Final Answer:
The delay of two billion years for the buildup of oxygen after the emergence of photosynthetic cells is attributed to the oxygen being initially absorbed by iron and other compounds, forming iron oxides and undergoing geological processes before accumulating in the atmosphere.
Step-by-step explanation:
The evolution of photosynthetic organisms marked a significant milestone in Earth's history. However, it took approximately two billion years for atmospheric oxygen levels to rise substantially. Initially, oxygen produced by photosynthetic cells was absorbed by iron and other elements in the Earth's crust, forming iron oxides. This process, known as the Great Oxidation Event, consumed the free oxygen, preventing its accumulation in the atmosphere.
Over time, geological processes and the saturation of oxygen sinks allowed free oxygen to accumulate, leading to significant changes in the Earth's atmosphere. The two-billion-year delay highlights the complex interplay between biological and geological processes in shaping the composition of the atmosphere over geological time scales.