32: Pumping Iron

For the past two episodes, we’ve visited a remote patch of seaside tundra in northern Quebec, in the lands of the Inuit. This area is called Nuvvuagittuq, and it is tied for the second oldest rocks on Earth, around 3.8 billion years ago, early March on the Earth Calendar. Nuvvuagittuq is our first tangible glimpse onto Earth’s surface- rocks that preserve physical traces of ancient seafloors. 

Last episode, we set the scene with volcanic rocks- our old friend basalt. By looking carefully at Nuvvuagittuq basalts, we learned two things: 1) This frozen tundra was once a chain of underseavolcanos, erupting dark lumpy pillow lavas. 2) These volcanos were probably next to a deepocean trench. At the very least, seawater was mixing with Earth’s mantle deep below to make rare volcanic rocks called boninites. Today, boninites are found on the islands of Guam, Iwo Jima, and others next to the Mariana Trench, the deepest part of the sea. 

So what else was going on 3.8 billion years ago at the bottom of the sea? Today, we’ll meet a completely different type of rock. This rock is a cornerstone of modern industry, one of the most important rocks we’ll meet on the show. And yet, it is a rock that is impossible to make on the modern Earth.

Part 1: Meet BIF

In Episode 2, we learned the three recipes for rocks: igneous, metamorphic, and sedimentary. Since then, we’ve focused on the first two recipes. The samples we’ve met have either been igneous rocks, made from molten lava and magma, or metamorphic rocks, the pressure-cooked remains of previous stones. Finally, it’s time to meet the last family: sedimentary rocks. If you can’t tell, I’m really excited for this one, since I’m a sedimentary geologist by trade. 

As a brief recap, sedimentary rocks form when loose particles settle and solidify in water. Usually, these particles are the broken pieces of other rocks: such as boulders, pebbles, sand, and mud. Sand turns into sandstone, mud turns into mudstone, etc. But there are other ways to make sediments, ones that don’t need to break down any rocks. Some rocks crystallize straight out of water, just like rock candy in a glass of sugar water. These special water-grown stones are called chemical sedimentary rocks, since they form when the chemistry of water changes. 

The best examples are limestone and rock salt. If you leave a glass of saltwater in the sun, salt crystals will eventually line the bottom. If you don’t clean your bathtub in a long time, lime will eventually form a crust. The same thing happens in nature- tweak the temperature, tweak the chemistry, and you’ll get new minerals popping out of water and settling to the bottom. 

Today, you can find limestones in coral reefs, and rock salt in dry seas, and we will see spectacular examples of both in later seasons. But there are some ancient rocks that cannot be made today. The world has changed too much. All the rocks we’ve met so far- the gneisses andtonalites, basalts and boninites, are still being made as we speak. But our next subject is literally an extinct rock, a window on a different age with very different chemistry. 

This rock is called banded iron formation, which is usually shortened to BIF. I’ll use both to keep things fresh.

I like the name banded iron formation- it tells you what the rock looks like and what’s in it. BIFs have beautiful alternating bands of dark gray and blood red, like a stone tiger pelt. BIFs are not the first stripey rock we’ve seen- in Episode 25, we met the Acasta Gneiss with zebra stripes of black and white. But even though the patterns look similar, they formed in completely different ways, and it’s worth a brief digression.

The Acasta Gneiss, and all gneisses on the planet, are metamorphic rocks. Their stripes were earned deep underground, literally squeezed like an accordion by intense pressure. In contrast, sedimentary rocks like BIFs earn their stripes in water. As grains of sand, mud, or iron fall to the seafloor, they form thin, flat layers. As more grains fall, new layers form, building upwards like a giant gritty layer cake. You can see these layers yourself if you dig down into a beach or look down in a construction pit. 

Sedimentary layers can be read like rings on a tree. Each layer is a snapshot of the ancient world, and as you work your way layer by layer, these snapshots turn into a movie, a story of change over time. This is exactly what scientists have done with ancient BIFs in Nuvvuagittuq, Quebec, 3.8 billion years old. But before we can read these stories, we need to know what the layers are made of in the first place. Along the way, we’ll learn just how important BIFs are to the modern world.

Part 2: From Iron to Steel 

Let’s start by learning what the red and gray layers inside a BIF are made of. While the rusty red layers are very striking, we won’t spend much time on them today for two reasons. First, these layers likely formed later in history. They don’t tell us as much about the ancient oceans. Second, despite their rusty appearance, the red layers don’t have that much iron. For reference, imagine a clear glass of water. Now add just a drop or two of food coloring. It doesn’t take a lot to turn the water red, and the same is true with certain minerals. We will return to these red layers, but not today. 

In contrast, the steely gray layers are rich in iron minerals, crystals with various recipes of iron and oxygen. The two most important minerals are magnetite and hematite- you don’t need to memorize them right now, but they will be frequent guests of the show. In brief, magnetite is magnetic, hematite isn’t, but together inside banded iron formations, these two characters are the backbone of the modern steel industry.

BIFs provide two-thirds of the world’s iron ore, and nearly all that iron is turned into steel. Steel is a crucial part of today’s world- it’s nearly impossible to avoid, from steel frames inside buildings and cars, to everyday items like cutlery and steel wool. As you go about your day, try to find all the objects that have steel around you. I would bet good money they all started as an ancient banded iron formation. 

Unlike gold or silver, iron is rarely found as a pure metal, ready to use immediately. For thousands of years, the best way to get pure iron was from a meteorite, as we learned way back in Episode 5. In Greenland, a single giant meteorite in supplied tools for Inuit across the Arctic.In ancient Egypt, King Tut’s tomb contained nineteen treasures made from iron meteorites, including a gold-hilted dagger which I think is impossibly cool. 

Pure metal iron is so rare because iron loves oxygen- once they meet, they’re hard to separate. Pure iron can make tools, but iron plus oxygen becomes crystals like our new friends magnetite and hematite, or just plain old rust. You can’t really make a dagger out of rust. 

And so, from the Iron Age to the present, one of humanity’s major quests is turning rust backinto usable metal. All you need to do is strip that pesky oxygen off, and you’re in business. This is usually done inside furnaces filled with carbon monoxide, which plucks the oxygen off rust. The end results are carbon dioxide, an old frenemy of the show, and metallic iron, which can now be turned into steel.

Which brings us back to the beginning- where does the original iron ore come from? For thousands of years, humans had to hunt for small rusty pockets of earth. Then, in 1844, the first banded iron formations were found in northern Michigan, and then around the world. The largest BIFs covers thousands of square miles, and thousands of feet thick. It was a bonanza. Today, BIFs supply more than a hundred million tons of iron each year. It is hard to overstate the importance of BIFs in your modern life.

And yet, BIFs are no longer forming today- all the ones we’re mining are billions of years old. What was different back then, and how were these BIFs made in the first place, especially the oldest ones in Nuvvuagittuq, 3.8 billion years ago?

Part 3: Life’s Leftovers

Despite their supreme economic importance, the origins of BIF are still debated.

But it gets better folks, the debate over BIFs is part of a much larger debate over early oxygen. It’s a regular nesting doll of debates over here. 

The oxygen debate is way too large to tackle in one episode, but here’s what we need to know today. Earth’s modern atmosphere has 20% oxygen. The Earth’s first atmosphere had none. Clearly, something happened in between. Scientists agree on what happened- life learned how to make oxygen- plants are still doing it today. What scientists don’t agree on is the timing. 

Some researchers think life was farting out oxygen very early in Season 2, where we are right now. Others argue for a very late arrival, Season 6 on the show. Most folks are somewhere in between. Oxygen will be a multi-season arc on this show.

Where do BIFs fit in? Most BIFs were formed in this wide window of time, starting in Season 2and disappearing after Season 6, mostly. We just learned that oxygen loves iron, they’re locked together inside every BIF. So, it’s no surprise that BIFs are a possible clue in oxygen’s early story. 

Today, we’ll briefly learn two ways to make a BIF. All three contestants start in the same place, but go in very different directions. It’s time to play:

Is This Your BIF?

Contestant 1 is the Classic Recipe, Iron + Oxygen. 

We start at the bottom of the ancient ocean. Last episode, we learned that Nuvvuagittuq in Quebec was a series of undersea volcanos. These volcanos were not only pumping out lava, theywere also boiling the water and rocks around them, creating a rich soup of minerals and elementsincluding iron. The dark, cloudy soup belched up into the deep sea through giant natural chimneys. These towers are called black smokers, and they’re still around today- to learn more about this primordial soup and how it kick-started life on Earth, check out Episode 22. 

But today, we’re interested in the iron. This marine iron wasn’t pure metal, and it wasn’t rust either. It was simply dissolved in the water, like sugar or salt. To turn this invisible iron into a BIF, an extra step is needed: oxygen. 

It’s hard to make lots of oxygen without life involved. Today, most oxygen is made by plants or algae, but neither were around 3.8 billion years ago. In fact, we don’t have well-accepted fossils of any life back then, though most folks think it was probably around. 

If we assume that life was present, there is one other oxygen-maker to consider: a special group of bacteria, cyanobacteria or cyanos for short. Cyanos are a large family of microbes, taking many shapes from long rods to raspberries to curling spirals. You can’t see individual cells without a microscope, but I guarantee you’ve seen their colonies on beaches and shores as frothy green pond scum. This pond scum is easy to confuse with more complex algae. For many years cyanos were mislabeled as “blue-green algae”- a name that’s stuck on many beach signs.

Apart from plants and algae, cyanobacteria are the only living things that make oxygen on planet Earth, and the first ones to do so. Many argue that plants literally stole this ability from cyanos, but that’s a story for another day. In fact, cyanobacteria will be our most frequent living guest on the show- they sit at the center of many ancient debates. 

Which brings us back to BIFs, to Nuvvuagittuq, 3.8 billion years ago. Do we see any fossil cyanos trapped inside the BIFs? No. In fact, many BIFs do not contain fossils, and when they do, we can’t easily ID them as specific bacteria. 

However, some would argue that the BIFs themselves are proof of ancient life. If you need lots of oxygen to make BIF, and you need cyanobacteria to make lots of oxygen, then you need cyanos to make BIF. 

Case closed, right? Well, if oxygen was floating around 3.8 billion years ago, we should see many other changes in chemistry. In a nutshell, we don’t, and we won’t for some time to come.

It turns out there are craftier ways to smash oxygen and iron together, which brings us to Contestant 2.

Contestant 2 starts in the same spot as Contestant 1, with black smokers belching iron into the sea. But if there’s no oxygen made by life, how can we make a BIF? 

Turns out, I’ve been a bit sneaky with my language, and for that I apologize. When you and I talk about oxygen every day, we’re usually talking about O2, the invisible gas that keeps us all alive. In science, we call this “free oxygen”, since it’s pure, uncut, free to mix around. Free oxygen is what plants, algae, and cyanobacteria make, and what we breathe. 

But there’s a lot of oxygen that’s bound to other elements. Oxygen plus hydrogen is water. Oxygen plus carbon is CO2. Oxygen plus silicon is quartz, and so on. In short, the ancient Earth didn’t have breathable air with free oxygen, but there were still plenty of other sources to steal from. 

Now dissolved iron can’t really steal oxygen from water or CO2 by itself- it needs help from somewhere else. It needs help from life.

That’s right, our second contestant still needs bacteria, it just doesn’t need cyanobacteria. It’s a process you can see today on a nature hike. 

If you walk through a swamp or a forest with a slow sluggish stream, you might notice something odd- patches of bright orange ooze on the riverbed. This is not a fungus, or some prankster’s trick- this is natural rust forming thanks to bacteria. 

Bedrock isn’t a biology podcast, so I’ll keep things simple- these bacteria use dissolved, invisible iron in the water as an energy source. Using this iron plus water, and CO2 as ingredients, the bacteria grow and produce rusty iron minerals as an orange by-product. To make this recipe, the bacteria need water with very little free oxygen, which would steal the iron all for itself. 

Because these microbes directly interact with iron, we call them iron-oxidizing bacteria. They won’t be as common as cyanos on the show, but they will be frequent guests. 

So, it this your BIF? Possibly! Remember, modern bacteria make small rusty patches in swamps all the time, almost like mini-BIFs. While swamp water isn’t the best analogue for the Archean ocean, they both have one thing in common- very little free oxygen. In some cases, swamps produce enough iron to mine commercially. It’s called “bog iron” and it was a decent source before their bigger, older BIF brothers were discovered. 

Let’s step back to Nuvvuagittuq, 3.8 billion years ago, the oldest BIFs on Earth. There are other hypotheses about BIF formation, but these are the Big Two: either cyanobacteria were farting out free oxygen which mingled with iron, or iron oxidizers were directly making rust themselves. For the moment, people are learning towards Option 2- the bog iron option, and I’m inclined to agree, though it’s not my expertise. 

In either case, this is really big news- even if we see a BIF with no fossils inside, there’s a decent chance life was kicking around 3.8 billion years ago. It’s not a direct fossil, but after our long, winding journey through life’s origins last season, it’s something we can hold on to. Life has made its’ mark on the planet.

Wouldn’t it be neat if we found some real fossilized cells in these BIFs, some feature we could point to and say “That is the oldest fossil on Earth”? 

Wouldn’t it indeed? In 2017, a team of researchers found something very interesting inside the Nuvvuagittuq BIFs. If they’re correct, it’s a landmark in paleontology. If they’re not, what the heck did they find? Tune in next time to see where that story leads us.

Summary

Banded iron formations are incredibly important rocks for researchers and for humanity as a whole. BIFs are a cornerstone of the modern steel industry, providing billions of tons of iron around the world. BIFs also give a unique glimpse into the early Earth, a world with vanishingly little free oxygen. 

Nuvvuagittuq in northern Quebec has one of the oldest banded iron formations on Earth, around 3.8 billion years old, March 3rd on the Earth Calendar. They’re not the largest BIF, but they give us the most tangible evidence yet for life’s presence, whether that life was green like plants or orange like bog iron. 

Next time, we’ll spend an entire episode on one heated debate: does Nuvvuagittuq have the oldest fossils on Earth?