Frozen Earth: The Once and Future Story of Ice Ages (25 page)

The widespread distribution of glacial features has long convinced geologists that this ice age was unusually harsh.
At least two and perhaps as many as four or five major icy episodes, separated by warmer intervals, have been identified during the long cold period.
But the big surprise about the Late Proterozoic glaciation came in the late 1950s and early 1960s, when researchers found that the magnetic properties of rocks associated with some of the glacial deposits indicated formation at very low latitudes.
As we have seen, during the present ice age, Northern Hemisphere glaciers have never pushed farther south than 40–45°, and even for the more severe Permo-Carboniferous Ice Age, there is no indication that ice reached much closer to the equator than 35° latitude.
In both cases, fossil evidence suggests that the tropics remained fairly warm.
If Late Proterozoic glaciers had existed near sea level in the tropics, that would indicate a very different ice age indeed, and initially many geologists were skeptical.
But as more and more data became available, the initial results were corroborated.
It seemed
inescapable that frigid climates had extended very close to the equator.
Thus was born the concept of Snowball Earth.

The idea that there was a truly global ice age, a Snowball Earth, is still controversial.
Geological evidence suggests that during most of our planet’s history, the Earth’s average temperature has stayed within rather narrow limits, and critics ask, Under what conditions could this have changed so that the Earth froze over completely, from pole to equator?
And, if it happened once, why haven’t we had more Snowball Earth episodes?
At the extreme of Snowball Earth conditions, the average surface temperature would have been closer to that of Mars than anything we are familiar with today—perhaps about –50°C.
Could even the primitive life of the Late Proterozoic have survived such extremes?
There is general agreement that the glaciation was severe, but as this is written, the jury is still out on whether or not a true Snowball Earth occurred.
It is nonetheless worth examining some of the major issues in the controversy.

The magnetic data are perhaps most crucial for the hypothesis, because they fix the latitude of the continents at the time of glaciation.
The term “Snowball Earth” itself was coined because magnetic measurements placed the glaciated regions in the tropics, so it is reasonable to assume that their reliability has been carefully scrutinized.
It is well known that magnetic measurements can be problematic, especially for old rocks, because they rely on a very accurate determination of the “frozen in” magnetic orientation that was captured when the rocks formed.
With care, the orientation can be measured quite accurately with respect to the rock’s current position on the Earth’s surface, but what happens if the rocks have been folded, or tilted, or otherwise moved from their original positions at some time over the past 700 million years?
And what if they have been deeply buried and heated, as often happens?
Heating can have a significant effect on the stored magnetism, and in extreme cases, heated rocks can be remagnetized—perhaps in an orientation that is completely different from the original.
Fortunately, there are ways around these problems.
Those who study the
magnetic properties of rocks have devised ingenious laboratory approaches that allow them to strip away the magnetic “overprinting” caused by heating and to recover the original magnetic orientation.
And because the layers of sedimentary rocks are always horizontal when they are first deposited, folded and tilted sedimentary rocks can be “unfolded” or “untilted” (not literally, but by applying a correction factor to the measured data) to recover their original orientation.
Doing this for several samples tilted or folded at different angles actually provides a very good test of the reliability of the measurements: if they all agree after unfolding, one can have a high degree of confidence in the results.

Magnetic measurements have been made on rocks from around the world that are associated with the Late Proterozoic glaciation.
They have been repeated in different laboratories with good agreement, and they show that the majority of these localities were at latitudes less than 30° at the time of glaciation, and none were farther than 60° from the equator.
In fact, there seem to have been no landmasses near either of the poles in the Late Proterozoic.

Even if glaciers existed at low latitudes, there is still the question of whether the evidence we have comes from ice in high mountains, or at low elevations.
This is not such a difficult question to answer, because, as already mentioned, most of the glacial effects that remain from the Late Proterozoic are preserved in sedimentary rocks.
It is fairly clear that some of the tillites were either deposited in shallow seawater at the edge of a continent, or so close to the shoreline that a slight change in sea level engulfed them in marine sediments.
The same is true of preserved glacial markings on bedrock.
Overall, the evidence is very strong that even the tropical ice sheets extended right down to sea level.

However, even such extreme climate conditions do not automatically lead to the central conclusion of the Snowball Earth theory: that the oceans were frozen too.
Evidence for that idea came first from an examination of ocean sediments from the Late Proterozoic by Joe Kirschvink, a geochemist at CalTech, who coined the term “Snowball Earth.”
In 1992, Kirschvink pointed out that peculiar sedimentary
deposits rich in iron, referred to as banded iron formations, or BIFs, occur in a number of localities around the world just at the time of the Late Proterozoic glaciation.
Geologists were familiar with BIF deposits from very early in the Earth’s history, but none were known for about a billion years before the Late Proterozoic glaciation, and none have formed since that time.
Their occurrence requires the buildup of very large amounts of dissolved iron in seawater, a phenomenon that cannot occur today because of the oxygen-rich atmosphere.
As anyone who has had to deal with rusty metal knows only too well, oxygen combines rapidly with iron and forms rust.
BIFs are basically rust deposits (with a few other components as well), and their restriction to the early part of the geologic record is thought to be due to the low concentration of oxygen in the atmosphere at that time.
Under such conditions, iron from the weathering of both continental and undersea rocks would accumulate in the oceans until it came into contact with oxygen—perhaps produced by photosynthetic algae living in surface waters—whereupon it would be oxidized and precipitate out as a BIF.
The occurrence of BIFs in the Late Proterozoic was an enigma, because by that time in the Earth’s history, there was enough atmospheric oxygen to prevent the necessary buildup of dissolved iron in the ocean.
But Kirschvink reasoned that if the ocean were frozen, preventing any exchange with the atmosphere, its oxygen content would be rapidly depleted.
Iron concentrations would increase to high levels, and BIFs would be deposited when the sea ice melted and oxygen from the atmosphere again began to exchange with the ocean.

Kirschvink’s proposal seemed reasonable, but it was not convincing to everyone.
Perhaps locally oxygen-poor basins—which occur because of restricted circulation even in today’s oceans—could have served as hosts for BIFs during Snowball Earth.
That would still not explain why they are absent before and after the Late Proterozoic, but it did cast some doubt on the frozen ocean hypothesis.
Then, in 1998, four Harvard University researchers, led by the geologist Paul Hoffman, published the results of a study they had made in northern Namibia,
which, they believed, made a strong case for the Snowball Earth theory.
The region showed clear evidence of low latitude (approximately 12°S) glaciation between about 760 and 700 million years ago.
An interesting and important aspect of their work was that the rock sequence they investigated indicated that the glaciation had both started and ended abruptly.
Immediately overlying the glacial deposits—as is the case at many other Late Proterozoic Ice Age localities—they found limestone-like sedimentary rocks of a kind that form only in warm, tropical waters.
Where they occur over glacial deposits, these distinctive formations have been termed “cap carbonates” by geologists.
They signify a rapid transition from very cold to very warm conditions.

The presence of cap carbonates in Namibia highlighted one of the issues consistently raised by critics of the Snowball Earth hypothesis, the problem of how the Earth could ever have thawed out again once it was completely ice-covered.
The high reflectivity of the snow and ice would have bounced much of the solar energy that normally warms our planet right back out into space.
An entirely frozen Earth, critics of the theory claimed, would have reached a climatic point of no return and could never have recovered.
But the cap carbonates in Namibia and other localities indicate that it did recover, and very rapidly at that.
Had the deep freeze really been as severe as the proponents of Snowball Earth would have it?
And if it was, just how had the climate changed abruptly from icy to tropical?

A possible solution to the permanently frozen Earth problem had actually been suggested by Joe Kirschvink several years before the work of Hoffman and his colleagues.
Kirschvink’s idea was that carbon dioxide, a “greenhouse” gas that traps solar energy in the form of heat in the atmosphere, would build up to high levels under Snowball Earth conditions, eventually leading to global warming and complete melting of the glaciers.
His reasoning went approximately as follows.
First, we know that the main source of CO
₂
to the atmosphere is volcanic eruptions, which spew out gases as well as lava.
Secondly, there are two removal mechanisms that keep CO
₂
roughly in balance—one is photosynthesis
(plants use CO
₂
to make organic tissue, and release oxygen into the atmosphere as a by-product), and the other is the chemical weathering of rocks on the surface.
This process will be explored in more detail later in this book, but in the simplest terms, CO
₂
is removed from the atmosphere because it dissolves in rainwater to make carbonic acid, which attacks and dissolves rocks.
Because there is no evidence to suggest that the level of volcanic activity—the source of carbon dioxide—was radically different in the Late Proterozoic compared with today, a buildup of atmospheric CO
₂
could only occur if there was a decrease in its rate of removal.
That is exactly what one would expect if the oceans were frozen—most of the photosynthetic organisms living in the sea would die, greatly diminishing one of the removal mechanisms, and the cycle of evaporation from the ocean and precipitation over land that drives the other, chemical weathering of rocks, would also cease.
Dry, desertlike conditions would prevail globally.
With the two primary CO
₂
removal processes greatly diminished or shut down altogether, its concentration in the atmosphere would be expected to rise to quite high levels.

The CO
₂
concentration that would be required to warm the Earth and melt a completely frozen ocean today is very high.
At the time of the Late Proterozoic Ice Age, even higher contents would have been necessary.
According to astronomers who investigate the life cycles of stars, our sun’s energy output has gradually increased over the Earth’s history and would have been significantly lower during the time of Snowball Earth.
Some researchers believe this was a factor in the initiation of the Late Proterozoic ice ages, but whether it was or not, a CO
₂
concentration several hundred times that of today’s atmosphere would have been required to thaw the completely frozen planet.
At those levels, melting would begin first at low latitudes, and, once begun, would proceed in a runaway fashion with the help of positive feedback.
Decreasing ice cover meant that more and more of the incident solar energy warmed the ocean instead of being reflected back into space.
Evaporation from the newly uncovered ocean increased atmospheric
humidity, which in turn intensified the greenhouse effect, because water vapor is an even more efficient trap for the sun’s energy than CO
₂
.
If this version of the Snowball Earth theory is correct, not only did the planet suffer through periods of extreme cold in the Late Proterozoic, it also endured brief but intense “super greenhouse” episodes when global temperatures soared far above anything experienced since, before returning to more normal levels as the CO
₂
balance was restored.

The work in Namibia by Hoffman and his colleagues supports the Kirschvink scenario of a buildup in atmospheric CO
₂
.
A centerpiece of their investigation is the chemical data they collected.
Like those who study the effects of Pleistocene Ice Age cycles in deep-sea sediments, they used the isotopic composition of sedimentary rocks to monitor environmental changes.
But instead of measuring oxygen isotopes, they examined isotopes of carbon.
These, like oxygen, can be fractionated when some process prefers one isotope over another.
During photosynthesis, the plankton that live in seawater extract carbon from the oceans, preferentially taking up one of the isotopes of carbon, carbon-12, relative to the other, carbon-13.
This leaves behind seawater enriched in carbon-13.
The Harvard researchers found that in the Namibian rocks deposited just before the ice age began, the carbon isotopes are consistent with this normal situation.
But in the glacial interval, and through hundreds of meters of the cap carbonates that overlay the glacial deposits, there is no longer evidence of an enrichment in carbon-13.
The logical conclusion is that photosynthesis had ceased, that the ocean was effectively “dead,” because it was frozen and had been that way through the several million years represented by the glacial and cap carbonate sediments.
The CO
₂
that would normally be consumed in photosynthesis instead built up in the atmosphere.