Questions: Evolution of Earth's Atmosphere and the Great Oxidation Event
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
What was the main source of Earth's early atmosphere after the planet formed?
AOxygen released by early plants
BGases released from Earth's interior by volcanic outgassing, possibly with some added by impacts of comets and asteroids
CGases captured from the solar wind
DEvaporation of the oceans, which already existed before Earth formed
Outgassing released water vapor, carbon dioxide, nitrogen, and other gases. Impacts may have added some volatiles. There was no significant free oxygen, because no process yet produced it in large amounts.
Question 2 Short Answer
Banded iron formations are layered rocks of iron oxides and silica, most of which formed before about 1.8 billion years ago. What do they suggest about early oceans and oxygen?
Think about your answer, then reveal below.
Model answer: Early oceans held large amounts of dissolved iron, which can stay dissolved only when there is little free oxygen. As oxygen from photosynthesis became available, it reacted with the dissolved iron and formed insoluble iron oxides that settled to the seafloor in layers.
Banded iron formations record oxygen being consumed as soon as it was produced. Once the dissolved iron and other oxygen sinks were largely used up, oxygen could build up in the atmosphere.
Question 3 True / False
Cyanobacteria were the first organisms to produce oxygen through photosynthesis, and their activity is the main reason free oxygen built up in Earth's atmosphere.
TTrue
FFalse
Answer: True
Oxygenic photosynthesis evolved in the ancestors of modern cyanobacteria. Plant chloroplasts descend from cyanobacteria that were taken up by other cells, so later plants and algae inherited the same process.
Question 4 Multiple Choice
Which change in the rock record after about 2.3 billion years ago is evidence for oxygen in the atmosphere?
AThe first appearance of granite
BThe appearance of red beds, sandstones colored by oxidized iron that formed on land
CThe disappearance of all sedimentary rocks
DThe first evidence of liquid water
Red beds form when iron in sediments exposed to the air is oxidized. Their spread after the Great Oxidation Event, along with the disappearance of easily oxidized minerals such as detrital pyrite from river deposits, shows oxygen was present at the surface.