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 Precambrian?
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
- first 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 and the debris formed the moon
*evidence = the moon has a composition similar to Earth’s mantle
When did the continental crust occur?
The crust was only present intermittently at 4.5 Ga
- started intermittent and ultramafic
What is Zircon Dating?
- 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 - aged 4.4 billion years (dated to the Hadean) - can use these for absolute dating
What is Jack Hills Zircon? What does it explain about the hydrosphere?
- oldest known piece of the Earth’s crust - 4.4 billion years old (found in Australia)
- evidence that the earth was cool enough for liquid water > oxygen isotopes tell us there was potential for the hydrosphere to form (about 4.3 Ga) and that there was land above sea level
Explain the significance of the Acasta Gneiss
- in NWT, Canada
- oldest *felsic rocks on Earth
- about 4 Ga
- marks the end of Hadean, beginning of Archean
What is the Nuvvuagittuz Greenstone Belt, Quebec
- argued to be the oldest *mafic rocks on Earth (debate on their actual age)
- about 4.3 Ga
- *lack of igneous zircons needed to reliably date them
Explain the origin of the hydrosphere and atmosphere
What allowed them to occur and what does it develop from?
- differentiation of the earth and the rotating core > caused the magnetic field to form > helped trap the gases, forming the atmosphere > this also let water out which caused the hydrosphere to form
**without the magnetic sphere, the atmospheric gases of the lightest elements would’ve been too light to keep here so they would’ve floated away and been attracted by the sun - the atmosphere develops from volcanic gases
- 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 compared to present day
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
- 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
- elements
- safe environment/good conditions to live
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
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?
They produced amino acids (monomers) from proteins in the lab
> modelled a primitive atmosphere and added a spark of lightning
> 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
What are submarine hydrothermal vents - what theory have they lead to?
- occur near seafloor spreading
- contain elements needed to form life
- the water is hot and metal-rich > could lead to formation of monomers
- polymerization could have occurred on surface of clay minerals > protocells deposited on seafloor
*Theory : life may have originated from these vents - thermal and chemical gradients could have synthesized organic molecules
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)
- asexual
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
- appearance of autotrophic organisms
> ex. cyanobacteria that form stromatolites > can photosynthesize
> stromatolites first appear 3.5-3 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 (evidence?)
- 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?
- distinctive layers 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 close to a narrow range of time
- conditions needed to form these rocks don’t exist anymore
