Question

about 2 billion years ago, oxygen began to build up in the atmosphere due to the evolution of photosynthetic organisms. which of the following is evidence of this?

• deep cracks in lithified mud from the mesozoic era.
• samples from gas bubbles trapped in cenozoic amber
• carbon dating of fossils from the precambrian period
• thick bands of iron oxides (rust) in precambrian rock layers

Explanation:

Oxygen in the atmosphere can react with iron to form iron oxides (rust). Thick bands of iron oxides in Precambrian rock layers indicate the presence of oxygen during that time, which is evidence of the evolution of photosynthetic organisms starting to build - up oxygen about 2 billion years ago. Deep cracks are not direct evidence of oxygen build - up. Gas bubbles in Cenozoic amber are from a much later time. Carbon dating of fossils doesn't directly show oxygen build - up in the atmosphere.

Answer:

thick bands of iron oxides (rust) in Precambrian rock layers