The Protoerzoic Earth

Question 1

What is the upper and lower ages of the boundaries to the Proterozoic?

Answer 1

2420Ma-541Ma

Question 2

What proportion of Earth history does the Proterozoic represent?

Answer 2

42%

Question 3

What key events took place during the Proterozoic?

Answer 3

-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 of 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

Question 4

What is the composition of the modern atmosphere?

Answer 4

78% N
21% O
<1% Ar, Xe, CO2, CO etc

Question 5

How does the Earth’s atmosphere compare to other planetary bodies in our solar system?

Answer 5

Significantly larger amounts of oxygen

Question 6

What is the origin of the gases other than oxygen in the atmosphere?

Answer 6

Volcanic activity

Question 7

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?

Answer 7

Photosynthesis and respiration add and remove, with photosynthesis being the most important contributor

Question 8

How did oxygen levels change through the Proterozoic? What caused the buildup of oxygen in the atmosphere?

Answer 8

Dramatic increase, loss of buffering system in the ocean as all reduced chemicals oxidised (BIF) meaning free oxygen escaped into the atmosphere

Question 9

When did the Great Oxidation Event occur and what are some of the phenomena that indicate that it took place?

Answer 9

During the paleo-proterozoic oxidisation of iron (II) occured to form BIF

Question 10

What was the nature of the atmosphere before the GOE and what is the evidence of no free oxygen?

Answer 10

High concentrations of nitrogen, alkenes, carbon monoxide and dioxide, lots of reduced chemicals, which would have easily been oxidised if there was free oxygen

Question 11

When did the ozone layer form? What does the ozone layer protect the Earth from?

Answer 11

2.4Ga during the GOE the ozone layer began to form, this protects the Earth from UV radiation (all of UVC and most UVB)

Question 12

What are the possible causes of the Neoproterozoic oxygenation event (NEO)?

Answer 12

-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

Question 13

What is a supercontinent?

Answer 13

One continent (stagnant lid), when all the continental plates assemble to form one large.

Question 14

What is the supercontinent cycle? What events (and evidence) are associated with continental break up and assembly of the supercontinents?

Answer 14

Formation and breakup of supercontinents
Assembly is driven by subduction zones, leading to the closure of ocean basins and the collision of continents.
Evidence includes orogenic belts (the Himalayas)
Breakup is caused by mantle convection and plumes of upwelling magma, creating rifts. Evidence includes rift valleys (e.g., East African Rift), large igneous provinces (LIPs), and magnetic striping on the ocean floor indicating seafloor spreading.

Question 15

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?

Answer 15

-A mountain-building event caused by the collision, compression, and deformation of Earth’s crust.
-Continental Collision Zones and Subduction Zones are where the most intense orogenesis occurs.
-Orogenic Belts: Regions with deformed and metamorphosed rocks, such as folded sedimentary layers and fault zones, high-pressure, high-temperature metamorphic rocks (e.g., schist and gneiss) formed during orogeny remain preserved, Granite Intrusions (large granitic bodies) associated with mountain-building processes are often found in former orogenic zones.

Question 16

What is a Large Igneous Province (LIP) and how do their emplacement relate to the supercontinent cycle?

Answer 16

A Large Igneous Province (primarily basalt) is a massive accumulation of igneous rock, formed by extensive volcanic activity over a geologically short period.
Breakup of Supercontinents: Mantle plumes can weaken continental crust, initiating rifting and contributing to the fragmentation of supercontinents. Assembly of Supercontinents: LIPs may result from enhanced mantle convection beneath assembling plates, releasing magma near convergent zones.

Question 17

What are the names of the Proterozoic supercontinents and the approximate ages of their formation and break up? When did the Grenville Orogeny occur and the assembly of which supercontinent is it associated with? When did the Iapetus Ocean open?

Answer 17

Columbia (Nuna): 2.0Ga- 1.3Ga.
Rodinia: 1.3Ga - 750Ma.
Pannotia: 600Ma- 540Ma.

Grenville Orogeny occurred between 1.3- 1.0 Ga, associated with the assembly of Rodinia, resulting from continental collisions.

Iapetus Ocean began to open around 550 Ma, following the breakup of Pannotia.

Question 18

What is the nature of the relationship between supercontinent formation, global sea level and global climate?

Answer 18

Supercontinent formations lowers sea level and creates extremes in climates

Question 19

What evidence from the rock record can be used to indicate the nature of ancient climates?

Answer 19

Sedimentary Rocks: Coal indicates warm, humid conditions and Evaporites form in arid, dry environments
Fossils: Tropical species indicate warm climates.
Paleosols (Ancient Soils): Chemical composition reveals rainfall and temperature.
Isotopes: Oxygen Isotopes (δ¹⁸O) in marine carbonates indicate past temperatures and ice volume. Carbon Isotopes (δ¹³C) reveal biological activity and CO₂ levels.

Question 20

What is meant by the term ‘snowball ‘Earth? What is the evidence for glaciation at the equator?

Answer 20

‘Snowball Earth’ refers to periods in Earth’s history when the planet was almost entirely covered in ice. These occurred during the Cryogenian period (720–635 million years ago).

Glacial deposits, such as tillites (rocks formed from glacial debris) and dropstones (rocks dropped by melting icebergs), have been found in rocks that were formed near the equator, based on paleomagnetic data.

Question 21

What is the age of the oldest (Palaeoproterozoic) glaciogenic deposits and the key causes of that glaciation?

Answer 21

The oldest known glaciogenic deposits date to 2.4–2.2 billion years ago, from the Huronian Glaciation. These are found in Canada, South Africa, and Australia, indicating a widespread glaciation.

The rise of atmospheric oxygen during the GOE removed methane from the atmosphere by oxidizing it into carbon dioxide, which is less effective at trapping heat. This significant cooling led to global glaciation.

Question 22

What is the age of the extended glacial event that occurred at the end of the Proterozoic and the key causes of that glaciation? Is it a single event or a series of events? What was the nature of the climate between the glacial periods?

Answer 22

720–635 million years ago. This period includes a series of glaciations
Reduction in Greenhouse Gases, Ice-Albedo Feedback and Volcanic and Biological Activity. These were separated by warmer interglacial periods, making it a series of events rather than a single glaciation.
The climate was warmer, with cap carbonates forming rapidly as ice melted, releasing CO₂ and causing a greenhouse effect.

Question 23

How has the length of the day and the distance between the Earth and the Moon changed through geological time?

Answer 23

About 4.5Ga, the day was lasting only 6–10 hours. Tidal friction caused by the Moon’s gravitational pull has slowed Earth’s rotation. By the end of the Proterozoic (~541Ma), the day was around 21 hours long. The Moon formed much closer to Earth, at an estimated distance of 20,000–30,000 km. Tidal interactions caused angular momentum transfer, gradually pushing the Moon away. The Moon is moving away from Earth at a rate of ~3.8 cm per year. Its current average distance is ~384,400 km.

Question 24

What are the key differences in tectonics between the early Precambrian and Proterozoic?

Answer 24

Early Precambrian (Hadean and Archaean):
The crust was primarily formed by magma ocean solidification. A less stable, non-plate tectonic system with mantle plumes, superplumes, and vertical tectonics, with no well-defined plate boundaries existed.

Proterozoic:
More stabilized cratons formed, and large continental landmasses began to assemble. Modern-style plate tectonics likely began, with evidence for subduction zones, continental collision, and the beginning of plate tectonic-driven processes. Larger, more complex supercontinents like Rodinia formed.

Question 25

How did the NOE, break up of Rodinia and warming climate affect the evolution of life?

Answer 25

Neoproterozoic Oxygenation Event:
increase in oxygen (800–541Ma) allowed the development of aerobic metabolic processes, fueling the evolution of eukaryotes and multicellular life. The rise in oxygen also led to the formation of the ozone layer, protecting life from harmful UV radiation and enabling organisms to thrive in shallow, sunlit waters and, eventually, on land.
Breakup of Rodinia: the rifting of Rodinia (750–600 Ma) led to the formation of new ocean basins, altering ocean circulation patterns and increasing habitat diversity. This also stimulated the evolution of new species in isolated environments.
Warming Climate: led to increased biological productivity in the oceans, promoted the evolution of multicellular organisms and eventually metazoans (animals), setting the stage for the Cambrian Explosion (541Ma).