Rock of Ages – Why Banded Iron Formations Are Far From Boring…

Two-and-a-half tonne banded iron formation at Hintze Hall, Natural History Museum, London. Artwork: NaturPhilosophieThis 2.5-tonne lump of rock is a banded iron formation.  It marks a turning point in the history of life on our beautiful planet.  A crucial chemical transition.  When oxygen started becoming abundant.  And life took its next step towards complexity… 

It’s after two o’clock.  Let’s take a trip back in time…

We’re 2 billion years ago… or thereabouts…  (You can’t be too specific with geological time.)

Free oxygen in the atmosphere reached an unprecedented high.

Free as a Rock

This geological sample is part of the artefacts on display in the Hintze Hall, at London’s Natural History Museum. 

And you can “read” it,  provided you can read a slab of rock the way an Earth scientist does.

About 2 metres along its base and 1.5 metre high, it represents a wonderful juxtaposition between the animate (complex life) and the inanimate (rock) and the very deep connection that exists between the two.

Those colourful wavy lines in the rock sample are bands of iron oxide (mostly haematite), interspersed with chert (silica).

Banded Iron Formations (BIFs)

Banded Iron Formation (BIFs) record a key chemical transition in the story of Earth – the geological time when oxygen started to become abundant… and the portal to the emergence of complex lifeforms was unlocked…

BIFs were laid down on ocean floors more than two billion years ago.

A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimetres in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in colour, of similar thickness, and containing microbands (sub-millimetre) of iron oxides.

BIFs account for more than 60% of global iron reserves, and can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine and the United States.

Until 1992, it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally.

Iron-rich waters would then form in isolation, subsequently coming into contact with oxygenated water.

Free Oxygen and Evolution

A free oxygen molecule, O<sub>2</sub>.Before photosynthesis evolved, Earth’s atmosphere had no free oxygen (O2).

Free oxygen is what gives us the diversity of life we have on Earth today.

Photosynthetic prokaryotic organisms began producing O2 as a waste product from respiration long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago.

Around 2 billion years ago (2.4 to 2.0 Ga) during the Paleoproterozoic Era, the increased oxygen level in the atmosphere had a profound change that would ultimately make complex life – such as the giant cetaceans – possible.

The Great Oxygenation Event

The O2 build-up in the Earth’s atmosphere over the Geological Timescale. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga). Stage 1 (3.85–2.45 Ga): Practically no O2 in the atmosphere. Stage 2 (2.45–1.85 Ga): O2 produced, but absorbed in oceans and seabed rock. Stage 3 (1.85–0.85 Ga): O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer. Stages 4 and 5 (0.85 Ga–present): O2 sinks filled, the gas accumulates. Source: Wikipedia

This “mass rusting” led to the deposition of iron oxide on ocean floors, forming the banded iron formations we see today.

Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event – this mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen.

At first, the oxygen produced would have been rapidly removed from the atmosphere by weathering of minerals, notably iron.

In the absence of plant life, the rate of oxygen production by photosynthesis was slower in the Pre-Cambrian.

The Pre-Cambrian

A Geological Timescale of the Pre-Cambrian Era (pЄ). Source: Wikimedia

The Precambrian (abbreviated , or Cryptozoic) accounts for 88% of the Earth’s geologic time.

It spans from the formation of Earth about 4.6 billion years ago (Ga) to the beginning of the Cambrian Period, about 541 million years ago (Ma).

PreCambrian = 88% of Geological Time

Back then, free oxygen O2 concentrations attained were less than 10% of today’s, and they probably fluctuated greatly.

Oxygen may even have disappeared from the atmosphere again around 1.9 Ga ago.

At current rates of primary production, today’s concentration of oxygen could be produced by photosynthetic organisms in 2,000 years.

Complex Life

These fluctuations in oxygen concentration had little direct effect on basic lifeforms, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period, 541 million years ago.

Aerobic metabolism being more efficient than anaerobic pathways, the presence of oxygen created new possibilities for life to explore.

Aerobic Metabolism

Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity.  It involves the conversion of organic molecules into carbon dioxide CO2 and water H2O.

Organic cells convert this biochemical energy from nutrients into Adenosine TriPhosphate (ATP) – a precursor to DNA and RNA – and release waste products.

Life = Growth + Metabolism

The energy released by respiration is used to drive other parts of metabolism to enable other metabolic chemical transformations to take place, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis, and to enable animals to move.

All of a sudden (in geological terms, at least), the presence of free oxygen O2 provided life with new opportunities.

The Alternative Theory

The Snowball Earth hypothesis provides an alternative explanation for these younger deposits.

Snowball Earth

An alternative mechanism for banded iron formations in the Snowball Earth Era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones. 

In a Snowball Earth scenario, the continents and seas at low  latitudes, were subject to a severe ice age circa 750 to 580 million years ago that nearly or totally depleted free oxygen.

Subsequently, dissolved iron accumulated at the bottom of oxygen-poor oceans, possibly from seafloor hydro-thermal vents.

Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.

Alternatively, geochemists even suggest that BIFs could form by direct oxidation of iron by microbial anoxygenic phototrophs.

Geologically Significant

Banded iron formations are indeed geologically significant.

They are the concrete record of a critical evolutionary shift in environmental conditions.  They mark a real turning point.

An indelible mark on the history of our Rock of Ages