Question

What process first began producing oxygen about 2.7 billion years ago? Why did the oxygen concentration remain low for 300 million years?

Short answer

Photosynthesis by cyanobacteria started producing oxygen. Low levels persisted due to absorption by oceans and rock oxidation.

Step-by-step solution

Understanding the Question

We need to identify which process began the production of oxygen about 2.7 billion years ago and why the concentration of oxygen remained low for so long afterward.

Identifying the Process

The process that first began producing oxygen around 2.7 billion years ago is photosynthesis, specifically by cyanobacteria, a type of bacteria that uses sunlight to convert carbon dioxide and water into glucose and oxygen.

Understanding Oxygen Accumulation

Initially, the oxygen produced by cyanobacteria did not accumulate in the atmosphere because it was absorbed by oceans and reacted with iron to form iron oxide, leading to the oxidation of available minerals.

Explaining Low Oxygen Concentration

Oxygen levels remained low because it took additional time, around 300 million years, for these sinks to become saturated. Only after oceans and rocks were fully oxidized could oxygen begin to accumulate significantly in the atmosphere.

Key concepts

Cyanobacteria

About 2.7 billion years ago, a remarkable group of microorganisms, known as cyanobacteria, began to change Earth's environment forever. These tiny bacteria are often referred to as "blue-green algae" due to their color and resemblance to algae, though, in reality, they are a distinct group of bacteria. Cyanobacteria are special because they have the ability to perform photosynthesis, just like plants.

• Photosynthesis is the process by which these organisms convert sunlight into chemical energy.
• They use sunlight to transform carbon dioxide ( ext{CO}_2) and water (H_2O) into glucose (a type of sugar) and oxygen (O_2).

This process is crucial because it started producing oxygen as a byproduct, introducing what would eventually become a critical component for life on Earth.

Cyanobacteria played a major role in shaping the planet's atmosphere and biosphere through this transformative process. Without them, the Earth's atmosphere might not have developed the oxygen levels needed to support complex life forms. Their ability to thrive in various environments, including oceans and freshwater bodies, enabled them to spread widely and contribute significantly to early oxygen production.

Oxygen Concentration

Initially, the oxygen concentration in the Earth's atmosphere remained surprisingly low, even though cyanobacteria began producing oxygen continuously. This raises a question: why didn't oxygen levels rise quickly?

The answer lies with certain geological and chemical processes underway at the time. Oceans and rocks acted as massive "oxygen sinks." Here's how:

• Oceans absorbed a lot of the oxygen produced, with vast amounts dissolving in seawater.
• Iron-rich minerals in the water reacted with oxygen to form iron oxides, which precipitated out and formed deposits known as banded iron formations.

This chemical interaction effectively locked away the oxygen, preventing it from entering the atmosphere. Until these reservoirs became saturated with oxygen, atmospheric levels remained low. This saturation process took significant time—about 300 million years. Only when this natural consumption of oxygen slowed down, could oxygen levels in the air begin to rise, setting the stage for more complex life to flourish.

Great Oxidation Event

The Great Oxidation Event (GOE) marks a time in Earth's history when the atmospheric oxygen concentration finally began to rise significantly. This major milestone occurred around 2.4 billion years ago, several hundred million years after the initial production of oxygen by cyanobacteria. Once the oceans and rocks could no longer absorb all the oxygen being produced, it started to accumulate in the atmosphere.

The rise in oxygen caused profound changes:

• More oxygen in the atmosphere led to clearer skies and more sunlight reaching Earth's surface.
• It allowed for the formation of the ozone layer, which protected the surface from harmful ultraviolet radiation.
• These conditions were favorable for the evolution of aerobic organisms that breathe oxygen.

The GOE is considered a turning point, as it paved the way for the evolution of more complex life forms on Earth. Although it brought challenges, like creating harsher environments for anaerobic organisms, it ultimately introduced a critical change in the planet's habitability, making it possible for life as we know it to evolve.