Past Climate of the South Central

Past Climates

Climate, like other parts of the Earth system, is not static but changes over time, on both human and geologic 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 Earth as a whole have varied through time, altering what kinds of climates are possible. The land that is now the South Central US has been arid, warm-temperate, and tropical at different times during the past 500 million years!

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 South Central. 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 find. However, we can still say something about the climate of the ancient Earth, in large part due to our knowledge of 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 (CO₂), and nitrogen to the atmosphere. As the Earth cooled enough for liquid water to form, the vapor formed clouds from which the rain poured forth in such a deluge as the planet will never experience again. These torrential rains were constant for millions of years, absorbing salt and other minerals from the soil 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 filled, 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 about 3.7 billion years ago, the surface of the Earth was cooling to form a solid crust of rock (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 more buoyant rock floats higher on the magma and is pushed around on the slow conveyor belts of mantle-formed rock (Figure 9.1). Denser rock forms oceanic plates that are lower and covered in water, and 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.

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

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, while during the warm periods there is little or no glaciation anywhere on Earth. Today, we are still in a cool period— although the world has been cooler than it is at present, it has been much hotter for much of its history (Figure 9.2). Through the shifting global climate and the movement of the continents, what is now the South Central has at times been submerged beneath a shallow sea, a verdant plain filled with swamps and rivers, and even buried under ice.

Figure 9.2: Changing global climate throughout the last 542 million years. These data were compiled using the ratios of stable oxygen isotopes found in ice cores and the carbonate skeletons of fossil organisms.

Snowball Earth

There is evidence suggesting that the entire surface of the planet has been covered in ice several times, a hypothesis called Snowball Earth (Figure 9.3). 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 towards a series of cooling feedbacks, causing ice to spread from pole to pole.

An ice-covered planet would remain in that state because almost all of the sun’s energy would be reflected back into space, but this did not happen on Earth because of plate 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. While these gases are usually removed from the atmosphere by organisms and the weathering of rocks, this was not possible through miles of ice! After millions of years, the concentrations of methane and CO₂ 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 derived 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, which was capable of combining with freshly thawed carbon sources to make more carbon dioxide, further enhancing the warming.

Figure 9.3: Snowball Earth periods during the Proterozoic.

For the next 1.5 billion years, the South Central, 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. About 850 million years ago, during the Cryogenian, the Earth entered a 200-million-year ice age, during which there were two more Snowball Earth cycles. Although the part of Rodinia that would eventually become North America was located near the equator, the fact that North America was at such a low latitude, yet had glaciers, is strong evidence that the Earth really did freeze over completely. However, no direct evidence for any of the Snowball Earth cycles comes from rocks in the South Central.

Life and Climate

With the start of the Paleozoic era, climates across the world were warm, and North America was located in the low and warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, North America moved northward; by about 480 million years ago, what would become the South Central US was located just below the equator (Figure 9.4). In Texas, the presence of Cambrian sandstone indicates that sediments were carried to the sea from the land to the northwest. East of the sandstone, a rich and diverse marine fauna, including trilobites, brachiopods, bivalves, sponges, and other invertebrates, is contained within dolomite and limestone units. Although sedimentary rocks of Cambrian age are only exposed in Texas, most of the South Central was probably home to warm, shallow seas that persisted into the Ordovician.

Figure 9.4: The location of the continents during the A) early and B) late Cambrian. Note the position of North America relative to the equator.

At the end of the Ordovician, from 460 to 430 million years ago, the Earth fell into another ice age, but Silurian and Devonian fossils in Oklahoma (including trilobites and brachiopods) indicate that the South Central still contained warm, shallow seas through the early Devonian. After an episode of uplift and erosion, however, the environment changed dramatically. The seas became deeper, and plankton productivity grew so high that it depleted all the oxygen from the seafloor and sediments. The lack of oxygen allowed organic matter to accumulate instead of decaying, leading to the deposition of black, carbon-rich shale. Drill cores show us that this rock occurs throughout the subsurface in the western two-thirds of Oklahoma and northern Texas; it is one of the richest sources of petroleum in those states.

From 430 to 300 million years ago, North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. Glaciation in the southern hemisphere occurred during the late Devonian, while the supercontinent Gondwana was located over the South Pole. At the same time, while the South Central States were still submerged, the oceans between Gondwana and North America began to close (Figure 9.5). By the early Carboniferous, ice capped the South Pole and began to expand northward. Although the Earth’s temperature fell during this time and the growing glaciers far to the south caused sea levels to drop, the northern part of the South Central returned to a warm, shallow sea with limestone and abundant marine life, including brachiopods, corals, and echinoderms, while the southern part accumulated thick deposits of sandstone and shale. By the late Carboniferous, North America had collided with Gondwana, advancing the formation of Pangaea— a supercontinent composed of nearly all the landmass on Earth. The Ouachita Mountains, remnants of what was once a chain of mountains that may have been as high and broad as the Tibetan Plateau, bear witness to this event. Sedimentary rocks indicate that most of the South Central was now covered by rivers and plains; Oklahoma, Kansas, and Arkansas had flourishing coastal swamps filled with vegetation that has since been transformed into rich coal beds by heat and pressure. Many Carboniferous rocks in the South Central, especially in Kansas, are cyclic, showing repeated episodes of sea level fall and rise as the ice cap in the Southern Hemisphere advanced and retreated.

During the Permian, as sea levels dropped, shallow marine waters gave way to lowland coastal areas, and most of the South Central also became terrestrial. The Permian Basin in western Texas remained marine, however, and an enormous barrier reef formed around its rim. Part of this reef can be seen at El Capitan in Guadalupe Mountains National Park (Figure 9.6). During the Permian, the climate was drier than that of the Carboniferous, and extensive salt and gypsum deposits indicate that evaporation rates were high. A shift in plant type—from water-loving ferns and horsetails to plants better adapted for drier conditions—is further evidence of arid conditions during this time. By the end of the Permian, the southern ice sheets had disappeared, and desert conditions existed in the core of the supercontinent (as indicated, for example, by what look like sand dunes preserved in sedimentary rocks).

Around 220 million years ago, the South Central moved north from the equator. The Earth remained warm and ice-free at the poles through much of the Mesozoic era, until worldwide temperatures began to dip again around 150 million years ago. After reaching its greatest size during the Triassic period, Pangaea began to break apart into continents that would drift toward their modern-day positions. The South Central’s climate gradually shifted, becoming wetter. Triassic rocks are known only from far western Oklahoma and Texas, where they contain a rich terrestrial and lake fauna of fishes, amphibians, and reptiles.

Figure 9.5: By the late Devonian (375 million years ago), the oceans between Gondwana and Euramerica had begun to close.

Jurassic outcrops in western Oklahoma are terrestrial and contain petrified wood, dinosaurs, and other reptiles indicative of lake environments, revealing that the environment there did not change much from the Triassic (except to become a bit wetter). Farther south, however, the breakup of Pangaea caused the Gulf of Mexico to rift open, flooding it with seawater. Because the climate was still relatively warm and dry, evaporation rates were high, and extremely thick deposits of salt accumulated there. These salt deposits have played a key role in trapping petroleum along the Gulf Coast. At the same time, the portion of the South Central that bordered the coastline began to subside, and thick deposits of coastal and marine sediments began to accumulate, a process that continues to this day.

Figure 9.6: El Capitan in western Texas, Guadalupe National Park. The large escarpment is part of a gigantic barrier reef that stretches into New Mexico under Carlsbad and back into Texas in the east. For most of this range, it is below the surface, but it crops out spectacularly here.

The Earth warmed near the beginning of the Cretaceous, and sea level rose. Throughout the Cretaceous, sea level was an average of 100 meters (330 feet) higher than it is today, largely as a result of water displacement by continental rifting and rapid sea-floor spreading. Shallow seaways spread over many of the continents, and by the start of the late Cretaceous, North America was divided in two by an inland sea known as the Western Interior Seaway (Figure 9.7). This sea flooded most of the South Central, covering older rocks and creating a wide belt of Cretaceous- and younger-aged rock that extends many hundreds of miles up the Mississippi River and covers all of Louisiana, about half of Texas, and parts of Arkansas, Oklahoma, and Missouri.

At the close of the Cretaceous, 65 million years ago, global climates (though still much warmer than those of today) were cooler than at the era’s start. At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when an asteroid or comet collided with Earth in what is now the northern Yucatán Peninsula in Mexico, just a few hundred miles away. The impact vaporized both water and rock, and formed tiny glassy spheres, called microtektites, from the melted rock. After that event, the climate may have cooled briefly (as suggested, for example, by an abundance of ferns), but it soon rebounded to a warmer state, and continued to warm into the Eocene.

Figure 9.7: The Western Interior Seaway.

By the early Cenozoic, the continents had approached their modern configuration, and India began to collide with Asia to form the Himalayas. The formation of the Himalayas had a significant impact on global climate, as the newly exposed rock began to serve as a sink to take up atmospheric CO₂. With the reduction of this greenhouse gas, global temperatures cooled. Antarctica moved south, and by 30 million years ago, temperatures were low enough that glaciers began to grow on its mountains. The South Central continued to accumulate sediment brought in by myriad rivers, including the antecedents to the Mississippi River. Sea level dropped, and the continued withdrawal of the sea is reflected in almost-parallel belts of progressively younger rocks that extend toward the Gulf Coast.

Eventually, a sheet of sea-ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since just 800,000 years ago, a type of equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces. The ice reached northern Missouri and northeastern Kansas during its maximum extent, while the rest of the South Central merely experienced a cooler climate than it does at present. The area was also somewhat wetter than it is today, with wetlands and forests covering much of what would later become grassland. Rocks of this age contain fossils of terrestrial vertebrates such as horses, camels, bison, mastodons, and mammoths. Much of the Mississippi River’s great delta and alluvial fan was deposited when the glacial ice melted, creating rivers that eroded older rocks as well as carrying sediments previously scoured by the glaciers.