Frozen Earth: The Once and Future Story of Ice Ages (23 page)

By the early 1970s, most of these difficulties were well understood.
Fortunately for research into the Pleistocene Ice Age, evaporation of water from the oceans—the process ultimately responsible for the supply of snow to the glaciers—and cold seawater temperatures both
act to change the oxygen isotope proportions in carbonate shells in the same direction.
Although it was still difficult to separate the two effects in a quantitative way, they at least reinforce one another, producing a stronger oxygen isotope signal than would temperature changes alone.

Research groups interested in glaciation and the Earth’s climate history scrambled to make oxygen isotope measurements on deep-sea sediments.
Two papers from these early studies were especially important in the debate over the astronomical theory of climate.
The first was by Wally Broecker and J.
van Donk, who were working at what was then the Lamont-Doherty Geological Observatory (now the Lamont-Doherty Earth Observatory) of Columbia University.
Published in 1970 in the journal
Reviews of Geophysics and Space Physics,
their paper had the title “Insolation Changes, Ice Volumes, and the Oxygen-18 Record in Deep-Sea Cores.”
Broecker and van Donk used the magnetic properties of the sediments to determine a timescale for the cores, and they showed that when their oxygen isotope analyses were plotted against this timescale, they exhibited a smooth and systematic variation over the past 400,000 years.

What was puzzling about this graph for advocates of the Milankovitch astronomical theory was that it showed several cycles of peaks and valleys, with each cycle lasting about 100,000 years (figure 16).
James Croll had predicted that the eccentricity of the Earth’s orbit, with a cycle close to 100,000 years, would be important for glaciation, but Milankovitch’s calculations of Northern Hemisphere temperatures had shown that the more important parameter is actually the tilt of the Earth’s axis of rotation.
The tilt changes through a cycle lasting approximately 41,000 years.
Combined high eccentricity and maximum tilt might result in especially severe glaciation, but according to Milankovitch, the tilt should be the determining factor.
Why didn’t the oxygen isotopes follow the tilt cycle rather than exhibiting regular 100,000-year variations?

Broecker and van Donk’s work was not the only study that showed the 100,000-year cycles.
Several other groups, some approaching the
problem from different perspectives, found the same thing.
Was the eccentricity really the important thing after all, or was there some unknown process at work with a 100,000-year cycle not connected in any way with the Earth’s orbit?
The debate about whether or not astronomical variations could be responsible for glaciation heated up once again.

As more and more oxygen isotope data accumulated, it became clear that the same variations found by Broecker and van Donk were present in deep-sea cores from all of the world’s oceans.
They were a global phenomenon, and they had to reflect the temperature changes and waxing and waning of the ice sheets that characterized the Pleistocene Ice Age.
The magnetic timescale was crucial for this conclusion, because sediments accumulate at different rates in different places.
The only way to be sure that the peaks and valleys in the oxygen isotope records occurred simultaneously throughout the globe was through accurate dating of each core using its magnetic properties.

The 100,000-year oxygen isotope cycles discovered by Broecker and van Donk are quite regular, but they are not perfectly smooth.
Superimposed on these long cycles are many smaller wiggles.
In a few cases, early workers examined cores that extended back to a million years or more, and in these they found that the prominent 100,000-year cycles seemed to die out between about 800,000 and one million years ago; before that there were also cyclical variations, but on a shorter timescale.
How could all of these features of the oxygen isotope graphs be explained?
Astronomical changes still seemed attractive because of the regular nature and planetwide occurrence of the variations.
But once again the question of reconciling the timing of glaciation with the well-determined orbital variations became an issue.

The problem was solved in 1976, in a paper that appeared in the journal
Science.
Using the technique of spectral analysis—an approach that is capable of disentangling multiple, superimposed, cyclical curves and retrieving the original characteristics of each type of cycle—James Hays, John Imbrie, and Nick Shackleton showed that the oxygen isotope record is actually made up of several distinct, superimposed cycles,
with timescales corresponding almost exactly to the predictions of the astronomical theory: a 100,000-year cycle that reflects changes in the eccentricity of the Earth’s orbit, another cycle of about 43,000 years, close to the timescale of changes in the tilt of the rotation axis, and one near 20,000 years that corresponds well with the wobble of the rotation axis.
Hays, Imbrie, and Shackleton titled their paper “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages.”
Finally, it seemed, the ideas that had been formulated first by Croll and then refined and extended by Milankovitch had been shown to be correct.
Exactly how the orbital variations get translated into glacial-interglacial temperature differences was still uncertain, because virtually every analysis that had been carried out concluded that these variations result in only very small changes in the amount of solar energy received on Earth.
But the close correspondence in timing between the astronomical cycles and the isotopic properties of deep-sea cores could not be denied.
It was unlikely to be a coincidence.
The repeated buildup and decline of the vast
Pleistocene ice sheets must have been linked directly to changes in the Earth’s position relative to the sun.
The Earth’s orbit is truly a pacemaker for climate.

Figure 16.
A representation of climate changes over the past 550,000 years, based on oxygen isotope analyses of deep-sea sediments.
The oxygen isotope “proxy” combines information about both temperature and the amount of glacial ice that exists on the continents.
It is obvious that cold and warm periods alternated on a roughly 100,000-year timescale over this period.

CHAPTER EIGHT

Our Planet’s Icy Past

Oxygen isotopes in deep-sea cores, together with a few other indicators of past climates, have given us a surprisingly clear picture of the coming and going of glaciers during the Pleistocene Ice Age.
But the current ice age occupies only the past few million years, an almost insignificant slice of our planet’s four-and-a-half-billion-year history.
What was the climate like for the rest of that vast sweep of time?
The “norm,” if it is possible to speak of such a thing, was one of warmth and little or no permanent ice.
However, there is good evidence that our small (by the standards of the universe) planet has experienced sporadic ice ages for at least the past three billion years.
Almost as soon as Louis Agassiz had pointed out the significance of glacial drift and other ice-produced effects in the 1830s, geologists began to find similar signs of ice ages in the more distant past.
The very first such reports came from India, where, trapped within layers of sedimentary rocks, deposits of glacial drift were found lying atop scratched and grooved bedrock.
Unlike Pleistocene drift in the Alps or in Canada, the ancient drift in India was not loose, but had been cemented and indurated into solid rock over hundreds of millions of years.
Such drift-turned-to-rock was termed “tillite” by geologists, employing a seventeenth-century word describing chaotic rock deposits that contain fragments of a variety of sizes.
Soon similar occurrences of tillites had been found in Australia, South Africa, and South America.
To early geologists, one of the most startling aspects of these discoveries was that many of the places showing evidence of past ice ages were tropical or subtropical.
In those pre-plate-tectonics days, when the continents were believed to be fixed and immobile, it was difficult to imagine tropical ice sheets.
It seemed reasonable enough to think that glaciers in the Alps had once been more extensive, or that there might have been ice in Scotland in the past, but an Earth with glaciers near the equator was hard to grasp.

Aside from the problem of ice in the tropics, a major difficulty for those attempting to characterize ancient ice ages was that much of the evidence is missing.
Even for the Pleistocene glaciation, erosion has obliterated some of the geological signs of ice action.
The most recent glacial advance of the Pleistocene ended only some twenty thousand years ago, and most of its effects on the landscape are still quite obvious.
But earlier Pleistocene glacial advances and retreats, even those that occurred only one or two hundred thousand years ago, are much more difficult to study, because the moraines and erratics and scratched bedrock from those episodes have not all been preserved intact.
Such difficulties are compounded many times over for the ice ages that occurred in the Earth’s very distant past.
But in spite of this, geologists have been able to identify at least four periods of severe glaciation that occurred long before the Pleistocene, all probably more intense than the current ice age.
The timing of these is shown schematically in figure 17—the earliest known dates to about 2.9
billion
years before the present, the most recent, 300 million years.
Some of the early ice ages are depicted here as a series of events stretching over several hundred million years; at these distant times in the past the uncertainty in dating is such that it is not clear whether these were actually discrete ice ages, or just especially severe intervals within an overall cold period.
In addition to the ones shown, several other times have been identified when the planet experienced cool periods, if not full-blown ice ages.

Figure 17.
The Earth’s major ice ages, as identified from glacial drift, tillite, varves, and glacially scoured bedrock.
Heights of the shaded bars give a rough indication of the intensity of these glacial periods, although the estimates are speculative for the glaciations between 2.2 and 2.4 billion years ago.
In addition to the four major ice age periods before the Pleistocene discussed in the text, a further cold period that occurred 450 million years ago has been identified and is shown here.
It is not possible to represent the durations of ice ages accurately on this small graph.

Traveling back into Earth’s history from the present, the first really major ice age that appears in the geologic record occurred about 300 million years ago.
This is the very same ice age for which evidence was uncovered in India and other southern continents in the nineteenth century.
It is worth considering for a moment what criteria must be satisfied before an ancient event can be called an ice age.
How is it possible to know, for example, that a tillite, or glacial scratching, results from global glaciation and not from local mountain glaciers?
Usually, at least three important characteristics must distinguish the evidence for glaciation.
First, the effects of ice sheets should be widespread, usually meaning that they occur on several, well-separated continents.
Secondly, the widespread glacial deposits must be contemporaneous.
And, finally, there should be evidence that the glaciation took place at low elevations in most localities.
This is not so difficult to establish as it might seem, because when ice sheets reach the sea, they drop glacial drift into the ocean, where it is preserved under later blankets of sediments.

The older the purported ice age, the more difficult it becomes to satisfy all of these criteria, especially the criterion of contemporaneity.
If a dating method is accurate to a few percent, the uncertainty in dating a glacial advance that occurred 100,000 years ago is only a few thousand years, but for a 300million-year-old tillite, it can be six to ten million years.
That’s several times the length of the entire Pleistocene Ice Age.
Furthermore, no method has yet been devised that can accurately date a glacially scratched surface, or an ancient tillite.
Usually, the best that can be done is to bracket the age by dating lava flows or volcanic ash layers that occur above or below the glacial deposits.
Sometimes, fossils in sedimentary rocks that accompany the glacial deposits are useful too, but usually these can only bracket the time of glaciation and do not date it directly.