15: Sea Change
“Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes:
Nothing of him that doth fade
But doth suffer a sea-change…”
-William Shakespeare, The Tempest
Last week, we took a water break, learning how asteroids brought ice crystals to Earth, which gradually melted into today’s oceans. Some folks think that water was present in our very first building blocks close to the sun, while others say that icy asteroids were flung from the outer solar system by Jupiter’s gravity, adding a late veneer of water-rich material. The chemistry of modern Earth water most closely resembles this outer family of cold asteroids. But as more recent evidence is uncovered, that story might be changing as we speak.
Today, we follow those asteroids back down to the Hadean Earth, 4.4 billion years ago, January 14th on the Earth Calendar. By this time, geologists know that liquid water was present on Earth’s surface, covering most or even all of the planet. How? Through the only pieces of Earth left from that time, our old crystalline friends the Jack Hills zircons from Western Australia.
This episode, the Jack Hills zircons will tell us how Earth changed from a hot world of magma oceans to a cooler planet with watery seas. Let’s start at the beginning, at the bottom of the brand-new Hadean ocean. It’s time to return to our imaginary submarine.
Part 1: Under the Sea
When last we left our submarine in Episode 7, it was floating on top of the magma ocean 4.5 billion years ago as the rogue planet Theia came crashing down above us. 100 million years later, we arrive at a very different scene.
Generations of meteor impacts have brought water from around the Solar System here, to Earth. At first, the surface is way too hot, and the water vaporizes into steam. But eventually, year after year, drop by drop, water begins to settle into the low places of the planet. Puddles become ponds, ponds become lakes, and lakes become a global ocean. Just like the Earth itself, the seas had very small beginnings. From small things, big things grow.
It’s nighttime on the Hadean ocean, 4.4 billion years ago. The full Moon hurtles overhead, the size of an apple at arm’s length. Its’ face has a few big craters, but not the giant dark splotches that will form the Man in the Moon. For now, the Moon looks more like a giant golf ball than our modern pockmarked friend. Let’s sink into the Moon’s reflection, and look at the new world beneath the ocean waves.
It's a quiet ride down to the ancient seafloor. There are no fish swimming out of the way, no strange glowing squid peeking through our portholes. No whale songs, no dolphin calls. Throughout the fathoms of the early ocean, all is silent and dark. It was a simpler time back then. There is no definitive evidence for life yet, but we’ll return to that idea at a later point.
Eventually, we reach the barren ocean floor. Apart from the lack of visible life, it looks remarkably like the bottom of today’s oceans. A vast plain of dark black basalt stretches before us in the submarine’s headlights. As we touch bottom, our view is suddenly obscured by a brown cloud of sediment. Our landing has kicked up some mud from the seafloor.
Seafloor basalts, Mozart Seamount, North Pacific Ocean
This dustup is the most exciting thing that’s happened to us this trip, but it’s also a sign of things to come. Almost all the rocks we’ve talked about until now have been igneous rocks, born from cooling magma or lava. We’ve seen dark basalt form at mid-ocean ridges, granites in magma chambers, and pale rhyolites in volcanic arcs.
In contrast, the sediment settling around our Hadean submarine is a product of water, not lava. Way back in Episode 2, we talked about sedimentary rocks, taking an imaginary walk from a beach into the modern ocean. On the beach, we saw that waves and rivers physically break down rocks to form sand. As we walked deeper, the grains became smaller, turning into soft mud beneath our feet.
This story is true for sediments close to shore, but way out here in the middle of the ocean, water uses a different technique to turn rocks into mud: chemical warfare.
A pale weathering surface on a dark basalt: water slowly changing minerals
If you put a rock into a glass of water, you could leave it there for years and it wouldn’t look like anything was happening. But on a slow microscopic level, the water and the rock are changing each other. Take our old friend olivine, for example. Olivine is a common mineral in ocean basalts and easily reacts with water, turning into completely different minerals. This transformation usually weakens the rock, breaking it down into tiny pieces of mud. Just as thousands of ocean waves turn a boulder into a grain of sand, thousands of years sitting in water will turn the same boulder into a pile of clay. If you’re an impatient person, you can see a similar process happening much faster with a piece of pure iron. Leave an iron rod sitting in water long enough, and it will slowly rust away.
Scientists call this process weathering- if you want to learn more, check out my interview with Dr. Ella Holme, where we talk about how olivine weathering is being used to remove CO2 from Earth’s atmosphere.
We can see weathered basalts across the modern seafloor, but how do we know this was going on in the Hadean 4.4 billion years ago? We don’t have the original basalts, and we certainly don’t have the original muds. As you’re probably sick of hearing by now, the only things we do have are the Jack Hills zircons. We’ve seen how different elements in these zircons tell different stories: uranium and lead provide an hourglass, hafnium and lutetium track the ancient crust. Today, I’ll introduce you to a far more familiar element that tells the tale of Earth’s earliest oceans. This is an element that is shared between basalt, water, clay, and zircon, making a gigantic ballroom dance across the Earth’s surface. This element is oxygen.
Part 2: The Fractionation Tango
When you think of oxygen, you might imagine the colorless gas entering your lungs this very second. This is pure oxygen, O2. Every breath you take contains around 21% pure oxygen. The lower limit for human survival is not much lower, only around 19%. Back in the Hadean, a breath of air would have less than .0002% oxygen, more than 100,000 times lower than today. Needless to say, if we time traveled back 4.4 billion years ago, we would need spacesuits to breathe. In fact, for 90% of Earth history, oxygen levels were too low for human survival. The breathable world is a very recent event, one we will not witness until the very end of our last season.
But oxygen is far more than just a gas. It loves to react and combine with other elements. The rust on a car contains oxygen plus iron. A brown apple slice contains oxygen plus carbon. The water in your glass contains oxygen plus hydrogen, the O in H2O.
Oxygen can be a fickle element, and often moves from one location to another- from gas in the air, to rust in the ground, to water in the ocean. But not all oxygen is the same- some atoms are heavier than others. I’m not going to bog you down with numbers yet. For the moment, all you need to know is that oxygen comes in two main flavors: light and heavy. Scientists call these varieties isotopes- you can think of them like dog breeds. A Chihuahua is much lighter than a Great Dane, but both are definitely dogs.
Light and heavy “isotopes” of dogs
The oxygen in a breath of air, a piece of rust, or a glass of water is a mixture of light and heavy isotopes: imagine a series of dog parks with different ratios of Chihuahuas to Great Danes. When oxygen moves between materials, different isotopes begin to separate. There are some places where lighter atoms prefer to sit, and others where heavier isotopes are more comfortable. Back to our dog park analogy, there are some areas where Chihuahuas can go that Great Danes can’t, and vice versa.
Scientists call this parting of the ways “fractionation”, from the Latin word meaning “to divide”. It has the same root as the words “fragment”, “fracture”, and yes, “fraction”. Fractionation is a term we will see throughout this podcast- it’s a cornerstone of geochemistry and deciphering Earth’s ancient past. Almost every element from hydrogen to uranium has light and heavy isotopes. Just like oxygen, these isotopes move between different materials, leaving distinct fingerprints. If you know how to read isotopes, you know how to read a rock.
Let’s take the idea of fractionation and apply it to the Hadean seafloor we just explored. Seawater is slowly weathering dark basalt into soft clay. Every ingredient in this recipe has some oxygen: the water, the basalt, and the clay. On a microscopic level, the gates have been flung open, and the oxygen atoms now have a choice: would they rather live in a mineral, or water?
In this case, the light oxygen isotopes, our atomic Chihuahuas, prefer to stay in the water. In contrast, the heavy oxygen isotopes, the nuclear Great Danes, would rather sit inside a mineral. In short, if you took a glass of seawater and some basalt from the ocean floor, the basalt would have a much heavier oxygen signal than the water.
So, now that oxygen has leapt from water into basalt, there’s only one step left: getting it into a zircon where scientists can measure it.
Part 3: From Water to Fire
On January 11, 2001, I was probably playing on a snowy schoolyard in southeastern Wisconsin, dreaming of dinosaurs and mammoths. Little did I know that on the same day, only an hour and a half to the west in Madison, Wisconsin, a research team was celebrating one of the landmark papers in science history. The paper was published in the journal Nature, the highest place a scientist can hope to reach. The authors were Simon Wilde from Perth, Australia, John Valley and William Peck from Madison, and Colin Graham from Edinburgh, Scotland. This was the article describing the oldest material on Earth, the zircon grain W74/2-36 from the Jack Hills that we met in Episode 10. As of 2022, no one has yet found anything older than 4.4 billion years.
That fact alone makes Wilde paper significant, but wait- there’s more. This paper is also tied for first to describe the earliest evidence of water on Earth. In fact, the other Jack Hills paper was published in the same issue of Nature, immediately behind Simon Wilde and the others. Sometimes two separate research groups arrive at the same idea at the same time. This second paper was written by Stephen Mojzsis and Mark Harrison from UCLA, and Robert Pidgeon from Perth. Later the same year, the Wisconsin group published a more detailed water paper led by William Peck.
The details differ slightly, but the 2001 papers of Wilde, Mojzsis, and Peck all conclude that water was present on Earth’s surface 4.4 billion years ago. All of them use oxygen isotopes from Jack Hills zircons to tell their story.
Let’s back up to the Hadean seafloor, where we left our submarine.
We’ve just learned that Earth’s new ocean is already starting to change the rocks around it, physically and chemically. At the bottom of the ocean, seawater is slowly turning dark basalts into clay. This weathering process involves an exchange of oxygen isotopes, with basalt getting all of the heavy atoms.
Various ways magma interacts underground: A is a magma chamber
To see the rest of the story play out, we have to press the fast-forward button. Our poor basalt gets buried deeper and deeper below the seafloor. Eventually, the dark rock melts into a magma chamber, an isolated bubble of molten material within Earth’s crust. As we saw in Episode 13, the magma slowly cools into a pale rock with large crystals, a rock that would look at home on a kitchen countertop: granite. All the heavy oxygen atoms that the basalt gathered on the seafloor are now shuffled into different minerals, including tiny purple zircons.
Fast forward even more, and this granite is eventually weathered away by water, just like the basalt before it. The only crystals tough enough to survive are the zircons. Unlike basalt, zircon doesn’t like to play with water, and greedily keeps the heavy oxygen atoms all for itself.
Which is a good thing! Without the zircons hoarding their precious cargo of ancient oxygen atoms, we would not have the story I just told you: the story of Earth’s earliest water.
At the beginning of this season, I described the Hadean as a time period that would fit well on the cover of a heavy metal album, with planetary collisions and magma oceans. But now, two hundred million years later, only two weeks on the Earth Calendar, the planet is cool enough to have liquid water. Don’t get me wrong, it’s still incredibly hot, with daily meteor showers, a gigantic moon, and days lasting only a few hours. But now there’s an ocean, with a calm seafloor and even mud. Who’s to say there couldn’t even be some life down there?
Next episode, we finish our tour of the Jack Hills zircons by examining that very question. Could life have survived the early Hadean? And if so, is there any evidence it was around back then?
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Images:
Nighttime Ocean: https://commons.wikimedia.org/wiki/File:Andaman_Sea_at_night,_Moonset,_Moonlight.jpg
Seafloor: https://commons.wikimedia.org/wiki/File:Pillow_balls_on_Mozart_Seamount.jpg
Weathering Basalt: https://commons.wikimedia.org/wiki/File:Spheroidal_weathering.jpg
Oxygen Cycle: https://commons.wikimedia.org/wiki/File:Global_Oxygen_Cycle.jpg
Dogs: https://commons.wikimedia.org/wiki/File:Big_and_little_dog_1.jpg
Magma Chamber: https://commons.wikimedia.org/wiki/File:Igneous_structures.jpg
Music:
La Mer by Claude Debussy: https://commons.wikimedia.org/wiki/File:La_mer_-_II._Jeux_de_vagues_-_Concert_Band_-_United_States_Air_Force_Band.mp3
Gymnopedie No. 1 by Erik Satie: https://commons.wikimedia.org/wiki/File:Gymnopedie_No._1_(ISRC_USUAN1100787).mp3
The Mastermind by Tiny Music
Whirlwind Dance by Sirus Music
TV Mambo by Daniel Belardinelli
Space 80s by Ilegot
Catacombs by Big Score Audio
Seven Days of Flying by Remember the Future