The Great Unconformity: Research points to glaciers being the culprit
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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).
[1] https://www.pnas.org/content/116/4/1136/tab-figures-data and see also Fig S16 in [4]
[2] https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/...
[3] https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse
[4] https://www.pnas.org/content/pnas/suppl/2018/12/26/180435011...
[5] https://doi.org/10.1016/j.epsl.2018.10.044 (see especially Fig. 7)
> 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.
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.
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.
Are you making a point by leaving human emissions of CO2 out of your list?
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]
Up thread, there seems to be a question about human civilizations being erased by the ice.
A billion years ago.
And there was a polite reply, from said geologist, not a comment about timescales.
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.
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.
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)?
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.
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.
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.
Maybe this is the simulation.
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).
For sure, I would love to have that simulation!
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.
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).
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.
[1] https://en.wikipedia.org/wiki/Earth_Story and https://www.youtube.com/watch?v=UFcKEcyWhGQ
I'd be happy to point you to the other episodes
[2] http://libgen.rs/book/index.php?md5=A3B163282F9B5A62A585C230...
[3] http://libgen.rs/book/index.php?md5=DD4B4B5E17D924EC237155BD...
[4] http://libgen.rs/search.php?req=Stephen+Jay+Gould&column=aut...
[5] https://brenhinkeller.github.io
[6] Alan Kay lecture "Is it really complex or did we make it complicated?" https://vimeo.com/82301919
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?
Caused by glacial erosion, the "freezing" of sediment deposition by ice coverage, or both?
Are there any estimates on glacial thickness or "heat map" of thicknesses across Pannotia?
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.
[1] https://www.researchgate.net/publication/228535120_A_new_Bur...
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!
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.
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.
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.
i just like to think about this sometimes. it brings me a deep sense of peace
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.
[1] Richard P Feynman - The Pleasure of Finding Things Out https://www.youtube.com/watch?v=7XA_NVn7XnE
[2] Fun To Imagine with Richard Feynman https://www.youtube.com/watch?v=P1ww1IXRfTA
This accounts for the absence of slood.