Tuesday, 13 January 2015

A Geochemical Explanation For The Delay Preceeding The Great Oxygenation Event

Beds of red iron oxide in Utah. This particular formation dates from
the Triassic rather than the time of the Great Oxygenation Event
The appearence of oxygen in the atmosphere 2.6 billion years ago was perhaps the event in Earth history, bar the planet's formation itself. The reactive molecule, the product of cyanobacterial photosynthesis, revolutionised the Earth's geochemistry creating great beds of red iron oxide on the ocean floor across the globe. Its effect on life was even more profound.

2.6 billion years ago the only organisms on the planet respired anaerobically. Oxygen, toxic to anaerobes, may well have resulted in the first mass extinction in planetary history: the so-called oxygen holocaust.

In its wake, bacteria evolved which took advantage of this new chemical environment. These newcomers respired aerobically, and in so doing, were able to generate far more energy than their anaerobic forebears. This gave them a crucial advantage in the drive to survive and reproduce. Today all complex organisms, whether they are single-celled or multi-celled, respire aerobically. In times of stress or exertion they may resort to anaerobic respiration, such as lactic acid formation in sprinters. evidence of their ancient heritage. Yet the higher energetic yield of aerobic respiration offers a distinct advantage.

One question which has puzzled geologists and geochemists for decades , is why it took so long for oxygen to become a significant component of the atmosphere. Genetic and fossil evidence indicates that oxygen-producing cyanobacteria evolved possibly three billion years ago. So why does oxygen only make its mark on the planet 400 million years later? Volcanic activity scrubbing the atmosphere of the gas has been a long-held hypothesis, but a new and highly plausible explanation has been put forward by geomicrobiologists from the University of Tubingen.

Cyanobacteria generate oxygen through photosynthesis
Dr. Elizabeth Swanner and Professor Andreas Kappler examined factors which would affect the rate of cyanobacterial growth. They found that soluble iron in the earliest oceans quickly combined with oxygen to form rust, producing reactive oxygen compounds in the process. These damage the biological molecules and make the cyanobacteria grow more slowly, producing less oxygen.

Oxides of iron are insoluble and are deposited on the seabed. It is for this reason that dissolved iron is virtually non-existent in today's oceans as any trace of the element is quickly oxidised and then deposited. Billions of years ago, before the appearance of oxygen in the atmosphere, dissolved iron was abundant - possibly due to hydrothermal activity on the seabed.

When photosynthesis evolved and oxygen appeared, the iron oxidised and came out of solution to form the beds of red iron oxide seen in the geological record 2.6 billion years ago. This also generated the reactive oxygen species responsible for inhibiting cyanobacterial growth. 'Too much iron in the presence of oxygen was harmful. You could say the early cyanobacteria poisoned themselves,' said Andreas Kappler. The volcanic scrubbing hypothesis works, but it is a bit woolly as it relies on a reduction of volcanic activity. If it was a sudden, singular change the reason for the change is unclear; if it was a trend then why do we see significant volcanic activity later on in the geological record? The iron hypothesis on the other hand provides a precise geochemical mechanism for the change.