The Beginning of Complex Life?
Introduction: A World of Ice
Imagine a world totally covered in ice. Vast continental glaciers span the land surface of the globe, while the surface of the deep ocean is covered with 1-2 km thick ice. The Earth has been in a global freeze for almost ten million years. Then, in less than one million years, the Earth is thawed out of its deep freeze and thrown into an ultra-warm greenhouse world, with average global temperatures of over 120 degrees.
Now imagine that not only is there strong evidence that this geologic process in fact occurred, but also that it probably happened as many as four or five times in the period from 750-550 million years ago. What's more, this series of global freeze/fry events quite possibly led to the rapid development of multi-cellular life as we know it today.
Snowball Earth: The Basic Theory
To understand how it is possible for the Earth to become fully glaciated for periods of several million years, it is first necessary to understand the mechanism by which the Earth's temperature and climate is principally regulated. Essentially, the vast majority of the Earth's warmth is derived from solar radiation. However, the Earth does not simply absorb all the energy projected by solar rays. Different environments on the Earth's surface vary in that some absorb more solar radiation than they reflect back into space, whereas others have a much higher rate of reflection than they do absorption. In geologic terms, this is measured in terms of a surface's albedo, which is simply the measure of how much radiation is reflected back into space. Surfaces that have a high albedo tend to reflect lots of energy, surfaces that have a low albedo tend to absorb lots of energy.
Oceans have very low albedos ~0.1, whereas ice has a very high albedo ~0.8. The remaining non-glaciated land surface tends to have an intermediate albedo, depending on the specific terrain, though it is always somewhere in between ~0.1 and ~0.8. The build up of oceanic and continental ice sheets leads directly to an exponential increase in the Earth's overall surface albedo. Once global ice sheets reach bellow ~30 degrees of latitude, the global albedo reaches a critical mass at which enough energy is being reflected back into space to allow average global temperatures to drop bellow freezing. The result is a globally glaciated "snowball" Earth.
Looks cold huh? 650 million years ago, this picture could have been taken at the equator.
The next question is, what could cause global ice sheets to reach such low latitudes? The Earth has mechanisms to prevent this global snowball event from occurring. Greenhouse gases, especially carbon dioxide, absorb solar radiation warming the planet up. Today, both photosynthesis and human respiration account for a large percentage of the carbon dioxide released into the atmosphere. However, 600-700 million years ago, when there was relatively little plant and animal life, the amount of carbon dioxide in the atmosphere was mainly determined by the balance between volcanic activity and chemical weathering. Volcanoes released carbon dioxide into the air, while chemical weathering changed carbon dioxide into carbonate.
If the Earth begins to cool, then the subsequent increase in glacial ice leads to an increase in the Earth's overall albedo, which leads to more cooling. However, this process is regulated by the fact that chemical weathering is prevented by intense glaciation. Thus while volcanoes continue to supply carbon dioxide to the atmosphere, the increase in the amount of glacial ice slows down the chemical weathering processes which normally reduce the level of carbon dioxide in the atmosphere. The increased amounts of CO2 in the atmosphere leads to global warming that offsets the increased ice albedo feedback. Again, this is because as a greenhouse gas, CO2 absorbs greater amounts of solar radiation.
What could interfere with this cycle enough to plunge the Earth into a deep freeze? The answer, unassumingly enough, is plate tectonics. Typically, throughout geologic history the land mass of the Earth has been distributed at relatively varied points of latitude. As the Earth cools and glacial ice forms on continents in high latitudes, the decrease in chemical weathering increases the CO2 concentration which offsets the cool-down. However, if the majority of the Earth's landmass was located almost exclusively near the equator at low level latitudes, as geologic evidence suggests it was during the period from roughly 600-700 million years ago, large ice sheets would be prohibited from forming in the warm low-latitude environments. As a result, chemical weathering would be almost completely unhindered due to the lack of glacial ice. The Earth would continue to slowly cool as the levels of carbon dioxide in the atmosphere decreased. Once ice sheets reached below 30 degrees of latitude, the ice albedo feedback would reach a critical mass, plunging the Earth into global glaciation.
With large portions of the Earth's landmass located near the equator 650 million years ago, the Earth slowly cooled despite the absence of much glacial ice. Chemical weathering reduced CO2 levels in the atmosphere. By the time that extensive glacial ice had formed at low latitudes, the ice albedo feedback reached a sufficient level to freeze the Earth. Although the sudden onslaught of ice halted chemical weathering, it would take a build up of almost ten million years for carbon dioxide levels to become high enough to offset the ice albedo feedback level and thaw the Earth out.
Assuming that there is sound geologic evidence to support the Snowball Earth theory thus far, the next question becomes how did the Earth escape this global glaciation? Once the ice albedo feedback reached the level it needed to plunge the Earth into an everlasting winter, what would thaw our little green planet out? The answer, of course, is carbon dioxide.
Once the ice albedo feedback was high enough to freeze the Earth, the absence of liquid water led to the absence of chemical weathering. Chemical weathering was the mechanism by which the amount of carbon dioxide was reduced from the atmosphere. A frozen Earth would lead to a build up of carbon dioxide in the atmosphere, unchecked due to the absence of chemical weathering. Eventually, the build up of CO2 itself would reach a critical mass at which point the Earth would start to thaw out. Because glacial ice at the equator would melt first and because there is more surface area at the equator, the thaw would also take place at an exponential rate. As the ice melted, the massive build up of CO2 in the atmosphere would lead to ultra-warm climate conditions, creating a greenhouse world. Relative to the present levels of CO2 in the atmosphere, it is estimated that the concentration would need to have been about 350 times greater to thaw the Earth out of a neoprotozoic global glaciation.
It is estimated that this freeze/fry process probably occurred several times between 750-550 million years ago. Ironically enough, as long as the majority of the Earth's landmass remained at subtropical latitudes, chemical weathering would continue to reduce the concentration of CO2 in the air to the point at which the ice albedo feedback level could freeze the Earth. Equally ironic, as long as the Earth remained frozen, chemical weathering would remain almost non-existent to the point at which CO2 could build up in the atmosphere to thaw the Earth out again.
But is there real geologic evidence to support this remarkable theory? While the geologic community has not unquestionably accepted as scientific fact that the Earth did go through a series of global freeze/fry events, the theory of the Snowball Earth has more and more become widely accepted throughout the geologic academic community, largely thanks to the work of Paul F. Hoffman, a professor of geology at Harvard University.
So what is the evidence that supports this contentious theory? There are a number of different pieces of geologic evidence that suggests Snowball Earth was part of the geologic past, though its staunch supporters will argue that it is all the evidence combined that really makes the theory sound. This is particularly so because the Snowball Earth theory simultaneously explains a series of relatively unrelated hereunto unexplained geologic anomalies.
1) "Cap" Dolstones
The global presence of neoprotozoic calcium carbonate saturated "cap" dolstones is strong evidence that in the period of time roughly 600-700 million years ago the Earth underwent several abrupt changes from periods of intense glaciation to periods of intense greenhouse gase-induced hot global climates.
Once the glacial ice started melting rapidly due to the reversal of the ice albedo feedback, the newly resumed and rapid hydrolic cycle led to massive chemical weathering of silicate rocks and dissolution of carbonate rocks. As the products of this weathering mixed with the ocean, they would neutralize its acidity thereby driving carbonate rich sediment to the surface ocean.
What was previously unexplained about cap dolstones was the seemingly paradoxical layering of glacial and warm-water sediments on top of each other. Only the rapid melting of large amounts of equatorial continental ice offers a plausible explanation for the close proximity of sediment produced under very different climactic conditions. It is the remarkable closeness of the layers that supports the geologic theory that the Earth underwent rapid periods of global warming in the aftermath of long periods of global freezing.
Paul Hoffman and Daniel Schrag, geologists and strong proponents of the Snowball Earth theory, point to cap carbonate abruptly above glaciomarine dropstones in Namibia.
2) Carbon Isotopic Excursions
Calcium carbonate primarily consists of two different isotopes of carbon: carbon-12 and carbon-13. During the periods of supposed glaciation, the amount of carbon-13 found in layers of calcium carbonate is significantly less than the layers of calcium carbonate above and below. What could explain this?
The main sources of carbon come from dissolved organic matter and volcanic activity. A higher content of carbon-13 in the composition of the dissolved inorganic carbon in seawater indicates that a higher amount of the carbon was derived from dead organisms. The content of carbon-13 emitted by volcanoes is about 1%, whereas the content of carbon-12 emitted is about 99%.
What could cause the level of carbon-13 in the composition of the DIC and calcium carbonate sediments to drop to lower levels? If the Earth entered a global ice age the mass extinction that would ensue would greatly reduce the amount of carbon-13. However, for the overall composition of the DIC to be so radically altered, the amount of organic carbon incorporated to the overall carbon content would have to be reduced for a period of at least several hundred thousand years. Only a global ice age could last long enough to alter the amount of carbon-13 in the DIC and in calcium carbonate sediments.
Banded-Iron Formations are marine deposits of sediment that are rich in iron oxides and sulfides. Typically, most BIF is found in layers of sediment older than ~1.85 billion years. This is because after this time in Earth history, the atmosphere contained enough oxygen to oxidize the iron in the ocean. How is it that over a billion years later, after no sign of BIF in marine deposits, such a substantial amount of it was discovered in layers of marine sediment 600-700 million years ago?
Something would have had to prevent the increasingly oxygen rich atmosphere from oxidizing iron deposits in the ocean. Had the Earth been frozen in glacial ice, this would have effectively reduced the interaction between the atmosphere and the deep ocean, allowing for oceanic deposits of iron rich in oxides and sulfides to accumulate. Given the specific time period in which these bands of FE2+ were deposited, it seems likely that they were caused by long periods of global freezing.
Those sedimentary marine deposits sure do look rich in iron oxides...what, you say they are only 650 million years old!? Incredulous, unless, of course, some bizarre mechanism could have prevented the oxygen rich atmosphere of the time from interacting with marine iron deposits...but what strange geologic process could do that?
4) Low-Altitude Glacial Ice Lines
Paleomagnetic evidence demonstrates that around the time period of these global freeze/fry events, glacial deposits in widely varied locations of very similar ages reveal that the glacial ice line reached sea level around the equator.
Given the climactic conditions of the Earth today, glacial ice can only exist near the equator at sea levels of at least 5000 meters. For glacial ice to reach such low altitudes, the Earth's climate would have to be much, much colder-the type of cold that would have existed on the Snowball Earth.
Perhaps the most interesting impact of this series of global freeze/fry events is the role it played in the evolution of complex multi-cellular life. The common characteristics of multi-cellular animal life as we know it today were more or less defined in the period from roughly 525-600 million years ago. According to the proponents of the Snowball Earth theory, the series of freeze/fry events that occurred could have jump-started evolutionary processes that led to a rapid advance in the development of complex life.
But how exactly does the Snowball Earth theory explain the development of complex life? The long periods of global glaciation (tens of millions of years perhaps) followed by the abrupt periods of global warming eradicated much of the simple forms of life that existed at the time. However, by forcing simple organisms to react to dealing with such harsh climactic conditions, these freeze/fry events could have paved the way for natural selection to take the course it did.
The Snowball Earth, By Paul F. Hoffman and Daniel P. Schrag
Snowball Earth, By Gabrielle Walker (Crown Publishers, New York: 2003)
Author: Alden F. Reed
Last revision: May 4, 2004
Copyright © 2004 Earlham College. Revised May 4, 2004 . Send corrections or comments to mailto:firstname.lastname@example.org