Paleoproterozoic Rust Belt

banded iron formation
Jaspilite banded iron formation (BIF) (Vulcan Iron-Formation, Paleoproterozoic, ~1.8 Ga; Iron Mountain, Menominee Iron Range, Upper Peninsula of Michigan, USA) by James St. John, (2011) CC BY-SA 2.0, via Flickr. https://flic.kr/p/oWHUNf%5D

As a human being living on Planet Earth in the 21st century, the physical surroundings of every place I encounter have been deeply modified by the passage of other human beings. Our cities and infrastructure, our farms and factories, are changing the chemistry, ecosystems and physical face of our planet. In Michigan, as in other Rust Belt places of former manufacturing glory, the shuttered factories and rusting machines are starkly visible as a symbol of decline and decay.

Rusty factory in Cleveland
Rust Belt Reflection by Bob Jagendorf, (2009) CC BY-NC 2.0 via Flickr. https://flic.kr/p/7eZ3p5

But humans are not the first organisms with the power to massively modify their environment. We’re not even the first to create a Rust Belt.

About 2 billion (that’s a 2 with 9 zeroes after it) years ago, colonies of photosynthetic bacteria figured out how to use sunlight to capture energy contained in the shared electron bonds between the hydrogen and oxygen atoms in water (H2O) molecules. This metabolic innovation opened up a whole new source of food for prokaryotic organisms, but there was a problem. The chemical reaction produced a toxic by-product: Highly reactive oxygen gas. The atmospheric oxygen (02) molecules exhaled by the bacteria, as well as the UV radiation from Sun, were harmful to the membranes of the living cells. However, the bacteria were safe as long as they stayed submerged under a protective layer of seawater.

However, a dramatic effect occurred when enough free oxygen gas accumulated to react with ferrous iron (Fe2+) ions dissolved in the oceans. The oxygen combined with the iron to create iron oxides (aka Rust.)
Let me emphasize this. The oxygen gas the bacteria were breathing out was not just poisoning the atmosphere with a toxic gas that would steal your electrons, it was literally rusting the planet. The precipitated iron oxides settled to the bottom of the ancient oceans, producing what geologists call Banded iron formations(BIFs).

I first learned the story of the oxygen-producing bacteria in my undergraduate biology class. It was shocking to me because it illustrated how tiny living things changed the face of the planet (i.e. “Once there was no oxygen, now our atmosphere is 21% oxygen *and* we need it to breathe!”) However, it was even more jarring to realize that this vital substance I take for granted, oxygen, was toxic to the first creatures who breathed it out. It seemed incredible that my ancient bacterial ancestors ever survived their own success in exploiting a new energy source, in the face of causing such widespread atmospheric pollution. (But that is a story for another blog post.)

In Michigan’s Upper Peninsula, the abundant iron ores that enabled the state’s mining and manufacturing industries of the 19th and 20th centuries come from Banded Iron Formations. The automobiles and steelworks of the Motor City owe their existence (in part) to the oxygen exhalations of 2 billion year old cyanobacteria.

This brings us to the modern parallel of how humans are contributing to changing the atmosphere by our burning of fossil fuels, super handy energy source in our civilization. While as oxygen-breathing organisms, we breathe out CO2, we’re adding it even more quickly to the atmosphere through burning the ancient sunlight stashes of coal, oil and natural gas. This year, we hit the atmospheric CO2 benchmark of 400 ppm. It’s been a long time (in human reckoning) since the atmosphere has held that concentration of CO2.

We’re seeing some of the effects in changing weather patterns, warming temperatures and increased ocean acidification (carbonic acid produced from greater concentrations of dissolved CO2, like in seltzer water.) The corals of the oceans’ great reef ecosystems have been hit pretty hard.  
I think about this each time I fill my the gas tank on my car, so I can drive to work. I think about how the oxygen produced by the ancient bacteria, in addition to rusting the oceans, triggered a possibly planet-wide ice age known as the Huronian Glaciation (named for sediments discovered in Lake Huron), aka “Snowball Earth.” If the ancient photosynthetic bacteria could change the face of the planet so drastically, what does that say about us humans who are at least beginning to understand the effects of our collective actions?

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