PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
Banded Iron Formations - A Deep Dive
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In this episode, we bring you a riveting Q&A session with GeoGirl Dr. Rachel Phillips and Dr. Joshua Davies, both experts in geology who explore the mysterious world of banded iron formations (BIFs). We dive deep into what BIFs are, when they formed, and their significance. Josh unpacks the debated origins of these rocks, discussing how they may be the most hotly contested rocks in the geological world due to our limited understanding of their formation process, despite their voluminous presence on Earth. We touch on how BIF layers represent cycles influenced by Milankovitch cycles, and whether microbial activity plays a role in their formation. Josh explains that while BIFs peters out around 1.8 billion years ago, there are unique conditions and debates on secondary processes that impact our current understanding. We humorously explore modern analogs, like weird anoxic lakes, and laugh about how our Earth would need a huge meteorite impact to revert to BIF-forming conditions—a scenario we're not quite rooting for! Finally, we ponder the possibility of extraterrestrial BIFs, symbolizing our endless curiosity about space and deep time. This one’s a dense, iron-packed dive for sure—just like BIFs!
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Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. Hey today, really excited to bring you another in our series of question and answer with uh GeoGirl Rachel Phillips. She's pinged her followers on her YouTube channel with questions about bandit iron formations. And so bringing together two of my favorite people to talk geology with, Dr. Joshua Davies. We've covered a little bit of his work on orbital forcing and talking about Milankovic cycles back in deep time. And we touched on banded iron formations a little bit, but today we're doing a deep dive with some questions that Rachel brought to us. So, Rachel, Josh, and me diving deep on banded iron formations. Let's go.
SPEAKER_03It sounds like we have a lot of wonderful experience and knowledge about Biffs, and that is what the viewers have brought us here today to talk about is these banded iron formations. And we will get into all the nitty-gritty questions that you guys had online. But to start with, can you just very simply explain what are banded iron formations for those that maybe haven't heard of them? And when did they form and why are they significant?
SPEAKER_00So there's there's so many, it's it's such a like a crazy topic, banded iron formations. Even just that simple question, what is a banded iron formation is gonna open it's a large box of things.
SPEAKER_02Okay, like can I uh uh ask you get a another question on this real quick, Josh, and and Rachel, you too. Is Biff the most debated rock there is? Because I struggled to come up with one that would be more hotly contested than that.
SPEAKER_00Yeah, I mean it's yeah, maybe you're right. I mean, there's we don't really know how exactly how they're formed, I guess. That's the main problem. I mean, and and it's such an important rock, but we just don't know. And so I mean, yeah, I would struggle to think of another one that's less well known and so I mean voluminous on the earth's surface. Yeah.
SPEAKER_03I mean, uh, even your field, Jesse, like the oldest rocks on earth, I feel like that's pretty hotly debated. But I feel like biffs is a funny thing. And this is why I like to start with like, you know, simply like, what are they and why are they significant? Because I feel like in textbooks, we're often taught this very simple story of like, oh, they represent when oxygen started rising in the atmosphere and oceans, and then they stopped forming, and that's when you know Earth was fully oxygenated, and then you know, the end. And then you kind of start looking into the literature and you're like, oh, oh, it's oh, it's not that simple.
SPEAKER_00Yes, that's right. Yeah, yeah. That's exactly. I mean, I think also, I mean, people group a lot of stuff into bif. When people, when when somebody asks what's a bif, most people are thinking about this rock type that's formed around about 2.5 billion years ago. And it's kind of interlayered silica, chert, essentially, and then iron oxides. That's that's kind of the most famous thing. But actually, that is a group of banded iron formations that are typically called the superior type. They're deposited on continental margins, essentially shallow to kind of shelf marine deposits, like around continents. Those ones that are like really significant in terms of volume of of rock produced, that's what most people think of as bit. But there's also another type which is older, kind of mostly in the Archean. These are called these algoma types. These are also kind of layered, a lot of iron oxide, some silica in there, not as nicely layered, usually more altered and metamorphosed, and much smaller in terms of their volume. Those are kind of found in really deep moraine settings, kind of abyssal planes associated with kind of hydrothermal vents, and have got they're kind of interbedded with a lot of volcanic rocks, a lot of mafic volcanics. So there's this kind of algoma type, which don't look at all the same to the kind of diff that everybody speaks about. And then there's other another kind called a granular iron formation, but they're they're all kind of actually the same kind of rock, formed in different environments.
SPEAKER_03Are the older ones also iron oxides? Or are they different Okay?
SPEAKER_00Yeah, yeah, they're iron oxides. I mean uh again, hematite or magnetite.
SPEAKER_03Okay.
SPEAKER_00And churred. But they're not as beautifully layered and they're typically metamorphosed. Um so when you metamorphose a rock that's not got a lot of minerals and it's not got a lot of different elements, you don't change it that much during the metamorphism. If you think of like a metamorphism of a of a pretty pure sandstone, you just end up with more quartz, right? Or like a really nice carbonate, you just kind of end up with more carbonate, just intergrown a little bit more. So these biffs, these old ones that are metamorphosed, they've still got the same kind of mineralogy, but they're a bit more messed up, I suppose.
SPEAKER_02A bit more discontinuous, too, right? I mean, the ones uh we've seen these in the field together, Josh. Which ones have you worked on most, Josh?
SPEAKER_00Me, which almost entirely the superior type, around about 2.5, the ones kind of more associated with the great oxidation event. That's kind of why BIFs are most are are famous, because there's this like massive, massive amount of this stuff deposited at one time associated with kind of big, big changes in kind of what's going on on the surface environment of the Earth.
SPEAKER_03With that in mind, just to simplify the kind of story, the idea behind the superior type, the the really massive deposits of biff right around the great oxidation event, is that these iron oxides formed in such abundance because you know, Earth before the great oxidation event, before we had this dramatic rise in oxygen, was relatively reduced. There was tons of reduced iron, reduced dissolved iron in the ocean, and then oxygen came about and oxidized a bunch of that forming these huge deposits. And so is it reasonable that I'm assuming that the previous deposited biffs were much less in abundance because it was probably just early whiffs of oxygen per se as photosynthesis started to spread more?
SPEAKER_00Yeah, I mean it's just non-stop debated problems with different. I mean, it's difficult to it's difficult to know the real answer to that.
SPEAKER_03Okay.
SPEAKER_00I mean, that's one. I mean, these algoma type, if we speak about them as these algoma, these small things. The people think that their depositional environment is really deep, deep in the ocean, where it's going to be dark, essentially. So maybe you've got iron interesting uh oxidation happening right at the surface.
SPEAKER_02Can we back out of this real quick and and sort of intro what you're referring to a little bit more and and and uh maybe work from like the the base model, the simple explanation for how bitforms and talk about oxidated uh oxidative iron, reduced iron, and and why does that matter? Why do you speak about that? Yeah, yeah. Okay before getting too much in the weeds, you know, right away.
SPEAKER_00Okay. Something super important, I suppose, before the atmosphere becomes oxidized, we we basically have a reduced atmosphere, and that means also there's not a lot of free oxygen in the oceans. Even though the ocean is made of H2O, there's a lot of oxygen there, there's not actually free oxygen hanging around. And so under those conditions, without very much free oxygen in the oceans or in the atmosphere, the ocean can do different things than it can do today. One of the things that it can do is it can dissolve a lot of iron, a lot of reduced iron. So if you've got no oxygen, the iron is typically less oxidized, it's not oxidized. And so there's a two-plus charge on the iron. Yeah, yeah. Oxidized iron has a three-plus charge. So two plus charged ion, uh iron um is an ion, confusingly. Um is soluble in water. So if you have reduced oceans or no oxygen in the atmosphere, you can start to fill the ocean with dissolved iron. The ocean's a huge, huge reservoir, essentially. And so you can slowly, over time, through just interaction between the the water and the rocks, I mean the the water is just constantly interacting with the rocks, it can leach out iron from the rocks, or rivers can bring in iron, and you can just slowly accumulate and fill up the oceans with iron, and they're just gonna be dissolved iron there. And so when you start to have oxygen, you change the iron from the two plus state, which is the reduced state, to the oxidized state, which is three plus. And at that point, iron no longer is soluble. It becomes it precipitates. And this is one of the main models for how you form huge, huge iron deposits. Oceans have just been accumulating iron for m billions of years, essentially. And then at some point you oxidize the iron and it just kind of rains out.
SPEAKER_02Recognizing that there's a lot of debate about these types of things, you need basically to get the the layering, the banded part of the banded iron formation, we need some kind of triggering mechanism, right? So we're sort of assuming that the silica part is the background depositional, uh, this is silica just kind of precipitating out of the ancient ocean, and then we have a like a triggering mechanism that induces precipitation of iron, then doesn't, then does, then doesn't, then it's sort of turning on and off, on and on and on and off, right? And so is that a good simple framing that could help us talk about the debate then a little bit about what that mechanism is?
SPEAKER_00Yeah, why not? I mean, that's definitely a model for how you get the layering in BIF. The idea is you kind of have homogeneous or kind of constant silica deposition, because also in these early oceans, the amount of silica dissolved in the ocean is also higher than today.
SPEAKER_01Yeah.
SPEAKER_00Because I mean, right now we have organisms that are using the dissolved silica in the oceans, like like diatons. So if you have a lot of silica, they're actually being taken out by these little organisms. Yeah. Back in the end of the Archean or the beginning of the Paleo Protozoic, you don't have that. There's no uh silica forming organisms. So there's a lot more silica available and you're just kind of precipitating it out.
SPEAKER_02This is actually just a chemical precipitate uh and not a biochemical precipitate.
SPEAKER_00Not yeah. That's the kind of the idea for the silica. And then one of the big ideas and main ideas is exactly what you're suggesting, Jesse. You have times when there's more iron oxidation and you're kind of raining out iron oxides, and then times when there's less. And so you have less iron oxide deposits, make more silica, and then maybe the iron oxide comes back, turns on, off, on, off. But there's there's other models, other people suggest different things. I mean, uh, you can also create this kind of layering in the sediment water interface. So if you kind of deposit like a kind of a blend of a blend of kind of amorphous silica plus these iron oxides at the same time, and then you start to compact them and make a rock out of it, you can actually form layers like that. Okay. At this time, there's bacteria around. And so bacterial action at the sediment water interface can also start to move elements around and create layers like that. There's other other people suggest this kind of stuff. It's not clear which is right.
SPEAKER_01I mean, I I yeah. Oh, there's a lot of things.
SPEAKER_00I mean, there's other people who think about even iron coming like deeper parts of the ocean that are more enriched in iron, and then they're kind of bringing up iron as a kind of a current, like a deep upwelling current rich in iron, that then suddenly interacts with surface waters that are more oxidized, and then raining out, and then this current kind of goes away and then comes back. Like, I mean, uh, we've got these kind of oscillating deep currents on the on the earth at the moment. So, I mean, that's another thing that people have suggested. I mean, I don't know what the right answer is, to be honest.
SPEAKER_03Well, yeah, I mean, like you say, it's probably a mixture of things. It makes sense to me, just because everything on Earth is very cyclic, that there would be these cycles, these oscillations of whether it be currents bringing in these dissolved chemicals or whatever it is. But it's so interesting that I didn't know like the sediment column, you know, bacterial thing, which we'll get into bacteria again in in a in a second. But I want to go back to something you started to say earlier that I think is really interesting because uh you, you know, we're talking about how the oxygen has to react with the iron to make these deposits, right? And what you mentioned earlier about the pre-grade oxidation event biffs that are the superior type, but they are there. They are technically a classification of biff. You mentioned, you know, some of these might have been deep water deposition. I'm curious how, you know, because we're we're used to oxygen being mostly present in the surface water, especially on early Earth where there was hardly any, and it was probably just these little oases of photosynthetic rich areas. Um, so I'm curious how they would have deposited in deep water, what kind of hypotheses there are around that, if there was any hydrothermal influence or you know, what we know about that.
SPEAKER_00Yeah, exactly. I mean, this is a big, a big question. I mean, um, so the the general idea for the main source of oxygen, I mean, in the in the when you have the big deposits of it is cyanobacteria. I mean, you're suddenly starting to take over and produce enough oxygen to create a oxidized atmosphere and also oxidize the water column. So you've got a lot of life, actually. I mean, a really significant amount of life to create the big bips. But then the early ones, like you're saying, there's these small things. And the general idea from most geologists is that in the early Archean, there's almost no oxygen around. But oxygen is a super reactive species. So if you produce oxygen, it reacts straight away. So if you start to have little areas where there are some cyanobacteria, you wouldn't necessarily see very much evidence for that other than just this quick reaction of the oxygen by a little bit of the iron and then it goes away. So you might be able to precipitate a little bit, a little bit of iron through some small amount of cyanobacterial blooms, just small things. There's also other ways to kind of create this reaction to form BIF, to change reduced iron to uh oxidized iron. There's types of bacteria that are not um cyanobacteria, not photosynthesizing bacteria. There's types of bacteria called uh photopherroths, for example. I don't know whether you know about these things. I love bacteria names.
SPEAKER_03I'm assuming photosynthesizing iron and oxidizing bacteria.
SPEAKER_00Yeah, essentially. Well, they they use light energy. So they mean they're they're in upper water, upper paddle water column. But instead of using water, the uh for separating water and using hydrogen for their electron donor, they actually use the transfer of energy from Fe2 plus to Fe3 plus to gain energy.
SPEAKER_01Okay.
SPEAKER_00And so they actually oxidize iron themselves. And these things, it's thought that these things evolved before cyanobacteria. But it's it's quite an energy-intensive process, so it's not like it's not like there was loads of them.
SPEAKER_01Okay.
SPEAKER_00So that I mean, maybe that's why you don't have a lot of death formed. There's also other ways to actually oxidize um iron. Another one, which is abiotic, is called uh UV photooxidation. So the great oxidation event, one of the main things that actually happened during this, I mean, we oxidized the atmosphere a little bit actually. But the main one of the main things is we produced an ozone layer. And that ozone layer protected the earth from high energy incoming uh solar radiation. So before before that, you had a lot of high energy uh radiation coming from the sun. And it was reaching the surface, it was penetrating into the ocean, and these high-energy particles they can actually oxidize, dissolve like Fe2 plus to Fe3 plus. So, I mean, this is a bit experimental work that's shown this. But these are all potential mechanisms to kind of produce BIF without having a like a massive step change in oxygenation of the atmosphere and the water. Yeah. So kind of ideas of how you might be able to make it earlier on in Earth's history.
SPEAKER_03Wow, absolutely.
SPEAKER_00I mean, there's there's a few little ideas there.
SPEAKER_03Yeah, we had a lot of questions about microbial involvement in BIF formation, whether it be before GOE or during or after. And one very enthusiastic comment said, talk about the relation of stromatolites and bips. With like a billion exclamation points. So can you talk more about maybe with the superior type? Like how how involved were microbes and or you know, cyanobacteria or other bacteria with the formation of bifs?
SPEAKER_00So I mean, uh, it's a really nice question. I mean, uh, you don't you what you don't see in the geological record is stromatolites kind of swamped by bif. That's not a thing. You don't stromatolites and biffs are not forming together, if that makes sense.
SPEAKER_02Would would mean what? Like it's not a direct relationship, but maybe there's an indirect relationship. But stromatolites are not and not in the sediment. Precipitating biff. Yeah, yeah, okay. Because I think this is one thing that people, and there might have been a question as well about this, Rachel, about how to identify biffs. And I think people think banded rocks might equal bif, but stromatolites are also banded. So that might lead to like a little bit of a conflation here of processes because bandit informations have it's not just straight linear bands, they're often a little bit contorted. In the case of some of the famous ones, there's folds in them and things which could on an outcrop scale maybe a little bit more stromatolite. And but you're saying that's not not the process, actually. Any curve, any structure you see in a biff is either post-depositional or like during diagenesis and sort of squishy sediment interface, not like microbial mat type stuff. Is that accurate?
SPEAKER_00Yeah, probably. There might be some microbial mat type activities and and some I mean that there's layering at all kinds of scales in BIF. This is something I've kind of worked on a little bit, is this scale, the variety of scale of layering and and cyclicity. Um so maybe at the kind of millimeter scale, you see potentially some microbial influences, but not really clear because I mean there's also diagenetic overprints and things. Um, but in general, you don't see that much. You don't see any kind of microbial mat interactions with bit. Okay, got it. Um, stromatolites, I guess we need to just quickly say exactly what they are. I mean, they're essentially microbial communities, microbial mat communities that kind of precipitate sediment, carbonate sediment, essentially. And they're cyanobacteria communities of single-celled organisms. Um, and so they are very much related to banded iron formations in that they're the kind of big communities of oxygen-producing cyanobacteria. And so we need a change in the amount of oxygen to make these massive banded iron formations. So evidence of like huge amounts of stromatolites are are present just before these big banded iron formation deposits, or kind of even interbedded with them. Right. I mean, the ones I've worked on are kind of in southern Africa and in Australia. There's there's big carbonate platforms with stromatolites, and then you get bif. You might then go back to to to stromatolites or something like that, and then more bif. But they're not they're not together. Okay. They're they're separate units, but the oxygen produced by these cyanobacteria is super important for the formation of the bif. Yeah, yeah.
SPEAKER_02Okay. I'm quick curious, Josh, what um the thicknesses, the the the sort of volumes here, the ones you've worked on are the superior type you said. How are there major differences between the different outcrops of superior type ones? Because I know you've worked in South Africa uh and Australia, Hammersley, right? A lot. Um are there major differences in either formation model or you know, the the type of iron and silica?
SPEAKER_00It's it's a it's a it's a really good question. There's a few things that are kind of at play here. Something that we've tried to look at a little bit is I mean, is there something that's kind of systematic in the layering of the bif? And we found parts of different banded iron formations that kind of these layers repeat themselves at kind of the four centimeter scale, like I mean, a kind of centimeter scale, the layers are appear appearing at kind of centimeter scale, and you can see this thing repeating itself, and like every whatever, like 20 centimeters, you have the same thing. So there are other kind of members of funded iron formation deposits where the layering is at the millimeter scale, where you have like tiny little interlayers of iron oxides and silica iron oxides, and these are all kind of wavy and things. Yeah, then there's other examples where you have huge amounts of iron oxide, yeah, and then a little bit of silica, and then mass like meters of iron oxide deposit. These are often the ones that are mined because they're I mean they're more iron-rich ton by ton. Um the idea for this stuff is one of the ideas, anyway, is actually that the iron has been remobilized and brought and moved around. So the layering at this kind of large scale, like meters, of a single deposit of iron, yeah, it's likely that that's secondary.
SPEAKER_02So that in that secondary, I'm curious your thoughts. Uh well, we can there's lots of weeds to go down into geochronology, but people have actually dated the hematite nowadays, right? And I'm curious your thoughts because the the ages of the hematite, those big uh the actual iron ore itself, give uranium lead ages, uh, and this is kind of a new technique, people dating hematite in this way, of around a billion years. So sort of a billion. Or more years after the original deposition. So you're saying this stuff has been remobilized and around. I mean upgraded to an ore level.
SPEAKER_00I mean it's a very new technique, this. The billion years, there's a there's quite a there's a variety of different ages in different upgraded BIFs.
SPEAKER_01Yeah.
SPEAKER_00And the ages themselves of these are kind of upgrading or hydrothermal events, it's it's not clear that they are linked to a a particular geological event. Oh for example, in the different areas. So it's it's not clear what's going on, I I would say exactly. Um that's a very common answer today, Josh.
SPEAKER_02What the heck?
SPEAKER_00Yeah, I don't I maybe we've got the wrong person on here.
SPEAKER_02Well, may maybe okay. I'll just to build on that real quick, real quick, Rachel. Uh let's I'll give you a question that I know Josh has an answer to. How about this? And it's related to your chronology. So, yeah, what's your name? Um, is so so you talked about these layers. I think people often assume, and this kind of came out of one of the questions to Rachel's that that the viewers to Rachel's channel said, is what does each layer represent? And I think there's a natural inclination for people to look at these layers and say and relate them to something like varves and say, Oh, that's seasonal. Each or tree rings, each one's a year or something like this. Is a seasonal variation. I know that's like an old older model, but also one that people talk about. And your your chronology work directly addresses this. So what can we say about that?
SPEAKER_00Just because you've got tight layers doesn't mean that it's a varve. When you use that term varve, it has a particular time scale associated with it, as as you suggested, Jesse. And there is also actually a cyclicity necessary in that. So sometimes at certain periods of time, maybe the layers are going to be more pronounced, sometimes they're going to be less pronounced. So this is actually essential to prove that something is a VAV. So you need to prove that there's cyclicity and you need to know how quick these layers are deposited. So my work has been looking at this depositional rate. And uh so we've kind of gone to different biffs and we found these kind of cryptic ash layers inside BIFs. And so we dated uh at high precision these ashes, and we found that the general depositional rate on a general scale is 10 meters per million years. And that depositional rate is similar to kind of uh marine sediments, uh typical marine sediments.
SPEAKER_02Um if these are VARs, it would need to be way higher than that. I mean, it may be kilometers per million years.
SPEAKER_00I mean Yeah, I mean the deep yeah, it would for the for the centimeter scale. If one silica layer and iron layers, that's one year. Yeah, I mean, then you I mean it it's extremely fast deposition rates, much, much, much faster than that. However, we found cyclicity inside the base. So we found that there are regular alternations, and these regular alternations kind of are uh are more expressed and less expressed, as you would kind of expect with varves, but the time scale for them is quite different. I mean, it's not on the yearly cycle where we're suggesting actually that there is the influence of Milankovic cycles, and these are in the kind of thousands of years. Okay. So that that means that there's a climatic influence on the city.
SPEAKER_02I'm trying to convert your rate, would you say 10 meters per million years? 10 million years, yeah. So how how thick or how what is the time represented by any individual layer of church or iron in that?
SPEAKER_00So we think that I mean, kind of like that at that kind of ten centimeter, four, or ten, twenty centimeter, this kind of scale. We're speaking about something that's around about twelve thousand years. Oh, okay. Something that kind of scale.
SPEAKER_02Okay, so we're talking about a thousand years for every centimeter layer, roughly. So somewhere in that in that bulb range.
SPEAKER_00Okay. And so cool. Our model involves like deposition, like just as as we were discussing before, the layers are kind of raining out of the oceans. But then there's also transformation of the sediment water interface to time. Kind of move around some of the elements essentially to create the final layering structure and create the final geochemical structure of the banded out formation. So there's kind of uh a change in iron deposited, more iron deposited, less iron deposited in general. And then there is kind of some interaction with um with bacteria at the sediment water interface to make the final structure of the banded out formation, potentially.
SPEAKER_03Okay, so on that, we already talked about how biffs are indirectly induced by things like stromatolite cyanobacteria, because they produce oxygen, but they're not like involved microbially with the formation of bifs. However, after deposition of the iron oxides to the seafloor at that sediment water interface, there's bacterial like redistribution into layers. Is that what we're saying?
SPEAKER_00So what we say is that there is some, I mean, because there's this this each if you look in detail at the geochemistry of the banded iron formations in these kind of packages of iron oxide and silica, and sometimes a little bit more um detrital material. If you look at the geochemistry of those in a kind of a profile sense, you see these variations in different redox sensitive elements. For example, sulfur. And so we we did a little bit of modelling that said, okay, to create this profile that we see of sulfur, for example, and the profile of iron oxide, um, or Fe2 plus and Fe3 plus that you see in the rocks, our model says that okay, you need to have kind of constant silica deposition, then you have to turn on and off iron raining down into the system. And then at certain time periods, you need to have microbial interaction that actually creates layers of uh essentially pyrite. I mean, because microbial interaction can can produce pyrite, which then will spike up your sulfur concentration. So there's these interactions between the biological activity that's happening in the sediment water interface with the various available ions, and that they can and cannot do their thing with reduced or oxidized iron at the same time. So we actually require this on-off thing of reduced and oxidized iron, and then you need this biological activity to kind of move some of the elements around, and then in the end, at the kind of time scale of uh of processional cycle, which in our model allows us to turn on and off the iron, deposit uh deposition, okay, you end up with the structure that you see in the banded iron formations. So I mean it's a it's it's not a model that's accepted by everybody.
SPEAKER_03That's so it's relatively new. How would Milankovic cycles like these are like orbital cycles and changes in Earth's orbit, tilt, wobble? How would that affect banded iron formation deposition?
SPEAKER_00Yeah, I mean it's a great question. I mean, uh that these things are so much more influential in the general surface environment than maybe people give them credit for. In our model, we have a little bit of evidence that the Milankovic cycles, they actually allow you to have essentially a more runoff at certain time periods. And if you have more runoff by runoff, I mean kind of input into the ocean from the continents. And that allows you to bring in nutrients into the shallow marine environment. And if you bring in nutrients into the shallow marine environment, whether that's like an upwelling or whatever, or coming in off the continents, you actually create cyanobacterial blooms. And that creates more oxygen, creates more deposition of oxidized iron. And so we suggest that we we see the cyclicity of that. And we say that maybe um one way to do that is to kind of have weaker or stronger monsoonal periods. Because we know that monsoons are something that are associated with well, they're they're they're influenced by Melankovich cycles. So in certain orbital configurations, you have stronger monsoons, and that means you have more runoff. You have more rain on the continents, bigger rivers, and you're bringing in more sediment into the shadow marine environment. So we kind of basically imply a similar type of process. Interesting. That's kind of how that's that's the model.
SPEAKER_03I mean, it's it's a little bit arm wavy, honestly. So my area of research back when I did, you know, more geochemical work was in reconstructing past oxygen oxygen concentrations using redox proxies, you know, looking at specifically like molybdenum sulfides and stuff. So I worked a lot with these sulfur bacteria, these sulfur-reducing bacteria and learned a lot about them. But have you heard of the Canfield ocean hypothesis? Yes, or like this idea, or yeah, my understanding of it is like it's this idea that when oxygen, you know, rose in the atmosphere, there was a lot more oxidative weathering of continental materials and things like sulfate went into the ocean, creating this big increase in activity of things like sulfate-reducing bacteria that utilize the sulfate, produce sulfide, and produce a lot of iron. And it's this idea that instead of you know the biffs stopping forming because they used up all the oxygen, the ocean rather kind of transitioned into this really sulfitic, this deep, deep water sulfitic mode because of all this oxidative weathering and input of sulfate and production of sulfide. And so I'm curious, given that this clearly continental input played a role with the layering or potentially how sulfide and sulfur, you know, would have played a role in that. Because you're talking about how it would have increased oxidized iron deposition, but would it have also increased like iron sulfide pyrite deposition?
SPEAKER_00So, yeah, sulfide. I mean, so our model, I mean, it's not it's not the same as the camp-field ocean model, but it it's it's based on something called reactive transport modeling. I don't know if you know about this. I mean, it's it's essentially modeling of different redox reactions that are possible in a sediment water column, essentially. And you can put inside this model organic matter and break that down, basically. And so then you can also impose different redox gradients in the model and then basically see what happens to the different elements if you leave them for a little bit of time. So that's basically what we did. And so if you add a little bit of organic matter into the sediment water interface at certain periods, you actually generate a lot more sulfide in the sediment water column. And so then that matches, I mean, we you get this profile that matches what we see in the rock record, essentially. So that that was kind of our the the reason behind our model. Because it's kind of based on observing the variations in the rock record and matching those variations with a model. So I mean, if the if those variations are all secondary, we're completely screwed. I mean, because our model is matching something that happened after the deposition. Um, but we we we kind of suggest that because we see this cyclicity in there and it's it it's very regular and it's querying on a periodicity that we kind of with our geochronology can show it's related to Milankovic cycles, it gives us a little bit more confidence that this process could be original. And like I say, we we to do this work, I mean we've we s we looked at loads of BIF and we kind of studied short core intervals. So I mean, this kind of stuff you don't see it everywhere. So I don't know how applicable it is to all BIF, but we kind of suggest that this could be an important process.
SPEAKER_02Uh can I clarify one thing here, Josh, just to to for the listener? Because I think that the there's a little you we could get very confused very quickly with the processes. But you're saying in your mind, secondary is like overprinting or upgrading. That's after the rock's been formed, that's a billion years later, fluids moving through and moving iron around. That's secondary. That's secondary, yeah. And that's pretty unlikely to form regular intervals uh that you're you're describing primary stuff would be both deposition, dropping out of the ocean, and also stuff happening at the sediment water interface. Basically, everything before it's a rock would be kind of primary in your mind. Is that right?
SPEAKER_00Everything before it's a rock, yeah.
SPEAKER_02And and the reason I'm speculating a little bit here, but the reason that you would say that uh the cyclic the cyclicity type stuff is probably primary is because it's much easier for cyclicity to be formed in a primary setting rather than like post-depositional hydrothermal fluid flow is very unlikely to create cyclicity that is like layer parallel. It's more likely to be related to faults and groundwater movement and massive overprinting rather than creating cycles in it. Is that is that fair?
SPEAKER_00Yeah, I mean that's that's exactly right. I mean, uh this this like super detailed stuff we're getting into here. I mean, we showed cyclicity in small sections of BIF. Yeah. But um our previous work showed on on a larger scale, on the meter scale, like a kind of four meters to twenty meters, that there is cyclicity at that kind of scale as well inside a big package of banded eye information. And this cyclicity is consistent over the entire bif deposit. I mean, we're speaking about hundreds of kilometers. I mean, this this same cyclical pattern you can follow for hundreds of kilometers.
SPEAKER_03That's also Milankovic related, do we think?
SPEAKER_00So, yeah, we use that as like evidence that it's Milankovic because you can't do that with like local faulting or local environmental stuff. I mean, you're not gonna have the same exactly the same thing for I mean 150 kilometers. It doesn't make sense.
SPEAKER_01Wow.
SPEAKER_00That's really so yeah, we kind of mapped this stuff, I mean, in multiple places over a whole basin.
SPEAKER_01Wow.
SPEAKER_00And showed the psychistic at the large scale and then went into the small scale.
SPEAKER_03That's really amazing. We've talked a lot about the layers, but I don't want to use up all our time on the layers. So the next one I was gonna hit on, because I think it relates to kind of, you know, we've already kind of set up the fact that there's really, really old BIFs. There's, you know, the superior type that begins around 2.5 billion years ago, stops forming around 1.8 billion years ago. Um, that's my 2014 textbook understanding, but correct me if that's been updated. Um, and then we see ages we talked about earlier for like 1 billion years ago, but that's all secondary stuff. All the primary formation was the older 1.8 plus billion-year-old biff formation. So the question that a lot of viewers had is why don't these superior types of biffs form anymore? And is it true that they are truly quote unquote extinct rocks? Are there modern analogs to biffs?
SPEAKER_00Well, I mean, uh, so biff formation, as we kind of I mean, in really in the weeds, in the details, maybe there's a lot of complexity in there and exactly how you get this layering or whatever. But if you stand back really far away, what you need is a reduced ocean, reduced atmosphere, and then you need to oxidize the system. And actually, your reduced ocean, you need to have had it for a long time so that you can kind of fill it up with a reduced iron. Yeah. That's what you need. It's a it's a chemical reaction. On the earth nowadays, I mean, the oceans are oxidized. Even in the deep ocean, I mean, it's mostly oxidized. So you don't form bif. But I mean, there are weird places where you've got these anoxic lakes. I mean, you have some in kind of Canada, like in the like northern climates where you've got like maybe a little bacteria, uh, like uh organic material ended up in the lake. You made it like anoxic, and um, it's hung around for a long time, and you can get a lot of iron in there. Then you can start depositing things that are a bit like biff. There's one apparently in Indonesia that I looked up. I don't know much about it, um, called uh Matano Lake, which is like a kind of a steep stratified lake with uh anoxic bottom waters, and it's actually iron-rich at the bottom, and it's actually actively precipitating iron and silica. So you can have like these natural environments.
SPEAKER_03On a local scale, it's possible with anoxic conditions where oxygen gets, you know, there's some sort of influx or flow of oxygen in there, and the iron gets oxidized periodically, and then that pours. But that's all local because globally we had our reduced planet, yeah, we oxidized it, we're done.
SPEAKER_02Yeah, yeah, okay. We're done. There's I think the what is the youngest Biff? I think it's Cambrian. There's a little bit of biff in the 500 and 520 MA or something like that. I can't remember the exact ages, but I think Josh, that's a good point. I hadn't thought about that. The lakes. I mean, any everybody in our department who studies protozoic stuff goes to these, like there's one in uh I think it's in upstate New York, Green Lake, or maybe it's actually Pennsylvania. I don't know. One of these. That's again one of these really weird settings. It's like a small little lake, super deep at the bottom, kind of becomes a modern analog for for what the oceans look like back in time. So they are extinct at the big scale. At the big scale, they're extinct. That small little local stuff, we could still get something.
SPEAKER_00And I mean, if they came back at the big scale, it would be very bad for us. We don't want them back. We don't really want them back.
SPEAKER_03Although it is frustrating as somebody who studies really ancient Earth that you have to use these tiny lake analogs that we don't have about it. But it's good for you know us to breathe.
SPEAKER_00So yeah, it's better for us to breathe.
SPEAKER_03Um actually, one viewer did ask would it be possible for Earth to go back to the conditions necessary for BIF formation? So could Earth go back to being fully reduced? Could oxygen all just you know up and leave?
SPEAKER_00I mean, it's possible you could do it. I mean, uh, but I mean the only way you're gonna do it is with, I would imagine, like a massive moon-style meteorite impact. To like resurface the earth, this kind of thing. I mean, you've got to kill everything, all plants, yeah, all cyanobacteria. Yeah. Yeah. So if you if you have like another moon forming impact, I would imagine you'd then after that kind of resurfacing. Be in a situation after you have left your oceans around for a long time. Possibly if life comes back. Because you also need to then produce more oxygen. I mean, you you've got to you can't just have anoxic, you've got to then become oxic again.
SPEAKER_03Yeah. Wow. That's interesting to think about. Oh, there was one question about biffs and cap carbonates. How are they related? Are they related? I guess you can define what are cap carbonates first.
SPEAKER_00Okay. Um so now um we're getting into a different type of bif.
SPEAKER_03Right.
SPEAKER_00Um, if we start speaking about cap carbonates. Cap carbonates. These are kind of famous, made famous because they're in the neo-protozoic. Essentially, we've got we've had these two very massive climate change events where the earth went into a kind of snowball state. Full glaciation. And so that creates this really crazy environment. It's really, really weird and and and quite cool, I guess. Um, these global snowball earth events. And so they're they're happening in the Cryogenian. So this actually allows you to kind of disconnect the ocean from the atmosphere because you cover the whole ocean with an ice sheet. And so then you stop the you stop oxygen from the atmosphere being able to get into the ocean. And so one of the things that happens in this situation, if you leave it for long enough, you can then use up all of the oxygen that's free in the ocean and then start to uh accumulate dissolved reduced iron in there again. So if you leave it for long enough, you can actually then have quite a lot of reduced iron in the ocean. And then if you get out of this climate state, you can kind of create a very intense greenhouse atmosphere. So then you kind of melt all of the glaciers and you end up putting oxygen again into the oceans, and then you dissolve and then you precipitate biff. Then you've kind of deposited these biffs under glaciers, and then right after that, all the ice is gone and your kind of equatorial regions are suddenly super hot again. So then you just precipitate loads of hot water carbonates, loads of them, a massive amount, right on top of something that was previously deposited under a glacier.
SPEAKER_02See you built up CO2 in the atmosphere that then as soon as the the snowball Earth is in the atmosphere ocean interaction again, you dissolve all that back in the ocean and don't.
SPEAKER_00Yeah. Cap carbonates. And so the funny thing is, is you've got this cold water deposit of the bif followed right, I mean, sequentially, by a hot water carbonate.
SPEAKER_01Yeah.
SPEAKER_00And that carbonate, then after the carbonate's all gone, the earth goes back to a little bit more of a normal environment. And so the cap carbonate is kind of one of the last things from the direct product of this kind of snowball thing. So that's kind of the relationship between the two. Again, they're like they're not interbedded, like the I mean the same thing with the stromatolites and the bif. They're they're clearly related, but it's not like biff is making carbonate all the other way around.
SPEAKER_03Yeah. Okay, amazing. So that was kind of a a small scale, well, global scale, but short time period example of when Earth kind of went back to this biff forming conditions. Really cool. Okay. And next question, I'm curious, or well, viewers were curious. Speaking of modern analogs of BIFs, is the formation of deep sea like polymetallic nodules, these iron and manganese oxides, at all similar to biff formation? And how is that related?
SPEAKER_00Yeah, I mean, that I I guess. We can be quick with this and we just say no. There we go.
SPEAKER_03Because there are iron oxides involved.
SPEAKER_00So how I mean, but it's, you know, associated with other metals and I mean no, we've got to think I mean the this is the the peop people whenever they see like I I think Jesse spoke about this. Uh you immediately think of bif and then you try and create the link with bif. These mm manganese oxide uh nodules on the on the sea floor, they're formed in really different environments. They they're they're they're deposited super super slowly out of the out of the water column essentially. And they've got to be made in an environment that's got such a low depositional rate that these things are not covered actually. I mean they they take millions of years to grow, yeah, but they don't actually have any sediment on them. So I mean there's been millions of years with with kind of nothing landing on them. So they actually precipitate naturally out of the water. So they're kind of leaching metals out of out of the water itself, which is kind of low concentration of these various metals.
SPEAKER_03Yeah, and they're kind of yeah, it's my understanding too. It's it's more of a I mean, it's a very open ocean, well-oxygenated system. It's not like it was anoxic and then got an influx of oxygen. It's you know, sitting there fully oxygenated and over time slowly manganese, iron, and other metals precipitate in these nodules.
SPEAKER_02Maybe this question is a good opportunity to highlight some of the complexity that we're talking about with Benad formations is related to the fact that the the precipitating minerals out of solution is a really complicated process. It's not just as simple as iron two plus dissolve, iron three plus drops out. I mean, that's a really simple form of it. But there's also this model, I'm curious, Josh, your thoughts on it. The model for bandit iron formation where you have is it greenolite as uh is the iron silica hydroxide or oxide? I can't remember if it's which one it is, but that you actually have direct precipitation of iron three plus or iron two plus, excuse me, iron two plus and a silicate mineral. So iron silicate mineral that is that stability field of that particular mineral overlaps in some ways with iron two plus dissolve. So it's an insoluble form of iron if you match it with silica.
SPEAKER_00Yeah, I mean the the greenolite story is kind of cool, actually. I mean, it it's something that's been observed with with a TEM. And TEM is one of our like our most small scale microscopes, essentially. It's looking things at the at the almost atomic level. I mean nanometer scale, you're looking at nanometer scale of green light in the colour. Green light is being found in in fifth. And so people have suggested because it's a kind of a combined a combination of iron and silica and oxygen, I mean, maybe that's actually the primary thing that's being deposited, and then you get the separation after the deposition of a kind of a greenolite gel type of thing.
SPEAKER_03Oh my god, that's so wild.
SPEAKER_02I mean, I guess I don't what is your take, Josh? I my understanding is this is not this is uh uh maybe an N-member model, maybe not the most widely accepted one, but potentially viable. I mean, what's your take on it?
SPEAKER_00Yeah, I mean I don't necessarily know. I I mean there's it's it's mostly being pushed by one particular research group, or maybe one or two research groups. I don't know, I don't mind it personally as a as a as a as a primary precipitate. I mean I I think it's a lot of sense to to having something that's actually iron and silica precipitating up.
SPEAKER_03Lots of questions still, lots of questions.
SPEAKER_00Very open research field, which is what I what I wanted to say actually, I mean there was a few questions I I I didn't actually I didn't mention before, but there's when we speak about bif, I mean we're really most people are they're going on about okay, silica, minerals, some church or jasper, and then iron oxides, either hematite, magnetite. But that that's I mean that's quite a simplistic view of what a bif is, even these superior types. There's a whole bunch of other stuff in there as well. I mean, quite a lot of the well, not a lot, I mean some of the bifs have the silicate minerals in them like uh rubicite. I don't know if you know what that is. It's a um it's a sodium-rich amphibol, probably secondary, secondary thing. It's really beautiful, it's kind of like a blue uh purpley colour, really beautiful. But alteration of that is what's it called? Crocodilite. Oh yeah. Do you know what this that is asbestos? Yeah, okay. So actually a lot of the world's asbestos comes from banded eye information. And yeah, I mean, so actually there's it it's it can be a little bit dangerous in Biff. Yeah. Um there's also tiger's eye. Tiger's eye, the famous gemstone, I mean semi-precious stone. That's actually a hydrothermal or silicate, it's basically a silicified version of that asbestos.
SPEAKER_03Oh, oh, I never knew.
SPEAKER_00And so tiger's eye also comes from Biff. I mean, so there's there's a lot of cool stuff in bif. Well, actually, there's also asbestos. So I mean people should be a little bit cautious.
SPEAKER_02You just don't grind it up, you know. Yeah, yeah.
SPEAKER_03Actually, you you bring up a good point because I forgot to ask this earlier, but a lot of the questions have to do with well, the composition. And we talked about how there's the silica, the chert bands, simplistically, and there's the iron oxides, whether it be hematite, magnetite. And so at a simple level, I've often seen, and my viewers also had came come across uh sources that said the red layers are actually not the iron oxide, they're the chert, and the gray layers are the iron oxide. Is that true? And why is that?
SPEAKER_02So it's true as well. This is I just I just need to highlight that this is the this is the Michigan one, because I'm from Michigan, I have to say the Michigan one from Jasper Knob. That that uh the Ishwiming iron formation and banana formation is the classic one. It's like the museum one you'll see. That's the red and silver. Yeah, red and silver, red and silver. Uh, and the the red is Jasper. It's quartz. I mean, it's probably Chalcedni, some mix of of quartz and chalcedony, um, with little iron pieces in it.
SPEAKER_03Okay, that's what I think. So that's only one type.
SPEAKER_00And that yeah, I mean, there's all kinds of things. You I mean, hematite can become red.
SPEAKER_03Right.
SPEAKER_00So, I mean, you can have kind of thinnish red bands that are iron oxide. You can have chert that's pure white. And so you can have chert that's red. You can have your iron oxide that's red, you can have your iron oxide that's kind of grey or silver. Yeah, yeah, yeah. Um, so it's it you actually can have all kinds of different colours. So it's not just this is why banded iron's such a famous rock.
SPEAKER_03Brings me to one of our last questions, which was like, how do you recognize it in the field? So it sounds like it's less good. It's it's there's a lot of ways that it can look. So, how could one recognize Biff in the field? What's the simplest way to find a biff?
SPEAKER_00Find Biff? Well, I mean, um to look up where it is.
SPEAKER_03I've got it.
SPEAKER_02I've got an answer. What do you what what do you have, Josh?
SPEAKER_00Well, yeah, I mean, uh the best one is look up where it is and go there. That's a very good one. Go to the museum. Look at the label. I mean, look at the label on there. Oh, that's biff. Okay. Yeah. Um so yeah, I mean it's super it's super dense. Yeah. I mean, obviously, magnet is a pretty good way to help you identify whether it's biff and it should have some layering and it should have a lot of magnetite in there. It's it's really dense. I mean, it's mostly iron and and and silica.
SPEAKER_02Um it's really hard. I think that's the point, Josh. It's really dense. That yeah, that's gotta be the answer. Because all the other layered rocks that we think of, stromatolites, you know, that's carbonate. That's light stuff. I mean, less dense. Sediments, very sediments, you know, even if you get some like layered hydrothermal quartz feature, you know, phenomenon, that's gonna be a lot lighter than anything that has iron with a mass of 56 sitting in it. So that this stuff, it's so it's just hard. I mean, it and I think it's it takes a little bit of a tuned skill to figure out what's a dense rock versus a not dense rock, but like it's pretty obvious when you hold in your hand. Like a fifth-size rock of biff is is heavy seriously.
SPEAKER_03Iron, yeah, iron rock with next to like quartz, and yeah, yeah, so heavy and layered is what we're looking for. Okay, okay. And then the final question, and I know we're we're over time, but the last question is are there extraterrestrial biffs or could there be biffs on other worlds?
SPEAKER_00Yeah, I mean, it's it's uh like a very speculative question for me. I mean, uh why not? Sure, why not? Why not? I mean, um the thing is we know we have an idea how you can create a biff, right? I mean, it's not something that has to be that that's unique to Earth. I mean, it's chemistry. So if you have the same conditions, there's no reason why you should not have BIF somewhere else. I can't tell you where we have one. I haven't seen another one, but um, it doesn't mean that there is not.
SPEAKER_03And we we know very few worlds that at least today are fully oxygenated. So I don't know about finding at le at least not a lot of deposits of bif, but it would I mean if we did, it would be it would have implications of like you know oxygenation at least at some point, which is really interesting.
SPEAKER_00So it would, yeah, it would it would indicate that the world has changed its oxidation state.
SPEAKER_03Yeah, yeah.
SPEAKER_00Well, which is really, really cool.
SPEAKER_03Yeah, especially because on Earth that was life driven, so that would be really interesting. All right. Well, thank you so much for answering all these questions questions, or at least discussing them.
SPEAKER_00I'm not sure I provided many real answers, but uh, but that's fine.
SPEAKER_03We love these deep dives, right? So I think people will really enjoy this. And yeah, thank you so much.
SPEAKER_02Yeah, bro. Thanks very much. Yeah, great. Thanks, Josh. Hey, that's wrap. Thanks for listening. You can follow us on all social media. We are at Plant GeoCast. Shoot us an email. We have uh contact us like on our website, planetgeocast.com. And uh please give us any uh hints, tidbits, topics you want us to cover. We're happy to hear those and dive into them. And stay tuned for more Planet Geoffrey.