The Great Unconformity: Research points to glaciers being the culprit

https://www.vice.com/en/article/wxdzzb/a-billion-years-of-time-are-mysteriously-missing-scientists-think-they-know-why

by cbkeller • 4 years ago

87 51 4 years ago

h2odragon• 4 years ago

Alternate theory: Earth was assembled by some subcontractor who skimped on construction costs where they thought no one would notice.

This accounts for the absence of slood.

neffy• 4 years ago

But Slartibartfast designed every single one of those Glacial Fjords personally!

74B5• 4 years ago

Let us thank the free market for his miraculous creation, amen.

ruined• 4 years ago

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

codesnik2• 4 years ago

peace? why?

morphle9• 4 years ago

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.

[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

[3] https://en.wikipedia.org/wiki/Sierra_del_Mugrón

morphle9• 4 years ago

Maybe we could explore this thought together. It could lead to new research.

cbkeller15• 4 years ago

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.

[1] https://doi.org/10.1073/pnas.2118682119

[2] https://doi.org/10.1073/pnas.1804350116

vrdabomb5717• 4 years ago

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?

cbkeller15• 4 years ago

Great question -- and yes!

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)

mynegation• 4 years ago

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?

cbkeller15• 4 years ago

Ground up and washed into the ocean -- and then ultimately subducted and made into new magmas!

throwawaylinux4• 4 years ago

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)?

cbkeller15• 4 years ago

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.

[1] https://en.wikipedia.org/wiki/Inland_sea_(geology)

[2] https://en.wikipedia.org/wiki/Sedimentary_basin

twhb• 4 years ago

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?

cbkeller15• 4 years ago

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).

ummonk• 4 years ago

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?

cbkeller15• 4 years ago

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.

errcorrectcode• 4 years ago

Thanks for sharing!

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?

morphle9• 4 years ago

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.

[1] https://www.researchgate.net/publication/228535120_A_new_Bur...

cbkeller15• 4 years ago

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!

throwaway81523• 4 years ago

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.

saiya-jin• 4 years ago

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.

cbkeller15• 4 years ago

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.

snidane• 4 years ago

Glaciers and big rocks? Obligatory Randall Carlson reference https://youtu.be/A2PzfWur6pY

The Great Unconformity: Research points to glaciers being the culprit

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