40: The Time War

We’re in the final arc of Season 2, focusing on SW Greenland. This is a land of mountains, fjords, and very old rocks, the most extensive and well-preserved of their age, the best we’ve seen so far. Let’s review what we’ve learned. These rocks have a name: the Itsaq Complex. In age, they’re 3.9 to 3.6 billion years old, Feb 23 to Mar 19 on the Earth Calendar. For the past two episodes, we’ve focused on just one type of rock, 70% of the Itsaq Complex. That rock is tonalite, a gray cousin of granite born in boiling magma chambers deep underground. Tonalites tell us there were likely islands rising from the ancient sea, the very beginnings of the massive modern continents. Just how those islands formed is a huge debate, one we’ll return to in a few episodes, armed with more evidence.

To gather that evidence, we need to stretch our legs and move on to new rocks and new questions. We’ve been cramped and squeezed deep in the crust and mantle, it’s time to return to the surface, the realm of oceans, volcanos, and life. This surface world is recorded in the remaining 30% of Itsaq rocks, everything that’s not tonalite. These remaining rocks are scattered around the region, smaller pockets of dark stone ranging from closet-size to stadium-size. These are the windows onto the ancient surface.

Sadly, we can’t look at every single one of these windows, we don’t have the time. Instead, we’ll focus on just one of these dark stone patches, a small corner of a small island off the Greenland shore. This island is one of the most contentious spots in early Earth history, and that’s saying something. For the past 30 years, two teams of researchers have been debating every aspect of this island: its’ age, its’ rock types, and if it holds traces of early life. Today, I’ll introduce you to these two teams- these folks will be our companions until the end of the season. If you like scientist stories, you’re in luck.

A caveat before beginning. It’s tempting to tell this story like a boxing match or competition, with winners, losers, and color commentary. We’ve done this for old folks like Anaxagoras or George Darwin, but they’re long dead- it’s easy to give them a playful ribbing. The researchers we’ll meet today (most of them) are still living and doing active research. When your subjects can listen to the show, politeness matters. I’ll still keep the debate entertaining, I’ll use my usual kitchen analogues to describe geology, but we’re going to treat the researchers with their due respect. Even if they’re proved wrong, they’ve done decades of intense fieldwork and chemistry which have advanced science.

With that in mind, let’s visit this contentious island and debate just how old it is.

Part 1: Akilia

In Episode 38, I described Greenland’s Itsaq Complex like a giant stone banner: a narrow strip of rock hanging from the white glaciers of the north to the blue Atlantic in the south. Let’s revisit that banner and meet our new home for the next few episodes.

The vast majority of the banner is dull gray tonalite, the focus of the last two episodes. Those tonalite stories were big and broad, occurring across the Earth Calendar from February to March, 3.9-3.6 billion years old. The tonalites were a constant background noise, a low, deep churning of magma over millions of years.

But now we’re going to get specific in place and hopefully in time. Scattered throughout the gray tonalite background are small patches of dark, multi-colored stone: black, green, brown, and red. We’ll learn what these rocks are next episode, today is a when story. All you need to know is these dark patches are snapshots of the ancient seafloor. They’ll tell us stories about early oceans, volcanos, and possibly life. These dark rocks were first described by our old friend Vic McGregor, the New Zealand geologist who opened this region to the world’s researchers. Vic started his fieldwork near the seaside capital of Nuuk, in the fjords and islands off the Atlantic Ocean. Check out Episodes 36 and 37 for a recap.

The island of Akilia. The oldest, contentious rocks are in the SW corner.

One island in particular caught Vic’s eye. On our imaginary stone banner, this island would be barely hanging from the ragged bottom edge, equally distant from either corner. This island is called Akilia, that’s A-K-I-L-I-A. The island of Akilia is small, a few dozen city blocks wide, about half an hour to walk end to end. The area that interested Vic, the area of intense future debate, is much smaller: the size of a large sports stadium like the Colosseum or Wrigley Field. Five minutes to walk across.

Vic described Akilia in 1977. The island and its’ treasures would remain undated for another two decades, until 1996. And now it’s time to introduce the first research team and their rock dates, the opening side of the Akilia debates. This team had many folks to start with and has grown to include many more. I want to give folks proper credit, but I also don’t want this to become a jumbled sea of names. Therefore, I’ll focus on certain individuals as the episodes progress. I like to call this the “Avengers” approach, introducing researchers as we assemble them together.

Allen Nutman, U of Wollongong, New South Wales

We start with Allen Nutman, the lead author of the 1996 paper with Akilia’s age. Nutman currently works at the University of Wollongong in New South Wales, and has been studying Greenland rocks for nearly fifty years. He has published over 100 papers just with the word “Greenland” in their title, and many more that don’t. Like Vic McGregor, Nutman worked extensively for the Geological Survey of Greenland, mapping the islands and fjords since 1974. So by 1996, he was already well-experienced with the area.

The 1996 paper also included Vic McGregor, one of his last before passing in 2000. Other co-authors included British and Australian researchers such as Clark Friend, Victoria Bennett, and Peter Kinny. We’ll meet them in future episodes, but for now, I’ll lump them all together as the Nutman crew. They will usually stand side by side in future debates.

So what did they find? How old was the ancient Akilia seafloor?

The Nutman crew were looking for zircon crystals, the best timepieces of the ancient Earth. If you want to measure a rock’s age, you need to find a zircon, or something like it. If you’re new to the show, please check out Episode 3, where I describe how scientists date rocks. It’s been ten episodes since we talked about zircons, so here’s a three sentence review. Zircon crystals form as magma chambers cool down. As crystals grow, they lock away uranium atoms inside their bodies. Over time, that radioactive uranium transforms into lead at a known rate, making a natural clock we can measure well beyond a human lifetime. 

Bam! That’s Rock Dating 101. But as we’ve seen in Seasons 1 and 2, not all rocks have zircons in them. The best rocks to look are granites and their cousins like tonalite, volcanic ashes, or certain sandstones. Unfortunately for the Nutman crew, the dark Akilia rocks, the ancient seafloors, didn’t have any zircons inside.

But! There was a clever solution. On Akilia Island, the Nutman crew found a vein of pale tonalite that sliced through the dark ancient seafloor. Like a new axe stuck in an old tree, the tonalite had to be younger than the seafloor it cut into. If the axe was made in 1900, then the tree is at least as old as 1899, and possibly much older. It’s not an exact age, but it’s better than nothing.

The Nutman crew analyzed the Akilia tonalite and found a wide range of dates. The rocks represented different generations of melting and remelting over time, like we saw last episode. The oldest date measured was 3.87 billion years old. On the Earth Calendar, that’s February 25, very early in Greenland’s story. Don’t forget the axe in the tree: the tonalite was February 25, which meant the dark ancient seafloor was a bit older. How much older? We don’t know, so we’ll play it safe and keep things at 3.87 billion years old. Even so, Akilia still represents Earth’s oldest surviving seafloor.

… Or does it?

Part 2: The Crystal TV

Martin Whitehouse, Nat. Museum of Sweden

The challenge to the Nutman crew came just two years later, in 1998 and 1999. These papers came from Martin Whitehouse in Sweden, and Balz Kamber and Stephen Moorbath in Oxford. Like the Nutman crew, this trio and others will usually stick together in their interpretations of Akilia’s age, rocks, and evidence for life. For today, we’ll call them the Whitehouse crew, since Martin Whitehouse usually takes point on the age questions. Side note: in Episode 37 we briefly met another member of Team Whitehouse, Stephen Moorbath, but his fascinating biography will come in a later episode.

Whitehouse got his Oxford degree in 1987, working in the USA and Norway before landing at the Swedish Museum of Natural History. His previous work investigated old Scottish gneisses- not quite Greenland age, but with their own complicated metamorphic history.  

Whitehouse brought a new tool to analyze zircons, one we haven’t seen before on the show, but I’ve actually used it in my own research. The machine is called a CL scope. CL is short for cathodoluminescence. I promise to just call it CL, but let’s pick apart that word first, as we usually do. For my older listeners, that word “cathode” might ring a bell- a cathode ray tube was the heart of old televisions. For my younger listeners, those are the boxy TVs your parents or grandparents still own. I grew up when flat screens were on the rise, but I still hold a soft spot for those old boxes.

The inside of a CRT TV, with the inside of the screen on the right

If you’ve ever wondered how an old TV works, here’s a very simplified explanation. Inside the TV, a tube would shoot beams of electrons at the screen. The inside of the screen was covered in different elements that would react to the electron beam. If the beam hit a tiny pad of copper, a green dot would appear. If it hit a pad of zinc, a blue dot would appear, and so on. Combining these colors would make images, and repeating this process many times every second would make a moving picture. For the days before home computers, this was a wildly advanced technology.

So how does this all relate to the CL scope? Was Martin Whitehouse watching a tiny TV on ancient Greenland rocks? In a way, yes. Like the TV, Whitehouse was shooting electrons at a flat surface- in this case, a polished zircon crystal. Certain elements would shine brighter, others would be darker. Difference number 1: the crystal wasn’t moving, so there was no moving image. Difference number 2: the images were black-and-white, like a really old TV.

A calm, greyscale image might sound incredibly boring, but the CL scope revealed much greater details inside tiny, sand-sized zircon grains. When Whitehouse cut and polished his zircons, he saw concentric rings like the inside of a tree. Just like tree rings, the smaller inner layers were older, the bigger outer layers were younger. Just like tree rings, you could look at a layer’s texture and tell if the crystal grew in liquid magma or in solid rock. This was all known before, but with a CL scope, you could now date individual rings with incredible precision, learning how a zircon grew over time.

Whitehouse did not invent the CL scope, but he was the first to use it on ancient Greenland zircons. By combining CL images, age dates, and a bit of extra chemistry, the Whitehouse team described a few interesting features.

Cross-sections of zircon crystals from Akilia Island, as viewed by CL scope. Note the older interior cores surrounded by newer rings. (from Whitehouse et al., 1999)

Many zircons showed three clear phases of growth: 3.8 billion years ago, 3.6 billion years ago, and 2.7 billion years ago. Don’t stress about the exact times, just imagine a forest where the trees started growing in February, then had massive growth spurts in March, then May on the Earth Calendar. Clearly, something happened in the forest in March and May.

This pattern wasn’t incredibly new: the Nutman team had already found similar dates in the Akilia Island rocks. In fact, the Whitehouse team purposefully picked the same locations for accuracy. The Nutman team had proposed a simple solution for these various dates: the crystals were born grew in liquid magma 3.87 billion years ago in February. After the magma cooled into tonalite, two pulses of metamorphism squeezed the solid rocks and made the middle and outer rings on the old zircons in March and May.

As the Whitehouse team looked further, they began to question such a simple story.

1: The crystals had far more middle ages than expected, March instead of old February on the calendar. Back to the forest analogy, if the trees were all planted in February, you would expect to see more February dates. Weird.

2: When the Whitehouse team examined these abundant March layers in the zircons, it looked like they had formed in molten, liquid magma. This contrasted with the Nutman hypothesis. According to Nutman, the rocks should have been solid by this time.

Team Whitehouse proposed a radical new idea: the oldest zircon crystals on Akilia Island had been made somewhere else long before. In other words, the crystals did not represent the rock’s true age, but the remnants of even older stones that had been melted and recycled into younger ones. If you’re confused, here’s an analogy.

Imagine you have a piece of granite sitting in your hand. This granite has a few tough zircon crystals inside, and those crystals tell you the rock is one million years old. Don’t memorize that date, just remember you have an old piece of granite. Step 2: take that old granite to a modern volcano and dump it into boiling hot lava. The wimpier crystals in the granite will melt away, but the tiny, tough zircon crystals will stick around- they can take the heat.

Not a piece of granite, but you get the point. Source: r/ThrowItIntoLava

Here's the point. As the brand new lava cools down into a brand new rock, those old geezer zircons are still hanging around- they’re literally grandfathered in. If you gave this rock to a geologist, the zircons would say “I’m one million years old”, but the rock itself is a baby. This would be a funny, but elaborate prank to play on a geologist friend.

If that’s still too much, here’s one final, one-sentence analogy. Just because your house has a TV from the 1950s doesn’t mean the entire place was built in the 1950s.

Let’s bring it back to Greenland. According to Nutman, the oldest zircons grew in place, 3.87 billion years ago, Feb 25 on the Calendar. According to Whitehouse, those old zircons were made somewhere else first, and were transplanted into younger magmas. In other words, a few crystals might be 3.87 billion, but the rock itself is only 3.65 billion, March 15 on the Calendar, three weeks later. If Team Nutman is right, this ancient seafloor and its possible evidence for life happened at the beginning of the Greenland story. If Team Whitehouse is right, it represents the end of that story.

So who is right: Team Nutman, or Team Whitehouse?

Part 3: The Inheritance Dispute

Stephen Mojzsis, University of Colorado-Boulder

In 2002 and 2006 a team from the University of California, Los Angeles brought new evidence to support Team Nutman. These researchers were Stephen Mojzsis (moy-zhis), Mark Harrison, and Craig Manning. We briefly saw them way back in Episode 15 on the Jack Hills zircons, and we’ll learn far more about them in future episodes. For the simplicity of names, I’ll call this California crew the Nutman supporters.

Here's the debate as it stands in the early 2000s: according to Nutman, Akilia Island is 3.87 billion years old, February 25 on the Earth Calendar, the oldest preserved seafloor in Earth history. Later dates in March and May record metamorphic events as solid rocks were squeezed and pressure-cooked.

According to Whitehouse, the same rocks are only 3.65 billion, March 15 on the Calendar- still very old, but not quite as impressive. Whitehouse doesn’t deny that there are really old zircons, he just thinks that they’re relicts from much older rocks that formed elsewhere .

Here comes the rebuttal, three counter-arguments the California crew used to support Nutman’s original idea.

1: Dissolution. If you put a crystal of salt into water, it will dissolve almost immediately. If you put a whole can of salt into water, many crystals will remain sitting around, and might even grow over time. The same concept works in magma: certain magmas will dissolve zircons, other magmas will help them grow.

 The Nutman supporters argued that for Whitehouse’s idea to work, the old, old zircons would have to survive for in molten magmas that would be actively trying to dissolve them, like a single salt crystal in fresh water. Zircons are tough, and can survive for long periods of time, but even they have their breaking, or dissolving points. According to Nutman, old magma cooled much earlier, preserving zircons before destruction.

2: Melting. When Whitehouse looked closely at Akilia zircons, he saw evidence for molten rock in March on the Calendar. This would seem to disprove my entire last point, but the California crew had an answer. Everyone agrees that the Akilia rocks are incredibly metamorphosed, squeezed and pressure-cooked to the edge of oblivion. In the most powerful, hottest forms of metamorphism, a small amount of rock does begin to melt. It doesn’t form a huge magma chamber, but small veins of molten stone.

The Nutman supporters argued that the Akilia rocks were made in February, but experienced intense metamorphism in March, with small but pervasive melting. It was this pressure cooking event that boosted the March signal over the original February dates.

3: Chemistry: We’re near the end of the show, so I won’t dive too deep into hardcore chemistry. Here’s the summary: a happy zircon that forms in a magma pool will have a different chemistry than a tortured zircon that forms under metamorphosis. When the Nutman supporters looked at different zircon rings in Akilia, only the oldest layers, the February dates, had chemistry that resembled magma chambers. The newer layers formed as an old rock was squeezed, not as a new rock was born.

Phew! Now that we’ve looked at that new batch of data, let’s return to the debate. In the year 2025, where do Team Nutman and Team Whitehouse stand on Akilia?

Neither team has conceded, at least not in writing. If you read Team Nutman papers, the Akilia rocks are older. If you read Team Whitehouse papers, the rocks are younger. However, as the years go on, Team Whitehouse has published fewer papers arguing over Akilia dates. They’ve moved on to different subjects in and beyond Greenland. For now, Team Nutman appears to hold the stage, but there’s always the chance new data could change everything.

Why do we care? What’s the difference between a few decimal places, between February and March? If Team Nutman is right, Akilia is by far the oldest seafloor on Earth, happening at the very beginning of the Greenland story, closer in time to the massive meteor showers from Earth’s early days. And that’s not all. In 1996, the same year that Team Nutman argued for the oldest seafloor on Earth, Team Nutman and Mojzsis’ California Crew joined forces to announce an even bigger discovery: possibly the oldest evidence for life on Earth. You better believe that Team Whitehouse had something to say about that.

Summary:

Nutman, Whitehouse, Mojzsis. These are three names out of many who have worked on Greenland’s oldest rocks. Whether their ideas turn out to be right or wrong, each has brought something to the scientific table: new fieldwork, new chemistry, or a new perspective. These teams will butt heads across hills and fjords all across Greenland. For the next few episodes, a tiny corner of a tiny island named Akilia is their stage. How old is this island? For now, the consensus seems to favor an older date, 3.87 billion years old, February 25 on the Calendar, the oldest preserved seafloor on Earth.

Next episode, we’ll finally learn what that seafloor was made of, and if it had any traces of early life.