The Protoerzoic Earth Flashcards
What is the upper and lower ages of the boundaries to the Proterozoic?
2420Ma-539Ma
What proportion of Earth history does the Proterozoic represent?
42%
What key events took place during the Proterozoic?
-Assembly and break-up of supercontinents as mantle cooled and plate tectonic processes evolved - Luminosity of sun increased
- Rise (and fluctuations) of atmospheric oxygen levels due to photosynthesis and burial off organic carbon in sedimentary rocks
- Oceans changes from reducing to aerobic as life evolved
- Major glaciations in palaeoproterzoic and neoproterozoic- linked to supercontinents and CO2 draw down from the atmosphere
What is the composition of the modern atmosphere?
78% N
21% O
<1% Ar, Xe, CO2, CO etc
How does the Earth’s atmosphere compare to other planetary bodies in our solar system?
Significantly larger amounts of oxygen
What is the origin of the gases other than oxygen in the atmosphere?
Volcanic activity
How is oxygen added to and removed from the Earth’s atmosphere? What is the most important source of
oxygen today? What are the major balancing mechanisms today?
Photosynthesis and respiration add and remove, with photosynthesis being the most important contributor
How did oxygen levels change through the Proterozoic? What caused the buildup of oxygen in the atmosphere?
Dramatic increase, loss of buffering system in the ocean as all reduced chemicals oxidised (BIF) meaning free oxygen escaped into the atmosphere
When did the Great Oxidation Event occur and what are some of the phenomena that indicate that it took place?
During the paleo-proterozoic oxidisation of iron (II) occured to form BIF
What was the nature of the atmosphere before the GOE and what is the evidence of no free oxygen?
High concentrations of nitrogen, alkenes, carbon monoxide and dioxide, lots of reduced chemicals, which would have easily been oxidised if there was free oxygen
When did the ozone layer form? What does the ozone layer protect the Earth from?
2.4Ga during the GOE the ozone layer began to form, this protects the Earth from UV radiation (all of UVC and most UVB)
What are the possible causes of the Neoproterozoic oxygenation event (NEO)?
-Increased Photosynthetic Activity
-Stabilisation of Continental Crust:
Reduced Continental Weathering and
Expansion of Landmasses
-Changes in Atmospheric Chemistry:
Reduction in Methane and Increased Carbon Burial
-The “Snowball Earth” Hypothesis:
end of Snowball Earth Glaciations (around 700–600 Ma), the warming climate and increased weathering could have enhanced nutrient cycling, which could have led to more photosynthetic production
-Changes in Ocean Chemistry:
reduced oceanic Iron availability and Deep Ocean Circulation Changes facilitated the mixing of oxygen-rich surface waters with deeper ocean layers, affecting global oxygen distribution
-Reduced Volcanic Activity
What is a supercontinent?
One continent (stagnant lid), when all the continental plates assemble to form one large.
What is the supercontinent cycle? What events (and evidence) are associated with continental break up and assembly of the supercontinents?
What is an orogeny? In which plate tectonic settings do the most intense orogenesis occur? Present day orogens are identified by finding regions of high, rugged peaks. How can geologists identify orogens that have been eroded away?
