Past Climate of the Midwest

Climate, like other parts of the Earth system, is not static but changes over time, on human time scales and even on much longer time scales. Latitude, for example, has a very direct effect on climate, so as the continents shift over geologic time, the climates on them also shift. Furthermore, the conditions on the Earth as a whole have varied through time, altering what kinds of climates are possible. What is now the Midwest has gone from being ice-covered to tropical and back during its long history!

Ancient climates are reconstructed through many methods. Written records and tree rings go back hundreds of years, glacial ice cores hundreds of thousands of years, and fossils and rocks that indicate different climates go back hundreds of millions of years. These clues, coupled with modeling and a knowledge of physics and chemistry, help climatologists put together an increasingly detailed history of the Earth’s climate, and of that of the Midwest. Unfortunately, we do not have as clear an understanding of climate for the earliest part of Earth history as we do for the later parts because the oldest rocks are much more difficult to come by. But for those very old times, we can still say something about the climate of the Earth as a whole, and that relates to the very important information related to atmospheric chemistry.

Ancient Atmosphere

Not long after the Earth first formed, more than 4.5 billion years ago, its atmosphere was composed mostly of hydrogen and helium. Volcanic activity and collisions with meteorites and comets added water vapor, carbon dioxide, and nitrogen. As the Earth cooled enough for liquid water to form, the vapor in the atmosphere formed clouds from which the rain poured forth in such a deluge as will never be repeated. These torrential rains were constant for millions of years, absorbing salt and other minerals from the earth as the rainwater coursed to the lowest areas, forming Earth’s oceans and seas.

At this time, the sun produced significantly less energy than it does today, so one might expect that once the oceans formed, they would continue to cool and eventually freeze. Yet temperatures stabilized, perhaps because there was a greater concentration of potent greenhouse gases in the atmosphere and less land surface to reflect light, so temperatures remained high enough for liquid water to exist. Indirectly, the ocean was responsible for the final ingredient of the modern atmosphere because it was home to the first life on Earth. Photosynthetic bacteria appeared perhaps as early as 3.5 billion years ago, but the abundant iron and organic matter quickly absorbed the oxygen they produced. After hundreds of millions of years, these sinks were exhausted, and free oxygen could finally build up in the atmosphere. With this addition, the modern atmosphere was complete, though the relative amounts of the gases composing it would, and still continue to, shift. The composition of the atmosphere and the huge volume of water on Earth are two of the most important factors affecting climate.

While the atmosphere was forming above, the surface of the Earth was cooling to form a solid crust of rock about 3.7 billion years ago (although there are indications that this process may have started as early as 4.4 billion years ago). Regardless of precisely when this took place, it represented the beginning of tectonic processes that have continued ever since. Molten rock from the mantle constantly wells up from deep fissures and solidifies into relatively dense rock, while less-dense rock floats higher on the magma and is pushed around on the slow conveyor belts of mantle-formed rock (Figure 9.1). The denser rock forms oceanic plates that are lower and covered in water, and the lighter rock forms continental plates, though part or all of a continental plate may be submerged under a shallow sea. The motion of these plates, the rearranging of the continents, and the amount and types of minerals exposed to the atmosphere play a huge role in the climate. Not only do the continents and oceans move through different climate zones, but the continents also affect climate based on their size, and the weathering of rock on the continents plays a large role in the composition of the atmosphere. For example, rock that is enriched in organic matter will release abundant amounts of carbon dioxide as it weathers, while rock rich in feldspar and mica will take up carbon dioxide.

Nearly one billion years ago, the Earth began fluctuating between warm and cool periods lasting roughly 150 million years each. During the cool periods, there is usually persistent ice at the poles; during the warm periods there is little or no glaciation anywhere on Earth. Today, we are still in a cool period. The world has been much hotter for much of its history, but it has also been a bit cooler. Through the shifting global climate and the movement of the tectonic plates, what is now the Midwest has at times been at the bottom of a shallow sea;; a plain with swamps, rivers, and grasslands;; and under very thick ice.

Figure 9.1: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Snowball Earth

There is evidence suggesting that several times the entire surface of the planet was covered in ice, a hypothesis called Snowball Earth (Figure 9.2). Glacial deposits discovered near Lake Huron and elsewhere show that starting about 2.4 billion years ago the entire surface of the Earth may have been covered in ice for as long as 300 million years, an event known in North America as the Huronian glaciation. At that time the continental plates made up less than half as much of the Earth’s surface as they do today and were unified as the continent Arctica. It may have been early life’s production of oxygen that reacted with and lowered the amount of the greenhouse gas methane in the atmosphere, which tipped the Earth toward a series of cooling feedbacks, causing ice to spread from pole to pole.

An ice-covered planet would stay ice-covered because almost all of the sun’s energy would be reflected back into space, but this did not happen on Earth because of tectonics: The Snowball Earth cycle was eventually disrupted by volcanic activity. While the Earth was covered in ice, volcanoes continued to erupt, dumping carbon dioxide and methane into the atmosphere. These gases are usually removed from the atmosphere by organisms and the weathering of rocks, but this was not possible through miles of ice! After millions of years, the concentrations of methane and carbon dioxide increased to the point that greenhouse warming began to melt the ice sheets. Once the melting started, more of the sun’s energy was absorbed by the surface, and the warming feedbacks began. Because the oceans had been covered, nutrients from volcanic gases and chemical changes in the rocks accumulated in the waters. Once they were re-exposed to light, a population explosion of cyanobacteria produced more and more oxygen capable of combining with freshly thawed carbon sources to make more carbon dioxide, further enhancing the warming. Rocks in the Midwest do not contain direct evidence for this Snowball Earth, or for Earth history for several tens of millions of years afterward.

For the next 1.5 billion years, the Midwest, free of ice, drifted around the surface of the Earth. A new supercontinent—Rodinia—formed, and the part that is now North America was stable, forming what is known as a craton, or continental interior relatively free of the folding and faulting that characterizes continental margins that are subjected to mountain building and other plate tectonic processes. The Midwest was under water for most of this time, and the simple lifeforms sustained on it produced the stromatolites, banded iron formations, and other evidence found in rocks of the Superior Upland Basin.

Figure 9.2: Snowball Earth periods during the Proterozoic.

About 850 million years ago, during the Cryogenian, the Earth entered a 200- million-year ice age. The part of Rodinia that would eventually become North America was near the equator. There were two more Snowball Earth cycles during this time. The fact that North America was at such a low latitude yet contained glaciers, is strong evidence that the Earth really did freeze over completely. These events are not recorded in Midwestern rocks.

Life and Climate

By 635 million years ago, the Earth had warmed again, and the North American continent, of which the Midwest was the central part, moved towards the equator. During the Ordovician, much of the Midwest was covered by very pure, quartz- rich sand. This sandstone, now known as the St. Peter Sandstone, is an enduring enigma to geologists because it is not clear how all the non-quartz minerals could have been removed. Quartz is extremely resistant to weathering, and it is often the last mineral left when other minerals have weathered away. This suggests that the climate was intensely wet and warm and that the sand was washed or blown (or both!) back and forth for a long time before being buried. This weathering is all the more remarkable because land plants, which play a huge role in the weathering of rock on the continents today, were only just evolving. But they were similar to lichens, and even lichens can contribute to the weathering of rock.

After deposition of this sand, fossil evidence in Wisconsin and elsewhere in the Midwest shows that, with the warmer temperatures and higher sea level, at least some of the Midwest continued to be covered by a warm inland sea, this time with limestone that had formed from the innumerable remains of living creatures as the predominate sediment. This sea persisted in some form for several hundred million years, and, despite a global dip in temperature from 460 to 430 million years ago during yet another ice age, it was warm enough to maintain tropical reef ecosystems for much of that time (Figure 9.3). These reefs were among the largest the world had ever seen, and one of the largest was in the Midwest. It grew around the shallow edges of a wide basin centered on Michigan. Today, the reef deposits (as limestone) can be found in much of the Midwest, but they are thickest in Indiana and Illinois—as thick as 300 meters (1000 feet)! Although much of this limestone is under the surface, limestone quarries throughout the area have yielded building stones that show the richness of the fauna that constructed these impressive structures. Many buildings in the Midwest have facings or walls made of stone quarried from these ancient reefs (Figure 9.4).

Figure 9.3: Life in the Silurian reefs.

Figure 9.4: A building in Indiana with a facing made of Bedford limestone.

Silurian deposits of salt centered on Michigan indicate that the North American climate experienced little precipitation during the warm period beginning 430 million years ago. Eventually, the salinity in the shallow seas of the ancient Midwest returned to normal in the Devonian, and a rich and diverse fauna occupied the sea floor, including reefs and other habitats. At the end of the Devonian, however, the fauna suffered a mass extinction that eliminated many of the more important groups of reef-builders and other animals that occupied the shallow seas. The causes of this mass extinction, which actually occurred in a series of steps, are still uncertain.

As the continent continued across the Tropic of Capricorn and the equator, the cycle of warming and cooling repeated yet again, and, by 360 million years ago, glaciers formed near the South Pole. Although the Earth’s temperature was falling during this time, the Midwest remained relatively warm, and the shallow, tropical seas continued to cover most of the Midwest until sea level dropped in the middle of the Carboniferous.

By this time, complex land plants had evolved and diversified, and they rapidly colonized the newly exposed landscape. Terrestrial fossils from this time show that the climate was humid and supported swampy forests. These swamps eventually became the coal deposits of the southern Midwest, especially in southern Illinois, Indiana, and Ohio. Farther north, the land was exposed and no record exists of this time, although it is likely that the area was traversed by rivers. The ice age that had started near the end of the Devonian around 360 million years ago intensified during the later part of the Carboniferous. Deposits in the southern part of the Midwest, in particular, show a cyclicity of rising and falling sea level that was caused by advance and retreat of the large ice cap in the Southern Hemisphere. This ice age lasted well into the Permian period, ending about 260 million years ago, when warm temperatures again became the norm.

Tectonic forces had by then pushed the Midwest above sea level, and erosional forces tended to dominate, so little direct evidence of the climate during this time is preserved, although adjacent areas have evidence that climate in the area was very warm and relatively arid. Worldwide temperatures, however, began to dip again around 150 million years ago, though perhaps not enough for ice sheets to form, and the tropics, presumably including the Midwest itself, became more humid. Pangaea, a supercontinent composed of nearly all the landmass on Earth, broke up into continents that would drift into increasingly familiar positions. By the Cretaceous, the world was heating up again, and a new body of water, the Western Interior Seaway (Figure 9.5), covered parts of North America, leaving fossils of tropical marine animals in Iowa and Minnesota.

Figure 9.5: The Western Interior Seaway.

Figure 9.6: Eskers are composed of sand and gravel deposited by streams that flowed under the ice, partially filling the sub-ice channel. When the ice melts, the sinuous deposit remains.

Figure 9.7: Kettle lakes formed where large, isolated blocks of ice became separated from the retreating ice sheet. The weight of the ice left a shallow depression in the landscape that persists as a small lake.

With the end of the Cretaceous and the extinction of the dinosaurs 65 million years ago, the world was cooling again. Antarctica moved south, and by 30 million years ago temperatures were low enough that glaciers were growing on its mountains. About 15 million years ago, ice covered much of that continent and had begun to form on Greenland. Eventually, by about 2 million years ago, a sheet of sea-ice formed over the Arctic, and other sheets spread over northern Asia, Europe, and North America and then pushed their way south. This is where the geologic record of climate in the Midwest picks up again.

Since just 800,000 years ago, a kind of equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces. During the ice’s maximum extent, it reached from the North Pole to where Chicago is now located, covering the northern half of the Midwest, while the southern half was far colder than it is today. The temperatures in areas not then covered in ice were moderated by its presence; the summers were much cooler, yet the winters were only a little cooler than they are today. The area was also somewhat wetter than it is today, with wetlands and forests covering much of what would later become grassland. The glaciers last retreated from the area around 10,000 years ago, leaving behind the Great Lakes and many geologic features that define the landscape of the upper Midwest today, including eskers (Figure 9.6), kettles (Figure 9.7), and thick deposits of sand and gravel. The climate was warmer and slightly drier, much like that we experience today.