Geology is all around us, scarcely thought of as we go about our lives. Yet, it affects everything we do as a civilization, as a society and as individuals. While barely appearing to change from day to day, it works to alter the course of evolution. Preserving a record of creatures and landscapes both ancient and forgotten, the story of our past is written in stone and waiting to be read. I offer a view of how I see our world and its inhabitants, both past and present, as seen through my lens.
Friday, February 17, 2012
The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II - The Rifting of Rodinia and the "Snowball Earth" Glaciations That Followed
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
Fire and Ice by Robert Frost, 1923
In my previous post on the Great Unconformity of the Grand Canyon, I discussed its contiguous stratigraphy and the regional events involved in their formation. In this follow-up post, in the light of current thought, I wanted to offer a more global interpretation of the events that occurred during its interval.
In the following photo we are peering into the Inner Gorge of the Grand Canyon, as seen from the South Rim. Can you specify the location of the Great Unconformity?
THE LATE PROTEROZOIC-CAMBRIAN INTERVAL Our Earth system experienced enormous change during the Late Proterozoic through the Cambrian. This is the time span that encompasses the events that lead up to and include the Great Unconformity. Recorded in the Earth's lithosphere is a history of the unconformity's missing time, of fragmented and lost ancient continents, of a planet frozen solid within a ball of ice, of a global flood of biblical proportions, and an epic tale about the creation, diversification and radiation of radically different new forms of life that appeared suddenly in a biological "Big Bang." It sounds religious, but it's not (although many believe that it is). Its geological, tectonic, climatological and biological.
Where shall we begin? Let's redefine the Great Unconformity of the Grand Canyon, briefly reprising my previous post.
THE GREAT UNCONFORMITY OF THE GRAND CANYON The photo below is taken of the Great Unconformity at arm’s length in a side canyon called Blacktail within the Grand Canyon. All unconformities are gaps-in-time, geological horizons where time and rock layers are forever lost (having been consumed by the forces of erosion) or simply never even formed. The only difference is that this one is massive. It's the mother of all unconformities. It's so important that it's written with capital letters, and that's not all. It's found not only in the Grand Canyon, but all across the globe. But not just anywhere. You have to know where to look. You have to know the geology, and plate tectonics helps. If you know that, you can truly go back in time.
The Great Unconformity resides within the contact between the horizontally-bedded, Early Cambrian Tapeats Sandstone and the vertically-folded, crystalline basement of the Early Proterozoic Vishnu Schist. Notice a conglomeritic lens of Tapeats "trapped" above the contact. Here and there, large chunks of schist and granite have been caught up in the flood that created the shoreline of the Tapeats.
You can almost hear the waves of the gradually rising Panthalassic Ocean (proto-Pacific Ocean) lapping on the shores of a new supercontinent (Laurentia), a remnant of the larger one (Rodinia) that has broken into pieces and whose pieces will one day reunite (Pangaea). One billion two hundred million years, give or take. That's the size of the time gap. That's the amount of time that's unaccounted for within that paper-thin space! A quarter of the Earth's history. Its magnitude is incomprehensible and it's missing!
MORE QUESTIONS THAN ANSWERS
What events happened during the Great Unconformity? How do we even begin to find out, if the strata holding the clues doesn't exist? Or does it? Why is the end of the Precambrian in the Grand Canyon marked by an unconformity? Why was the interval so long? Is the unconformity found globally? Are any aspects of a Snowball Earth glaciation recorded in the Chuar Group? What is Snowball Earth? What lifeforms, if any, existed during the prolonged interval of the unconformity? How did they survive such a crisis? Did the geological events of the period favor or disfavor their evolution and diversification? What caused the global high seas following the rifting of Rodinia that flooded the Grand Canyon region and capped the Great Unconformity? Did the rifting of Rodinia somehow influence the climate, glaciation and even the evolution of life?
Let's try to answer these questions. But know that we're treading on thin ice when it comes to a universal consensus regarding many of the issues! The subject of this post is fraught with controversy and riddled with differing opinions and perspectives. Several of the concepts addressed may in time be proven to be completely erroneous, including the concept of Snowball Earth which has its opponents. Many of the issues and hypotheses discussed lack complete agreement amongst earth scientists. That said...
THE GREAT UNCONFORMITY'S CONTIGUOUS STRATIGRAPHY
Viewed from the South Rim, the billion-year-plus Great Unconformity of the Grand Canyon resides in the contact between the steep, dark, gnarled cliff of Vishnu Schist and the overlying, coarse-bedded, elongate cliffs of the Tapeats Sandstone. Injected under intense heat and pressure well over a billion years ago, prominent dikes of pinkish, pegmatitic Zoroaster Granite can be seen snaking through the schist. Together, they comprise the Granite Gorge Metamorphic Suite and constitute the crystalline basement of the Grand Canyon. The Great Unconformity is sandwiched between these two rock formations. Not quite visible from this perspective, the Colorado River carves the Inner Gorge from the right to the left.
IF THE STRATA DEPOSITED DURING THE INTERVAL OF THE GREAT UNCONFORMITY IS MISSING, THEN HOW DO WE KNOW ANYTHING ABOUT THE EVENTS THAT TRANSPIRED DURING IT? Buried within the Great Unconformity and distributed in a discontinuous, patchwork-depositional pattern along the Colorado River is a largely fault-controlled sequence of rocks called the Grand Canyon Supergroup. The entire group was deposited during the Middle and Late Proterozoic. Its vertical height at over 12,000 feet is greater than the depth of the Grand Canyon from the rim to the river. Imagine that!
In regions where huge blocks (horsts) of the Supergroup were uplifted by faulting, the Supergroup has virtually eroded away, placing the Early Proterozoic Vishnu Schist in intimate contact with the Cambrian Tapeats Sandstone (as seen in the above photo). And in places where sections of the Supergroup were down-dropped by faulting, it survived erosion, protected if you will, as if buried in a grave (graben). Please visit my previous post for photo examples.
This fortuitous circumstance of geological preservation has provided us with many of the details of what transpired during the Great Unconformity's massive gap of time. By the way, we owe special thanks to the erosive power of the Colorado River system (and lots of mass wasting) to have excavated the Supergroup from its providential burial.
WHERE STRATIGRAPHICALLY IS THE CONTACT THAT ENCOMPASSES THE GREAT UNCONFORMITY?
The red line in the diagram depicts the massive, unconformable contact at the bottom of the Grand Canyon’s Paleozoic column between the Early Cambrian Tapeats Sandstone and the underlying basement structures. Where the Tapeats Sandstone contacts the Early Proterozoic Vishnu Schist (solid blue arrow), resides the Great Unconformity. No intervening deposits of the Grand Canyon Supergroup are preserved. Where the Tapeats contacts various members of the Middle and Late Proterozoic Grand Canyon Supergroup (dotted blue and dotted green arrows), a “great” unconformity of time exists of perhaps 600 to 700 million years, depending on the specific layer of the Supergroup in contact. Where Bass Limestone, lowermost of the Supergroup, rests on the Vishnu Schist (solid green arrow), a “lesser” unconformity exists of about 200 million years. More time is missing from the stratigraphy of the canyon than is present!
WHEN DID THE EVENTS SURROUNDING THE GREAT UNCONFORMITY OCCUR?
Geology is totally preoccupied with time. Its time is fractally divided into increasingly smaller units of measurement such as eons, eras, periods, epochs, ages, and so on. Geologic time is measured in increments of thousands, millions and billions of years. Anything less is useless. The nomenclature of the time-stratigraphic subdivisions surrounding the Great Unconformity hold some clues to the events that occurred. The timeframe that brackets the unconformity extends from the Early Proterozoic (when the region's crystalline basement was annealed to the core of early Rodinia) through the Middle Cambrian (when rising global seas flooded the margin of the young Laurentian craton). So let's get fractal.
(Modified from the GSA 2009 Geologic Time Scale)
THE PROTEROZOIC EON The Proterozoic Eon (2500 to 542 Ma) is neatly divided into three eras: the Early (Paleo-), the Middle (Meso-) and Late (Neo-) Proterozoic. The word Proterozoic is derived from the Greek word for "earlier life" in deference to the Phanerozoic of "visible life." During the end of the Proterozoic, life became even more "visible" when it became multi-cellular. That was actually before the Phanerozoic began (and before the Cambrian).
THE LATE PROTEROZOIC ERA The Late Proterozoic Era (1000 to 542 Ma) is further subdivided into three periods: the Tonian, Cryogenian and the Ediacaran. These three periods are dated chronometrically, stratigraphically (a work in progress for the Precambrian) and bio-stratigraphically (for the Ediacaran). At first, the periods might seem to have somewhat cryptic names, but their names are actually giveaways regarding the events that happened during them. It's easy to overlook, but the Late Proterozoic was a very long interval of time, comparable in length to the Phanerozoic Eon. As we shall see, the era encompasses an incredibly eventful period in Earth history involving the atmosphere, the oceans, the climate, and life itself.
THE TONIAN PERIOD
The Tonian period (1000 to 850 Ma) derives its name from the Greek word meaning to “stretch” and refers to the extension of the continental plate of Rodinia that led to its eventual breakup. The period begins roughly with the end of the Grenville Orogeny, the final event in the formation of Rodinia. Active rifting of Rodinia began by the mid-Cryogenian.
THE CRYOGENIAN PERIOD The name for the Cryogenian period (850 to 630 Ma) is derived from the Greek word for “cold.” The period includes at least two major glaciations: the Sturtian and Marinoan. These glaciations are at the center of a controversial and hotly debated concept concerning the Earth locked into the icy grip of a frozen "snowball." The Cryogenian period ended with Rodinia actively rifting apart. Given the timing of Rodinia's rifting and the onset of glaciation, might there exist a relationship?
THE EDIACARAN PERIOD The Ediacaran (formerly referred to as Vendian, a Eurasian term) period (630 to 542 Ma) is the newest period to be named to the geologic time scale which hasn't had a change (other than date clarifications) in over 120 years. Unlike the cryogenic “icehouse” of the previous period, it was characterized by a warm and humid “greenhouse” climate. This period is known for its large-scale radiation of multicellular life, typified by eponymous metazoans first documented in the Ediacara Hills of the Flinders Range of South Australia but now known to occur at other sites globally. Might there be Ediacaran flora preserved within remnants of the Supergroup, buried within the Great Unconformity of the Grand Canyon?
THE CAMBRIAN PERIOD The stratum overlying the Great Unconformity was deposited during the Cambrian Period, the first period of the Paleozoic Era and at the start of the Phanerozoic Eon. The Greek word-root for Phanerozoic means "visible life," since it was believed that life first appeared during the Cambrian. That's when life became "macro-scopic", multi-cellular (multicellularity was actually acquired in the latest Proterozoic) and calcified (and well preserved in the fossil record). Complex, highly diversified life in the Cambrian appeared with an "explosion" especially in light of eukaryotes that hadn't evolved extensively beyond a single-cellular (unicellular) stage over a billion years after their first appearance. Is it merely a coincidence that life emerged in the Cambrian following the Ediacara and the glaciations of the Late Proterozoic? Is there a grandiose "scheme" operating here of tectonics, glaciation and genetic emergence?
WHAT'S ALL THIS HAVE TO DO WITH THE GREAT UNCONFORMITY OF THE GRAND CANYON? Clearly, lots of things were happening during the unconformity's gap in time both regionally and globally. Let's try to make some sense of it. It hasn't escaped the attention of geologists, sedimentologists, climatologists, paleontologists and geochemists that the events surrounding the Great Unconformity somehow might have come under the influence of Rodinia and its dissociation. As anticipated, Rodinia's fragmentation resulted in a plethora of new landforms: shelves, shores, continents and oceans, etc. But its break-up involved more than just the creation of new geography. It triggered a radical change in the entire Earth system between the time of its rifting through the deposition of the Cambrian sequences that bracket the Great Unconformity.
Fortunately, for those of us looking into the Grand Canyon for answers, the Grand Canyon Supergroup buried within the Great Unconformity documents the supercontinent’s final formation, its ultimate rifting apart, indications of widespread glaciations and early vestifes of life. The deposits overlying the Great Unconformity record biblically-remniscent global floods and new lifeforms of the Cambrian explosion that ushered in the Phanerozoic Eon.
WHY IS THE END OF THE PRECAMBRIAN OFTEN MARKED BY AN UNCONFORMITY (AND NOT JUST IN THE GRAND CANYON)?
The supercontinent of Rodinia assembled orogenically between 1300 and 900 Ma. The Grenville Orogen marked its final synthesis, a protracted global-scale, mountain-building, crust-generating event that involved virtually all continental blocks known to exist at the time. Once fully formed, Rodinia lasted about 150 million years, like the supercontinent of Pangaea that followed. Its assembly left Rodinia standing ‘tall’ and buoyant with new crust that 'floated' in a tectonic ‘sea’ of molten mantle well above that of oceanic crust.
Continental crust has an average density of 2.8g/cm3; whereas, oceanic crust comes in at 2.9-3.0. The difference makes continents 'float' higher than oceans (their ocean basins that is). It’s why ocean basins are filled with water, by floating lower. Floating high, newly formed continents such as Rodinia are subject to erosion. In the case of Rodinia, erosion was prolonged, and as we shall see later, compounded by other events happening at the time that. It is thought (but not by all).
Why is the Great Unconformity found globally? Massive Rodinia began to break up along its western margin between Laurentia and Australia-East Antarctica, and along its southeastern margin with the Amazonia craton perhaps as early as 750 Ma. Rodinia's rifted, continental siblings possess the signature of the Great Unconformity. Its global presence is attributable to Rodinia rifting apart and sending its eroded, crystalline basement on a worldwide, tectonic journey.
OUTSIDE OF THE GRAND CANYON LET'S LOOK AT THE GREAT UNCONFORMITY OF THE ADIRONDACK LOWLANDS IN NEW YORK STATE
The Great Unconformity is a global entity, not just found within the Grand Canyon or in the Southwest. In my birth state of New York, the Precambrian-Cambrian unconformity is found in the Adirondacks lowlands. Somewhat south of Lake Champlain in the photo, a basal Cambrian Potsdam Sandstone unconformably overlies Middle Proterozoic Grenvillian gneiss. The crystalline basement was generated during the Grenville Orogeny that occurred from one end of Rodinia to the other during the Proterozoic. The Potsdam Sandstone is the geo-equivalent of the Tapeats Sandstone in the Grand Canyon.
In the photo, thin sandstone bedding of the Potsdam can be seen close to the contact, while its uppermost surface has been heavily eroded, glacially polished and inscribed by glacial striae during the Pleistocene. After Rodinia rifted apart on its eastern shore, the Potsdam was deposited on Laurentia's subsiding shelf bordering the Iapetus Ocean. The region at the time of deposition during the Cambrian was blanketed by a time-trangressive sequence during the Sauk trangression, analagous to the Tonto Group in the Grand Canyon comprising the Tapeats in its basalmost formation. Beginning in the Early Cretaceous and even within the last 20 million years the Adirondacks have experienced a prolonged unroofing history. This has removed the Early Paleozoic sequence from the apex of the Adirondack's slowly rising dome leaving remnants of the Potsdam Sandstone (and the Great Unconformity) scattered about on its periphery.
Interestingly, the Great Unconformity of the Adirondack lowlands is situated about 150 feet above sea level; whereas, the Great Unconformity of the Grand Canyon lies generally above 3,000 feet. Its low elevation in New York State is related to its relatively stable depositional-locale on the eastern Laurentian shore (in spite of Taconic, Acadian and Alleghenian orogenic events regionally throughout the Paleozoic); whereas, in the Grand Canyon, Cenozoic uplift of the Colorado Plateau has brought the Great Unconformity to its lofty perch, related to Farallon Plate subduction beginning in the latest Jurassic.
LET'S VISIT THE GREAT UNCONFORMITY OF CAPE TOWN, SOUTH AFRICA
As mentioned, the Great Unconformity is found globally. Wherever a Proterozoic (and in some cases late Archaean) crystalline basement of Rodinia's rifted continents (the two largest of which were Laurentia and Gondwana) were covered by a marine, Cambrian sandstone deposit of the Sauk transgression, the Great Unconformity lurks within the contact.
The Great Unconformity in the photo is located near Cape Town, South Africa. The road is built right on the contact of the unconformity. Note the Jurassic dike running through the granite at bottom center. Jurassic dikes are also found injected into the crystalline basement of New York State related to Pangaean rifting.
This photo and the one below was contributed by Wayne Ranney, geologist, author, lecturer and guide.
This is the Great Unconformity in Petra, located within the country of Jordan. The region was part of the Arabian-Nubian Shield. Most of its terranes are believed to represent island arcs and micro-continental fragments that accreted to western Gondwana during the Late Proterozoic.
LATE PROTEROZOIC SNOWBALL EARTH
A CHILLING HYPOTHESIS Through much of its history, our planet has experienced ice ages, from the Archean to the present. In fact, we're currently experiencing modern-day glacial conditions, albeit in the form of an interglacial period, which is estimated to end in 80,000 years or so with the return of continental glacial ice. Glaciations of the Phanerozoic were largely polar events with the exception of three major, mid-latitudinal intervals (for the record during the Ordovician, Carboniferous and Cenozoic).
(Courtesy of SnowballEarth.org)
During the Proterozoic, several glaciations, the first about 2.2 Ba, were thought to have reached well beyond the poles to within 11º of latitude of the equator, while two or more of which (the previously mentioned Sturtian, Minoan and possibly not global Gaskiers) occurred in the Late Proterozoic within 5º of the equator. Following Rodinia's break-up, these glaciations are thought to have been global in their extent and significant in their magnitude. Collectively, they have been colorfully and aptly assigned the catchy phrase "Snowball Earth" in 1992 by Kirschvink (and "Slushball Earth" by Cowen, 2000). In 1998 and 2002 Hoffman and Schrag unified, popularized and championed the concept known as the Snowball Earth hypothesis.
MOTHER EARTH BECOMES A GLACIAL MIRROR
The hypothesis claims that the Cryogenian glaciations were so severe that global mean temperatures reached from -20º C to -50º C (-75º F) with the oceans frozen to a depth of 1 km in equatorial as well as polar latitudes. That's a weather report very similar to Mars on a nice day! The resulting glaciations produced a "runaway ice-albedo" feedback (Budyko, 1969), a situation where the sun's radiation would have been reflected back to space by the ice's highly-reflective mirror-like surface. The planet's albedo rises at a faster rate with low-latitudinal ice, because direct sunlight strikes a much larger surface area. The end result is theorized to have created a self-perpetuating "icehouse" scenario.
The consequences of such a severe and prolonged low-latitude glaciation were profound. With the hydrologic cycle and continental weathering shutdown, photosynthetic and biologic productivity would have stalled considerably in the anoxic oceans, but not totally. Surviving within specialized environments called "refrugia" such as hot springs on the sea floor or under the ice itself (where photosynthesis was still possible), life (then represented largely by unicellular protozoa and filamentous bacteria) persisted.
THE RIFTING OF RODINIA AND THE GLACIATIONS OF THE CRYOGENIAN: A TEMPORAL COINCIDENCE OR A TECTONIC RELATIONSHIP TOO DIFFICULT TO IGNORE?
THE RIFTING OF RODINIA The supercontinent of Rodinia was assembled through multiple orogenic events between ~1300 and 900 Ma, and persisted, as previously mentioned, for another 150 million years after its assembly. Although the rifting apart of Rodinia might have begun as early as 825 Ma, its break-up on the present-day western and eastern margins occurred initially at ~750 and after ~600 Ma, respectively.
Rifting essentially left Laurentia as the new continental core of proto-North America, flanked by passive margins on its boundaries. The geometric configuration of the continents adjacent to Laurentia's margin on the west currently remains ill-defined resulting in a plethora of proposed paleographic reconstructions (such as Missing-Link and the acronyms AUSWUS, SWEAT and AUSMEX) based on the orientations of Australia and East Antarctica. By 750 to 700 Ma, most of the world's new continents were clustered at low- to moderate-latitudinal positions.
(Modified from Balgord thesis, 2011)
"BABY, IT'S COLD OUTSIDE" The mid-latitudinal locale of the new continental fragments coincided with the first major global glaciation of the Cryogenian period, the Sturtian (~720 to 660). By ~650 to 630 Ma, the dispersing continental blocks became even further aligned along the paleo-equator. By then, the second widespread, low-latitude glaciation called the Marinoan (~655 to 635) was in full swing. A third glaciation, the Gaskiers, is thought to have occurred at ~580. This period of Cryogenian glaciation is referred to as the Neoproterozoic glacial interval from ~720 to 580 Ma. Note that the precise number (possibly even four or more) and timing of these glaciations remains problematic.
GLACIAL EVIDENCE The equatorial cluster of continents that occurred over 600 million years ago obviously no longer exists, the continents having tectonically drifted and shifted into their contemporary positions. However, convincing evidence of a snowball state has come from compilations of paleolatitudes and the relics of glacial debris left behind when the ice formed and melted, which are exposed at locations throughout the globe on virtually every continent. Earth scientists have offered variable interpretations for the evidence especially the cap carbonates.
The geological evidence includes tillites, diamictites, dropstones (marine ice-rafted debris), banded iron formations (BIFs), cap carbonates and glacial markings such as polish and striae. Each deposit tells the story of a particular aspect of glaciation. Geochemical evidence includes depleted carbon 13 isotopic excursion values which indicates a progression from an ocean full of life to one devoid of life during glaciation. Note the arrows in the diagram below showing atmospheric carbon dioxide excursions during the Sturtian and Marinoan glaciations. Also a metalliferous, reducing ocean chemistry resulted from the isolation of the oceans from the atmosphere, which had direct consequences for marine biology in the era immediately after the snowball event.
(Adapted from Bartley and Kah, 2004)
A PLAUSIBLE PLATE TECTONIC-GLACIATION CAUSATION AND THE ROLE OF CARBON DIOXIDE
We've acknowledged the dramatic changes that took place during the Late Proterozoic. Those changes include tectonic, climatic and biologic events. One can't help but ask the obvious question "Is there a coincidence between the rifting of Rodinia and the global glaciations of the Cryogenian Period?" The following is the overwhelming but not sole response to the question.
The global glaciations of the Cryogenian are a reflection of and response to the climatic and geographic variables that existed during the Late Proterozoic. To that extent, a low-latitudinal aggregation of Rodinia’s rifted continents allowed for warmer terrestrial temperatures and higher rates of silicate weathering. Under these conditions atmospheric carbon dioxide concentrations were reduced, thereby decreasing the greenhouse effect and global temperatures. Normally, the climate system would have been stabilized at a new and colder state with global cooling lowering the silicate weathering rate, but two circumstances contributed to a sustained high silicate weathering rate and cooling of the climate.
(From a Lecture by Hoffman and Schrag, 1999)
Rodinia, which began its fragmentation ~830 Ma and continued for 200 million years, produced a multitude of smaller continental fragments that were wetter compared to the arid climate of the former supercontinent's interior. Subsequently, the fragments exhibited increased weathering rates and the deposition of both carbonate and shale with organic carbon along rift basins and margins. A major source of calcium ions and carbonate deposition was provided by the extensive eruption of easily weathered basaltic lava on multiple continents.
The combined effects of tropical continents, supercontinental break-up and extensive eruptions of basalt are thought to have favored cooling, and that led to sea ice at higher latitudes. The growth of ice increased the albedo which initiated a runaway feedback state. This in turn led to cooler temperatures that expanded the ice equatorially. Voila, Snowball Earth!
ARE THERE OTHER POTENTIAL MECHANISMS CONTRIBUTING TO A SNOWBALL EARTH CONDITION? It will come as no surprise that numerous additional models have been suggested as causes for low-latitude glaciations in the Late Proterozoic. Remember, this is a subject riddled with controversy and lack of consensus. Here are just a few. Terrestrial models include a mantle superplume-induced 90º rotation of Rodinia which brought the continent into equatorial latitudes (Li, 2004), a "zipper-rift" hypothesis involving the diachronous rifting of Rodinia (Eyles, 2002), a decreased period of Earth's magnetic intensity (Maruyama, 2008) allowing an increase in cosmic ray-showers which enhances atmospheric cloud formation (Svensmark, 1998) and a subsequent drastic decrease in the surface temperature of the Earth, and plate tectonic causations relating to the rate of seafloor-spreading and maximum continentality (Veuvers, 1990). Extra-terrestrial models include high Earth obliquity (Williams, 1975, 1993), reduced solar luminosity during the Proterozoic, increased cosmic gamma rays from a galactic star burst in the Early and latest Proterozoic, and long-term periodicities in global tectonic phenomena linked to motions of the solar system through the galaxy (Rampino, 1986).
WHAT EVENT(S) TRANSPIRED TO DEFROST THE PLANET?
While the ice continued to advance, the equatorially-situated continents continued to consume atmospheric carbon dixoide as their silicate rocks weathered. By the time the earthly snowball was complete, the atmosphere had little carbon dixoide left to act as a greenhouse gas. So how did Earth escape from its frozen grip?
The Snowball Earth hypothesis further predicts that the planet defrosted when the high albedo of the reflective, ice-covered oceans was overcome by volcanic outgassing of enormous quantities of carbon dioxide into the atmosphere in the absence of sinks for carbon. Normally, this endless supply of carbon is offset by the erosion of silicate rocks. The chemical breakdown of the rocks converts carbon dixoide to bicarbonate which is washed to the oceans.
So, the enormous reservoir of volcanic carbon dioxide "rocketed" (Cowen) the entire planet into an intense greenhouse "hothouse" period from its former "icehouse" condition.The resultant global warming is thought to have violently deglaciated the planet in the short interval of 10 million years. Catastrophic, rapid warming and deluges of acid rain would have immediately precipitated calcium carbonate into the warming seas and created Snowball Earth's signature (albeit enigmatic) juxtaposition of greenhouse "cap" carbonates overlying icehouse glacial sediments. Ultimately, photosynthesis and weathering brought down the carbon dioxide levels, allowing the world to make a biological recovery.
The majority of earth scientists agree on the massivity and timing of the enigmatic Cryogenian glaciations but disagree on aspects such as their globality, the triggers that caused them, the drivers that perpetuated them, and even the conditions necessary to exit the frozen, snowball state. It's conceivable that in regards to icehouse intervals, several proposed circumstances must coincide to initiate a feedback condition favorable for glaciation.
IF THE PLANET MADE A RECOVERY FROM ITS FROZEN GRIP, WHY DID THE CYCLE OF GLACIATION REPEAT ITSELF (POSSIBLY THREE OR MORE TIMES)? The answer lies in the geographic distribution of the continents, their equatorial distribution that initiated the Snowball Cycle to begin with. We see that a pattern has emerged in that a glacial cycle is initiated by geography and ended by volcanic eruption (Cowen). Thus, the planet would have been 'rescued' only when tectonics would have created a less favorable glaciogenic distribution of the continents, which they eventually drifted into.
WHAT'S TO PREVENT A SNOWBALL EARTH SCENARIO FROM RECURRING IN THE FUTURE? It seems ironic to be asking such a question, while basking in the warmth of an interglacial period possibly (or probably) accentuated by man's release of greenhouse gases into the atmosphere. The likely (and safest) answer to the question is not for many tens of millions of years, since the contemporary arrangement of the continents is distributed in the middle to upper latitudes where carbon dioxide-absorbing chemical weathering is retarded. When the process of plate tectonics eventually re-assembles the continents equatorially (as it has done in the past and will undoubtedly do in the future), snowball earth conditions may again recur. Perhaps the diminishing luminosity of our aging sun or the fragmentation of the next supercontinent will pull the trigger.
MORE KNOWLEDGE BEGETS MORE QUESTIONS
Lot's of questions are still unanswered! Does the biota, stratigraphy and geo-chemistry of the Chuar Group within the Grand Canyon (that we know was deposited at low-latitudes when Rodinia was breaking up) preserve a record of snowball glaciation? Who were the survivors of the snowball global crisis? In fact, how did they survive? Did the severity and duration of the glaciations of the Cryogenian have an impact on the course of their evolution and diversity? Does the great global flood of the early Paleozoic have a Rodinian tectonic connection? Did the Snowball Earth event exert controls on the revolutionary changes in the evolution of life forms during the Cambrian Explosion?
ONE CLOSING THOUGHT A contemporary quote comes to mind by Lynn S. Fichter, a professor in the Department of Geology and Environmental Science at James Madison University. He said, " Nothing in geology makes sense except in the light of tectonics." Theodosius Dobzhansky, a prominent geneticist and evolutionary biologist now deceased, previously stated, "Nothing in biology makes sense except in the light of evolution." It is clearly evident that the Late Proterozoic was one of the most remarkable periods in the history of the Earth. Its events exemplify the intricate interplay of tectonics, the ever-changing environment and the ever-evolving biosphere.
I see the necessity for one more post on the events of the Great Unconformity...Part III.
"Assembly, configuration, and break-up history of Rodinia: A synthesis" by Li et al, Precambrian Research, 2007.
"Snowball Earth" by Hoffman and Schrag, Scientific American, January 2000.