Milankovitch Cycles Part 2: Agassiz’s Boulders and Earth’s Orbital Cycles


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The Beginning of the Search: Agassiz’s Boulders

This is second in a series by RCA Member David Horne.

For hundreds of years, people in northern and central Europe had been noticing large, out-of-place boulders that seemed to have been dropped in a field, or popped up from underground. “Erratics,” they came to be called. Legends attributed the boulders to giants, trolls, and the Devil. Eventually, by the 1700s, the notion began to take hold that these boulders could have been transported by the agents of water: huge floods, ice or mud. Some suggested Noah’s Flood as a likely culprit. By the early 1820s many natural history philosophers, scientists, came to focus on ‘ice’ as the likely mechanism of transport. But the source of all the ice was a mystery. Also by this time, reports were coming in from continents on both hemispheres, of similar observations. One scientist even speculated that some not understood circumstance caused a change in Earth’s rotational axis, resulting in a severe change in the climate in Northern Europe and the attendant development of huge sheets of ice. But it was only speculation. There was no understanding that ice and glaciers had covered large portions of the Earth, not once but repeatedly, and that these events seemed to occur in cycles.

At the end of the last Glacial Epoch, a huge ice dam impounding the waters of Great Lake Missoula broke, refroze, and broke again on several occasions sending flood waters down the Columbia Basin. These “Bretz Floods” backed up water in the Willamette Valley several hundred feet deep and carried with them large erratics, and most of our soil. The soil making the Valley so fertile, comes from Eastern Washington, for which we thank them. Here is a good recent article to start with.

At the end of July, 1837, at the annual meeting of the Swiss Society of Natural Sciences in Neuchatel in the Jura Mountain region of Switzerland, Louis Agassiz, the young president of the association and former fish fossil expert turned geologist, presented a paper to the august assembly — on boulders. He explained that the large, faceted, and oddly scratched boulders (erratics) littering the area around Neuchatel, were rocks which had been transported long distances by glaciers. He was not the first to suggest this, but he went much further.

Agassiz’s investigations had led him to conclude that large portions of northern Europe and North America had cycled through periods when they had been covered by enormous sheets of ice extending down from the Polar Regions. And, he believed, this had occurred more than once. Agassiz’s theory received mixed response; at least one scientist suggested Agassiz return to his study of fossil fish. It required 30 more years of “lively” debate amongst very large egos, and investigations by a widespread assortment of unique personalities and adventurers, before the glacial theory came to be accepted. But by the mid-1860s, it was accepted, at least in its general outlines.

Within a few years of Agassiz’s proposing his theory, scientists began looking for a scientific explanation of the cycles which caused glaciers to cover much of the Northern Hemisphere. The speculation came to focus on the amount of radiation received by the Earth from the Sun. Scientists began looking for cyclical changes in solar radiation – changes large enough to drive planet-wide temperature cycles and large enough to cause continent-wide sheets of ice to form.  

A Theory Begins

At first, many theories for the glacial cycles were advanced: Earth’s passage through a large dust cloud in space occupying a part of its orbit, dust particles from space falling on Earth, and the concentration of carbon dioxide in the atmosphere. Carbon dioxide has the unique property of being relatively transparent to the ultraviolet short wave radiation received from the sun which heats Earth and its oceans, and burns your skin even on a cloudy day. But it is relatively opaque to the long wave infrared radiation that we feel as "heat." The result is that, after the earth and waters are heated by the higher energy ultraviolet radiation, the CO2 in the atmosphere impedes the radiation of the infrared heat from the Earth back into space. The higher the concentration of CO2, the more heat is trapped heating the planet even further. And colder ocean waters hold more CO2 in solution than warmer waters; warmer waters release the gas into the atmosphere. Scientists however, saw no cause for cyclical variations in CO2 corresponding to the cycles of glaciations. A few scientists finally began looking at the relationship between Earth’s orbital mechanics and the amount of solar radiation it received as a potential driver of these periods of glaciations, ice ages.

By the closing decades of the 19th Century, scientists began focusing in on the details of Earth’s orbital mechanics as the explanation for the long-term climate swings and the periods of ice cover, glaciations. The key element considered by the scientists as the ultimate cause of ice ages and long term climate cycles, was the amount of heat, solar radiation, received by the Earth, termed “insolation” (incoming solar radiation). The question they asked was: what would cause the amount of radiation to cycle through periods of increase followed by decrease, sufficiently large enough that large portions of the Earth would periodically accumulate massive sheets of ice.

In the first years of the 20th Century, Milutin Malankovich, a remarkable, multi-talented genius, building on the work of predecessors, put the pieces of the puzzle together. He published a detailed theory, called the Milankovich cycles (sometimes the Milankovich-Croll cycles after the two scientists who formulated the basic concepts and mathematics). He developed a complex mathematical model which enabled him to calculate cyclical temperature variations at different latitudes, based on Earth’s orbital positions during the various cycles — at cloud top level. His model predicted that these cyclical changes were the driver of Earth’s climate swings and Glacial Periods over the last several million years. Scientists began looking for evidence of his predictions in Earth’s geologic history. They began looking for the “markers” of Earth’s orbital cycles in the geologic record. To these cycles, our story now turns.

Earth’s Orbital Cycles

We all learn that Earth orbits the sun and as it does so it revolves on its axis. But what is not emphasized is that every aspect of Earth’s orbit about the sun changes in repeating cycles of tens or hundreds of thousands of years. By the middle of the 1870s, the major features of Earth’s orbital mechanics had been described in mathematical detail, back for approximately 1 million years. I have divided the discussion into two parts: Rotational Mechanics and Orbital Mechanics.

Rotational Mechanics

Axial Tilt/Obliquity

Amateur astronomers know that Earth’s axis is inclined 23.5 degrees in relation to its plane of orbit around the sun. During Earth’s yearly orbit around the sun, the direction of the tilt remains constant with respect to the background stars. The result is that at one point of its orbit, the North Pole points away from the sun, currently in December — and at the opposite point of Earth’s orbit, the pole points more towards the sun, currently in June.

This of course impacts the amount of solar radiation received at certain latitudes by spreading it out over a larger surface area. The angle of tilt of the axis slowly changes over a period or cycle of 41,000 years from 22.1 degrees to 24.5 degrees. It also defines the 66.5 degrees of latitude of the Arctic and Antarctic circles (23.5 + 66.5 = 90).

Earth is not alone in its axial tilt. Other planets exhibit an axial tilt: Mars’ varies as much as 60 degrees over millions of years and its cycle is more erratic compared to Earth’s. The initial causes of the tilt are still a matter of debate. But the extent of the tilt of Earth, and other planets seems to be related to various gravitational interactions involving the sun and other planets. 

This leads to the second feature of Earth’s orientation during its orbit: precession.

Rotational Precession

While Earth spins on its axis every 24 hours, it is acted upon by the gravity of the sun and moon on a not perfectly round Earth. This introduces a slow “wobble” into the spin. So, while the Earth is orbiting around the Sun tilted on its axis, the arrow of its axis describes or moves in a slow circle. A spinning top is the analogy most used. This wobble is termed precession and is separate from and occurs in addition to the obliquity cycles of the axial tilt. Precession, in other words, occurs around the axis determined/set by the axial tilt. Precession occurs because of the due to the gravitational pull of the Sun and Moon on the bulge of Earth’s equator, the Earth is not a perfect sphere. 

The “wobble,” precession, occurs on a much shorter time frame than the axial tilt cycle.  The result is that Earth’s orientation in its orbit, as seen against the background stars, is always changing due to the precession. And this determines which star is the “pole star.” Around 2500 BCE, the North Pole of Earth’s axis pointed near the star Thuban in Alpha Draconis. The precession is a circular motion, the location in the sky where the axis points describes a circle over time in a clockwise direction, taking approximately 26,000 years to describe a complete circle. The following diagram illustrates that circle against the background of the constellations.  

And, as we will see, while this is occurring, Earth’s orbit itself is rotating in the opposite direction which cause the “dates” of the equinoxes to sift forward or “precess.” This is termed the Precession of the Equinoxes, and we will pick up and continue in the next article with this and the remainder of Earth’s orbital features.