A banded iron formation in South Africa records a profound change in Earth’s environment. I took the photo below many years ago while examining an iron deposit near the town of Messina, the northernmost town in South Africa located just south of the Limpopo River on the border of Zimbabwe.
The rock in the photo shows light bands of quartz alternating with dark bands of magnetite, an iron oxide. The folds record soft-sediment deformation and other structural changes since the rock was deposited on the sea floor approximately 2 billion years ago.
The magnetite is the result of an environmental change, but the story itself begins about a billion years before the rock was formed. The story is about how a life-form changed the planet.
Life leaves a signature. We now think that life began on Earth almost 4 billion years ago, and it left a signature consisting of a special combination of carbon isotopes. The first known life-form on Earth was the bacterium. Bacteria are still with us because they are good chemists and very adaptable. They don’t change body shape, but they do change chemical processes in response to the environment.
The first bacteria developed and lived at crushing ocean depths near undersea volcanoes where they derived sustenance from hydrogen sulfide emitted by the volcanoes. As continents rose out of the primordial sea, these earliest bacteria worked their way to shallow water near land, and started to use carbon dioxide and sunlight. The oldest known fossils, called stromatolites, are remnants of bacterial mats. The earliest stromatolites are dated at about 3.5 billion years before present. For about one billion years, bacteria were the only life-form on earth.
Bacteria give off oxygen, and after a billion years, that process caused an environmental crisis. About 2.5 billion years ago, oxygen levels in the ocean reached some critical level which caused iron and manganese to precipitate. All of the world’s large iron deposits, called Banded Iron Formations, formed between 2.5- and 1.8 billion years ago, and none have formed since. After oceanic iron was used up, oxygen increased in the atmosphere. The oxygen began to destroy methane, a very powerful greenhouse gas, and the reaction produced carbon dioxide, which is 62 times less effective at warming the surface of the planet. Loss of methane plunged the planet into a profound ice age that lasted for about 30 million years. The bacteria retreated to equatorial habitats and again toward warm volcanic vents. Populations became isolated and some changed; they became more organized into a new life form called Eukaryotic microbes. Fossil Eukaryotes appear in the iron formations, initially as single cells, then as multicellular chains up to 4″ long. Life was getting big. The Eukaryotes would eventually become animals, plants, and fungi.
It is estimated that the current mass of bacteria on Earth exceeds that of all other life-forms combined.
The rocks record the history of this planet and it has always been fascinating to me to figure out their story.