48: The Impossible Rock

Last week, I introduced a new rock to the show: dolomite. Most folks haven’t heard of dolomite, but it has a more famous sibling: limestone. Both dolomite and limestone are usually dull gray, both form in oceans and lakes, and both are great places to find fossils. But dolomite has some major differences from limestone. Dolomite is tougher physically and chemically. It’s also much, much harder to make than limestone. Dolomite should be incredibly common today, but it’s very hard to find. Scientists also have trouble making dolomite in the lab: one experiment failed to grow dolomite after 32 years in extremely favorable conditions.

However, when we look at ancient rocks, dolomite is all over the place! What gives? This paradox is called The Dolomite Problem: dolomites are rare today, but very common in the past, and we don’t exactly know how to make it. Many ideas have been proposed over 200 years of research.

Today, we examine the dolomite problem through some of Earth’s oldest dolomites. These rocks are 3.75 billion years old, or March 7 on the Earth Calendar. They sit in the middle of the Isua region in southwest Greenland, a land of rocky tundra and rolling hills we’ve been exploring for the past few episodes. You might ask: why do we care? I didn’t even know dolomites existed until last episode! We care because dolomites are the best rocks to find fossils on this show. May I remind you that after 48 episodes and nearly 1 billion years of time, we still haven’t found a fossil yet, at least not one everyone agrees on. If we know how dolomites form, we might be able to find those elusive traces of life. And some folks claim that the dolomite itself is a sign of life.

Part 1: Locked Away

Let’s start by comparing the ingredients of dolomite and its sister limestone. Limestone has a simple list: calcium plus carbon plus oxygen. Limestone is a relatively common rock today, especially in warm, tropical waters. You can find thick deposits from the Great Barrier Reef to the Bahamas to the Persian Gulf.

Dolomite only has one extra ingredient: magnesium. It’s this one element that makes dolomite such a headache to make. To see why, we need to shrink down to the atomic scale. I want you to imagine a single atom of magnesium like a round orange fruit, hovering in front of you. Got it? Good. Now, let’s compare magnesium with its’ sister calcium. If magnesium is an orange, calcium is a grapefruit- slightly larger, but very similar.

Atomic models of magnesium (Mg) and calcium (Ca) interacting with water molecules (the “Mickey Mouse” heads). Note how difficult it is to pull water off of magnesium compared with calcium! From Jahnen-Dechent & Kettler 2012, CKJ

All by themselves, our magnesium orange and our calcium grapefruit aren’t that different. The key difference is how they interact with water. On our imaginary atomic scale, water would not be dripping wet- it would be a bunch of sticky balls like wads of duct tape flying around, attaching and detaching themselves to other atoms. It turns out, water really likes magnesium and sticks to it like flies on a rotting orange. Because magnesium is slightly smaller, it’s surrounded thick cloud of sticky water molecules. In contrast, calcium is slightly larger and has a more manageable swarm of water molecules.

If I want to make some limestone, all I have to do is peel off a bit of water from my big calcium atom. It’s a little bit of work, but not too bad. Trying to make dolomite is a different story. Looking over at magnesium, I can hardly see it through the dogpile of sticky water molecules. It’s a lot harder to pull the water away. If I can’t access magnesium through all that water, I simply can’t make dolomite. This is a cornerstone of the Dolomite Problem- there’s a decent amount of magnesium in the ocean, but it’s locked away behind water. Atoms, atoms everywhere, but not a drop to drink.

Epsom salt, or magnesium sulfate

But wait! It gets worse. Let’s say you’re able to peel all those water atoms off magnesium, which takes a lot of energy. Just as you’re ready to build dolomite, bam! In swoops another molecule to steal your magnesium away, like a monkey stealing your precious orange. This molecule is sulfate. Sulfate is a very important material and deserves its’ own episode, but I don’t want to get too off track and we’re already deep in chemistry, so here’s the gist.

Sulfate is a mix of sulfur and oxygen. It’s very common in nature, and it loves to stick onto atoms like our precious magnesium. Sulfates are very common in our everyday lives: in shampoo, plaster, batteries, and so on. In fact, when sulfate so rudely steals away our magnesium, it makes a product you can find at the store: Epsom salt. Epsom salt isn’t in the spice aisle, it’s found by medicine. Some folks put powdered Epsom salt in their bathwater. They claim it soothes sore muscles and helps your skin. Research doesn’t really support that, but I’m not wading into a medical discussion today- we’re already far into the weeds. Let’s center ourselves and return to today’s theme of dolomite.

There are other aspects to the Dolomite Problem, ones we’ll tackle later in the episode. The point for now is this: dolomite needs magnesium to form. Magnesium is a fairly common element, but it’s locked away by other molecules- either water or sulfates like Epsom salt. If we want to make dolomite, we need something that can smash through all these barriers, that can cut straight to the magnesium. If geology and chemistry can’t solve the dolomite problem, perhaps biology can.

Part 2: Made By Microbes 

I’m a geobiologist by trade, so I’m always excited when we talk about life. Before we see how modern life can make dolomite, let’s recap the story of ancient life on this show. We’re currently sitting 3.75 billion years ago. On our imaginary Earth Calendar, that’s March 7, still very early in the game, but we’re long past Earth’s earliest days. Nearly every geologist and every biologist agrees that life was around by this time. The problem is finding solid proof.

Let’s say you find a weird old rock. If you think your pet rock is Earth’s oldest fossil, you need to prove that only life could make it, nothing else. Time and time again, we’ve seen researchers bring their pet rocks to test: diamonds from Australia, iron from Canada, and graphite from Greenland. All these crystals could have been formed by life, but there are alternate explanations that don’t require life at all. As long as there’s an alternative, these rocks are only possible fossils. It’s tough news to hear, but we need to be rigorous when looking for Earth’s oldest fossil. We haven’t found it yet, but we’re still looking.

Which brings us back to dolomite. What can dolomite tell us about life?

A sulfate-reducing bacteria. Scale is half a micron.

Our story starts in the modern world. Remember, dolomite is extremely hard to make today. We just learned why last section: the key ingredient magnesium is locked away by two molecules: water and sulfate. Water is a double-edged sword: we’ve seen how it blocks access to magnesium, but you still need some water to build dolomite crystals, like rock candy in sugar water. In short, we can’t look for dolomite in dry deserts.

So researchers focused on that other pesky obstacle: sulfate. They were looking for natural ways to get rid of sulfate, and this is where life comes in. It turns out, there is a special group of bacteria that consumes sulfate for energy. The more they consume, the more free space is left for magnesium, leaving room to make dolomite crystals. This group of bacteria will be incredibly important in future seasons, so let’s learn more about them.

These bacteria are helpfully called sulfate-reducers. They’re some of the most common bacteria on Earth. You may not be able to see them, but you have certainly smelled them. Have you ever walked by a swamp and smelled rotting eggs bubbling up from the mud? Those are sulfate reducers. Have you dug down into a beach and found a dark black smelly layer? Those are sulfate reducers. What are they doing, and why does it smell?

Sulfate-reducing bacteria colony in an Italian cave.

Life needs energy to live. For example, you must eat food and breathe oxygen every single day. The food and air gives you the energy to walk, talk, and listen to geology podcasts. But to give you that energy, the food and the air must change inside you. The carbon-rich food turns into carbon dioxide, while the oxygen-rich air turns into water vapor. Whenever you breathe out, you’re releasing these waste products back into the world.

Sulfate reducing bacteria are doing something very similar, but smellier. Like you, they need carbon-rich food. A sulfate-reducer would be happy eating a salad or a burger. But instead of breathing in oxygen like you do, these bacteria breathe in sulfate. Inside their tiny bodies, that sulfate breaks apart, releasing energy and changing into a different chemical: smelly, stinky hydrogen sulfide. Whenever you smell a rotten egg odor, that’s the smell of hydrogen sulfide. There’s a reason that this gas smells so bad: it’s very toxic to humans. Your body is literally telling you to get the heck away. Smelling a few rotten eggs won’t kill you, but you shouldn’t hang around them for too long.

The scent of rotten eggs might be noxious to you or me, but for dolomite researchers, it’s the sweet smell of success. This stench means that sulfate reducing bacteria are doing their jobs, clearing their environment of those pesky sulfate molecules. The more sulfate they devour, the more magnesium is liberated to make dolomite crystals, solving the dolomite problem. Adding insult to injury, those bacteria take their captured sulfate and fart it out as stinky hydrogen sulfide.

That’s a nice metaphor, but here’s a gentle reminder: the bacteria are not trying to make dolomite. In other words, it’s not their goal to grow crystals. All the bacteria want to do is gather sulfate for energy, just like you want to breathe in the air. The dolomite growth is simply a byproduct of their dinners. In fact, if bacteria had a brain and a choice, they probably wouldn’t want to make dolomite. If dolomite grows long enough, it can imprison bacterial cells, entombing them inside crystal coffins. It’s like getting buried alive in a room full of pizza boxes. Not pleasant stuff- maybe you should consider cleaning your room.

Whether they want to or not, sulfate-reducing bacteria are one good way to form modern dolomite. You might ask: if these bacteria are everywhere, then why isn’t dolomite everywhere? Well, I might have been a bit generous with “everywhere”. If you dig down into soil, or a beach, or under the seafloor, you’ll probably find a stinky layer of sulfate reducers. But they’re less common up on the surface, for one big reason: oxygen. Oxygen is toxic for most bacteria species, including our new sulfur-loving friends. The bacteria are also limited by the amount of carbon-rich food around- those burgers and salads I mentioned earlier. Just like you can’t survive on oxygen alone, sulfate-reducers can’t live on sulfate alone.

These bacteria are common, but fairly picky- they can only thrive in certain environments. This chain of rules and restrictions means that dolomite only forms in special nooks and crannies of the modern ocean.

Dolomite-rich layers, 3.75 billion years old, Isua, Greenland. From Nutman et al., 2010, Precambrian Research

It also appears that dolomite was forming in the nooks and crannies of the ancient ocean, 3.75 billion years ago. If you’ve forgotten all about our old Greenland rocks, I wouldn’t blame you- this has been a chemistry and biology heavy episode. Back on the Greenland tundra, dolomites are fairly rare, not nearly as abundant as all those volcanic rocks we’ve seen. For decades, researchers wondered just how those rare Greenland dolomites got there. To make a long debate short, the latest and most detailed study was in 2010. This research was done by an old friend of the show, Dr. Allen Nutman- a man who has appeared nearly eight episodes in a row!

In 2010, Nutman’s team proposed that the Greenland dolomites were likely made by microbes. They clearly formed on the seafloor, and as we’ve seen, the best way to make seafloor dolomite was with the help of bacteria. Note! They did not claim to find any fossils: no cells, no pond scum, not even any leftover carbon. The argument was still circumstantial, there were no bodies remaining.

At this point, you’re probably bracing yourselves. Whenever someone makes a big claim about early life, other scientists usually dogpile on with criticism, trying to find any cracks in the logic. Weirdly, folks have remained quiet. No one has made any huge attacks on the 2010 paper, but also no one has championed this paper as the oldest evidence for life. Even Team Nutman doesn’t promote it that much. They’ll reference the 2010 study in recent papers, but only briefly, never as a core argument.  

We’ve seen vigorous debates over life in earlier episodes, why not here?

To me, it feels like the scientific community is waiting for the other shoe to drop. Here’s what I mean. The dolomite problem is big and complex. Microbes were the best solution in 2010, and it’s still a real possibility, but there are still folks working on this problem. If any dolomite alternative is found that doesn’t need life, it’s back to the drawing board.

Which brings us to 2023, just two years ago. An international team of researchers made headlines with one simple statement. They had solved the dolomite problem without life.

Part 3: Rinse and Repeat

For this final section, we’re on the cutting edge of geology, brave new frontiers.

In coming years, this work might be supported and hailed as the best solution for the Dolomite Problem. It might be shot down as another dead end. My best guess is somewhere in between: it’s a big step forward, and the Dolomite Problem doesn’t have one easy answer. Things in science rarely do- in fact the Dolomite Problem is more accurately described as several smaller problems. There are many reasons why dolomite is a brat.

In Part 1, I described one major aspect of the problem: it’s hard to free magnesium from the shackles of other molecules. No magnesium, no dolomite. But let’s say you’re able to free some magnesium and finally start building a dolomite crystal. The next problem comes during the construction phase, as you’re building the crystal atom by atom.

This new problem is easy to visualize if we use our orange and grapefruit analogy from Part 1. In this case, smaller oranges are magnesium atoms, and larger grapefruits are calcium atoms. Let’s start simple, with just calcium. Let’s say you’re working at a grocery store, and you’re piling up grapefruit on a cart. That’s fairly easy: after laying down one bottom layer of grapefruit, you can settle the next layer on top. Each top grapefruit is nestled in the nooks between its lower neighbors, nice and stable. If you keep repeating this over and over, you can build a great pyramid of grapefruit, stable, geometric, and locked into place.

Models comparing calcite and dolomite crystals (modified from Land & Lyndon, 1982, AAPG Notes 24)

Crystals build themselves in similar patterns, on a microscopic level. If you look inside limestone, you’ll find neat, ordered rows of grapefruit, I mean calcium atoms. There are other atoms involved like carbon and oxygen, but the point is, everything is stable and well-behaved. Let’s return to our fruit stand and see how dolomite is different.

In this dolomite scenario, you have grapefruit and oranges to place on your cart: calcium and magnesium. There’s one more rule you get from your grocery store boss. You must stack one layer of grapefruit, then one layer of oranges, grapefruit layer, orange layer, etc. Maybe your boss is an artsy sort of person, but this makes life harder for you. If just one grapefruit or one orange is out of place, the pyramid fails, you can’t make the next layer without going back and removing the wonky fruit. It takes longer to build, it’s easy to make a mistake, and eventually you might just walk away and quit.

As silly as this sounds, it’s quite similar to how dolomite crystals grow. Again, there are other atoms involved, but here are the basics. To make a dolomite crystal, you need to make layers of calcium then magnesium, calcium then magnesium. If a few atoms are in the wrong layer, the crystal stops, it can’t grow any farther, or it takes a long time.

Photo by Yam Chhetri.

Let’s call this new problem the Stacking Problem. It means that even if you have all the ingredients free and ready, dolomite is still very hard to grow. It’s this Stacking Problem that was addressed in 2023.

A joint team of researchers from the University of Michigan and the University of Sapporo in Japan published an article in the journal Science, one of the highest places you can go. Led by Dr. Jonsoo Kim, this team took a theoretical idea in the dolomite community and put it to the test with both math and real crystal growth. I’m going to start by explaining what they did using our fruit analogy one last time, then the actual science.

Let’s return to our fruit cart. Just like last time, we have a bunch of grapefruit and a bunch of oranges. Just like last time, our picky boss wants alternating layers of each fruit. But now, there are two workers on the job: yourself and a neat-freak coworker. For the first few layers, you give a decent try: it’s mostly right, but screw the man, you have better things to do. A few pieces of fruit are out of place You leave your half-finished work behind for the day, and your co-worker comes in for their shift. They diligently reorganize your work, removing any random pieces left over. Neat and tidy. The next day you come back and work on some new layers, not perfect but the pyramid builds a little more. When you leave, your coworker tidies it up yet again. It’s still not fast or efficient, but at least a pyramid is built.

How does this work in the real world? Dolomite grows best in alternating water conditions: crystallizing then dissolving, crystallizing then dissolving. The crystallizing stages build the crystal randomly, the dissolving stage remove any impurities, allowing future growth. Dr. Kim’s team tested this by literally shooting a laser at a dolomite crystal, on again off again for over two hours. The crystal only grew 100 nanometers, smaller than a bacteria, but that’s still better than other experiments!

Clearly, there aren’t superpowered lasers firing off in nature, but there are conditions where water chemistry changes frequently such as tidal flats or flowing groundwater. Even microbes can change water chemistry on a daily basis, which brings us back to our original question. Bacteria are still a strong candidate for making dolomite, especially in Earth’s early past. However, the cutting-edge research from Dr. Kim reminds us that there can are other recipes waiting to be discovered, ones that might not need life at all.

Model showing generations of dolomite growth over millions of years, replacing calcium (blue) with magnesium (orange). From Kim et al., 2023, Science.

Who knows? Perhaps there will be a great merging of these ideas in the future, that microbes are still the best way to make dolomite, that they help fluctuate water conditions on top of all their other chemical tricks. Only more experiments will tell, and when they do, you’ll be the first to know.

Are the Greenland dolomites evidence for life 3.75 billion years ago? For now, the jury is still out. The answer might be microbes, or it might be changing water conditions. It’s worth keeping an eye on these rocks but for now, our quest for the first fossil continues. Fortunately, we won’t have to wait long before our next candidates. Greenland still has many more possible fossils to offer, but that’s a story for next episode.

Summary:

Dolomite is one of the toughest rocks to make, and yet it is incredibly common in Earth’s ancient past. It’s one of the best places to find fossils, but how it forms is still a mystery.

This Dolomite Problem has plagued geologists for two hundred years, but recent studies are beginning to chip it away. The problem is really several smaller problems. Some problems can be solved by microbes, smashing through barriers in their constant search for food and energy. Dolomite can also form in shifting water chemistry, as crystals grow and dissolve repeatedly over time. Are Earth’s oldest dolomites in Greenland evidence for life? Maybe, maybe not. As long as there’s a non-living option, we can’t call these rocks fossils just yet.

But if you’re a fan of life, don’t despair! The next few episodes will focus on other fossil debates in Greenland’s Isua region, we’ve only just begun. Next episode, we move forward in time to 3.72 billion years ago. March 10 on the Earth Calendar. We’ll examine flecks of carbon that have the chemical signatures of life, but are they the real deal? Stay tuned to find out!