The Atmospheric Oxygen at 1.9 billion years ago, was percent that of the modern level. A- 10% B- 15% C- 70% D- 50%

To answer the question about the levels of atmospheric oxygen 1.9 billion years ago, we can analyze the historical data regarding oxygen levels in Earth’s atmosphere during different geological periods.

Explanation of Historical Context

1. Understanding Early Earth's Atmosphere:

   • The Earth's atmosphere has undergone significant changes since its formation. Early in Earth's history, particularly around 2.5 billion years ago, it had very low levels of oxygen. The Great Oxidation Event (GOE) occurred around 2.4 billion years ago, which was when oxygen began to accumulate in the atmosphere due to photosynthesizing cyanobacteria.

2. Levels of Oxygen During the Proterozoic Eon:

   • During the time around 1.9 billion years ago, atmospheric oxygen levels were still quite low compared to modern levels. Estimates suggest that the oxygen content was significantly less than what we experience today.

3. Comparison of Options:

   • A- 10%: This could be a reasonable estimate but still may be too high for that time.
   • B- 15%: This is also a possibility but likely not accurate for the period in question.
   • C- 70%: This figure is too high when compared to modern levels.
   • D- 50%: This is beyond the estimated amounts of atmospheric oxygen for that time.

Conclusion

The modern atmospheric oxygen level is about 21%. Research indicates that 1.9 billion years ago, the levels of atmospheric oxygen were far lower, with estimates suggesting they were around 10% to 15% of the modern levels.

Final Answer

The correct answer is A- 10%.