Geologist here -- happy to answer any questions about this! The underlying paper [1] and the older one it builds on [2] are both open access. The rj-MCMC for the time-temperature inversion used an existing program from the thermochron community, but we made most of the figures in Julia.
I'm curious if we see other unconformities related to other Ice Ages. If receding glaciers scraped away the earth and led to this big of a gap, shouldn't this have happened again when the glaciers receded after more recent ice ages, like in the Pleistocene?
Pleistocene glaciation in the northern hemisphere has been a lot shorter (so far) than the Cryogenian glaciations, but it is probably not a coincidence that the outline of Canada's "Precambrian shield" basically matches the outline of the Laurentide ice sheet (check out Figure 5 of the 2019 paper [1]). We're probably only talking about scraping off no more than a couple hundred meters of sedimentary rock formerly covering the shield, but that's about what you'd expect.
More broadly, as one author noted long before us [2] it turns out that most of the places on Earth where there is a lot of Precambrian crystalline basement exposed at the surface today (i.e., where later sedimentary rocks have been scraped off, one way or another) were glaciated either recently or in the Late Paleozoic Ice Age [3], which hit much of Gondwana (see also Fig S16 here [4]).
More recently, another group of researchers using thermochronology in Antarctica [5] found evidence of several kilometers of exhumation during the Late Paleozoic Ice Age, as well as perhaps 1-2 km during the last ~35 Myr of Cenozoic glaciation (n.b., Antarctica has been glaciated for a good bit longer than we've been having ice ages in the northern hemisphere).
I'm going to be a bit fanciful here, but it's a serious question. What implications does this have, if any, for the possibility of human civilizations older than currently known being completely erased by the ice? Is that plausible? How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization?
> How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization?
it would take a lot. Even in the snowballs, when we had ice on every continent, there are still plenty of basins (mostly at the continental margins) where syn-glacial sediments are preserved; that is in part how we know the glaciations happened. While we have maybe 1/5th as much sedimentary rock volume per unit time prior to the end of the unconformity, that still leaves a lot!
At the time of the Cryogenian, both the fossil record and DNA-based molecular clocks suggest we didn't have multicellular animal life until after at least the first (Sturtian) glaciation. And we're talking basically just sponges (porifera) at first.
Of course, it's not impossible we could have another snowball in the far future (probably unlikely for several reasons, but never say never), and the question of what that would do to the record of modern human civilization is an interesting one.
The short answer is "I don't know", but I think it would be hard to erase all traces without something a good bit more severe than the erosion that produced the Great Unconformity.
This brings us to a question I wanted to ask: Given that a lot of rock is missing from this period, how likely is it that the abruptness of the Cambrian explosion is an artifact of missing evidence? How likely is it that we would even recognize a distinct Cambrian period, if four-fifths of the Cambrian rocks were missing?
Ah, good questions. Although we are missing a lot of rock, paleontologists have spent a lot of time looking through the non-missing parts. So far, they have found a lot of really interesting things, such as the macroscopic soft-bodied fossils of the Ediacaran biota [1], but even so there's really very little in the way of shelly fossils until the Cambrian, and their appearance really does seem to be quite sudden as far as we can tell!
I’m under the impression that we’re still in the quaternary ice age. What makes a snowball impossible now? The division of the ocean in two that restricts convection and the large polar continent that provides a convenient place for ice sheets to increase the Earth’s albedo both suggest, to my admittedly limited understanding, that a severe glaciation isn’t out of the cards. I ask not to challenge but to learn.
Edit: I can’t remember the name right now, but there are also a bunch of solar cycles that affected past glaciations. I’d be grateful for information on that too.
We are still in an icehouse period, yes! However, a few things may make it a bit harder for that to develop into a full snowball, currently:
1) While having continents at the poles makes it easier to have icesheets at all, it also makes it harder for them to grow into a full snowball. This is because
a) covering the continents with ice shuts down silicate weathering (and silicate weathering consumes CO2, so that's a stabilizing negative feedback)
b) the difference in albedo between water and sea ice is greater than the difference in albedo between land and ice. So if you can get cold enough to start making sea ice at the poles, you should get a stronger positive feedback of cooling -> higher albedo -> more cooling
During the Neoproterozoic, most or all of the continents seem to have been near the equator, so silicate weathering could keep going until sea ice reached the "point of no return" of the sea ice-albedo feedback [e.g. 1]
2) The biosphere is pretty different today than it was last time we had a snowball, and there is some reason to think that evolutionary developments like land plants and pelagic calcifiers may make the climate system more stable than it was 700 million years ago.
None of that is to say it's impossible though! The solid earth acts slowly, but it's a big lever, so hypothetically if you could somehow crank silicate weathering up high enough and volcanic degassing down low enough, you could probably still in principle reach the tipping point again.
For your last question, you are probably thinking of Milankovitch cycles [2] -- those are definitely going strong as well, though generally not strong enough to get us into or out of a snowball state.
That certainly doesn't help either! To some extent though, anthropogenic emissions are dangerous more for their rate than their absolute magnitude; in the long run, once we stop emitting, silicate weathering will take back over "soon enough" -- it's just that "soon enough" in this case means ~5 myr and probably a mass extinction later.
The other one I forgot to mention is that the sun is a bit brighter now than it was 700 Myr ago (by perhaps a few percent). Go back another two or three billion years to the Archean and the difference would have been bigger -- to the point that we have some trouble explaining why there weren't a lot more snowballs back then [1]
Is it possible from geological evidence to confirm that Sun was dimmer billions years ago? I am asking as stability of Earth orbit cannot be taken for granted for such periods. For example, we could underestimate the effects of solar wind in past that could have pushed Earth, or the could be an interaction with passing close stars.
Ah, so there are many things that it is hard to be absolutely certain of in geology, but changing Earth's orbit is at least very very hard; even the kinetic energy from things like the Chixulub impact are far too small to have a significant effect. The "moon-forming impact" in the most common model of the origin of the moon might be more on the right order of magnitude, but there don't seem to have been any of those more recently than about 4.51 Ga. An astronomer could say more, but solar luminosity is also relatively well understood from studying other main-sequence stars of various ages.
The most common solutions involve high concentrations of organic greenhouse gases like methane as well as high CO2, but it's always possible there are other possibilities that have not yet been considered.
We do not have good models of the rate Sun has been losing hydrogen especially on scale of billions of years. So from that we do not have a precise answer how heavier was Sun in past. But heavier Sun implies that Earth was closer compensating for the dimmer younger Sun.
As I understand according to the current estimates this not enough to avoid the cold Earth problem, but there are way too much uncertainty. But if we do not have way to read the brightness from geology alone, that can be an answer.
I have to say I was thinking to myself this person identified as a geologist, so an event that started 200 years ago probably isn’t a major concern. In the sense of geologic timescales I mean.
Thank you! So I get it that glaciers eroded miles of rock from the surface. But where is all that material? Was it moved to the mounds elsewhere? (where?). Was it ground into gravel, dust or silt and washed into the ocean?
When I was a kid I remember older grade-school science books telling about ice ages. According to the books, there had been 4 (or maybe 5) ice ages in the past, the last one was N years ago (maybe 20,000 or something), and there weren't going to be any more. And I was like "how can they know there won't be any more?".
Do I mis-remember what those books said? Did geologists actually believe something like that? I guess those books must have been from the 1970s or earlier, but still.
I imagine there have been tons of ice ages similar to last one which ended only... 10k years ago? Where I come from, the mountains were cut pretty deep even from this last one which was pretty insignificant on long-term scale. And of course all previous ones left their marks, but the deep scratches were mostly removed by later ones.
Maybe its about defining what a news-worthy ice age means. It for sure shouldn't be just snowball Earth type when most of the planet apart from a bit of equatorial places froze solid white.
I'm not aware of any time when that was a mainstream belief, but there might be a gap in my history-of-science knowledge.
In any case, while we're currently in an "interglacial" period with respect to the Pleistocene glacial cycles, we're actually still in an "icehouse" period by the big-picture definition, since there are continental icesheets on Greenland and Antarctica. By contrast, a lot of the rest of the Phanerozoic has been characterized by "hothouse" climates, where there are no major continental icesheets anywhere on Earth.
It's possible that anthropogenic CO2 emissions will prevent us from going back into a colder "glacial" period when we otherwise next would have ~80 kyr from now, but geologic time is long and there is a lot of it still in front of us.
Noob question but are these layers in general built up from sediments eroded mostly from higher ground, volcanic activity, or both (or something else)?
In any case, if glaciers rapidly eroded kilometers of rock on a continent-wide or global scale, could evidence in similar layers be found for all that sediment when it is deposited by the glaciers? And if so, might that look different expected by the other theory (tectonic activity)?
Ah yes, quite right on the first part! The catch is that to be preserved, those sediments have to be deposited somewhere else on the continental crust, say in epicratonic seas [1] or in subsiding basins [2]. If the sediments wash all the way off the continental shelf and onto the ocean crust, then they'll ultimately get subducted into the mantle!
The catch with glaciers is that putting a lot of big glacial ice sheets onto the continents takes that water out of the oceans, and lowers sea level -- meaning a bunch of places where you could normally preserve sediments on the continents will be above sea level during the glaciations (AKA above "base level"), and more of those sediments will be washed off the continents entirely. There still some places where you can find the fossilized glacial till from these Cryogenian glaciations (which is in part how we know they happened), but it's basically only in the tectonic basins at the margins of the continent that were subsiding fast enough to stay below base level and not get eroded away.
Thanks, great answer. So it's not that sediments in the ocean can't be found so much as they don't stay around that long you'd find enough of it to be able to account for that massive glacial erosion event, due to tectonic movement?
Yeah, the oldest ocean crust on Earth today is only about 180 million years old, and the older it gets the colder and denser (and thus more prone to subduct) it gets. And when ocean crust subducts it generally (with some caveats) takes the sediments that have accumulated on top with it.
So we still have ocean crust from when T. rex lived, for instance, but none from when trilobites lived, and definitely none from the time of these glaciations.
I didn't know that the colder and denser ocean crust gets it thus is more prone to subduct. Does that also imply the denser crusts subducts deeper towards the earth center or does the colder crust prevent it from going deeper? Or does the angle of friction of ocean crust with the underside of the continental crust dominate the angle of subduction deep into the mantle?
How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc.
Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know.
Older ocean crust does indeed generally subduct at a steeper angle! Relatedly, if you have ocean crust that is relatively young and hot enough but is still colliding with continental crust, then you can get "flat-slab" subduction [1]
Once the crust gets deeper in the mantle, eventually metamorphic phase transitions become the most important factor. In some seismic images, it looks like subducted slabs make it all the way to the core-mantle boundary (where they may contribute to the source of new mantle plumes).
To create that simulation I would need to team up with scientist who have enough credibility, reputation get the funding for programming that simulation (see my other comments on this thread). My part, the open source code would cost $25K to create. Collection and readying all the geology, all paleontology and part of biology science papers can partially be done with machine learning like Google's scholar search engine until you get a list of all relevant papers that humans must parse to find the timeline and spatial movements of rocks and materials we know about. A nice job for students.
Still, it would be a cheap research project with big science returns like the Hubble Ultra-Deep Field, Dark Energy Survey or Sloan Digital Sky Survey did for cosmology.
Thank you for your answers, I now have a few weeks of science papers to read and follow up on.
> How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc. Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know.
The title and start of the article make it sound like it was a complete mystery that was solved now.
However when continuing to read the article it appears that there were competing theories to explain it, and the new research has bolstered evidence for one of the two main theories.
Am I correct in assuming that the latter is a more accurate assessment of the subject?
Short answer: it's complicated, and not a binary, but perhaps a gradual process of "becoming less of a mystery". The history of the subject is actually pretty interesting, and like most things in science gets more nuanced the more time you spend on it.
Do we have (open source) data or simulations that trace the paths of rocks and other material masses through time and space? You mention that we can account for 1/5th of the sediments of eroded rocks from this periode between 1,7 billion and 550 million years. Do we have accounts for part of the 4/5th that have turned into sediments deposited elsewhere. You already answered this, my question is more wide in scope. Has science put all those paths of material through time and space into a single database or simulation yet? For example, the Cambrian explosion was detected by study of fossils at the Burgess Shale. We can trace the rock from that quarry back in time from today to 508 million years ago when this Stephen Formation was formed in undersea mud slides burying the animals that became fossils [1]. We can trace cratons back to there origin, map megafloods and trace the path of tectonic plates through time. If we made a list of every cubic meter of earth, mainly the upper crust of the earth, we could probably trace back all the ocean crust and some of the continental crusts. By labeling all the bits we do know about (or have theories on) we could, by process of elimination, map out what parts we know about and what we don't know yet and where we have not looked at all. The simulation would show animations of the movements and transformations of each cubic meter throughout the last 10 billion years with an margin for error of the data at each point on the timeline. It would look like the plate tectonic simulations but at a much smaller scale and linking all the scientific papers related to this cubic meter of material through time and space.
This would suggest where to go look for new histories in the rock and where the edges of regions are where we could find new unconformities.
I'm eager to write the simulation/database software to do this mapping in time and space.
I would love it, but no one has quite managed it yet! There are simulations of, e.g., plate motions, but even those get quite uncertain if you go back more than about a billion years.
Forward models of the flow of the atmosphere, oceans, or even the mantle all exist, but with harsh tradeoffs between resolution and how much geologic time you can simulate. For this reason, we have coupled atmosphere-ocean models, but not really atmosphere-ocean-mantle models (because the timescale of mantle convection is just too much slower than the other two). I would really love to see more integrated whole-earth-system modelling, and that's probably something I'll try to work on in the future, but we may have a long way to go!
The way to integrate all these simulations at different time and spatial scales and different models would be at a meta-model or (lack of a better word) meta-simulation level.
That is what I propose. It would allow your future research to retrofit in the partial simulations we have today.
I think it requires cross-disciplinary collaboration between my kind of 'computer science' with mostly geologists, paleontologist and a few planetairy scientists, cosmologists and evolutionairy biologists.
Maybe documentairies like Cosmos would fund part of it for they also make many such animations.
How did the erosion affect the entire planet so thoroughly? Is it plausible we’ll someday find a place where the geological record of the missing years is still intact?
Ah, so while there were ice sheets on every continent during these glaciations, there are actually quite a lot of places where sedimentary rocks survived the glaciations - perhaps as much as a fifth of the area of the continents (back-of-the-envelope, given that there is about a fifth as much preserved sedimentary rock per unit time prior to the end of the unconformity).
Generally a lot of these regions where rocks from the "missing" interval are well-preserved are at the margins of the (paleo)continents. This is for two reasons, as far as we can tell: (1) erosion by continental icesheets is more hit-or-miss near the margins; "hit" if you're in an outlet ice stream, but "miss" if you're not, since marginal ice tends to be "cold-based" and generally not very erosive at the margins outside of the outlet ice streams; (2) the paleo-continental margins are where all the tectonic activity was at the time -- and while tectonic uplift won't help any, tectonic subsidence can help a lot if it makes a basin subside below sea level (which will protect against erosion).
cbkeller, thank you so much for your answers!
Its rare I read such good answers on the grand scale implications of someone's research.
When you study plate tectonics, watch the Earth Story documentary [1], read Earth System History [2], Five Kingdoms by Margulis [3] or the books of Steven J. Gould [4] you get an overview of the history of planet Earth that geology, paleontology, biology and astronomy sciences pieced together in the last 150 years.
But then you never hear about the new breakthrough research that happens after these publications. But now I have learned about a new mayor piece of the puzzle by these comments of cbkeller [5], thank you very much!
If there is anything I could assist with, like writing complex (simulation) software in just a few thousand lines of code [6] for your research, you can reach me at morphle at ziggo dot nl.
it's notable that the missing time period is longer than the entire time period since, including basically all of known evolution. immediately prior to the discontinuity, there is only very simple life, and immediately after, life is significantly more complex, but also highly constrained, as after a mass extinction.
i just like to think about this sometimes. it brings me a deep sense of peace
Read his book or watch Feynman explain [1][2] for one of many answers.
Pondering these questions and learning about them gives joy, excitement, peace of mind and is my great hobby. So much so that I recently purchased 21 hectare of mountain for $9000 and emigrated there so I can study erosion (geology), wildlife, flowers and forest (biology and evolution) and dig for fossils in my own garden. And still read science and hacker news online.
Feel free to come and visit, either camping or residing in a tiny house on my mountain[3]. I'll play science and nature guide in the Sierra de Cazorla, Segura and other mountain ranges.
To have your own mountain just to dig it up and ponder about stuff! What an usual idea. I'll remember your comment, thank you. Who knows, maybe I'll pass by sooner or later.
[1] https://doi.org/10.1073/pnas.2118682119
[2] https://doi.org/10.1073/pnas.1804350116