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Forum » SpaceEngine » Science and Astronomy Discussions » Snowball Earth
Snowball Earth
midtskogenDate: Tuesday, 20.08.2013, 20:52 | Message # 1
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[Split from here]

Quote (HarbingerDawn)
A pretty good documentary about Snowball Earth.

It left me wondering if the geological evidence really needs the entire earth to have been glaciated. Let's say that a deep ice age produced ice caps down to 40 degrees latitude or around there. And the remaining open water was pretty cool. Then I see no reason why not icebergs could carry stones and deposit them at the equator.





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HarbingerDawnDate: Tuesday, 20.08.2013, 21:28 | Message # 2
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But what would have prevented the ice from advancing to the equator in that case? If ice had advanced that far south it would pass the tipping point where the increased albedo would produce a runaway cooling effect until the entire planet had frozen, which would only be broken after very long periods of greenhouse gas buildup.




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midtskogenDate: Wednesday, 21.08.2013, 05:53 | Message # 3
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Quote (HarbingerDawn)
But what would have prevented the ice from advancing to the equator in that case? If ice had advanced that far south it would pass the tipping point where the increased albedo would produce a runaway cooling effect until the entire planet had frozen, which would only be broken after very long periods of greenhouse gas buildup.

I'm suggesting that there could be a state where the glaciation and cooling are great enough for icebergs to survive to the equator, but not so much that a tipping point is reached. The Wikipedia page quotes sources saying that it wont happen until glaciation reaches 25-30 degrees latitude, while it doesn't seem far fetched to me that icebergs could travel from 40 degrees and reach or even cross a cool equator. Perhaps even from closer to the poles if currents are favourable and temperatures didn't fall/rise too steeply with latitude.





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WatsisnameDate: Wednesday, 21.08.2013, 06:32 | Message # 4
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Evidence for snowball earth and the extent of glaciation isn't simply stones dropped by icebergs. (If it was, then snowball earth would be a very questionable hypothesis from the beginning). There is a variety of evidences from which geologists can determine if an area was affected by glaciation, as well as the latitude of that location via paleomagnetic studies.

The problem was that although there were lots of data showing ice extended very far from the poles, none could be found at equatorial latitudes. But apparently that evidence has now been discovered (this being just a few years ago). I actually didn't know about this until just now as I looked it up, and whether or not the geology community as a whole is convinced by it I don't know either.





 
midtskogenDate: Wednesday, 21.08.2013, 07:33 | Message # 5
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The documentary was surely an oversimplification (and the manuscript was obviously not written by a scientist), and therefore failed to address obvious counter questions to the claims given.

We don't really need icebergs to carry stones from the poles either. Perhaps the land around the equator, at least above a certain elevation, had ice sheets, but the ocean was open also providing precipitation needed to feed the ice sheets. Then not only rocks out of place would be expected but also evidence of other glacial deposits and moraines. And life would still have sunshine at sea. Oceans would have low albedo preventing them from freezing over around the equator, but large continents (in particular Pangaea) could have crossed a tipping point trapping them in ice. High elevation inland areas could have been the seed which the oceans at the same latitude lack. Wouldn't this dynamic state rather produce more glacial deposits than a snowball earth with ice rink oceans that would also likely lock continental ice into a static state?

The documentary tried to make a turning point for the whole snowball earth idea out of the fact that ice, given the right circumstances, can become pretty transparent, hence allowing photosynthesis beneath it. And the proof was to be found in presumably freshwater lakes in the dry valleys of Antarctica. One thing is that lakes and oceans are different, the latter being subject to currents constantly cracking and stacking ice. While global ice rink oceans may be possible, I can't help thinking that other possibilities involving open water may assume less.





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Edited by midtskogen - Wednesday, 21.08.2013, 07:39
 
WatsisnameDate: Wednesday, 21.08.2013, 09:49 | Message # 6
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I don't know all the answers. smile I can say that evidence does suggest that the hydrologic cycle completely shut down, or very nearly so. This fits with the buildup of volcanic CO2 as an escape mechanism, and the resulting cap carbonates, massive weathering rates, and high temperature regime afterward. This in turn suggests most if not all of the ocean was covered in ice at the peak of the event.

The equatorial ocean also does not necessarily need to remain unfrozen to feed the land ice, because salt water freezes at lower temperature than fresh water. So one might imagine that the ice sheets had enough time to build before the oceans froze completely.

I wish we had some geologists on this forum; these would be excellent questions to ask of them!





 
midtskogenDate: Wednesday, 21.08.2013, 15:55 | Message # 7
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Quote (Watsisname)
I wish we had some geologists on this forum; these would be excellent questions to ask of them!

Yes, though I'm not sure if geologists would be the best to answer if the global ice rink assumption is likely.

What we really need is to find some gravitational lenses at the right distances to bend light back towards us and really, really powerful telescopes. biggrin





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WatsisnameDate: Wednesday, 21.08.2013, 20:52 | Message # 8
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I don't think they are assuming that. It took a long time and the finding of very powerful evidence just to convince the community that the snowball earth hypothesis was credible, and the question was, and I believe still is, debated as to whether or not the ice really covered the whole surface.

Geologists are the best people to ask these questions because they understand the subject, the data, and the history of the research far better than we do. They are the ones best equipped to conclude what things are known or remain to be known. I think the same goes for all fields in academia.





 
midtskogenDate: Wednesday, 21.08.2013, 21:03 | Message # 9
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Quote (Watsisname)
I don't think they are assuming that. It took a long time and the finding of very powerful evidence just to convince the community that the snowball earth hypothesis was credible, and the question was, and I believe still is, debated as to whether or not the ice really covered the whole surface.

I'm not sure if you saw the documentary, but its argument was: global glaciation would prevent light from passing through the sea ice which would kill life very quickly, but life survived. The solution to that problem was crystal clear ice which would let light through even if it is many metres thick.





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Edited by midtskogen - Wednesday, 21.08.2013, 21:04
 
WatsisnameDate: Wednesday, 21.08.2013, 22:54 | Message # 10
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Sorry, I can't watch videos until I return from vacation to faster internet. :/ But geologists have been looking into that particular question for over a decade. Early studies found that a globally iced-over ocean was not inconsistent with the continuity of photosynthesis. The equatorial ice need not be crystal clear, for it would also be fairly thin -- less than 10 meters -- thus allowing sufficient transmission of light. Newer studies bring those results into question though, so the matter is not settled.

Relevant papers can be found here and here.

So to the best of my knowledge, right now evidence suggests global ice coverage, but problems remain.

Shall this be split into a new Snowball Earth topic?

edit: Oops, did more reading and found a newer paper with conclusions contrary to what I first wrote. Post updated accordingly.





 
midtskogenDate: Thursday, 22.08.2013, 07:35 | Message # 11
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The papers are paywalled, but the first one doesn't seem to include any real world measurements of ice transparency, and the second uses the dry valley lakes as the model. The question remains, how do oceans get a lake type of ice (and a very specific kind of lake ice as well)? Speculating, I would imagine the Earth's ocean and air circulation having to shut down as open water disappears eliminating that kind of stress on the ice. And it's easy to imagine that less open water means less weather. Then, the ice must grow very slowly to achieve purity. We also have to assume that the tides aren't great enough to disturb the ice. The alternative is to explain that tropical continental ice caps can exist without any runaway cooling in the oceans. In our current ice age, excluding the interglacials, I think glaciers grow in the tropics above 3000 metres or so (Hawaii seems to have a 100 m thick cap during glaciation periods). Unless we in recent geological times have been at the brink of runaway cooling, it doesn't seem to be a huge stretch that Pangaea could have formed an ice cap extending to the tropics if it was slightly cooler. It would be interesting to see a paper discussing why that is a less likely option.




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WatsisnameDate: Thursday, 22.08.2013, 10:44 | Message # 12
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Quote (midtskogen)
Unless we in recent geological times have been at the brink of runaway cooling, it doesn't seem to be a huge stretch that Pangaea could have formed an ice cap extending to the tropics if it was slightly cooler. It would be interesting to see a paper discussing why that is a less likely option.


Actually it does seem like a huge stretch because Pangaea didn't exist until several hundred millions years later. tongue The most recent snowball earth episodes happened after (or during, as it was a long process) the breakup of Rodinia. But I must say I don't think either of us are well suited for this level of speculation.

I'm looking at the problem now from the climate modeling side. According to these researchers, climate models consistently show the vast amounts of the ocean froze over, but do not agree as to whether or not the sea ice reached the equator. Furthermore, the more detailed models are the ones with the largest gap. So in short, they agree with the 'snowball earth, slushball ocean' scenario. Caveat, though, this was from 2002 and I've yet to find more recent studies of this sort.

I have not found anything suggesting that land ice did not reach the equatorial region. That's not surprising because the evidence is pretty strong on this point.





 
midtskogenDate: Thursday, 22.08.2013, 17:11 | Message # 13
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Quote (Watsisname)
Actually it does seem like a huge stretch because Pangaea didn't exist until several hundred millions years later.

I do not claim to know much about geology, so I try to look up things before I post. But that doesn't help if I misread... Pangaea formed in the late Paleozoic era, while snowball earth happened just before that era. Thanks for the correction.

Do we know anything about precession over geological time? I tried to google, but didn't find anything trustworthy on the matter.





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WatsisnameDate: Friday, 30.08.2013, 21:07 | Message # 14
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Hey, sorry for the delay in writing back -- this last week has been hectic for me. wacko

Anyway, by precession, do you mean just the periodicity, or are you interested in the axial tilt angle as well? I don't know of any geological evidence that could directly tell us what the precession rate was (what would record it?), and I haven't seen anything in the literature that answers it either. But I suppose maybe (big maybe) it could be figured out from knowledge of the moon's orbit, since as I understand it it is the moon that drives precession.

On that note, we do have at least a reasonably good understanding of the evolution of the moon's orbit through geologic time. One avenue for figuring this out is from theory and modeling of the tidal evolution, though these are by no means complete models (the physics is extremely complex) and so the degree to which they agree with reality is probably not excellent. There is also geologic evidence (tidal rhythmites) that directly tell us the length of day, month, and year as far back as 2.4 billion years ago.





 
midtskogenDate: Saturday, 31.08.2013, 00:00 | Message # 15
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I meant the axial tilt angle. If that was larger smaller than what we have had in the past 100,000 years, that would greatly affect seasons and ice ages. Today's wobble isn't big, but it apparently matters.

I don't know what mechanism would change it, but several 100 million years is a lot.





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