Oman’s view of the Snowball Earth

A post by Chris Rowan The latest Accretionary Wedge, being hosted at Geology Happens, asks: what are you working on? This seemed to me to be a good excuse to finally write something about this whole Snowball Earth thing that I’m currently researching. More musings will hopefully follow in the future.
Why did visit I Oman last year? Because in Oman you can find Late Neoproterozoic rocks, between about 750 and 600 million years in age, that contain sequences like this:

Snowball1.jpg
Fiq-Hadash diamictite-carbonate couplet, Oman

There are two very different rock units in this outcrop. The lower part consists of a diamicite: a sedimentary rock that contains both very fine particles (the dark grey matrix) and also large pebbles, cobbles and even occasionally boulders, and possibly everything in between (this range of different grain sizes means that is a poorly sorted sediment).

Mirbat_DMa.JPG
Close up of Fiq diamictite, Huqf Group, Oman

The buff coloured unit deposited directly above this is a carbonate rock, more specifically dolomite; it is crystalline, and generally quite fine grained.

Hadash cap carbonate, Huqf Group, Oman

This is not just an outcrop of extremes because of the obvious lithological differences between the two units. Diamictites are often interpreted as having being deposited in a glacial environment*. Bedrock underneath a glacier is very efficiently ground up into fine powder by the slowly moving ice, while larger rock fragments are also swept along, frozen within the main body of the glacier itself. Thus, when the ice melts at the end of a glacier, fine and coarse material will all get mixed up in the same deposit. However, whilst this suggests that the lowermost unit was deposited in a very cold, icy climate, the carbonate appears to have been directly chemically precipitated from seawater, something which only happens in warm, tropical conditions – think the Bahamas. There is no sign of a large time gap between the formation of these two units, so if these interpretations are correct, this outcrop records an extremely abrupt climatic shift from very cold conditions to very warm ones.

Climatic interpretation of Fiq-Hadash diamictite-carbonate couplet


This is not a one-off occurrence. A composite stratigraphic section of the Neoproterozoic of Oman shows that there are actually two diamictite/carbonate couplets, and older one about 700 million years old, and a younger one about 630 million years old (the younger one is the one pictured above).

Composite straitgraphy of Neoproterozoic of Oman
Composite stratigraphy for the Neoproterozoic of Oman. Rieu et al., 2007, Fig. 7.

And if that wasn’t interesting enough, there’s also the question of where on the planet Oman was when this sequence was deposited. Limited palaeomagnetic studies indicate that the diamictites and the carbonates carry an ancient magnetization with a very shallow, almost horizontal dip, which you would only find if the rock carrying that magnetization formed close to the equator (magnetic field lines are horizontal at the equator, and vertical at the poles).
So, 700 million years ago, the bit of continental crust that would eventually become Oman was close to the equator. Yet despite this, it was covered in glaciers. Then, suddenly, it wasn’t. Instead, much more in keeping with its tropical latitude, it was covered by a shallow, warm lagoonal sea. 70 million years later, the same thing happened again: Oman was apparently still at a low latitude, yet the glaciers encroached once more, until they once more abruptly vanished.
Diamictites of around the same ages, and interpreted to have similarly glacial origins, are found on every continent on Earth, and many of them are also capped by warm-water carbonates. Palaeomagnetic evidence suggests at least some of them formed at very low latitudes. Thus the global picture appears similar to that seen in Oman: extreme glaciations, with ice caps extending to far lower latitudes than even during the last Ice Age 20,000 years ago, followed by an extremely abrupt deglaciation and the deposition of limestone where once there was ice.
So how do we explain this extreme climatic picture? Perhaps the most provocative hypothesis is the ‘Snowball Earth‘, which posits that the widespread glacial sequences record times when the entire Earth froze over. This is actually not so far-fetched if extensive ice sheets did indeed reach equatorial regions. If you cover a continent or ocean with highly reflective ice, solar radiation that would otherwise be absorbed is instead reflected back into space, reducing the surface temperature. Reducing the temperature leads to the growth of more reflective ice, which leads to further falls in temperature, which leads to more growth…** This negative ice-albedo feedback effectively means that once ice sheets extend to within about 30 degrees of the equator, they will quickly, and irreversibly, continue to grow until the entire globe is frozen over.

SBEsequence.jpg

The real problem is what happens then. The high planetary albedo will keep temperatures extremely low, so how do you melt all that ice? According to the Snowball Earth hypothesis, plate tectonics comes to the rescue. A surface scum of ice does not stop rifting or subduction, so there will still be active volcanoes producing carbon dioxide. Because a global freeze would effectively halt both continental weathering and photosynthesis, which are the main ways that carbon dioxide is taken out of the atmosphere, so the concentration of this greenhouse gas would steadily increase over geological time. Eventually, the atmosphere would trap enough heat to counteract the cooling effect of ice albedo, and the Snowball Earth would start to melt. At this stage, CO2 would perhaps make up about 12% of the atmosphere, more than 300 times its current concentration. Once the ice had melted, the full effect of this would be felt, leading to a fearsome ‘supergreenhouse’ which more than explains the apparent transition to warm water conditions recorded by the cap carbonates.
To call this hypothesis ‘provocative’ would be an understatement. But is it true? The sequences in Oman also illuminate several of the problems which may yet undermine the whole grand and sweeping story. The first problem is the assumed correlation between all of the different glacial deposits. Working out the relative ages of more modern sediments relies heavily on the use of index fossils, but prior to the appearance of mineralised body parts at the dawn of the Cambrian, the fossil record is so patchy as to make this impossible. Unless there are any conveniently datable volcanic horizons, this means that there are often large error bars on our age estimates. In Oman, the main glacial unit in southern Oman was correlated with the youngest of the two glacial formations in the north, so was thought to be around 630 million years old – until the almost eroded remnants of a younger glacial formation were found above it. With two glacial formations in both the southern and northern Oman, the proposed correlation has changed, with the newly discovered unit being 630 million years old, and the more prominent diamictite below it being 700 million years old. Making it 70 million years older is actually well within the chronological error bars, and herein lies the problem: if the current dating uncertainties are such that geologists can encounter problems correlating Neoproterozoic sequences on the same block of crust, it is even more difficult to confidently correlate those sequences to others on entirely different continents.

Old and new correlations of Oman Neoproterozoic sequences
The Ayn diamictite unit in South Oman was originally correlated with younger Fiq formation in the north, but the discovery of the younger Shareef formation in the south led to this correlation being revised.

The palaeomagnetic data pointing to low latitude glaciations are also not quite as clear cut as we would like. As rocks get older, the chances of them getting remagnetised by later igneous or tectonic activity increases. There are ways of testing whether the magnetisation you are measuring actually formed at the same time as the rock did; in the case of Oman the results of these tests were a little inconclusive.
The group that I’m currently part of is attempting to address some of these uncertainties, in Oman and elsewhere. More dating studies can narrow the error bars and improve correlations. More comprehensive paleomagnetic sampling can prove or disprove the presence of remagnetisations. And, on a broader scale, we’re interested in the reason why the Late Neoproterozoic seemed to have been a time of such extreme climatic shifts (which definitely seems to have been the case, even if you don’t buy the ‘hard’ Snowball Earth where the oceans completely froze over). Over geological timescales, one of the fundamental controls on the Earth’s climate is tectonics: the position of the continents and mountain belts determines the route oceanic and atmospheric currents which move heat between the poles and the equator. The Snowball Earth period appears to coincide with the break-up of the supercontinent Rodinia, but quite how all the different crustal fragments were arranged within this older version of Pangaea, and the timing and sequence of its dismemberment, are still quite poorly understood. By looking at the whole of the Neoproterozoic record, and not just the glacial deposits themselves, our project is aiming to try and work out was was going on tectonically and see what geological changes might have influenced the climatic deterioration that resulted in the severe glaciations towards the end of the Neoproterozoic. Oman is only a small piece of the puzzle, but all of the pieces are needed to see the true picture of the Neoproterozoic world.
*this is not the only way that diamictites can form; other processes, such as landslides and other debris flows, can produce similar looking sediments. Other indicators – such as clasts with ice-carved striations, are required to definitively establish a glacial origin.
**Today, we have to worry about this feedback in the opposite direction, as warming temperatures in the polar regions reduce the area of ice reflecting solar radiation straight back into space, leading to further warming, leading to further melting…

SBE_timescale.png

Categories: climate science, deep time, fieldwork, geology, outcrops, palaeomagic, past worlds, Proterozoic
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Comments (20)

  1. Cool contribution! I have one question though. Please bear with me if it sounds ignorant but I never worked nor read much about this period of time. How do you know the dolomite is primary? You mention is was directly chemically precipated. How do you know it was not a dolomitised limestone that has undergone diagenetic destruction of the primary texture and components? Can it be excluded that the limestone is not perhaps a biological origin and later suffered a comlete fabric destruction by diagenesis and dolomitisation?

  2. Sili says:

    Very interesting!
    Stupid question: could impacts and or increased albedo due to ‘space dust’ be a cause for the remelting? (Both of those would presumably leave telltale isotopic/elemental traces, so I assume the answer is no.)

  3. David Marjanović says:

    A limestone of biological origin? In the Cryogenian? That would have to be a stromatolite, right? Could a stromatolite completely lose its easily recognizable structure?

  4. Right. I didn’t consider that! Totally forgot where I was stratigraphically. That makes it rather unlikely. But besides stromatolies didn’t we already have sponges growing? I belief to recall a paper dating the earliest sponges to the cryogenian.
    Please don’t get me wrong. I generally accept the theory. I am just curious to learn more of the details and the how and why.

  5. Eric Lund says:

    Interesting writeup, but I have one question.
    You infer from the horizontal magnetic field at the time the rocks were formed that the rocks were at the equator. Strictly speaking, that would be the magnetic equator, which in general does not coincide with the geographic equator. Today the dipole axis is tilted about 11 degrees with respect to the spin axis. Do we have any good way of knowing what it was in the geologic past? For instance, at Neptune the dipole axis is tilted about 60 degrees off the spin axis. Was this ever the case on Earth, or are we sure that the Earth’s dipole axis has always been (other than perhaps during reversals) not more than 15 or 20 degrees off the spin axis?

  6. megan says:

    I what gets me is that NO Creationist theory can analyze or try to even create a logical workable hypothesis to test for these geological things/let alone biological data except “The WIZARD (gawd) did it……in 6000yrs”. That they even pretend they have alternate theory that should be taught in schools is criminal and should be illegal, imho. To me if there is a ‘god’ the billions of years and slow development and processes are the proof and glory of IT’S creation than human childish desert nomadic tribal myths. I find the older religions believing literally in a sun god from which all life comes is THE REALITY of what we find in science. It’s just there is no self aware directive or consciousness from our miniature point of view as granular beings on one of the planets out of billions lucky to have the conditions to create self replicating lifeforms out of the vast stores of energy from the stars.

  7. John Vreeland says:

    As a rule creationists have no idea that these things get studied or these discussions take place. To them National Geographic and Scientific American are professional scientific journals. If thy do encounter real scientific work they treat is as if we were rearranging the deck chairs on the Titanic.

  8. Birger Johansson says:

    Isn’t Oman also the place where you can study a rare patch of very old ocean crust that escaped subduction by getting folded on top of continental crust?

  9. Colin says:

    Super-duper-stupid question, but it was bugging me as I read your fantastic piece.
    How do we know that this region didn’t undergo a significant drop in altitude over the time period we’re considering? Would there be evidence of that in the rock formations that would rule it out?

  10. Lab Lemming says:

    Re 9:
    These are marine sediments, so they were deposited underwater.
    Re 8: Yes, but the ophiolite (ocean lithosphere thrust onto a continent) is much younger.
    Re 5: There is a alternative hypothesis that drastic changes to Earth’s axial tilt and/or magnetic field happened at about this time, but that theory suffers from the lack of a compelling mechanism.
    Re 3, 4: The dolomites are abiogenic. Sponge molecular fossils are known from the Huqf Supergroup, but physical spicules don’t appear until the Ediacaran.
    Re 2: There’s a paper somewhere that clains to see an iridium spike associated with melting. Basically, the snowball cover ice collects lots of small and medium impacts for millions of years, then when the ice melts, all that stuff dissolved into the ocean. If I recall, there are doubts as to the conclusivity of the analytical results.
    Re 1: There is no shortage of arguments about the genesis of cap carbonates. A few different ideas:
    http://geology.gsapubs.org/content/29/5/443.short
    http://palaios.geoscienceworld.org/cgi/content/abstract/20/4/348
    http://rparticle.web-p.cisti.nrc.ca/rparticle/AbstractTemplateServlet?calyLang=eng&journal=cjes&volume=38&year=2001&issue=8&msno=e01-046

  11. Chris Nedin says:

    I don’t think you need to have large temperature changes to deposit the cap carbonate. Carbonate is reverse soluble, meaning cold water contains more dissolved carbonate than warm water. Also the higher the partial pressure of carbon dioxide in the atmosphere the greater the solubility of carbonate. When sea water temperatures started to rise, the dissolved carbonate would quickly become out of equilibrium. With no obvious method of balancing, this would build up until chemical precipitation occurred. This would also be forced by high carbon dioxide levels in the atmosphere, which would fall as precipitation occurred and more carbon dioxide dissolved into the oceans to replace dissolved carbonate removed by precipitation, resulting in a decrease in the partial pressure of carbon dioxide and a decrease in the solubility of carbonate in sea water. This would result in large scale precipitation of carbonate around the world without extreme changes in temperature.
    The cap carbonates appear primary as they show the same characteristics around the world – even in areas with differing subsequent geological histories.

  12. Modeler says:

    This might be a stupid question or reinventing the wheel – wouldn’t a good test for remagnetisation of the diamictite be comparing the magnetisation of the sediment itself with the magnetisation of the larger cobbles and boulders?
    If the diamictite had been remagnetised, then the included cobbles and boulders would share their magnetisation; if there had been no remagnetisation, then the fine-grained material would show one magnetic direction, and the cobbles and boulders would have random magnetic directions.

  13. DDeden says:

    Couldn’t Oman have been part of tectonic equatorial coastal infrequently active volcano system (Kilamanjaro has glaciers) which became inactive and heavily eroded to shallow reef atolls accumulating thick limestone, then reactivated for a period? The Red Sea and Persian Gulf are rich in warmwater coral reefs today.

  14. Chris Rowan says:

    Wow, some excellent comments and questions!
    Regarding the cap carbonates: yes, they formed in a period well before biomineralisation became widespread as a means of carbonate production, so the carbonate is an inorganic precipitate. There are some stromatolitic horizons, but most of it seems to have directly precipitated from sea water, much like the “whitings” seen in the water in places like the Bahamas. As far as I know, dolomisation occurs during diagenesis, so the cap carbonate was originally precipitated as calcite or aragonite. Although these things appear to record some very weird ocean geochemistry, so I could be wrong about that.
    @Eric: although the magnetic field is inclined away from the spin axis on human timescales, when you average out its secular variation over more than about 10,000 years it reduces to a dipole centered on the geographic poles; in other words, the magnetic equator matches the physical equator. We have reasonably good evidence that this has been the case for at least the past two billion years of Earth history, although some have proposed that non-dipole contributions to the field, which could result in significant asymmetries, could result in shallow inclinations even if the formation formed at moderate latitudes. There is no strong evidence that this is the case, however.
    @Modeler: You have actually described one of the tests that can be used to check for remagnetisations. Unfortunately, in Oman at least, most of the cobbles consist of gneiss and granites, which do not hold a stable magnetisation.
    @DDeden: It is certainly true that you don’t necessarily need global glaciation to have ice caps at low latitudes; you just need a high mountain range (look at the Himalayas). I think the argument is that the apparent geographic extent of low latitude glacial deposits rules this out. Personally, I’m not entirely convinced that the amount of reliable low latitude palaeomagnetic data is extensive enough to rule this out.

  15. Concering the whitings there are some who argue on the basis of isotope studies that whitings are essentially of biological origin or at least biological induced. Being as untrained on Snowball Earth as I am, are there any studies adressing that part? Instead of a direct precipation from seawater I could imagine a massive growth of algae or other carbonate skeletal organisms in the follow-up. Of course the oceans might be too acidic for this to happen bc of the immense CO2 volumes present. Hmm…interesting.
    The presence of stromatolites support pretty shallow water. Dolomitisation could be a result of very early diagenesis. I really need to read more about this I notice.
    Very interesting work you do!

  16. Passerby says:

    Cool blog topic.
    Strange chemistry of covariation in C and O isotopic and odd sulfate budgets. Several papers suggest that similar deposits are not evidence of global marine environment, but rather, large silled, saline-stratified, enclosed basins with limited marine water intrusion.
    Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean. Bristow and Kennedy (2008)
    Geology 36(11):863-866.
    http://en.wikipedia.org/wiki/Ediacaran

  17. Omega Centauri says:

    9: I’m guessing that high altitude glacier deposits are rarely incorporated into the sedimentary record. Mountain environments are usually erosive environments, i.e. rock is eroded and carried away to the lowlands, where sedimentary sequences are deposited. Do you need geologic subsidence to create a sedimentary basin? Mountains can only be present with significant uplift.

  18. Lab Lemming says:

    Just outta curiosity, what is the lowest latitude LGM fjord. Ignoring the issue that a sea level glacial feature today might have still bee grounded by a hundred meters, it’s a first order estimate.
    A quick look at the map gives me the following for the apline division and the ice sheet division:
    Alpine: Gulfo de Ancud, Chile (42S)*
    Icesheet: Hudson valley (41N)
    So at least in our ice age, alpine glaciers don’t reach sea level at latitude vastly smaller than icecaps. Now, if you instead look at ice-rafted debris, then you might have a different story, but that doesn’t show up on a world atlas…
    * This is a guess from looking at the map. anyone with actual knowledge of the area is free to shoot me down.

  19. CherryBomb says:

    oooh! Pretty outcrop.

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