Frozen Earth: The Once and Future Story of Ice Ages (19 page)

However, the ice-dammed lakes that caused both the Scablands and Siberian floods were midgets compared to Lake Agassiz, the vast lake that formed along the margin of retreating Pleistocene ice sheets in central Canada.
Its shorelines shifted around considerably during its more than 4,000-year-long history, but at one time or another, it stretched eastward from Saskatchewan across Ontario and into Quebec, and from Hudson Bay south into Minnesota and North Dakota.
At its maximum, it is estimated to have held more than twice the volume of the Caspian Sea, the Earth’s largest present-day lake.
As the glaciers gradually
retreated northward at the end of the last glacial period, drainage from Lake Agassiz periodically changed.
Every so often, a new, lower outlet to the sea would become available and many thousands of cubic kilometers of water would be released quite suddenly.
Over its history, the lake drained eastward through the St.
Lawrence River to the Atlantic, southward through the Mississippi to the Gulf of Mexico, northwest along the Mackenzie River to the Arctic Ocean, and, finally, north through Hudson Bay.
Lake Agassiz contained thousands of years of stored precipitation, and during its last major draining—nearly 8,500 years ago—it released an amount of water that was roughly one hundred times the peak volume of Lake Missoula.
How catastrophic this event was is not known.
Unlike Lake Missoula, Lake Agassiz didn’t drain through a single, ice-blocked valley, however, but leaked into Hudson Bay through the margin of the gigantic retreating ice sheet.
This may have softened its impact on the landscape (there is no visible surface evidence of flooding as there is in the Scablands or Siberia).
Nevertheless, those who have mapped out the shifting shorelines of the glacial lake conclude that it drained suddenly, not in a series of small steps.

Even if the draining of Lake Agassiz didn’t leave a record of giant potholes and ripple marks, it may have had another, quite different, effect: it may have influenced the ice age climate of Europe and North America, and perhaps the entire Earth.
That is one of the reasons there is currently great interest in the history of this particular glacial lake.
It is also another example of the fascinating interconnections that occur in science.
How could Lake Agassiz, large though it was, have a profound influence on the climate of an entire hemisphere?
The answer, in very general terms, is that the sudden introduction of a huge volume of fresh water into the ocean has the potential to change oceanic circulation—and oceanic circulation is closely tied to climate.

The Gulf Stream brings salty, relatively warm water into the North Atlantic Ocean, and as that water moves north and is cooled by the frigid arctic air, it becomes quite dense and sinks.
That is what keeps the Gulf Stream operating: more water is continually drawn from the
south to replace the water that sinks.
But cooling the Gulf Stream is a two-way street; heat released from the water warms the air, and helps give Scandinavia, the British Isles, and indeed Europe as a whole, relatively mild climates.
If the Gulf Stream circulation were to slow down, or stop altogether, the European climate, especially in the north, would resemble that of Siberia.

That may be what happened when Lake Agassiz suddenly drained into the North Atlantic.
Introduction of such a large amount of low-density fresh water would have reduced the density of the North Atlantic surface water considerably, preventing it from sinking and thus slowing down (or perhaps even shutting off completely) the Gulf Stream.
There is evidence that such a scenario may have occurred more than once.
Each of the major changes in drainage that affected Lake Agassiz would have had a profound effect on ocean circulation.
Ice cores from Greenland show that cold periods closely follow these drainage shifts, with two of the largest temperature drops in the past 15,000 years occurring near 12,800 and 8,200 years ago—approximately the times when Lake Agassiz released large amounts of fresh water into the North Atlantic, first when its drainage shifted from the Mississippi to the Great Lakes and the St.
Lawrence, and later when it finally drained into Hudson Bay.
Although it is difficult to trace past fluctuations in ocean circulation through direct measurements, these coincidences in timing constitute strong circumstantial evidence that there were such changes, and that climate was affected.

It may be that the interaction between melting glaciers and ocean circulation was more or less continuous, and that rapid release of meltwater from large lakes such as Lake Agassiz were simply short-term, speeded-up events in an ongoing process.
As the climate warmed, ice melting would have supplied more fresh water to the North Atlantic, which would have slowed the transport of warm waters from the south, leading to a decrease in average temperatures.
In the colder climate, retreat of the ice sheets would have halted temporarily, or the glaciers might even have advanced again.
Eventually, as the overall global
climate continued to warm, the ice would have begun to melt again.
Such oscillations, driven by melting of glaciers around the northern rim of the Atlantic Ocean, would explain why there is evidence in the Channeled Scablands of multiple episodes of freezing and thawing of the ice dam that blocked the drainage of Lake Missoula.

The glacial floods chronicled by Harlan Bretz and others were cataclysmic events of the sort that occur only rarely, even on the very long timescale of our planet’s history.
It is quite clear from Bretz’s early papers that he recognized the uniqueness of the Channeled Scablands and the processes that were responsible for their unusual morphology.
But he most certainly could not have foreseen that half a century later, because of his work, he would become a guru for planetary geologists, and that his studies in the Scablands would be heavily cited in connection with similarly gigantic floods on another planet.
Beginning in the 1970s and continuing up to the present, a series of spacecraft missions to Mars have beamed back images of features on the red planet that are remarkably like those of the Channeled Scablands.
Although there are some dissenters, many planetary scientists are convinced that flowing water, some of it quite likely in the form of huge floods, have played a role in shaping the landscape of Mars.
NASA has even sponsored conferences and field excursions to the Channeled Scablands in order to learn more about the processes that occurred during the Lake Missoula floods.

There is no running water on the surface of Mars today.
At mid latitudes the average temperature is about –75°C, colder even than on the Earth’s Antarctic Ice Sheet.
In addition, the atmospheric pressure on Mars is only 1 percent of that on Earth, and there is almost no water vapor in the atmosphere.
Any liquid water that appeared on the Martian surface would quickly freeze or evaporate into the atmosphere.
Mars does have polar ice caps, however, composed of both water and carbon dioxide ice.
There is also good evidence for water stored in the ground as permafrost, and—based on some of the most recent high resolution images—there may even be some real glaciers, partly covered with rocky debris.

In the simplest of descriptions, the geography of Mars comprises a southern highland region that has been heavily cratered by impacts, and a smoother, lower-lying northern section.
The higher crater density in the southern highlands implies that this region is older than the less-cratered lowlands to the north.
But some of the images that have been sent back from Mars suggest that the northern plains are smooth, not because they are less cratered, but because they are filled with sediment—there is evidence of craters and other features buried beneath the surface.
Mars has frequent large dust storms—the very first mission to Mars, in 1971, encountered one that didn’t clear to reveal the surface for months—so it is possible that the northern sediments were transported by wind.
But it is also the case that some of the largest channels on the planet, with numerous features that suggest flowing water, seem to empty into the northern lowlands.
Could this region once have been a sea of standing water, and could the sediment that now blankets the lowlands have been transported into this sea by giant floods?
This is a question that has excited the imagination of both scientists and the general public, not least because the presence of liquid water on the Martian surface would enhance the possibility that extraterrestrial life has developed there.

In 1997, a spacecraft called the Mars Global Surveyor was put into orbit around Mars.
On board is a camera that can resolve features on the surface down to about 1.5 meters in size.
There is also a laser-based altimeter that can measure the height of the surface to a resolution of about 10 meters.
Together, these instruments are giving planetary scientists a wealth of new information about the Martian landscape on a scale that was never before possible.
The most recent data both confirm and extend earlier results that point to large-scale flooding at some time in Martian history.
Information from the laser altimeter has been especially important, because this instrument makes it possible to map the elevation of Martian channels in great detail.
Six of the largest channels on Mars—all of them estimated to be several billion years old—enter the northern lowlands and then suddenly become very indistinct.
The
laser altimeter measurements indicate that this occurs at about the same elevation for each of the channels, in spite of the fact that they are widely separated geographically.
It has been suggested that this happens because the channels reached an ancient shoreline.
They continue, indistinctly, far out into the northern plain, but these faint traces could have been made by the same type of sediment-laden “turbidity current” that carried sediments from the Missoula floods 1,000 kilometers out into the Pacific.
If this interpretation is correct, if the channels really did empty into a standing body of water, it would have been a vast ocean covering 25 million square kilometers of the northern part of Mars to a depth of 560 meters.
That would require a Martian climate very different from today’s, even if, as some have suggested, the ocean persisted over a long time period only because its surface was frozen, keeping evaporation to a minimum.

The high-resolution images show that many of the Martian channels exhibit all the features of those in the Channeled Scablands.
They often have an anastomosing pattern, widening and narrowing along their path.
They flow around teardrop-shaped hills reminiscent of the residual loess hills in the Scablands.
They show linear grooves and scouring marks similar to those produced in the Columbia River basalt by the Missoula floods.
There is evidence of large cataracts.
These features, especially when considered together, seem to require large-volume flooding.
Estimates made by different scientists of the amount of water involved, however, range very widely, from channel flow rates not much more than that of the Mississippi River to truly catastrophic floods with peak flow rates nearly two hundred times greater than the largest Lake Missoula or Siberian floods.

One of the reasons why Martian flood volumes are so uncertain is that the source of the water is unknown.
Just as Bretz’s opponents seized upon this issue during the Scablands debate, so too critics of Martian flooding have focused on the source problem.
There is no sign that there were glacier-dammed lakes above the Martian channels.
However, some of them begin suddenly in regions of “chaotic terrain”
that have been interpreted as areas where the ground was disrupted by sudden melting of subsurface ice, perhaps by volcanic heating.
In fact, volcanic heating is the most plausible process for quickly melting large volumes of ice on Mars and initiating floods.
Like Bretz, those who argue for catastrophic flooding on Mars point to the field evidence as proof enough, regardless of the source for the water, although for Mars, their “fieldwork” requires interpreting images rather than actually tramping over the ground and making direct observations.
In spite of the similarities between the Scabland landscape and the features of the Martian channels, it will most likely require a breakthrough in understanding the source of surface water on Mars to convince the skeptics that catastrophic floods really took place there, just as it took Pardee’s giant ripple marks to demonstrate that Lake Missoula supplied the water for the Scablands floods.

CHAPTER SEVEN

The Ice Age Cycles

Just a few years before Bretz published his first paper on the Scablands, a book appeared in Europe that was to have a lasting impact on thinking about the Pleistocene Ice Age.
It was written by a Serbian mathematician who, in all probability, had never seen a moraine or an erratic boulder.
But he was a gifted theorist who was ambitious and eager to take on large, unsolved problems in the sciences.
The mathematician was Milutin Milankovitch, and he eventually settled on climate as a field that seemed ripe for the application of mathematical principles.
Today, you will find his name in textbooks on geology, climatology, and the environmental sciences.
Milankovitch revived and refined James Croll’s ideas about the connection between ice ages and changes in the Earth’s orbit around the sun.
The repeated cold-warm cycles that are reflected in the advance and retreat of glaciers during the ice ages are now usually referred to as Milankovitch cycles.

Milankovitch and his twin sister were the eldest of seven children, born in 1879 into a well-to-do family in the town of Dalj, which sits alongside the Danube in what is now Croatia.
The family had lived in Dalj for generations, and when Milutin was growing up, it was a pleasant and peaceful village, surrounded by fertile farmland, part of the Austro-Hungarian Empire.
Milankovitch’s education began at home,
where he was taught by a governess, but at age ten, he was sent away to a secondary school in another town, where he stayed with relatives.
“Sent away” is perhaps too strong a term; the school was only fifteen kilometers away, a shorter distance than people today travel to a favorite restaurant or a movie, but in Milankovitch’s childhood, transport was slow, and he boarded with relatives.
He was an indifferent student who found schoolwork easy and not very challenging, and he didn’t work especially hard.
Still, at the end of his second term, when the reports were being given out, he was called to the front of the class and proclaimed the top student.
This came as something of a surprise to Milankovitch, and it placed upon him, he said later, a “moral obligation” to excel.
And excel he did, especially in mathematics, graduating with top honors in the summer of 1896, at age seventeen.
But then he was faced with a problem: What to do next?