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Fraser
2005-Sep-29, 07:49 PM
SUMMARY: With the "Snowball Earth" hypothesis, scientists have proposed that our planet was once encased under a thick layer of ice and snow. Life could only survive huddled around hot vents deep under water. But now scientists have found fossil evidence of creatures that lived during this period, but were photosynthesizing. This means they needed to live under thin enough ice for sunlight to get through. It's possible that the entire planet wasn't encased in ice, instead there were large patches of thin ice, or even open water near the equator.

View full article (http://www.universetoday.com/am/publish/ever_snowball_earth.html)
What do you think about this story? post your comments below.

cran
2005-Sep-29, 10:59 PM
:D
Thank you!

Greg
2005-Sep-30, 03:27 AM
This appears to be a quality study and the results are quite believable. I never really liked the idea of a Europa-like Earth since complex life should have taken a really big hit (bigger even than it did) and yet it bounced back relatively quickly. To me this suggested a pool or resivoir of complex organisms that did quite well despite the extreme events on the planet. This implies an oasis of relatively warm habitat with full exposure to the sun.

Samantha Carter
2005-Sep-30, 05:40 PM
The news in the press release linked to by the OP is not terribly radical, and in fact only marks the latest salvo against the "hard snowball" Earth that Paul Hoffman and Dan Schrag of Harvard have been promoting since 1998. (Too bad the press release author doesn't seem to have known that...)

There is ample geological evidence to demonstrate that a hard snowball was impossible to achieve and sustain. A hard snowball glaciation would require the hydrologic cycle to be shut off (no precipitation, no running water at the surface), by the Harvard team's estimation, for several million years at a minimum. That supposition is not supported by thick glacial deposits in shallow marine settings, for which you would have to have actively moving ice sheets (lubricated by meltwater) grinding across the landscape and pushing sediment into the sea. Layers of pebbles and cobbles in deeper water sediments of that time are exactly analogous to ice-rafted detritus (IRD) associated with the most recent ice age, and that requires open ocean for icebergs to drift across and dump bits of entrained gravel as they melt.

See, for example, http://calvin.st-andrews.ac.uk/external_relations/news_article.cfm?reference=264

Paleobiologists have also long said that a totally iced-over ocean was incompatible with the the survival of life in any appreciable amount, and it was one of the first criticisms that the Harvard team was forced to address in an adaptation of their hypothesis. They had to grant that in the very least, small open ice-free areas had to exist to provide refugia for life. What the linked article touches on is the question of how large you can allow a refugia to be before you just have to admit there was open ocean.

This is an informative page: http://www.geology.ucdavis.edu/~cowen/HistoryofLife/slushball.html

Furthermore, most 3-D global climate models using reasonable boundary conditions for the time period aren't able to produce anything near a totally-frozen ocean (sea ice generally doesn't advance below 45 deg. latitude). Granted, no model portrays the physical system perfectly, but it should be yet another head's up that the "hard snowball" is problematic.

Examples:
http://www.spaceref.com/news/viewpr.html?pid=1895
http://www.giss.nasa.gov/research/briefs/sohl_01/
http://www.nytimes.com/2001/06/19/science/19SNOW.html?ex=1128225600&en=ff41b4b7c5adcc1b&ei=5070&ex=1113364800&en=0c45972937afcc8a&ei=5070 (NY Times, free registration required)

By the way, for those who haven't been in on the history of the controversy, the following links are a good layperson's intro to the Snowball Earth hypothesis.

Part 1 - http://www.suite101.com/article.cfm/everyday_geology/99898
Part 2 - http://www.suite101.com/article.cfm/everyday_geology/99899
Part 3 - http://www.suite101.com/article.cfm/everyday_geology/100705


So... bottom line for me is, slushball Earth is fine, hard snowball Earth is not. (As if you couldn't tell by now. ;) ) The really interesting questions that still need addressing are: 1) how many of these severe glacial intervals did we really have, 2) why did they happen when they did, and 3) what cause(s) led to them in the first place, and how did the Earth get back out to a more globally temperate climate?

daddy
2005-Oct-01, 12:43 AM
Edited to remove religious content.

cran
2005-Oct-01, 02:48 PM
daddy, I think you might be in the wrong thread...

Greg
2005-Oct-02, 06:13 AM
Very nice post, Samantha. The links are greatly appreciated. This is the kind of thing I like to see on the forum and try to provide when I am able to find the time.

eburacum45
2005-Oct-02, 07:07 PM
Here is another one...
http://www.palaeos.com/Proterozoic/Neoproterozoic/Cryogenian/Snowballs.html

This is another possible Earth-like world condition that we might find when we start detecting terrestrial extrasolar planets;
the early slush ball or snowball scenario might have been caused by interactions between oxygen produced by photosynthesisng organisms and methane (which is a greenhouse gas); the end of the snowball/slushball scenario might have been caused by buildup of volcanic CO2 .
If we discover a cold, largely ice-covered Earth-sized world it does not necessarily mean that life is not present. In fact the freeze might even be caused by life itself.

cran
2005-Oct-03, 12:17 AM
Excellent summation, Samantha Carter ... have you studied the so-called 'Snowball Earth' periods before (eg, 610-790 MaBP?, 22GaBP?)?



This is another possible Earth-like world condition that we might find when we start detecting terrestrial extrasolar planets;
the early slush ball or snowball scenario might have been caused by interactions between oxygen produced by photosynthesisng organisms and methane (which is a greenhouse gas); Indeed :)



the end of the snowball/slushball scenario might have been caused by buildup of volcanic CO2. hmmm >bites tongue<



If we discover a cold, largely ice-covered Earth-sized world it does not necessarily mean that life is not present. In fact the freeze might even be caused by life itself. Indeed :)

JonClarke
2005-Oct-03, 05:41 AM
Yes, there was glacial activity in the late Neoproterozoic but there is no reason to suppose a global snowball.

People like Kath Grey showed several years ago that the acritarch diversity (photosynthetic plankton fossils) did not change significantly through the supposed snowball earth period (Marinoan and Sturtian glacation) in Australia. If I were cynical I would say that Kath's research was ignored my the media because she does not work in the US. Once US researchers find this information it is suddenly big news.



Jon

cran
2005-Oct-03, 12:30 PM
As I understand it, Jon, what got the 'Snowball Earth' originators all worked up was the finding of glacial activity in low latitudes ... there was some weird reasoning that Ice Ages and glaciation had to spread from the poles ... and, if you'll pardon the pun, everything kind of 'snowballed' from there...

Why the hypothesis gained any credence at all remains a mystery to me, but it has been effectively rebuffed by many workers over the past few years (including, I believe, by one of the originators...) - perhaps a 'catastrophe-loving' media might have something to do with it?

PS. I missed a decimal point in my previous post - it should have read "2.2GaBP"

Samantha Carter
2005-Oct-03, 01:28 PM
Thanks for the kind words, Greg. I hadn't expected to be offering up any sort of insights so early in my history here, but perhaps it was fate. ;)


Excellent summation, Samantha Carter ... have you studied the so-called 'Snowball Earth' periods before (eg, 610-790 MaBP?, 22GaBP?)?
Indeed I have. Studied it, lived it, breathed it, slept it... :)


If I were cynical I would say that Kath's research was ignored my the media because she does not work in the US. Once US researchers find this information it is suddenly big news.
No need to be cynical in this case, Jon. It's more the simple fact that Kath Grey is Australian; NSF funds Americans; NSF is going to issue press releases focused on the work it has supported, not scientific results anywhere in the world. I wouldn't expect CSIRO to be issuing press releases about non-Australians' work either.

Moreover, NSF and NASA have entered a new period of mutual interest in topics astrobiological, and the debate over the nature of life during snowball Earth intervals does actually fit under that umbrella. So it's not surprising, in a way, that NSF is making a big deal about just another increment in the anti-hard snowball body of evidence.

That science journalism in the US is often little more than the parroting of press releases is another matter. (I'm not slagging Universe Today here, because it's a volunteer effort. This criticism is aimed at the major news services and publishers who have the resources to do much better.)


Why the hypothesis gained any credence at all remains a mystery to me, but it has been effectively rebuffed by many workers over the past few years (including, I believe, by one of the originators...) - perhaps a 'catastrophe-loving' media might have something to do with it?
In my opinion, the Hoffman and Schrag vision of a snowball Earth (Hoffman et al., 1998; Hoffman and Schrag, 2001, 2002) took off like gangbusters for a few reasons:
The hypothesis was huge and sweeping, and appeared at first glance, especially to those unfamiliar with the details, to explain a whole slew of questions (sort of a "grand unified theory" of Neoproterozoic climate). It's this very aspect of the hard snowball hypothesis that makes it hard to pick apart, because any decent challenge, of necessity, can only address one aspect.
Paul Hoffman went on an incredible lecture tour, promoting the hard snowball wherever he went. If you have ever seen Paul speak, you'll know that he is actually quite a charismatic speaker, and projects an air of extreme self confidence in his work. Besides, if a National Academy member is so confident in his ideas, why shouldn't the rest of us be? ;)
The popular book by Gabrielle Walker, and the unbelievably one-sided BBC Horizon program produced about the snowball Earth hypothesis, helped cement an admittedly evocative portrait of a bizarre period in Earth's past, one that does appeal to catastrophe-loving media and their audience both.

Dan Schrag may have become less vocal in his support of the hard snowball, but to my knowledge he hasn't actually abandoned it. Certainly Paul Hoffman has not; I suspect he views it as one of the crowning achievements of his career, and he's not about to let go that easily...

cran
2005-Oct-04, 02:36 AM
Hi Samantha Carter,
thanks for the reply. If you've prepared a summary of your findings and ideas, I would dearly love to read it; I will gladly offer a copy of my essay on what I didn't like about the 'Snowball Earth' scenario (for Flinders University - 2001) in exchange - I'm particularly interested in filtering out the contradictions that seem to proliferate... :)

I've just done a quick search through my science news archives, and found the following ... I'll apologise now for my earlier misrep... :o

- from News@ Nature

"Snowball fight - 6 April 2000 by Philip Ball

"Geologists of the nineteenth century concluded that in the last Ice Age glaciers reached much further down alpine valleys. But they did not know the half of it. Imagine their surprise if they'd discovered evidence of glacial ice near the Earth's equator. Yet this is just what, in 1986, Joe Kirschvink of the California Institute of Technology, Pasadena, discovered in Adelaide in Australia... Kirschvink published his so-called 'Snowball Earth' findings in 1992... The Soviet scientist M. I. Budyko proposed one possible cause: runaway global cooling... Budyko's theoretical models of the Earth's climate suggested that this feedback could pass a point of no return, leaving the planet to freeze over... it was lent strong support by a report in 1998 from geologist Paul Hoffman of Harvard University, Boston, Massachusetts, and colleagues. They found evidence from carbonate rocks in Namibia that, about 700 million years ago, biological activity almost disappeared in the surface oceans for millions of years... The geological record from Namibia and elsewhere implies that there may have been up to five of these freeze-thaw cycles, the last one ending about 575 million years ago. This timing corresponds to a massive diversification of the fossil record and the appearance of multicelled animals."


- from University of Toronto - 5 June 2000

"Equatorial Water May Have Provided Means Of Survival

"... In a paper to be published in the May 25 edition of Nature, University of Toronto physics professor Richard Peltier and Texas A&M oceanographers William Hyde, Thomas Crowley and Steven Baum note that the late Proterozoic era (600-800 million years ago) was the most important period of evolution for multi-cellular creatures. However, this period was also a time in Earth's history that has come to be referred to as the Snowball Earth. At that time, the planet was thought to be completely ice-covered. Geological and paleomagnetic evidence indicates that for alternating periods, the Earth was completely covered by ice sheets over the continents and sea ice over the oceans, followed by sudden warming trends that melted the ice... The late Proterozoic period was also a time when the supercontinents Rodinia and Pannotia [? I don't know this one - Cran] formed and subsequently rifted and disassembled. Located over the south rotational pole in the position of present-day Antarctica, these supercontinents were made up of the current land masses of Africa, South America, Antarctica, Australia, Greenland, Laurentia and parts of Asia. According to Peltier, the entry of the Earth into the snowball state required not only the weak sun and atmospheric carbon dioxide levels not significantly higher than present-day, but also this high degree of polar continentality. "
[This article (among other similar ones) seems to ignore the finding of others that these lands were equatorial, or at least provides for a polar land-link to lower latitudes - Cran]


- from Geological Society of America - 4 December 2001

"A Curve Ball Into The Snowball Earth Hypothesis?
"... Martin Kennedy, from the University of California, Riverside, has just tossed a curveball into the Snowball Earth theory with new data he reports in the December issue of GEOLOGY... Kennedy and his colleagues' most recent research reveals that life in the oceans during the "snowball" event basically went on as usual. This new data is difficult to reconcile with the effects on life an entirely ice-covered ocean would have imposed, and this fundamentally challenges the Snowball hypothesis... For the last six years, Kennedy has collected limestone and dolomite rocks from Precambrian glacial deposits to establish a record of carbon isotopic variation through the glacial interval. These data indicate consistent positive isotopic values from glacial rocks in northern Namibia, central Australia, and the North American Cordillera... His evidence shows that the carbon isotope 13C to 12C ratio was actually higher during the glaciation indicating the presence of a healthy and productive marine ecosystem. This ratio dropped only after the ice had melted and this suggests that other influences other than those proposed in the Snowball hypothesis must have been active."


-from BBC Science News - 6 March, 2002

" 'Snowball Earth' theory melted'

Drs Dan Condon, Tony Prave and Doug Benn say they have found evidence of sedimentary material, which could only have been derived from floating ice on open oceanic waters... The team examined a suite of rocks called Port Askaig Tillite, south west of Oban in Scotland and directly north of Jura, which is supposed to record Snowball Earth glaciations... Dr Prave said: "What is interesting about the Port Askaig Tillite, is that it was the first rock unit ever described (in 1871) to be attributed to these ancient glaciations, thus the roots of the Snowball Earth model are firmly grounded on Scottish soil." ... The team examined rocks in Namibia upon which the Snowball Earth theory is based, as well as samples from County Donegal, Ireland, and Death Valley in California, which would have formed during the glaciations... "What is important and different about our findings is that we have found physical evidence for the glacial rocks being deposited in an environment where there were areas of open seas, contradictory to the 'hard' Snowball model." "
[This is probably the article I mis-remembered as linked with the 'originators' - Cran]


One thing that does amuse (or bemuse) me is the citing of extensive vulcanism as both the means of originating and ending the so-called 'Snowball Earth'... :think:

Samantha Carter
2005-Oct-04, 05:29 PM
cran, I don't yet have a written summary of all my thoughts and ideas on the snowball Earth, although it is on my list of things to do (seriously). If you're interested in some feedback on your essay, I'd be happy to do that on or off the boards, whatever your preference (my email's in my profile).

And it does get to be a bear to keep track of everything associated with this little hypothesis, doesn't it? :)

Just a couple quick comments on the news items you have summarized -

That first item from News@Nature is unfortunately replete with mistakes, at least one of which (the misrepresentation of Budyko's work) was actually started by Paul Hoffman and not widely corrected.

In the Hyde et al. paper, Pannotia (http://en.wikipedia.org/wiki/Pannotia) refers to a late Neoproterozoic supercontinent that succeeded Rodinia. I haven't seen widespread adoption of this term, to be honest. Also, Hyde et al. do refer to a continental configuration that extends from the south polar region up to equatorial latitudes because that is an appropriate reconstruction for the late Neoproterozoic and the then-presumed timing of the Varanger/Marinoan glacial interval as ~590 Ma. Chandler and Sohl (2000) used a similar reconstruction (see the second modeling link in my first post). With a revision of the dates for the Marinoan (now ending ~635 Ma), some updating of that reconstruction will be required.

By the way, the business of subaerial volcanism bringing snowball Earth intervals to an end was really a byproduct of a 1992 Nature paper by Ken Caldeira and Jim Kasting, discussing a theoretical runaway icehouse condition on Earth and what would be needed to overcome it. It was not a good move on Hoffman and Schrag's part to make the hard snowball model partly reliant upon the C&K model results, because the C&K model was not a sufficiently good analog for the particulars of Neoproterozoic climate. (The C&K model results became irrelevent to the hard snowball anyway, once the concession for open patches of ocean was made).

cran
2005-Oct-05, 10:51 PM
Thanks again, Samantha;
re Pannotia, like others, I saw that supercontinent as an extension of Gondwana... just hadn't come across the name before. :)
I'd probably better PM the essay to you - it's a bit long for posting here, and from memory, it may wander 'off-topic'... besides, I can best handle embarassment in small doses...:o
My disc copy was corrupted by a virus from the university computer (and subsequently removed by antivirus software) - so, if after re-jigging my new computer so it talks to my scanner I still have OCR, I'll scan the relevent pages back into the computer and copy it to you... otherwise, I'm in for a couple of days of typing... >thinks< unless my PDF software actually works...
So, please bear with... it will happen. (I'll add any of my supervisor's comments if they turn up in that section)

Samantha Carter
2005-Oct-06, 09:19 AM
No worries, cran, I'll be happy to read your work whenever you're ready. :)

By the way, I meant to ask, you were at Flinders as a student? I used to know some folks there, although they may have all retired by your time (2001).

cran
2005-Oct-06, 10:40 PM
Yes, for 3 years ... still haven't graduated (only part time) - I've only shifted campuses ... lately I've been at Curtin Uni in Perth, but now that I'm living out in the wheat belt, I'll have to rethink my strategy - I've PM'd most of the reply... "off-topic" and all that...

Have you looked at the earlier 'Snowball' periods? - I think one was around 2.2GaBP, another around 2.6GaBP - they might be stronger candidates for a 'hardball' case ... though there is even less publicly available info as far as I can tell...