How Great Was the Great Oxidation?

OlaPicture this, Amara: no blue sky, just this thick orange haze sitting over an ocean that looks almost purple.

AmaraPurple ocean? Okay, I'm listening.

OlaIron-rich water, sulfur in the air, and if you breathed it, well, you couldn't, because there was basically nothing to breathe.

AmaraSo we're doing full villain origin story atmosphere right out the gate.

OlaPretty much. Cyanobacteria down there are quietly making oxygen, and the rocks and oceans just soak it right up like a sponge that never fills.

AmaraA patient sponge. For how long, though?

OlaThat's the thing we're supposedly building to.

AmaraRight. Because, okay, get this, we've been promising this exact episode since the show started. The tagline practically begs for it.

OlaIt really does.

AmaraSo tonight, we're finally doing it. Ola and I are digging into the moment oxygen actually filled Earth's atmosphere about two point four five billion years ago.

OlaThe Great Oxidation Event.

AmaraThe one and only. But here's the twist we didn't promise.

OlaOxygen wasn't enough?

AmaraNo. We're putting the evidence on trial because it turns out the people who study this aren't even agreed on how much oxygen showed up.

OlaWait, seriously?

AmaraSeriously. Wait for it.

OlaNow I need to know how firm the textbook version even is. So the textbook version of this story is basically a bumper sticker.

AmaraOh, here we go.

OlaNo, listen. The confident version says oxygen shows up, sticks around, and that's the ending. Filed under solved.

AmaraAnd that's the whole plot?

OlaThat's the whole plot, at least the version most people got taught. The claim is that this is the moment oxygen finally becomes a permanent part of the air, not just a passing spike.

AmaraPermanent as in it never dips again?

OlaPermanent as in it's baked into the story we promised to finally tell on this show. A done deal, case closed.

AmaraOkay, but done deals make me nervous.

OlaThey should, because from that same starting point, most standard rundowns describe it as a staged process, not a switch flipping.

AmaraStaged like a play?

OlaStaged like an actual model. Five stages, one after another, and the stretch geologists call the Great Oxidation Event proper runs from that opening moment until about one point eight five billion years ago.

AmaraSo roughly six hundred million years of what? Oxygen just piling up?

OlaPiling up and then getting eaten. That's the part people skip. Oxygen climbs to somewhere between a tenth and a fifth of what we breathe right now, and then a huge chunk of it gets pulled straight back down. Oceans soak it up. Sea floor rock locks it away.

AmaraWait, so it rises and then partly reverses?

OlaPartly reverses, yeah. That's the summary you'll find in most overviews of this stage. Rise, draw down, repeat, until the air finally settles into something like a permanent balance.

AmaraSo even the event isn't really one clean event?

OlaIt's a six-hundred-million-year tug-of-war that the oxygen side eventually wins.

AmaraOkay, but the way it's taught in school-

OlaIs clean. One arrow, one date, one grand finale. Very tidy.

AmaraSuspiciously tidy, actually, for something that dragged on that long and left that many chemical fingerprints behind in the rock.

OlaSo what does climbing to even a tenth of today's levels actually mean for something living down in that ocean? That's still a jump from basically nothing, even before any of it gets pulled back out.

AmaraRight. Going from zero to even a sliver of oxygen changes everything for organisms built to use it, and just as much for the ones that never adapted at all.

OlaRight. And that's exactly the confidence we're supposed to be questioning here.

AmaraSo walk me through why anyone would doubt oxygen went from a tenth to a fifth of today's levels.

OlaIt's less about doubting the direction and more about doubting how precisely anyone can date the start of that climb.

AmaraOoh, go on.

OlaThere's a new carbon study that says the clock on this whole thing might be off.

AmaraOff how? A little or full rewrite the chapter amounts?

OlaThat's what we're about to find out. It's carbon-13 doing the talking this time, not oxygen.

AmaraOoh, plot twist already, and we're not even fifteen minutes in.

OlaFifteen minutes in and the settled tidy story already has a crack running through it.

AmaraA crack made of carbon.

OlaA crack made of carbon. Okay, so two geologists at the University of Victoria took a fresh run at this whole timeline.

AmaraOkay, so two geologists at the University of Victoria took a fresh run at this whole timeline.

OlaFresh how? They didn't dig up new rock samples, did they?

AmaraSort of. They combined field geology with statistical modeling of ocean floor chemistry from billions of years ago, and they linked a carbon-13 spike to the Great Oxidation Event at two point four five billion years ago.

OlaWait, same starting date as the whole event?

AmaraSame date. Except now it's not just oxygen showed up. The carbon cycle itself is getting rewired at that exact moment.

OlaCarbon-13, the heavy isotope. Photosynthesis prefers the light one, so when that ratio moves-

AmaraSomething big just changed in what's producing carbon and what's burying it, exactly

Speaker 3So what's actually new compared to what we already had?

AmaraThe timing. Their revised model shows major shifts in the ancient carbon cycle happening earlier than expected, and it's not landing alone. It's coinciding with ballooning atmospheric oxygen and a stretch of global glaciation.

Speaker 3Hold on. Glaciation and rising oxygen at the same time?

AmaraI know. Sounds backwards.

Speaker 3It does.

AmaraI know. The working idea is oxygen eats atmospheric methane. Methane's a greenhouse gas. Kill the greenhouse, you freeze the planet.

Speaker 3So one isotope spike is carrying oxygen, climate, and carbon chemistry all on its back?

AmaraPretty much.

Speaker 3Okay, but walk me through why that particular combination matters so much. Oxygen eating methane isn't just some side reaction, is it? That's the whole thermostat for the planet at that point.

AmaraBasically, yeah. Methane's doing a lot of heavy lifting keeping the early Earth from freezing, so the moment oxygen starts scrubbing it out of the sky, you lose that blanket fast. And this model's saying that scrubbing lines up with the carbon shift, not sometime long after it.

Speaker 3That's a lot of weight for one signal.

AmaraIt is, but that's kinda the move here. If the clock on this piece can shift earlier than the textbook says, what else in the story is actually locked down?

Speaker 3Right. Because if the timing's soft in one spot-

AmaraMaybe it's soft everywhere.

Speaker 3You say that like you already know the answer.

AmaraI do, sort of. Somebody wrote an entire paper asking that exact question about the field's own numbers.

Speaker 3Wait, an actual paper questioning itself like that?

AmaraNot itself, the whole body of work, every estimate anyone's made about how much oxygen this event actually produced.

Speaker 3Hmm. So it's not just when this happened that's up for grabs.

AmaraNope. It's how much oxygen, how fast, how far it spread, the numbers under the numbers.

Speaker 3And this came out recently?

AmaraYeah, this year. It goes after the estimates other scientists have leaned on for decades.

Speaker 3That's a bold thing to publish.

AmaraIt's a bold title too, honestly.

Speaker 3Bold enough that I want to know if it's actually backed up or if it's just a spicy abstract.

AmaraOh, it's backed up. Stick around.

Speaker 3Now I want to know what it actually found.

AmaraSame. Let's get into the paper itself because the range it comes back with is not small.

Speaker 3Give me a number.

AmaraNot yet. You'll wanna sit down for this one.

Speaker 3Fine, fine. Go. Okay, so the paper actually has a title. It's called Revisiting the Great Oxidation Event. Wait, they used the word great in the title on purpose?

AmaraOn purpose. It's a 2026 review in Communications Earth & Environment led by a researcher named Crockford, and it goes after the field's own numbers.

Speaker 3Goes after them how?

AmaraThe paper states flat out that the defensible range of possible atmospheric oxygen levels spans orders of magnitude.

Speaker 3Orders of magnitude, plural?

AmaraPlural. Not double, not triple. Powers of 10 apart from each other.

Speaker 3So two labs could each publish a number for how much oxygen was actually in the air back then and be 100 times apart and both get cited as fact?

AmaraBasically, yeah.

Speaker 3That's-- Okay, that's a lot bigger gap than I expected for something this famous.

AmaraRight? This is supposed to be the headline event of the whole planet history, and the review is saying we don't actually agree on the size of it.

Speaker 3What's the mechanism, though? Why would estimates be that far apart?

AmaraThe paper doesn't hand you one villain. It's pointing at how much of this rests on proxy data, rocks standing in for something nobody measured directly.

Speaker 3Right, because nobody had a sensor sitting in the Archean atmosphere.

AmaraExactly. So every number is inference stacked on inference, and different proxies disagree with each other.

Speaker 3And the review's answer to that is what? Throw up their hands?

AmaraNo. The same review argues narrowing those divergent scenarios should be a priority for future research. So it's not we give up. It's we haven't reconciled this yet, and we should.

Speaker 3Which is a very polite way for scientists to say our numbers are a mess.

AmaraVery polite.

Speaker 3Okay, so let's actually sit with this for a second because I think it changes what we were told earlier in the episode.

AmaraGo on.

Speaker 3If the confidence interval on oxygen is that wide, then the whole Staged, tidy-sounding version, climb, peak, drawdown, that's not wrong exactly, but it's a shape drawn with a very shaky ruler.

AmaraI'd go further. It's not that the shape is wrong. It's that we don't actually know how tall the mountain is.

Speaker 3Careful. That's basically a contrast frame.

AmaraFine. I'll rephrase. We know Oxygen went up. We do not agree by how much.

Speaker 3Which matters because Great is doing a lot of work in Great Oxidation Event. Great compared to what if the compared-to number moves by a factor of 100?

AmaraThat's the actual question the title is asking, and it's not rhetorical. Crockford and colleagues aren't being cute. They mean it.

Speaker 3So does this whole thing collapse, or is it just messier than the textbook made it sound?

AmaraI don't think it collapses. Cyanobacteria still happened. Oxygen still shows up in the rock record.

Speaker 3But the confidence we were handing this story is bigger than the evidence can carry.

AmaraThat's fair. And if the headline number is this contested, I'm wondering what happens when you look at the other proxies people use to back it up.

Speaker 3Right. Because if oxygen levels are this uncertain, are the other lines of evidence any more solid, or do they just add more noise?

AmaraThat's exactly where I wanna go next.

Speaker 3Okay, so get this. While the review papers got everyone side-eyeing the numbers, there's a Phosphorus study that just adds a whole new layer.

AmaraA Phosphorus study? Interesting. A twenty twenty-five study looked at phosphate trapped in carbonate rock pulled from sites all over the planet and argues that marine Phosphorus, biological productivity, and atmospheric Oxygen were rising and falling together somewhere between two thousand four hundred thirty and two thousand six hundred million years ago.

Speaker 3So the food supply for the organisms making the Oxygen is doing the same dance as the Oxygen itself.

AmaraRight. Phosphorus feeds the algae, the algae photosynthesize, Oxygen goes up. Take away the Phosphorus, and the whole engine stalls.

Speaker 3Makes sense on paper. Does the study trust its own numbers, though?

AmaraThat's the fun part. No. The same paper admits its oxygen estimates can swing a lot even within their own error bars because nobody has a solid handle on how much oxygen was flowing in from rivers or leaking up from the mantle.

Speaker 3Wait, the mantle is putting oxygen into this system too?

AmaraApparently enough to matter and not enough that anyone's pinned down the number.

Speaker 3So even the people building a new proxy are flagging their own inputs as shaky.

AmaraYep.

Speaker 3Alright, here's one that really made me sit up. A PNAS paper on Canada's Belcher Group.

AmaraOoh, tell me.

Speaker 3Sulfate isotopes from those rocks suggest primary productivity. Basically, how fast life was pumping out organic matter crashed hard somewhere around two thousand four hundred to two thousand fifty million years ago.

AmaraCrashed after what?

Speaker 3After an oxygen overshoot. Their idea is oxygen spiked higher than the system could handle, and that helped trigger the collapse that ended the whole event.

AmaraSo it's not a slow fade-out. It's a spike and then a crash.

Speaker 3That's their proposal. But even they say the timing and the trigger for that crash are poorly constrained.

AmaraPoorly constrained is becoming the theme of this episode.

Speaker 3It really is.

AmaraOkay, but here's the one that broke my brain a little. MIT researchers found evidence that some early life forms may have been using oxygen hundreds of millions of years before the Great Oxidation Event actually locked oxygen into the atmosphere permanently.

Speaker 3Wait, organisms breathing oxygen before the oxidation event that's supposed to have introduced breathable oxygen?

AmaraExactly. Pockets of it, oasis probably long before the main event. The MIT team is calling it some of the earliest evidence of aerobic respiration on the planet.

Speaker 3So the starting line isn't even a line. It's smudged.

AmaraIt's smudged.

Speaker 3Phosphorus wobbling with oxygen, a productivity crash nobody can date precisely, and oxygen breathers showing up before the party officially started.

AmaraWhen you stack it like that, I don't know how anyone writes one clean sentence about this event.

Speaker 3I think that's exactly the question we've been circling all episode. Is great even the right word anymore? Or is that just the label stuck to something nobody's actually pinned down?

AmaraWell, we've got timing in question, scale in question, and now even the starting gun is in question.

Speaker 3So which is it? Big deal, overhyped, or somewhere we haven't even named yet?

AmaraI think it's time we actually said it out loud.

Speaker 3So verdict time.

AmaraThe jury's back.

Speaker 3Does it still deserve the word great?

AmaraHonestly, I think it earns the name for the outcome, not for how clean the story is.

Speaker 3See, that's where I land differently. The event happened, fine, but the paper we keep coming back to literally poses the question in its title: How great was Earth's Great Oxidation? And its own answer is a spread of estimates that differ by orders of magnitude.

AmaraRight. That's not a footnote. That's the headline.

Speaker 3So when a field asks its own confidence out loud like that, I don't think you get to keep the adjective for free.

AmaraBut think about what actually changed on the planet. Iron oceans, methane skies. Then a permanent gas nobody could put back in the bottle. That transformation is real, even if the yardstick measuring it is wobbly.

Speaker 3Sure, the outcome's real. I'm questioning the confidence, not the event.

AmaraFair. So maybe it's great in scale, shaky in specifics.

Speaker 3That's basically the whole episode in one sentence.

AmaraWe should trademark that.

Speaker 3What gets me is how many separate lines of evidence all land on the same word: uncertain. The timing's uncertain. The amount's uncertain. Even who was breathing oxygen first is uncertain.

AmaraAnd every single study we found this episode admits it in the fine print. Nobody's hiding it.

Speaker 3Which honestly makes me trust them more.

AmaraSame. Scientists flagging their own blind spots is a better sign than a tidy number with no error bars.

Speaker 3And that's honestly the difference between science and marketing. A marketer never puts an error bar in the fine print.

AmaraTrue. Though I'd argue that's also why the story keeps getting flattened into a bumper sticker by the time it reaches a classroom.

Speaker 3So where do you actually land? Keep the name or retire it?

AmaraKeep it. Rename the confidence, not the event.

Speaker 3I'd go further. I'd say it's great the way a rough draft is great. The idea holds. The details are still being fought over.

AmaraOoh, I like that.

Speaker 3Thank you. Thank you.

AmaraDon't let it go to your head.

Speaker 3Too late.

AmaraOkay, but zoom out for a second. This show promised listeners a Great Oxidation Event episode, and I think what we actually delivered is better than the postcard version.

Speaker 3Agreed. We didn't hand anyone a settled story

Speaker 4We handed them a live argument with the receipts

OlaWhich is honestly the more honest thing to do with two-and-a-half-billion-year-old rock records.

Speaker 4So listeners, next time someone says the Great Oxidation Event like it's a closed case

OlaAsk them which oxygen estimate they're using

Speaker 4Savage

OlaI learned from the best

Speaker 4All right, let's send them off properly

OlaOne planet, one enormous atmospheric shift, and a field still arguing about how big it really was

Speaker 4That's not a bad note to end on

OlaIt's the true one Right

Speaker 4So where does that leave us genuinely? The atmosphere changed. That part isn't up for debate. What's up for debate is how confidently anyone can put a number on it

OlaRight

Speaker 4And I think that's the honest ending here. The planet did something enormous, and the people measuring it are still arguing about the ruler

OlaWhich if you squint sounds a lot like the fog I described at the start of this episode

Speaker 4Oh, here we go

OlaNo, I mean it. That hazy iron-rich ocean I painted for you, turns out the science is just as hazy

Speaker 4The planet was murky. The papers are murky. Somehow that feels appropriate

OlaPoetic even

Speaker 4If you've got a favorite epoch or you think we got something wrong, and someone always does

OlaWrite in

Speaker 4Subscribe, leave a review. It actually helps people find the show

OlaAnd email us, hello@heymato.com. Tell us which number you'd have picked or which epoch you think deserves its own episode next

Speaker 4We'll argue about it

OlaWe always do