Iron and Oxygen: A Tale

The Iron Record Of Earth’s Oxygen / Science News.

Oxygen and iron can both exist in aqueous solution, but not together in great quantities. Instead they form insoluble oxides that precipitate out i.e. they make rust! Earth’s oceans haven’t always been as iron poor nor as oxygen rich as they are today. For about 2 billion years – from about 4.4 Gya to 2.3 Gya – they were full of dissolved iron and short on oxygen. As the Science News article, linked to above, reports new research is giving us deeper insights into the “Great Oxygenation Event” of that post-Archean, early Paleo-Proterozoic Era. For example, about 2.316 Gya enough oxygen had built up to form an ozone layer, fundamentally changing the prospects for life ever since.

Geological understanding is built on applying what we know of present day processes to past events – a maxim usually put as “the Present is the Key to the Past”. One present day key-to-the-past is an oxygen-poor lake in Indonesia. Its surface waters are oxygenated, but short on nutrients, then below ~120 metres the water is full of dissolved iron, no oxygen and alive with “photoferrotrophs” – bacteria that use photosynthesis to get energy from oxidising dissolved iron. They live by rusting the iron in the deep lake waters. A feature of the early days of oxygen’s rise are “Banded Iron Formations”, BIFs, from which most of the world’s commercial sources of iron/steel are derived. Basically BIFs are huge rust deposits and the “photoferrotrophs” of the Archean/Paleo-Proterozoic seem to be partially responsible.

What a strange, and humbling, debt we owe to such obscure bacteria. Their metabolic wastes are the ruddy feedstock of the first stage of manufacturing many of modern day industry and construction’s products. The steel skeletons of the Colossii of a modern City come from bacterial chemistry concentrating iron oxides over millennia. And frequently BIFs show a banding effect, now believed to have been caused by seasonal chemistry changes – more rust deposited in warm weather, more silica in cold. But the BIFs keep many more mysteries that geologists have yet to tease out of them…