Question

What arguments favor the idea that little atmospheric oxygen existed on Earth until slightly before 2 billion years ago?

Short answer

Geological evidence, biological factors, the absence of the ozone layer, MIF signatures in sulfur isotopes, and changes in ocean chemistry all support the idea that little atmospheric oxygen existed on Earth until slightly before 2 billion years ago. These factors indicate that the early Earth environment was oxygen-poor, allowing for the emergence of anaerobic lifeforms and specific geological formations that are indicative of low oxygen levels. The rise of oxygen-producing cyanobacteria and the subsequent changes in Earth's atmosphere, ozone layer, and ocean chemistry further solidify this argument.

Step-by-step solution

1. Geological Evidence

One argument favoring the idea of little atmospheric oxygen on early Earth is the presence of reduced iron in ancient rocks, which can only form in an oxygen-poor environment. Examples include banded iron formations (BIFs) and red beds. These formations indicate that the early Earth’s atmosphere was rich in methane and lacked significant amounts of oxygen.

2. Biological Factors

Early lifeforms on Earth were anaerobic, meaning that they did not require oxygen for growth and were able to survive in oxygen-poor environments. The appearance of aerobic lifeforms in the fossil record, such as cyanobacteria, which produce oxygen through photosynthesis, occurred around 2 billion years ago. This further supports the idea that atmospheric oxygen levels were low before this time period.

3. Absence of Ozone Layer

Before the rise of atmospheric oxygen, there was no ozone layer to protect Earth's surface from harmful ultraviolet (UV) radiation. This implies that life had to be either anaerobic or adapted to withstand high levels of UV radiation. The emergence of oxygen-producing cyanobacteria led to the eventual formation of the ozone layer, providing protection for life on Earth and supporting the idea that atmospheric oxygen levels were low before 2 billion years ago.

4. MIF (Mass Independent Fractionation) Signatures

Another argument supporting the idea of low atmospheric oxygen levels until around 2 billion years ago is the existence of MIF signatures in sulfur isotopes found in ancient rocks. The presence of these MIF signatures indicates that sunlight-driven, oxygen-free chemical reactions occurred in Earth's atmosphere. After the Great Oxygenation Event (GOE) around 2 billion years ago, these MIF signatures disappeared, indicating that significant amounts of atmospheric oxygen had accumulated.

5. Changes in Ocean Chemistry

The chemistry of ancient oceans also provides evidence supporting the idea of little atmospheric oxygen on early Earth. Studies have shown that the ratio of chromium isotopes in ancient marine sediments changed significantly around 2 billion years ago, indicating a shift in ocean chemistry that coincided with the rise of oxygen levels in the atmosphere. This evidence suggests that Earth transitioned from an oxygen-poor to an oxygen-rich environment around this time. In conclusion, various arguments favor the idea that little atmospheric oxygen existed on Earth until slightly before 2 billion years ago, including geological evidence, biological factors, the absence of ozone layer, MIF signatures, and changes in ocean chemistry. These factors provide a strong foundation for our understanding of Earth's early atmosphere and the development of life on our planet.