Bands of Iron

On a wintry day late last year, I visited the Museum of Natural History in New York City. While touring the geology wing, I came across this boulder-sized chunk of a rock formation:

A banded iron formation from the geology exhibit of the Museum of Natural History. Photo credit to Erich Vieth.

It was out in the open with no ropes or glass around it, inviting visitors to touch it. I brushed a hand across its polished surface, which was as smooth and cool as a sheet of glass. Nothing about that touch hinted at the stone’s age or history; yet it had traveled down immense vistas of time to come here, to our era, so that I could see and touch it on that day. And in the moment of that touch, I knew, I as a modern Homo sapien was briefly reunited with predecessors ancient beyond imagining, perhaps some that date back almost to the origin of life on Earth itself.

The curious, gorgeously colored strata of this stone are called banded iron formations. The dark bands are layers of metallic iron oxide compounds such as magnetite and hematite, while the reddish layers are silica-rich quartz minerals like chert, jasper and flint. Banded iron formations occur almost exclusively in very ancient rocks, and are common in strata dating to between 2.5 billion and 1.8 billion years ago. This is the period commonly called the Precambrian, although its more technical name is the Proterozoic Eon.

True multicellular life first appears in the fossil record at the very end of the Proterozoic, in the form of the bizarre and famous Ediacaran biota that would become the precursors of the Cambrian explosion. But for most of the Proterozoic, the most common fossils are stromatolites: puffy accretions of sedimentary rock laid down by vast colonies of bacteria.

The Earth in this eon was a different place. Most notably, from chemical and geological evidence, we know that its atmosphere had no oxygen. The only life was colonies of purple bacteria, making a living using the chain of chemical reactions called photosystem I, which converts light, carbon dioxide and hydrogen sulfide into sugar and releases sulfur as a byproduct. But the Proterozoic was when this began to change: this was the time when evolution invented photosystem II, the more advanced version of photosynthesis that uses water and carbon dioxide to make sugar, liberating oxygen as a byproduct. This is the very same set of reactions that sustains all green plants, and ultimately all animal life, today, two and a half billion years later.

At first, oxygen was an annoyance to Proterozoic life, but it soon became a menace. Unlike today, there were no oxygen-breathing animals to expire carbon dioxide and close the cycle, and so it quickly built up in the atmosphere as photosynthetic bacteria spread and thrived. To us, it’s the breath of life, but to these bacteria, it was a deadly toxin.

At the same time, another process was taking place. Weathering of the Earth’s primordial rocks had been releasing iron, most of which washed down to the sea and ended up as iron ions dissolved in the oceans. Until then, that iron had had nothing to react with, but when it encountered oxygen, the two chemically combined into iron oxides like magnetite and hematite. These compounds are insoluble, and when they formed, they precipitated out and sank to the ocean bottom, gradually building up those dark silver layers.

With iron reactions steadily removing oxygen from the atmosphere, anaerobic bacteria thrived for a time. But eventually, there was no more free iron. Once that point was reached, oxygen started to build up in the atmosphere. Heedless, the bacteria kept churning it out – until a toxic tipping point was reached, and the Earth’s atmosphere was changed to such an extent that it became poisonous to Earth’s life. The consequence was mass death among the planet’s abundant bacterial colonies – an oxygen holocaust that knocked life back down to nearly nothing. Only a few anaerobes survived, in isolated nooks and crannies where the deadly gas did not reach.

After this catastrophe, the planet would have seen several million years of relative quiet. In this life-poor era, layers of silica minerals were deposited on the ocean floor. But in the meanwhile, erosion continued to free up iron atoms, which slowly scrubbed the atmosphere and oceans of oxygen. Eventually, the world was cleansed, and life bounced back, spreading from its refuges to once again cover the planet. Of course, this exuberance contained the seeds of its own downfall – bacteria still spewed out the waste oxygen that they could not abide – and the cycle repeated, not just once but many times. Each time, a layer of iron oxides was deposited, followed by a layer of iron-poor silicates in the aftermath. And that leads me back to the Natural History Museum, on that cold winter day where I stood and brushed a hand across a banded iron formation.

Looking at this stone, you get some idea of the dizzying vistas of geological time, as well as the turmoil that life has endured to reach the present day. Each of those colorful red and silver layers represents what was, in its own era, a disaster beyond imagining, one that reset life to its starting point. Each of those layers, as well, is a silent testament to life’s tenacity in the face of overwhelming odds. Of course, the cycles of growth and destruction did not last forever. Eventually, evolution found a way, as evolution nearly always does, and oxygen was tamed to become a power source in an entirely new metabolic cycle. The oxygen-breathers arose, the remaining anaerobes retreated to the deep crevices of rocks and the sea, and life found a new equilibrium, with the balance of the atmosphere permanently changed. All the oxygen we breathe today is biologically produced, a tangible proof of life’s power to reshape its own world.

As well, these banded iron formations may be a metaphor for our own foolhardiness. In our time, we too are changing the composition of the planet’s atmosphere, this time through the release of greenhouse gases. In the process, we are becoming the first species since the ancient photosynthetic bacteria to have such a global effect. The danger we face may not be as severe – but it is severe enough. Those bands of iron are not only a record: they are a warning of what happens when life reshapes its own environment without thought for the consequences.

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About Adam Lee

Adam Lee is an atheist writer and speaker living in New York City. His new novel, City of Light, is available in paperback and e-book. Read his full bio, or follow him on Twitter.

  • Tycho the Dog

    Fascinating and humbling. But what’s even more extraordinary is that if you look hard enough, you can make out an exact likeness of Jesus H. Christ riding a horse. Holy smoke, it’s a miracle…

  • nogrief

    Ebon, I can’t comment because your account has left me speechless! Many years ago my sons and I rafted down the Grand Canyon. Thanks for rekindling the same feelings of awe as I felt on that trip.

  • meta.user

    A wonderful meditation on life and time and change. And although global warming has made me aware of carbon dioxide’s double capability to help and harm life, it’s interesting to be reminded that oxygen also has those same properties.

  • Christopher

    Those bands of iron are not only a record: they are a warning of what happens when life reshapes its own environment without thought for the consequences.

    I suppose that’s one way to look at it, but I have a different one: these formations show what life is capable of surviving – remember that without said oxygen (toxic to the cyanobacteria that emitted it) later (and far more diverse) lifeorms would never have come into existence (including our species). The lesson I take away form the banded iron formations is that which is poison to one may be nurishment to others; no matter how many things we release in the environment that are “poisonous” (to us and life forms like us), some life forms out thete will survive (even thrive) in the new ecology that our “toxins” make possible.

    Just as with the cyanobacteria, with or without our species life will go on – running the course it began all those billions of years ago, without any regard to what happened to those forms that nature had selected out.

  • Lynet

    Wow! That was an amazing story. Thanks for brightening up my morning with a little transcendence.

  • Eric

    I’d just like to say that purple sulfur bacteria are a litle more robust than this post makes them seem. Yes, the die offs that make banded iron would have been massive, but there would still be plenty of anerobes all over the place. The descendants of these bacteria are sometimes quite visible in freshwater aquariums with thick substrates. Their colonies can be quite visible as a black band a few inches down. Perhaps these bacteria are more robust than than purple sulfur bacteria of old and have metablolic tricks for living in an aerobic world.

    I grow anerobes in mud as a hobby. I have a couple of tanks that even do a closed nitrate cyce.

  • Maynard

    Great example of how delicate and how strong life is, all at the same time. And a great comparison to our own situation with climate changes.

    I hope I end up part of a geological formation that future, intelligent earthlings consider beautiful.

  • BlackSun

    A timely and poetic post. If the lowly bacteria could reshape the biosphere to its own demise, then the Earth’s atmosphere is certainly not “too vast” to be impacted by our civilization and its waste carbon.

  • TommyP

    Toss some banded iron at young earth creationists. I’d love to see what their “explanation” would be for such formations. Probably…the biblical flood.

    That rock is beautiful, and it is very humbling to know about how much time is involved in forming these things!

  • Leum

    Their explanation for everything is the Deluge. Speaking as a geosciences major, I find that offensive. If there had been a global flood it would have left distinct traces in similarly aged strata worldwide–we’d have to be blind not to notice it. Furthermore, the things they attribute to the Deluge are not and can not be effected by water!

  • Waialeale Mike

    Recently, I read an article about research into the cultural differences in the way numbers are visualized, understood. For example, how could a culture whose largest number is five, comprehend 54×12 for example.

    But even within a culture, how could somebody who believes time began 8000 years ago comprehend 4,000,000,000 years and understand that his 8000 years is a dust mote compared to the actual age of the Universe(our’s anyway–another billion or so universes might have popped into existence since I start pecking away at this msg ).

    Even big number cruncher folks have problems getting a handle on even a mere million, preferring to just add exponents. It’s hard to appreciate that every time you add a zero, the number is ten times larger, that a billion is a thousand millions.

    And infinity, they’re really clueless. Do they really believe they’ll be basking in the glow of the invisible old fart in the sky a trillion trillion years from now — which isn’t even a wart on a gnat’s ass compared to infinity. After the stars fade burn out, the universe rips apart or crunches back into itself, they’ll still be chatting with Jerry Falwell. Whatever (while the merciful old fart fries my butt)

    I’ve just one itty bitty concern. After death, I’ll only find out if I’m wrong.

  • Steve Bowen

    I’ve just one itty bitty concern. After death, I’ll only find out if I’m wrong.

    and that would be a great adventure. Probably won’t happen though.

  • D

    Wow. That is heart-rendingly beautiful.

    Knowing what those stripes mean is so enriching, so mind-blowing, so wonderful – I can’t fathom the mind that would shut out such knowledge. How can one see the beauty caused by such inconceivably vast cycles of death and rebirth, and dismiss it as nothing but eye-candy from the Great Entertainer? Such a mindset is truly antithetical to science in its willful ignorance… heresy, I dare say.

  • Ebonmuse

    I hope I end up part of a geological formation that future, intelligent earthlings consider beautiful.

    I just have to say that I totally agree with this. :)

    As it stands, I rather suspect that humanity’s strata will be identifiable by the high percentage of heavy metals and plastic.

  • Leum

    Human activity has such a strong affect on geosystem that there’s a movement among geologists to name the later portion of the Holocene (the one we’re in) the Anthropocene Epoch. Human activity displaces more sediment than all natural forces combined, has destroyed glaciers, leveled mountains, started an extinction event that’s killing off species at a faster rate than the Permian-Triassic Extinction (the largest mass extinction in Earth’s history), raised the amount of CO2 in the ocean and atmosphere, and more. Heavy metals and plastics will be only one means of identifying anthropic strata.

  • Eric

    Well, at least some ancient cultures had the means to imagine very large numbers. The Mayan calendar counted years by the thousands, though their calendar cycle ends in 2012. The Vedas speak of cycles in the billions of years. Archimedes invented a kind of exponential notation to number the grains of sand filling a sphere with a radils of the distance from the earth to the sun. Persians were familiar with Greek and Hindu large numbers and made some pretty accurate astronomical calculations based on even earlier data.

    I somwtimes think we need to make a ew calendar that has its year zero the earliest dates recorded in Persian data taken from earlier Assyrian data.

  • Spanish Inquisitor

    As it stands, I rather suspect that humanity’s strata will be identifiable by the high percentage of heavy metals and plastic.

    But who’s going to be around to cut a big slab of it and put it in a museum?

  • velkyn

    “But who’s going to be around to cut a big slab of it and put it in a museum?”

    very large intelligent cockroaches :)

    I love this banded iron, also called “tiger” iron in the jewelry trade. It makes a lovely pendant.

  • Mike Cope

    In South Africa we have a banded ironstone which is quartz with a high percentage of iron. I suspect it’s different from this stone.

    You can see some pictures of an artefact made from the material here:

  • Tommykey

    But who’s going to be around to cut a big slab of it and put it in a museum?

    Why, WALL-E of course!

  • Andrew MW

    Great post, and congrats on the recent award!

    My question is: how come the bacteria didn’t reach an equilibrium vis a vis the amount of oxygen in the atmosphere? That seems to be a more likely scenario than the wild see-sawing you describe here.

    Thanks again.

  • Etnier

    A wonderful essay which I’ve been pondering all day, since finding it through 3QD.

    Congratulations on the award: richly-deserved.

  • Chris

    this remains my favorite blog post ever; I’ve read it more than once since it first came out. somehow captures the beauty, distressing weight, and immenseness of the world.