental crust occur?
The crust was likely only present intermittently on early Earth, 4.5 Ga
- it was intermittent to begin with and likely started off ultramafic
Oceans may have been developing as early as 4.4 Ga
What is Zircon Dating? What are the absolute dating ranges?
- no rocks of Hadean age are known on Earth
- but we do have some crystals!
- some sedimentary rocks found in Australia with the mineral zircon (ZrSiO4) have a numerical age of 4.4 billion years (dated to the Hadean)
Absolute dating:
- half life: 4.5 billion years
- dating range: 10 million - 4.5 billion
What is Jack Hills Zircon?
- oldest known piece of the Earth’s crust 4.4 billion years old
- *earliest indication of Earth’s crust existing
- evidence that the earth was cool enough for liquid water
- quartz-pebble metaconglomerate
- found in Australia
What can Jack Hills Zircon tell us about the hydrosphere
Oxygen isotopes - formed from magma that had freshwater in it > tells us there was potential for the hydrosphere to form
- there was land above sea level
- Hydrosphere (about 4.3 Ga)
- land interacted with seawater
Explain the significance of the Acasta Gneiss
- in NWT, Canada
- oldest *felsic rocks on Earth (Zircon U-Pb)
- repeated periods of felsic magma and subsequent thermal metamorphism
- about 4 Ga
- marks the end of Hadean, beginning of Archean (very pivotal rocks in terms of geological time)
What is the Nuvvuagittuz Greenstone Belt, Quebec
- argued to be the oldest *mafic rocks on Earth (debate on their actual age)
- Sm-Nd isotopes
- about 4.3 Ga
- *lack of igneous zircons needed to reliably date them due to basaltic composition and low-Zr content
Explain the origin of the hydrosphere and atmosphere
What allowed them to occur and what does it develop from?
- prior to differentiation, there was no magnetic field > magnetic sphere around the earth helped form the atmosphere > differentiation of the earth and the rotating core caused the magnetic field to form which helped trap the gases and some of what came out was water which caused the hydrosphere to form
**without the magnetic sphere, the particles from the sun would’ve stripped away the atmospheric gases > bc early atmosphere was made up of the lightest elements they would’ve been too light to keep here - the atmosphere develops from volcanic gases > erupting volcanoes emit mostly water vapor, carbon dioxide, hydrogen, nitrogen, ammonia, methane
- no free oxygen yet: outgassing
- when the earth becomes cool enough, moisture condenses and accumulates
What were the oxygen and salinity levels of the atmosphere and hydrosphere at the time?
- early atmospheres were less than 1% of modern oxygen levels
- hydrosphere was very salty, if not saltier than it is now
What are some theories for the source of water on earth?
- outgassing from earth’s interior > originally contained hydrogen and oxygen that could’ve formed water
- bombardment by meteorites and icy comets
- Archean oceans existed but we are unsure of size and how salty they were
*fringe theory > came from extraterrestrial beings? maybe came from asteroids and comets that had water on them
What are the years in the Archean time period?
4-2.5 billion years ago
List the main events during the Archean
- starts with Acasta Gneiss
- chemical evidence for life
- Greenstone belt
- steady origin of continental crust (not intermittent anymore)
- oldest well-documented stromatolies
- age of most greenstone belts, North America
What is life? What are the requirements for life?
organisms that have two features > metabolism and reproduction
requirements:
- energy > potentially volcanic lightning or UV radiation
- elements (ex. carbon)
- safe environment/good conditions to live
What is earth’s atmosphere made up of?
nitrogen
oxygen (but no oxygen originally)
argon
carbon dioxide
water vapor
trace amounts of other gases
What are the 5 kingdoms of life
animals
plants
protists
fungi
monera (bacteria and archaea)
How old are the oldest fossils?
Oldest are about 3.5 Ga, but chemical evidence indicates organisms existed at 3.8 Ga
What is Abiogenesis?
Life arising from non-living matter
- how life first formed > involved many small steps from first life to something like a bacterium
What is the Panspermia hypothesis?
life originated outside of our planet and was brought here by impacts from comets and meteorites
How did Miller and Urey test abiogenesis - explain their experiments
What are modern animals made up of?
(1953) they produced nearly all amino acids (monomers) found in proteins in the lab
> they modelled a primitive atmosphere and added “lightning” - a spark
> showed that amino acids form easily - able to synthesize them
*modern animals are made up of polymers (DNA, RNA), which hasn’t been made in a lab yet
Explain the experimental production of thermal proteins and microspheres
- we can synthesize polymers - linking together of monomers but we need a system for them to be able to link up and have longevity
What are submarine hydrothermal vents - what theory have they lead to?
- occur near seafloor spreading
- contain elements needed to form life > H, C, N, O and S
- the water is hot and metal-rich > the heat could lead to formation of monomers
- polymerization could have occurred on surface of clay minerals > protocells deposited on seafloor
- We still have them today and there are Extremophiles → organisms able to exist in extreme environments
What were the first organisms? their characteristics? What are Archaea
First organisms were Prokaryotes
- unicellular
- anaerobic (don’t need oxygen which is good bc there wasn’t any yet)
- heterotrophic (can’t make own food)
- lots of molecular ATP (energy) in oceans
- asexual - offspring have genes from only one parent
Archaea (type of Prokaryote):
- similar to bacteria
- live in extreme environments
- do not exist as fossils in Archean, but chemical evidence exists
Explain the 2 processes that released oxygen into the atmosphere
when did this start?
Photochemical dissociation:
- UV radiation disrupts H2O in upper atmosphere, releasing oxygen and hydrogen
- released enough oxygen to start formation of ozone layer
Photosynthesis:
- performed by organisms that take in carbon dioxide, water, and sunlight to produce organic compounds and oxygen > earliest evidence shows this was 3.5Ga
Explain how/when free oxygen started appearing more
- There was little to no free oxygen in Archean
- as O2 was increasing, C02 decreased
- appearance of autotrophic organisms
> ex. cyanobacteria that form stromatolites > can photosynthesize
> stromatolites first appear 3.5-3 Ga, not common until 2.3 Ga - the great oxygenation event marks the transition into the Proterozoic > free oxygen in the atmosphere at the beginning of Proterozoic
Describe the great oxygenation event
- 2.4 billion years ago
- rapid rise in atmospheric oxygen
- increase in abundance of photosynthetic bacteria
- evidence: banded iron formations
What are banded iron formations?
- Red chert layers interlayered with iron oxide layers made up of hematite and magnetite
(distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor red chert) - Huge point in the rock record that can draw you back to this specific time → conditions needed to form these rocks don’t exist anymore → draws you close to a narrow range of time
What are red beds
- continental red beds (ex. in mexico)
- 2.3 Ga
- free oxygen in atmosphere
- iron precipitates on land
