banded iron formations

Fossil blue-green algae.

Another image from the Nikon small world photomicrography competition, winning 5th place in 1993, depicts a fossilised section of the type of organism that gave the world free oxygen three billion years ago or more. Without these life-forms, who incidentally are not algae but cyanobacteria, none of life as we know it would exist. The oxygen from these first photosynthesisers first filled the oceans, resulting in the banded iron formations from where we mine that metal, and then the atmosphere, paving the way for the rise of oxygen using marine and terrestrial life. In the process, they incidentally poisoned off most of the existing ecosystem, since it couldn’t tolerate free oxygen. They survive as what we now call extremophiles.

The magnification is 10 times, and the lighting used that known as brightfield, which is direct illumination from below the sample.


Image credit: Norm Barker

Telling Us, “Cyanobacteria at Work”

From a focus issue of Nature Geoscience - April 2014 Volume 7 No 4

Today, life on Earth depends on the availability of free oxygen, whether in the atmosphere, oceans or aquatic systems. However, oxygen concentrations were low and variable for most of the first four billion years of Earth’s history. In this April 2014 web focus, we bring together a collection of research and review articles as well as opinion pieces that trace the origins of oxygenic photosynthesis and the factors that allowed oxygen to accumulate in the oceans and atmosphere

The BIF (banded iron formation) shown above was the subject of a geochemical analysis of its manganese, molybdenum and iron. The results add to growing evidence that the evolution of oxygenic photosynthesis may have predated the oxygenation of the Earth’s atmosphere by half a billion years or more.  

Scientific paper by Planavsky et al, Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event - behind a paywall at Nature Geoscience - April 2014 Volume 7 No 4

IMAGE: approximately 2.95-billion-year-old Sinqeni banded iron formation, located on the campus of the University of Witswatersrand in Johannesburg, South Africa // © CALLAN BENTLEY/Northern Virginia Community College

Banded Iron

This giant slab of sedimentary rock from Ontario, Canada, records a time from the Earth’s distant past when evolving life profoundly influenced the planet’s evolution. It offers evidence that even some of the world’s largest landmasses were at one time covered by water. This nearly 3-billion-year-old banded iron formation shows that the atmosphere and ocean once had no oxygen. The oxygen that is now in the Earth’s atmosphere was not there at the beginning. Early life began to generate oxygen by converting the Sun’s energy into food. Photosynthetic organisms were making oxygen, but that oxygen reacted with the iron dissolved in seawater to form iron oxide minerals on the ocean floor, creating banded iron formations. 

This boulder, with its layers of red jasper and iron magnetite, was formed billions of years ago as part of that process. Its presence in the Hall of Planet Earth is a reminder that life made our atmosphere breathable. The dark layers in the rock are mainly composed of magnetite (Fe30), while the red layers are chalcedony, a form of silica (SiO) that is colored red by tiny iron oxide particles. Some geologists suggest that the layers formed annually with the changing seasons. 

See more from the Museum’s Hall of Planet Earth.